WO2015000944A1

WO2015000944A1 - A process of converting oxygenates to olefins and a reactor for that process

Info

Publication number: WO2015000944A1
Application number: PCT/EP2014/064037
Authority: WO
Inventors: Jeroen Van Westrenen; Sivakumar SADASIVAN VIJAYAKUMARI; Leslie Andrew Chewter; Ye Mon Chen
Original assignee: Shell Internationale Research Maatschappij B.V.; Shell Oil Company
Priority date: 2013-07-02
Filing date: 2014-07-02
Publication date: 2015-01-08

Abstract

A method of converting a feedstock comprising oxygenates to olefins comprising: providing the feedstock to an oxygenate to olefins conversion reactor; contacting the feedstock with a molecular sieve catalyst at oxygenate to olefin conversion conditions; and removing and separating olefins from the reactor wherein the reactor comprises one or more inner metal surfaces having an average roughness of at least 0.1 mm.

Description

A PROCESS OF CONVERTING OXYGENATES TO OLEFINS AND A REACTOR FOR

THAT PROCESS

Field of the Invention

The invention relates to a process for converting oxygenates to olefins and to a reactor suitable for carrying out that process. The reaction zone has an inner metal surface with an average roughness of at least 0.1 mm.

Background

Oxygenate-to -olefin ("OTO") processes are well described in the art. Typically, oxygenate-to -olefin processes are used to produce predominantly ethylene and propylene. An example of such an oxygenate-to-olefm process is described in US Patent Application Publication No. 2011/112344, which is herein incorporated by reference. The publication describes a process for the preparation of an olefin product comprising ethylene and/or propylene, comprising a step of converting an oxygenate feedstock in an oxygenate-to -olefins conversion system, comprising a reaction zone in which an oxygenate feedstock is contacted with an oxygenate conversion catalyst under oxygenate conversion conditions, to obtain a conversion effluent comprising ethylene and/or propylene.

US 6,737,556 describes a common problem with methanol to olefins (MTO) reaction systems. In a typical MTO reactor system, undesirable by-products may be formed through side reactions. For example, the metals in conventional reactor walls may act as catalysts in one or more side reactions. The patent describes various methods for preventing metal catalyzed side reactions of the methanol in the feedstock, including passivating the metal surface with sulfur-containing chemicals; using a metal alloy containing specific amounts of nickel, chromium, aluminum, or copper; using a non-metal insulating material selected from ceramics, fire brick, high temperature calcium silicate, alumina and silica-alumina ceramics, diatomaceous silica brick and cements and fillers; and maintaining a lower temperature in the feed introduction section.

US 7,763,766 further describes this problem and describes the issue of metal catalyzed coking that occurs when converting an oxygenate in a fluidized bed reaction zone. The patent describes prior art techniques such as adding water or using stainless steel equipment to prevent the formation of coke. The patent describes preventing the formation of coke by using a protective layer that is resistant to metal catalyzed coking. Suitable protective layers can be formed by materials including tin, chromium, antimony, aluminum, germanium, bismuth, arsenic, gallium, indium, lead, copper, molybdenum, tungsten, titanium, niobium, zirconium, tantalum, hafnium, silver, gold, platinum and mixtures, intermetallic compounds and alloys, as well as silicon and alumina. The protective layer can be applied by painting, electroplating, cladding, spraying, chemical vapor deposition, and sputtering.

US 2012/0108876 describes the use of a reactor system for an oxygenate to olefins conversion that has a contact surface made of a material of the formula MX where M is a metal and X is C, or M is a metal or Si and X is N. The publication teaches that these materials provide excellent suppression of methanol decomposition, and that they can be provided as coatings on carbon steel.

Summary of the Invention

The invention provides an oxygenate to olefins conversion reactor comprising an inner metal surface having an average roughness of at least 0.1 mm.

The invention further provides a method of converting a feedstock comprising oxygenates to olefins comprising: providing the feedstock to an oxygenate to olefins conversion reactor; contacting the feedstock with a molecular sieve catalyst at oxygenate to olefin conversion conditions; and removing and separating olefins from the reactor wherein the reactor comprises one or more inner metal surfaces having an average roughness of at least 0.1 mm.

The invention also provides a method of preventing decomposition of a methanol feed to an oxygenate to olefins conversion reactor comprising placing a protective layer of carbonaceous material on at least a portion of the inner metal surface of the reactor, feed nozzle or other apparatus that contacts the methanol as it is fed to the reactor wherein the inner metal surface has an average roughness of at least 0.1 mm.

Brief Description of the Drawings

Figure 1 depicts results from Example 1.

Detailed Description

Contrary to the prior art processes which teach various methods of preventing coke formation in the reactor, it has now been found that it is advantageous to form a layer of carbonaceous material on the internal surfaces of the reactor before or during the initial operation of the process. The formation of the carbonaceous layer prevents additional metal catalyzed side reactions, and this layer is better suited than the protective layers taught by the prior art. The prior art teaches the use of special metals or non-metal surfaces, which can be expensive and/or require additional steps during installation as well as additional

maintenance and inspection to check the integrity of the protective layer. The oxygenate to olefins process will be described herein and a more complete description of the invention and its advantages will follow.

The oxygenate to olefins process receives as a feedstock a stream comprising one or more oxygenates. An oxygenate is an organic compound that contains at least one oxygen atom. The oxygenate is preferably one or more alcohols, preferably aliphatic alcohols where the aliphatic moiety has from 1 to 20 carbon atoms, preferably from 1 to 10 carbon atoms, more preferably from 1 to 5 carbon atoms and most preferably from 1 to 4 carbon atoms. The alcohols that can be used as a feed to this process include lower straight and branched chain aliphatic alcohols. In addition, ethers and other oxygen containing organic molecules can be used. Suitable examples of oxygenates include methanol, ethanol, n-propanol, isopropanol, methyl ethyl ether, dimethyl ether, diethyl ether, di-isopropyl ether,

formaldehyde, dimethyl carbonate, dimethyl ketone, acetic acid and mixtures thereof. In a preferred embodiment, the feedstock comprises one or more of methanol, ethanol, dimethyl ether, diethyl ether or a combination thereof, more preferably methanol or dimethyl ether and most preferably methanol.

The oxygenate to olefins process may, in certain embodiments, also receive an olefin co-feed. This co-feed may comprise olefins having carbon numbers of from 1 to 8, preferably from 3 to 6 and more preferably 4 or 5. Examples of suitable olefin co-feeds include butene, pentene and hexene.

Preferably, the oxygenate feed comprises one or more oxygenates and olefins, more preferably oxygenates and olefins in an oxygenate:olefin molar ratio in the range of from 1000: 1 to 1 : 1, preferably 100: 1 to 1 :1. More preferably, in a oxygenate : olefin mo lar ratio in the range of from 20: 1 to 1 : 1, more preferably in the range of l8: l to 1 : 1, still more preferably in the range of 15 : 1 to 1 : 1, even still more preferably in the range of 13 : 1 to 1 : 1. It is preferred to convert a C4 olefin, including recycled C4 olefins, together with an oxygenate, to obtain a high yield of ethylene and propylene, therefore preferably at least one mole of oxygenate is provided for every mole of C4 olefin.

The olefin co-feed may also comprise paraffins. These paraffins may serve as diluents or in some cases they may participate in one or more of the reactions taking place in the presence of the catalyst. The paraffins may include alkanes having carbon numbers from 1 to 10, preferably from 3 to 6 and more preferably 4 or 5. The paraffins may be recycled from separation steps occurring downstream of the oxygenate to olefins conversion step. The oxygenate to olefins process may, in certain embodiments, also receive a diluent co-feed to reduce the concentration of the oxygenates in the feed to suppress side reactions that lead primarily to high molecular weight products. The diluent should generally be non- reactive to the oxygenate feedstock or to the catalyst. Possible diluents include helium, argon, nitrogen, carbon monoxide, carbon dioxide, methane, water and mixtures thereof. The more preferred diluents are water and nitrogen with the most preferred being water.

The diluent may be used in either liquid or vapor form. The diluent may be added to the feedstock before or at the time of entering the reactor or added separately to the reactor or added with the catalyst. In one embodiment, the diluents is added in an amount in the range of from 1 to 90 mole percent, more preferably from 1 to 80 mole percent, more preferably from 5 to 50 mole percent, most preferably from 5 to 40 mole percent.

During the conversion of the oxygenates in the oxygenate to olefins conversion reactor, steam is produced as a by-product, which serves as an in-situ produced diluent.

Typically, additional steam is added as diluent. The amount of additional diluent that needs to be added depends on the in-situ water make, which in turn depends on the composition of the oxygenate feed. Where the diluent provided to the reactor is water or steam, the molar ratio of oxygenate to diluent is between 10: 1 and 1 :20.

The oxygenate feed is contacted with the catalyst at a temperature in the range of from 200 to 1000 °C, preferably of from 300 to 800 °C, more preferably of from 350 to 700 °C, even more preferably of from 450 to 650°C. The feed may be contacted with the catalyst at a temperature in the range of from 530 to 620 °C, or preferably of from 580 to 610 °C. The feed may be contacted with the catalyst at a pressure in the range of from 0.1 kPa (1 mbar) to 5 MPa (50 bar), preferably of from 100 kPa (1 bar) to 1.5 MPa (15 bar), more preferably of from 100 kPa (1 bar) to 300 kPa (3 bar). Reference herein to pressures is to absolute pressures. A wide range of WHS V for the feedstock may be used. WHSV is defined as the mass of the feed (excluding diluents) per hour per mass of the catalyst. The WHSV should preferably be in the range of from 1 hr^-1 to 5000 hr ¹.

The process takes place in a reactor and the catalyst may be present in the form of a fixed bed, a moving bed, a fluidized bed, a dense fluidized bed, a fast or turbulent fluidized bed, a circulating fluidized bed; or riser reactors, hybrid reactors or other reactor types known to those skilled in the art may be used. In another embodiment, more than one of these reactor types may be used in series. In one embodiment, the reactor is a riser reactor. The advantage of a riser reactor is that it allows for very accurate control of the contact time of the feed with the catalyst, as riser reactors exhibit a flow of catalyst and reactants through the reactor that approaches plug flow.

The feedstocks described above are converted primarily into olefins. The olefins produced from the feedstock typically have from 2 to 30 carbon atoms, preferably from 2 to 8 carbon atoms, more preferably from 2 to 6 carbon atoms, most preferably ethylene and/or propylene. In addition to these olefins, diolefms having from 4 to 18 carbon atoms, conjugated or nonconjugated dienes, polyenes, vinyl monomers and cyclic olefins may be produced in the reaction.

In a preferred embodiment, the feedstock, preferably one or more oxygenates, is converted in the presence of a molecular sieve catalyst into olefins having from 2 to 6 carbon atoms. Preferably the oxygenate is methanol, and the olefins are ethylene and/or propylene.

The products from the reactor are typically separated and/or purified to prepare separate product streams in a recovery system. Such systems typically comprise one or more separation, fractionation or distillation towers, columns, and splitters and other associated equipment, for example, various condensers, heat exchangers, refrigeration systems or chill trains, compressors, knock-out drums or pots, pumps and the like. The recovery system may include a demethanizer, a deethanizer, a depropanizer, a wash tower often referred to as a caustic wash tower and/or quench tower, absorbers, adsorbers, membranes, an ethylene-ethane splitter, a propylene-propane splitter, a butene- butane splitter and the like.

Typically in the recovery system, additional products, by-products and/or

contaminants may be formed along with the preferred olefin products. The preferred products, ethylene and propylene are preferably separated and purified for use in derivative processes such as polymerization processes.

In addition to the propylene and ethylene, the products may comprise C4+ olefins, paraffins and aromatics that may be further reacted, recycled or otherwise further treated to increase the yield of the desired products and/or other valuable products. C4+ olefins may be recycled to the oxygenate to olefins conversion reaction or fed to a separate reactor for cracking. The paraffins may also be cracked in a separate reactor, and/or removed from the system to be used elsewhere or possibly as fuel.

Although less desired, the product will typically comprise some aromatic compounds such as benzene, toluene and xylenes. Although it is not the primary aim of the process, xylenes can be seen as a valuable product. Xylenes may be formed in the OTO process by the alkylation of benzene and, in particular, toluene with oxygenates such as methanol.

Therefore, in a preferred embodiment, a separate fraction comprising aromatics, in particular benzene, toluene and xylenes is separated from the gaseous product and at least in part recycled to the oxygenate to olefins conversion reactor as part of the oxygenate feed.

Preferably, part or all of the xylenes in the fraction comprising aromatics are withdrawn from the process as a product prior to recycling the fraction comprising aromatics to the oxygenate to olefins conversion reactor. In a preferred embodiment, the oxygenate to olefins conversion reactor is operated as a riser reactor where the catalyst and feedstock are fed at the base of the riser and an effluent stream with entrained catalyst exits the top of the riser. In this embodiment, gas/solid separators are necessary to separate the entrained catalyst from the reactor effluent. The gas/solid separator may be any separator suitable for separating gases from solids. Preferably, the gas/solid separator comprises one or more centrifugal separation units, preferably cyclone units, optionally combined with a stripper section.

The reactor effluent is preferably cooled in the gas/solid separator to terminate the conversion process and prevent the formation of by-products outside the reactors. The cooling may be achieved by use of a water quench.

Once the catalyst is separated from the effluent, the catalyst may be returned to the reaction zone from which it came, to another reaction zone or to a regeneration zone.

Further, the catalyst that has been separated in the gas/solid separator may be combined with catalyst from other gas/solid separators before it is sent to a reaction zone or to the regeneration zone.

Catalysts suitable for use in the conversion of oxygenates to olefins may be made from practically any small or medium pore molecular sieve. One example of a suitable type of molecular sieve is a zeolite. Suitable zeolites include, but are not limited to AEI, AEL, AFT, AFO, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, EUO, FER, GOO, HEU, KFI, LEV, LOV, LTA, MFI, MEL, MON, MTT, MTW, PAU, PHI, RHO, ROG, THO, TON and substituted forms of these types. Suitable catalysts include those containing a zeolite of the ZSM group, in particular of the MFI type, such as ZSM-5, the MTT type, such as ZSM-23, the TON type, such as ZSM-22, the MEL type, such as ZSM-11, and the FER type. Other suitable zeolites are for example zeolites of the STF-type, such as SSZ-35, the SFF type, such as SSZ-44 and the EU-2 type, such as ZSM-48. Preferred zeolites for this process include ZSM-5, ZSM-22 and ZSM-23.

A suitable molecular sieve catalyst may have a silica-to-alumina ratio (SAR) of less than 280, preferably less than 200 and more preferably less than 100. The SAR may be in the range of from 10 to 280, preferably from 15 to 200 and more preferably from 20 to 100.

A preferred MFI-type zeolite for the oxygenate to olefins conversion catalyst has a silica-to-alumina ratio, SAR, of at least 60, preferably at least 80. More preferred MFI-type zeolite has a silica-to-alumina ratio, SAR, in the range of 60 to 150, preferably in the range of 80 to 100.

The zeolite-comprising catalyst may comprise more than one zeolite. In that case it is preferred that the catalyst comprises at least a more-dimensional zeolite, in particular of the MFI type, more in particular ZSM-5, or of the MEL type, such as zeolite ZSM-11, and a one- dimensional zeolite having 10-membered ring channels, such as of the MTT and/or TON type.

It is preferred that zeolites in the hydrogen form are used in the zeolite-comprising catalyst, e.g., HZSM-5, HZSM-11, and HZSM-22, HZSM-23. Preferably at least 50wt%, more preferably at least 90wt%, still more preferably at least 95wt% and most preferably 100wt% of the total amount of zeolite used is in the hydrogen form. It is well known in the art how to produce such zeolites in the hydrogen form.

Another example of suitable molecular sieves is siliocoaluminophosphates (SAPOs). SAPOs have a three dimensional microporous crystal framework of P02+, A102-, and Si02 tetrahedral units. Suitable SAPOs include SAPO-17, -18, 34, -35, -44, but also SAPO-5, -8, -11, -20, -31, -36, 37, -40, -41, -42, -47 and -56; aluminophosphates (A1PO) and metal substituted (silico)aluminophosphates (MeAlPO), wherein the Me in MeAlPO refers to a substituted metal atom, including metal selected from one of Group IA, IIA, IB, IIIB, IVB, VB, VIB, VIIB, VIIIB and lanthanides of the Periodic Table of Elements. Preferred SAPOs for this process include SAPO-34, SAPO-17 and SAPO-18. Preferred substituent metals for the MeAlPO include Co, Cr, Cu, Fe, Ga, Ge, Mg, Mn, Ni, Sn, Ti, Zn and Zr.

The molecular sieves described above are formulated into molecular sieve catalyst compositions for use in the oxygenates to olefins conversion reaction. The molecular sieves are formulated into catalysts by combining the molecular sieve with a binder and/or matrix material and/or filler and forming the composition into particles by techniques such as spray- drying, pelletizing, or extrusion. The molecular sieve may be further processed before being combined with the binder and/or matrix. For example, the molecular sieve may be milled and/or calcined.

Suitable binders for use in these molecular sieve catalyst compositions include various types of aluminas, aluminophosphates, silicas and/or other inorganic oxide sol. The binder acts like glue binding the molecular sieves and other materials together, particularly after thermal treatment. Various compounds may be added to stabilize the binder to allow processing.

Matrix materials are usually effective at among other benefits, increasing the density of the catalyst composition and increasing catalyst strength (crush strength and/or attrition resistance). Suitable matrix materials include one or more of the following: rare earth metals, metal oxides including titania, zirconia, magnesia, thoria, beryllia, quartz, silica or sols, and mixtures thereof, for example, silica-magnesia, silica-zirconia, silica-titania, and silica- alumina. In one embodiment, matrix materials are natural clays, for example, kaolin. A preferred matrix material is kaolin.

In one embodiment, the molecular sieve, binder and matrix material are combined in the presence of a liquid to form a molecular sieve catalyst slurry. The amount of binder is in the range of from 2 to 40 wt%, preferably in the range of from 10 to 35 wt%, more preferably in the range of from 15 to 30 wt%, based on the total weight of the molecular sieve, binder and matrix material, excluding liquid (after calcination).

After forming the slurry, the slurry may be mixed, preferably with rigorous mixing to form a substantially homogeneous mixture. Suitable liquids include one or more of water, alcohols, ketones, aldehydes and/or esters. Water is the preferred liquid. In one embodiment, the mixture is colloid-milled for a period of time sufficient to produce the desired texture, particle size or particle size distribution.

The molecular sieve, matrix and optional binder can be in the same or different liquids and are combined in any order together, simultaneously, sequentially or a combination thereof. In a preferred embodiment, water is the only liquid used.

In a preferred embodiment, the slurry is mixed or milled to achieve a uniform slurry of sub-particles that is then fed to a forming unit. A slurry of the zeolite may be prepared and then milled before combining with the binder and/or matrix. In a preferred embodiment, the forming unit is a spray dryer. The forming unit is typically operated at a temperature high enough to remove most of the liquid from the slurry and from the resulting molecular sieve catalyst composition. In a preferred embodiment, the particles are then exposed to ion- exchange using an ammonium nitrate or other appropriate solution.

In one embodiment, the ion exchange is carried out before the phosphorous impregnation. The ammonium nitrate is used to ion exchange the zeolite to remove alkali ions. The zeolite can be impregnated with phosphorous using phosphoric acid followed by a thermal treatment to H+ form. In another embodiment, the ion exchange is carried out after the phosphorous impregnation. In this embodiment, alkali phosphates or phosphoric acid may be used to impregnate the zeolite with phosphorous, and then the ammonium nitrate and heat treatment are used to ion exchange and convert the zeolite to the H+ form. Alternatively to spray drying the catalyst may be formed into spheres, tablets, rings, extrudates or any other shape known to one of ordinary skill in the art. The catalyst may be extruded into various shapes, including cylinders and trilobes.

The average particle size is in the range of from 1-200 μιη, preferably from 50-100 μιη. If extrudates are formed, then the average size is in the range of from 1 mm to 10 mm, preferably from 1.5 mm to 7 mm.

The catalyst may further comprise phosphorus as such or in a compound, i.e.

phosphorus other than any phosphorus included in the framework of the molecular sieve. It is preferred that a MEL or MFI-type zeolite comprising catalyst additionally comprises phosphorus.

The molecular sieve catalyst is prepared by first forming a molecular sieve catalyst precursor as described above, optionally impregnating the catalyst with a phosphorous containing compound and then calcining the catalyst precursor to form the catalyst. The phosphorous impregnation may be carried out by any method known to one of skill in the art.

The phosphorus-containing compound preferably comprises a phosphorus species such as P0₄ ^3~, P-(OCH₃)₃, or P2O5, especially P0₄ ^3~. Preferably the phosphorus-containing compound comprises a compound selected from the group consisting of ammonium phosphate, ammonium dihydrogen phosphate, dimethylphosphate, metaphosphoric acid and trimethyl phosphite and phosphoric acid, especially phosphoric acid. The phosphorus containing compound is preferably not a Group II metal phosphate. Group II metal species include magnesium, calcium, strontium and barium; especially calcium.

In one embodiment, phosphorus can be deposited on the catalyst by impregnation using acidic solutions containing phosphoric acid (H₃P0₄). The concentration of the solution can be adjusted to impregnate the desired amount of phosphorus on the precursor. The catalyst precursor may then be dried. The catalyst precursor, containing phosphorous (either in the framework or impregnated) is calcined to form the catalyst. The calcination of the catalyst is important to determining the performance of the catalyst in the oxygenate to olefins process.

The calcination may be carried out in any type of calciner known to one of ordinary skill in the art. The calcination may be carried out in a tray calciner, a rotary calciner, or a batch oven, optionally in the presence of an inert gas and/or oxygen and/or steam

The calcination may be carried out at a temperature in the range of from 400 °C to 1000 °C, preferably in a range of from 450 °C to 800 °C, more preferably in a range of from 500 °C to 700 ° C. Calcination time is typically dependent on the degree of hardening of the molecular sieve catalyst composition and the temperature and ranges from about 15 minutes to about 2 hours.

In a preferred embodiment, the calcination is carried out in air at a temperature of from 500 °C to 600 °C. The calcination is carried out for a period of time from 30 minutes to 15 hours, preferably from 1 hour to 10 hours, more preferably from 1 hour to 5 hours.

The calcination temperatures described above are temperatures that are reached for at least a portion of the calcination time. For example, in a rotary calciner, there may be separate temperature zones that the catalyst passes through. For example, the first zone may be at a temperature in the range of from 100 to 300 °C. At least one of the zones is at the temperatures specified above. In a stationary calciner, the temperature is increased from ambient to the calcination temperatures above and so the temperature is not at the calcination temperature for the entire time.

The calcination is carried out on a bed of catalyst. For example, if the calcination is carried out in a tray calciner, then the catalyst precursor added to the tray forms a bed which is typically kept stationary during the calcination. If the calcination is carried out in a rotary calciner, then the catalyst added to the rotary drum forms a bed that although not stationary does maintain some form and shape as it passes through the calciner.

The equipment in the reaction zone used for the above described process is typically made of metal, and it is preferred to be able to use the lowest cost metal available for use under the temperature and pressure conditions required by the process. As described previously, oxygenates undergo metal catalyzed side reactions that can result in the production of coke and other by-products, thus reducing the yield of desired products of the process. To prevent the metal catalyzed side reactions, a protective layer of carbonaceous material is formed on the metal surfaces in the reaction zone. The reaction zone includes equipment used for this process which is exposed to one or more of the feed stream, catalyst, intermediate reactants and products at elevated temperatures that may be high enough to cause metal catalyzed side reactions. The metal surfaces of the following equipment may be coated with the protective layer: reactor, feed introduction devices, feed distribution devices, reactor internals, cyclones, conduits, diplegs, standpipes and any associated heat exchange equipment. Additionally the protective layer may be formed on the feed inlet pipe or at least any portion that transports oxygenate at an elevated temperature where metal catalyzed side reactions may occur.

The protective layer is a carbonaceous material and is preferably coke, which may be produced by metal catalyzed side reactions and/or by other reactions carried out inside the reactor. The coke forms on the metal surfaces in the reaction section. Once the protective layer is present, the metal catalyzed side reactions cease as the oxygenate no longer contacts the metal surfaces.

The carbonaceous protective layer is preferably formed soon after startup of the process. As the process equipment is heated up and oxygenate feed is added, the

carbonaceous layer may begin to form on the metal surfaces in the reaction zone. The coke is formed by the metal catalyzed side reactions and the coke adheres to the metal surface as it is formed.

If the metal surface is rough then the coke will adhere better to the surface. Further a rough surface will prevent coke spalling during temperature cycles when the metal and coke expand differently. This can occur when the unit is started up or shut down and the temperature is increased or lowered. The coke and metal have different thermal expansion properties, so during this temperature cycle, the coke tends to separate from the metal, often in large pieces. Coke spalling can result in damage to downstream equipment or plugging of downstream equipment, and in this case, it also results in damage to the protective layer of carbonaceous material.

The metal surface will preferably have an average roughness of at least 0.1 mm, more preferably at least 0.15 mm and most preferably at least 0.2 mm. The average roughness of the metal surface is preferably in the range of from 0.11 mm to 10 mm, more preferably in the range of from 0.15 mm to 5 mm, and most preferably in the range of from 0.2 mm to 3 mm. Average roughness, Ra, is the arithmetic mean of the absolute departures of a roughness profile from the mean line of measurement, as defined in ISO 4287.

In one embodiment, as the process is started up, the temperature is raised to a temperature above the normal operating temperature to increase the rate of formation of the carbonaceous protective layer. This temperature may be maintained until the protective layer is sufficiently formed on one or more of the metal surfaces in the reaction zone.

In another embodiment, the amount of diluent fed to the reactor is decreased below the normal operating amount to increase the rate of formation of the carbonaceous protective layer. The diluent is preferably increased to its normal level once the protective layer is sufficiently formed in the reaction zone. While the prior art described the use of a protective layer in this type of process, the protective layers described in the prior art are expensive and result in increased time for construction, installation and maintenance.

Examples

Example 1

In Example 1, a carbonaceous layer is formed on the inside of a 316L stainless steel tube and the subsequent reduction in methanol decomposition is demonstrated. A feed comprising 6 vol. % methanol was introduced into the stainless steel tube at a temperature of 100 °C. The stainless steel tube did not contain any catalyst. The temperature in the tube was increased to 600 °C at a rate of 2.5 °C/min. When the temperature reached 600 °C, the temperature was maintained for one hour. Then the temperature in the tube was decreased to 350 °C. During the time of the test, the effluent from the tube was analyzed using a mass spectrometer to determine the amount of methanol, hydrogen, carbon dioxide and water. The results from the mass spectrometer are provided in Figure 1. Line A represents the amount of methanol in the effluent. Line B represents the amount of hydrogen in the effluent. Line C represents the amount of C02 in the effluent. Line D represents the amount of water in the effluent.

As can be seen from Figure 1 , as the temperature of the tube increased, a point was reached where the methanol being fed to the tube began to decompose and form additional hydrogen, water and C02. In addition to the methanol decomposition, a protective layer of carbonaceous material was being formed on the walls of the tube. Even though the temperature continued to increase and was maintained at 600 °C, the amount of methanol reaching the tube exit increased showing that less methanol was being decomposed. This shows the effectiveness of the protective carbonaceous layer that was formed in the tube.

Claims

1. An oxygenate to olefins conversion reactor comprising an inner metal surface having an average roughness of at least 0.1 mm.

2. The reactor of claim 1 wherein the inner metal surface has an average roughness of from 0.15 mm to 10 mm.

3. The reactor of claim 1 wherein the inner metal surface has an average roughness of from 0.15 mm to 4 mm.

4. The reactor of claim 1 wherein at least a portion of the inner metal surface is coated with a layer of carbonaceous material.

5. The reactor of claim 4 wherein the carbonaceous material is coke formed by a

decomposition reaction of the oxygenates and/or olefins.

6. The reactor of claim 4 wherein the carbonaceous material is coke formed by the

reaction of oxygenates in the presence of a molecular sieve catalyst.

7. The reactor of claim 1 wherein the inner metal surface is the inner metal surface of one or more of the reactor walls.

8. The reactor of claim 1 wherein the inner metal surface is the surface of one or more of the feed nozzles.

9. A method of converting a feedstock comprising oxygenates to olefins comprising: a) providing the feedstock to an oxygenate to olefins conversion reactor;

b) contacting the feedstock with a molecular sieve catalyst at oxygenate to olefin conversion conditions; and

c) removing and separating olefins from the reactor

wherein the reactor comprises one or more inner metal surfaces having an average roughness of at least 0.1 mm.

10. The method of claim 9 wherein the inner metal surface has an average roughness of from 0.15 mm to 10 mm.

11. The method of claim 9 wherein the inner metal surface has an average roughness of from 0.15 mm to 4 mm.

12. The method of claim 9 wherein the inner metal surface is coated with a layer of

carbonaceous material.

13. The method of claim 12 wherein the carbonaceous material is coke formed by a

decomposition reaction of the oxygenates.

14. The method of claim 12 wherein the carbonaceous material is coke formed by the reaction of oxygenates in the presence of a molecular sieve catalyst.

15. The method of claim 12 wherein the layer of carbonaceous material is formed by raising the temperature in the reactor to a temperature above the oxygenate to olefin conversion conditions.

16. The method of claim 15 wherein the temperature in the reactor is raised to a

temperature above 400 ° C.

17. A method of preventing decomposition of a methanol feed to an oxygenate to olefins conversion reactor comprising placing a protective layer of carbonaceous material on at least a portion of the inner surface of the reactor, feed nozzle or other apparatus that contacts the methanol as it is fed to the reactor wherein the inner metal surface has an average roughness of at least 0.1 mm.

18. The method of claim 17 wherein the inner metal surface has an average roughness of from 0.15 mm to 10 mm.

19. The method of claim 17 wherein the inner metal surface has an average roughness of from 0.15 mm to 4 mm.

0. The method of claim 17 wherein the protective layer of carbonaceous material is formed by metal catalyzed methanol decomposition reactions.