CA1088576A - Process for recovering dichloroethane - Google Patents

Abstract

PROCESS FOR RECOVERING DICHLOROETHANE

ABSTRACT OF THE DISCLOSURE
In a process for preparing dichloroethane by oxychlorination of ethylene, dichloroethane entrained in the plant off-gases is recovered by absorption in an alkyl aromatic solvent having at least 12 carbon atoms and a boiling point greater than 190°C.

Description

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BACRGROUND OF TIIE INV~NTION
U.S. Patent 3,689,582 discloses a proce~s for reco~ering d~chloroethane produced by the oxychlorination of ethylene in air by absorbing dichloroethane in an alkyl benzene ab~orbent having a boiling point within the range of 130 to 190C., preferably from 135 to 180C. The u~e of this solvent has not beQn completely satisfactory.
In fact, foul~ng and plugging of the apparAtu9 i8 frequent, probably becau~e of the formation of polymers and thi~ in-volve~ the use o~ pump~ equipped ~ith ~pecial ~eal~ and gasket~, as well as ~he u5e of filters suitable to retain solid particles. The frequency of the plant ~hut-downs reaches an ~xtremely troublesome level. In addition thero is often the o~cape to the atmosphere of foam~
entrained by the wa~te gases. The presence of foam~ in materialJ di~charged to thé atmosphere is dop~nd~nt in part on humlaity and representJ a drawback alJo from the viewpoint of the absorption efficiency at the high temper-atures at which the solvent is e~ployed. A further defi-¢ioncy iJ in ~olvent lo~ses that increase a~ a function of the fr~quency of ~topping the pro¢es~.
SUMMAR~ OF THE INVENTION
In a proce~s for preparing dichloroethane by oxy-chlorination of ethylene, dichloroethane entr~ined in the off-ga~e~ is recovered in a solvent of alkylbenzenes con-taining at lea~t 12 carbon atom~ and having boiling point~greater than 190C.
DETA$LED DESCRIPTION
In preparing dichloro~thane in an ethylene oxy-chlorinat~on plant, ethylene is converted into dichloro---2-- ~L

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ethane through catalytic reaction with hydrochloric acid and oxygen, or other oxygen-contalning gases, according to the equstion:
C2H4 + 2~Cl + 1/~ 2 > C2H4Cl2 Usually the oxygen i8 fed in excess in the form of air, and after separation of dichloroethane and reaction water, there is a large amount of inert qas still contalning con-siderable amounts of useful product dichloroethane. These residual amountJ of dichloroethane are recovered by absorption with ~uitablo solvents.
The~e solvont~ are alkylbenzenes containing at least 12 and up to 16 carbon atomss and having boiling points greater than 190C., for example 195 to 255C.
Particularly useful are those alkylbenzenes having boiling points of about 195 to 235~., more preferably, about 200 ~i to 225C. An oxample of u~eful solvent is dii~opropyl ; benz-ne or mixturos of diisopropyl benzenes hav~ng bo~ling polnt~ in the range of about 204 to 210C. The solvent may contain at le~st 60~ by weight of ortho-, meta- or para-diisopropyl benzene, or mixtures thereof. It w~s quite surprisinq in view of the clear teaching of U.S.
Patent 3,689,582 to find the diisopropyl benzenes 80 effec-tive.
~hese solvents are inert toward dichloroethane and are exceptionally ~table at operating tomperatures.
The solubility of dichloroethane vapors in these solvents is very hi~h at low temperatures, even for very low part~al pressures of dichloroethane. The separation of dichloro-ethane from the solution however is completely satisfactory when thi~ is done under vacuum at pressures below about 300 mm of Hg because of the very great differences between the vapor pre~sure of dichloroethane and the solvent. Since lower pressures are important, plant operations have been effective at 200 mm of Hg and while they may go aJ low as S 10 mm of Hg, a more useful range will be from about 50 mm ~-~
of Hg to le~s than 300 mm of ~g. The lower the pressure, the lower the temperature in the reboiler and one thus avoids problems related to operating the reboiling at higher temperatures.
When using the defined solvents, as the diiso-propyl benzene solvents, and desorbing at pres~ures below 300 mm of Hg, i.e., about 200 mm of Hg, diiJopropyl ben-zene has been used in the claimed process on a commercial ~cale for two years ~about 16,000 hours) with nono of the problem~ a~soclat~d with ~e of the solvents of the prior art. It i~ noted that in 3,689,582, the stability of that solvent ~a~ reported as 2000 hours.
~XAMPLE I
An ethyleno fod stream was oxychlorinated with hydrochloric acid and air, and most of the resulting di-chloroethane was condensed and separated, along with the reaction water, by cooling undor a pressure of 3 kg/cm2.
The re~idual ga~es, containing 3.75~ by weight of dichloro-. ~
ethane, were introduced into the bottom of an absorption column that operated at 2.5 kg/cm2. For each kg of in-` flowing ga~, 0.31 kg of an alkylbenzene~ mlxture having the following characteri~tics were fed to the top of t~e -column:
pecific heat at 150C. 0.577 kcal/kg/C
~ .
~ ~ 30 diisopropylbenzenes content 81.9% by weight .

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di~tillation range between 195 and 255C.
(ASTM D-850) density (15/4C.) 0.8685 (ASTM D-1298) visco~ity at 50C. 12. centistokes (ASTM D-445) flash point (Pensky Martens 71C. ~ASTM D-93) method) alkylbenzenes having.12 or 95.78 by weight more carbon atoms cracking starting temperature >450C.
The solution leaving the column bottom, rich in dichloro-sthane, was fed to a desorptlon or stripping column complete with reboiler operating at 200 mm of Hg; the recovered dichloroethane containing le~s than 100 ppm of alkyl-benzenes flowed ol~t from the top of this column., while the ~ .
hot alkylbenzenes (at 160C.) flowed out from the bottom;
these alkylbenzeneQ preheated the ~olution to be stripped and were then cooled down to 8C. before being fd again to the ab~orption column top, where the te~perature was kept at 20C. and whore the residual gases containing 15 ppm of dichloroethane were separated. After a 10,000-hour run the equipment exhibited in~ignificant soiling and no solid~ in ~uspension in the solvent were noticed. In practice the solvent losse~ did not exceed 0.7 kg per ton of dichloroethane produced.
E3aHPLE II
The test of Example I was repeated, but with the following variations: the solvent ~as cooled down to 2C.
before being introduced into the absorption column, and the temperature at the top of said absorption colu~n wa~
; ~ept at 14-C. The result was that the dichloroethane content of the gases ~ented to the atmosphere was lower ~ S 7 than 3 ppm and the solvent consumption decrea~ed to less than 0.5 kg per ton of dichloroethane produced. Also -under these conditions no foam formed.

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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Process for preparing dichloroethane by oxy-chlorination of ethylene, in which the dichloroethane entrained by waste gases is recovered by absorption in a solvent, characterized in that said solvent consists essentially of alkylbenzenes having at least 12 carbon atoms and boiling points greater than 190°C.

2. A process of Claim 1 wherein said solvent contains 12 to 16 carbon atoms and has a boiling point range of about 195°C. to 235°C.

3. Process according to Claim 2, characterized in that at least 60% by weight of said solvent consists of ortho, meta or para-diisopropylbenzene or of mixtures thereof.

4. A process of Claim 1 wherein the solvent containing dichloroethane flows to a desorption column operating at a pressure less than 300 mmHg to recover dichloroethane from the solvent.

5. A process of Claim 3 wherein the solvent containing dichloroethane flows to a desorption column operating at a pressure of about 50 to 200 mmHg to recover dichloroethane from the solvent.