WO1993012200A1

WO1993012200A1 - Method for simplifying quench and tar removal facilities in steam crackers

Info

Publication number: WO1993012200A1
Application number: PCT/US1992/010402
Authority: WO
Inventors: David Beattie; Ian Clarke; John Reuben Messinger; Frank Roberto; Robert David Strack; Terence Clive Swatton; Rimas Virgilijus Vebeliunas
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1991-12-11
Filing date: 1992-12-03
Publication date: 1993-06-24
Also published as: AU3151793A

Abstract

A method is disclosed for quenching the effluent from hydrocarbon pyrolysis units and removing resultant heavy oils and tars to prevent their accumulation in the recirculated quench water and excess quench water. Gaseous pyrolysis effluent is initially cooled to a target temperature close to or at the dew point of water at the pressure in the effluent stream. The gaseous effluent is then flashed upon entry into a separation vessel. Conditions in the separation vessel are such that any heavy oils and tars condense and are removed from the system as a substantially water-free concentrate. Following initial cooling, the gaseous effluent is largely free of the heavy oils and tars capable of forming a stable oil/water emulsion and the effluent is water quenched to complete the cooling process. By preventing the formation of a stable oil/water emulsion upon water quenching of the gaseous effluent, the disposal problems associated with a high volume stream of oil-contaminated water are eliminated. In addition, fouling in the recirculating quench water circuit and excess quench water system is reduced. The disclosed method is particularly applicable to removal of the heavy oils and tars produced when the pyrolysis products of mid-range hydrocarbons, such as liquefied petroleum gases and light naphthas, are quenched.

Description

METHOD FOR SIMPLIFYING QUENCH AND TAR REMOVAL FACILITIES IN STEAM CRACKERS

SPECIFICATION BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to the field of hydrocarbon processing, more particularly a method for quenching the effluent from pyrolysis units and removing the heavy oils and tars to prevent their accumulation in the recirculating quench water and net purge water.

2. Description of Related Art

The production of light olefins (ethylene, propylene, butenes) from various feedstocks utilizes the technique of steam pyrolysis, or cracking.

Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules which tend to react and form heavy oils and other high molecular weight products known as tars.

The further formation of heavy oils and tars is minimized by rapidly reducing the temperature of the effluent exiting the pyrolysis unit to a level at which the heavy oil and tar-forming reactions are greatly slowed. This temperature reduction is normally accomplished by cooling the pyrolysis effluent in transfer time exchangers (TLE's) that are located immediately downstream of the pyrolysis unit. In some cases pyrolysis effluent is rapidly quenched by injecting a quench liquid such as, but not limited to, water. Despite efforts to prevent heavy oil and tar formation, these materials often deposit on the walls of the quench chamber, the quench liquid piping, and indirect heat exchange devices which are typically used to remove heat from the quench liquid. This impedes flow and requires costly shutdowns for cleaning. Furthermore, if a quench liquid such as water is used, the heavy oils and tars may form stable emulsions which make it difficult to dispose of excess quench water in an environmentally acceptable manner. The quantity of heavy oil and tar formed increases when heavier feedstocks are cracked. One technique used to further quench pyrolysis unit effluent and remove the resulting heavy oils and tars employs a water quench tower in which the condensibles are removed at near ambient conditions. This water quench technique has proven effective when cracking light gases, primarily ethane. An alternative and more complex technique utilizes an oil quench with fractionation to remove the heavier tars, followed by a water quench to remove other condensibles and complete the cooling. This technique is most practical for heavy oil crackers which produce large quantities of heavy tars. Neither of these techniques is, however, entirely satisfactory for use in mid-range crackers which crack liquefied petroleum gases or light naphthas and produce relatively little heavy oil and tar. Some of the heavy oils and tars produced when the pyrolysis effluent of these feedstocks is quenched have approximately the same density as water and can form stable oil/water emulsions. Emulsion formation can render water quench operations ineffective, make disposal of excess quench water in an environmentally acceptable manner difficult and complicate heavy oil and tar disposal. The relatively simple water quench technique is thus unsuitable under these circumstances.

Alternatively, a primary fractionator would prevent the formation of oil/water emulsions by removing the heavy oils and tars in the primary oil quench stage. Such a system would, however, be more costly to construct and operate than a simple water quench system. Additionally, the system may not generate sufficient heavy oil to allow it to replenish its own quench oil, some of which must be continuously removed to dispose of accumulated tars. Operation of a primary fractionator under these conditions would require the added expense of an external supply of quench oil. Furthermore, logistical difficulties are presented if the cracker is not located adjacent to a facility capable of providing quench oil and removing spent oil.

Because of the necessity to economically crack mid-range hydrocarbons such as liquefied petroleum gases and light naphthas, there is a need for a simplified method for quenching pyrolysis unit effluent and removing the resulting heavy oils and tars. SUMMARY OF THE INVENTION

The present invention is directed to a method for quenching the effluent from pyrolysis units and removing the heavy oils and tars which result as a by-product of the quench process. The invention is particularly applicable to removal of the heavy oils and tars produced when the pyrolysis products of liquefied petroleum gases and light naphthas are quenched. The heavy oils and tars resulting from the use of these mid-range feedstocks have approximately the same density as water and can form stable oil/water emulsions if the pyrolysis effluent is water quenched. The invention utilizes a three-step process, without primary fractionation, to remove the heavy oils and tars from the pyrolysis effluent upstream of the final water quenching to prevent the formation of an oil/water emulsion. In this invention pyrolysis dilution steam is not condensed until the final quenching after most of the heavy oils and tars have been removed. Thus, the excess quench water, which requires further cleaning for reuse or disposal, and the recirculating quench water used in the final quench tower do not come into contact with the heavy oils and tars.

Initial cooling of the pyrolysis effluent takes place in the transfer line exchangers. The gaseous pyrolysis effluent from the transfer line exchangers is further cooled by the direct addition of a small quantity of water. The water addition may be achieved in one of two ways - either directly into the pipeline conveying the pyrolysis effluent to a primary separation vessel or into the primary separation vessel itself. The target temperature, after the water addition, will be close to, or at, the water dew point of the stream entering the primary separation vessel in which the heavy oils and tars will be condensed and removed. A portion of, or all, of the added water is vaporized and, as a consequence, the pyrolysis effluent is cooled resulting in condensation of the heavy oils and tars. With the target temperature maintained close to, or at, the water dew point, water condensation is prevented but some of the added water may go with the heavy oils and tars.

Next, the condensed heavy oils and tars (plus a small quantity of the added water) are removed from the primary separation vessel as a concentrate. In the event that there is some water with the concentrate then this stream is routed to a dedicated water/hydrocarbon separation vessel; the water phase is then recycled back to the pyrolysis effluent water addition point.

Lastly, with the gaseous effluent stream essentially free of the heavy oils and tars capable of forming a stable oil/water emulsion, the stream passes overhead from the primary separation vessel into the secondary vessel (or water quench tower) where it is fully water quenched to complete the cooling process. In the water quench tower recirculating quench water is used and most of the pyrolysis dilution steam is condensed.

The present invention is thus able to provide an effective and economical means of quenching the pyrolysis products of mid-range hydrocarbons such as liquified petroleum gases and light naphthas.

Further, without the necessity of a primary oil quench stage, the present invention is able to prevent the formation of oil/water emulsions in the quench tower recirculating quench water and excess quench water when the pyrolysis products of mid-range hydrocarbons are water quenched.

Still further, the present invention enables pyrolysis units cracking mid-range hydrocarbons to operate successfully without the necessity of an external source of quench oil.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more fully understood when the following detailed descriptions of the embodiments are considered with the accompanying drawings in which:

FIG. 1 is a schematic flow diagram of the quench process of the present invention utilizing water addition directly into the pipeline and a dedicated primary separation vessel. The primary separation vessel could be either the bottom part of the quench tower or an entirely separate vessel. FIG. 2 is a schematic flow diagram of the quench process of the present invention in which the water contacts the pyrolysis effluent directly in the primary separation vessel itself and directly above this vessel is the water quench tower.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a line 10 supplies a hydrocarbon feed and dilution steam to a pyrolysis zone 12 wherein the hydrocarbon is heated to cause thermal decomposition of the molecules. The pyrolysis process occurring in pyrolysis zone 12 produces some molecules which tend to react to form heavy oils and tars. The gaseous pyrolysis effluent after cooling in transfer line exchangers,exits the pyrolysis zone 12, through line 14. The gaseous pyrolysis effluent in line 14 is quenched to maintain a specified target temperature at the inlet 16 to the primary separation vessel 18. The target temperature must be high enough to prevent the precipitation of heavy oils and tars in line 14. In this embodiment liquid water is injected through line 20 into line 14 at a rate sufficient to maintain a target temperature just above the dew point of water at the pressure condition at the flash zone inlet 16. For the typical pyrolysis effluent of mid-range hydrocarbons, such as liquefied petroleum gases and light naphthas, and typical operating pressures, the target temperature would be in the range of 105°C. to 130°C.

The gaseous pyrolysis effluent stream next enters the primary separation vessel 18. In vessel 18 pressure and temperature conditions are maintained such that any water in the gaseous pyrolysis effluent stream plus the injected water remains in the vapor phase while the heavy oils and tars condense.

The condensed heavy oils and tars, which are free of water and light hydrocarbons, are removed as a concentrate from the vessel 18 through the tar removal line 22. The removal process may be either continuous or intermittent. A diluent liquid may be injected into vessel 18 through the diluent injection line 24. The purpose of the diluent liquid is to prevent plugging of the tar removal line 22 in the event that the condensed material is solid or has a very high viscosity.

The gaseous pyrolysis effluent exits vessel 18 through line 26 and proceeds to the secondary vessel (water quench tower) 28. At this stage of the process the gaseous pyrolysis effluent is sufficiently free of the heavy oils and tars capable of forming a stable emulsion with water that a simple water quench may be used to complete the cooling/condensing process. Upon entering the quench zone 28 the pyrolysis effluent is further cooled with recirculating quench water supplied through line 30. In this embodiment the quench zone 28 is a water quench tower of the standard design as is well known in the art.

The quench water is removed from the quench tower 28 through line 20 and flows to an oil/water separation drum (not shown) . From this latter drum four liquid streams are withdrawn - light oil, recirculating quench water (to become line 30) , excess quench water (line 34) plus any heavy oils/tars. A portion of the recirculating quench water is injected into line 14 to control the temperature of the pyrolysis effluent as it exits the pyrolysis zone 12.

FIG. 2 is a schematic flow diagram of the quench process of the present invention in which the water contacts the pyrolysis effluent directly in the primary separation vessel itself and directly above this vessel is the water quench tower. A line 10 supplies hydrocarbon feed and dilution steam to the pyrolysis zone 12 wherein the hydrocarbon is heated to cause thermal decomposition of the molecules. The pyrolysis process occurring in the pyrolysis zone 12 produces molecules which tend to react to form heavy oils and tars. The gaseous pyrolysis effluent exits the pyrolysis zone 12 through line 14 after cooling in transfer line exchangers to a temperature in the range of 190 to 240°C. The pyrolysis effluent is then further cooled in the primary separation vessel by direct contact with a small quantity of water which is injected through line 44. The temperature in the bottom of the primary separation vessel is at the water dew point. For the typical pyrolysis effluent of mid-range hydrocarbons, such as liquefied petroleum gases and light naphthas, and typical operating pressures, the target temperature in the bottom of the primary separation vessel would be in the range of 90 to 100°C. A portion of the water that is added is vaporized and passes, with the hydrocarbon, as stream 26 into the water quench tower.

Cooling of the hydrocarbon and condensing of heavy oils and tars is promoted in the primary separation vessel by the use of internal heat transfer devices (packing or baffles) .

The condensed heavy oils, tars and a portion of the added water are withdrawn from the primary separation vessel through the tar removal line 22. A diluent liquid may be injected into the primary separation vessel 18 through the diluent injection line 24. The purpose of the diluent liquid is to prevent plugging of the tar removal line 22 in the event that the condensed material is solid or has a very high viscosity.

The condensed material is routed to the water/hydrocarbon separation vessel 40. Water is removed from vessel 40 and returned to the top of the primary separation vessel 18 through line 44. The condensed heavy oils and tars, which are relatively free of water and light hydrocarbons, are removed from vessel 40 through line 42.

A slipstream of cooled quench water from line 30 is added to the water withdrawn from the water/hydrocarbon separation vessel 40 and routed to the primary separation vessel 18 through line 44. This water, from two sources, is the cooling medium for vessel 18.

The gaseous pyrolysis effluent (stream 26) exits the primary separation vessel 18 and enters the secondary vessel 28 which is a water quench tower of the standard design as is well known in the art. At this stage of the process the gaseous pyrolysis effluent is sufficiently free of heavy oils and tars capable of forming a stable emulsion with water that a simple water quench may be used to complete the cooling/condensation process. Upon entering the secondary vessel 28 the pyrolysis effluent is quenched with water supplied through line 30. Having performed its cooling duty, the quench water is withdrawn from the secondary vessel 28 through line 34 for reuse or disposal.

Two configurations of the invention have been described for the purpose of disclosure. However, numerous further changes in the details of the process and arrangements of the parts will be readily apparent to those skilled in the art and are encompassed within the spirit of the invention and the scope of the appended claims.

Claims

CLAIMS :

1. A method for quenching gaseous effluent from a hydrocarbon pyrolysis unit to a temperature at which the effluent is stable, comprising: passing the gaseous effluent to a primary vessel; controlling the temperature of the gaseous effluent at the inlet to said primary vessel such that high molecular weight heavy oils and tars, formed by reactions among the constituents of said effluent, condense; conducting to a conventional quench tower that portion of the effluent remaining in a gaseous state; and reducing the temperature of the gaseous effluent in said quench tower to a point at which the effluent is chemically stable, said temperature reduction occurring by direct contact with a quench liquid introduced into said quench zone.

2. The process of claim 1 wherein the temperature of the gaseous effluent at the inlet to said vessel is controlled close to, or at, the dew point of water at the pressure condition at the inlet to said primary vessel.

3. The process of claim 1 wherein the temperature of the gaseous effluent at the inlet to or within said primary vessel is controlled by means of direct contact with a quench fluid.

4. The process of claim 3 wherein said quench fluid is water. 13

5. The process of claim 4 wherein said quench water is recycled and is conducted (with the addition of fresh quench water) either to the inlet pipework or into the primary vessel itself.

6. The process of claim 1 wherein said heavy oils and tars are removed from said primary vessel or downstream water/hydrocarbon separation vessel in the form of a substantially water-free concentrate.

7. The process of claim 1 wherein a diluent liquid capable of fluxing heavy oils and tars is injected into said primary vessel.

8. The process of claim 1 wherein the gaseous effluent of said hydrocarbon pyrolysis unit is produced by pyrolyzing a mid-range hydrocarbon, including any liquefied petroleum gas or light naphtha, and the temperature of the gaseous effluent at the exit from the pyrolysis zone is the range of 190 to 240°C.

9. The process of claim 1 wherein heavy oils and tars are condensed and removed prior to the gaseous pyrolysis effluent entering a conventional water quench tower, thereby avoiding an oil/water emulsion in the recirculated quench water and/or excess quench water.