US2288070A - Hydrocarbon conversion process - Google Patents

Description

M. T. CARPENTER HYDROCARBON CONVERSION PROCESS A Filed Aug. 8, 1939 June 3o, 1942.

Patented `lune 30, 1942 rtree Standard Oil Company, Chicago, Ill., a

tion of Indiana corpora- Application August 8, 1939, Serial No. 288,988 i 5 Claims.

This invention relates to hydrocarbon conversion processes for the manufacture of high quality motor fuel and it pertains more particularly to an improved system for fractionating normally liquid from normally gaseous hydrocarbons and utilizing the' separated fractions in said conversion processes more efficiently than has heretofore been possible.

An object of the invention is to increase the yield and quality of motor fuels obtainable from petroleum and petroleum products and from similar hydrocarbons obtainable from shale, coal or other carbonaceous materials by hydrogenation, carbon monoxide synthesis, etc.

A further object is to provide an improved fractionation system for high pressure hydrocarbon conversion processes, to minimize compression costs of such fractionation systems and to effect savings in investment costs and operating eX- penses.

A further object is to provide a new and eficient method and means whereby fractionation of various components is effected under such pressure conditions that savings may be eifected in the use of stripping steam in the fractionation of gasoline fromgas oil, and savings may be effected in the compression costs in the fractionation of normally gaseous hydrocarbons and the return of said hydrocarbons to the conversion system.

A further object is to improve the effectiveness and efciency of such hydrocarbon processes as gas reversion, pyrolytic cracking, thermal and catalytic reforming, polymerization, etc. More specific objects of the invention include:

1. Low pressure separation of cycle stock and gasoline, and better fractionation between the two, whereby less gasoline will be recycled to the furnace than in the high pressure system.

2. Avoidance of flash separation as heretofore required in the primary separator of cracking systems.

3. Low temperature operation in the evaporator and bubble towers, which in turn considerably reduces the amount of expensive alloy tubin and equipment.

4. Effective utilization of transfer line exchangers whereby much smaller furnaces can be employed than have heretofore been necessary.

Other objects of the invention will be apparent as the detailed description proceeds.

In practicing my invention I discharge the conversion products from a furnace, preferably through transfer line exchangers, into a high pressure tower maintained at about 300 to 400 pounds per square inch, wherein a light overhead fraction consisting of all of the propane and lighter hydrocarbons, along with most of the butane and some of the heavier hydrocarbons lighter than gas oil are separated from a bottoms fraction consisting of tar, cycle stock and asubstantial amount of the gasoline, The overhead fraction is stabilized without substantial decrease in pressure for recovery of its gasolinekcontent, and the remaining gases, while still `at, the high pressure, are further stabilized for the separation of propane and butane from hydrocarbons lighter than propane. The C3-C4 fraction, which is still at the high pressure, is passes to the same or a different conversion system which may be gas jreversion, alkylation, polymerization, etc.l The bottoms from thehigh pressure separator are released to low pressure before separation of cycle stock, gasoline and tar respectively so that sharp fractionation of these components can be effected and so that a negligible amount of gasoline will be recycled to the furnace with the gas oil.

In other words, I provide operating conditions in the high pressure separation zone which result in overhead which is substantially free of cycle stock, and a bottoms which is free of propane and substantially free of butane. The overhead is separated in two steps at high pressure into the gasoline, recycle condensibles and fuel gas. The bottoms from the high pressure separator are fractionated in two steps at low pressure into tar, cycle stock and a gasoline which under preferred conditions requires no stabilization. The invention will be more clearly understood from the following detailed description read in connection with the accompanying drawing which forms a part of this disclosure and which is a diagrammatic flow sheet of the application of my invention to a gas reversion process.

Gas oil from storage tank I0 is introduced by pump Il and line l2, together with condensibles (chiey C3 and C4 hydrocarbons) from line I3 and cycle stock from line I4 through heat eX- changer l5 into coils I6 of furnace l1. For gas reversion the pressure should be about 1000 to 3000 pounds per square inch, although pressures of the order of 750 pounds per square inch may be employed where the conversion is chiefly pyrolytic cracking and where the condensibles are passed to a separate alkylation or polymerization system. In coils I6 the stock is heated to a temperature which in the case of gas reversion may be about 950 to 1100" F., preferably about 1020 F. I-lot transfer line products at about this temperature may be passed by line i8 through a transfer line exchanger l wherein they are cooled to about 800 to 900 F. and then passed through pressure reduction valve I9 and introduced into high pressure separator which is maintained at a pressure of about 300 to 400 pounds per square inch, preferably at about 325 to 350 pounds. It should be understood, of course, that the transfer exchanger may be omitted and the hot products passed directly through the pressure reducing valve I9 to the separator.

The high pressure separator 20 is provided with a suitable reflux means 2l which may consist of cooling coils or means for introducing cool reflux liquid. The separator is also provided with suitable bubble plates, baille plates or packing material 22. The bottom temperature in this separator should be from about 750 to 900 F., i. e. sufficiently high to insure the elimination of all propane and most of the butane from the bottoms leaving this separator tower. high pressure separator is maintained at a temperature of about 300 to 800 F., preferably within the lower part of this temperature range, to insure the elimination of all gas oil from the overhead products. I have found that by operating the high pressure evaporator tower at 300 pounds with a bottom temperature of 800 F. that the bottoms contain practically no methane or ethane, less than .1% of propane, and only about .2% of total butanes.

The overhead fraction from separator 2Q, which fraction contains substantially all of the normally gaseous hydrocarbons and a considerable amount of gasoline fraction, is withdrawn through line 23 to gasoline stabilizer 24 which is maintained at about the same pressure as the separator. Stabilizer 24 is provided with suitable reflux means 25 at the top. This means is illustrated as a cooling coil but in actual practice I prefer to employ reflux condensate in accordance with conventional practice. The base of the tower is provided with suitable reboiler or heating means 26. The temperature in this stabilizer will vary with the different stocks and desired products but a top temperature of about 110 F. and a bottom temperature of about 430 F. have been successfully employed. Stabilized gasoline is withdrawn from the base of the stabilizer through line 21.

The removed C1 to C4 hydrocarbons are withdrawn overhead through line 28 and introduced into condensibles stabilizer tower 29 which is likewise at substantially the same pressure as separator 20 and gasoline stabilizer 24. The condensibles stabilizer is provided with refrigerated reflux means 30 at its top and reboiler or heating means 3| at its base. The top temperature is preferably rather low, i. e. of the order of 0 to 40 F., and the bottom temperature may range from about -10 to about 125 F. The exact temperature conditions in this stabilizer will be determined by the sharpness with which it is desired to separate fuel gases from C3, C4 hydrocarbons, the 4fuel gases being with-drawn overhead thru line 32 and the condensibles being Withdrawn thru line 33 to receiver 34. From this receiver the condensibles may be withdrawn thru line 35 and pump 3S for recycling thru line I3 to the gas reversion system. If, however, the invention is applied to a conversion process other than gas reversion these condensibles may be passed by line 3l to a suitable polymerization, alkylation or other conversion system.

Bottoms from high pressure separator 20 are introduced thru line 38 and pressure reduction The top of the valve 39 to tar flash tower 40, which is preferably operated at about 50 pounds per square inch with a bottom temperature of about 750 to 900 F. and a top temperature of about 600 to 800 F., i. e. under such conditions that all of the gasoline and gas oil fractions are taken overhead. It will be understood, of course, that suitable reflux means 4I, and if necessary, heating means 42, may be employed in this ash tower. Tar is withdrawn through line 43 and the gasoline and gas oil vapors are withdrawn through line 44 to bubble tower 45 which is likewise operated at the relatively low pressure of about 40 or 50 pounds per square inch. The bubble tower will, of course, be provided with suitable reflux means 45 and, if necessary, with suitable reboiler means 41, and the exact temperature conditions in the tower will depend upon the stock charged and the products desired. A top temperature of 320 F. and a bottom temperature of 450 F. have given satisfactory results.

Since the gasoline is substantially free from propane and lighter hydrocarbons the overhead from tower 45 may simply be passed through line 43 and condenser 49 to receiver 50, from which it may be withdrawn through line 5i to storage. A gas vent line 52 is preferably provided and in case small amounts of propane get into this system, the gasoline may be pumped through line 53 by pump 54 to stabilizer 24. Generally speaking, however, in the preferred operation of the invention it is unnecessary to stabilize the gasoline from receiver 50 since substantially all of the propane and lighter hydrocarbons have been removed therefrom in high pressure separator 2).

By operating bubble tower 45 at the relatively low pressure and temperature I not only effect marked savings in equipment costs, but obtain a sharp separation of gasoline from gas oil so that gas oil Which is recycled through line 55 and pump 56 to line I4 or storage tank I0, will be substantially free from gasoline. It should be noted that although stripping steam may be employed in the bubble tower, it is not essential as it would be in the case of fractionation at higher pressures.

Other advantages obtained by this fractionation system include the elimination of the necessity of stabilizing the gasoline from high pressure separator bottoms. The relatively heavy gasoline from line 5I may be blended with the relatively light gasoline from line 21 to obtain a blend of any desired volatility.

It will thus be seen that I have accomplished the objects of the invention as hereinabove set forth. The absence of condensibles in the gasoline-gas oil fractionation makes possible the use of high temperatures at low pressures; were it not for the removal of the ethane, propane and butane in the high pressure fractionation step the temperatures required for efiicient gas oilgasoline fractionation might be close to 0r above the critical temperatures of some of these condensibles. Marked savings in compression are eifected by maintaining the gases and vapors at a pressure of about 300 pounds or higher throughout the entire series of steps, so that liqueed condensibles from line 34 may be handled by a .liquid pump when it is desired to subject them to high pressures for polymerization, gas reversion, etc. Compression costs are thus markedly decreased.

While the invention has been described in connection with gas reversion, it should be understood that it is equally applicable to other hydrocarbon conversion processes, particularly to cracking, polymerization, reforming and alkylation. Similarly, the invention is not limited to the particular operating conditions hereinabove set forth since many departures therefrom and modii'ications thereof will be apparent to those skilled in the art.

I claim:

1. In a hydrocarbon conversion system wherein hydrocarbons are subjected to elevated temperatures and pressures for effecting said conversion, the method of fractionating the products of conversion Which comprises introducing said products into a separator zone at a pressure within the approximate range of 325 to 350 pounds per square inch, maintaining a temperature at the top of said separator zone Within the approximate range of 300 to 600 F., maintaining a temperature at the bottom of said separator zone Within the approximate range of '750 to 900 F., said conditions in said separating zone effecting the separation of an overhead fraction containing substantially all of the C1 to C4 hydrocarbons along with substantial amounts of gasoline and a bottoms fraction containing substantially all of the tar and gas oil with substantial amounts of gasoline hydrocarbons, separately recovering gasoline and condensible hydrocarbons from said overhead fraction Without substantial reduction of pressure, and separating the bottoms fraction into gasoline and gas oil at low pressure.

2. The method of obtaining relatively sharp fractionation between condensible Cs-Cl hydrocarbons, gasoline hydrocarbons and gas oil hydrocarbons, which method comprises introducing a mixture of said hydrocarbons into a separating zone at a pressure Within the approximate range of 325 to 350 pounds per square inch, withdrawing liquids from said zone at a temperature within the approximate range of 750 to 900 F., withdrawing gases and vapors from said zone at a temperature within the approximate range of 300 to 600 F., reducing the pressure on liquids Withdrawn\from said zone to a pressure Within the approximate range of atmospheric to 50 pounds per square inch, fractionating the gasoline from the gas oil under said low pressure conditions, stabilizing the gasoline which is included in the gases and vapors Withdrawn at high pressures from the separating zone and removing C1 and C2 hydrocarbons from the stabilizer gases and vapors leaving said stabilizing step.

3. The method of operating a gas reversion system Which comprises heating a mixture of gas oil and C3--C4 hydrocarbons to a temperature Within the approximate range of 950 to 1100o F., under a pressure within the approximate range of 750 to 3000 pounds per square inch, passing the hot products to a pressure reduction valve to lower the pressure of said products to a pressure within the approximate range of 300 to 350 pounds per square inch, separating said products at said pressure to an overhead fraction containing substantially all of the C1 to C4 hydrocarbons and a substantial quantity of gasoline hydrocarbons, and a bottoms fraction containing all of the tar and gas oil with substantial quantities of gasollne hydrocarbons, reducing the pressure of the bottom fraction to a pressure within the approximate range of atmospheric to 50 pounds per square inch and separating tar, gas oil and gasoline respectively from said bottom fraction said low pressure, recycling said gas oil to said ating step, removing C1 to C4 hydrocarbons n the gasoline in said overhead fraction Without substantial reduction in pressure, removing C1 and C2 hydrocarbons from the C3 and C4 hydro; the fraction which has thus been separat. from gasoline without substantial reduction in pressure and at a low enough temperature to permit liquefaction of the C3-C4 hydrocarbons, and returning said liquid C3-C4 hydrocarbons by a liquid pump to the original heating step.

4. The method of converting hydrocarbon gases and liquids to high quality motor fuels which comprises heating a mixture of said gases and liquids to temperatures within the approximate range of 1000 to 1100 F, at pressures within the approximate range of 1000 to 3000 pounds per square inch, separating the reaction products in a high pressure zone at a pressure Within the approximate range of 300 to 350 pounds per square inch into a gas vapor fraction containing substantially all of the C1 to C4 hydrocarbons and a part of the gasoline, and into a liquid fraction containing the rest of the gasoline and all of the heavier components, separating the gasoline components from the gas vapor fraction without materially decreasing the pressure thereon, separating the C3--C4 hydrocarbons from the remainder of the gas-vapor fraction under subfstantially the same pressure in a stabilizer zone operated at low temperature, whereby the C3-C4 hydrocarbons are separated as liquids, pumping said liquids back to the heating zone for further conversion, reducing the pressure on the liquid fraction from said rst separation step and separating said fraction into gasoline, gas oil and a fraction heavier than gas oil, and pumping said gas oil component back to the heating zone for further conversion with the C3-C4 hydrocarbons.

5. The process of claim 4 wherein the gasoline from the low pressure fractionation step is pumped into the gasoline stabilizer zone for the removal of any small amount of propane that was not removed with the gaseous fraction in the rst separation step.

MORRIS T. CARPENTER.

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