US2463482A - Separation of olefins - Google Patents

Description

Patented Mar. l, y1949 SEPARATION F OLEFINs Alfred W. Francis, Woodbury,

Socony-Vacuum Oil Company,

N. J., assignor to Incorporated, a

corporation of New York Application March 13, 1947, Serial No. 734,509

(Cl. ZEG-677) 14 Claims. l

This invention relates to a process for separation of olefins from mixtures thereof with saturated aliphatic hydrocarbons and is particularly concerned with the recovery of normally gaseous olens from aqueous solutions of silver salts prepared by scrubbing a mixture of olens and parains with such aqueous silver salts.

It is well known that olens are selectively absorbed from hydrocarbon mixtures by aqueous solutions of certain metal salts such as the salts of silver, copper, mercury and the like. The copper salts normally require the presence of activating compounds which may be designated as cosolvents, for example, various amines, amides and other derivatives of ammonia. The copper salts also absorb carbon monoxide and are highly sensitive to a number of other contaminants. Further, solutions of copper salts which are suitable for the present purpose are in general highly viscous in nature and are therefore diflicult to use.

The present invention is concerned with an irnprovement on the process of separating and purifying oleiins by absorption in aqueous solutions of water-soluble silver salts as typified by silver nitrate and silver fluoride. These solutions require no co-solvents, do not adsorb carbon monoxide to any appreciable extent and are highly selective with respect to olens. The olens may be recovered from the silver salt solutions by heating or evacuation or by the extraction process disclosed in U. S. Patent No. 2,077,041 issued April 13, 1937, to Davis and Francis. Heating is objectionable in that elevated temperatures tend to induce reduction of the silver salts by the action of hydrocarbons and/or metal Walls of the apparatus. Evacuation is diflicult to control and is also subject to losses of silver solution at points of transfer and handling.

The process of the above-mentioned patent avoids many of these difficulties but still involves mechanical losses of the expensive silver solution. According to the present process the olefins are recovered from the aqueous solution by stripping with carbon dioxide or nitrous oxide, preferably under pressures about equal to the pressure prevailing in the absorption zone. By use of this expedient it becomes possible to operate the process while retaining the silver solution in a closed circuit under substantially constant pressure which avoids losses of silver. According to a preferred embodiment circulation through the system is induced by means of a magnetically operated pump which avoids the necessity of packing through which silver salt solutions may be lost.

The carbon dioxide or nitrous oxide may be employed in either gaseous or liquid phase, depending upon the process and temperature prevailing and it may be noted that other inert compounds which are gaseous under the conditions of the process may also be employed.

When carbon dioxide is used in liquid phase, the process depends upon the solubility of hydrocarbons in liquid carbon dioxide and it may be noted that the olens are completely miscible with liquid carbon dioxide in all proportions.

Gaseous carbon dioxide acts as a stripping agent in much the same manner as stripping steam is employed in conventional hydrocarbon fractionation operations. For this purpose, other inert gases such as nitrogen, hydrogen and air may be used with similar result.

It has been found that carbon dioxide is inert to the various components of the system under consideration. It does not react with silver nitrate in neutral or acid solution to give silver carbonate or any other precipitate even when the carbon dioxide is under high pressure or in liquid form.

Carbon dioxide is relatively insoluble in concentrated silver nitrate solution and it has been found that silver nitrate solutions stronger than 50% dissolve less than 0.8% by weight of liquid carbon dioxide. It may be noted that water will dissolve 6.0% by weight of carbon dioxide when in equilibrium with liquid carbon dioxide. In general, very strong solutions of the silver salts are preferred since the solubility of olens in aqueous silver salts is roughly proportional to the concentration of silver salt. Any concentration of a soluble silver salt gives some effect and the dilute solutions are operative. However, the concentrated, preferably saturated solutions are greatly preferred because of the greater overall eiliclency obtained by the use of silver solutions having a high solvent power for oleiins.

Nitrous oxide is similar to carbon dioxide in the above'respects and it is to be understood that where carbon dioxide is mentioned herein, nitrous oxide is also contemplated as an equivalent thereof.

If a pure olefin is desired as the final product it is, of course, necessary to charge a relatively narrow fraction of the hydrocarbon mixture from which the olefin is to be recovered. Thus, the hydrocarbon mixture should be fractionally distilled to separate a narrow boiling cut containing only one olefin. For example, in the case of ethylene, the gas charged to the absorption stage may also contain methane, ethane, propane, hydrogen,

carbon monoxide and carbon dioxide. Sulphur compounds such as hydrogen sulfide and mercaptans will react with the silver salt and should be removed from the charged gas by caustic scrubbing. Acetylene also is undesirable and should be removed if present, as by selective hydrogenation.

The solvent power of aqueous silver salts for olefins varies with pressure, concentration oi' the silver salt solution and mol fraction of the olefin in the gas. The effect of these factors is well illustrated by consideration of the solubility of propylene in aqueous silver nitrate solutions.

In 71% aqueous silver nitrate at 25 C., the solubility of propylene was found to be 8 volumes at atmospheric pressure and 185 volumes (reduced to atmospheric pressure) when using liquid propylene, vapor pressure 11.5 atm., about twice as much as that expected (92 volumes) if the solubility were proportioned to pressure. This amounts to 18.7 g. propylene in 100 g. of silver nitrate solution or 1.06 moles per mole of silver nitrate.

In 53% aqueous silver nitrate at 0 C. the solubility of ethylene was found to be 63 volumes at 40 atmospheres (just under the vapor pressure) and 200 volumes at 60 atmospheres (900 lbs.). The latter is 12% by weight of the solution or 1.37 moles per mole oi' silver nitrate.

The disproportionate gas solubilities noted in the above examples, though unexpected, are not unreasonable in view of analogous observations on solubility of ammonia in water and carbon dioxide in organic solvents reported in the literature. For example, in 1 g. of water at 20 C. are dissolved 1.5 g. ammonia at 4 atmospheres and 17 g. or over eleven times as much at 8 atm. (Seidell, Solubilities of Organic and Inorganic Compounds. D. Van Nostrand, New York, ed. III, vol. I, pp. 1035-6). Similarly, carbon dioxide is nearly three times as soluble in benzene or toluene at 40 kg. per sq. cm. as it is at 20 kg., even at 35 C., which is above the critical temperature, 31 C. (Seidell, supra, p. 234). These abnormal solubility isotherms are probably related to the densities of the gas or liquid solute layers which are likewise I disproportionate to the pressures in the neighborhood of the vapor pressures.

When the olen is not pure, the solubility in silver nitrate is approximately proportional to the mole fraction rather than the partial pressure, as shown in the following table:

lgcllign Olean Diluent @251071170 Per cent by 0.945 Propylene wei"MISJ 25 C. is 18.7% by weight in 71% silver nitrate and 9.6% in 39% silver nitrate solution.

A nearly saturated solution of silver nitrate is thus favored in order to increase the capacity. In fact propylene combines with solid anhydrous silver nitrate to form an easily flowing liquid complex containing about 1.58 moles of propylene to one of silver nitrate. It freezes at 0 C. On re duction of pressure, it evolves propylene quantitatively, leaving crystals oi silver nitrate.

A higher extraction temperature is suggested by the rapid increase in solubility of silver nitrate with temperature, so as to get a higher capacity for oleiins. This is opposed by a slightly lower molar capacity of the silver nitrate, as illustrated by the solubility of propylene in 71% silver nitrate at 25 C., 18.7% at 0 C., 21.2%; and at about 25 C. 23%. Other objections to an elevated temperature are a slightly increased tendency toward contamination of the solution by corrosion of the vessel or by reduction; and the risk of local cooling which might cause crystallization and plugging of a passage. Room temperature and about 70% silver nitrate are recommended. Solution of olefin in silver nitrate evolves heat and raises the temperature slightly, but desorption would produce a slight refrigeration.

To test the possibility of unfavorable reaction of silver nitrate with carbon dioxide under pressure, a thick glass tube was charged with 50% silver nitrate and solid carbon dioxide, sealed and warmed to room temperature to melt both layers, which remained as clear, colorless layers after shaking. This indicated no formation of insoluble silver carbonate. .The increase in volume of the lower aqueous layer corresponded to a. solubility of about 0.8% CO2. A similar experiment with water had indicated a solubility of about 6.0% by weight. On the other hand, carbon dioxide was found to be completely miscible with paraffin hydrocarbons at 50 C. and with olefins at still lower temperatures.

The invention contemplates adsorption of olefins from a gas containing the same in admixture with parafilns by contacting the gas with an aqueous solution of a silver salt, preferably under increased pressure and then stripping the enriched silver salt solution by intimately contacting the same with carbon dioxide in either liquid or gaseous phase. As an alternative, the invention contemplates stripping by the use of inert gases generally. Preferably both the absorption and stripping stages are carried out by countercurrent contacting. In the case of inert gases, including gaseous carbon dioxide, the gas should boil at a temperature substantially removed from the boiling point of the olefin. For example, in the recovery of ethylene from mixtures of ethane the process, in eifect, involves substitution for ethane of a gas which is more readilyseparated by distillation. Carbon dioxide is greatly preferred because it is readily removed by washing with Water, thus dissolving the carbon dioxide which may then be recovered by heating or pressure reduction for return to the system. It is, of course, generally desirable to dry carbon dioxide so returned in order that it shall not affect the concentration of the silver salt solution.

Referring now to the annexed drawings, there is shown apparatus for practice of the present process in diagrammatic form. A gas mixture such as ethane and ethylene free of acetylene obtained by fractionation of cracked gases is scrubbed for removal of sulphur compounds and compressed to a pressure of about 900 lbs. per square inch. Gaseous mixtures which can be liqulfled at room temperature are generally compressed to a pressure sufficient to liquefy the same before charging of the process. The compressed gas is supplied by line I to an absorber II through which it rises counter-current to a silver nitrate solution admitted by line I2. The olefin is thereby absorbed at the high pressure in the silver nitrate solution and the denuded gas, in this case ethane, is discharged by vent'l pipe I3 from the top of absorber II. The enriched silver nitrate solution is withdrawn from the bottom of absorber II and transferred by pump I4 through line I to the top of the desorber I6 through which it passes downwardly counter-current to a stream of carbon dioxide supplied from line I1. The carbon dioxide strips the absorbed olefins from the `silver nitrate solution and a mixture of carbon dioxide and ethylene is taken off by pipe I8 for recovery of the olefins. The lean silver nitrate solution is then withdrawn from the bottom of desorber I6 and returned to the top of absorber II by line I2.

This entire circuit including absorber Il, desorber I6, pump I4 and the connecting lines together with such heat exchangers as may be required by the process are maintainedl under an elevated pressure about equal to the pressure of the compressed feed gases. It will be understood that the process may be operated at a lower pressure, even down to atmospheric pressure, but any reduction in pressure involves a corresponding reduction in absorption efciency and the invention preferably involves the use of elevated pressures upwards of 600 lbs. per square inch and preferably upwards of 900 lbs. per square inch. Loss of the expensive silver nitrate solution through packings and the like is preferably avoided by the use of some such device as a magnetic pump to induce circulation without employing moving parts projecting through the walls of any part of the system. The pump may be any one of several types, as is most convenient, the main requirements being (1) a piston of magnetic material, such as iron, with a corrosion resistant surface such as silver plating or a stainless steel sheath; (2) a cylinder of non-magnetic and corrosion resistant material, such as stainless steel; (3) a solenoid with intermittent current, or moving physically with a reciprocating motion, surrounding the cylinder, to actuate the piston; and (4) two check valves, one of which may be in the piston, and one at the end of the cylinder. One form is described in Ind. Eng. Chem. 34, 937 (1942).

Other means for providing circulation without possibility of leakage can be devised. It might be accomplished with thermal convection. Or the circulating pump might be operated with a sealed induction motor. Or the stuffing boxes of a conventional pump might be sealed against leakage by glands with high pressure gas.

Returning now to the mixture of carbon dioxide and ethylene discharged through line IB, this is transferred to a stripper I9 wherein the mixture is washed with water to separate the carbon dioxide and yield relatively pure ethylene discharged by pipe 20. The solution of carbon dioxide in water is transferred by line 2| from the bottom of stripper I9 to a flash tower 22 wherein the pressure is sharply reduced to evolve carbon dioxide. Th carbon dioxide is compressed to the pressure of the absorber-desorber system by compressor 23, cooled in heat exchanger 24 and returned to the desorber through line I1. Such 6 drying of the carbon dioxide as may be necessary can be carried out in the cooler 24 or by the use of suitable deslccant chambers in the return line. Water from the bottom of flash-tower 22 may be recycled to the stripper I9 through pump 25, heat exchanger 26 and return feed line 21.

The pressures prevailing in the cycle including stripper I9 and ash tower 22 need not necessarily be closely controlled. However, since this is a substantially closed cycle, it is convenient to operate the same at pressures approaching those prevailing in the absorber-desorber system. The pressure in the flash tower 22 must, of course, be substantially lower than that in stripper I9 in order to obtain a good separation of carbon dioxide. Thus, the stripper I9 may be operated at a pressure of about 900 lbs. per square inch and the water-carbon dioxide mixture may be flashed to 100 lbs. per square inch or less in the tower 22.

I claim:

l. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures countercurrent with a concentrated aqueous solution of silver nitrate at high pressure and thereafter stripping the olefins from said aqueous solution by contacting said solution countercurrent with liquid carbon dioxide substantially at said high pressure and recovering olens from solution with said carbon dioxide by washing lwith water.

2. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures countercurrent with a concentrated aqueous solution of a water-soluble silver salt at high pressure and thereafter stripping the olefins from said aqueous solution by contacting said solution countercurrent with liquid carbon dioxide substantially at said high pressure and recovering olefins from solution with said carbon dioxide by washing with water.

3. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a concentrated aqueous solution of silver nitrate and thereafter stripping the olefins from said aqueous solution by contacting said solution with gaseous carbon dioxide.

4. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises'contacting said mixtures with a concentrated aqueous solution of a water-soluble silver salt and thereafter stripping the olefins from said aqueous solution by contacting said solution with gaseous carbon dioxide.

5. A process for separation of olefins from mixtures thereof lwith saturated hydrocarbons which comprises contacting said mixtures with a concentrated aqueous solution of silver nitrate and thereafter stripping the olefins from said aqueous solution by contacting said solution with liquid carbon dioxide.

6. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a concentrated aqueous solution of a water-soluble silver salt and thereafter stripping the olefins from said aqueous solution by contacting said solution with liquid carbon dioxide.

'1. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a substantially saturated aqueous solution of silver nitrate and thereafter stripping the olefins from said aqueous solution by contacting said solution with carbon dioxide.

8. A process for separation of oleiins :from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a substantially saturated aqueous solution of a watersoluble silver salt and thereafter stripping the olens from said aqueous solution by contacting said solution with carbon dioxide.

9. Aprocess for separation of olefns from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with ay concentrated aqueous solution of silver nitrate in an absorption zone and thereafter stripping the oleflns from said aqueous solution by contacting said solution with carbon dioxide in a desorption zone at about the same pressure as that prevailing in said absorption zone.

10. A process for separation of olens from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a concentrated aqueous solution of a Water-soluble silver salt in an absorption zone and thereafter stripping the olefins from said aqueous solution by contacting said solution with carbon dioxide in a desorption zone at about the same pressure as that prevailing in said absorption zone.

11. A process for separation of olens from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a concentrated aqueous solution of silver nitrate and thereafter stripping the olefins from said aqueous solution by contacting said solution with carbon dioxide.

12. A process for separation of olefins from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures with a concentrated aqueous solution of a water-so1uble silver salt and thereafter stripping the olefins from said aqueous solution by contacting said solution with carbon dioxide.

13. A process for separation of olens from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures countercurrent with concentrated aqueous silver nitrate solution at high pressure in an absorption zone and thereafter stripping olens from said solution by contacting the same countercurrent -with an inert gas in a desorption zone maintained at about said high pressure prevailing in said absorption zone.

14. A process for separation of olens from mixtures thereof with saturated hydrocarbons which comprises contacting said mixtures countercurrent with concentrated aqueous silver nitrate solution at high pressure in an absorption zone and thereafter stripping olefins from said solution by contacting the same countercurrent with nitrous oxide in a desorption zone maintained at about said high pressure prevailing in said absorption zone.

ALFRED W. FRANCIS.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Germany Mar. 10, 1932 Number Number

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