US3239458A - Recovery of light hydrocarbons from refinery gases - Google Patents

Description

March 8, 1966 BAUMANN ETAL 3,239,458

RECOVERY OF LIGHT HYDROCARBONS FROM REFINERY GASES Filed July 30, 1962 COMPRESSOR |6 l7 l3 l5 REFLUX DRUM VIRGIN NAPHTHA LEAN GAS |0- 2| i 26 COOLER ,WFRACTIONATOR CONTACTOR CONTACTOR Patent Attorney Inventors United States Patent 3,239,458 RECOVERY OF LIGHT HYDROCARBONS FROM REFINERY GASES George P. Baumann, Sparta, and Jerome G. Freiling,

Livingston, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed July 30, 1962, Ser. No. 213,299 4 Claims. (Cl. 208344) This invention relates to the recovery of light hydrocarbons from refinery gases formed during refining and converson of petroleum oils and deals particularly with the recovery of light hydrocarbons such as propane, propene, butanes, and butenes (C and C.,) from lower boiling gaseous products. While the invention in some of its broader aspects has a more general application, it is particularly applicable to the recovery of such light hydrocarbons from the gaseous products obtained during cracking of petroleum oil and more specifically in low pressure cracking processes such as catalytic cracking.

According to conventional procedure, the vapor products from cracking operations are first passed to a product fractionator in which the higher boiling normally liquid products are fractionally condensed. The top temperature of this fractionating tower is controlled to remove overhead uncondensed vapor products including naphtha and lower boiling components. This temperature may range from 225350 F. depending on the tower pressure.

These overhead vapors are then passed to a condenser operated at temperatures which will condense the major portion of the naphtha components. The products from the condenser, comprising the liquid condensate together with uncondensed vapors including substantial amounts of C s and C s, are then passed to a separator or reflux drum in which the raw naphtha condensate is separated from the uncondensed gases. These uncondensed gases will normally contain substantial quantities of valuables C and C, hydrocarbons which may form valuable components in the gasoline product, LPG product, or possible petrochemical feedstocks.

According to common custom, these uncondensed wet gases are first compressed and then passed into an absorption tower through which the gases pass upwardly in countercurrent contact with a higher boiling oil such as a heavy naphtha fraction which is capable of absorbing the C and C hydrocarbons contained in the gases. The fat oil containing these components is withdrawn from the bottom of the absorber and then sent to a distillation or desorber tower where the light C and C components are vaporized and separated from the oil. The lean oil freed of the C -C components is then returned to the top of the absorber tower for reuse.

While the typical procedure briefly outlined above provides an effective method for recovering the C -C hydrocarbons from the refinery light ends, such an installation with its complex of absorber and desorber towers, heat exchange equipment, recycle lines, pumps, etc. represents a substantial investment in equipment, particularly in small refineries located in remote areas where only sufficient C or C hydrocarbons need to be recovered to supply the necessary low boiling components for the gasoline or to meet requirements for a 'very small amount of LPG product. The main purpose of a cracking unit in such area is to produce gasoline and LPG to supply the needs of the surrounding area. In such cases no facilities can be economically provided for utilizing the excess C s and C s which may be formed during the cracking operation. Consequently it has only been practical to recover enough C components to provide a finished gasoline of desired volatility and boiling range.

The main purpose of this invention is to provide a more 3,239,458 Patented Mar. 8, 1966 simple and less expensive apparatus for recovering the C and C hydrocarbons from the gases. Briefly, according to this invention the light ends removed from the naphtha reflux or separating drum receiving products from the product fractionator of a cracking unit are first compressed to :a pressure substantially above the separating pressure and again remixed and re-contacted with the naphtha fraction separately removed from the reflux drum while at substantially the same temperature at which the original separation was effected. For example, the gases may be compressed to a pressure between 25 and 200 pounds per square inch higher than the separating pressure. Under the higher pressure, this naphtha fraction is capable of absorbing or taking up additional quantities of the C and C components present in the gases. Following the contacting, the gaseous products, partially depleted of C and C hydrocarbons by contacting with the cracked naphtha, are separated from the naphtha and are further contacted with a virgin naphtha fraction which absorbs additional quantities of the C and C hydrocarbons. These two separate contacting stages, first with the cracked naphtha and thereafter with virgin naphtha, are capable of recovering adequate quantities of the C and C hydrocarbons to supply the necessary amount for finished gasoline and LPG product. The cracked naphtha and the virgin naphtha, after being used for contacting the wet gases, may then be passed to separate or common stabilizing facilities wherein the naphthas are subjected to distillation and fractionation to produce a final gasoline product of desired boiling range and volatility. Stabilizer overhead C and C plus small amounts of lighter gases, may be then further processed for LPG.

From the above, it will be seen that according to the present invention it is unnecessary to provide facilities for first absorbing the lighter components of the gases followed by desorbing facilities for removing the gases from the absorbent oil. Consequently the equipment necessary for effecting the desired removal of these fractions can be greatly reduced and, in the illustration that follows, re quiring only a compressor, two small coolers and two drums (or, alternately, a single, small drum partitioned into two compartments).

The invention will be best understood by reference to the accompanying drawing which is a diagrammatic illustration of basic equipment necessary for carrying out the invention.

Referring to the drawing, the reference character 10 designates a product fractionating tower adapted to receive overhead vapor products through line 11 from a cracking or distilling unit (not shown). For example, the vapor products passing through line 11 may be obtained from the distillation of crude or vapor products from catalytic cracking, thermal or coking operations, or any combination thereof. However, for illustrative purposes, the specific embodiment will be directed to the recovery of C and C hydrocarbons from catalytically cracked gases. In such case the temperature at the top of the fractionating tower is controlled according to conventional methods to condense all the products boiling above the end point of naphtha. For example, the temperature of the fractionating tower may commonly be held from 250 to 300 F. The condensed products are removed from the bottom of the fractionating tower through line 12. If desired, separate side cuts may be removed from the tower by the provision of separate trap out trays, not shown in the drawing.

Uncondensed products including the naphtha components are removed overhead from the product fractionator through line 13 and are passed through the condenser 14 in which the products are cooled to substantially atmospheric temperature suflicient to condense the higher boiling naphtha components. denser 14 then pass to a reflux drum 15 in which the uncondensed vapors are allowed to separate from the condensate formed in the condenser. gases and vapors are removed from the reflux drum through line 16 and passed to a compressor 17.

The naphtha condensate from the reflux drum 15 is withdrawn through line 18 and passed to a pump 19. A portion of this condensate is returned to thetopof'the product fractionator through line 20 to provide reflux therefor according to conventional practice. The remainder of the condensed naphtha fraction is pumped through line 21 and is intermixed with the compressed gaseous components removed from the reflux drum in line 22. The resulting mixture is then passed to a cooler 23 which serves to remove the heat of compression from the gases and brings the temperature .ofthe mixture to,

substantially atmospheric. The resulting mixture is then passed into a contacting drum 24 in which the gases and naphtha are allowed to remain in contact with each other for a period sufiicient to remove a substantial amount of the C and C hydrocarbons originally contained in the gases. drum, as illustrated in the drawing, or may take the form of a battle tower through which the-gases and naphtha pass in countercurrent contact with each other.

Following the contacting of the gases with the catalytic naphtha, the uncondensed gases are removed from the contactor through line 25 and then intermixed with a stream of virgin naphtha introduced through line 26'. This mixture is then passed through a gas cooler 27 cooled to remove the heat of absorption and the resulting mixture is then passed to a separate, contactor 28 wherein additional light hydrocarbons are absorbed in the virgin naphtha fraction. The unabsorbed gases substantially depleted of the light C and C hydrocarbons are removed overhead from the contactor 28 through line 29 and may be rejected from the system as fuel.

The catalytic naphtha is removed from the bottom. contactor 24 through line 30, and the virgin naphtha fraction is removed from the contactor 28 through line 31 These fractions are then sent to suitable distillation and stabilizing equipment (not shown) to produce products of desired vapor pressure and volatility. In some cases both fractions may be treated in a common stabilizer, and in other cases it may be desirable to provide separate facilities for stabilizing the two products. The stabilizer overhead product containing C s and C s as well as lighter hydrocarbons is sent to fractionation facilities for production of specification LPG. The stabilization of: these products and fractionation of the overhead cuts for LPG is carried out according to a customary procedure The uncondensed This contacting vessel may be in the form of a The products from conand, for purposes of simplicity, has not been illustrated in the drawing. 7 The amountof light C and C hydrocarbons absorbed by the catalytic and virgin naphthas in the separate con-:

tactors 24 and 28 will depend upon the pressure maintained in these drums. The higher the pressure, the more complete the separation of these fractions. This is illustrated in the table given below;

TABLE I Approximate Recoveries Compressor Discharge From the above table, it will be seen that when operating at a discharge pressure of 183 pounds, approximately 95% of the butanes are recovered from the gases and 92% of the propanes. The amount of butanesnecessary tobe recovered to supply the necessary volatility and vapor pressure to. the gasolineandto be usedas 'LPG will vary in different locations. The preferred pressure may range from 50 to 200 pounds pen-square inch at the compressor discharge. a

It is particularly desirable to carry out the process according to the. particular sequence" described; jnarnelygby first contacting the light gases with crackednap'htha and thereafter with the virgin naphtha. By operating'in this manner, the gasesarefir'st contacted with liquid of relatively low absorption capacity and thereafter'with liquid of much higherv absorption capacity.

For exampleythelcatalytic naphtha removedrfrom the relativelylow-pressurerefiux drum i's already fairlyrich in'butanes and thereforeits ability to absorb additional butanes is correspondingly limited.

content which has a relatively low absorption efficiency for the C .s and Cgs which 'are naturally parafiinic. The virgin naphtha used-in the final stage'has a much higher The naphtha, in

absorption capacity for C s and C s.

addition to being relativelyfree of .C' s' and C s, is

usually of paraffinic nature;

Furthermore, by using 'virgin naphtha in :the I final stage, the light naphtha components carried over with the gas and normally lost WillI ber lOW; octanecomponents; originally contained in the virgin naphtha rather than the relatively high' octane components"fromvthe 7 Catalytic,

naphtha. Having described the 1 preferred embodiment of the invention,-it will be undcrstood thatit coversother such.

last-named naphtha condensate fraction under a pressure 1;

substantially higher than thea'pressure at which said gaseous fraction was separated from said naphtha condensate, thereafter recontacting said gaseous fractionwith said naphtha condensate at said elevated pressure to thereby absorbinsaid naphtha fraction additional quantities of C and C4hydrocarbons, thereafterT separating ;the. last-.

named naphtha fraction containing additional absorbed C and C hydrocarbons from the remaining gases, subf sequently contacting the: remaining gases with an un-.. cracked virgin naphtha fraction at; temperature and pressure conditions which absorb additional 0 andC com-. 'ponents in said uncracked naphtha and thereafter sepa- 'rating the virgin naphtha ;.containin'g said additional absorbed C anacr components;- from the remaining unabsorbed 1 gases.

2. 'A process for. recovering components: containing 3 and 4 carbon atoms per molecule from a normally gaseous fraction obtained from a catalytic cracking of petro leum oil wherein vapor products from said catalytic crack-: ing are-initially fractionated tov condense and separate the higher boiling :components from uncondensed naphtha and lower boiling gases. wherein theuncondensed fraction is further cooledto condense a naphtha fraction which is separated from the I uncondensed. gases and wherein it said iuncondensed gases contain a substantial amount of C and-C components, which comprisesplacing said last-named gases under, a pressure substantially higher than the pressure ,at which said gases were sepa- Furthermore, .the catalytic naphtha contains a relatively high aromatic rated from said naphtha fraction, thereafter recontacting said gases with said naphtha fraction while at said elevated pressure to thereby absorb in said naphtha fraction additional quantities of C -C components, thereafter again separating the naphtha fraction containing additional absorbed C -C components from the remaining gases, subsequently contacting said last-named gases with an uncracked virgin naphtha fraction at temperature and pressure conditions which will efiect additional absorption of C and C components from said gaseous fraction and thereafter separating said last-named virgin naphtha fraction from the remaining gases.

3. The invention defined in claim 1 wherein said gaseous fraction is recontacted with said catalytic naphtha 6 per square inch higher than the pressure at which the gaseous fraction was initially separated from said naphtha fraction.

4. The invention defined in claim 1 wherein the contacting of said gaseous fractions with said catalytic naphtha and uncracked naphtha is carried out at substantially atmospheric temperature.

References Cited by the Examiner UNITED STATES PATENTS 2,745,889 5/1956 Johnston et a1. 208-344 DELBERT E. GANTZ, Primary Examiner.

fraction while at a pressure between 25 and 200 pounds 15 ALPHONSO D. SULLIVAN, Examiner.

Claims (1)

1. A PROCESS FOR RECOVERING C3 AND C4 COMPONENTS FROM A LIGHTER GASEOUS FRACTION OBTAINED FROM THE CRACKING OF PETROLEUM WHEREIN VAPOR PRODUCTS RESULTING FROM SAID CRACKING ARE INITIALLY FRACTIONATED TO CONDENSE AND SEPARATE HIGHER BOILING COMPONENTS FROM UNCONDENSED NAPHTHA AND LOWER BOILING GASES WHEREIN THE UNCONDENSED NAPHTHA AND GASES ARE FURTHER COOLED TO CONDENSE THE NAPHTHA FRACTION WHICH IS SEPARATED FROM THE GASEOUS FRACTION AND WHEREIN SAID GASEOUS FRACTION CONTAINS A SUBSTANTIAL AMOUNT OF C3 AND C4 COMPONENTS, WHICH COMPRISES PLACING SAID LAST-NAMED GASEOUS FRACTION AND SAID LAST-NAMED NAPHTHA CONDENSATE FRACTION UNDER A PRESSURE SUBSTANTIALLY HIGHER THAN THE PRESSURE AT WHICH SAID GASEOUS FRACTION WAS SEPARATED FROM SAID NAPHTHA CONDENSATE, THEREAFTER RECONTACTING SAID GASEOUS FRACTION WITH SAID NAPHTHA CONDENSATE AT SAID ELEVATED PRESSURE TO THEREBY ABSORB IN SAID NAPHTHA FRACTION ADDITIONAL QUANTITIES OF C3 AND C4 HYDROCARBONS, THEREAFTER SEPARATING THE LASTNAMED NAPHTHA FRACTION CONTAINING ADDITIONAL ABSORBED C3 AND C4 HYDROCARBONS FROM THE REMAINING GASES, SUBSEQUENTLY CONTACTING THE REMAINING GASES WITH AN UNCRACKED VIRGIN NAPHTHA FRACTION AT TEMPERATURE AND PRESSURE CONDITIONS WHICH ABSORB ADDITIONAL C3 AND C4 COMPONENTS IN SAID UNCRACKED NAPHTHA AND THEREAFTER SEPARATING THE VIRGINI NAPHTHA CONTAINING SAID ADDITIONAL ABSORBED C3 AND C4 COMPONENTS FROM THE REMAINING UNABSORBED GASES.

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