US20090050530A1

US20090050530A1 - Process and Apparatus for Steam Cracking Hydrocarbon Feedstocks

Info

Publication number: US20090050530A1
Application number: US12/144,305
Authority: US
Inventors: David B. Spicer; Arthur R. Di Nicolantonio; James M. Frye; Richard C. Stell; James N. McCoy; Robert D. Strack
Original assignee: ExxonMobil Chemical Patents Inc
Current assignee: ExxonMobil Chemical Patents Inc
Priority date: 2007-08-21
Filing date: 2008-06-23
Publication date: 2009-02-26

Abstract

The present disclosure provides a process for treating a hydrocarbon feedstock comprising: (a) feeding the hydrocarbon feedstock at a linear velocity equal to or less than 0.9 m/s to a first preheating zone in the convection section of a steam cracking furnace; (b) preheating the hydrocarbon feedstock in the first preheating zone to vaporize less than 99 wt. % of the hydrocarbon feedstock to form a vapor-liquid mixture; (c) separating at least a portion of the vapor-liquid mixture to form a vapor fraction and a liquid fraction; and (d) feeding at least a portion of the vapor fraction to the steam cracking furnace.

Description

RELATIONSHIP TO OTHER APPLICATIONS

This application claims benefit of and priority to International Patent Application Serial Number PCT/US2007/018486, filed Aug. 21, 2007.

FIELD OF THIS DISCLOSURE

This disclosure pertains to a method for the manufacture of light olefins in a steam cracking furnace or a pyrolysis furnace, more particularly to a process of steam cracking a hydrocarbon feedstock containing at least 0.01 wt. % low-volatile compounds.

BACKGROUND OF THIS DISCLOSURE

Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a steam cracking furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products, including olefins, leave the steam cracking furnace for further downstream processing.
Conventional steam cracking systems have been effective for cracking high-quality feedstocks such as natural gas liquids, (NGL's), gas oil and naphtha. However, steam cracking economics sometimes favor cracking low cost heavy feedstock such as, by way of non-limiting examples, condensates, which is an associated oil occurring in small quantities with the production of gas from gas fields, crude oils, atmospheric resids, also known as atmospheric pipestill bottoms, and vacuum gas oils, crude oil, vacuum gas oil and atmospheric resid contain high molecular weight, low-volatile components with boiling points in excess of 590° C. and/or sometimes coke precursors with boiling points in excess of 760° C. The low-volatile components and/or coke precursors of these feedstocks would lay down as coke in the convection section of conventional steam cracking furnaces as the lighter components were vaporized. Only very low levels of low-volatile components and coke precursors can be tolerated in the convection section downstream of the point where the lighter components have fully vaporized because the coke deposition normally fouls tubes in convection section which lowers the heat transfer efficiency and increase the pressure drop in the tubes.
Additionally, some naphthas are contaminated with heavy crude oil containing low-volatile components and coke precursors. Conventional steam cracking furnaces do not have the flexibility to process residues, crude oils, or many residue- or crude-contaminated gas oils or naphthas which are contaminated with low-volatile components and coke precursors.
To address coking problems, U.S. Pat. No. 3,617,493, incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230 and 590° C. The vapors are cracked in the steam cracking furnace into olefins, and the separated liquids from the second flash tank are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize 50% of the heavy feedstock with superheated steam and the removal of the residual, separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
U.S. Pat. No. 5,190,634, incorporated herein by reference, discloses a process for inhibiting coke formation in a furnace by preheating the feedstock in the presence of a small, critical amount of hydrogen in the convection section. The presence of hydrogen in the convection section inhibits the polymerization reaction of the hydrocarbons thereby inhibiting coke formation.
U.S. Pat. No. 5,580,443, incorporated herein by reference, discloses a process wherein the feedstock is first preheated and then withdrawn from a preheater in the convection section of the pyrolysis furnace. This preheated feedstock is then mixed with a predetermined amount of steam (the dilution steam) and is then introduced into a vapor-liquid separator to separate and remove a required proportion of the low-volatile components and coke precursors as liquid from the separator. The separated vapor from the vapor-liquid separator is returned to the pyrolysis furnace for heating and cracking.
U.S. Pat. No. 6,632,351, incorporated herein by reference, discloses a process for pyrolyzing a crude oil feedstock or crude oil fractions containing pitch feedstock, and a pyrolysis furnace, comprising feeding the crude oil or crude oil fractions containing pitch feedstock to a first stage preheater within a convection zone, wherein said crude oil or crude oil fractions containing pitch feedstock is heated within the first stage preheater to an exit temperature of at least 375° C. to produce a heated vapor-liquid mixture, withdrawing from first stage preheater the vapor-liquid mixture to a vapor-liquid separator, separating and removing the gas from the liquid in the vapor-liquid separator, and feeding the removed gas to a second preheater provided in the convection zone, further heating the temperature of said gas to a temperature above the temperature of the gas exiting the vapor-liquid separator, introducing the preheated gas into a radiant zone within the pyrolysis furnace, and pyrolyzing the gas to olefins and associated by-products.
U.S. Pat. No. 7,097,758, incorporated herein by reference, discloses a process to increase the non-volatile removal efficiency in a flash drum in the steam cracking system. The gas flow from the convection section is converted from mist flow to annular flow before entering the flash drum to increase the removal efficiency. The conversion of gas flow from mist flow to annular flow is accomplished by subjecting the gas flow first to at least one expander and then to bends of various degrees and forcing the flow to change directions at least once. The change of gas flow from mist to annular helps coalesce fine liquid droplets and thus increases the efficiency with which they are removed from the vapor phase.
U.S. Pat. No. 7,138,047, incorporated herein by reference, discloses a process for feeding or cracking hydrocarbon feedstock containing non-volatile hydrocarbons comprising: heating the hydrocarbon feedstock, mixing the hydrocarbon feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase, and varying the amount of the fluid and/or the primary dilution steam stream mixed with the hydrocarbon feedstock in accordance with at least one selected operating parameter of the process, such as the temperature of the flash stream before entering the flash drum.
U.S. patent application Ser. No. 11/068,615, filed Feb. 28, 2005, incorporated herein by reference, describes a process for cracking hydrocarbon feedstock which mixes hydrocarbon feedstock with a fluid, e.g., hydrocarbon or water, to form a mixture stream which is flashed to form a vapor phase and a liquid phase, the vapor phase being subsequently cracked to provide olefins, and the product effluent cooled in a transfer line exchanger, wherein the amount of fluid mixed with the feedstock is varied in accordance with a selected operating parameter of the process, e.g., temperature of the mixture stream before the mixture stream is flashed.
U.S. application Ser. No. 10/851,434, filed May 21, 2004, incorporated herein by reference, and U.S. Provisional Application Ser. No. 60/573,474, filed May 21, 2004, incorporated herein by reference, describe a process to increase the non-volatile removal efficiency in a flash drum used in a steam cracking system, the flash drum having a lower boot comprising an inlet for introducing stripping steam, a ring distributor for recycle quench oil, anti-swirl baffles, and a grate.
There is therefore a need for a novel and energy efficient process for steam cracking hydrocarbon feedstocks with low level of coke formation. The present inventors surprisingly find that the coke formation in the first preheating section of the first preheating zone is negligible when the feedstock is fed at an inlet linear velocity below a threshold value and the feedstock is preheated to a temperature below a threshold value. Furthermore, the coke formation in the first preheating zone is minimized so long as at least 1 wt. % of the hydrocarbon feedstock exits the first preheating zone in liquid phase. This disclosure therefore offers a steam cracking process capable of processing hydrocarbon feedstock with minimum coke formation in the preheating zone and low pressure drop for the feedstock to flow through the convection section by optimizing the linear velocity of the feedstock entering the preheating section of a steam cracking furnace.

SUMMARY OF THIS DISCLOSURE

In some embodiments, the present disclosure provides a process for treating a hydrocarbon feedstock comprising:

- (a) feeding the hydrocarbon feedstock at a linear velocity equal to or less than 0.9 m/s to a first preheating zone in the convection section of a steam cracking furnace;
- (b) preheating the hydrocarbon feedstock in the first preheating zone to vaporize equal to or less than 99 wt. % of the hydrocarbon feedstock to form a vapor-liquid mixture;
- (c) separating at least a portion of the vapor-liquid mixture to form a vapor fraction and a liquid fraction; and
- (d) feeding at least a portion of the vapor fraction to the steam cracking furnace.

According to one embodiment, the convection section comprises multiple banks of heat exchange tubes and the hydrocarbon feedstock flows inside the tubes.
In a preferred embodiment, the hydrocarbon feedstock is fed to the first preheating zone at a linear velocity in the range of 0.05 to 0.85 m/s, preferably from 0.1 to 0.8 m/s, and more preferably from 0.1 to 0.75 m/s.
In one preferred embodiment of this disclosure, the first preheating zone comprises a first preheating section and a second preheating section, wherein the hydrocarbon feedstock is supplied to the first preheating section at a pressure in the range of 790 to 1825 kPa-a (kilopascal absolute), preferably in the range of 790-1450 kPa-a, more preferably in the range of 790-1400 kPa-a, even more preferably in the range of 790-1200 kPa-a, and most preferably in the range of 790-1100 kPa-a, and a temperature in the range of 25 to 250° C. to form a preheated hydrocarbon product exiting the first preheating section at a temperature in the range of about 100 to 350° C., and then at least a portion of the preheated hydrocarbon product is supplied together with a first diluent stream to the second preheating section to form the vapor-liquid mixture exiting the first preheating section at a temperature in the range of 350 to 500° C. and comprising at least 1 wt. % liquid phase based on the total weight of the hydrocarbons in the vapor-liquid mixture.
In some aspects, the hydrocarbon feedstock comprises one or more of steam cracked gas oil and residues, gas oils, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, atmospheric resid, heavy resid, C₄'s/residue admixture, naphtha/residue admixture, Fischer-Tropsch liquids, Fischer-Tropsch gases, Fischer-Tropsch waxes, and low sulfur waxy resid. In one embodiment, about 10 to 99.99 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887. In another embodiment, about 10 to 95 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.
In one embodiment, the present disclosure also provides a process for cracking a hydrocarbon feedstock to light olefins in a steam cracking furnace having radiant section burners and a convection section, the convection section comprises a first bank, a second bank and a third bank of heat exchange tubes, the process comprising:

- (a) feeding the hydrocarbon feedstock in at least 99 wt. % liquid phase at a pressure in the range of 790 to 1825 kPa-a, preferably in the range of 790-1450 kPa-a, more preferably in the range of 790-1400 kPa-a, even more preferably in the range of 790-1200 kPa-a, and most preferably in the range of 790-1100 kPa-a, and a temperature in the range of 25 to 250° C. to the first bank of heat exchange tubes provided in the convection section, with a linear velocity in the range of 0.1 to 0.9 m/s;
- (b) preheating the hydrocarbon feedstock to form a preheated hydrocarbon product having a temperature less than 350° C.;
- (c) supplying at least a portion of the preheated hydrocarbon product with a first diluent stream to the second bank of heat exchange tubes to vaporize at least a portion of the hydrocarbon feedstock to form a vapor-liquid mixture having a temperature in the range of 350 to 500° C. and comprising at least 1 wt. % liquid based on the total weight of the hydrocarbons in the vapor-liquid mixture;
- (d) separating at least a portion of the vapor-liquid mixture from step (c) to form a vapor fraction and a liquid fraction;
- (e) feeding at least a portion of the vapor fraction from step (d) to the third bank of heat exchange tubes and further to radiant section of the steam cracking furnace to from a product comprising the light olefins,
  wherein the hydrocarbon feedstock comprises one or more of steam cracked gas oil and residues, gas oils, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, atmospheric resid, heavy resid, C4's/residue admixture, naphtha residue admixture, and low sulfur waxy resid.

In some embodiments, the vapor-liquid mixture has a temperature in the range of 400 to 500° C. and comprises at least 2 wt. % liquid based on the total weight of the hydrocarbons in the vapor-liquid mixture and wherein 50 to 99.99 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887. In other embodiments, the vapor-liquid mixture has a temperature in the range of 425 to 500° C. and comprises at least 3 wt. % liquid based on the total weight of the hydrocarbons in the vapor-liquid mixture and wherein 40 to 99.99 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887. In yet other embodiments, the vapor-liquid mixture has a temperature in the range of 435 to 500° C. and comprises at least 4 wt. % liquid based on the total weight of the hydrocarbons in the vapor-liquid mixture and wherein 30 to 99.99 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887. In still yet other embodiments, the vapor-liquid mixture has a temperature in the range of 450 to 500° C. and comprises at least 5 wt. % liquid based on the total weight of the hydrocarbons in the vapor-liquid mixture and wherein 10 to 99.99 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.
There is now provided apparatus adapted for steam cracking a hydrocarbon feedstock to light olefins, wherein 10 to 99.99 wt. % of the hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887, the apparatus comprises:

- (a) a steam cracking furnace comprising radiant section burners adapted to provide radiant heat and hot flue gas and a convection section having a first bank, a second bank and a third bank of heat exchange tubes;
- (b) means for feeding the hydrocarbon feedstock to the first bank of heat exchange tubes with a linear velocity in the range of 0.1 to 0.9 m/s;
- (c) means for maintaining the hydrocarbon feedstock exiting the first bank of heat exchange at a temperature less than 350° C.;
- (d) means for supplying a first diluent stream together with at least a portion of the preheated hydrocarbon product from (c) to the second bank of heat exchange tubes adapted to vaporize less than 99 wt. % of the hydrocarbon feedstock to form a vapor-liquid mixture;
- (e) means for maintaining the vapor-liquid mixture exiting the second bank of heat exchange tubes at a temperature in the range of 350 to 500° C.;
- (f) a vessel adapted to separate at least a portion of the vapor-liquid mixture from step (e) in to form a vapor fraction and a liquid fraction; and
- (g) means for feeding at least a portion of the vapor fraction from step (f) to the third bank of heat exchange tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow diagram of a steam cracking furnace of this disclosure.

DETAILED DESCRIPTION OF THIS DISCLOSURE

The present disclosure relates to a process for heating and steam cracking a hydrocarbon feedstock to produce light olefins, e.g., ethylene and/or propylene. Typical products of a steam cracking furnace include, but are not limited to, ethylene, propylene, butenes, butadiene, benzene, hydrogen, methane, and other associated olefinic, paraffinic, and aromatic products. Ethylene is the predominant product, typically in the range of 15 to 30 wt. %, based on the weight and composition of the vaporized feedstock. The process of this disclosure comprises preheating a hydrocarbon, mixing the preheated hydrocarbon with a diluent stream comprising at least one of steam, water, N₂, H₂, and hydrocarbon(s) to form a mixture, further preheating the mixture to form a vapor-liquid mixture, separating at least a portion of the vapor-liquid mixture in a vessel to form a vapor fraction and a liquid fraction, and feeding at least a portion of the vapor fraction to the steam cracking furnace for further heating and cracking.
Unless otherwise stated in this disclosure, all percentages, parts, ratios, etc., are by weight. A reference to a compound or component includes the compound or component by itself, as well as in combination with other compounds or components, such as mixtures of compounds. Further, when an amount, concentration, or other value or parameter is given as a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of an upper preferred value and a lower preferred value, regardless whether ranges are separately disclosed.
As used herein, “low-volatile components”, sometimes referred as non-volatile components or resids, are the fraction of the hydrocarbon feed with a nominal boiling point above 590° C. as measured according to ASTM D-2887. This disclosure works well with a hydrocarbon feedstock containing 0.01 to 90 wt. % low-volatile components. As used herein, “coke precursors”, are the fraction of the hydrocarbon feed with a nominal boiling point above 760° C. as measured according to ASTM D-2887. This disclosure works well with a hydrocarbon feedstock containing 0.01 to 90 wt. % coke precursors. The boiling point distribution of the hydrocarbon feed is measured by Gas Chromatograph Distillation (GCD) according to ASTM D-2887.
The term “substantially liquid phase” as used herein means at least 99 wt. %, preferably at least 99.5 wt. %, even more preferably at least 99.9 wt. %, and most preferably at least 99.99 wt. %, liquid phase. For example, a stream in substantially liquid phase means that at least 99 wt. %, preferably at least 99.5 wt. %, even more preferably at least 99.9 wt. %, and most preferably at least 99.99 wt. %, of the stream is in liquid phase.
The term “vapor fraction” as used herein means a fraction predominately, preferably at least 75 wt. %, more preferably at least 90 wt. %, even more preferably at least 95 wt. %, in vapor phase. The term “liquid fraction” as used herein means a fraction is predominate, preferably at least 75 wt. %, more preferably at least 90 wt. %, even more preferably at least 95 wt. %, in liquid phase.
The term “predominately” or “predominate” as used herein means more than 50 wt. %. For example, a diluent stream comprises predominately steam means that the diluent stream comprises more than 50 wt. % steam.

Hydrocarbon Feedstock

The hydrocarbon feedstock can comprise at least a portion, such as in the range of 0.01 to 90 wt. %, 1 to 90 wt. %, or 5 to 90 wt. %, of low-volatile components and coke precursors. Such feedstock could comprise, by way of non-limiting examples, one or more of steam cracked gas oil and residues, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, Fischer-Tropsch gas oils, Fischer-Tropsch waxes, natural gasoline, distillate, virgin naphtha, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphtha to gas oil condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, atmospheric residue, heavy residue, C₄'s/residue admixtures, naphtha/residue admixtures, hydrocarbon gas/residue admixtures, hydrogen/residue admixtures, gas oil/residue admixtures, crude oil, and low sulfur waxy resid.
The hydrocarbon feedstock can have a nominal end boiling point of at least 315° C., generally greater than 510° C., typically greater than 590° C., for example greater than 760° C. The economically preferred feedstocks are generally low sulfur waxy residues, atmospheric residues, naphthas contaminated with crude oil, various residue admixtures, and crude oils.
The gas to liquid (GTL) technologies, such as SMDS, AGC-21 and SSPD processes for production of middle distillates show a great potential for fuel alternatives and higher value products. The product of any Fischer-Tropsch gas to liquid process may further be subjected to, optionally hydrotreating, fractionating to Fischer-Tropsch liquids (also called Fischer-Tropsch naphtha), Fischer-Tropsch gas oils (also called Fischer-Tropsch gases), and Fischer-Tropsch waxes. Fischer-Tropsch naphtha, Fischer-Tropsch gas oils and Fischer-Tropsch waxes produced by these GTL processes are attractive for steam cracking applications because of their high concentration of normal paraffin components. The high paraffinic content of the Fischer-Tropsch liquids and the Fischer-Tropsch gases allows them to be cracked at very high severities not normally seen for conventional feedstocks.
In some embodiments, the process of this disclosure finds to be useful to process a feedstock comprise at least 1 wt. % of at least one of Fischer-Tropsch liquids, Fischer-Tropsch gases, Fischer-Tropsch waxes, crude oils, crude oils fraction. In other embodiments, the process of this disclosure finds to be useful to process a feedstock comprise at least 1 wt. % of at least one of Fischer-Tropsch liquids resids, Fischer-Tropsch gases resids, fraction of Fischer-Tropsch liquids, and fraction of Fischer-Tropsch gases.

Process Description

This disclosure is described below while referring to FIG. 1 as an illustration of one of many embodiments of this disclosure. It is to be understood that the scope of the disclosure may include any number and types of process steps between each described process step or between a described source and destination within a process step.
The steam cracking furnace may be any type of conventional olefins steam cracking furnace operated for production of lower molecular weight olefins, especially including a tubular steam cracking furnace. The tubes within the convection zone of the steam cracking furnace may be arranged as a bank of heat exchange tubes in parallel, or the tubes may be arranged for a single pass or multiple passes of the feedstock through the convection zone. At the inlet, the feedstock may be split among multiple single pass tubes, or may be fed to one single pass tube through which all the feedstock flows from the inlet to the outlet of the tubes, and more preferably through the whole of the convection zone. Preferably, the first preheating zone comprises at least one single pass bank of heat exchange tubes disposed in the convection zone of the steam cracking furnace. In a preferred embodiment, the convection zone comprises less than 20 passes tube having two or more banks through which the hydrocarbon feedstock flows. Within each bank, the tubes may be arranged in a coil or serpentine type arrangement within one row, and each bank may have several rows of tubes.
The number of pass(es) of heat exchange tubes disposed in the convection zone of the steam cracking furnace useful for this disclosure is in the range of 1 to 20. In some embodiments, the number of pass(es) of heat exchange tubes disposed in the convection zone of the steam cracking furnace useful for this disclosure is 2, 4, 6, 8, 10, 12, 14, 16, 18, or 20. In other embodiments, the number of pass(es) of heat exchange tubes disposed in the convection zone of the steam cracking furnace useful for this disclosure is 1, 3, 5, 7, 9, 11, 13, 15, 17, or 19.
In some embodiments, the steam cracking furnace 1 useful for this disclosure, as illustrated in FIG. 1, comprises a convection section 3 and a radiant section 13. The radiant section 13 comprises radiant burners which provide radiant heat and hot flue gas 12. The convection section 3 of the steam cracking furnace 1 comprises a first preheating zone 5 and a second preheating zone 11. The first preheating zone 5 comprises a first preheating section 7 and a second preheating section 9. The first preheating zone and the second preheating zone comprise multiple banks of heat exchange tubes. In one embodiment, the first preheating section 7 comprises a first bank of heat exchange tubes 15, the second preheating section 9 comprises a second bank of heat exchange tubes 17, and the second preheating zone 11 comprises a third bank of heat exchange tubes 19. The steam cracking furnace 1 also comprises a vessel 53. It is to be understood that the steam cracking furnace 1 may include any number of process equipments, such as, pump(s), valve(s), injection point(s), meter(s), gauge(s), and controlling device(s).
A hydrocarbon feedstock 31 comprising at least a portion, such as 0.01 to 90 wt. %, 1 to 90 wt. %, or 5 to 90 wt. %, of low-volatile components and coke precursors is supplied to and preheated in the first preheating section 7 of the first preheating zone 5 in the convection section 3 of a steam cracking furnace 1. The heating of the hydrocarbon feedstock can take any form known by those of ordinary skill in the art. However, it is preferred that the heating comprises indirect contact of the hydrocarbon feedstock in the first preheating section 7 with hot flue gases 12 from the radiant section 13 of the furnace. This can be accomplished, by way of non-limiting example, by passing the hydrocarbon feedstock through the first bank of heat exchange tubes 15 located within the first preheating section 7.
The pressure at which the hydrocarbon feedstock is fed to the inlet of the first preheating section in the convection zone is maintained to ensure a pressure less than 1825 kPa-a, preferably less than 1480 kPa-a, more preferably less than 1400 kPa-a, and most preferably less than 1200 kPa-a. In some embodiments, the pressure and temperature at which the hydrocarbon feedstock is fed to the inlet of the first preheating section in the convection zone is maintained to ensure a pressure in the range of between 790-1825 kPa-a, more preferably from 790-1480 kPa-a, yet more preferably in the range of 790-1450 kPa-a, even more preferably in the range of 790-1400 kPa-a, yet even more preferably in the range of 790-1200 kPa-a, and most preferably in the range of 790-1100 kPa-a, and a temperature in a range from 25 to 250° C., typically from 50° C.-200° C. The feeding rate at which the hydrocarbon feedstock is fed to the inlet of the first preheating section in the convection zone is controlled to maintain an inlet linear velocity of the hydrocarbon feedstock less than 1.1 m/s, preferably less than 1 m/s, more preferably less than 0.9, yet more preferably from 0.05 to 0.9 m/s, still yet more preferably from 0.1 to 0.9 m/s, and even more preferably from 0.2 to 0.8 m/s.
In preferred embodiments of this disclosure, the inlet linear velocity of the hydrocarbon feedstock is less than 0.9 m/s. In other embodiments, the inlet linear velocity of the hydrocarbon feedstock is in the range of 0.05 to 0.9 m/s. The following inlet linear velocities of the hydrocarbon feedstock are useful lower inlet linear velocity limits: 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8. The following inlet linear velocities of the hydrocarbon feedstock are useful upper inlet linear velocities limits: 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2 and 0.1. The inlet linear velocity of the hydrocarbon feedstock ideally falls in a range between any one of the above-mentioned lower limits and any one of the above-mentioned upper limits, so long as the lower limit is less than or equal to the upper limit. The inlet linear velocity of the hydrocarbon feedstock may be present in an amount in the range of 0.05 to 1 in one embodiment, alternatively 0.1 to 0.5, alternatively from 0.4 to 0.9, alternatively 0.5 to 0.85, alternatively 0.2 to 0.5, alternatively and from 0.5 to 0.6 in another embodiment.
We surprisingly find that the coke formation in the first preheating section of the first preheating zone is negligible. Furthermore, the coke formation in the first preheating zone is minimized so long as at least 1 wt. % of the hydrocarbon feedstock exits the first preheating zone in liquid phase. Therefore, the inlet linear velocity of the hydrocarbon feedstock may be selected to maintain optimum heat transfer efficiency and low pressure drop. An appropriate linear velocity for a particular feedstock improves both heat transfer efficiency and reduces pressure drop downstream of the first preheating section.
The preheated hydrocarbon feedstock 33 exits the first preheating section 7 and then is optionally mixed with a fluid 35. The fluid can be a liquid hydrocarbon, water, steam, or mixture thereof. The preferred fluid is water. The temperature of the fluid can be below, equal to or above the temperature of the preheated feedstock. The mixing of the preheated hydrocarbon feedstock and the fluid can occur inside or outside the steam cracking furnace 1, but preferably it occurs outside the furnace. The mixing can be accomplished using any mixing device known within the art.
The preheated feedstock exits the first bank of heat exchange tubes 15 at a temperature in the range of 100 to 350° C., preferably in the range of 150 to 325° C., more preferably in the range of 160 to 300° C., and most preferably in the range of 170 to 300° C. In one preferred embodiment, the preheated hydrocarbon feedstock 33 exits the first preheating section 7 in substantially liquid phase.
In a preferred embodiment in accordance with the present disclosure, a first diluent stream 37 is mixed with the preheated hydrocarbon feedstock. In some embodiments, the first diluent stream comprises at least one of steam, water, nitrogen, hydrogen, and hydrocarbons. Preferably the first diluent stream comprises at least one of steam and water. The first diluent stream can be preferably injected into the preheated hydrocarbon feedstock before the resulting stream mixture enters the second preheating section 9 of the first preheating zone 5 in the convection section 3 of a steam cracking furnace 1 for additional heating by radiant section flue gas.
The first diluent stream can have a temperature greater, lower or the same as the preheated hydrocarbon feedstock but preferably greater than that of the preheated hydrocarbon feedstock and serves to partially vaporize the preheated hydrocarbon feedstock. Alternatively, the first diluent stream is superheated before being injected into the preheated hydrocarbon feedstock.
The mixture of the preheated hydrocarbon feedstock, the first diluent stream, and optionally the fluid, is further heated in the second preheating zone 9 in the convection section 3 of a steam cracking furnace 1 to produce a vapor-liquid mixture. The heating can be accomplished, by way of non-limiting example, by passing the feedstock mixture through the second bank of heat exchange tubes 17 located within the second preheating zone 9 and thus heated by the hot flue gas from the radiant section of the furnace. The thus-heated mixture 39 leaves the convection section as a mixture stream.
The vapor-liquid mixture stream 39 temperature is limited by highest recovery/vaporization of volatiles in the feedstock while avoiding coking in the furnace tubes or coking in piping and vessels conveying the mixture from the vessel to the furnace. The selection of the vapor-liquid stream 39 temperature is also determined by the composition of the feedstock materials. When the feedstock contains higher amounts of lighter hydrocarbons, the temperature of the mixture stream 39 can be lower. When the feedstock contains a higher amount of low-volatile hydrocarbons, the temperature of the vapor-liquid mixture stream 39 should be higher. By carefully selecting a mixture stream temperature, the present disclosure can find applications in a wide variety of feedstock materials.
Typically, the temperature of the vapor-liquid mixture stream 39 is set and controlled at between 315 and 510° C., preferably between 370 and 490° C., more preferably between 400 and 480° C., and most preferably between 430 and 475° C. These values will change with the boiling curve and the concentrating volatiles in the feedstock.
The amount of liquid phase in the vapor-liquid mixture stream 39 is calculated based on the total weight of the hydrocarbons in the vapor-liquid mixture stream 39. The vapor-liquid mixture stream 39 comprises at least 1 wt. % liquid. The amount of liquid phase in the vapor-liquid mixture stream 39 is limited by highest recovery/vaporization of volatiles in the feedstock while avoiding coking in the furnace tubes or coking in piping and vessels conveying the mixture from the vessel to the furnace. The selection of the vapor-liquid stream 39 liquid content is also determined by the composition of the feedstock materials. When the feedstock contains higher amounts of lighter, hydrocarbons, the liquid content of the mixture stream 39 can be set lower. When the feedstock contains a higher amount of low-volatile hydrocarbons, the liquid content of the vapor-liquid mixture stream 39 should be set higher. By carefully selecting the liquid content of the mixture stream, the present disclosure can find applications in a wide variety of feedstock materials.
In some embodiments, the liquid content of the vapor-liquid mixture stream is in the range of from 1 wt. % to 99 wt. %. In other embodiments, the liquid content of the vapor-liquid mixture stream is in the range of from 2 wt. % to 60 wt. %. In yet other embodiments, the liquid content of the vapor-liquid mixture stream is in the range of from 5 wt. % to 30 wt. %. The following liquid contents of the vapor-liquid mixture stream are useful lower liquid contents limits: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 15. The following liquid contents of the vapor-liquid mixture stream are useful upper liquid contents limits: 99, 90, 80, 70, 60, 50, 40, 30, 25, 20 and 15. The liquid content of the vapor-liquid mixture stream ideally falls in a range between any one of the above-mentioned lower limits and any one of the above-mentioned upper limits, so long as the lower limit is less than or equal to the upper limit.
In a preferred embodiment, a diluent stream 41 comprising at least one of steam, water, nitrogen, hydrogen, and hydrocarbons, preferably predominately steam and/or water, is first heated in a bank of heat exchange tubes 43 to a desire temperature, preferably superheated. The resulting diluent stream 45 is withdrawn from the convection section 3 and optionally, splits into a second diluent stream 47 which is mixed with the vapor-liquid mixture 39 withdrawn from the second preheating section 9 before the vessel 53 and a bypass diluent stream 49 which bypasses the vessel and, instead is mixed with the vapor fraction from the vessel before the vapor fraction is cracked in the radiant section of the furnace. In one embodiment, the present disclosure can operate with all diluent stream 45 used as flash second diluent stream 47 with no bypass stream 49. Alternatively, the present disclosure can be operated with all diluent stream 45 directed to bypass stream 49 with no second diluent stream 47. In a preferred embodiment in accordance with the present disclosure, the ratio of the second diluent stream 47 to bypass stream 49 should be preferably 1:20 to 20:1, and most preferably 1:2 to 2:1. The second diluent stream 47 is mixed with the vapor-liquid mixture stream 45 to form a flash stream 51 before flashing in the vessel 53.
Preferably, the secondary diluent stream is superheated in a superheating section 43 in the furnace convection before splitting and mixing with the vapor-liquid mixture. The addition of the flash stream 47 to the vapor-liquid mixture stream 39 ensures the vaporization of nearly all volatile components of the mixture before entering the vessel 53.
The mixture 51 of the vapor-liquid mixture and second diluent stream is then introduced into a vessel 53 for separation into two fractions: a vapor fraction comprising predominantly volatile hydrocarbons and a liquid fraction comprising predominantly low-volatile hydrocarbons. The vapor fraction is preferably removed from the vessel 53 as an overhead vapor stream 55. The vapor stream 55, preferably, is fed back to the second preheating zone 11 of the convection section 3 of the steam cracking furnace 1 for optional heating and further supplying through crossover pipes 59 to the radiant section of the steam cracking furnace for cracking. The liquid fraction of the separation is removed from the vessel 53 as a bottoms stream 57.
The flash is conducted in at least one vessel. Preferably, the flash is a one-stage process with or without reflux. The vessel 53 is normally operated at 275 to 1400 kPa-a pressure and its temperature is usually the same or slightly lower than the temperature of the mixture 51 before entering the vessel 53. Typically, the pressure of the vessel 53 is 275 to 1400 kPa-a and the temperature is 310 to 510° C. Preferably, the pressure of the vessel 53 is 600 to 1100 kPa-a and the temperature is 370 to 490° C. More preferably, the pressure of the vessel 53 is 700 to 1000 kPa-a and the temperature is 400 to 480° C. Most preferably, the pressure of the vessel 53 is 700 to 760 kPa-a and the temperature is 430 to 480° C. Depending on the temperature of the flash stream, usually 50 to 95% of the mixture entering the vessel 53 is vaporized to the upper portion of the vessel 53, preferably 60 to 95%, more preferably 65 to 95%, and most preferably 70 to 95%.
In the vessel 53, the vapor fraction 55 usually contains less than 400 ppm of coke precursors, preferably less than 100 ppm, more preferably less than 80 ppm, and most preferably less than 50 ppm. The vapor fraction is very rich in volatile hydrocarbons (for example, 55-70 vol. %) and steam (for example, 30-45 vol. %). The boiling end point of the vapor phase is normally below 760° C.
The vapor fraction stream 55 continuously removed from the vessel 53 is preferably superheated in the steam cracking furnace lower convection section 11 to a temperature in the range of, for example, 430 to 650° C. by the flue gas 12 from the radiant section of the furnace. The vapor fraction is then introduced to the radiant section of the steam cracking furnace to be cracked.
The vapor fraction stream 55 removed from the vessel 53 can optionally be mixed with a bypass steam stream 49 before being introduced into the furnace lower convection section 11.

APPARATUS EMBODIMENTS

There is now provided apparatus adapted for steam cracking a hydrocarbon feedstock to light olefins, comprising

- (a) a steam cracking furnace comprising radiant section burners adapted to provide radiant heat and hot flue gas and a convection section comprised of a first bank, a second bank and a third bank of heat exchange tubes;
- (b) means for feeding the hydrocarbon feedstock to the first bank of heat exchange tubes with a linear velocity in the range of 0.1 to 0.9 m/s;
- (c) means for maintaining the hydrocarbon feedstock exiting the first bank of heat exchange at a temperature less than 350° C.;
- (d) means for supplying a first diluent stream together with at least a portion of the preheated hydrocarbon product from (c) to the second bank of heat exchange tubes adapted to vaporize less than 99 wt. % of the hydrocarbon feedstock to form a vapor-liquid mixture;
- (e) means for maintaining the vapor-liquid mixture exiting the second bank of heat exchange tubes at a temperature in the range of 350 to 500° C.;
- (f) a vessel adapted to separate at least a portion of the vapor-liquid mixture from step (e) in to form a vapor fraction and a liquid fraction; and
- (g) means for feeding at least a portion of the vapor fraction from step (f) to the third bank of heat exchange tubes and further to the radiant section of the steam cracking furnace.

The means for feeding in steps (b) and (g) can be any conventional pumping mechanism, or piping for transporting materials. The means for maintaining in steps (c) and (e) can be any conventional mechanism for controlling temperature, pressure, flowrate, feedback control, and/or control valve(s). One mechanism for maintaining the vapor-liquid mixture exiting the second bank of heat exchange tubes at a temperature in the range of 350 to 500° C. is the injection of a fluid, such as water, to the preheated hydrocarbon product from (c) prior to (d). The vessel in step (f) can be any type of container, tank, or drum capable of separating the vapor-liquid mixture from step (e) in to form a vapor fraction and a liquid fraction. In one embodiment, the vessel in step (f) is a flash drum. In another embodiment of this disclosure, the vessel in step (f) is at least one of column, pipe, distillation tower, flash tower, and tank.

EXAMPLES

The following examples illustrate some of the embodiments of this disclosure and are not intended to limit the scope of the disclosure. Comparative examples 1, 2, 3, 4 and examples 1, 2, 3, and 4 are prophetic examples which were simulated using the modeling program Simulated Sciences ProVision Version 6.0 and 7.1, wherein the ProVision Version 7.1 was used for hydraulics simulation. Example 2A and example 4A were results obtained in a plant facility.
The following feedstocks, A, B1, C, and D1 were used for the simulation as shown in Table 1. The feedstocks B2 and D2 were tested in examples 2A and 4A. These feedstocks were characterized using 1) ASTM D 86 (A Standard Test Method for Distillation of Petroleum Products at Atmospheric Pressure) for liquid volume percentage boiling point curve; and/or 2) ASTM D 2887 (A Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography) method for weight percentage boiling point curve, which is a graph of temperature versus mass-percent distilled curve corresponding to a laboratory technique which is defined at 15/5 (15 theoretical plate, 5:1 reflux ratio) or TBP. All molecular weight values are weight average molecular weights.

TABLE 1

Feed:	A	B1	B2	C	D1	D2

Specific Gravity (g/ml)	0.8769	0.821	0.8302	0.8566	0.9082	0.8787
D86 IBP (0.5 vol %), (° C.)	63	62	122	236	309	277
D86 5 vol %, (° C.)			143			315
D86 10 vol %, (° C.)	131	99	172	290	362	346
D86 20 vol %, (° C.)			219			377
D86 30 vol %, (° C.)	225	159	257	319	403	398
D86 40 vol %, (° C.)			288			413
D86 50 vol %, (° C.)	307	240	318	342	434	431
D86 60 vol %, (° C.)			347			455
D86 70 vol %, (° C.)	400	316	375	364	466	490
D86 80 vol %, (° C.)			405			553
D86 90 vol %, (° C.)	535	472	456	394	508	711
D86 95 vol %, (° C.)			515			815
D86 EP(99.5 vol %), (° C.)	662	626	643	440	546	871
Molecular Weight,	210	163	250	293	422	479
TBP (15/5) IBP (0.5 wt %), (° C.)	−1	−11	79	196	251	208
TBP (15/5) 5 wt %, (° C.)	79	35	119	232	335	307
TBP (15/5) 10 wt %, (° C.)	119	73	154	287	360	346
TBP (15/5) 20 wt %, (° C.)	186	118	219	316	396	388
TBP (15/5) 30 wt %, (° C.)	238	157	264	324	421	416
TBP (15/5) 40 wt %, (° C.)	285	221	301	333	442	437
TBP (15/5) 50 wt %, (° C.)	333	251	336	350	462	457
TBP (15/5) 60 wt %, (° C.)	384	289	368	366	482	485
TBP (15/5) 70 wt %, (° C.)	438	350	401	378	503	534
TBP (15/5) 80 wt %, (° C.)	500	425	438	389	529	626
TBP (15/5) 90 wt %, (° C.)	606	535	503	413	558	847
TBP (15/5) 95 wt %, (° C.)	685	630	590	440	580	950
TBP (15/5) EP (99.5 wt %), (° C.)	799	777	959	465	621	1032
Viscosity @ 49° C., 896 kPa-a, (CP)	4.1564	1.708	3.21	5.0996	37.479	42.21

Comparative Example 1

Feed A, a crude oil feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This crude oil feedstock A which has a specific gravity 0.8769 ml/g, and an average molecular weight of 210, is fed at a temperature of 127° C., a pressure of 2413 kPa-a and a rate of 111.8 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock A, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight rows of tubes. The feedstock A is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 1.28 m/s. The feedstock A is heated to a temperature of 181° C. and exits at a pressure of 2393 kPa-a in all liquid phase. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about 21 kPa.
The heated feedstock A exits the first bank of heat exchange tubes 15 in liquid phase and is mixed with a flow of 30 tons/hr of steam. After mixing with steam, a portion of the hydrocarbon feedstock is vaporized to form a vapor-liquid mixture having 71 wt. % liquid phase based on the total weight of the combined stream of hydrocarbon feedstock and steam.
The vapor-liquid mixture is subsequently fed to a second bank of heat exchange tubes 17 with a tube diameter about 13% bigger than the tube diameter of the first bank of heat exchange tubes 15. The vapor-liquid mixture is fed to the second bank of heat exchange tubes 17 at a linear velocity of 12 m/s, wherein the vapor-liquid mixture is further heated to a temperature of 458° C., and exits the second bank of heat exchange tubes 17 at that temperature and at a pressure of about 952 kPa-a. At the exit of the second bank of heat exchange tubes 17, the liquid weight percentage exiting the second bank of heat exchange tubes 17 is now reduced down to 10 wt. % of the entire stream. The pressure drop across the second bank of heat exchange tubes 17 in the convection section is about 1448 kPa. The combined pressure drop across both the first bank of heat exchange tubes 15 and the second bank of heat exchange tubes 17 in the convection section is 1469 kPa.
The vapor-liquid mixture exits the second bank of heat exchange tubes 17 in the convection section of the steam cracking furnace at a linear velocity of about 35 m/s and mixes with a flow of about 2.7 tons/hr of steam superheated to 482° C. at a pressure of 952 kPa-a. The resulting vapor-liquid mixture flows to a vapor-liquid separator 53 at a temperature of 458° C. and a pressure of 811.7 kPa-a and having a liquid weight percentage of 7 wt. % of the entire stream due to the addition of superheated steam.

Example 1

Feed A, a crude oil feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This crude oil feedstock A which has a specific gravity 0.8769 ml/g, and an average molecular weight of 210, is fed at a temperature of 127° C., a pressure of 958 kPa-a and a rate of 111.8 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock A, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock A is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 0.55 m/s. The feedstock A is heated to a temperature of 181° C. and exits at a pressure of 967 kPa-a in all liquid. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about −9 kPa (the negative pressure drop is partially due to gravity).
The heated feedstock A exits the first bank of heat exchange tubes 15 in liquid phase and is mixed with a flow of 30.5 tons/hr of steam at 1142 kPa-a and 211° C. After mixing with steam, a portion of the hydrocarbon feedstock is vaporized to form a vapor-liquid mixture having 70.6 wt. % liquid phase based on the total weight of the combined stream of hydrocarbon feedstock and steam. The vapor-liquid mixture is subsequently fed to a second bank of heat exchange tubes 17. The vapor-liquid mixture is fed to the second bank of heat exchange tubes 17 at a linear velocity of 11.9 m/s, wherein the vapor-liquid mixture is further heated to a temperature of 458° C., and exits the second bank of heat exchange tubes 17 at that temperature and a pressure of about 819 kPa-a. At the exit of the second bank of heat exchange tubes 17, the liquid weight percentage of the hydrocarbon feedstock exiting the second bank of heat exchange tubes 17 is now reduced down to 10 wt. % of the entire stream. The pressure drop across the second bank of heat exchange tubes 17 in the convection section is about 145 kPa. The combined pressure drop across both the first bank of heat exchange tubes 15 and the second bank of heat exchange tubes 17 in the convection section is 136 kPa.
The vapor-liquid mixture exits the second bank of heat exchange tubes 17 in the convection section of the steam cracking furnace at a linear velocity of about 34.7 m/s and mixes with a flow of about 2.7 tons/hr of steam superheated to 482° C. at a pressure of 819 kPa-a. The resulting vapor-liquid mixture flows to a vapor-liquid separator 53 at a temperature of 458° C. and a pressure of 811.7 kPa-a and having a liquid weight percentage of 7 wt. % of the entire stream due to the addition of superheated steam.

Comparative Example 2

Feed B1, a light crude oil feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This crude oil feedstock B1 which has a specific gravity 0.821 ml/g, and an average molecular weight of 163, is fed at a temperature of 88° C., a pressure of 1896 kPa-a and a rate of 93.4 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock B1, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock B1 is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 1.23 m/s. The feedstock B1 is heated to a temperature of 144° C. and exits at a pressure of 1875 kPa-a in all liquid. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about 21 kPa.
The heated feedstock B1 exits the first bank of heat exchange tubes 15 in liquid phase and is mixed with a flow of 27 tons/hr of steam. After mixing with steam, a portion of the hydrocarbon feedstock is vaporized to form a vapor-liquid mixture having 63 wt. % liquid phase based on the total weight of the combined stream of hydrocarbon feedstock and steam.
The vapor-liquid mixture is subsequently fed to a second bank of heat exchange tubes 17 with a tube diameter about 19.4% bigger than the tube diameter of the first bank of heat exchange tubes 15. The vapor-liquid mixture is fed to the second bank of heat exchange tubes 17 at a linear velocity of 10 m/s, wherein the vapor-liquid mixture is further heated to a temperature of 446° C., and exits the second bank of heat exchange tubes 17 at that temperature and at a pressure of about 855 kPa-a. At the exit of the second bank of heat exchange tubes 17, the liquid weight percentage exiting the second bank of heat exchange tubes 17 is now reduced down to 5 wt. % of the entire stream. The pressure drop across the second bank of heat exchange tubes 17 in the convection section is about 1027 kPa. The combined pressure drop across both the first bank of heat exchange tubes 15 and the second bank of heat exchange tubes 17 in the convection section is 1048 kPa.
The vapor-liquid mixture exits the second bank of heat exchange tubes 17 in the convection section of the steam cracking furnace at a linear velocity of about 26 m/s and mixes with a flow of about 5.5 tons/hr of steam superheated to 473° C. at a pressure of 855 kPa-a. The resulting vapor-liquid mixture flows to a vapor-liquid separator 53 at a temperature of 446° C. and a pressure of 889.5 kPa-a and having a liquid weight percentage of 4 wt. % of the entire stream due to the addition of superheated steam.

Example 2

Feed B1, a light crude oil feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This crude oil feedstock B1 which has a specific gravity 0.821 ml/g, and an average molecular weight of 163, is fed at a temperature of 88° C., a pressure of 979 kPa-a and a rate of 93.4 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock B1, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock B1 is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 0.49 m/s. The feedstock B1 is heated to a temperature of 144° C. and exits at a pressure of 989 kPa-a in all liquid. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about −10 kPa (the negative pressure drop is partially due to gravity).
The heated feedstock B1 exits the first bank of heat exchange tubes 15 in liquid phase and is mixed with a flow of 26.6 tons/hr of steam at 1142 kPa-a and 211° C. After mixing with steam, a portion of the hydrocarbon feedstock is vaporized to form a vapor-liquid mixture having 63 wt. % liquid phase based on the total weight of the combined stream of hydrocarbon feedstock and steam.
The vapor-liquid mixture is subsequently fed to a second bank of heat exchange tubes 17 with a tube diameter about 44% bigger than the tube diameter of the first bank of heat exchange tubes 15. The vapor-liquid mixture is fed to the second bank of heat exchange tubes 17 at a linear velocity of 10.5 m/s, wherein the vapor-liquid mixture is further heated to a temperature of 446° C., and exits the second bank of heat exchange tubes 17 at that temperature and at a pressure of about 896 kPa-a. At the exit of the second bank of heat exchange tubes 17, the liquid weight percentage exiting the second bank of heat exchange tubes 17 is now reduced down to 5 wt. % of the entire stream. The pressure drop across the second bank of heat exchange tubes 17 in the convection section is about 117 kPa. The combined pressure drop across both the first bank of heat exchange tubes 15 and the second bank of heat exchange tubes 17 in the convection section is 107 kPa.
The vapor-liquid mixture exits the second bank of heat exchange tubes 17 in the convection section of the steam cracking furnace at a linear velocity of about 26.4 m/s and mixes with a flow of about 5.5 tons/hr of steam superheated to 473° C. at a pressure of 896 kPa-a. The resulting vapor-liquid mixture flows to a vapor-liquid separator 53 at a temperature of 446° C. and a pressure of 889.5 kPa-a and having a liquid weight percentage of 4 wt. % of the entire stream due to the addition of superheated steam.

Example 2A

Feed B2, a light crude oil feedstock, having the properties listed in Table 1 above, was used as the hydrocarbon feedstock for this example. This light crude oil feedstock B2 which has a specific gravity 0.8302 ml/g was fed at a temperature of 115° C., a pressure of about 1355 kPa-a and a rate of 61.5 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock B2, being all liquid at this point, was routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock B2 was fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 0.36 m/s. The feedstock B2 was heated in the first bank of heat exchange tubes 15 in the convection section 3 and exited at an estimated 96 wt. % liquid phase.
The heated feedstock B2 exiting the first bank of heat exchange tubes 15 was mixed with a flow of 11.6 tons/hr of water at 2999 kPa-a and 138° C. and a flow of 2.4 ton/hr steam at 1138 kPa-a and 191° C. After mixing with water and steam, a portion of the hydrocarbon feedstock was vaporized to form a vapor-liquid mixture having an estimated 77 wt. % liquid phase based on the total weight of the combined stream of hydrocarbon feedstock and steam.
The vapor-liquid mixture was subsequently fed to a second bank of heat exchange tubes 17. The vapor-liquid mixture was fed to the second bank of heat exchange tubes 17 at an estimated linear velocity of about 1.07 m/s, wherein the vapor-liquid mixture was further heated to a temperature of 421° C., and exited the second bank of heat exchange tubes 17 at that temperature and at a pressure of about 834 kPa-a. At the exit of the second bank of heat exchange tubes 17, the liquid weight percentage exiting the second bank of heat exchange tubes 17 was now reduced down to an estimated 8 wt. % of the entire stream. The pressure drop across both the first bank of heat exchange tubes 15 and the second bank of heat exchange tubes 17 in the convection section was about 521 kPa.

Comparative Example 3

Feed C, a heavy atmospheric gas oil (HAGO) feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This feedstock C which has a specific gravity 0.8566 ml/g, and an average molecular weight of 293, is fed at a temperature of 99° C., a pressure of 910 kPa-a and a rate of 95 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock C, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock C is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 1.33 m/s. The feedstock C is heated to a temperature of 256° C. and exits at a pressure of 862 kPa-a in all liquid. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about 48 kPa.
The heated feedstock C exits the first bank of heat exchange tubes 15 in liquid phase and having a linear velocity of 32 m/s.

Example 3

Feed C, a heavy atmospheric gas oil (HAGO) feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This feedstock C which has a specific gravity 0.8566 ml/g, and an average molecular weight of 293, is fed at a temperature of 99° C., a pressure of 876 kPa-a and a rate of 95 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock C, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock C is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 0.82 m/s. The feedstock C is heated to a temperature of 256° C. and exits at a pressure of 862 kPa-a in all liquid. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about 14 kPa.
The heated feedstock C exits the first bank of heat exchange tubes 15 in liquid phase and having a linear velocity of 31.7 m/s.

Comparative Example 4

Feed D1, a low sulfur vacuum gas oil (LSVGO) feedstock, having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This feedstock D1 which has a specific gravity 0.9082 ml/g, and an average molecular weight of 422, is fed at a temperature of 110° C., a pressure of 724 kPa-a and a rate of 68 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock D1, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock D1 is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 1.31 m/s. The feedstock D1 is heated to a temperature of 292° C. and exits at a pressure of 683 kPa-a in all liquid phase. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about 217 kPa.
The heated feedstock D1 exits the first bank of heat exchange tubes 15 in liquid phase and having a linear velocity of 17 m/s.

Example 4

Feed D1, a low sulfur vacuum gas oil (LSVGO), having the properties listed in Table 1 above, is used as the hydrocarbon feedstock for this example. This feedstock D1 which has a specific gravity 0.9082 ml/g, and an average molecular weight of 422, is fed at a temperature of 110° C., a pressure of 730 kPa-a and a rate of 68 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock D1, being all liquid at this point, is routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock D1 is fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 0.3 m/s. The feedstock D1 is heated to a temperature of 292° C. and exits at a pressure of 758 kPa-a in all liquid. The pressure drop across the first bank of heat exchange tubes 15 in the convection section is about −28 kPa (the negative pressure drop is partially due to gravity).
The heated feedstock D1 exits the first bank of heat exchange tubes 15 in liquid phase and having a linear velocity of 17.4 m/s.

Example 4A

Feed D2, a low sulfur waxy resid (LSWR) feedstock, having the properties listed in Table 1 above, was used as the hydrocarbon feedstock for this example. This light crude oil feedstock D2 which has a specific gravity 0.8787 ml/g was fed at a temperature of 93° C., a pressure of about 925 kPa-a and a rate of 65 tons/hr to the entrance of the first bank of heat exchange tubes 15 in the convection section 3. The feedstock D2, being all liquid at this point, was routed through the first bank of heat exchange tubes 15 having eight parallel passes of tubes. The feedstock D2 was fed to the entrance of the first bank of convection section heat exchange tubes 15 at a linear velocity of 0.44 m/s. The feedstock D2 was heated in the first bank of heat exchange tubes 15 in the convection section 3 and exited at an estimated 100 wt. % liquid phase.
The heated feedstock D2 exiting the first bank of heat exchange tubes 15 was mixed with a flow of 2.6 tons/hr of water at 1100 kPa-a and 120° C. and a flow of 15.6 ton/hr steam at 925 kPa-a and 210° C. After mixing with water and steam, a portion of the hydrocarbon feedstock was vaporized to form a vapor-liquid mixture having an estimated 94.6 wt. % liquid phase based on the total weight of the combined stream of hydrocarbon feedstock and steam.
The vapor-liquid mixture was subsequently fed to a second bank of heat exchange tubes 17. The vapor-liquid mixture was fed to the second bank of heat exchange tubes 17 at an estimated linear velocity of about 23.75 m/s, wherein the vapor-liquid mixture was further heated to a temperature of 455° C., and exited the second bank of heat exchange tubes 17 at that temperature and at a pressure of about 827 kPa-a. At the exit of the second bank of heat exchange tubes 17, the liquid weight percentage exiting the second bank of heat exchange tubes 17 was now reduced down to an estimated 32 wt. % based on the total weight of hydrocarbon in the entire stream (estimated 25 wt. % liquid phase based on the total weight of the entire stream). The pressure drop across both the first bank of heat exchange tubes 15 and the second bank of heat exchange tubes 17 in the convection section was about 98 kPa.
The following table (Table 2) lists all pressure drops for comparative examples 1-4 and examples 1-4. In summary, by supplying feedstock to the first bank of heat exchanger at a linear velocity less than 1.1 m/s, lower pressure drop across both the first bank and especially the second bank of heat exchanger can be achieved. The pressure drop for the second bank of heat exchange tubes of the Examples 1 and 2 are about 9 times less than the pressure drop for the second bank of heat exchange tubes of the Comparative Examples 1 and 2. Because of the low pressure drop, the process of this disclosure has the advantage of supplying hydrocarbon feedstock at a lower inlet pressure which saves energy required for the steam cracking process. Furthermore, the lower inlet pressure results in lower outlet pressures at the exits of the first bank and second bank of the heat exchange tubes, which has the advantage of using first and second diluent streams with lower pressures. By lowering the pressure required for the first and the second diluent streams, the process of this disclosure offers the advantage of energy saving and steam cracking efficiency.

TABLE 2

Pressure drop for	Pressure drop for	Combined pressure
the 1^stbank of	the 2^ndbank of	drop for both 1^stand
heat exchange	heat exchange	2^ndbank of heat
tubes (kPa)	tubes (kPa)	exchange tubes (kPa)

Comparative Example 1	21	1448	1469
Example 1	−9	145	136
Comparative Example 2	21	1027	1048
Example2	−10	117	107
Example 2A			521
Comparative Example 3	48	NA	NA
Example 3	14	NA	NA
Comparative Example 4	41	NA	NA
Example 4	−28	NA	NA
Example 4A			98

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this disclosure, and without departing from the spirit and scope thereof, can make various changes and modifications of this disclosure to adapt it to various usages and conditions.
While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that this disclosure lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention.

Claims

1. A process for treating a hydrocarbon feedstock comprising:

a. feeding said hydrocarbon feedstock at a linear velocity equal to or less than 0.9 m/s to a first preheating zone in the convection section of a steam cracking furnace;

b. preheating said hydrocarbon feedstock in said first preheating zone to vaporize less than 99 wt. % of said hydrocarbon feedstock to form a vapor-liquid mixture;

c. separating at least a portion of said vapor-liquid mixture to form a vapor fraction and a liquid fraction; and

d. feeding at least a portion of said vapor fraction to said steam cracking furnace.

2. The process of claim 1, wherein said hydrocarbon feedstock is fed in substantially liquid phase.

3. The process of claim 1, wherein said first convection section comprises multiple banks of heat exchange tubes and said hydrocarbon feedstock flows inside said tubes.

4. The process of claim 1, wherein said linear velocity is in the range of 0.05 to 0.85 m/s.

5. The process of claim 1, wherein said linear velocity is in the range of 0.1 to 0.80 m/s.

6. The process of claim 1, wherein said first preheating zone comprises a first preheating section and a second preheating section, wherein said hydrocarbon feedstock is supplied to said first preheating section at a pressure in the range of 790 to 1480 kPa-a and a temperature in the range of 25 to 250° C. to form a preheated hydrocarbon product exiting said first preheating section at a temperature in the range of about 100 to 350° C., and then at least a portion of said preheated hydrocarbon product is supplied with a first diluent stream to said second preheating section to form said vapor-liquid mixture exiting said first preheating section at a temperature in the range of 350 to 500° C., at least 1 wt. % said vapor-liquid mixture based on the total weight of hydrocarbons in said vapor-liquid mixture is in liquid phase, and at least 60 wt. % said vapor-liquid mixture based on the total weight of hydrocarbons in said vapor-liquid mixture is in vapor phase.

7. The process of claim 1, wherein 10 to 99.99 wt. % of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

8. The process of claim 1, wherein said preheated hydrocarbon product is in substantially liquid phase.

9. The process of claim 1, wherein said vapor-liquid mixture has a temperature in the range of 400 to 500° C. and comprises at least 2 wt. % liquid based on the total weight of hydrocarbons in said vapor-liquid mixture and wherein 50 to 99.99 wt. % of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

10. The process of claim 1, wherein said vapor-liquid mixture has a temperature in the range of 450 to 500° C. and comprises at least 5 wt. % liquid based on the total weight of hydrocarbons in said vapor-liquid mixture and wherein 10 to 99.99 wt. % of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

11. The process of claim 1, further comprising mixing a secondary diluent stream with said vapor-liquid mixture prior to said step (c).

12. The process of claim 11, wherein a secondary diluent stream comprises steam.

13. A process for cracking a hydrocarbon feedstock in a steam cracking furnace having a convection section, said convection section comprises a first bank, a second bank and a third bank of heat exchange tubes, said process comprising:

a. feeding said hydrocarbon feedstock having at least 1 wt. % low volatile components and at least 0.1 wt. % coke precursors to said first bank of heat exchange tubes provided in said convection section, with an inlet linear velocity equal to or less than 0.9 m/s;

b. preheating said hydrocarbon feedstock to form a preheated hydrocarbon product having a temperature below 350° C.;

c. supplying at least a portion of said preheated hydrocarbon product with a first diluent stream to said second bank of heat exchange tubes to vaporize at least a portion of said hydrocarbon feedstock to form a vapor-liquid mixture having a temperature in the range of 350 to 500° C. and comprising at least 1 wt. % liquid based on the total weight of hydrocarbons in said vapor-liquid mixture;

d. separating at least a portion of said vapor-liquid mixture from step (c) to form a vapor fraction and a liquid fraction; and

e. feeding at least a portion of said vapor fraction from step (d) to said third bank of heat exchange tubes.

14. The process of claim 13, further comprising mixing a second diluent stream to said vapor-liquid mixture prior to step (d).

15. The process of claim 13, further comprising mixing said vapor fraction with a third diluent stream comprising steam prior to step (e).

16. The process of claim 13, wherein 10 to 95 percent of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

17. The process of claim 13, wherein said hydrocarbon feedstock comprises one or more of steam cracked gas oil and residues, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, Fischer-Tropsch gases, Fischer-Tropsch waxes, distillate, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, atmospheric resid, heavy resid, C₄'s/residue admixture, naphtha residue admixture, and low sulfur waxy resid.

18. A process for cracking a hydrocarbon feedstock to light olefins in a steam cracking furnace having radiant section burners and a convection section, said convection section comprises a first bank, a second bank and a third bank of heat exchange tubes, said process comprising:

a. feeding said hydrocarbon feedstock at a pressure in the range of 790 to 1480 kPa-a and a temperature in the range of 25 to 250° C. to said first bank of heat exchange tubes provided in said convection section, with a linear velocity equal to or less than 0.9 m/s;

b. preheating said hydrocarbon feedstock to form a preheated hydrocarbon product having a temperature less than 350° C.;

19. The process of claim 18, wherein said vapor-liquid mixture has a temperature in the range of 400 to 500° C. and comprises at least 2 wt. % liquid based on the total weight of hydrocarbons in said vapor-liquid mixture and wherein 50 to 99.99 wt. % of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

20. The process of claim 18, wherein said vapor-liquid mixture has a temperature in the range of 425 to 500° C. and comprises at least 3 wt. % liquid based on the total weight of hydrocarbons in said vapor-liquid mixture and wherein 40 to 99.99 wt. % of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

21. The process of claim 18, wherein said vapor-liquid mixture has a temperature in the range of 450 to 500° C. and comprises at least 5 wt. % liquid based on the total weight of hydrocarbons in said vapor-liquid mixture and wherein 10 to 99.99 wt. % of said hydrocarbon feedstock boils below 590° C. measured according to ASTM D-2887.

22. The process of claim 18, wherein the total pressure drop for said first bank of heat exchange tubes is less than 100 kPa calculated by subtracting pressure at the inlet point of said first bank of heat exchange tubes from the exit point of said first bank of heat exchange tubes.

23. The process of claim 18, wherein the total pressure drop for said second bank of heat exchange tubes is less than 500 kPa calculated by subtracting pressure at the inlet point of said second bank of heat exchange tubes from the exit point of said second bank of heat exchange tubes.

24. Apparatus adapted for steam cracking a hydrocarbon feedstock to light olefins, comprising:

a. a steam cracking furnace comprising radiant section burners adapted to provide radiant heat and hot flue gas and a convection section having a first bank, a second bank and a third bank of heat exchange tubes;

b. means for feeding said hydrocarbon feedstock to said first bank of heat exchange tubes with a linear velocity equal to or less than 0.9 m/s;

c. means for maintaining said hydrocarbon feedstock exiting said first bank of heat exchange tubes at a temperature less than 350° C.;

d. means for supplying a first diluent stream together with at least a portion of the preheated hydrocarbon product from (c) to said second bank of heat exchange tubes adapted to vaporize less than 99 wt. % of said hydrocarbon feedstock to form a vapor-liquid mixture;

e. means for maintaining said vapor-liquid mixture exiting said second bank of heat exchange tubes at a temperature in the range of 350 to 500° C.;

f. a vessel adapted to separate at least a portion of said vapor-liquid mixture from step (e) in to form a vapor fraction and a liquid fraction; and

g. means for feeding at least a portion of said vapor fraction from step (f) to said third bank of heat exchange tubes.

25. The apparatus of claim 24, further comprising means for mixing a second diluent stream to said vapor-liquid mixture prior to (f).