Process for the preparation of an olefinic product from an oxygenate (Shell)

CATEGORY: OXYGENATE-TO-OLEFIN (OTO)
Process for the preparation of an olefinic product from an oxygenate (Shell)

Type	Patent
Inventor	Jeroen van Westrenen
Inventor	Sivakumar Sadasivan VIJAYAKUMARI
URL	Free Full Text Source:  http://www.google.com/patents/US20140187835
Assignee	Shell Oil Company
Patent Number	US20140187835 A1
Issue Date	Jul 3, 2014
Abstract	The invention relates to a process for the preparation of ethylene and/or propylene comprising: (a) an oxygenate conversion step wherein a gaseous effluent comprising olefins is obtained; (b) subjecting the effluent to water removal and compression steps; (c) acid gas removal from the effluent obtained in step (b), wherein the gaseous effluent is treated with a caustic solution and a non-aqueous liquid stream comprising aromatic C7+ hydrocarbons is added to the caustic solution to control the formation of red oil, to obtain a treated gaseous effluent and a spent liquid phase comprising spent caustic solution, aromatic C7+ hydrocarbons and high molecular weight contaminants; and (d) separating an aqueous phase and a non-aqueous phase from the spent liquid phase; (e) purifying the non-aqueous phase by membrane separation using a hydrophobic non-porous or nano-porous membrane; and (f) separating the olefinic product from the treated gaseous effluent. BACKGROUND TO THE INVENTION Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks including ethane, propane, naphtha and hydrowax. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into for instance methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process. In an OTO process, an oxygenate such as methanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and converted into ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate such as methanol is converted into higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The effluent from the reactor comprising the olefins, any unreacted oxygenates such as methanol and dimethylether and other reaction products such as water may then be treated to provide separate component streams. Unreacted oxygenates can be separated from the reaction effluent, for instance by contacting with a cooled aqueous stream in a quench tower. In order to increase the ethylene and propylene yield of the process, the C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene. Due to the high temperatures in the reaction zone and the acidity of the catalyst, a portion of the oxygenates such as methanol may unavoidably decompose thermally or catalytically into oxides of carbon, i.e. carbon monoxide and carbon dioxide in the gaseous form. The carbonaceous deposits on the catalyst can be removed by the periodic regeneration of the catalyst by heating it with an oxidising gas such as oxygen, in order to burn off the deposits. Carbon dioxide generated during the OTO process is an acid gas which is thus present in the effluent from the reactor. In order to prevent contamination of the olefinic product and problems associated with the formation of solid carbon dioxide during the separation of the olefinic product into olefinic component streams, which may be carried out at cryogenic temperatures, carbon dioxide should be removed from the reaction effluent and from the gaseous effluent from the quench tower before separation into olefinic component streams. This is typically done by washing the gaseous effluent with a caustic solution in a caustic tower. Carbonyl compounds, such as aldehydes and ketones, in particular acetaldehyde, are commonly generated by the catalyst in side reactions and are also found in the effluent from the reactor. Carbonyl compounds may build up in the caustic solution used to remove carbon dioxide and other acid gases. The basic components of the caustic solution, such as hydroxide ions, can catalyse the aldol condensation and subsequent dehydration reactions of particularly acetaldehyde to form unsaturated aldehydes such as acrolein, especially at higher pH, such as a pH above 9. Unsaturated aldehydes may polymerise when allowed to accumulate in the caustic solution and if the aldol condensation reaction is left unchecked, a viscous oily polymer can be formed, known as ‘red oil’, which is insoluble in the caustic solution and can deposit on equipment internals, causing fouling and leading to maloperation of the caustic tower containing the caustic solution. WO 2007/111744 discloses a process for oxygenate conversion to olefins with enhanced carbonyl recovery. A recycle or circulated water stream is treated with a sulphite-containing material in order to form a treated water stream with an appropriately reduced or minimised carbonyl, in particular aldehyde, content. The sulphite-containing material is added to the oxygenate absorber zone. The oxygenate-rich water stream containing unreacted sulphite and bisulphite addition compounds produced in the oxygenate absorber zone is passed to an oxygenate stripper zone to be separated into an oxygenate-containing stream and a recycle water stream. The oxygenate-containing stream can be returned to the oxygenate conversion reactor. The recycle water stream can be passed to a wash water stripper to recover oxygenates and provide a bottoms water stream comprising unreacted sulphite and bisulphite addition compounds which can be passed to the effluent treatment zone for the treatment of the reactor section effluent. The recycle water stream can also be passed to the oxygenate absorber zone for the treatment of the compressed oxygenate conversion effluent stream. A disadvantage of the process of WO2007/11174 is, however, that a stream comprising unreacted sulphite and bisulphate addition compounds is treated in the effluent treatment system of the OTO process. This may result in undesired release of acetaldehyde from its addition compound, since the formation of formaldehyde addition products is favoured. Moreover, a sulphur-containing waste stream will be formed in an olefins-to-oxygenate process that normally produces a sulphur-free effluent.

Process for the preparation of an olefinic product comprising ethylene and/or propylene (Shell)

CATEGORY: OXYGENATE-TO-OLEFIN (OTO) 
Process for the preparation of an olefinic product comprising ethylene and/or propylene (Shell)

Type	Patent
Inventor	Sivakumar SADASIVAN VIJAYAKUMARI
Inventor	Jeroen van Westrenen
URL	Free Full Text Source:  http://www.google.com/patents/US20140187824
Assignee	Shell Oil Company
Patent Number	US20140187824 A1
Issue Date	Jul 3, 2014
Abstract	An oxygenate to olefins (OTO) process, comprising the steps of: (i) purifying an oxygenate feedstream comprising one or more ionic contaminants by contacting the feedstream with a membrane, resulting in the formation of a retentate and a permeate separated by the membrane, which permeate is a purified oxygenate stream which contains a lower ionic contaminant concentration than the original oxygenate feedstream; (ii) introducing the purified oxygenate stream into an oxygenate to olefins reaction zone; and (iii) contacting the purified oxygenate stream with a molecular sieve catalyst in the oxygenate to olefins reaction zone to form a product stream comprising olefins. BACKGROUND TO THE INVENTION There is a large market for light olefin products ethylene and propylene which are, for example, used in the production of plastics. Light olefins are traditionally produced via the cracking of petroleum feedstocks, e.g. through catalytic cracking, steam cracking, or some combination of the two processes. However, as petroleum feedstocks from crude oil face increasing prices, other sources of ethylene and propylene are becoming more attractive. It is known to produce olefins from oxygenates via an oxygenate to olefins (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can, for example, be converted to methanol or dimethyl ether (DME), both of which are suitable oxygenate feedstocks for an OTO process. Oxygenates can also be produced from biomass. In an OTO process, an oxygenate such as methanol is provided to a reaction zone comprising a suitable conversion catalyst, and converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the methanol is converted to higher hydrocarbons including C4+ olefins. The catalyst is usually a molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 describes a process for making olefins from methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. U.S. Pat. No. 6,797,851 describes a process for making ethylene and propylene from an oxygenate feed using two or more zeolite catalysts. OTO processes can encounter problems due to the catalyst being fouled, for example due to coke deposition on the catalyst. Coke herein refers to a carbonaceous composition which next to the prevailing carbon may also contain hydrogen and other elements. Conventional catalyst regeneration techniques can be employed to remove the coke. It has now been realised that the purity of the oxygenate feed may also affect the viability of the catalyst. In particular, the catalyst appears to be susceptible to deactivation by contaminants in the oxygenate feedstream such as metals. US 2011/014404 describes a process for improving the quality of an oxygenate feedstream to an oxygenate to olefins conversion reactor by passing the feedstream to a feed fractionation column. This process therefore involves distilling the feedstream, in order to separate the oxygenate from undesired impurities such as sodium hydroxide (NaOH). It is an aim of the invention to provide an improved oxygenate to olefins (OTO) process.

Process for removing oxygenate from an olefin stream (Shell)

CATEGORY: OXYGENATE-TO-OLEFIN (OTO)
Process for removing oxygenate from an olefin stream (Shell)

Type	Patent
Inventor	Sivakumar SADASIVAN VIJAYAKUMARI
Inventor	Jeroen van Westrenen
URL	Free Full Text Source:  http://www.google.com/patents/US20140187836
Assignee	Shell Oil Company
Patent Number	US20140187836 A1
Issue Date	Jul 3, 2014
Abstract	The present invention relates to a process for removing oxygenate from an olefin stream comprising oxygenate, comprising providing to an oxygenate recovery zone the olefin stream comprising oxygenate and a liquid solvent comprising: (a) butanol; (b)alkyl tert-alkyl ether; or (c) alkyl tert-alkyl ether and butanol, treating the olefin stream comprising oxygenate with the liquid solvent, and retrieving from the oxygenate recovery zone at least one oxygenate-depleted olefinic product stream comprising olefin and a spent liquid solvent comprising at least part of the oxygenate. BACKGROUND OF THE INVENTION Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks including ethane, propane, naphtha and hydrowax. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into for instance methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process. In an OTO process, an oxygenate such as methanol is provided to a reaction zone of a reactor comprising a suitable conversion catalyst whereby the oxygenate is converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate, such as methanol, is converted to higher hydrocarbons including C4+ olefins and paraffins. The effluent from the reactor comprising the olefins, any unreacted oxygenates such as alcohols or ethers, particularly methanol and dimethylether and other reaction products such as water may then be treated to provide separate component streams. Unreacted oxygenates, in particular methanol, can be separated to a certain extent from the reaction effluent, for instance by contacting with a cooled aqueous stream in a quench zone. In order to increase the ethylene and propylene yield of the process, the C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene. In patent application WO 03/020678, a process for the removal of dimethylether from an olefinic stream is disclosed. In the process of WO 03/020678, the olefinic stream comprising dimethylether is first separated into a first stream comprising dimethylether and lighter boiling point compounds and a second stream comprising C4+ olefin and higher boiling point hydrocarbons. The stream comprising dimethylether subjected to an extractive distillation using an extraction solvent to remove at least part of the dimethylether. Methanol may for instance be used as a solvent. A similar process is described in US patent application No. 20090223870, a liquid phase containing hydrocarbons and oxygenates is charged to a separation vessel and separated into a light gaseous fraction and a heavier C4+ fraction. The light gaseous fraction together with a gaseous stream. is subjected to an extractive distillation with an extraction solvent, which dissolves the oxygenates, to remove at least part of the oxygenates from the combined gaseous stream. The preferred solvents are methanol or NMP. Where a gaseous stream is contacted with a liquid solvent, inevitably part of the liquid solvent will evaporate, due to its vapour pressure. As a result the combined gaseous stream is contaminated with the solvent. Although NMP has the advantage that it has a low vapour pressure, i.e. as much as 100 times lower than methanol, a disadvantage of using NMP is that it is typically not readily available at the process site and thus must be provided externally. Methanol may be more readily available to use a solvent, however, due to the high vapour pressure of the methanol, the light olefin rich, dimethylether lean overhead vapour stream will comprise substantial amounts of methanol as a contaminant. When methanol is diluted in a non-polar environment, such as the light olefin rich overhead vapour stream, its properties are no longer determined by its ability to form hydrogen bonds with other polar compounds. Rather, the methanol properties are determined based on its molecular weight. Consequently, methanol when diluted in a non-polar environment behaves similar to a C3 hydrocarbon. In the subsequent treatment of the light olefin rich, dimethylether lean overhead vapour stream to isolate ethylene and propylene product streams such diluted methanol will accumulate in the ethylene and propylene product streams. Methanol-contaminated ethylene and propylene is less suitable as a feedstock for preparing olefin derivatives such as polyethylene or polypropylene. Removing, the diluted methanol from the ethylene and propylene product is difficult and energy consuming. U.S. Pat. No. 7,132,580 discloses a methanol to olefin catalytic conversion process including the selective recovery and recycle of dimethylether and methanol from the effluent stream of the reactor. After the reactor effluent stream is charged to a quench zone, the resulting cooled overhead vapour stream can be compressed. The compressed stream can then be passed to a separation zone to recover a vapour stream which is then passed to a dimethylether absorption zone. The vapour stream is contacted with a dimethylether selective solvent containing methanol at scrubbing conditions effective to produce a liquid solvent bottom stream containing methanol, dimethylether, water and substantial and undesired amounts of ethylene and propylene and a light olefin rich, dimethylether lean overhead vapour stream containing methanol. The liquid solvent bottom stream further treated to remove a substantial portion of ethylene and propylene contained in the stream. According to U.S. Pat. No. 7,132,580, the use of a dimethylether selective solvent containing methanol in the dimethylether absorption zone necessarily results in a vapour stream that is saturated with methanol at the conditions prevailing at the top of the dimethylether absorption zone. As mentioned above, due to the properties of the diluted methanol in the light olefin rich, dimethylether lean overhead vapour stream, part of the methanol will end up as a contaminant in the ethylene and propylene product streams. Consequently, unless additional steps are taken to rigorously remove methanol from the light olefin rich, dimethylether lean overhead vapour stream, the light olefin product may be contaminated with methanol. The process of U.S. Pat. No. 7,132,580 therefore requires a secondary methanol absorption zone in which the light olefin rich, overhead vapour stream is contacted with an aqueous solvent at scrubbing conditions to remove methanol to produce a dimethylether-lean and methanol-lean overhead vapour product stream comprising ethylene and propylene and a bottom stream containing methanol and aqueous solvent. Nowowiejski et al. (Nowowiejski et al., An overview of oxygenates in olefins units in relation to corrosion, fouling, product specifications, and safety, Presentation at American Institute of Chemical Engineers 2003 Spring National Meeting, New Orleans, USA, in particular page 16) disclose the risk of methanol breakthrough in a C3 splitter even where the feed to the C3 splitter only contains small amounts of methanol. According to Nowowiejski et al., methanol, entering a C3 splitter producing a polymer grade propylene product, will concentrate in the C3 splitter around the 90 to 95 percent propylene zone in the C3 splitter. If methanol levels in the C3 splitter build up over time, a minor upset or change in operating conditions may result in off-spec methanol contaminated propylene product. A need exists to provide an improved process for the removal of oxygenates from hydrocarbon streams, in particular hydrocarbons streams containing ethylene and propylene. Preferably, a process that mitigates the contamination of the light olefin rich overhead vapour stream with additional methanol.