US1720070A - Method of cracking oil in liquid phase - Google Patents

Description

July 9, 1929.

J. C. BLACK METHOD OF CRACKING OIL IN LIQUID PHASE Filed Oct. 26 1926 INVENTOR ATTM 1,720,070 PATENT OFFICE.

JOHN C. BLACK, OF DESTREHAN, LOUISIANA.

METHOD OF CRACKING OIL IN LIQUID PHASE.

Application filed October 26, 1926. Serial No. 144,347.

This invention is directed to a process of cracking oil, particularly to a process of cracking oil in the liquid phase.

In my Patent $51,456,419 is described a process of cracking oil wherein oil is passed through coils in a furnace and subjected to a cracking heat for a prolonged period of time, .while maintaining the oil under pressure sufficient to overcome, substantially, the

vapor pressure of the oil.

It-is an object of this invention to devise an improved process whereby the oil may be subjected to said cracking heat for a. prolonged period of time, and to increase the period of time during which the oil is subjected to the aforementioned cracking heat.

It is well known that the cracking reaction is a function of both time and temperature. At any given temperature the amount of oil decomposed is proportional to the time during which the oil is kept at this cracking temperature. A greater decomposition is obtained for a given period of time byincreasing the temperature at which the oil is maintained, providing, of course, that the temperature is above the minimum cracking temperature of the oil.

It has been recognized that a process as described in my aforementioned patent wherein the oil is heated in liquid phase at a cracking temperature, allows for more efficient transfer of heat to the oil. This is so because none of the heat is absorbed as latent heat of vaporization of the oil, all of the absorbed heat going to endothermic heat of the cracking reaction. Again, the coefficient of heat transfer is smaller for liq uid oil than for vaporous oil, and the specific heat of liquid oil is greater than vaporous oil.

I have found also, that in addition to the above advantages the character of the gas0- line formed by this liquid phase cracking is entirely different and of a better grade than that obtained by either a vaporous phase, or combined liquid and vaporous phase cracking method.

An additonal advantage of this liquid phase cracking resides in the fact that a greater degree of conversion is obtainable since the oil can be maintained in the cracking coil for a longer period of time, since the mass velocity is smaller than in combined liquid and vapor phase cracking in tubes, or in vapor phase cracking. By operating at this greater period of time 1 may either get an increased decomposition at any given temperature, or by lowering the temperature and increasing the length of time I may get the same conversion as would be obtained by operating at a higher temperature and a shorter period of time. The advantage which resides in operating at lower temperature and increased lengths of time arises from the fact that at lower temperatures a much smaller amount of carbon is deposited than is deposited at higher temperatures.

There is, however, a limit to the length of time and to the temperature at which an oil may be subjected to such liquid phase cracking systems. The oil, under an imposed mechanical pressure, may be decomposed up to a point where the generated vapor pressure of the cracked oil at the temperature employed, exceeds the imposed mechanical pressure upon the oil. Any further decomposition would cause a vaporization of the oil in the tubes. The operation would then be liquid phase, through the first portion of the tubes,'until the point is reached, in the progress through the tubes, where the concentration of the light hydrocarbons in the oil is such, that at the temperature to which the oil is subjected, their vapor tension overcomes this super-imposed pressure. From that point on the operation is that of a combined vaporous and liquid phase, and may even, at a further point, become vapor phase cracking. The result of this is a deposition of carbon in the tubes, a great increase in the production of permanent gases, and decrease in the heat efficiency of the process, and a variation in the character of the gasoline obtained usually showing an increase in the specific gravity of the gasoline, although the gasoline may not be of decreased volatility and also an increase in the olefine content of the gasoline.

The deposition of carbon results both from carbon of decomposition and also is a result of the concentration of high boiling, tarlike bodies in the oil, after a part of the oil has become vaporized. These tars settle out upon the tube surfaces and become carbonized rapidly, the tubes becoming overheated, whereupon further skin cracking results. This accelerates, and a very harmful operation is soon reached where gassing results and tubes burn out.

It is an object of this invention to prevent this increase in vapor tension of the oil to a point where it exceeds the imposed mechanical pressure upon the oil, while yet allowlng the accumulation of these light hydrocarbons and a further decomposition of the heavier constituents of the partially cracked oil. The process in general comprises, heating oil under an imposed mechanical pressure at a chosen cracking temperature and for a period of time, to a point where the cracked oil has avapor pressure approximating the imposed mechanical pres sure; diluting the oil at this point and continuing the cracking of the diluted oil at the said imposed mechanical pressure for a further period of time. i

The invention will be better understood by reference to the accompanying drawing which shows a schematic arrangement of ap paratus for carryingout the process of this invention.

1 is a tank containing the oil to be cracked. 2is a line connecting this tank with pump 3.

2 is a valve in said line. 4 is a line connecting pump 3 with heat exchanger A. 5 is a cross- :5 over line connecting heat exchanger A with heat exchanger B. '6 is a line connecting heat exchanger B with heat exchanger C. 7 is a line connecting heat exchanger C with coil 8 situated in furnace 9. 10 is a cross-over line 1 connecting coil 8 with coil 11 situated in furnace 12. 13 is an exit line connecting coil 11 with heat exchanger C. 14 is a line connecting heat exchanger C with evaporator 15. 14' is a valve in said line. 16 is afractionating tower, such as a bubble tower, connecting with evaporator 15. 17 are bubble trays 'g-i'n'said tower. 18 is a condensate trap in tower 16. 19 is an analyzer for refluxing condensate. 20' is a line connecting tower 16 with condenser 21. 22 is a lookbox in condensate run-down line 23 which connects to tank 24. 25'is a gas vent connected to lookbox 22. 26 is a line connecting the trap 18 with the upper trays 28 in still 27 29 is a vapor line connecting still 27 with condenser 30- 31 is .a gas vent connected to lookbox 32 in condensate run-down line 33 which line connects with ta k 34. 35 is aline connecting tank 34 with pump 36. 37 is a line connecting pump 36 with the upper trays 17 in tower'16. 38 are steam coils in still 27. 39

is a line connecting the liquid portion of still 27 with tank 42, via heat exchanger A, line 40 and pump 41. 43 is a line connecting tank 42 with intake side of pum 3. 43 is a valve in said line. 44in which t ere is a valve 44 is a line connecting evaporator 15 with heat exchanger B. 45 is aline connecting heat exchanger B with pump 46. 47 is a line connecting pump 46 with tank 48. 49 is a bypass around exchanger C connecting lines 6 and 7. 49 is a valve in said by-pass. 50 is a by-pass line connecting the exit side of pump 3 with coil 11 situated in furnace 12. 50 is a valve in said line. Coil 11 is connected to line 50 by meansof a series of crossover connections 51, 52, and 53 in which are a number of valves 51", 52 and '53. Pyrometers 55, 56, 57, 58, 59, and 60 are situated on either side of the point of connection of these cross-overs with the coil. 54 is a pyrometer at the outlet of coil 8. 61 is a pyrometer at the outlet of coil 11. 62 is a pressure gauge placed immediately before the pressure reducing valve 14.

The operation of this system is as follows: Oil contained in tank 1 is pumped by means of pump 3 through'line 4, heat exchangers A, B, and C to coil 8. In this coil the oil is raised to the desired cracking temperature which is measured by pyrometer 54. It then passes through coil 11 where it is maintained at, at least, said cracking temperature for a prolonged period of time. Instead of raising the oil to the cracking temperature at the exit of coil 8 it may be raised to this cracking temperature part way through this coil and maintained at the cracking temperature through the rest of the coil and through coil 11. The oil then passes through heat exchanger C where it is partially cooled and the cooling regulated by regulating the amount of oil passing through heat exchanger C by the proper manipulation of valve 49, in by-pass 49. The oil then passes from heat exchanger C to evaporator 15, past-the pressure reducing valve 14'. The pressure of the oil during the heating step is regulated by the regulation of this valve 14. The oil on injection in evaporator 15 is distilled, the unvaporized fraction being removed through line 44 and valve 44 and through heat exchanger B, and sent to storage tank 48 by pump 46. The vapors rising from 15 are fractionated in tower 16 by a proper regulation of the analyzer 19 and by the introduc-' tion of reflux oil through 37, as will appear below. The condensate trapped out in 18 I which consists of fractions heavier than gasoline, still contains gasoline fractions, and is sent via line 26 to still 27 where the hot c011- densate is distilled andfractionated by aid of steam 38 and bubble trays 28. The vapor issuing from 29 consists of the heavy gasoline fractions. These are condensedin condenser 30, collected through lookbox 32 and line 33 in tank 34. Uncondensed gases are removed via line 31. This condensate in tank 34 is sent up via pump 36 and line 37 to act as the reflux aforementioned. The unvaporized portion remaining in still 27 is removed via line 39 and valve 39 through heat exchanger A in line 40 and stored in 42, by means of the pump 41. This condensate may be Stored for future cracking at higher temperature, or may be recycled to mingle with the oil removed from tank 1, by

, manipulation of valve 43 in line 43. Vapors issuing from the tower 16 which consist of either pressure distillate or crude gasoline,

are condensed in condenser 21, collected via lookbox 22 in line 23 into tank 24. Unvaporized gases uncondensed in condenser 21 are removed via line 25.

The temperatures and time during which the oil is heated are deliberately chosen so that the oil will reach a vapor pressure exceeding the imposed pressure on the oil. Therefore, at some point in its progress through the coil,

for instance, in coil 11, the oil will have reached a partially vaporized condition. When this happens, a pyrometer situated in the oil stream will begin to fluctuate. This is due to the fact that this pyrometer is in the region where it is at one moment subjected to an atmosphere of vapor, and, at another moment, to an atmosphere of liquid oil. The heat transfer from vapor to the pyromcter is not as good as from liquid oil, and the pyrometer begins to fluctuate up and down. All pyrometers situated in-the coils from this point to the outlet will show the same effects, or will show a falling tem perature. Supposing this effect is first noted on pyrometer 55. Oil is injected at this point via line and cross-over 53, valve 53' eing open, and valves 51 and 52 being closed. It will then appear that pyrometers 56, 57, 58, 59 and 60 become steady due to the addition of relatively cool and uncracked oil which acts as a solvent for the low boiling hydrocarbons, and thus reduces the vapor pressure of the oil to a point where it does not exceed the imposed mechanical pressure. The oil then passes through the remaining portion of the coil where it is raised back to its cracking temperature, and the cracking is continued for an increased length of time. Should it appear that after this injection, that whereas pyrometers 55 and 56 are steady, 57 has begun to fluctuate, as also have 58, 59, and 60, oil may be injected via cross-over line 52 to reconvert the partially vaporized oil stream back to a liquid phase. The pyrometers then become steady again. By so manipulating the oil stream and injecting cool oil, or relatively cool and uncracked oil to dilute the oil at the point where the partial vaporization of the oil occurs, the oil may be maintained in liquid phase throughout its course through the coils, and subjected for a greater length of time to cracking in the liquid phase at the chosen cracking temperature. The injected oil may be relatively cool oil or may be preheated to the cracking temperature aforementioned and this relatively uncracked oil injected into the stream to dilute the more cracked oil and so reduce its vapor pressure.

" Instead of injecting the oil at the point where the pyrometers show a fluctuation where the cracked oil has reached a vapor presure such as to cause vaporization, the oil ma. be injected at a point in front of this p ace, that is at an earlier point. This will have the same effect and if the subsequent heating is not too prolonged no vaporization will occur. Likewise the injection may be made at a place after the point of incipient vaporization to obtain a slight degree of vapor phase cracking if the advantage of vapor phase cracking is desired. This amount of vapor phase cracking must, however, be strictly limited to prevent the accumulation of carbon and excessive permanent gas formation.

The oil by means of this operation can therefore be maintained for a greater length of time at its reaction temperature, or the oil may be maintained at a higher reaction temrature than would be possible if this injection of oil .were not employed. Having determined the position of the point of injection in the operation of the system for any given oil, time, and temperature, it will be no longer necessa to have all the crossovers originally emp oyed. The system may be built only with those cross-overs actually needed. N or is it necessary to start in with a coil containing a number ofcross-overs, such as 51, 52, and 53, since it would be possible to weld into the coil any cross-overs which might appear to be necessary on the inspection of the operation of these pyrometers. It is usually undesirable to have a large number of connections or a large number of valves on a coil operating under the high pressures and temperatures usually employed in this system, which pressures are in the neighborhood of one thousand pounds and the temperature between approximately 800 F. and 900 F.'

The exact locations of these cross-over lines and the length of time during which the oil may be exposed to heat at the cracking temperature, will depend upon the character of the oil cracked and the temperature and pressure employed. Thus, in cracking gas oil at a temperature inthe neighborhood of 820 F. to 840 F. at pressure approximately 1,000 pounds, the oil may be subjected for approximately twenty minutes without substantial vaporization in the coils. If the heating is continued beyond this point for any considerable length of time, it will be found that oil must be injected into the coils. The length of time during which the oil may then be heated without its flashing into vapor phase will depend upon the amount and character of the oil injected. However, if one operates at a temperature in the neighbor hoodof 850 F. to 880 F. the length of time during which the oil may be subjected to heat at this temperature will be materially less, sincethe rate of cracking reaction is considerably augmented and it ma be found that the injected oil must be intro uced after eight or ten minutes of cracking reaction.

The ides to determining the presence of vapor piiise cracking are as stated above, fluctuation of the pyrometers an increase in the amount of carbon and fixed gases formed, an

increase in the specific gravity and olefine content of the gasoline. The surest guide is the fluctuations in the pyrometers placed in the coils, as will appear to those skilled in the art.

The above is not .to be taken as limiting my invention but merely as illustrative of the best manner of carrying out my invention, which I claim to be:

1. A. process of cracking oil which comprises heating oil in a confined stream under superatmospheric pressure to crack said oil, measuring the temperature of said oil by pyrometers at a plurality of places along said stream, and injecting oil at points in said stream where fluctuations in temperature, as indicated by said pyrometers,'are observed.

2. A process of cracking oil which comprises heating oil to a cracking temperature,

maintaining at least said cracking temperature while passing said oil under superatmosplieric pressure in a confined stream, measuring the temperature of said oil by pyrometers situated at a plurality of points along said confined stream, and injecting relatively cool oil in the region where fluctuations in temperature, as indicated by said pyrometers, are observed.

3. A process of cracking oil which comprises heating 011 in a confined stream under superatmospheric pressure to crack said oil,

measuring the temperature of said oil by pyrometers at a plurality of places along said stream, and injecting relatively uncracked oil 1n the region in said stream where fluctuation along said confined'stream and injecting relatively uncracked oil in the region where fluctuatlons in temperature, as indicated by sald pyrometers, are observed.

5. A process of cracking oil which comprises heating oil in a confined stream under superatmospheric pressure to crack said oil, measuring the temperature of said oil by pyrometers at a plurality of places along said stream, and injecting relatively cool and uncracked oil in the region in said stream where fluctuations in temperature, as indicated by said pyrometers, are observed.

6. A process of cracking oil which comprises heating oil to a cracking temperature, maintaining at least said cracking temperature while passing said oil under a superatmospheric pressure in a confined stream, measuring the temperature of said oil by pyrometers situated at a plurality of points along said confined stream, and in ecting relatively cool and u'ncracked oil in the region where fluctuations in temperature, as indicated by said pyrometers, are observed.

In testimony whereof I aflix my signature.

JOHN C. BLACK.

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