EP0031609A1

EP0031609A1 - A process for recovering heat from the effluent of a hydrocarbon pyrolysis reactor

Info

Publication number: EP0031609A1
Application number: EP19800201154
Authority: EP
Inventors: John Edward Gwyn
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1979-12-21
Filing date: 1980-12-04
Publication date: 1981-07-08
Also published as: ES8204155A1; DE3065919D1; ES497920A0; EP0031609B1; CA1145776A

Abstract

A process of efficiently recovering heat from hydrocarbon pyrolysis effluent (3), whilst lessening coking problems, by cooling the effluent (3) in two quenchers, in the first (4) indirectly to at least 540°C with production of high pressure steam (6) and in the second (8) to at least 370°C, the second quencher (8) comprising a direct quench section (9) giving an effluent quench liquid mixture of at least 400°C, followed by an indirect quench section (11) producing high pressure steam (13); then, the effluent quench liquid mixture is fractionally distilled (17) and a bleed stream (22) from the distillation (17) is cooled and returned to the distillation. Coking materials may be removed from the bottoms (21) of the fractionating column by adding aromatic hydrocarbons (27) and separating off the precipitate then formed; the bottoms thus treated can be used as quench liquid (19).

Description

The invention relates to a process for recovering heat from the effluent of a hydrocarbon pyrolysis reactor.
Pyrolysis of liquid hydrocarbons is well-known and involves heating the hydrocarbons to a temperature that is high enough to thermally decompose larger molecules to form smaller molecules. Pyrolysis may be accomplished with a diluent, such as steam, to produce more favourable product distribution. Pyrolysis produces a highly unsaturated and very unstable product, hereinafter called "the effluent from the pyrolysis process", or simply "the effluent".
The effluent is usually rich in alkenes, alkadienes, alkynes and other highly unstable compounds, and these compounds very easily form high molecular weight products which may be identified collectively as "coke" or "tar". Such products are not desirable and to avoid forming them it is essential to reduce the temperature of the effluent quickly to a stable temperature, that is, to a temperature that is so low that rapid reactions of unstable compounds with each other do not take place.
In at least one process of this type, the effluent is stabilized by indirect heat exchange in stages, while in another process, the effluent is first precooled indirectly, and then stabilized by direct heat exchange with a liquid quench. In the latter process, the bulk of the heat absorbed by the quench liquid is removed in the later fractional distillation of the effluent and quench liquid, a significant portion of the heat removal being accomplished by separation of a bleed stream, heat exchange of the stream, and return of at least a portion of the bleed stream to the fractional distillation zone. This procedure, however, suffers from the deficiency that only low pressure steam may be generated, as well as requiring a large volume of bleed.
According to British patent specification No. 1,503,871 the effluent is quenched at temperatures of up to 4000C, the effluent and quench liquid contacting the walls of a shell and tube exchanger to generate high pressure steam. However, this procedure also does not utilize efficiently the high quality energy present in the pyrolysis effluent.
In view of the above a need exists for a process that lessees coking problems while providing efficient heat recovery. The invention satisfies this need.
Accordingly, the invention provides a process for recovering heat from the effluent of a hydrocarbon pyrolysis reactor by indirectly quenching the effluent in a first quencher to a temperature of at least 540^oC, with simultaneous production of a heated fluid; passing the quenched effluent at a temperature of at least 5400C to a second quencher comprising a direct quench moderator section communicating with an indirect quench section providing heat transfer to water, and contacting the quenched effluent first in the moderator section with a suitable quench liquid to cool the quenched effluent and produce an effluent quench liquid mixture having a temperature of at least 400°C, and then indirectly quenching the mixture of the effluent and quench liquid in the indirect quench section of the second quencher with simultaneous production of high pressure steam, and producing a quenched effluent and quench liquid mixture having a temperature of at least 370°C; passing mixture of the quenched effluent and quench liquid as a feed to a fractional distillation zone, and fractionally distilling the feed; continuously removing a bleed stream from the lower portion of the fractional distillation zone, passing the bleed stream to a heat exchanger and recovering heat from the bleed stream, and producing a cooler bleed stream, and returning at least a portion of the cooler bleed stream to the fractional distillation zone.
Preferably, at least a portion of the quench liquid is separated from the mixture of the quenched effluent and quench liquid, withdrawn from the second quencher, and returned to the upper portion of the second quencher.
As used herein, the word "indirect", as applied to heat exchange methods, implies that the medium to which heat is initially principally transferred does not contact the higher energy level material, heat transfer being accomplished through an intermediate medium such as a tube wall or other barrier.
Any hydrocarbon material can be pyrolysed for the process according to the present invention; heavier hydrocarbon materials are particularly suitable. Preferred feedstocks include gas oil and pitch. The particular procedure employed in the pyrolysis of the hydrocarbon feed forms no part of the invention, and any suitable method that produces a high temperature effluent may be employed.
As indicated, the temperature of the effluent from the pyrolysis unit will normally exceed 760°C. Temperatures on the order of from 780°C to 800⁰C are common for pyrolysis of gas oils and temperatures of 815⁰C to 925⁰C are employed for high temperature, short contact time pyrolysis. In accordance with the invention, the effluent will first be indirectly quenched to a temperature of at least 5400C; this temperature is preferably below about 650°C and is particularly in the range of from 590 to 650°C. This high quality energy present may be utilized, by indirect heat exchange, for any heating desired, and, in particular, the production of high pressure steam.
An important feature of the invention is the concept of limited heat exchange in the first quencher. In order to minimize coking, heat exchange is regulated in such a manner that, while valuable heat is recovered, the effluent is not cooled to such an extent that coking occurs. Thus, heat exchange is limited so that the temperature of the effluent, after indirect heat exchange with the heat exchange medium in the first quencher, does not approach the temperature of the incoming heat exchange medium. This "limited" exchange, in conjunction with the quenching procedures described herein, permits recovery of valuable heat energy with minimal coking.
Mechanically, the limited surface exchanger represents the simplest design. For example, tube-in-tube heat exchangers are particularly suitable. Performance may be maintained by extending the length of the exchanger tubes. Caution must be exercised that the cooler end of the tube will not stay at a low enough temperature that coking will plug the tube.
The partially cooled effluent, with a significant heat content extracted, is passed to the second quencher. This quencher may contain a continuous wet film quench unit similar to that described in U.S. patent specification No. 3,907,661. However, because the invention aims at enhanced heat recovery, the following modifications of the procedure of U.S. patent specification No. 3,907,661 are required.
As noted, the effluent is cooled in the second quencher to a temperature of at least 370°C. This is accomplished by suitable quench liquid temperature and volume, and by appropriate design of the quencher. In particular, the quencher is divided into two principal sections, a direct quench moderator section and an indirect quench section. In the moderator section, the quenched effluent from the first quencher is contacted with a suitable quench liquid in such quantity as to drop the temperature of the effluent to a value of at least 400°C. This temperature, depending on the nature of the liquid, will normally be in the range of 400°C to 500°C, preferably 400°C to 475⁰C, and more preferably 425°C to 475°C. Preferably, the quench liquid will be sprayed into the effluent, this procedure having the advantage of wetting the walls of the moderator section and inhibiting coking therein. Additionally, the quench liquid in the effluent-quench liquid mixture advantageously provides liquid for wetting the walls of the indirect quench section of the second quencher and inhibiting coking thereon. The ratio of quench liquid to effluent is varied to provide the temperatures desired, a ratio of about two parts quench liquid to one part effluent, on a weight basis, being acceptable.
As indicated, the moderator section communicates with an indirect heat exchange section. The quench liquid effluent mixture passes from the moderator section into this section, where it is cooled to a final temperature of at least 370°C. Preferably, a shell and tube configuration is employed, high pressure water being brought into indirect heat exchange with the quench liquid effluent mixture for production of high pressure steam. Additional quench liquid may be added to ensure a continuous film on the exchanger walls. By maintaining conditions to produce a quenched effluent (heat exchanger outlet) temperature of at least 370°C, the high quality energy present in the effluent may be effectively recovered. Quenched effluent temperatures of 425°C to 400°C are preferred.
The quench liquid employed may vary in composition, subject to the requirement that it does not completely vaporize at the temperatures employed for quenching and the unvaporized portion remains liquid. Suitable hydrocarbonaceous liquids must be compatible with the effluent, and normally will include such highly aromatic liquids as aromatic residual oils, gas oils, etc. Fractionator bottoms may be used, and pyrolysis pitch is preferred. Those skilled in the art, given the requirements set forth herein, may select the appropriate quench liquid with little difficulty. Preferably, the bottom fraction of the fractional distillation zone is used as quench liquid in the direct quench moderator section of the second quencher.
After quenching, the pyrolysis effluent still retains significant quantities of heat which may be utilized. This heat may be utilized in the fractional distillation of the effluent.
According to the invention, the effluent is passed to a fractional distillation zone for separation of the effluent into desired products. Prior to the entry into the fractional distillation zone, the quench liquid (or a portion thereof) may be separated from the effluent, or the effluent and quench liquid (or a portion thereof) may be cooled by heat exchange, as desired. Preferably, the effluent and a portion of the quench liquid sufficient to maintain wetted transfer line walls are forwarded directly to the fractional distillation zone. Procedures employed in fractionating such effluents are known in the art, and form no part of the invention.
If the effluent has been sent to the fractionation unit without cooling, the quantity of heat supplied to the fractionation unit will be too great, and will not permit proper operation of the unit unless appropriate measures are taken. Accordingly, the invention provides that a bleed stream of liquid is removed from the lower portion of the fractional distillation zone, the stream is subjected to heat exchange, preferably indirectly with water, and the cooled stream is returned to the fractionation unit. The heat is thus recovered, as desired, preferably as low temperature steam. Accordingly, the invention provides effective recovery of heat present in the pyrolysis effluent. Additionally, the amount of quench liquid required is reduced by use of the first quencher, and the pressure drop in the heat exchanger of the second quencher is also reduced.
The process according to the present invention has the advantage that inexpensive quench liquids, such as pitch, fractionator bottoms, etc., may be employed as quench liquid. However, because such materials may contain coke and high molecular products which might tend to form coke, and because the pyrolysis effluent does contain such materials, it may be desirable to provide a method to prevent buildup of tars and coke in the quench liquid or reduce the possibility of coking in the quench system. The following two embodiments of the present invention provide such methods.
According to the first embodiment abottcmfraction of the fractional distillation zone before introduction thereof into the direct quench moderator section of the second quencher is contacted with a light aromatic hydrocarbon liquid to produce a mixture of light aromatic hydrocarbon, fractionator bottoms and insoluble materials, the insoluble materials are removed from the latter mixture to produce a mixture of light aromatic hydrocarbon liquid and fractionator bottoms, which mixture is used as quench liquid in said direct quench moderator section. A portion or all of the bottom fraction may be treated in this manner.
According to the second embodiment a portion and preferably all of the portion of the quench liquid, separated from the mixture of the quenched effluent and quench liquid, withdrawn from the second quencher, before introduction thereof into the direct quench moderator section of the second quencher is contacted with a light aromatic hydrocarbon liquid to produce a mixture of light aromatic hydrocarbon liquid, quench liquid and insoluble materials, the insoluble materials are removed from the latter mixture to produce a mixture of light aromatic hydrocarbon liquid and quench liquid, which mixture is used as quench liquid in said direct quench moderator section.
The stream to be contacted with the light aromatic hydrocarbon liquid may be cooled to prevent vaporization of the aromatic hydrocarbons. The insolubles may then be removed, for example, by filtra- tionorcentrifugation. The liquid is then suitable for use as a quench liquid. Similarly, if a portion of the quench liquid is separated from the mixture issuing from the quench zone (such as in a knockout drum), at least a portion thereof may be treated with the light aromatic hydrocarbon to remove insolubles.
The composition of the aromatic hydrocarbon liquid utilized for rejecting the tars, etc., may be varied widely. In general, light aromatic hydrocarbon liquids, normally mixtures of light aromatic hydro- hydrocarbons, may be employed. The aromatic materials may then be recovered as part of a gasoline fraction from the fractionation column. In general, streams containing benzene, toluene, or aromatic gasoline fractions are preferred. A preferred source of such materials is, of course, a bleed stream from the fractionation column employed. The ratio of light aromatic hydrocarbon to quench liquid may be varied widely, it being necessary only to supply such volumes of aromatic liquid as to precipitate the coke. In general, ratios of 2 to 5 of light aromatic hydrocarbon to quench liquid may be used, with ratios of 3 to 4 being preferred.
The invention is further illustrated by means of Figures 1 and 2. Values in these figures are calculated.
The embodiment of Figure 1 lessens coking problems while providing efficient heat recovery.
According to Figure 1, gas oil is introduced via a line 1 into a high temperature pyrolysis reactor 2 and thermally cracked to form an effluent containing alkenes. Temperatures in the pyrolysis reactor 2 will, for example, range from an inlet value of 540°C to an outlet value of 800°C and will produce an effluent leaving the reactor having a temperature of from, for example, 780°C to 800°C. The effluent is passed via a line 3 into a first quencher 4. The first quencher comprises a limited surface heat exchanger such as a tube-in-tube heat exchanger. Steam at high pressure is employed as coolant, and may be introduced via a line 5 at the cooler end of quencher 4. High temperature steam is recovered at the hotter end of quencher 4 via a line 6. In the first quencher 4, the temperature of the effluent is reduced to 595⁰C, while generating high pressure steam at 650°C and 10.1 MPa.
From the first quencher4, the effluent passes through a line 7 to a second quencher 8. The effluent is contacted in a direct quench moderator section 9 of the quencher 8 with quench liquid from a line 10, preferably sprayed in as shown. Preferably, a segment of line "0 provides quench liquid for wetting the walls of line 7. The temperature of the effluent quench liquid mix is lowered in moderator section 9, to about 450°C.
From moderator section 9, the effluent quench liquid mixture passes through a tube in shell exchanger 11, where heat exchange is maintained by indirect contact through the tube walls with high pressure water entering via a line 12 and exiting via a line 13. Upon passing through the tube section, the temperature of the effluent quench liquid mixture is lowered to a value of at least 370⁰C. Simultaneously, the heat exchange produces steam at about 3150C in line 13. Steam in line 13 goes to a steam drum 14, where it may be forwarded to further use, e.g., via line 5 or via a line 15 for use outside the present process, and water is recycled via line 12 to the exchanger of the second quencher 8.
From quencher 8, the effluent is passed via a line 16 to a fractional distillation column 17. Preferably, provision is made for removal of and recycle of a portion of the quench liquid prior to entry into the fractionation column 17. As shown in the drawing, a knockout drum 18 cooperates with the bottom of the second quencher 8 to remove a quantity of quench liquid from the effluent quench liquid mixture. From knockout drum 18, the quench liquid is recyled, via a line 19, through lines 10.
In fractional distillation column 17 the effluent is separated into the desired fractions. The lowest boiling materials are removed via a line 20. Alkenes, i.e., ethylene, and propylene, are separated from the upper portions of the column (not shown), while bottoms or other fractions may be removed and returned via a line 21 for use as the quenching liquid. Column control, per se, including reflux, forms no part of the invention, and is within the skill of the art.
In accordance with the invention, however, a bleed stream is withdrawn via a line 22 and passed to a heat exchanger 23. Heat exchanger 23 is preferably a water, shell and tube exchanger. Steam is generated for useful applications, and at least a portion of the cooled bleed stream is returned via a line 24 to fractional distillation column 17. The amount of bleed and return are regulated, in conjunction with quench liquid recycle, and column reflux, to provide efficient fractionation and effective utilization of the heat in the quenched pyrolysis effluent.
The embodiment of Figare 2 provides a method to prevent buildup of tars and coke in the quench liquid and reduces the possibility of coking.
Similar numbers refer to similar elements of Figure 1. For example, the effluent from the pyrolysis reactor is conducted at a temperature of at least 540⁰c via line 7 to the second quencher 8. The quench liquid passes via a line 25 to a contactor 26 wherein it is contacted with a light aromatic fraction from a line 27. The quench liquid may be cooled before contacting, in order to reduce the vaporization of the light aromatic hydrocarbon. Recycle or make-up liquid from fractionator 17 may be added via line 21 prior to or during entry into contactor 26 (dotted lines). Contactor 26 does not have to be a separate unit; the fraction may be injected in line 25. In any event, tars, etc., tend to precipitate from the bottoms liquid, and the combined streams are passed through a line 28 to a centrifuge 29 wherein the insolubles are removed via a line 30. Centrifuge 29 may be a filter instead of a centrifuge. The quench liquid is then passed via line 19 to the second quencher 8. The utilization of the light aromatic hydrocarbon has the added advantage of providing additional moderation in this quencher. Quench liquid in line 21 may be added to line 19 prior to entry into quencher 8. Bottom product is withdrawn from the fractionation column 17 via a line 31.

Claims

1. A process for recovering heat from the effluent of a hydrocarbon pyrolysis reactor, characterized by indirectly quenching the effluent in a first quencher to a temperature of at least 540°C, with simultaneous production of a heated fluid; passing the quenched effluent at a temperature of at least 5400c to a second quencher comprising a direct quench moderator section communicating with an indirect quench section providing heat transfer to water, and contacting the quenched effluent first in the moderator section with a suitable quench liquid to produce an effluent quench liquid mixture having a temperature of at least 400°C, and then indirectly quenching the mixture of the effluent and quench liquid in the indirect quench section of the second quencher with simultaneous production of high pressure steam, and producing a quenched effluent and quench liquid mixture having a temperature of at least 370 C; passing mixture of the quenched effluent and quench liquid as a feed to a fractional distillation zone, and fractionally distilling the feed; continuously removing a bleed stream from the lower portion of the fractional distillation zone, passing the bleed stream to a heat exchanger and recovering heat from the bleed stream, and producing a cooler bleed stream, and returning at least a portion of the cooler bleed stream to the fractional distillation zone.

2. A process as claimed in claim 1, characterized in that bottom fraction of the fractional distillation zone is used as quench in the direct quench moderator section of the second quencher.

3. A process as claimed in claim 1 or 2, characterized in that at least a portion of the quench liquid is separated from the mixture of the quenched effluent and quench liquid, withdrawn from the second quencher and said portion is passed to the upper portion of the second quencher.

4. A process as claimed in claim 3, characterized in that the quenched effluent and remaining quench liquid, if any, are cooled prior to entry into the fractional distillation zone.

5. A process as claimed in any one of the preceding claims, characterized in that the effluent is quenched in the first quencher to a temperature in the range of from 590 to 650°C.

6. A process as claimed in any one of the preceding claims, characterized in that the heated fluid in the first quencher is high pressure steam.

7. A process as claimed in claim 6, characterized in that the high pressure steam produced in the indirect quench section of the second quencher is used as quench fluid in the first quencher.

8. A process as claimed in any one of the preceding claims, characterized in that in the moderator section a mixture of effluent and quench liquid having a temperature in the range of from 400 to 4750C is produced.

9. A process as claimed in claim 8, characterized in that in the moderator section a mixture of effluent and quench liquid having a temperature in the range of from 425 to 475°C is produced.

10. A process as claimed in claim 9, characterized in that in the indirect quench section of the second quencher a quenched effluent and quench liquid mixture having a temperature in the range of from 400 to 4250C is produced.

11. A process as claimed in any one of claims 2 to 10, characterized in that bottom fraction of the fractional distillation zone before introduction thereof into the direct quench moderator section of the second quencher is contacted with a light aromatic hydrocarbon liquid to produce a mixture of light aromatic hydrocarbon, fractionator bottoms and insoluble materials, the insoluble materials are removed from the latter mixture to produce a mixture of light aromatic hydrocarbon liquid and fractionator bottoms, which mixture is used as quench liquid in said direct quench moderator section.

12. A process as claimed in any one of claims 3 to 10, characterized in that the portion of the quench liquid, separated from the mixture of the quenched effluent and quench liquid, withdrawn from the second quencher, before introduction thereof into the direct quench moderator section of the second quencher is contacted with a light aromatic hydrocarbon liquid to produce a mixture of light aromatic hydrocarbon liquid, quench liquid and insoluble materials, the insoluble materials are removed from the latter mixture to produce a mixture of light aromatic hydrocarbon liquid and quench liquid, which mixture is used as quench liquid in said direct quench moderator section.

13. A process as claimed in claim 11 or 12, characterized in that the light aromatic hydrocarbon liquid is a bleed stream from the fractional distillation zone.

14. A process as claimed in any one of claims 11to 13, characterized in that a ratio of light aromatic hydrocarbon liquid to quench liquid to be contacted with the aromatic liquid in the range of from 2 to 5 is maintained.

15. A process as claimed in claim 1, substantially as hereinbefore described with reference to the drawings.