Process for removing light components from an ethylene stream

Abstract

A process for removing light components from an ethylene stream may include providing a dried ethylene stream containing ethylene, ethane, CO, CO.sub.2, H.sub.2, CH.sub.4, and C.sub.3+ hydrocarbons. The process may include sending the dried ethylene stream to a stripper to produce an overhead stream containing ethylene, CO, H.sub.2 and CH.sub.4, and a bottom stream containing ethylene, ethane, CO.sub.2, and C.sub.3+ hydrocarbons. The gaseous phase on top of the stripper may be condensed in a heat exchanger cooled by a refrigerant stream to get a first gaseous phase and a first liquid phase. The first gaseous phase may be condensed in a heat exchanger cooled by liquid ethane or liquid ethylene to get a second gaseous phase containing ethylene CO, H.sub.2 and CH.sub.4 and a second liquid phase. The first and second liquid phases may be the reflux of the stripper.

Claims

1. A process for removing light components from an ethylene stream comprising: a) providing a dried ethylene stream (A) comprising ethylene, ethane, CO, CO.sub.2, H.sub.2, CH.sub.4, C.sub.3+ hydrocarbons and optionally oxygenates; b) separating said dried ethylene stream (A) in a separation means that is a demethanizer or a stripper to form: an overhead gaseous stream (B) comprising ethylene, CO, H.sub.2 and CH.sub.4; and a bottom stream (C) comprising ethylene, ethane, CO.sub.2, C.sub.3+ hydrocarbons and optionally oxygenates; c) cooling the overhead gaseous stream (B) to a temperature ranging from −10° C. to −45° C. to form a first gaseous stream (D) and a first liquid stream (E), wherein the cooling is performed with a refrigerant stream that comprises a mixture of liquid and optionally C.sub.3 to C.sub.4 gaseous hydrocarbons in a first condenser; d) further cooling the first gaseous stream (D) to a temperature ranging from −10° C. to −45° C. lower than the temperature of step c) to get a second gaseous stream (F) comprising ethylene CO, H.sub.2 and CH.sub.4 and a second liquid stream (G), wherein the cooling is performed with liquid ethane or liquid ethylene in a second condenser, wherein the liquid ethane or the liquid ethylene has a pressure ranging from 30 kPag to 500 kPag; e) sending the first and second liquid streams (E) and (G) to the separation means as a reflux and; either i) f) sending said bottom stream of step b) to a deethanizer to produce: a bottom stream comprising ethane, C.sub.3+ hydrocarbons and optionally oxygenates; and an overhead stream consisting of ethylene and CO.sub.2, wherein a portion of a liquid ethylene in a reflux drum of the deethanizer is expanded and used for the cooling in (d); and d) sending said overhead of step f) to a fixed bed CO.sub.2 adsorption zone to recover an ethylene stream free of CO.sub.2; or (ii) f1) sending said bottom stream of step b) to a fixed bed CO.sub.2 adsorption zone to recover a stream free of CO.sub.2, then sending said stream free of CO.sub.2 to a deethanizer to produce: a bottom stream comprising ethane, C.sub.3+ hydrocarbons and optionally oxygenates; and an overhead stream consisting of ethylene free of CO.sub.2.

2. The process according to claim 1, wherein the refrigerant stream comprises liquid and optionally gaseous propane and/or liquid and optionally gaseous propylene, wherein the liquid propane or the liquid propylene has a pressure ranging from 30 kPag to 200 kPag.

3. The process according to claim 1, wherein during the cooling of the first gaseous stream (D) the liquid ethylene in step (d) returns to gas phase and is recycled.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a flow diagram of a process in accordance with one or more embodiments.

(2) FIG. 2 depicts a flow diagram of a process in accordance with one or more embodiments.

(3) FIG. 3 depicts a flow diagram of a process in accordance with one or more embodiments.

(4) FIG. 4 depicts a flow diagram of a process in accordance with one or more embodiments.

DETAILED DESCRIPTION OF THE INVENTION

(5) As regards the oxygenated contaminants also referred to as oxygenates, one can cite, methanol, ethanol, C3 alcohols; ethers such as diethylether and methyl ethyl ether and dimethylether; carboxylic acids such as acetic acid; aldehydes such as acetaldehyde; ketones such as acetone; and esters such as methyl esters; and vinyl derivatives. Particularly problematic oxygenate contaminants in an alcohol dehydration are aldehydes.

(6) As regards the ethylene stream (A) of step a), it can be originating from the dehydration of ethanol. Said dehydration can be made in one or more ethanol dehydration reactors. As regards alcohol dehydration, such process is described in WO-2009-098262, WO-2009-098267, WO-2009-098268 and WO-2009-098269 the content of which is incorporated in the present application. The present invention is very efficient for the purification of ethylene produced by dehydration of ethanol.

(7) The outlet of said dehydration reactor comprises essentially ethylene and steam as well as minor amounts of oxygenates, ethane, CO, CO2, H2, CH4 and C3+ hydrocarbons. “Minor amounts” means the weight ratio of ethane+CO+CO2+H2+CH4+C3+ hydrocarbons to ethylene is less than 20/80 and most of time less than 10/90.

(8) Said outlet of dehydration reactor is initially cooled, typically in a quench tower employing water as the quench medium. In the quench tower, most of the water contained in the outlet of dehydration reactor is condensed and is removed from the bottom of the tower as a liquid water bottom stream. A part of said water bottom stream is cooled in a heat exchanger and recycled as quenching medium to the top of the quench column. The part of the water bottom stream which is not recycled as quenching medium may contain a part of the oxygenates and mostly unconverted ethanol if any. Said stream can be treated in a stripping column to recover a pure water stream. Ethylene, oxygenates, ethane, CO, CO2, H2, CH4 and C3+ hydrocarbons are removed from the top of the quench tower at a pressure typically such as 1 to 16 bars absolute (0.1 MPaa to 1.6 MPaa) and are referred to as the contaminated ethylene stream. Advantageously said contaminated ethylene stream is successively compressed and cooled in one or more steps to remove the major part of water, further fed to a fixed bed drying zone and finally to the process of the invention.

(9) In the previous compression steps the recovered water contains a part of the oxygenated contaminants and hydrocarbons dissolved. The contaminated hydrocarbon stream can also be cooled before the first compression step and water recovered. In an embodiment the water recovered upon each cooling further to a compression step and upon cooling, if any, before the first compression step is sent to a stripping column to produce an overhead stream comprising essentially oxygenated contaminants and hydrocarbons and an essentially pure water bottoms stream. Optionally the overhead stream is burned to destroy the oxygenated contaminants and recover heat.

(10) After the compression steps the contaminated ethylene stream is further fed to a fixed bed drying zone and finally to the process of the present invention. The fixed bed drying zone is known in itself.

(11) As regards the stripper, the purpose of said stripper is to recover an overhead comprising essentially H2, CH4 and CO. It is advantageously a distillation column.

(12) As regards the operating conditions, the man skilled in the art of hydrocarbon distillation can select the operating conditions in view of the proportion of light components in the ethylene feed to the stripper and of the thermodynamics properties of the cooling fluids. The basis of the present process is to use mainly propane or propylene to condense the top of the stripper and to “finish” the condensation by cooling with ethane or ethylene to reduce the amount of ethylene which escapes with the light components in the stripper overhead. Advantageously the part of the cooling energy in the second condenser, the one supplied by ethane or ethylene, on top of the stripper is up to 10% of the total cooling energy required on top of the stripper.

(13) The stripper has to be at a pressure high enough to operate at temperatures which are not too low to use mainly liquid propane or liquid propylene as cooling fluid on top. A stripper to recover an overhead comprising H2, CH4 and CO and essentially liquid ethylene at the bottoms operating at 40 barg (4 MPag) has an overhead temperature of around 0 to −10° C. and a bottom temperature of around 0° C. The same stripper operating at 21 barg (2.1 MPag) has an overhead temperature of −30° C. and a bottom temperature of around −24° C. These temperatures and pressures are a function of the proportion of H2, CH4 and CO in the ethylene stream (A) and mainly of the proportion of H2.

(14) As regards to the temperature of stream (D) i.e. the temperature on the top of the stripper and after the first condenser, it ranges from −5° C., −10° C. or −15° C. to −45° C., −40° C. or −35° C. As regard to the temperature of the refrigerant stream of step c) it ranges from −10° C.; −15° C. or −20° C. to −50° C., −45° C. or −40° C. As regards to the temperature liquid ethane or ethylene used in step d), it ranges from −60° C.; −65° C. or −70° C. to −80° C., −85° C. or −90° C.

(15) As regards the first embodiment and the fixed bed CO2 adsorption zone, it can be any component capable to selectively remove CO2. By way of example it is an available commercial fixed bed adsorption (PSA for pressure swing adsorption or TSA for temperature swing adsorption) using molecular sieves or basic oxides, supported basic oxides, high surface area carbons, organo-metallic framework components (MOF's) or mixture thereof. The molecular sieves are preferably low silica zeolites, having 8 (among which zeolite A) or 12 membered (among which zeolite X) rings and exchanged with alkali, alkaline earth or lanthanide cations. Other molecular sieves are crystalline titanosilicates (ETS family materials). Supported basic oxides are preferably, alkali, alkaline earth or lanthanide oxides supported on high surface area carbons, alumina, silica, zirconia or titania, clays. The removal of CO2 can be carried out with a liquid stream or with a gaseous ethylene stream depending on the pressure and temperature. A stream essentially free of CO2 is recovered. As only trace amounts of CO2 have to be removed from the ethylene, the preferred process cycle is of the thermal swing adsorption (TSA) type. Adsorption of CO2 can be performed on two or more fixed bed adsorbent. Said fixed bed adsorbent, once saturated with CO2, can be regenerated, while the main stream is treated on the other adsorption bed or any combination. During regeneration the desorption produces a stream which can be treated anywhere. In a TSA process cycle, the regeneration is done while sweeping the saturated adsorbent with an inert gas by increasing the temperature until desorption of the CO2 occurs. Eventually the saturated adsorbent can be replaced by new adsorbent and the saturated adsorbent either be disposed of or regenerated ex-situ for further use. “Essentially” has to be interpreted in the light of the further use of ethylene. Should ethylene is to be polymerized or oligomerized CO2 has to be 1 ppm vol or less and preferably 0.5 ppm vol or less.

(16) In an embodiment the pressure of the C2 splitter also referred to as a deethanizer is selected to have a temperature of the C2 splitter/deethanizer bottoms such as there is no oligomerization or polymerization of the oxygenates. By way of example said temperature should not exceed 150° C. and advantageously not exceed 100° C. This temperature is function of the pressure and of the proportion of oxygenates in the mixture of oxygenates+ethane+C3+ hydrocarbons. The higher the proportion of oxygenates the higher the temperature. The higher the pressure the higher the temperature is. The C2 splitter/deethanizer is advantageously a distillation column.

(17) A process according to the first embodiment is described on FIG. 2. The contaminated ethylene stream (A) comprising essentially ethylene, ethane, CO, CO2, H2, CH4, C3+ hydrocarbons and optionally oxygenates has been dried and sent to the stripper (also referred to as a demethanizer) to produce an overhead stream comprising essentially C2H4, CO, H2 and CH4, a bottom stream comprising essentially ethylene, ethane, CO2, C3+ hydrocarbons and optionally oxygenates,
said bottom stream of the stripper is sent to the deethanizer to produce a bottom stream comprising essentially ethane, C3+ hydrocarbons and optionally oxygenates, an overhead stream consisting essentially of ethylene and CO2,
said overhead of deethanizer is sent to a fixed bed CO2 adsorption zone to recover an ethylene stream essentially free of CO2. A part of liquid ethylene in the reflux drum (also known as decanter) of the deethanizer is expanded and sent as a cooling fluid to condense the first gaseous phase on top of the stripper.

(18) In an embodiment the stripper (demethanizer) and the C2 splitter/deethanizer are operating at the same pressure except the pressure drop between the demethanizer and the C2 splitter/deethanizer for transfer of fluids. Advantageously the pressure is ranging from 15 to 45 barg (1.5 MPag to 4.5 MPag).

(19) In a specific example the pressure of the stripper ranges from 15 to 35 barg (1.5 MPag to 3.5 MPag) and the pressure of the deethanizer and the CO2 adsorbers is about 1 or 2 barg (0.1 MPag to 0.2 MPag) less corresponding to the pressure drop due to pipes and equipment. In this range of pressure the temperature on top of stripper and after the first condenser ranges from −20 to −30° C., the temperature on bottom of stripper ranges from −15 to −25° C., the temperature on top of deethanizer and after the condenser ranges from −30 to −20° C. and the temperature on bottom of deethanizer ranges from 75 to 85° C.

(20) In a specific example the pressure of the stripper ranges from 20 to 25 barg (2.0 MPag to 2.5 MPa) and the pressure of the deethanizer and the CO2 adsorbers is about 1 or 2 barg (0.1 MPag to 0.2 MPag) less corresponding to the pressure drop due to pipes and equipment. In this range of pressure the temperature on top of stripper and after the condenser ranges from −22 to −26° C., the temperature on bottom of stripper ranges from −20 to −24° C., the temperature on top of deethanizer and after the condenser ranges from −27 to −22° C. and the temperature on bottom of deethanizer ranges from 78 to 82° C.

(21) In another specific example the pressure of the stripper ranges from 30 to 45 barg (3.0 MPag to 4.5 MPag) and the pressure of the deethanizer and the CO2 adsorbers is about 5 to 25 barg (0.5 MPag to 2.5 MPag) less. Advantageously the pressure of the deethanizer ranges from 15 to 30 barg (1.5 MPag to 3.0 MPag). In this range of pressure the top of stripper is condensed at a temperature ranging from −20 to −45° C., the temperature on bottom of stripper ranges from −5 to 5° C., the temperature on top of deethanizer ranges from −25 to −35° C., is condensed at a temperature in the range −25 to −35° C. and the temperature on bottom of deethanizer ranges from 75 to 85° C.

(22) Preferably the pressure of the stripper ranges from 25 to 35 barg (2.5 MPag to 3.5 MPag) and the pressure of the deethanizer and the CO2 adsorbers ranges from 20 to 25 barg (2.0 MPag to 2.5 MPag). In this range of pressure the top of stripper is condensed at a temperature ranging from −10 to −35° C., the temperature on bottom of stripper ranges from −5 to −25° C., the temperature on top of deethanizer ranges from −28 to −32° C., is condensed at a temperature in the range −28 to −32° C. and the temperature on bottom of deethanizer ranges from 50 to 80° C.

EXAMPLES

Example 1, According to the Invention

(23) The process according to FIG. 2-3 is operated. The results are on the following table 1:

(24) TABLE-US-00001 TABLE 1 stream number Flowrate kg/h 1 2 3 4 5 6 7 H2 12 12 12 0 0 0 0 CO 2 2 2 0 0 0 0 METHANE 1 1 1 0 0 0 0 ETHYLENE 25119 25954 73 25881 24987 59 835 ETHANE 15 15 15 14 0 1 CO2 8 8 8 0 C3+ 767 720 720 720 H2O 197 TOTAL 26121 26712 88 26624 25001 779 836

(25) Stream 1 is the outlet of the quench following the ethanol dehydration, stream 7 is recycled in the compression zone located between the quench and the stripper as shown on FIG. 3.

(26) 25119 kg C2H4 are produced, 73 kg are lost in stream 3 and 59 kg are lost in stream 6 which means about 0.5% are lost.

Example 2, Comparative

(27) The process according to FIG. 4 is operated. The results are on the following table 2.

(28) TABLE-US-00002 TABLE 2 stream No on FIG. 4 1 2 3 Stripper Stripper Stripper feed bottoms purge Temperature ° C. 15 −20 −24 Pressure bar g 22 22 22 H2 kg/h 0.1 0.1 CO kg/h 1 1 CO2 kg/h 1 1 ethane kg/h 5 5 ethylene kg/h 25091 25013 78 acetaldehydes kg/h 18 18 C3+ kg/h 325 325 Total kg/h 25441.1 25362 79.1 stream No on FIG. 4 5 4 Deethanizer 6 Deethanizer vapor Ethylene bottoms distillate product Temperature ° C. 80 −24 20 Pressure bar g 21 21 20 H2 kg/h CO kg/h CO2 kg/h 1 ethane kg/h 5 5 ethylene kg/h 18 24995 24995 acetaldehydes kg/h 18 C3+ kg/h 325 Total kg/h 361 25001 25000

(29) By comparison with ex 1 there are much less light components in the stripper feed, as a consequence a condenser cooling at −24° C. is enough. To get −24° C. on the process side the cooling fluid can be liquid propane at 0.5 barg. The C2H4 loss at the stripper overhead is 78 kg/h. In ex 1 to have a C2H4 loss of 73 kg/h at the stripper overhead a second condenser fed with liquid ethylene at −65° C. is required.

(30) 25091 kg C2H4 are produced, 78 kg are lost in stream 3 and 18 kg are lost in stream 4 which means about 0.5% are lost.