Title:
Naphtha steam-cracking quench process
United States Patent 3923921


Abstract:
In quenching the hot gaseous effluent from a naphtha steam-cracking operation employing transfer line heat exchangers, improved heat recovery is obtained by recycling a high boiling fraction, i.e., filtered steam-cracked tar bottoms, to the quench point in addition to quenching with a steam-cracked gas oil fraction recovered from the quench tower. The use of the high-boiling fraction, i.e., tar bottoms, in conjunction with the steam-cracked gas oil fraction to quench the effluent which has previously passed through a transfer line heat exchanger, maintains the temperature of the effluent passing to the quench tower in the range of from about 500° to about 650°F. while insuring a liquid phase on the walls of the transfer line to prevent fouling, and thus allows additional high-level heat recovery in the quench tower.



Inventors:
KOHFELDT WALTER C
Application Number:
05/365052
Publication Date:
12/02/1975
Filing Date:
05/30/1973
Assignee:
Exxon Research & Engineering Co. (Linden, NJ)
Primary Class:
Other Classes:
208/48Q, 208/100, 208/101, 208/130, 585/613, 585/648, 585/950
International Classes:
B01D51/10; C10G9/00; (IPC1-7): C07C3/30; C10G9/36
Field of Search:
208/130,48Q,100,101 260
View Patent Images:
US Patent References:
3676519QUENCH PROCESS1972-07-11Dorn et al.
3647907N/A1972-03-07Sato et al.
3597494N/A1971-08-03Bigache et al.
3580838N/A1971-05-25Lutz
3180904Process for the manufacture of olefins1965-04-27Fischer et al.
2943041Processing of steam-cracked naphtha light end products1960-06-28Johnston et al.
2366521Method of removing coke deposits from high-temperature oil lines1945-01-02Guichet
2340778Process for producing olefins and motor fuel1944-02-01Steward et al.



Primary Examiner:
Gantz, Delbert E.
Assistant Examiner:
Spresser C. E.
Attorney, Agent or Firm:
Caulfield, Donald C.
Parent Case Data:


This is a continuation of application Ser. No. 119,485 filed Mar. 1, 1971, now abandoned.
Claims:
What is claimed is

1. In a process of treating a hydrocarbon feed stock in a steam-cracking furnace at elevated temperatures and low hydrocarbon partial pressures to form unsaturated hydrocarbon products wherein a quenching medium is introduced into the effluent from the furnace and thereater passing said effluent through a transfer line heat exchanger to the quench tower, the improvement which comprises passing said effluent from the furnace through a transfer line heat exchanger zone to lower the temperature of said products below about 850°F. and thereafter contacting said products passing to the quenching and fractionation zone with:

2. The process as described in claim 1 wherein the hydrocarbon feedstock is a naphtha fraction boiling in the range from about 100° to about 350°F.

3. The process of claim 1 wherein a portion of the steam-cracked gas oil fraction which has been cooled to a temperature in the range of from about 350° to about 400°F. is passed through a second heat exchanging zone in order to further generate steam in the range of from about 10 to about 30 psig.

Description:
FIELD OF THE INVENTION

This invention relates to a method of quenching a hot gaseous effluent of steam-cracked naphtha products being transferred from a cracking coil outlet into a quench tower for separation of the products into fractions. More particularly, this invention relates to an improved steam-cracking quench process wherein the effluent from the furnace is first passed through a heat exchanging zone and thereafter quenched with a circulating quench oil fraction, i.e., steam-cracked gas oil recovered as a distillate fraction from the quenching and fractionation zone, i.e., quench tower and with a high boiling fraction such as filtered steam cracker tar bottoms recycled from the quench tower to the quench point. Employing such a novel quench system allows the temperature of the effluent passing to the quench tower to be maintained at a temperature in the range of from about 500° to about 650°F. while at the same time maintaining a liquid phase on the wall of the transfer line or order to prevent fouling thereof. Accordingly, high-level heat recovery is obtained by first passing the effluent from the furnace through a transfer line heat exchanger and, after employing the quench system heretofore described, passing the effluent at a preferred temperature in the range of from about 525° to about 600°F. into the quench tower such that a significant amount of high-level heat is further recovered when the circulating quench oil fraction is removed from the quench tower, passed through one or more heat exchangers and thereafter employed as a quenching medium at the transfer line quench point and in the quench tower itself.

DESCRIPTION OF THE PRIOR ART

In steam-cracking virgin naphtha fractions to produce high yields of C2 to C5 olefins and diolefins using cracking conditions at high temperatures and low pressures, quick quenching and fast separation of products are necessary. It is necessary to quench the product from the cracking zone; that is, chill, cool them suddenly and rapidly to a lower temperature to prevent or minimize side reactions which reduce yields of desired products and increase yields of undesired products.

In the prior art there are numerous disclosures of different quenching agents or mediums and among these are high-boiling hydrocarbons, low-boiling hydrocarbons, water, steam, and the like. In many cases fouling occurs at or beyond the quench point or region and the process must be terminated to clean the equipment. Simultaneously, when naptha-cracking operation economics favor the use of transfer line heat exchangers to quench the effluent from the steam-cracking furnace, it is important to maximize the level of heat recovered in the quenching process in order to generate high pressure steam for turbine drivers and the like.

In conventional naphtha steam-cracking quench systems, the effluent is quenched with a steam-cracked oil distillate fraction recovered from the quench tower and recycled to the quench point. The use of this fraction as the quench oil lowers the temperature to a range of from about 525° to about 600°F., permitting substantial recovery of high-level heat, i.e., recovering X BTU per hour as 125 to about 145 psig steam generation in the quench tower equipment. However, when one or more transfer line heat exchangers are employed to initially cool the effluent from the furnace and to recover a substantial amount of heat, i.e., about 0.85 X BTU/hr. as 1200 to about 1800 psig high-pressure steam, a considerably lower temperature after oil quench must be employed in order to maintain the liquid phase on the walls of the transfer line passing to the quench tower in order to prevent fouling and plugging of the transfer line. This operation lowers the temperature of the effluent in the transfer line to from about 350° to about 425°F. such that only a low-level heat recovery is economical in the quench tower, i.e., 0.15 X BTU/hr. as 25 psig steam.

SUMMARY OF THE INVENTION

It has now been discovered that improved heat recovery can be realized in a naphtha steam-cracking quench process employing a transfer line heat exchanger to initially quench the effluent from the furnace. Now, in accordance with the instant invention, a high-boiling fraction such as filtered steam-cracked tar bottoms recovered from the quench tower is recycled along with a steam-cracked oil distillate fraction recovered from the quench tower to the quench point in order to maintain the temperature of the effluent in the transfer line in the range of from about 525°F. to about 600°F. while at the same time maintaining a liquid phase on the wall of the transfer line passing to the quench tower in order to recover 0.85 X BTU/hr. as about 1800 psig steam and 0.15 X BTU(hr. as 120 to about 140 psig steam. Thus, the instant quenching scheme allows the effluent to pass into the quench tower at a sufficient temperature to provide for substantial high-level heat recovery in the quench tower equipment and at the same time provides a liquid phase on the wall of the transfer line passing from the quench point to the quench tower in order to prevent plugging and fouling therein.

The manner of quenching the high temperature steam-cracked naphtha products and obtaining the quick separation of these products into suitable fractions while allowing for an improved heat recovery will be further understood by reference to the accompanying drawing.

Referring to FIG. 1, which shows a diagrammatic flow plan of the naphtha steam-cracking quench system of the instant invention, a hydrocarbon feedstock 1 is passed by line 2 into a preheat and cracking coil located within the cracking furnace 3, wherein the cracking coil is exposed to high intensity radiant heat. The preferred hydrocarbon feedstock is a naphtha fraction containing principally C5 -C10 saturated aliphatic hydrocarbons, i.e., paraffins or naphthenes, boiling principally in the range from about 100° to 350°F. The feedstock may have a somewhat narrower boiling range, e.g., in the range of from about 100° to about 160°F.

A suitable proportion of steam passing by way of line 4 is added to the hydrocarbon feed to make the resulting cracking mixture contain from about 40 to about 65 mole % steam, thus substantially lowering the partial pressure of the hydrocarbons. In the cracking coil located within furnace 3, the naphtha hydrocarbons mixed with steam are heated to an outlet temperature in the range of from about 1450° to about 1650°F., preferably in the range of from about 1500° to 1600°F. The total pressure of the cracked reaction mixture is in the range of from about 1.5 to 3 atmospheres, and preferably less than 30 pounds per square inch absolute. The residence time of the cracked reaction mixture of steam and hydrocarbons in the cracking coil is in the range of from about 0.1 to about 0.6 seconds, and more preferably for about 0.3 to about 0.5 seconds. On leaving the outlet of the coil, the cracked reaction of products is transferred by way of line 5 to transfer line heat exchanger 6. The temperature (cot) of the reaction products leaving the outlet of the coil is in the range of from about 1450° to 1650°F., and more preferably from about 1500° to 1600°F. After passing through heat exchanger 6, the effluent is cooled to a temperature below about 850°F., and preferably to a temperature in the range of from about 650° to about 815°F. The heat recovered by the effluent passing through the heat exchanger generates steam in the range of from about 600 to about 1800 psig steam, preferably from about 1400 to 1800 psig.

The effluent having been quenched in the transfer line heat exchanger to a temperature in the range from about 650° to about 815°F. is passed by way of line 7 to the quench tower 8. A steam-cracked oil fraction is withdrawn from the lower portion of the quench tower 8 at a temperature in the range of from about 450° to 550°F. through line 9 and is passed by pump 10 through line 11 for cooling in heat exchanger 12. Heat recovered in heat exchanger 12 generates steam in the range of from about 100 to about 150 psig. The cooled oil distillate fraction leaving the heat exchanger 12 at a temperature in the range of from about 350° to about 400° F. through line 13 becomes divided into two streams. One stream of the cooled oil distillate fraction is passed through line 14 for further cooling in the second heat exchanging zone 15. Heat recovered in heat exchanger 15 generates steam in the range of from about 10 to about 30 psig. The oil distillate cooled in the heat exchanger 15 is passed by line 16 into the top section of the quench tower for partial cooling of the vapors flowing upwardly through quench tower 8. A sufficient amount of the oil distillate having a temperature in the range of from about 350° to about 400°F. is passed from line 13 into line 14 and is injected at one or more points into line 7 to effect the lowering of the temperature in the effluent passing from the transfer line heat exchanger to a temperature in the range of from about 525° to about 600°F. The steam-cracked oil distillate fraction which is injected into line 7 by way of line 14 has a preferred boiling range of from about 350° to about 750°F., and more preferably from about 450° to about 650°F. The flow rate of the oil distillate from line 14 into line 7 is in the range of from about 0.3 to about 0.8 parts by weight of the oil fraction per one part by weight of the effluent from the furnace, the oil fraction having a temperature in the range of from about 350° to about 400°F.

The bottoms fraction is withdrawn from quench tower 8 at a temperature in the range of from about 525° to about 600°F. through line 17 and is passed by pump 18 through line 19 to a filter 20. The bottoms product recovered from quench tower 8 to line 7 comprises steam-cracked tar bottoms. The coke and other carbonaceous particles present in said product are removed in the filter 20 and tower bottoms pass by way of line 21 for cooling in heat exchanger 22. A sufficient amount of the cooled high-boiling fraction is recovered from heat exchanger 22 and passed by way of line 23 through line 24 and injected into line 7 at one or more points in order to maintain a liquid phase on the wall of line 7. The boiling point of the material, i.e., steam-cracked tar bottoms passing by way of line 24 is in the range of from about 550° to about 800°F. The flow rate from line 24 into line 7 is in the range of from about 0.01 to about 0.03 parts by weight of the bottoms product passing by way of line 24 per one part by weight of the sum of the effluent and oil distillate, the temperature of the high-boiling material passing by way of line 24 being in the range of about 130° to about 250°F.

The mixture of the cracked products and the steam-cracked oil fraction and high-boiling bottoms fraction passing by way of line 7 is preferably introduced into the bottom of the quench tower at a temperature in the range from about 525° to 600°F.

gaseous vapors containing steam-cracked hydrocarbon products boiling below about 450°F., steam, and hydrogen is taken overhead from the tower 8 through line 25. This gaseous stream is passed by way of line 25 into condenser 26 which is operated at a sufficiently low temperature to condense out water and hydrocarbons having more than about 7 carbon atoms per molecule. The condensate is then passed by way of line 27 into a separation tank 28 wherein the liquid condensate is settled so that a lower water layer can be withdrawn to line 29 and condensed oil can be withdrawn from an upper liquid layer through line 30. Uncondensed gaseous hydrocarbon products containing principally olefins and diolefins having up to about 6 carbon atoms per molecule are withdrawn from vessel 28 to line 33 to be subjected to light ends processing i.e., recover ethylene, propylene, butenes, butadienes and the like. A portion of the condensed oil withdrawn from settling drum 28 by way of line 30 is passed by way of line 31 into the upper portion of quench tower 8 to form a reflux medium for the top of the tower.

The interior of quench tower 8 is equipped with plates 32 for obtaining contact between the liquid and vapor but allowing for the fast flow of materials. The necessary cooling is obtained in the upper part of the quench tower 8 by the injection of the cooled oil distillate fraction introduced to the tower by way of line 16 and by introducing the condensed oil introduced by way of line 31. Each of these streams is introduced in suitable amounts and at the proper temperatures at several space points in order to obtain the optimum cooling and fractionation. The temperature of the vapors at the top of the tower 8 is set to avoid condensation of water in the upper part of the tower.

A preferred mode of operation is illustrated further by the following example.

EXAMPLE

While various virgin naphtha fractions may be employed as the feedstocks of the instant invention, a preferred feed contains hydrocarbons boiling principally in the range of 100° to 300°F. The feed is cracked at temperatures of about 1500°F. in the presence of a sufficient amount of steam to make the hydrocarbon partial pressure about 14 lbs. per square inch absolute. The cracked products leave the outlet coil at about 1500°F. and are then introduced into the transfer line heat exchanger. The cracked products are recovered from the transfer line heat exchanger at a temperature in the range of from about 650°F. to 815°F. The effluent is then quenched by a steam-cracked gas oil distillate fraction being removed from the lower section of the quench tower, and after passing through a heat exchanger is injected into the quench point or points at a temperature of about 375°F., the proportion of steam-cracked oil quench being about 0.5 times the weight of the admixed hydrocarbon products which are being quenched. Simultaneously, a steam-cracked tar bottoms fraction having a boiling point of about 750°F. is removed from the bottom of the quench tower, and after being filtered and passed through a heat exchanger such that the temperature of the tar fraction is about 200°F., is injected at said temperature into the quench points in the transfer line, the proportion of tar bottoms quench oil being 0.02 times the weight of the admixed cracked hydrocarbon products and steam-cracked oil quench which are passing in the transfer line to the quench tower.

The amount of steam-cracked gas oil quench and tar bottoms quench injected into the transfer line is sufficient to maintain the temperature in the transfer line at about 525° to 600°F., while maintaining a liquid phase on the wall of the transfer line in order to prevent coking and plugging of the transfer line. The quenched effluent is then introduced into a combined quenching and fractionation zone, the gaseous products being cooled by being brought successfully into contact with the cooled distillate fractions either removed or recovered from the quenched tower. The gaseous stream that is recovered from the top of the quenched tower is taken overhead and passed into a settling drum to separate out water, condensed oil, and uncondensed gaseous hydrocarbon products containing principally olefins and diolefins.

A sidestream may be withdrawn from the quench tower to remove cycle oil boiling in the intermediate range between the overhead and bottoms products. This may be necessary to control the boiling range of the distillate oil to permit generation of 100 to 150 psig steam. The sidestream is usually steam stripped to recover absorbed light hydrocarbon products.

The above-described novel quench system results in recovering about 0.85 X 1800 psig steam by passing the gaseous effluent from the furnace to the initial transfer line heat exchanger. Additional heat is recovered i.e., 0.15 X BTU/hr, in the range of from about 100 to about 150 psig steam in initially cooling the steam-cracked gas oil distillate fraction removed from the middles of the quench tower, before recycling as a quench oil to the transfer line. Additionally, a lower level of heat recovery, in the range of from about 10 to about 30 psig steam is recovered from the steam-cracked gas oil stream which is being passed to the upper part of the quench tower for cooling and reflux purposes.