CS218342B1 - A method for treating degasses resulting from the process of ethylene hydration to ethano - Google Patents

Abstract

The invention relates to a method for processing off-gases from the production of synthetic ethanol by direct hydration of ethylene into a raw material suitable for further petrochemical production, in particular the alkylation of benzene with styrene into ethylbenzene. Impurities contained in the raw off-gases from direct hydration, in particular oxygen compounds such as water, ethanol and diethyl ether, are separated from the off-gas by condensation after lowering its temperature at the expense of the heat of vaporization of light liquid hydrocarbons such as propane, propylene, butane or butenes, or mixtures thereof.

Description

The present invention relates to a process for treating off-gases from ethanol production by direct hydration of ethylene such that off-gases are suitable as feedstock for further petrochemical production.

Synthetic ethanol is usually produced by direct hydration of ethylene with steam at a pressure of 6-10 MPa, a temperature of 260-310 ° C. The hydration reaction is catalyzed by phosphoric acid, supported on a solid support. The synthesis product is an aqueous solution containing about 15% ethanol; only 3.5 to 5% of ethylene is converted in one pass through the reactor and is therefore circulated in the process unit. In order to prevent the accumulation of inert components in the circulating ethylene, part of it is discharged from the system in the form of a high pressure off-gas.

The aqueous ethanol solution is separated from the circulating ethylene after cooling to a temperature of about 40 ° C at said ethanol synthesis pressure of direct hydration, i.e. 6-10 MPa. For further processing of this aqueous solution, its pressure is reduced by throttling to 0.1-1.0 MPa, the temperature at this throttle remaining roughly constant. When the pressure is reduced, dissolved gas in the form of a low pressure off-gas is released from the aqueous ethanol solution.

Both high-pressure and low-pressure off-gas contain, in addition to ethylene, significant concentrations of ethanol, water and other components, which are formed as by-products of the synthesis, in particular diethyl ether and butene. The presence of these components, in particular of the oxygen compounds, i.e. water, ethanol and diethyl ether, prevents the direct use of off-gases as a petrochemical feedstock and even prevents them from being added to the intermediate products of the polymerization purity ethylene production line.

It has now been found that off-gases from the production of synthetic ethanol by direct hydration of ethylene can be treated by lowering their temperature at the expense of the evaporative heat of a liquid hydrocarbon containing three or four carbons per molecule at 0 ° C to -45 ° C. a mixture of impurities, i.e. water, ethanol, diethyl ether and other compounds with a higher boiling point than ethylene, which is optionally further processed to ethanol and diethyl ether. The de-liquefied waste gas is fed as a feedstock for petrochemical production, such as the production of ethylbenzene by alkylation of benzene, or is introduced for hydrocarbon separation or added to the fuel gas.

In a plant producing ethanol by direct hydration of ethylene, other hydrocarbons having 3 or 4 carbons per molecule are usually treated. These hydrocarbons such as propane, propylene, butanes and butenes are mostly transported and stored in a liquid state. Advantageously, therefore, the distribution of any of these liquid hydrocarbons may be utilized, removed in the required amount at a location close to the synthetic ethanol production unit, and its evaporative heat used to cool off the off-gas from the ethanol synthesis. The gaseous hydrocarbon which is used for cooling is then returned to the process of production, separation or processing.

In some cases, the use of off-gases treated by the process of the invention may be expedient to further reduce the content of oxygen compounds, especially diethyl ether. It has been found that this reduction in the oxygen content of the compounds can be achieved by washing at least one of the off-gases prior to reducing the temperature with a mixture of ethanol and water containing at least 50% by weight. ethanol.

In the sub-case of using a low pressure off-gas from ethanol synthesis for the production of ethylbenzene, which is described in more detail in the following example, propylene is used to cool off the off-gas treated by the process of the invention. The amount of this propylene is a fraction of the amount of ethylene to be processed. Compared to the alternative option where the gas was returned to the olefin production line, the amount of propylene returned instead of contaminated ethylene is reduced, its processing in the line is at a higher temperature level than ethylene, and above all does not yield oxygen compounds that could compromise the olefin quality intended for the production of polyolefins.

Example:

In the example, the treatment of low pressure off-gas from ethanol synthesis is reported.

In the production of 1000 tons of synthetic ethanol (calculated on 100% ethanol), 25 tons of low-pressure off-gas are separated off, this off-gas having an ethylene composition of 92.73% by weight.

ethanol 1.38% wt.

diethyl ether 2.27 wt.

butene-1 2.27 wt.

water 1.35 wt.

The low-pressure off-gas is cooled to -30 ° C at a pressure of 0.5 MPa by evaporating propylene in a tube heat exchanger. The propylene consumption is 15.43 tons, the propylene vapors are introduced into the pyrolysis gas separation line.

The treated off gas has the following composition after liquid separation:

ethylene 97.28 wt.% ethanol 100 ppm wt., i.e. 0.010 wt.

diethyl ether 5910 ppm by weight, i.e. 0.591 wt.

butene-1 2.12 wt.

water 30 ppm by weight, i.e. 0.003 wt.

This offgas is used as a completely suitable raw material, for example, for the production of ethylbenzene by alkylation of benzene on a Friedel-Crafts catalyst, and 84 t of ethylbenzene is produced.

The separated liquid, which is divided into two layers - water and ether - has the following overall composition:

% ethylene 3.96 wt.

ethanol diethyl ether butene - 1 water

28.09% wt. 35.10 wt. 5.20% wt. 27.65 ° / p wt.

The molar ratio of the separated liquid to the low pressure off-gas before treatment is 0.04.

This liquid mixture of substances is subjected to further processing to obtain ethanol and diethyl ether.

Claims (2)

Process for the treatment of off-gases produced in the process of direct hydration of ethylene to ethanol, characterized in that the off-gas temperature is reduced at the expense of the evaporation heat of a liquid hydrocarbon, preferably containing three and / or four carbons per molecule, at 0 ° C to -45 ° C. separates the condensed mixture of impurities, i.e. water, ethanol, diethyl ether and compounds with a higher boiling point than ethylene, which is preferably processed into ethanol and / or diethyl ether, and the degassed waste gas is fed as feedstock for petrochemical production, preferably ethylbenzene alkylation of benzene, and / or is fed to the hydrocarbon separation or added to the fuel gas. 2. A process according to claim 1, wherein at least one of the off-gases is washed with a mixture of ethanol and water containing at least 50% by weight ethanol before the temperature is reduced.