US2407386A - Recovery of secondary olefins from acid extracts - Google Patents

Description

Patented Sept. 10, 1946 RECOVERY OF SECONDARY OLEFINS FROM ACID EXTBACTS Harold W. Scheeline, Elizabeth, J., assignor to Standard Oil Development Company, a corporation of Delaware Application February 14, 1942, Serial No. 430,872

3 Claims. (01. 260-677) I This invention relates to improvements in the separation and recovery of secondary olefins from mixtures of olefins and saturated hydrocarbons which are associated in gaseous and light liquid hydrocarbon mixtures and more particularly the separation of olefins having from itself (e. g. isobutane) and (2) difficulty in re- 3 to 5 carbon atoms to the molecule from mixcovering the desired product after pyrolysis. tures of hydrocarbons or mixtures consisting Pure olefinic hydrocarbons have been sepamainly of olefins and saturated hydrocarbons. rated from naphthas containing the olefins di- The separation of olefins by fractional distillae' luted with paraffinic hydrocarbons of approxition from saturated hydrocarbons having the mately the same boiling point by treating the same number of carbon atoms to the molecule, naphtha with sulfuric acid under such conditions as in the case of mixtures produced by cracking of acid strength, temperature, etc., as to selecor pyrolysis of petroleum oils or other bituminous tively extract the olefins into the acid phase. materials, is very diiiicult. In most cases, it is The acid extract containing olefins may then be impossible for all practical purposes, for example, subjected to conditions whereby the olefin is reto fractionally separate by distillation propylene covered and the acid returned to the extraction boiling at 48.2 C. from propane which boils at steph has en done heretofore by S a e 45.0 C. The separation of 4 carbon atom oledilution with water followed by steam stripping. fins and saturated hydrocarbons that are found It is W611 known that Sulfuric acid y a y in a cracked hydrocarbon mixture is still more poiymeriZe the olefins and vis Well known that, difiicult, as ey a e ore numerous and have the olefin-sulfuric acid reaction products the following boiling points: are heated without sufficient dilution, polymer- Bomng Points 0 C ization and secondary oxidation reactions occur, Isobutane such as the formation of tar or carbon, with lsobutnne decomposition of the sulfurlc ac1d to sulfur diox- 1 bute;1e ide and water. It is also known that under these conditions excess or uncombined sulfuric acid Butadlene 4"75 increases the formation of polymers. n'Buta'ne 3o disadvantage of extract dilution and steam strip- Z'butene' t g ping process is in the expense involved in hanz'butene C15 dling, reconcentrating and reducing the carbon The 5 carbon atom group of pentanes and pen- Co e of the 11013 dilute acidtenes is even more complex and the separation According to the present invention the secof these olefins from the associated saturated olefins y be recovered from Olefinhydrocarbons byphysical means has heretofore sulfuric acid solution without any substantial not been accomplished. loss due to polymerization and secondary oxida- There are many chemical processes in which it tion and without the dilution or heating of the is a decided advantage to use a substantially pure acid extract. The following description will be olefin, unmixed with inert diluents, such as sat- 4 related to normal butene separation but the prourated hydrocarbons. For example, in chemical cess described will also have application to the reactions with the gaseous olefins in which presrecovery of other secondary olefins, such as prosure is advantageous, the effective pressure is pylene, normal amylene, etc. normally the partial pressure of the reactive Sulfuric acid of about 70% to 90%, preferably olefin in the mixture. In such cases, substan- 83%, concentration is used to absorb the secontially pure olefins permit much lower pressures dary olefins. This may be done at an elevated to be employed and less gas to be compressed pressure in the liquid phase or at a lower presthan in the case of gas mixtures poor in reactive sure in the gas phase. olefin. Also, in many cases the olefin reaction When normal butenes react with H2804, the may be reversible, the proportion of the desired resulting extract has been found experimentally product being dependent upon the concentrato contain some butene as such, some as secontion of the reactive olefin in the mixture. In dary butyl alcohol, and some a mono-or dibutyl such cases, the dilution of the olefin with an sulfate. The extract is found to exert a considinert diluent, such as a saturated hydrocarbon, 5 erable vapor pressure of normal butylene, de-

may so affect the yield of the desired reaction product as to render the process unprofitable. In addition, in cases where butenes are to be subjected to pyrolysis, as in butadiene manu-. facture, dilution with paraflins causes (1) loss of the paraflin, which is a valuable material in Also, the

pending on the concentration of hydrocarbon and acid in the extract and on the temperature. Extracts of tertiary olefins, such as isobutylene, on the other hand, contain the olefin predominately as the tertiary alcohol only and exert a negligible olefin vapor pressure.

The separation and recovery of secondary ole fins from strong sulfuric extracts is accomplished by the counter-current action of a saturated hydrocarbon gas having at least 20 C. difference in the boiling point from the secondary olefins such as propane or by extracting the olefins from the strong olefin-sulfuric acid extract by the countercurrent action of an inert liquid solvent such as pentane or hexane. It has been found that an extract of 83% sulfuric acid containing 1 mol of olefin per mol of sulfuric acid at 30 C. is in equilibrium with a liquid naphtha containing 11% normal butene, while a similar extract containing 0.2 mol of olefin per mol of sulfuric acid is in equilibrium with a naptha containing 1.2% normal butene. fin vapor pressures of these two extracts are calculated to be 4.2 and 0.46 lb. per square inch, respectively. Therefore an 83% sulfuric acid extract containing 1 mol of the olefin per mol of th sulfuric acid can be stripped at 30 C. and at atmospheric pressure by passing it down a tower oountercurrently through a stream of gaseous propane and the gas leaving the top of the tower should contain approximately 25% normal butene and 75% propane. The butene is then recovered by fractionation and the propane recycled. The extract leaving the bottom of the tower contains only 0.2 mol of olefin per mol of sulfuric acid and is recycled directly to the olefin absorption equipment.

Alternately, a strong extract is treated countercurrently with liquid pentane and the liquid pentane leaving at the top of the extraction tower contains approximately 10% normal butene. The butene is recovered by fractionation and the pentane recycled to the extraction tower. When some alkyl sulfate or alcohol is carried over mechanically or otherwise by the pentane, it is readily separated as bottoms in the fractionating tower and is recycled to the extraction step along with the pentane. The lean acid extract from th extraction tower is recycled directly to the olefin absorption equipment.

The stripping or the extraction process requires a suitable length of time as the reversion of alcohol or alkyl sulfate to olefins is relatively slow. Since more time is generally allowed in extraction equipment than in stripping equipment, there are certain mechanical advantages in recovering the olefin by extraction. The extraction or stripping operation may be carried out at substantially atmospheric pressure and at temperatures below that at which substantial polymerization of the olefins in the acid extract will take place, probably from to 60 C.

The invention will be more clearly understood on reading the following description with reference to the accompanying drawing.

Numeral 1 designates the reactor into which is passed, at the upper part, sulfuric acid of about 83% concentration b means of pipe 2, and a mixture of hydrocarbons having four carbon atoms to the molecule and containing both butane and a secondary olefin is passed by means of pipe 3 into a lower part of the reactor. 'An agitator Therefore the corresponding ole- 4 is provided and cooling coil 5 to regulate the temperature. The mixture of sulfuric acid and the hydrocarbons is then passed through pipe 8 into a settler l where the unabsorbed hydrocarbons are removed by means of pipe 8 consisting mainly of butane. The sulfuric acid extract is removed from the settler 1 by means of pipe 9 and passed to the upper part of the liquid extraction tower ill. In the lower part of the liquid extraction tower Ill is passed a liquid parafiinic hydrocarbon solvent, such as pentane, by means of pipe ll, which passes in countercurrent flow to the sulfuric acid in the tower. The sulfuric acid, with most of the olefins removed, is then recycled by means of pipe l2 to the reactor l. The liquid pentane with the olefin solution is removed from the extraction tower by means of pipe l3 and passed to a fractionating tower M where the secondary olefins are distilled over to pipe 15 and the liquid paraffinic hydrocarbon solvent is removed by means of pipe I l and recycled to extraction tower ill. The fractionating tower i4 is provided with a condenser I? at the upper part and a heating coil [8 at the lower part.

I claim:

1. Improvements in the separation and concentration of a secondary olefin from a hydrocarbon mixture containing the said secondary olefin which comprises contacting a hydrocarbon mixture containing a secondary olefin with sulfuric acid of about 75%0% concentration, thereby forming a solution of the secondary olefin in said acid, extracting the secondary olefin from said acid solution by means of a saturated hydrocarbon compound, the said saturated hydrocarbon compound having at least 20 C. difference in boiling point from the secondary olefin, capable of dissolving the secondary olefin and separable from the said secondary olefin by fractionation.

2. Improvements in the separation and concentration of a secondary olefin from a hydrocarbon mixture containing the said secondary olefin which comprises contacting a hydrocarbon mixture containing a secondary olefin with sulfuric acid of about 75%-90% concentration, thereby forming a solution of the secondary olefin in said acid, extracting the secondary olefin from said acid solution by means of a saturated hydrocarbon compound in the liquid phase, the said saturated hydrocarbon compound having at least 20 (Indifference in boiling point from the secondary olefin, capable of dissolving the secondary olefin and separable from the said secondary olefin by fractionation.

3. Improvements in the separation and concentration of a secondary olefin from a hydrocarbon mixture containing the said secondary olefin which comprises contacting a hydrocarbon mixture containing a secondary olefin with sulfuric acid of about '75%90% concentration, thereby forming a solution of the secondary olefin in said acid, extracting the secondary olefin from said acid solution by means of a saturated hydrocarbon compound in the vapor phase, the said saturated hydrocarbon compound having at least 20 C. difference in boiling point from the secondary olefin, capable of dissolving the secondary olefin and separable from the said secondary olefin by fractionation.

HAROLD W. SCHEELINE.

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