WO2014064172A2

WO2014064172A2 - Process for recovery light molecules from olefinic feedstream

Info

Publication number: WO2014064172A2
Application number: PCT/EP2013/072212
Authority: WO
Inventors: Laurent LEMINEUR; Dominique Pascal; Wolfgang Garcia; Sylvain GOURVIL
Original assignee: Total Research and Technology Feluy SA
Current assignee: TotalEnergies Onetech Belgium SA
Priority date: 2012-10-24
Filing date: 2013-10-23
Publication date: 2014-05-01
Anticipated expiration: 2015-04-24
Also published as: WO2014064172A3

Abstract

The present invention relates to a process for purifying methanol to olefin effluents. It consists in the purification of the valuable olefins from the fuel gas (i.e. methane and hydrogen) with the help of separation steps, distillation steps and a final washing in an absorption column of only a small part of the extracted fuel gas.

Description

PROCESS FOR RECOVERY LIGHT MOLECULES FROM OLEFINIC

FEEDSTREAM

[Field of the invention]

The present invention relates to the purification of a stream containing hydrocarbons with at least two atom carbons (C2+), said hydrocarbon being saturated or unsaturated, from light molecules such as methane and hydrogen.

Unsaturated hydrocarbon, i.e. olefins, are traditionally produced from petroleum feedstocks by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s), such as ethylene and/or propylene, from a variety of hydrocarbon feedstocks. The feedstocks generally used include naphtha and ethane. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.

The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. The MTO (methanol- to-olefins) process produces light olefins such as ethylene and propylene as well as light hydrocarbons such as methane and ethane and light molecules such as hydrogen. Said MTO process is the conversion of methanol or dimethylether by contact with a molecular sieve. The interest in the methanol to olefins (MTO) process is based on the fact that methanol can be obtained from various carbonated feedstocks such as coal or natural gas by the production of synthesis gas which is then processed to produce methanol.

The effluent produced by a MTO process is a complex mixture comprising the desired light olefins, light hydrocarbons, oxygenates, heavier hydrocarbons and large amounts of water. The separation and purification of this mixture to recover the light olefins and other valuable by-products is critical to the overall efficiency and cost effectiveness of the process. In particular, it is important that the purification scheme produces products that are substantially free of impurities, which could adversely affect downstream processing which use those products. Purification of the stream produced by the MTO process hereinafter called olefinic stream is a rather complex, expensive and energy intensive process. To ensure the overall economical viability of the process, the use of an optimized separation process is required.

Light molecules (hydrogen and methane) are present in streams produced by a steam cracker and a MTO process at different concentration level. In the case of a steam cracker, the content of methane and hydrogen reaching the separation section is in the range of 35 to 50 % mol whereas in the case of MTO, the content of methane and hydrogen reaching the purification section is a range of 2 to 15 % mol based upon the total stream exiting the MTO unit. Consequently purification section traditionally used downstream a steam cracker cannot be directly transposed for the stream produced by the MTO process. In the case of a steam cracker, for high content of hydrogen and methane, the removal of light molecules is generally performed via cryogenic distillation or methane distillation (demethanation). The known purification processes require to lower temperature at cryogenic levels (i.e. below -100°C and generally below -1 10°C) which requires the use of an ethylene refrigeration cycle. Such cycle is a highly energy consuming process and requires substantive investment. Thermal integration of the purification installation appears as a crucial aspect of the economical liability of the process. The present invention relates to a process and an installation allowing separation of light components such as methane and hydrogen in a stream containing olefins and saturated hydrocarbons issued from a MTO plant or a MTO- OCP combined plant. OCP stands for Olefin Cracking Process. [Background of the invention]

US 7361799 B2 describes a process to separate water and oxygenates from a MTO effluent stream. The vapour product stream comprises C2 to C4 olefins, C5+ hydrocarbons, at least one oxygenate and water. In the process, the vapour product stream is cooled to remove water there from and produce a first vapour effluent stream. The first vapour effluent stream is then cooled and compressed to produce a condensed liquid effluent stream comprising C5+ hydrocarbons and at least one oxygenate, and a residual vapour effluent stream comprising C2 to C4 olefins. At least part of the condensed liquid effluent stream is contacted with a liquid water-containing stream in a liquid-liquid contacting device to at least partly separate said condensed liquid effluent stream, or portion thereof, into an aqueous phase rich in said at least one oxygenate and an organic phase rich in said C5+ hydrocarbons.

US 2005/0283038 describes a process for producing an olefins stream from a first vapour effluent stream from MTO reaction section. The vapour effluent stream comprises C2 and C3 olefins, C4 hydrocarbons, and C2 to C6 carbonyl compounds. In the process, the temperature and pressure of the first vapour effluent stream are adjusted to produce a second vapour effluent stream having a pressure ranging from about 100 psig to about 350 psig (790 to 2514 kPa) and a temperature ranging from about 70°F to about 120°F (21 to 49°O), said second vapour effluent stream containing about 50 wt. % or more C4 hydrocarbons based upon the total weight of C4 hydrocarbons in the first vapour effluent stream. The second vapour effluent stream is then washed with a liquid alcohol-containing stream to produce a third vapour effluent stream. The third vapour effluent stream is then washed with liquid water to provide a fourth vapour effluent stream comprising the C2 and C3 olefins and about 1.0 wt. % or less C2 to C6 carbonyl compounds.

These documents do not make mention of the possibility to separate methane and hydrogen from the hydrocarbon stream produced by the MTO unit. The prior art documents cited above are strongly focused on oxygenates purification. However MTO effluents also contain significant amount of light molecules such as hydrogen and methane that required to be separated. Separation of those light molecules can be performed in a demethanizer. Typical purification scheme including demethanizer of steam cracker effluent can for instance be found in the Kirk-Othmer encyclopaedia of Chemical Technology 5^th edition Vol. 10 p 611 -613. In this usual design, effluent purification is performed in a demethanizer.

US 2009/01 12037 relates to an improved method for recovering a purified ethylene product and optionally a purified hydrogen product from the effluent of an autothermal cracking reactor. The process consists of cracked gas chilling, rough separation of a hydrogen-rich stream, demethanization, separation of ethylene from the demethanizer bottoms product, and final purification of the ethylene product. Hydrocarbons heavier than ethylene, including ethane, propylene, and propane are recycled to the ATC reactor. This document makes no mention of the MTO process. It also does not make mention of the operating temperatures of the stream during the separation of methane and hydrogen from the olefinic stream however from other prior art document, it is known to operate at temperatures below - 90°C.

For example FR 2957931 relates to the treatment of a stream originating from a steam cracker (and not of the treatment of a stream originating from a MTO process), involving a demethanizer to remove methane and hydrogen. The process described involves different separation and washing steps of the olefinic stream before entering the demethanizer. It can be noted that the separation and washing steps are conducted at a temperature below -90°C. For example, it describes a stream being introduced in an ethylene adsorption column, said stream having previously been cooled to a temperature between -1 10°C to -120°C.

Similarly US 2006/0004242 relates to an olefin recovery process and plant that heat pumps the front-end distillation tower at a relatively low pressure, with good ethylene recovery and energy consumption. It mentions the use of temperature ranging from -95°C to -130°C to separate tail gas (i.e. to separate methane and hydrogen from the hydrocarbon stream produced by the MTO). Operating at these ranges of temperatures implies the use of an ethylene refrigeration cycle. Moreover, this document does not relate to the treatment of MTO effluents.

A process of purification of a stream containing methane and hydrogen and originating from a MTO process is described in DE 3220998. After separation of a C3 fraction, the incoming stream produced by the MTO is subjected to low temperature gas decomposition using a C1/C2 separating column and an ethylene- ethane separating column. The head of the C1/C2 separating column is operated at temperatures below -95°C and a part of the methane removed from the head of this column is fed back into the incoming stream produced by the MTO.

The traditional methods for separating ethylene from an olefin containing stream uses cryogenic flash stages and distillation (i.e. operating temperature below -90°C). However, working at these temperatures pose safety problems and require a high consumption of energy.

Indeed to reach these temperatures, several of these processes use an ethylene refrigeration cycle. The use of ethylene refrigeration cycle and the use of cryogenic temperature associated can result in unstable and potentially dangerous operating conditions. Olefin-containing streams produced by a MTO process may inevitably contain trace amounts of nitrogen oxides, including NO and NO2. Nitrogen oxides may originate from the regeneration section of the MTO where the carbon present on the catalyst is burned with air (i.e. in the presence of 02 and N2) at elevated temperature therefore forming NO and NO2. Typically, nitrogen oxides are inert; however, under cryogenic conditions, these compounds may further react to form N2O3, which is highly reactive. For example, even trace amounts, N2O3 may combine and react with poly-unsaturated olefins, such as butadiene present in an olefin-containing stream, to form highly unstable gum compounds. Such compounds are a major safety and operability concern, as they may cause runaway reactions and even explosions. A solution is given by US 2010/0105973 which replace said traditional methods involving a succession of separation/ distillation steps at cryogenic temperature by distillation using hydrocarbons absorbents. This document describes the separation of methane from an ethylene stream with an extractive distillation. The solvent used comprises hydrocarbons C2 to C4. The inlet stream can contain nitrogen oxides, the extractive distillation is run under conditions avoiding the formation of N203 from nitrogen oxides. Formation of N203 is avoided when temperature are above -100°C.

However, the implementation of this alternative method implies the use of large quantities of solvent and the installation of significantly big absorption columns in order to treat the incoming stream to reach the required level of purity. The treatment of such large quantities of solvent induces also a decrease of the overall efficiency of the process. Consequently this alternative method is expensive to implement and not satisfying in term of efficiency.

The present invention aims at providing a process and an installation for purifying a MTO stream that addresses the above-discussed drawbacks of the prior art.

[Brief summary of the invention] In a first aspect, the present invention relates to a process for purifying a gaseous stream (A), the gaseous stream (A) comprising:

- hydrocarbons with at least two carbon atoms,

- methane and hydrogen,

- contaminants comprising oxygenated contaminants and C02 and optionally at least one component of the following list: H20, CO, H2S ; the process comprising the following steps:

a) separating the gaseous stream (A) into a first gaseous stream (B) and a first liquid stream (C) ; b) separating the first gaseous stream (B) into a second gaseous stream (D) and a second liquid stream (E) ;

c) separating the second gaseous stream (D) to get third gaseous stream (F) comprising methane and hydrogen, and a third liquid stream (G)

d) conducting a distillation on the second liquid stream (E) and/or the third liquid stream (G) to recover a fourth gaseous stream (H) and a fourth liquid stream (I) comprising hydrocarbons with at least two carbon atoms the process being remarkable in that stream (A) is produced by a methanol to olefin process, in that the operating temperature of the second gaseous stream (D) is greater than -90°C and in that the separation of the second gaseous stream (D) in step c) is conducted by washing said second gaseous stream (D) by a solvent.

As it can be seen by, the invention combines a succession of separation/ distillation steps with the washing of a fraction of the stream with a solvent. In doing so, the invention allows to implement a separation/distillation process at an operating temperature greater than -90°C, at least for the distillation/separations steps, while maintaining a high degree of purity on the final products. At the same time as the inventive method implements a washing step of the stream by solvent on a fraction only of the olefin stream and not on the totality of the stream. Indeed it has been found that it is possible to separate a large fraction of ethylene by distillation steps conducted at a temperature greater that -90°C and that only a small fraction of the stream is to be submitted to a further treatment with solvent in order to achieve the required purity on the final products. According to the invention the ratio of the flow rate of the second gaseous stream (D) to the gaseous stream (A) is at most 1/5, preferably at most 1/6. Thus the invention allows working at a temperature greater than -90°C without the need of expensive installations and use of large quantities of solvent.

The inventive process is dedicated to the treatment of MTO stream that contains less methane and hydrogen than the stream originated from steam cracker. Thus according to an preferred embodiment of the invention the stream (A) is produced by a methanol to olefin process, or by a methanol to olefin process with at least part of the effluent of the methanol to olefin process being treated in an olefin cracking process. Therefore, advantageously, the stream (A) comprises up to 20 %mol of methane and hydrogen relative to the total molar content of stream (A) preferably up to 10%mol, and/or the stream (B) comprises up to 70%mol of methane and hydrogen relative to the total molar content of stream (B) preferably up to 60% mol.

In an embodiment of the invention, the step a) of separating the gaseous stream (A) into a first gaseous stream (B) and a first liquid stream (C), and/or the step b) of separating the first gaseous stream (B) into a second gaseous stream (D) and a second liquid stream (E), comprises the steps of cooling the gaseous stream (A) and/or (B) and separating the liquid fraction obtained from the remaining gaseous fraction to produce a gaseous stream and a liquid stream, with preference said stream (A) and/or (B) is cooled to a temperature ranging from - 30°C to - 90°C

Preferably, the step of cooling the gaseous stream (B) is conducted by an heat exchange with the fourth gaseous stream (H), said fourth gaseous stream (H) having been previously subjected to cooling step to a temperature ranging from - 30°C to -100°C, more preferably said cooling step is conducted by expansion of said gaseous stream (H).

In another embodiment, before the step a) of separating the gaseous stream (A) into a first gaseous stream (B) and a first liquid stream (C), the gaseous stream (A) is subjected to a step of removal of the contaminants, with preference said step of removal of the contaminants comprises at least one of :

- compressing and cooling gaseous stream (A) to condensate H20 and removing said condensed H20, if any;

- removing the oxygenated contaminants by a stripping step;

- removing C02 and/or H2S by a caustic washing step. In another embodiment, the first liquid stream (C) is further subjected to a step of distillation to recover a gaseous stream, and a liquid stream with at least with two carbon atoms, preferably the distillation step of the first liquid stream (C) is conducted together with the step d) of conducting a distillation on the second liquid stream (E) and/or the third liquid stream (G) to recover a fourth gaseous stream (H) and a fourth liquid stream (I) comprising hydrocarbons with at least two carbon atoms. In another embodiment, the fourth liquid stream (I) comprising hydrocarbons with at least two carbon atoms are further subjected to a step of recovering hydrocarbons comprising:

- separating by a fractionation step the fourth liquid stream (I) into a fifth stream (J) comprising hydrocarbons with at least 3 atom carbons and a sixth stream (K) comprising hydrocarbons with two carbon atoms, preferably by distillation via a de-ethanizer unit,

- separating in a fractionation step the fifth stream (J) comprising hydrocarbons with two carbon atoms into unsaturated hydrocarbon stream (M) comprising ethylene and saturated hydrocarbon stream (N) comprising ethane;

- recovering stream (J) comprising hydrocarbons with at least three atom carbons and unsaturated hydrocarbon stream (M) and saturated hydrocarbon stream (N). Preferably, the solvent of step c) used to wash the second gaseous stream (D) is ethane, preferably the ethane used is originated from saturated hydrocarbon stream (N). In another embodiment of the invention, the fourth gaseous stream (H) is recycled into gaseous stream (A), preferably before the step of removal of contaminants from the gaseous stream (A) according to the previously mentioned embodiment. In another embodiment of the invention, the operating temperature of one or more of the second and third liquid stream (E) and (G) and of the third gaseous stream (F) is greater than -90°C.

Preferably, the oxygenated contaminants consist of alcohols, ethers, carboxylic acids, aldehydes or any combination.

More preferably, the stream (K) exiting the de-ethanizer (1 1 ) is sent to a selective hydrogenation unit to convert the possible acetylene traces.

According to a second aspect the invention relates to an installation for implementing the process according to embodiments described above comprising:

- a first separator unit to separate a incoming gaseous stream (A) into a first gaseous stream (B) and a first liquid stream (C);

- a second separator unit to separate the first gaseous stream (B) into a second gaseous stream (D) and a second liquid stream (E);

- a third separator unit to separate the second gaseous stream (D) into a third gaseous (F) and a third liquid stream (G);

the first and second separation unit comprising at least one cooler device followed by a gas/liquid separator;

remarkable in that the third separator unit is an absorption column (9).

In another embodiment of the second aspect of the invention, the installation is remarkable in that it further comprises distillation means (8) to separate hydrocarbon with at least two carbon atoms from hydrogen and methane, preferably said distillation means are a demethaniser (8), preferably it further comprises means to convey one or more of the first liquid stream (c), the second liquid stream (E) and the third liquid stream (G) said distillation means (8), more preferably at least two of the first liquid stream (c), the second liquid stream (E) and the third liquid stream (G) are conveyed to the same distillation means (8). In a second aspect of the invention, the installation is preferably characterized in that it further comprises means to collect the overhead stream (H) exiting the distillation means (8) and to convey said stream (H) to expanding means (10) in order to cool said stream (H), and means to convey said cool stream (H) in the cooling device of the first or second separation unit.

The present invention can either be used to revamp an existing plant or it can be used in a new plant.

All embodiments of the first aspect of the invention relating to the process described in the above part are linked together and should be considered in combination with each other and they are also linked and should be considered in combination with all the embodiments related to the second aspect of the invention relative to the installation. Figure 1 describes one possible embodiment of the invention. The main stream (A) originates from a methanol to olefins (MTO) process or a MTO process with at least part of the effluent of MTO process being treated in an olefin cracking process (OCP). The main stream (A), being at a temperature of about 50°C, is firstly compressed in a compression means (1 ) like for instance a compressor. It is preferably subjected to a step of removal of the contaminants comprising a purification in an oxygen removal unit (2) to remove the oxygenated contaminants by a stripping step, and the removal of C02 and H2S in the C02 removal unit (3) by implementation of a caustic washing step using for instance a caustic wash tower. The temperature is then adapted in a cooler device like for instance the heat exchanger (4) to reach at most -30°C. The temperature is maintained at a temperature greater than -90°C. A liquid and a gaseous fraction are then produced and separated using a gas/liquid separator like the splitter (5) to form a first gaseous stream (B) and a first liquid stream (C). The separator or splitter (5) makes a first separation of the fuel gas (i.e. methane and hydrogen) from the C2+. The fuel gas (i.e. methane and hydrogen) are mainly contained in first gaseous stream (B), whereas the C2+ are mainly contained in the first liquid fraction (C). The first liquid stream (C) is subjected to a step of distillation in a distillation means (8) being preferably a demethanizer/stripper (i.e. a distillation with a re-boiler on the bottom but no re-condenser on the top) to remove the remaining methane and hydrogen. The fuel gas exits the distillation means (8) in a stream (H) as overhead flow and a purified stream (I) containing the C2+ is recovered as bottom flow.

The first gaseous stream (B) is cooled down to a temperature of about -85°C in a cooler device (6) being for instance a heat exchanger. The cold used in this cooler device (6) comes advantageously from the decompressed flow (Η') as described below. This further cooling of the first gaseous stream (B) creates a liquid and a gaseous fraction. The stream (Β') obtained at the exit of the cooler device (6) is then sent to a gas/liquid separator (7) being for instance a splitter. The liquid and the gaseous fractions are separated in this gas/liquid separator (7) to obtain a second liquid stream (E) and a second gaseous stream (D). The gas/liquid separator (7) makes a second separation of the fuel gas from the C2+. The fuel gas (i.e. methane and hydrogen) are mainly contained in second gaseous stream (D), whereas the C2+ are mainly contained in the second liquid fraction (E). The second liquid stream (E), is subjected to a step of distillation in distillation means (8) to remove the remaining methane and hydrogen contained. The fuel gas exit the distillation means (8) in a stream (H) as overhead flow and a purified stream (I) containing the C2+ is recovered as bottom flow.

The second gaseous stream (D), having a temperature of about -85°C, is subjected to a third separation step to remove the remaining ethylene. The separation is conducted by washing stream (D) by a solvent on an absorption column (9). The absorption column (9) finishes the separation started in the gas/liquid separator (5) and (7). The absorption column (9) is fed with a solvent stream (L). The solvent used is preferably ethane. Advantageously, the ethane used as solvent is a product of the process that is recycled as it will be seen later. On the top of the absorption column, a stream (F) containing purified fuel gas (i.e. methane and hydrogen) is obtained.

It is to be noted that the stream (D) exiting gas/liquid separator (7) has low ethylene content and is relatively small compared to stream (A). Indeed, the ratio of the flow rate of the second gaseous stream (D) to the gaseous stream (A) is at most 1/5, preferably at most 1/6. Consequently the requirements for the absorption column (9) are relatively low: the separation can be performed with a relatively small column and the absorbent flow (L) is also small. The capital expenditure (CAPEX) associated with the column remains therefore small and the stream (G) recycled to the distillation means (8) stays also small.

One can see that during the separation performed in the distillation means (8), the gas/liquid separator (5), the gas/liquid separator (7) and the absorption column (9), the temperature stays always above -100°C, therefore there is no risk of N2O3 formation.

The stream (G) exiting the bottom of the absorption column (9) is subjected to a distillation step and therefore sent to the distillation means (8) to remove the remaining ethylene and methane and hydrogen contained. The fuel gas exit the distillation means (8) in a stream (H) as overhead flow and a purified stream (I) containing the C2+ is recovered as bottom flow.

The stream (H) obtained in the top of the distillation means (8) has a temperature of about - 30°C. In a preferred embodiment, it is expanded in an expanding means being for instance a turbo expander (10) to obtain a cooled stream (Η') at a temperature of about -95°C. This temperature is obtained without the use of ethylene refrigeration cycle, thus lower capital expenditure (CAPEX) is required to set up the installation. The interest of reaching such temperature is not in a separation step but to save energy by using this stream to cool stream (B) in a cooler device (6) being for instance a heat exchanger. A stream (H") obtained at the exit of the cooler device (6) has a temperature of about -40°C. Another advantage of this step of expanding is that it allows stream (H") to be recycled by being mixed with stream (A) at the beginning of the purifying process, as both stream will show similar pressure.

The stream (I) obtained at the bottom of the distillation means (8) is sent to a de- ethanizer (1 1 ) to recover a bottom stream (J) containing mainly C3+ and a top stream (K) containing mainly ethane and ethylene. Stream (K) is further sent to a C2 splitter to separate ethylene on the top (stream (M)) from ethane on the bottom (stream (N) and (L)). Part of the ethane obtained in the bottom of the C2 splitter is used in the absorption column (9). Other possible solvents that can be used in the absorption column (9) are saturated paraffins from C2 to C4.

In an embodiment (not represented on fig. 1 ), the stream (K) exiting the de- ethanizer (1 1 ) is sent to a selective hydrogenation unit to convert the possible acetylene traces. Acetylene being a poison for the polymerization catalysts, it is required to limit to a maximum level of 5ppmv its content in the ethylene stream. Selective hydrogenation is an adequate option to purify an ethylene stream.

Altogether the distillation means (8), the gas/liquid separator (7) and the absorption column (9) allow separating the fuel gas from the C2+ fraction with the same specifications as a usual industrial design of a steam cracker plant. Good heat balances are obtained with the process design of the present invention.

[Detailed description of the invention]

As regards the MTO process, such process is described in WO-2008- 1 10526, WO-2008-1 10528, WO-2008-1 10530, WO-2009-016153, WO-2009- 016154, WO-2009-016155, WO-2009-092779, WO-2009-092780, WO-2009- 092781 , the content of which is incorporated in the present application. The MTO process has also been described in US 2006 0235251 , WO 2005 016856, US 2006 0063956, US 2006 0161035, US 6207872, US 2005 0096214, US 6953767 and US 7067095, the content of which is incorporated in the present application. The C4+ olefins produced in the MTO process may also be cracked in an olefin cracking process. Such process is described in EP1036133, EP92091 1 the content of which is incorporated in the present application.

The effluent produced by a MTO and Quench processes is a hydrocarbon stream comprising light molecules. The composition of stream (A) consists of water, hydrogen (H2), methane, ethylene, ethane, propylene, propane and C3+ (i.e. hydrocarbons having at least three carbon atoms). In an embodiment, water is present in the stream with a concentration between 1 to 10% mol (molar percent), preferably between 2 and 8 % mol based upon total molar content of the stream (A). Hydrogen (H2) is present in the stream with a concentration between 1 and 10 % mol, preferably between 2 and 8 % based upon total molar content of the stream (A). Methane is present in the range of 1 to 5 % mol, preferably in the range of 2 to 4 % mol based upon total molar content of the stream (A). The C2 molecules (ethane + ethylene) are present within a concentration range of 20 to 50% mol, preferably between 25 and 35% mol based upon total molar content of the stream (A). The heavier molecules (C3+) constitute the rest of the stream.

The effluent stream (A) coming from the MTO + Quench section or from the MTO + OCP + Quench section has a temperature in the range of 10 and 90°C, preferably in the range of 30 and 60°C and a pressure in the range of 10⁴ Pa to 5 10⁵ Pa (0.1 to 5 bara), preferably between 10⁵ to 3 10⁵ Pa (1 and 3 bara).

Ethylene and propylene are particularly desirable olefins but it has been found that their yields in the MTO process are reduced by the production of medium weight hydrocarbons such as C4, C5 and C6 olefins, as well as some heavier components. Methods are needed to alter the product distribution in the MTO process for making light olefins to provide processing flexibility. Methods are sought to reduce the production of C4, C5 and higher olefins from the MTO process relative to the production of ethylene and propylene. Therefore an olefin cracking process (OCP) is combined with the MTO process to crack the C4, C5 and higher olefins from the MTO process. Such yield improvements of ethylene and propylene significantly improve the economies of the methanol to olefins process.

As regards to the OCP process, it consists in selective cracking of olefin- rich stream. The process is selective toward the formation of light olefins. It enables improvements of the ethylene and propylene yields from the MTO unit. Details about the OCP catalyst and process can be found in the patent EP 1036139; EP 1036135; EP 1036133 and WO 99/29805, content of which is incorporated in the present application.

As regards the oxygenated contaminants one can cite alcohols such as methanol, ethanol, C3 alcohols; ethers such as dimethyl ether, diethylether and methyl ethyl ether; carboxylic acids such as acetic acid, propanoic acid and butyric acid; aldehydes such as acetaldehyde; ketones such as acetone; and esters such as methyl esters. Particularly problematic oxygenate contaminants downstream the MTO process are dimethyl ether (DME) and acetaldehyde. The presence and concentrations of these by-products may vary depending, for example, on the feedstock qualify, the type and size of reactor, the reaction conditions, and the type and condition of the catalyst used.

As regards to the oxygenated contaminants removal one can cite the process described in the patents WO 2004/048300; US 7288692; US 7361799; US 7332639; US 2004/0039239 the content of which are incorporated in the present application. In WO 2004/048300, the oxygenated contaminants are separated by heating and converted in the corresponding olefins. In US 7288692, oxygenated contaminants removal is performed via gaseous phase washing with water containing solvent or alcohol containing solvent. The patent US 7361799 consists in cooling the vapour products stream to remove water and recovering a first vapour effluent. Cooling and compressing the said first vapour effluent to produce a condensed liquid effluent stream and finally washing this stream with a liquid water containing stream. The patent US 7332639 consists in the separation of the gaseous fraction from the liquid fraction and washing the gaseous fraction with a liquid alcohol-containing stream and finally washing the gaseous fraction washed fraction obtained with a water containing stream.

As regards to the carbon dioxide and to the H2S, it requires removal. For example, an olefin product specification may require removal of carbon dioxide from the methanol-to-olefins reactor effluent. Further, exposure of the carbon dioxide containing stream to below-sublimation temperatures may result in equipment damage and frozen piping. Methods commonly known and used in the industry, such as caustic solution treatment or amine absorption, may be used to remove C02 from the methanol-to-olefins reactor effluent. In some embodiments, the reactor effluent may be contacted with a caustic solution to separate at least a portion of the carbon dioxide present in the reactor effluent. If necessary, the reactor effluent may be additionally compressed prior to the carbon dioxide removal stage. C02 can also be removed in an available commercial fixed bed adsorption (PSA for pressure swing adsorption or TSA for temperature swing adsorption) using molecular sieves or basic oxides or mixture thereof. A stream essentially free of C02 is recovered. Said fixed bed can be regenerated, during regeneration the desorption produces a stream which can be treated anywhere. A non limiting example of C02 removal can be found in US 7135604. As regards to H2S, such contaminant is a well known catalyst poison. It is also a highly toxic molecule. H2S is generally withdrawn from industrial stream with process like the caustic wash and transformed with process well known in the art such as Claus unit for instance. The H2S content may vary in stream (A) from 1 ppm up to 2% mol base upon the total molar content of stream (A). H2S generally originates from the OCP unit.

As regards to water, it can lead to a number of problems. For example, cooling and/or compressing the reaction effluent may result in formation of water condensate that can damage equipment and freeze pipes. Therefore, dehydration of the reactor effluent to remove water using one of a number of techniques commonly used in the industry may be required or may be optionally performed based on process schemes and temperatures employed. In some embodiments, a molecular sieve dryer may be used for separating at least a portion of the water, drying the reactor effluent. In other embodiments, a chemical desiccant such as glycol may be used for drying the reactor effluent. In yet other embodiments, a portion of the water in the reactor effluent may be condensed and the remaining effluent may be dried. Other dehydration techniques commonly known and used in the industry may also be used. If necessary, the MTO+Quench effluents effluent may be additionally compressed prior to the water removal stage. Optionally the stream containing the oxygenated contaminants is burned and heat is recovered.

As regards the distillation means being preferably demethanizer/stripper, it shall be understood has being a particular "fractionation tower" or "distillation column" having no condenser on the top and no reflux. More precisely, the demethanizer/stripper is a conventional distillation column containing a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing. The demethanizer/stripper section also includes re-boilers on the bottom of the column (not represented) but it presents no condenser on the top of the column and no reflux. The re-boiler on the bottom of the column heats and vaporizes a portion of the liquids flowing down the column to provide the stripping vapours which flow up the column to strip the liquid product. In the configuration of the present invention, the presence of a condenser on the top of the column would require an ethylene cryogenic cooling and there is no need of a reflux on the top of the column because of the presence of the adsorption column. As it is well known in the art, reflux can be used at various height of the column to improve the separation yield. Non limiting example of demethanizer/stripper processes can be found in EP 0980502; EP 1 137903 and WO 03/040633 which are incorporated thereby by reference. The demethanizer/stripper of the present invention operates at temperature between 20 and 80°C on the bottom of the column and between -10 and -90°C on the top of the column. The operating pressure of the demethanizer/stripper is between 2 10⁶ to 35 10⁶ Pa (20 and 35 bara).

The overhead stream exiting the demethanizer/stripper (stream (H)) is essentially constituted of hydrogen, methane, ethylene and ethane. Hydrogen concentration is in the range of 2 to 10 % mol based upon the total molar content of stream (H); methane concentration is in the range of 30 to 50 % based upon the total molar content of stream (H); and C2 (ethylene and ethane) in the range of 40 to 60% based upon the total molar content of stream (H). Traces of C3+ may also be present.

The bottom stream exiting the demethanizer/stripper (stream (I)) is essentially constituted of C2+ (ethylene, ethane and C3+). Ethylene and ethane are in the range of 30 to 50 % mol in the effluents based upon the total molar content of stream (I). C3+ content is in the range of 50 to 70% mol based upon the total molar content of stream (I).

As regards to the absorption column, conventional absorption systems can be used in this invention. In one embodiment, the absorption system uses packed columns, although plate absorption columns may also be used. In another embodiment, the absorption column has a liquid inlet located at a top portion of the absorption column. The absorbent liquid is evenly distributed across the top of the column. Desirably, an even distribution of the absorbent liquid is accomplished by using a distributor plate or spray nozzles. At the bottom of the absorption column is a gas inlet where the hydrocarbon stream, enters the absorption column. The vapour components move up the column counter-current to the liquid absorbent moving down the column. This is known as counter-current absorption. The packing or plates in the column provides a surface for intimate contact between the vapour and liquid components within the column. In a counter-current absorption column, the concentration of soluble gasses in both the liquid and vapour phases is greatest at the bottom of the column, and lowest at the top of the column. The outlet for the liquid is at the bottom of the absorption column, typically below the gas inlet. The outlet for the gas phase lean in the gasses most soluble in the liquid absorbent is at the top of the absorption column, typically above the liquid inlet. In an embodiment of the invention, ethylene in stream D is about less than 10% mol, preferably less than 5% mol based upon the total molar content of stream (D), of ethylene stream B'.

As regards to the heat exchanger classical heat exchanger or cryogenic unit like Plate Fin Heat Exchanger (PFHE) or Brazed Aluminium Heat Exchanger (BAHX) in cold box as used in petrochemicals plants can be used. Usual heat exchanger with particular insulation allowing adequate heat exchange of the flows can be used. Design and use of cold boxes or cryogenic units are well known in the art. In particular, the heat exchanger (6) transfers cold from stream (Η') to stream (B). Typical temperature of stream (Η') ranges from to -1 10°C to -80°C, preferably from -90°C to -100°C. After heat exchange in the unit (6), the typical temperature of stream (H") ranges from -50 to -20°C, preferably from -40 to -30°C. Before entering the heat exchanger (6), typical temperature from stream (B) is in the range from - 20 to -40°C, preferably from -30 to -40°C. After the thermal exchange in the unit (6), the temperature of stream (Β') ranges from -105 to -85°C, preferably from -100 to -90°C.

As regards to the absorption solvent, any absorbing medium allowing good separation of the fuel gas from the C2+ can be used. Hydrocarbons can be used. More preferably short chain hydrocarbons in C2-C4. Even more preferably ethane as it presents a low boiling point. Absorption being an exothermic process, part of the absorption heat can be released via ethane vaporization inside the absorption column. Thereby ethane vaporization advantageously limits the heat treatment outside the column. As regards to the turbo expander, it is well known by the person skilled as an apparatus used to expand a high pressure gas while recovering work. The work recovered can be used to drive a compressor or to produce electricity.

The turbo expander is driven by a high pressure gas source. The high pressure gas source becomes reduced in pressure to a lower pressure gas source as a consequence of gas expansion that drives the turbo expander. The energy recovered by the turbo expander can be either used to produce electricity with an alternator or it can be used to run a compressor. Advantageously the alternator or the compressor runs in the same shaft as the turbo expander. By way of example, the turbo expander (10) may be linked with the compressor (1 ) in order to improve the overall energy balance.

As regards to the de-ethanizer, it is can also be referred to as a "fractionation tower" or a "distillation column". The de-ethanizer is a conventional distillation column containing a plurality of vertically spaced trays, one or more packed beds, or some combination of trays and packing. The de-ethanizer section also includes reboilers or reflux (not represented) which heat and vaporize a portion of the liquids flowing down the column to provide the stripping vapours which flow up the column to strip the liquid product. As it is well known in the art, reflux can be used at various height of the column to improve the separation yield.

As regards to the ethylene splitter, it is can also be referred to as a "fractionation tower" or a "distillation column". It is a conventional separation unit used to separate ethane from ethylene well known in the art.

The term "traces" refers to a concentration in the range of 0.001 %wt to 0.1 %wt. The term "essentially" refers to a composition containing at least 80 wt % of the said product, preferably between 85 wt % and 95 wt %, more preferably above 95 wt %. In another way, the present invention can be described as it follows:

Embodiment 1 , the present invention is a process for purifying a gaseous stream

(A) comprising:

(i) C2+ hydrocarbons comprising olefins and

(ii) fuel gas consisting mainly of methane and hydrogen and

(iii) oxygenated contaminants and

(iv) C02, optionally CO and H2S and

(v) optionally water

said process comprising:

a) compressing the gaseous stream (A) to condensate at least the optional

H2O

b) removing the oxygenated contaminants (iii) in oxygenate removal unit (2) and removing CO2 and optionally CO and H2S (ii) in dedicated unit (3) and removing the optional water and optionally adapting the working temperature in heat exchangers or any combination

c) recovering a stream (Α') sending said stream (Α') to a separator to get a liquid stream (C) and a gaseous stream (B) and removing water from (Α') or from

(B) and (C) or any combination

d) sending said liquid fraction (C) to a demethanizer (8)

e) cooling said gaseous fraction (B) in a heat exchanger (6) to produce a stream (Β'), sending (Β') to a separator (7) to get a purified liquid fraction (E) comprising mainly C2+ and a gaseous fraction (D) comprising mainly fuel gas

f) sending said purified liquid fraction (E) into said demethanizer (8) g) washing said gaseous stream (D) with an hydrocarbon in an absorption column (9) thereby recovering an overhead gaseous stream (F) consisting mainly of fuel gas and a liquid bottom stream (G) consisting mainly of C2+ and the hydrocarbon used for washing

h) sending said liquid stream (G) into said demethanizer (8)

i) recovering on the top of said demethanizer (8) a gaseous stream (H) comprising C2+ and fuel gas j) expanding stream (H) in conditions effective to cool down the stream in a dedicated turbo expander unit (10) to get a stream (Η')

k) sending said expanded stream (Η') in said heat exchanger (6) to cool down the gaseous stream (B) obtained in step a) to get a stream (H")

I) recycling the stream (H") obtained in step k) upstream the said compression section of step a)

m) recovering the bottom of the demethanizer (8) a stream (I) comprising mainly C2+. Embodiment 2, a process according to embodiment 1 wherein the stream (I) is sent to a de-ethanizer (1 1 ) to get a C3+ bottom flow (J) and an overhead (K) constituted mainly of ethane and ethylene and the flow (K) is sent to a C2 splitter (12) where ethylene is separated on the top and ethane on the bottom. Embodiment 3, a process according to embodiment 2 wherein part of the ethane obtained in said C2 splitter (12) is used as absorbent in the absorption column (9).

Embodiment 4, a process according to any of the preceding embodiments wherein the said olefins stream (A) is produced by a methanol to olefins (MTO) process.

Embodiment 5, a process according to any of the preceding embodiments wherein the said MTO process is based on methanol and dimethyl ether.

Embodiment 6, a process according to any of the preceding embodiments wherein the said olefins stream (A) is produced by an olefin cracking process.

Embodiment 7, a process according to embodiment 1 wherein the solvent used in said absorption column of step g) is ethane. Embodiment 8, a process according to embodiment 2 wherein a acetylene selective hydrogenation is performed on stream (K).

Embodiment 9, a process according to embodiment 1 wherein the oxygenates contaminants (iii) consist of alcohols, ethers, carboxylic acids, aldehydes.

[Example]

A process simulation was performed based on the flow diagram presented in figure 1. The calculations were performed with PRO/II VERSION 8.3.3 ELEC V7.0 from SIMSCI with the thermodynamic package standard EOS SRK. The parameter characterizing the apparatus (efficiency of compressor, expanders etc) were those known by the person skilled in the art i.e. realistic for industrial simulations. The normalized results obtained are presented in the following tables. Table 1. Main properties of the flow rates (A) to (G)

Table 2. Main properties of the flow rates (H) to (N)

The energy consumption of the design of the invention was of 506 kcal/ kg light olefins (ethylene and propylene). In comparison, the design generally used (as described in the Kirk-Othmer encyclopaedia of Chemical Technology 5 edition Vol. 10 p 61 1 -612) was of 583 kcal/ kg light olefins (ethylene and propylene). The present invention allows an improvement of the energy consumption. In both cases, the ethylene and propylene recovery were higher than 99.7%.

Table 1 clearly shows that the ratio of the flow rate of the stream (D) to the stream (A) is of about 0.12 which is less than 0.17 (i.e. 1/6).

Claims

1. Process for purifying a gaseous stream (A), the gaseous stream (A) comprising:

- hydrocarbons with at least two carbon atoms,

- methane and hydrogen,

e) separating the gaseous stream (A) into a first gaseous stream (B) and a first liquid stream (C) ;

f) separating the first gaseous stream (B) into a second gaseous stream (D) and a second liquid stream (E) ;

g) separating the second gaseous stream (D) to get third gaseous stream (F) comprising methane and hydrogen, and a third liquid stream (G)

h) conducting a distillation on the second liquid stream (E) and/or the third liquid stream (G) to recover a fourth gaseous stream (H) and a fourth liquid stream (I) comprising hydrocarbons with at least two carbon atoms the process being characterized in that stream (A) is produced by a methanol to olefin process, and in that the operating temperature of the second gaseous stream (D) is greater than -90°C and in that the separation of the second gaseous stream (D) in step c) is conducted by washing said second gaseous stream (D) by a solvent.

2. Process according to claim 1 further characterized in that the ratio of the flow rate of the second gaseous stream (D) to the gaseous stream (A) is at most 1/5, preferably at most 1/6. 3. Process according to claim 1 to 2 further characterized in that - stream (A) comprises up to 20 %mol of methane and hydrogen relative to the total molar content of stream (A) preferably up to 10%mol, and/or

- stream (B) comprises up to 70%mol of methane and hydrogen relative to the total molar content of stream (B) preferably up to 60% mol.

Process according to any of claim 1 to 3 further characterized in that stream (A) is produced by a methanol to olefin process with at least part of the effluent of the methanol to olefin process being treated in an olefin cracking process.

Process according to claim 1 to 4 further characterized in that

- the step a) of separating the gaseous stream (A) into a first gaseous stream (B) and a first liquid stream (C), and/or

- the step b) of separating the first gaseous stream (B) into a second gaseous stream (D) and a second liquid stream (E),

comprises the steps of cooling the gaseous stream (A) and/or (B) and separating the liquid fraction obtained from the remaining gaseous fraction to produce a gaseous stream and a liquid stream, with preference said stream (A) and/or (B) is cooled to a temperature ranging from - 30°C to - 90°C

Process according to any of claim 1 to 5 further characterized in that the step of cooling the gaseous stream (B) is conducted by an heat exchange with the fourth gaseous stream (H), said fourth gaseous stream (H) having been previously subjected to cooling step to a temperature ranging from - 30°C to -100°C, preferably said cooling step is conducted by expansion of said gaseous stream (H).

Process according to any of claim 1 to 6 further characterized in that before the step a) of separating the gaseous stream (A) into a first gaseous stream

(B) and a first liquid stream (C), the gaseous stream (A) is subjected to a step of removal of the contaminants, with preference said step of removal of the contaminants comprises at least one of :

- removing the oxygenated contaminants by a stripping step;

- removing C02 and/or H2S by a caustic washing step.

Process according to any of the claim 1 to 7 further characterized in that the first liquid stream (C) is further subjected to a step of distillation to recover a gaseous stream, and a liquid stream with at least with two carbon atoms, preferably the distillation step of the first liquid stream (C) is conducted together with the step d) of conducting a distillation on the second liquid stream (E) and/or the third liquid stream (G) to recover a fourth gaseous stream (H) and a fourth liquid stream (I) comprising hydrocarbons with at least two carbon atoms.

Process according to any of the claim 1 to 8 further characterized in that the fourth liquid stream (I) comprising hydrocarbons with at least two carbon atoms are further subjected to a step of recovering hydrocarbons comprising:

- separating in a fractionation step the fifth stream (J) comprising hydrocarbons with two carbon atoms into unsaturated hydrocarbon stream (M) comprising ethylene and saturated hydrocarbon stream (N) comprising ethane; - recovering stream (J) comprising hydrocarbons with at least three atom carbons and unsaturated hydrocarbon stream (M) and saturated hydrocarbon stream (N).

10. Process according to any of claim 1 to 9 further characterized in that the solvent of step c) used to wash the second gaseous stream (D) is ethane, preferably the ethane used is originated from saturated hydrocarbon stream (N) according to claim 9.

1 1. Process according to any of claim 1 to 10 further characterized in that the fourth gaseous stream (H) is recycled into gaseous stream (A), preferably before the step of removal of contaminants from the gaseous stream (A) according to claim 7.

12. Process according to any of claim 1 to 1 1 further characterized in that the operating temperature of one or more of the second and third liquid stream (E) and (G) and of the third gaseous stream (F) is greater than -90°C.

13. Installation for implementing the process according to any of claims 1 to 12 comprising

characterized in that the third separator unit is an absorption column (9).

14. Installation according to claim 13 characterized in that it further comprises distillation means (8) to separate hydrocarbon with at least two carbon atoms from hydrogen and methane, preferably said distillation means are a demethaniser (8), preferably it further comprises means to convey one or more of the first liquid stream (C), the second liquid stream (E) and the third liquid stream (G) to said distillation means (8), more preferably at least two of the first liquid stream (C), the second liquid stream (E) and the third liquid stream (G) are conveyed to the same distillation means (8).

15. Installation according to claim 14 characterized in that it further comprises means to collect the overhead stream (H) exiting the distillation means (8) and to convey said stream (H) to expanding means (10) in order to cool said stream (H), and means to convey said cool stream (H) in the cooling device of the first or second separation unit.