Process and device for separating ethane and ethylene from a...

Details Process and device for separating ethane and ethylene from a... Process and device for separating ethane and ethylene from a...

2000-08-16

2002-01-22

Preisch, Nadine (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

With preliminary treatment of feed

C208S340000, C208S341000, C208S347000, C208S350000, C208S351000, C208S130000, C208S313000, C585S259000, C585S800000, C585S802000, C585S809000, C585S650000, C095S169000, C095S187000, C095S188000, C095S206000, C095S209000, C095S238000, C095S240000

Reexamination Certificate

active

06340429

ABSTRACT:

The invention relates to a process for separating ethylene and ethane from a hydrocarbon steam-cracking effluent that contains in particular ethane, ethylene and acetylenic compounds.
The production of ethylene and propene by steam-cracking of hydrocarbons uses processes that make it possible to separate the ethylene and the propene of lighter gases that are contained in the effluents of cracked gases. Crude ethylene and propene (C
2
/C
3
fractions) also contain undesirable acetylenic compounds that should be recovered. When these compounds are desired as co-products, they can be extracted by a solvent. Such processes are, however, very dangerous because of the instability of highly concentrated acetylenic compounds.
The prior art is illustrated by Patent Applications U.S. Pat. No. 3,755,488, EP-A-0 825 245 and WO-93 24428.
The acetylenic compounds conventionally are converted into ethylene and propene by hydrogenation. A process for separating ethylene from methane via at least one distillation column (demethanizer) whose top fraction is condensed at a very low temperature by the ethylene is known by Patent U.S. Pat. No. 4,900,347.
These condensation conditions require the use of stainless steel material and consume a lot of energy.
A process of another type (ALCET, registered trademark) that is less expensive was described by LAM, W. K., AICHE Spring National Meeting April 1986, New Orleans. It comprises, in a series, a distillation stage (deethanizer, in English, to draw off C3
+
hydrocarbons at the bottom of the column, or depropanizer, in English, to draw off C
4
+
hydrocarbons at the bottom of the column), a compression stage of the top gaseous fraction, a stage for hydrogenation of this gaseous fraction, a stage for separating a gaseous phase that is introduced into a solvent absorption column, and a liquid phase that is recycled as reflux. At the top, the absorption column delivers a light phase that contains hydrogen and methane that is separated by condensation with propane and/or propene, and at the bottom, the column delivers a solvent phase that contains the desired C
2
compounds. This solvent phase is then regenerated, the solvent is recycled in the absorption column, and the desired C
2
compounds are recovered as feedstock of a subsequent downstream treatment, of polymerization, for example.
In such an ALCET process, the stages of compression and heating as well as the stage for hydrogenation of the acetylenic compounds take place in the presence of the entire top gaseous fraction that contains in particular hydrogen, carbon monoxide and methane. This involves larger-size equipment and larger investments. In addition, the reaction heat that is involved in the hydrogenation reactor and the fact of operating in vapor phase with excess hydrogen ensures that the temperature of the reactor has a tendency to increase, which can impair the selectivity of the hydrogenation reaction of the acetylenic compounds, whereby the ethylene can be partly hydrogenated in turn. To eliminate this, the ALCET process is carried out in the presence of two hydrogenation reactors with intermediate cooling.
These hydrogenation reactions can be accompanied by the formation of polymers (green oil) that gradually foul and deactivate the catalyst. Because the reactions are carried out in gaseous phase, these compounds cannot be washed and eliminated at least in part.
Finally, the presence of CO and H
2
can result in the formation of methane and water that it is necessary to eliminate in the downstream condensation treatment.
One of the objects of the invention is to eliminate the drawbacks of the prior art, in particular to obtain a mixture that contains at least 85% by weight of ethylene and that can be used directly for the synthesis of polyethylene and plastics.
Another object is to carry out at least in part a hydrogenation in liquid phase, which is very selective and which essentially eliminates all of the triple-bond compounds and the diene compounds.
It was noted that by first carrying out a stage where a steam-cracking effluent is absorbed by a solvent and in particular the one that is obtained from a furnace, for example, a ceramic furnace that operates at a very high temperature, then a hydrogenation stage in mixed liquid phase and vapor phase of the effluent at the bottom of the absorber and finally stages for separating effluents that are produced and that comprise a stage for regenerating solvent that do not use cryogenic condensations, a final product of ethylene and ethane of excellent quality was obtained at a reduced cost.
More specifically, the invention relates to a process for separating a mixture that consists essentially of ethane and ethylene from a hydrocarbon steam-cracking effluent, whereby the effluent comprises hydrogen, methane, ethylene, ethane, acetylene, methylacetylene, propadiene, propene and hydrocarbons with at least 4 carbon atoms. The process is characterized in that:
Said feedstock (
1
) is absorbed in at least one absorption column (
7
) by a cooled solvent phase (
9
) under suitable absorption conditions, and a gaseous phase (
50
) that contains in particular hydrogen and methane at the top of the column and a partly liquid phase at the bottom of the column that contains the solvent that is enriched with ethylene, ethane, acetylene, methylacetylene, propadiene, propene and hydrocarbons with at least 4 carbon atoms are recovered;
the liquid phase is hydrogenated in at least one catalytic hydrogenation zone (
15
) in the presence of hydrogen and a hydrogenation catalyst under suitable hydrogenation conditions, and at least one liquid phase that is at least partly hydrogenated and that essentially does not contain acetylene is recovered.
The following stage sequence is carried out:
a) Said liquid phase that is at least in part hydrogenated is circulated in at least a first distillation column (
16
), and there is recovered: at the top of the column, a gaseous phase that is condensed to separate a non-condensed vapor phase and a liquid phase that contains hydrocarbons with two carbon atoms as reflux; by a lateral draw-off, said mixture (
17
) that consists essentially of ethane and ethylene; and at the bottom of the column, a hydrocarbon-enriched solvent phase (
19
) with at least 3 carbon atoms (C
3
+
)
b) The solvent phase is regenerated in at least a second distillation (regeneration) column (
22
), and a C
3
+
-enriched fraction is recovered, and at the bottom, a regenerated solvent phase is recovered;
The solvent phase is cooled, and it is at least partly recycled in the absorption column.
By carrying out the hydrogenation of a partly liquid phase that contains many fewer light compounds (H
2
, CH
4
) than the hydrogenation feedstock according to the ALCET process upstream from the solvent absorption stage, the temperature of the exothermic reaction that, moreover, is carried out toward 80° C. is monitored much better. In addition, a much more selective reaction is obtained, without loss of ethylene, in a reactor of smaller size and with a catalyst whose service life is increased because the polymeric compounds are washed by the liquid phase and eliminated by a downstream purge.
According to a characteristic of the process, the gaseous phase that contains in particular methane and hydrogen, obtained from the absorption column, is condensed at least in part to deliver a liquid phase (
52
) that is recycled at least in part as reflux in the column, and a vapor phase (
11
) that is high in methane and hydrogen.
This reflux can contain the cooled solvent that supplies said column when supply of solvent of the column is connected to the output of the gaseous phase that is to be condensed and not directly to the top of the absorption column. The solvent is generally cooled between −10° C. and −60° C. before it enters the column.
According to another characteristic of the process, the liquid phase at the bottom of the absorption column can be reheated by heat exchange with the regenerated solvent phase that is obtained

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