US1981842A - Process for cracking heavy stocks - Google Patents

Description

1934- P. c. KEITH, JR

PROCESS FOR CRACKING HEAVY STOCKS Filed March 27, 1931 Patented Nov. 20, 1934 UNITED STATES PATENT OFFICE PROCESS FOR CRACKING HEAVY STOCKS I Delaware Application March 27, 1931, Serial No. 525,772

8 Claims.

My invention relates to a process for increasing the yield of lighter components in a hydrocarbon oil and more particularly to a process for obtaining a high yield of desired low gravity components from a relatively high gravity charging stock.

Previously in the processing of relatively hea charging stocks such a large percentage of coke has been formed in the system that the runnin time or as it is called, the time in which the unit is on stream, has been comparatively short. For this reason it has sometimes been difiicult to obtain a successful commercial operation by cracking such stocks. If the charging stock has been a low gravity crude, it has previously been necessary to recover the usually small percentage of light constituents present by a separate topping operation. This, of course, called for an expensive outlay of equipment and further decreased the possibility of a successful commercial processing of such stocks.

My present process consists in breaking the viscosity of the charging stock to increase the percentage of components suitable for charging stock to a cracking coil, utilizing the same fractionating equipment for the oil discharged from the viscosity breaking and cracking units and furthermore stripping the original charging stock of its virgin gasoline.

One of the'primary objects of my invention is to provide a process in which a relatively heavy charging stockmay be continuously run for a long period and a high percentage of desirable light products obtained. I

Another object of my invention is to provide a process in which a topping, viscosity breaking and cracking operation may be carried on with a minimum capital outlay.

A further object of my invention is to provide a process in which a maximum amount of naphtha may be obtained from a charging stock at a single operation.

My method and apparatus are disclosed in the following description and accompanying drawing, the single figure of which shows a diagrammatic outline of an apparatus suitable for carrying out my process. Referring more particularly to the drawing in which like reference characters denote like parts, my apparatus consists primarily of a stripping tower 1, a viscosity breaking pipe still 2, an evaporator or vaporizing tower 3, a fractionating tower 4, a cracking coil and a conversion chamber 6. The fractionating tower 4 and the vaporizing chamber 3 are fitted I in their upper part with the pre-heating coils 7 and 8 respectively. The line 9 passing through the pre-heater 10 connects the pro-heating coils with the stripping tower 1. The line 11 connected to the top of the stripping tower discharges into a condenser 12 which is connected to a sep- 69 arating drum 13. From the bottom of the stripping tower there is a line 14 which connects with a tank 15. This tank 15 is connected with the viscosity breaking unit 2 by the line 16 having in it the pump 1'1. The products from the viscosity breaking unit may be discharged through the line 18 controlled by the valve 19 into the vaporizing tower 3. The line 20 is connected to the top of the vaporizing tower and directs the vapors into the side of the fr'actionating tower. The line 21 connects to the top of the fractionating tower and discharges into a condensing coil 22 which is connected to the separating drum 23. The line 24 connects the bottom of the fractionating tower with an accumulator tank 25. The flow through the line 24 is controlled by a liquid level control 26. The accumulator 25 is connected by means of the line 2'7 in which there is a pump 28 to the cracking coil 5. This cracking coil is connected by means of the line 29 to the soaking drum or conversion chamber 6. This line is controlled by the pressure reducing valve 31. To the bottomof the vaporizing chamber there is connected theline 32 which discharges I through the heat exchanger 10. The flow through this line is controlled by the valve 33 which is sensitive to the liquid level control 34. Also connected to the line 32 is the line 35 in which there is the pump 36. This line is connected to the line 16 between the pumpout drum or tank 15 and the viscosity breaking unit 2. This line is controlled by the valve 37. Beneath the preheating coil 8 in the vaporizing chamber there has been placed the pan 38. Liquid from this pan is carried through the line 39 controlled by the valve 40 to the accumulator 25. Steam may be admitted to the bottom of the vaporizing chamber through the line 41.

I have also shown a small stripping drum 42 which is fitted with fractionating plates in the conventional manner and which is arranged so that condensate from the pan 43 in the stripping tower 1 passes through the line 44 into it. The top of the stripping drum is connected to the upper part of the stripping chamber 1 by means of the vapor line 45. Steam may be admitted into the lower portion of the stripping drum through the line 46. The line 47 is connected to the bottom of the shipping drum and may either pass to storage through the line 48 controlled by the valve 49 or else pass through the line 50 in which there is the pump 51 to the accumulator 25.

In the operation of my device, a heavy crude, for example, Venezuela crude, or reduced stock, would pass into the pre-heating coils 7 in the top of the fractionating tower 4 Where it would pick up heat and act as a cooling medium for forming reflux condensate within the tower. The oil would then pass into the pre-heating coil 8 in the top of the evaporating tower 3 where further pre-heat is picked up and where it again forms a condensate which may be collected if desired, and drawn off to the accumulator as will be described later. The oil passes from this pre-heating coil through a bottoms heat exchanger 10 where further heat is picked up from the bottoms being drawn off from the evaporator tower. The pre-heated stock which has obtained a temperature preferably between 500 and '700 degrees F. is discharged into the stripping tower 1. The lighter components are flashed in this tower and are drawn off through the vapor line 11 and condensed in the condenser 12. The condensate and uncondensible gases are separated in the separator 13 from which the uncondensible gas may be withdrawn and from which the virgin naphtha liquid may be drawn ofi to storage. The stripping tower is fitted with fractionating trays or plates 1a in the usual manner so that a close separation may be obtained. Steam may be admitted to this tower to further facilitate the removal of the desired light components. The

stripped charging stock is withdrawn from the bottom of the tower 1 through the line 14 and passes into the pumpout drum or tank 15 from which tank the charging stock for the viscosity breaking coil is withdrawn through line 16 by means of the pump 17. Oil passes to this still with the pre-heat of between 400 and 650 degrees F. and preferably at a temperature of about 575 degrees F. The oil is discharged from this pipe still at a temperature of between 700 and 850 degrees F. with a preferred temperature of about 780 degrees F. This temperature is sufficient to break the viscosity of the oil, forming an increase in the percentage of suitable charging stock for the cracking coil 5. The oil passes through the line 18 controlled by the valve 19 to the evaporator 3. The pressure on the pipe still may be any suitable pressure for example between 200 and 500 pounds per square inch, preferably 400 pounds per square inch. The pressure is reduced, of course, at the valve 19 to the pressure in the fractionating system of preferably 50 pounds, although, of course, it is to be understood that any other desired pressure may be used without departing from my invention. The oil discharged from the pipe still into the evaporator immediately has certain of its components flashed or vaporized. The heavier components remain liquid or are condensed and fall to the bottom of the tower from which they may be withdrawn as fuel oil. The vaporized portions pass upwards through the tower where they come into contact with the pre-heating coil 8 which recondenses certain of the heavier portions. The remaining vapors pass through the line 20 into the fractionating tower 4. This fractionating tower is fitted with trays, bell caps or other means to facilitate fractionation, designated by the dotted lines 4a. The vapors passing up through the tower are fractionated and the heavier constituents condensed. The vapors in the upper part of the tower come in contact with the pre-heating coils 7 which take up a portion of the heat from the vapors, condensing certain of them to form a reflux condensate. There may, of course, be other means used to form sumcient reflux condensate to carry on the fractionation under the most eflicient conditions. Vapors pass from the top of the tower into the line 21 and thence into the condenser 22 which discharges them into the separating drum 23 in which the uncondensibles may be separated from the light distillate constituents.

Charging stock for the cracking system is separated out in the lower portion of the fractionating tower 4. This stock has characteristics preferably of the nature of gas oil. It may be withdrawn through the line 24 to the accumulator 25. I have shown in my drawing a liquid level control 26 which automatically regulates the discharge of this stock from the bottom of the fractionating tower. Pressure between the accumulator 25 and the fractionating tower 4 is equalized by the vapor line 25a. The oil in the accumulator is withdrawn through the line 27 by means of the pump 28 and discharged to the cracking coil 5. I will describe later the various means for controlling the character of the stock in the accumulator 25.

The oil is discharged from the cracking coil at the temperature of between 800 and 900 degrees F., preferably however, at a temperature of about 840 degrees F. The cracked oil is discharged through the line 29 to the soaking drum 6 where further conversion is allowed to take place. A pressure of preferably 750 pounds is maintained on this cracking system by means of the pump 28 and the valve 31. The conversion products are discharged from the soaking drum 6 into the evaporator 3 through the line 30. It is seen that this evaporator is the same unit to which the oil was discharged from the viscosity breaking still 2. Here again the cracked products due to their contained heat and due to the reduction in pressure are vaporized to a great extent. The heavy unvaporized or recondensed products drop to the bottom of the evaporator and are drawn off through the line 32 and may be passed through the bottoms heat exchanger 10. The flow of oil through the line 32 is controlled by the valve 33 which is sensitive to the liquid level regulator 34. This bottoms, if desired, may also pass through the line 35 and by means of the pump 36 be forced into the line 16 discharging to the viscosity breaking unit. By this means I am able to further break the viscosity of the oil bottoms formed in the evaporating tower. Such a method of operating would of course, depend upon the characteristics of the stocks and the manner in which the system was being operated. For example, if a large amount of oil bottoms were being formed in the evaporator and the bottoms were of a character which permitted their further cracking without detriment to the viscosity breaking unit, it would be highly advisable to obtain further charging stock for the cracking unit 5 by breaking their viscosity in the cracking coil 2. This coil, of course, discharges to the evaporator where the light products formed would be separated and the heavier products would be withdrawn from the system to storage through the line 32. It would be, of course, possible to simultaneously withdraw a portion of the fuel oil bottoms from the system and return a portion through the line 35 to the viscosity breaking unit.

The vapors formed from the cracking operation pass through the fractionating system in a manner similar to that described in connection with the viscosity breaking unit inasmuch as the systems are the same. A pan 38 may be placed below the pre-heating coils 8 and condensate I formed by this coil may be collected and withdrawn through the line 39 controlled by the valve 40 to the accumulator 25. This stock might have characteristics similar to the stock collected in the bottom of the fractionating tower 4 or of course, might be of slightly heavier nature. The collecting and removal of this stock in the evaporator tower permits a close control of the characteristics of the charging stock in the accumulator 25 and furthermore, relieves a portion of the load upon the tower 4, in that a certain percentage of the charging stock to the cracking coil is short circuited around the fractionating tower and sent directly to the accumulator. Steam may be admitted to the lower portion of the evaporator tower through the line 41 so as to facilitate the complete removal of light constituents which are present in the bottoms and which should pass from the tower as overhead stock.

In connection with the stripping tower l, I have shown a stripping drum 42 which may be fitted with fractionating trays or plates of the usual type. This drum is used to obtain a close fraction or cut of a product such as kerosene or furnace oil which is collected on the pan 43 in the stripping tower and withdrawn through the line 44 to the stripping drum. The fractionation in this tower may be facilitated by means of steam which may be admitted through the line 46. The vapors separated may be returned to the stripping tower through the line 45 and the liquid bottoms withdrawn through the line 47. This bottoms may be taken from the system to storage or otherwise, through the line 48 controlled by the valve 49. .They may also be passed through the line 50 by means of the pump 51 to the accumulator 25.. By this arrangement I am able to putinto the cracking coil charging stock which is withdrawn from the accumulator,

a relatively light stock, and thereby obtain another control on the characteristics of the stock to the cracking unit.

It is seen, therefore, that I have obtained an apparatus and process in which several operations are combined and carried on which previously necessitated separate processing equipment with the result I have made a considerable saving in expense and time. Such a saving is of especial importance in the processing of heavy crudes or similar charging stock inasmuch as due to the excessive coke formation in cracking such stocks it has been previously impossible to keep the apparatus on stream as long as can be done with lighter and better grades of crude or charging stock.

By means of the viscosity breaking unit, I obtain a quantity of stock suitable for charging stock to the cracking unit over that originally present in the oil. Thus even with a very heavy original charging stock, I am able to obtain a relatively large percentage of good charging stock for the cracking unit and so greatly in- It is also seen that I am able to obtain a closecontrol of the products and stock charged to the cracking and viscosity breaking units.

It will become apparent to those skilled in the art that there are certain modifications which may be made in my process without departing from the spirit of my invention for example I do not wish my process to be limited to the running of a heavy charging stock although the process is of special importance in connection with heavy charging stocks having normally present only a small amount of light distillate components suitable for charging further the cracking coil. limited by the accompanying specification and drawing, but only by the appended claims.

What I claim is:

l. The method of converting relatively high boiling hydrocarbon oils into lower boiling products which comprises flowing a distillate oil formed in the operation in a confined stream through a heating zone to raise said stream of oil to a cracking temperature and effect cracking thereof into lower boiling products while maintaining said stream under superatmospheric pressure, thereafter discharging the resulting stream of cracked products into an enlarged separating zone wherein cracked vaporous and non-vaporous products are separated, separately withdrawing vapors from said separating zone and passing them to a fractionating operation wherein higher boiling constituents thereof undesired in the final product are condensed and thereby separated as reflux condensate, withdrawing and condensing the fractionated vapors to form the desired low boiling product, continuously passing a stream of cool fresh relatively heavy oil containing heavy residual constituents and lighter volatilizable constituents, in indirect heat exchange with hot cracked products derived from the separating operation taking place in said separating zone to I, therefore, do not wish to bethereby heat said fresh oil to a distilling temperature, thereafter discharging the stream of preheated fresh oil into a separate and independent distilling zone wherein volatilizable constituents of said fresh oil separate as vapors by the contained heat of the said preheated crude oil and the remainder of said fresh oil is collected in the form of a heavy residue, separately withdrawing said heavy residue from said independent distilling zone and flowing it in a confined and independent stream through a heating zone to raise it to a decomposing temperature and effect a reduction therewith, passing reflux condensate from said fractionating operation to said cracking operation as charging stock therefor, and separately withdrawing from said independent distilling zone volatilized distilled products of said fresh oil and adding them to said reflux condensate stock being charged to said cracking operation.

2. The method of converting relatively high boiling hydrocarbon oils into lower boiling products which comprises flowing a distillate oil formed in the operation in a confined stream through a heating zone to raise said stream of oil to a cracking temperature and effect cracking thereof into lower boiling products while maintaining said stream under superatmospheric pressure, thereafter discharging the resuiting stream of cracked products into an enlarged separating zone wherein cracked vaporous and non-vaporous products are separated, separately withdrawing vapors from said separating zone and passing them to a fractionating operation wherein higher boiling constituents thereof undesired in the final product are condensed and thereby separated as reflux condensate, withdrawing and condensing the fractionated vapors to form the desired low boiling product, continuously passing a stream of cool fresh relatively heavy oil containing heavy residual constituents and lighter volatilizable constituents, in indirect heat exchange with hot cracked products derived from the separating operation taking place in said separating zone to thereby heat said fresh oil to a distilling temperature, thereafter discharging the stream of preheated fresh oil into a separate and independent distilling zone wherein volatilizable constituents of said fresh oil separate as vapors by the contained heat of the said preheated fresh oil and the remainder of said fresh oil is collected in the form of a heavy residue, separately withdrawing said heavy residue from said independent distilling zone and flowing it in a confined and independent stream through a heating zone to raise it to a decomposing temperature and effect a reduction of viscosity of said residues and the production of additional charging stock for said cracking operation, thereafter discharging the stream of residue products into said separating zone wherein the lighter constituents thereof separate as vapors and admix with the vaporous products from the said cracking operation and are fractionated therewith, passing reflux condensate from said fractionating operation to said cracking operation as charging stock therefor, and independently fractionating the fresh oil vapors evolved in said independent distilling zone and adding the resulting reflux condensate to the aforesaid reflux condensate being charged to said cracking operation.

3. A process in accordance with claim 2 wherein non-vaporous products are withdrawn from said separating zone first-mentioned, and are combined with the heavy residue withdrawn from said independent distilling zone and charged to said heating zone second-mentioned.

4. A process in accordance with claim 2 wherein a portion of the said non-vaporous products is withdrawn from said separating zone first-mentioned and isolated from the process while another portion of said non-vaporous products is withdrawn from said separating zone first-mentioned and is combined with the heavy residue withdrawn from said independent distilling zone ing thereof into lower boiling products while maintaining said stream under superatmospheric pressure, thereafter discharging the resulting stream of cracked products into an enlarged separating zone wherein cracked vaporous and nonvaporous products are separated, separately withdrawing non-vaporous products from said separating zone and discharging them from the system, separately withdrawing vapors from said separating zone and passing them to a fractionating operation wherein higher boiling constituents thereof undesired in the final product are condensed and thereby separated as reflux condensate, withdrawing and condensing the fractionated vapors to form the desired low boiling product, continuously passing a stream of cool relatively heavy oil containing heavy residual constituents and lighter volatilizable constituents, oil in indirect heat exchange with hot vaporous products derived from the separating operation taking place in said separating zone to thereby heat said fresh oil to a distilling temperature, thereafter discharging the stream of preheated fresh oil into a separate and independent distilling zone wherein volatilizable constituents of said fresh oil separate as vapors by the contained heat of the said preheated fresh oil and the remainder of said fresh oil is collected in the form of a heavy residue, separately withdrawing said heavy residue from said independent distilling zone and flowing it in a confined and independent stream through a heating zone to raise it to a decomposing temperature and efiect a reduction of viscosity of said residues and the production of additional charging stock for said cracking operation, thereafter discharging the stream of residue products into said separating zone wherein the lighter constituents thereof separate as vapors and admix with the vaporous products from the said cracking operation and are fractionated therewith, passing reflux condensate from said fractionating operation to said cracking operation as charging stock therefor, and independently fractionating the fresh oil vapors evolved in said independent distilling zone and adding the resulting reflux condensate to the aforesaid reflux condensate being charged to said cracking operation.

7. The method of converting relatively high boiling hydrocarbon oils into lower boiling products which comprises flowing a distillate oil formed in the operation in a confined stream through a heating zone to raise said stream of oil to a cracking temperature and effect cracking thereof into lower boiling products while maintaining said stream under superatmospheric pressure, thereafter discharging the resulting stream of cracked products into an enlarged separating zone wherein cracked vaporous and nonvaporous products are separated, separately withdrawing non-vaporous products from said separating zone and discharging them from the system, separately withdrawing vapors from said separating zone and passing them to a fractionating operation wherein higher boiling constituents thereof undesired in the final products are condensed and thereby separated as reflux condensate, withdrawing and condensing the fractionated vapors to form the desired low boiling product, continuously passing a stream of cool fresh relatively heavy oil containing heavy residual constituent and lighter volatilizable constituents, in indirect heat exchange with hot vaporous and non-vaporous products derived from the separating operation taking place in said separating zone to thereby heat said fresh oil to a distilling temperature, thereafter discharging the stream of preheated fresh oil into a separate and independent distilling zone wherein volatilizable constituents of said fresh oil separate as vapors by the contained heat of the said preheated fresh oil and the remainder of said fresh oil is collected in the form of a heavy residue, separately withdrawing said heavy residue from said independent distilling zone and flowing it in a confined and independent stream through a heating zone to,

raise it to a decomposing temperature and effect a reduction of viscosity of said residues and the production of additional charging stock for said cracking operation, thereafter discharging the stream of residue products into said separating zone wherein the lighter constituents thereof separate as vapors and admix with the vaporous products from the said cracking operation and are fractionated therewith, passing reflux condensate from said fractionating operation to said cracking operation as charging stock therefor, and independently fractionating the fresh oil vapors evolved in said independent distilling zone and adding the resulting reflux condensate to the aforesaid reflux condensate being charged to said cracking operation.

8. The method of converting relatively high boiling hydrocarbon oils into lower boiling products which comprises flowing a distillate oil formed in the operation in a confined stream through a heating zone to raise said stream of oil to a cracking temperature and effect crackin thereof into lower boiling products while maintaining said stream under superatmospheric pressure, thereafter discharging the resulting stream of cracked products into an enlarged separating zone wherein cracked vaporous and nonvaporous products are separated, separately withdrawing non-vaporous products from said separating zone and discharging them from the system, separately withdrawing vapors from said separating zone and passing them to a fractionating operation wherein higher boiling constituents thereof undesired in the flnal products are a condensed and thereby separated as reflux condensate, withdrawing and condensing the fractionated vapors to form the desired low boiling product, continuously passing a stream of cool fresh relatively heavy oil containing heavy residual constituents and lighter volatilizable constituents, in indirect heat exchange first with vapors in said fractionating operation and then with hot non-vaporous products withdrawn from said separating zone to thereby heat said fresh oil to a distilling temperature, thereafter discharging the stream of preheated fresh oil into a separate and independent distilling zone wherein volatilizable constituents of said fresh oil separate as vapors by the contained heat of the said preheated fresh oil and the remainder of said freshoil is collected in the form of a heavy residue, separately withdrawing said heavy residue from said independent distilling zone and flowing it in a confinedand independent stream through a heating zone to raise it to a decomposing temperature and effect a reduction of viscosity of said residues and the production of additional charging stock for said cracking operation, thereafter discharging the stream of residue products into said separating zone wherein the lighter constituents thereof separate as vapors and admix with the vaporous products from said

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