Chen, John Q. (UOP LLC, 25 East Algonquin Road P.O. Box 501, Des Plaines Illinois, 60017-5017, US)
Bjorklund, Bradford L. (UOP LLC, 25 East Algonquin Road P.O. Box 501, Des Plaines Illinois, 60017-5017, US)
Chen, John Q. (UOP LLC, 25 East Algonquin Road P.O. Box 501, Des Plaines Illinois, 60017-5017, US)
BACKGROUND OF INVENTION
 The present invention relates generally to a method of catalyst conservation in an Oxygenate-To-Olefin (OTO) Process utilizing a fluidized oxygenate conversion zone and a relatively expensive catalyst containing such as an ELAPO molecular sieve and the use of a wet scrubbing step that recovers these contaminating catalyst particles in a scrubbing liquid.  The worldwide petrochemical industry is concerned with the production of light olefin materials for use in the production of numerous important chemical products. Light olefins include ethylene, propylene and mixtures thereof. The major source for these materials in present day refining is the steam cracking of petroleum feeds. The art has long sought a source other than petroleum for the raw materials needed to supply the demand light olefin materials. The prior art has focused on different procedures for catalytically converting oxygenates such as methanol into the desired light olefin products. The major focus of routes to produce these desired light olefins has been on methanol conversion technology. US-A- 4387263 which was filed in May of 1982 reports a series of experiments with methanol conversion techniques using a ZSM-5-type of catalyst system wherein the problem of DME recycle is a major focus of the technology disclosed. The good yields of ethylene and propylene reported in the '263 patent were accompanied by substantial formation of higher aliphatic and aromatic hydrocarbons. To limit the production of this heavier material the patentees of the '263 patent proposed to limit conversion to less than 80% of the methanol charged to the MTO conversion step thereby necessitating means for recovering and recycling not only unreacted methanol but also substantial amounts of a DME intermediate product. The '263 patent taught a DME and methanol scrubbing step utilizing a water solvent in order to efficiently and effectively recapture the unreacted methanol and the intermediate DME.  US-A-4587373 teaches the need for higher pressure when operating commercial equipment to maintain reasonable equipment sizes and mass flow rates Which in turn causes dissolution of substantial quantities of DME in the heavy hydrocarbon oil by-product recovered from the liquid hydrocarbon stream withdrawn from the primary separator. The '373 patent also discloses diversion of the methanol feed to the DME absorption zone to recover DME and thereby reduce the size of the scrubbing zone.  US-A-5095163, US-A-5126308 and US-A-5191141 teach the use of a non-zeolitic molecular sieve as a catalytic material to reduce the amounts of undesired C4"1" hydrocarbon products produced relative to a ZSM-5 type of catalyst system. The non-zeolitic molecular sieve generally comprises a metal aluminophosphate (ELAPO) and more specifically a silicoaluminophosphate molecular sieve (SAPO).  US-B-6403854, US-A-6166282 and US-A-5744680 point to the use of a fluidized reaction zone along with a fluidized regeneration zone as the preferred commercial solution to the problem of effectively and efficiently using an ELAPO or SAPO-type of catalyst system in OTO service. The use of this technology gives rise to a substantial problem of solid- vapor separation to efficiently separate the particles of the fluidized catalyst from the vapor components exiting the OTO conversion zone. US-A-6166282 shows a series of three cyclonic separation means to separate spent OTO catalyst from the product effluent stream. There still remains a very substantial problem of OTO or catalyst contamination of the product effluent stream withdrawn from the fluidized conversion zone.  US-A-5744680 discloses the use of a wet scrubbing step on the cooled effluent stream from an OTO conversion zone to remove ELAPO molecular sieve-containing catalyst particles from this effluent stream but merely teaches the withdrawal of the catalyst-containing bottom stream from the wet scrubbing step for further unspecified treatment. US-A-6121504 uses wet scrubbing to quench the effluent stream from the OTO conversion zone and produce a bottom stream which is recirculated to the wet scrubbing stream except for a drag stream that enters a stripping zone for purposes of heat recovery. US-B-6403854 exemplifies a quench arrangement for the hot effluent stream recovered from the OTO conversion zone with first stage that removes catalyst fines entrained in the product effluent stream. US-B-6870072 discloses the problem of product effluent contamination with catalyst particles and uses a wet scrubbing zone to remove these contaminating particles but no means of recovery and reuse of the catalyst particles.  A substantial economic problem for the OTO process is the amount of fresh catalyst that must be added to the OTO or fluidized conversion zone in order to maintain the catalyst inventory in the OTO conversion system when the product effluent stream contains substantial amounts of contaminating catalyst particles. This problem of effluent contamination by catalyst particles is because of the relatively high expense ELAPO or SAPO molecular sieves. Currently the equivalent cost of an ELAPO-containing catalyst system is expected to cost 5 to 40 times that of a zeolitic system. This invention addresses the problem of reducing the loss of catalyst particles from a fluidized OTO conversion zone to decrease the consumption of relatively expensive catalyst thereby improving the economics of the OTO conversion process.  The solution envisioned and provided by the present invention to this catalyst loss problem involves the use of a wet scrubbing step designed to recover substantially all of the product effluent contaminating catalyst particles and to provide a slurry of these catalyst particles in a scrubbing liquid such as water with subsequent recycle of at least a portion of the catalyst particles contained in the resulting slurry to the OTO conversion zone or to the associated deactivated OTO catalyst regeneration zone thereby recapturing the catalytic activity of these contaminating catalyst particles and diminishing the need for adding fresh catalyst to make-up for catalyst losses. It has been found that the catalytic activity of these effluent-contaminating catalyst particles will survive not only the hydrothermal shock associated with the introduction of these relatively hot particles contained in this product effluent stream into a relatively cool wet scrubbing zone which captures these particles by immersion in a scrubbing liquid which is typically aqueous but also the direct return of these particles, after an optional concentration step, to either the relatively hot OTO conversion zone or to the relatively hot spent OTO catalyst regeneration zone without any additional treatment. Quite surprisingly the catalytic activity of these recaptured catalyst particles survives the hydrothermal shocks associated with introduction into the wet scrubbing zone as well as the thermal shocks associated with return to the OTO conversion zone or to the associated catalyst regeneration zone. Thus far the prior art failed to show the reuse of these product effluent-contaminating catalyst particles in order to recapture their catalytic activity. The invention thus substantially improves the economics of the overall process by diminishing the need for fresh catalyst make-up to compensate for this source of catalyst loss.
SUMMARY OF THE INVENTION
 The primary objective to the present invention is to provide a technically feasible solution to the problem of catalyst loss in the product effluent from an OTO reaction zone utilizing a fluidized reactor system and a relatively expensive catalyst system. A secondary objective is to improve the economics of an OTO process that utilizes a fluidized reaction zone in combination with a relatively expensive catalyst system containing an ELAPO molecular sieve by diminishing the requirement for make-up fresh catalyst.  This invention solves the problem of the loss of expensive ELAPO-containing catalyst particles in the product effluent stream withdrawn from the fluidized OTO conversion zone by the use of a direct recycle step to return at least a portion of the resulting catalyst slurry from the effluent stream wet scrubbing step to either the OTO conversion zone or to the associated regeneration zone thereby recapturing the activity of these catalyst particles. An associated finding is that an ELAPO-molecular sieve-containing catalyst system tolerates the hydrothermal shock associated with the wet scrubbing step which is typically run at a temperature considerably lower than the temperature of the entering effluent stream and also tolerates the thermal shock associated with recycling at least a portion of the relatively cool slurry of catalyst particles back to the hot OTO conversion zone or to the hot catalyst regeneration zone and that the activity of an ELAPO molecular sieve-containing particles survives immersion in a scrubbing liquid when covered with a protective layer of carbonaceous deposits. It is believed that the carbonaceous deposits or coke deposited on the catalyst particles in the OTO conversion zone may insulate the ELAPO molecular sieve from the adverse effects of immersion in a scrubbing liquid that is typically aqueous in nature.  A first embodiment of the invention is a process for the catalytic conversion of a feedstream containing an oxygenate by contacting a catalyst containing an ELAPO molecular sieve in a fluidized conversion zone including recovery and recycle of contaminating catalyst particles from the product effluent stream withdrawn from the fluidized conversion zone. The process contacts the feedstream with the fluidized catalyst in the fluidized conversion zone at conversion conditions effective to form a mixture of the activated catalyst particles and olefmic reaction products. In a vapor-solid separating zone containing one or more vapor-solid cyclonic separators, at least a portion of the deactivated catalyst particles are separated from the resulting mixture to form a stream of deactivated catalyst particles and a conversion zone product effluent stream containing light olefins, unreacted oxygenates, H2O, other reaction products and undesired amounts of contaminating catalyst particles. The resulting product effluent stream passes to a wet scrubbing zone and therein contacts a scrubbing liquid under scrubbing conditions effective to form a substantially catalyst- free vaporous overhead stream containing light olefins, unreacted oxygenates, olefmic by-products and water and a liquid bottom stream containing a mixture of the contaminating catalyst particles and the scrubbing liquid. At least a portion of the stream of deactivated catalyst particles separated in the vapor- solid separating zone is passed to a regeneration zone and therein contacted with an oxidizing gas stream under oxidizing conditions effective to form a stream of regenerated catalyst particles. At least a portion of the liquid bottom stream produced in the wet scrubbing zone is recycled either to the OTO conversion step or the catalyst regeneration step. At least a portion of the stream of freshly regenerated catalyst particles recovered from the regeneration step is recycled to the OTO conversion zone.  A second embodiment differs from the first embodiment by passing at least a portion of the liquid bottom stream recovered from the wet scrubbing zone to a liquid-solid separating zone containing one or more liquid-solid separating means operated under separating conditions effective to produce a solid-rich stream containing a relatively rich slurry of contaminated catalyst in the scrubbing liquid and a relatively solid-lean stream containing scrubbing liquid. At least a portion of the solid-rich stream is then recycled to the OTO conversion step or to the deactivated catalyst regeneration step in order to recapture the activity value of the contaminating particles.  A highly preferred embodiment of the present invention comprises an OTO conversion process as described above in the first embodiment wherein the oxygenate present in the feedstream is methanol or dimethylether or a mixture thereof and wherein the ELAPO molecular sieve is a SAPO molecular sieve having its crystal structure corresponding to SAPO-34 or SAPO- 17 and wherein the scrubbing liquid used in the wet scrubbing zone is water optionally containing an alkaline reagent compatible with SAPO-34 and SAPO- 17 in an amount sufficient to neutralize a significant portion of any acidic by-products of the oxygenate conversion reaction that are present in the product effluent stream withdrawn from the OTO conversion zone.  A heat integrated embodiment of the present invention involves the process as described above in the first embodiment wherein the conversion zone product effluent stream which is at a temperature of 350° to 600°C upon exit from the OTO conversion zone is substantially cooled between the conversion step and the wet scrubbing step by use of steam generation and/or by indirect heat exchange against the oxygenate feedstream in order to recapture at least a portion of the exothermic heated reaction liberated in the OTO conversion zone by using it to preheat and vaporize at least a portion of the feedstream charged to the OTO process. BRIEF DESCRIPTION OF THE DRAWING
 The FIGURE is a process flow diagram of a preferred integrated embodiment of the present invention.
TERMS AND CONDITIONS DEFINITIONS
 The following terms and conditions are used in the present specification with the following meanings: (1) A "portion" of a stream means either an aliquot part that has the same composition as the whole stream or a part that is obtained by eliminating a readily separable component therefrom. (2) An "overhead" stream means the net overhead recovered from the specified zone after recycle of any portion to the zone for reflux or any other reason. (3) A "bottom" stream means the net bottom stream from the specified zone obtained after recycle of any portion for purposes of reheating and/or reboiling and/or after any phase separation. (4) A "vapor" stream means a stream containing one or more components in the gaseous state. (5) The term "light olefins" means ethylene, propylene and mixtures thereof. (6) The expression "ELAPO" molecular sieve means a material having a three-dimensional microporous framework structure of ALO2, PO2 and ELO2 tetrahedral units having the empirical formula:
where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof, x is the mole fraction of EL and is at least 0.005, y is the mole fraction of Al and is at least 0.01 z is the mole fraction of P and is at least 0.01 and x+y+z = 1. When EL is a mixture of metals, x represents the total amount of the metal mixture present. Preferred metals (EL) are silicon, magnesium and cobalt with silicon being especially preferred. (7) The expression "SAPO molecular sieve" means an ELAPO molecular sieve wherein the EL element is silicon as described in US-A-4440871.
DETAILED DESCRIPTION OF THE INVENTION
 In the instant OTO process, the feedstream comprises one or more oxygenates. The term "oxygenate" is employed herein to include alcohols, ethers, and carbonyl compounds (e.g. aldehydes, ketones, carboxylic acids, and the like). The oxygenate feedstock preferably contains at least one oxygen atom and 1 to 10 carbon atoms and, and preferably, contains from 1 to 4 carbon atoms. Suitable oxygenates include lower straight or branched chain alkanols, and their unsaturated counterparts. Representatives of suitable oxygenate compounds include methanol, dimethyl ether (DME), ethanol, diethyl ether, methylether, formaldehyde, dimethyl ketone, acetic acid, and mixtures thereof.  In the OTO conversion step of the present invention, the oxygenate feedstock is catalytically converted to hydrocarbons containing aliphatic moieties such as methane, ethane, ethylene, propane, propylene, butylene, and limited amounts of other higher aliphatics by contacting the feedstock with an ELAPO-containing catalyst. A diluent may be used to maintain the selectivity of the catalyst to produce light olefins, particularly ethylene and propylene at ratios of 1 mole of oxygenates to 0.1-5 moles of diluent. The preferred diluent is steam.  The oxygenate conversion step of the present invention is preferably conducted such that the oxygenate feedstock is contacted in a vapor phase in a reaction zone with a ELAPO molecular sieve catalyst at effective conversion conditions to produce olefinic hydrocarbons, i.e., an effective temperature, pressure, weight hourly space velocity (WHSV) and, optionally, an effective amount of diluent. The OTO step is affected for a period of time sufficient to produce the desired light olefin products. The oxygenate conversion step is effectively carried out over a wide range of pressures, including autogenous pressures. Pressures generally range between 0.1 atmospheres (10.1 kPa) and 100 atmospheres (10.1 MPa), with preferred pressures between 0.5 atmospheres (50.6 kPa) and 20 atmospheres (2.0 MPa). The pressure will more preferably range from 1 to 10 atmospheres (101.3 to 1013.3 kPa). The pressures referred to herein are exclusive of any diluent and refer to the partial pressure of the oxygenate feedstock. The temperature which may be employed in the oxygenate conversion step may vary over a wide range depending in general range from 350° and 6000C.  Preferred ELAPO catalysts have relatively small pores and a substantially uniform pore structure, e.g., substantially uniformly sized and shaped pore with an effective diameter of less than 5 angstroms. A preferred ELAPO molecular sieve is one in which the element (EL) content varies from 0.005 to 0.2 mole fraction and in which EL is silicon (usually referred to as SAPO). The SAPOs which can be used in the instant invention are preferably any of those described in US-A-4440871; US-A-5126308, and US-A-5191141. Especially preferred SAPOs include the SAPO-34 and SAPO- 17 structures with SAPO-34 being most preferred.  The ELAPO catalyst is preferably incorporated into solid particles containing one or more matrix materials in which the catalyst is present in an amount effective to promote the desired oxygenate conversion reactions. Matrix materials are preferably selected from the group consisting of binder materials, filler materials, and mixtures thereof in an amount selected to provide desired properties, e.g., desired catalyst dilution, mechanical strength, and the like to the solid particles. Such matrix materials are preferably porous in nature and may contribute to or promote one or more of the desired oxygenate conversion reactions- particularly the conversion of methanol to DME. If matrix materials, e.g., binder and/or filler materials, are included in the catalyst composition, the molecular sieves preferably comprise 1 to 99 percent, more preferably 5 to 90 percent and still more preferably 5 to 60 percent, by mass of the total composition.  The oxygenate conversion reactions deposit, a carbonaceous material, i.e., coke, in an amount of 1 to 20 mass-% and more commonly 1.5 to 9 mass-%. The carbonaceous deposit material reduces the number of available active sites on the catalyst which thereby affects the extent of the conversion. During the OTO conversion step a portion of the coked catalyst is withdrawn from the OTO reaction zone and passed to a regeneration step where it is regenerated with an oxygen-containing medium (e.g. air) to remove at least a portion of the carbonaceous material and returned to the oxygenate conversion reaction zone. Depending upon the particular catalyst and conversion, it may be desirable to substantially remove the carbonaceous material e.g., to less than 0.5 mass-%, or only partially regenerate the catalyst, e.g., to from 1 to 3 mass-% carbonaceous material. Preferably, the regenerated catalyst will contain 0 to 3 mass-% and more preferably from 0 to 1 mass-% carbonaceous material (i.e. coke). During regeneration there can be additional oxidation of sulfur and in some instances nitrogen compounds along with the removal of any contaminant metal materials from the catalyst. Regeneration conditions can be varied depending upon the type of ELAPO catalyst used and the type of contaminant material present upon the catalyst prior to its regeneration.  A fluidized bed type of system creates the unique problem of recovering the ELAPO catalyst particles from the product effluent stream. A fluidized system continuously transports large amounts of finely divided catalyst particles between a reaction zone and a regeneration zone that in the OTO reaction zone are admixed with the oxygenate feedstream in an amount which is conveniently measured in terms of a WHSV calculated on the basis of mass hourly flow rate of the sum of the mass of oxygenate reactants passed to the MTO conversion zone plus any other oxygenate or hydrocarbon reactants present in the feed or recycle streams divided by the mass of the ELAPO catalyst present in the OTO conversion zone. WHSV in the fluidized OTO conversion zone can range from 0.1 to 100 hr* with best results obtained within the range of 0.5 to 40 hr"l. The strongly exothermic reactions will increase temperatures across the OTO reaction zone by 100° to 400°C (180° to 720°F) and the catalyst circulation rate between the reactor and the regenerator will ordinarily be set at a minimum level to hold average coke on the ELAPO catalyst entering the conversion step in the range of 1 to 20 mass-% of the active ingredient in the catalyst and more preferably in the range of 1.5 to 9 mass-%. The fluidized catalyst recirculated to the OTO conversion reactor is intimately admixed with the vaporous oxygenate containing feedstream for the time dictated by the desired WHSV and thereafter the resulting vapor-solid mixture is quickly separated in to provide a stream of deactivated catalyst that at least in part is charged to the regeneration zone thereby completing the catalyst circulation loop. To achieve proper fluidization of the ELAPO catalyst system, the catalyst particles are typically in a particle size distribution of 1 to 150 microns with the average particle size usually set in the range of 20 to 100 microns and preferably in the range of 65 to 85 microns. Interaction between catalyst particles and between catalyst particles and equipment surfaces will degrade the particle size distribution over time and generate a significant amount of catalyst fines.  A fluidized bed catalyst system with a fast-fluidized reactor system is particularly preferred and shown in US-A-6166282. The fluidized catalyst system of this invention may use several stages of catalyst separation as shown in the '282 patent. At least one of these stages can be located in a separate surge vessel. Although any catalyst separation may be used, typically each stage of separation will comprise a cyclonic separation action that employs a tangential discharge of the mixture of reaction products and catalyst particles. Multiple stages of separation may involve closed-couple cyclones by inputting separated vapor stream from an upstream cyclone directly into a downstream cyclone. Depending on the exact fluidization conditions, reaction zones of the type shown in the '282 patent can produce product effluent streams with a catalyst particle concentration ranging from 0.01 to 0.1 mass-% and more typically from 0.015 to 0.05 mass-% of the product effluent stream. These quantities of effluent-contaminating catalyst particles represent over time a significant loss of the relatively expensive ELAPO catalyst system.  The present invention is further described in reference to an MTO embodiment using a preferred SAPO-34. The MTO embodiment utilizes methanol as the principal source of the oxygenate reactant. The SAPO-34 catalyst ordinarily used in a fluidized reactor system has an average particle size of 65 to 85 microns.  The fluidized MTO reaction zone using a SAPO-34 catalyst is operated at conditions, which include a temperature of 350° to 600°C (662° to 1112°F) with the preferred range being 450° to 550°C (842° to 1022°F). The pressure used in the MTO conversion step is typically in the range of 138 to 1000 kPa (20 to 145 psia) and preferably from 170 to 345 kPa (24.7 to 50 psia). WHSV for use in the MTO conversion zone can range from 0.1 to 100 nr~l, with the best results obtained in the range of 0.5 to 20 hr 1. The external catalyst circulation for the MTO reactor and the regenerator is the same as that previously described.  The regeneration step for MTO conversion step will ordinarily use one of the previously described oxidative techniques for removing the necessary amount of coke from the catalyst prior to recirculation to the conversion zone. The equilibrium value of coke on catalyst again primarily establishes the circulation rate between the conversion zone and the regeneration zone and SAPO-34 based catalyst systems run quite successfully at conversion levels of 95% or higher and result in a coke made of 0.6 to 10.4 mass-% of methanol equivalent and more typically 2 to 5 mass-% of methanol equivalent. Preferably the methanol feed stream is 95 to 99.9 mass-% methanol. Although the methanol feedstock charged to the MTO conversion step can use a diluent as described, ordinarily the only diluent will comprise the autogenously produced steam. The kinetics of the reactions in the MTO reaction zone form DME extremely fast and result in the formation of one mol of a steam diluent for every 2 mols of methanol that react to produce DME. While diluent use benefits the control of the methanol reactant partial pressure of the methanol reactant, it disadvantageously increases the volume of the reaction zone providing additional material requiring separation from the products. The preferred diluent for the MTO conversion step is steam derived from the water that inevitably contaminates the methanol feed stream and the recycle oxygenate streams, hi many cases, the crude methanol feed stream may contain up to 20 wt-% water and will bring substantial diluent into the system. DETAILED DESCRIPTION OF THE DRAWING
 The attached FIGURE schematically shows the interconnections and interactions between the six zones that form a preferred embodiment of the invention. A line 7 charges MTO conversion zone 1 with a methanol-containing feedstream that preferably is a substantially vaporous stream. A line 21 charges zone 1 with a stream of fluidizable SAPO-34- containing catalyst particles. Fresh catalyst is typically added to line 21 via an interconnecting line (not shown) in an amount sufficient to replenish the fluidizable catalyst inventory that circulates in and through zone 1 and to and from associated regeneration zone 4. Zone 1 operates in a fast-fluidized mode of operation preferably using the fast-fluidized bed reactor shown in US-A-6166282. Zone 1 operates in accordance with the MTO conversion conditions previously discussed to produce a mixture of deactivated catalyst particles and olefmic reaction products. This mixture travels up the riser section of the reaction zone (not shown) and goes through a series of three stages of vapor-solid separation operations to produce a stream of deactivated catalyst particles and a conversion zone product effluent stream containing light olefins, unreacted oxygenates, H/jO, other reaction products and undesired amounts of contaminating catalyst particles. The highly exothermic MTO reaction in zone 1 coats the outer surface of the catalyst particles with a layer of carbonaceous material that deactivates the catalyst particles at least in part.  At least a portion of these catalyst particles have their activity restored in catalyst regeneration zone 4. A catalyst stream circulates between zone 1 and zone 4 via lines 22 and 21. Following cyclonic separation, at least a portion of the deactivated catalyst material is stripped of volatile hydrocarbons in zone 1 and passed via line 22 to regeneration zone 4 wherein at least a significant portion of the carbonaceous deposits are oxidatively removed to produce a stream of regenerated catalyst particles which flow via line 21 for reuse in zone 1. Despite the use of one or more vapor-solid cyclonic separators, a significant amount of catalyst particles are present in the product effluent stream. These contaminating catalyst particles present a substantial risk to downstream compression means and therefore must be removed prior to olefmic product recovery in downstream recovery and purification zones. The degree of contamination of the product effluent stream by these catalyst particles corresponds to 0.01 to 0.1 mass-% of the effluent product stream.  The product effluent stream passes from zone 1 via line 8 to a catalyst wet scrubbing step performed in zone 2 and contacts a scrubbing fluid to remove the contaminating catalyst particles contained therein. The relatively hot product effluent stream preferably undergoes cooling prior to entering zone 2 via one or more techniques (not shown) such as passage through a steam generation vessel (i.e. a boiler) and/or conventional indirect heat exchange against the methanol feedstream. Preferably the cooling will reduce the product effluent stream temperature below the point of creating excessive flashing of the scrubbing fluid in zone 2 but not below the dew point of the stream to prevent precipitation of catalyst particles and the forming of fouling deposits on the walls of line 8. The product effluent stream leaves zone 1 at a temperature in the range of 350° to 600°C and preferably sufficient cooling steps will reduce its temperature to about 110° to 300°C when it enters zone 2. The temperature of the product effluent stream will not of course be reduced  hi wet scrubbing zone 2, the catalyst-contaminated effluent product stream is contacted with a descending stream of liquid scrubbing fluid which enters an upper region of zone 2 via line 9 and flows countercurrent to the ascending vapor-solid effluent stream. Zone 2 may include suitable means for increasing vapor-liquid contact such as appropriately sized solid packing materials as well as suitably designed trays and/or baffles. Zone 2 preferably uses an aqueous scrubbing solution that is pumped around zone 2 in a pump-around loop represented by line 9. This scrubbing solution undergoes substantial cooling (not shown) in line 9 prior to reintroduction to facilitate a partial quench of the vaporous effluent product stream entering zone 2. Condensation of at least a portion of the water by-product from the MTO conversion reaction in zone 2 or in a downstream vessel such as zone 3 can replenish an aqueous scrubbing solution. The by-products of the MTO conversion zone can include one or more acidic material such as organic acids which are preferably neutralized at least in part via the addition to the aqueous scrubbing fluid of a suitable alkaline material that is compatible with the SAPO-34 catalyst in amounts sufficient to neutralize at least a substantial portion of these acidic materials in order to prevent corrosion and fouling of the wet scrubbing zone as well as the downstream flow conduits and equipment. This alkaline reagent such as a suitable amine can be added to the scrubbing liquid circulating in line 9 via an alkaline reagent injection line (not shown). Zone 2 operates at scrubbing conditions effective to produce an overhead vapor stream 10 which is substantially free of particles of the SAPO-34 catalyst system and essentially comprises the olefmic and other hydrocarbon products of the MTO conversion reactions occurring in zone 1 plus all or a substantial portion of the by-product water and any unreacted oxygenates such as methanol and DME that accompany this material. The liquid bottom stream produced in zone 2 comprises a mixture of the contaminating catalyst particles in the scrubbing liquid. Depending on operating conditions, a portion of the water by¬ product produced in MTO conversion zone 1 as well as a portion of the water soluble products and reactants may be removed from zone 2 along with the contaminating catalyst particles via line 9. Zone 2 also functions as a first stage quench zone reducing the temperature of the vapor overhead stream withdrawn via line 10 by 10 to 200°C relative to the temperature of the material entering zone 2 via line 8. The circulation rate of scrubbing liquid around and through zone 2 via line 9 produces a mass ratio of the scrubbing liquid to the entering product effluent stream of 0.5:1 to 3:1 and preferably of 0.8:1 to 1.5:1. Preferably wet scrubbing zone 2 will concentrate the catalyst particles such that the circulating catalyst slurry contains 0.5 to 5 mass- % catalyst particles and preferably 0.35 to 0.65 mass-% catalyst particles. Preferably line 16 continuously withdraws a drag stream from this circulating scrubbing fluid (or catalyst slurry) via line 9 and that is passed to a catalyst slurry dewatering zone 5 to further concentrate this slurry of catalyst particles in scrubbing fluid prior to its recirculation to the MTO conversion zone 1 or to regeneration zone 4. Where scrubbing zone 2 is operated at conditions that result in the flashing of scrubbing fluid and the addition of water, make-up water can be added to line 9 via line 25.  The overhead vapor stream recovered from scrubbing zone 2 via line 10 is passed to effluent quench zone 3 that further cools the vapor stream to remove the residual portion of the by-product water not recovered in wet scrubbing zone 2 and any scrubbing fluid flashed off in zone 2. Line 10 may have one or more intercoolers located therein (not shown) that can further lower the temperature of the overhead stream from the wet scrubbing zone 2 and allow heat integration with various other streams charged to one or more of the conversion zones. The quenching fluid used in zone 3 is preferably water and it is injected into the upper region of zone 3 where it counter currently contacts an ascending vaporous stream entering the zone via line 10. The quenching conditions maintained in zone 3 are sufficient to further quench the vaporous portion of the product effluent stream to further reduce the water content and unreacted oxygenate content of the stream form line 10. Zone 3 produces an olefm-rich overhead vapor stream containing trace amounts of unreacted oxygenates (primarily methanol and DME) which is passed via line 12 to downstream facilities for further purification and recovery of the light olefins. The liquid bottom stream from zone 3 is an aqueous quenching stream that is preferably pumped around zone 3 via line 11 through pumping means (not shown) and further cooled via cooling means (not shown) to produce a quenching medium with a temperature of 20° to 75°C prior to injection into quenching zone 3. A water-rich drag stream is withdrawn from this circulating quenching fluid loop via lines 11 and 13 and at least a portion of this drag stream is charged to the upper region of oxygenate stripping zone 6. In the case where zone 2 operates to consume water, at least a portion of this drag stream can pass to zone 2 via lines 13, 25, 26 and 9.  Oxygenate stripping zone 6 strips any unreacted oxygenates such as methanol and DME from the aqueous streams to produce a relatively pure water stream which is withdrawn from the bottom of the stripping zone via line 14 and is available for further use in the process. Stripping zone 6 is operated at oxygenate stripping conditions effective to produce an overhead vapor stream which exits zone 6 via line 15 and comprises a significant portion of the net unreacted oxygenates recovered from the effluent stream from zone 1 in zones 2 and 3 and available for recirculation to zone 1 via line 7. All or a portion of overhead stream 15 may be routed to zone 3 or into line 12, via transfer lines (not shown) to consolidate the unreacted oxygenate recovery and recycle with the downstream purification of the overhead stream in line 12.  Optional catalyst slurry dewatering zone 5 further concentrates the catalyst particles recovered in wet scrubbing zone 2 prior to recycle of these recovered particles to conversion zones 1 or regeneration zone 4. Zone 5 diminishes the amount of scrubbing fluid that accompanies the recycled stream of catalyst particles. Zone 5 will comprise a set of one or more solid-liquid separating means operating in series or in parallel or a combination thereof to dewater the catalyst slurry that enters the upper region of zone 5 via line 16. Suitable liquid- solid separating means include hydrocyclones, filters, centrifuges, slurry settlers and combinations of one or more of these separating means. Liquid-solid cyclones or hydrocyclones are preferred for this application. In some heavy load operations zone 5 may connect together a series of one or more trains of liquid-solid cyclones. Line 18 withdraws a solid-rich stream from zone 5. Line 17 withdraws a solid-lean liquid stream from zone 5.  The solid-lean stream from line 17 primarily comprises the aqueous scrubbing liquid used in zone 2 with trace amounts of very fine particles of the SAPO-34 catalyst that contaminated the effluent stream withdrawn from MTO conversion zone 1. As previously explained, at least a portion of this overflow stream is preferably passed via lines 25 and 9 to wet scrubbing zone 2 when zone 2 is operated with net water consumption. A drag stream taken from line 17 prevents the build-up of fine catalyst particles in the circuit between zones 2 and 5. This drag stream is passed downstream for recovery of any unreacted oxygenates contained therein and for suitable disposal of the very fine particles of catalyst contained therein. The solid-rich stream withdrawn from dewatering zone 5 via line 18 is a relatively concentrated slurry of SAPO-34 catalyst particles in scrubbing liquid and the catalyst particles will comprise 5 to 30 mass-% of this underflow stream. The underflow stream 18 contains catalytic SAPO-34 material that is reused in promoting additional MTO conversion reactions thereby substantially lessening the amount of fresh catalyst that is necessary to add to zone 1 to maintain the inventory of circulating catalyst that passes in and through zones 1 and 4. This underflow stream can reinj ect these recovered catalyst particles at a number of different points in the circulating catalyst stream flowing in and around zones 1 and 4. This material can be directly injected into MTO conversion zone via line 18 wherein the water contained in this stream will serve as diluent to the MTO conversion reaction. Preferably injection into the MTO conversion zone is into a stripping zone which serves a part of the catalyst disengagement zone. Alternatively or in combination, this injection of this recycled catalyst stream can pass directly into the regeneration zone via line and 19. A less preferred choice injects these recovered catalyst particles directly into a return conduit in the form of a standpipe which is at a relatively high temperature and contains regenerated particles. Line 20 schematically illustrates passage of this material from line 18 to line 21 which for this purpose represents a standpipe.  An oxygen-containing regeneration enters gas zone 4 via line 24. An oxidation reaction is performed in regeneration zone 4 utilizing sufficient oxygen to establish combustion in regeneration zone 4 thereby converting the deactivating carbonaceous deposits into water and CO2 which are eliminated from the process as flue- gas via a line 23. Line 23 will also typically contain an amount of very fine catalyst particles that are removed from this flue- gas stream prior to its release into the atmosphere utilizing various separation techniques. Flue-gas line 23 constitutes the principal means for elimination from the circulating catalyst stream utilized in zones 1 and 4 of any very fine particles that are produced by attrition and fragmentation of the SAPO-34 catalyst system. The present invention recovered approximately 75 to 99 plus mass-% of the catalyst particles exiting MTO conversion zone 1 via line 8 for recycle to MTO conversion zone 1 and for further use therein. This stands in sharp contrast to the prior art schemes that utilize the wet scrubbing zone but discarded the resulting recovered catalyst particles into a waste stream apparently not recognizing that these catalyst particles retain substantial activity for promoting the desired MTO conversion reaction.
 To demonstrate the protective effect of coke content on the activity-stability characteristics of an MTO catalyst-containing SAPO-34 when it is immersed in an aqueous scrubbing solution in accordance with the wet scrubbing step of the present invention, an experiment was conducted in which a sample of a typical non-zeolitic MTO catalyst was subjected to an aqueous immersion step with and without the protective benefit of a layer of coke deposits. In addition, a second control catalyst was tested in order to benchmark the activity-stability performance of a coke-free catalyst that had not been subjected to an aqueous immersion step.  The composition of the typical non-zeolitic MTO catalyst was 40 mass-% SAPO- 34, 40-mass-% kaolin clay and 20 wt-% silica-alumina binder. The SAPO-34 material was synthesized in accordance with the methodology specified in US- A- 5191141. This catalyst in a coke-free form had a piece density of 1.075 g/cc and an average particle size of 75 microns (i.e. micrometers). This catalyst had been used to catalyze MTO conversion reactions in a fluidized reaction zone until it had accumulated an equilibrium level of coke which corresponded to 6 mass-% carbonaceous deposits. The catalyst was then stripped of volatile material and then divided into 3 portions. The first portion, hereinafter referred to as Catalyst A, was a sample of the coke-containing catalyst as recovered from the fluidized pilot plant unit. Three separate 50 gram samples of Catalyst A were then taken and subjected to a sequence of a water immersion step followed by a regeneration step conducted under the conditions specified below to prepare a coke- free catalyst for testing in the activity-stability test below. The second portion of the starting catalyst, hereinafter referred to as Catalyst B, was subjected to the regeneration step described below to prepare a coke- free material which was then subjected to the aqueous immersion step described below. The third portion, hereinafter referred to as Catalyst C, was subjected to the regeneration step described below in order to produce a coke-free catalyst which was tested directly in the activity-stability test described below without any exposure to the aqueous immersion step in order to provide a fully regenerated control catalyst for the experiment. Catalysts B and C are both control catalysts.  The water immersion step in this experiment subjected a 50 gram sample of the particular catalyst to immersion in 100 to 500 grams of distilled water at a temperature and a holding time which is specified in Table 1. In all cases sufficient pressure was maintained on the mixture of catalyst and water to maintain a liquid-phase condition for the period of immersion.  The regeneration step used in the experiment subjected the coke-containing catalyst sample to a drying step involving exposure to a dry gas stream for 3 to 12 hours at a temperature of 100° to 1200C. The resulting dried catalyst sample was then subjected to a treatment with an air stream at coke combustion conditions which included a temperature of 650°C for a coke combustion period of 5 hours which in all cases was sufficient to remove substantially all of the coke deposits.  The resulting regenerated catalysts were then subjected to an MTO conversion breakthrough test designed to measure catalyst activity-stability which involved loading 10 grams of the regenerated catalyst sample into a fixed bed pilot plant reactor and charging thereto a vaporized feed comprising 80 mass-% methanol and 20 mass-% distilled water (i.e. steam) present as a diluent. During the test, the reactor was maintained at a pressure of 138 kPa (5 psig), a WHSV of 2.5 hr~l (based on grams of methanol charged per hour/grams of SAPO- 34 present in the reactor) and a reactor inlet temperature of 435°C (8150F) measured near the point of introduction of vaporized feed. The test in all cases was conducted until one mass-% of unreacted oxygenates (i.e. methanol and/or dimethylether) was detected in the product effluent stream at which point the conversion level in the reaction zone had dropped to 99 mass-% of the methanol charged. This point of oxygenate breakthrough into the effluent stream was measured in terms of hours on stream (HOS) and the number for each of the catalyst samples is reported in Table 1. In this breakthrough test the magnitude of the HOS number is a good reproducible measure of the activity-stability of the particular catalyst sample in an MTO conversion application.  The results of a series of experiments assessing the effect of an immersion step on the activity-stability of the SAPO-34 catalyst samples is set forth in Table 1. TABLE l Effect of Aqueous Immersion on the Activity-Stability of SAPO-34 MTO Catalyst
* Catalyst B was regenerated prior to the immersion step so it was coke-free in this step.
 The activity-stability test results presented in Table 1 demonstrate the expected adverse effect of an aqueous immersion step on the performance of an unprotected MTO catalyst. Evidence to support this is found by comparing the HOS results for run 5 (i.e. performed with Catalyst C that had not been exposed to the aqueous immersion step) with the results for run 4 (performed with Catalyst B which had no protective layer of coke deposits when subjected to aqueous immersion) where a decrease of 8.8 mass-% in HOS was recorded. This HOS measure is a sensitive indicator of relative activity-stability and a decrease of 8.8% corresponds to a substantive drop in activity-stability.  Quite surprisingly, runs 1, 2 and 3 dramatically demonstrate that the aqueous immersion step does not damage the performance of the three samples of Catalyst A that were tested since all of these runs Catalyst A had a protective layer of coke deposits at a level of 6 mass-% when it was subjected to the aqueous immersion step. The HOS results for the three samples of Catalyst A clearly show that their activity-stability was equal to or comparable to the activity-stability measured in run 5 for Catalyst C which is the control catalyst which was not exposed to an aqueous immersion step. Data presented in Table 1 for runs 1, 2 and 3 thus provide convincing evidence that the harmful effect of aqueous immersion can be avoided with a SAPO-34 MTO catalyst system if the aqueous immersion is conducted with a catalyst that contains a protective coat of coke deposits.
 In order to study the effect of thermal shock on a wet scrubbing MTO catalyst that had been withdrawn from an aqueous immersion step, an experiment was conducted wherein two 50 gram portions of Catalyst A were exposed to an aqueous immersion step conducted at a temperature of 40°C for a period of 48 hours as explained above. The resulting wet scrubbing catalyst in the case of run 6 was subjected to a thermal shock step which essentially involved quickly placing the wet scrubbing catalyst that was withdrawn from the immersion step in a box oven that was maintained at 65O0C and an ambient pressure at a period of 5 minutes. These conditions were chosen to simulate a worst case scenario when the catalyst that was withdrawn from the wet scrubbing step is immediately exposed to a high temperature condition in the MTO conversion step or the regeneration step of the present invention. The temperature differential was set at a level of 61O0C in order to accomplish this objective. In run 6, the catalyst was subjected to the immersion step followed by the thermal shock step followed by the regeneration step that was described in Example 1. In run 7, the sample of Catalyst A was not subjected to the thermal shock step but was in fact subjected to the wet scrubbing immersion step followed by the regeneration step. Both of these samples of catalyst were then subjected to the MTO conversion breakthrough test designed to measure activity-stability which was described above in Example 1.  The results of the experiment are presented in Table 2 wherein the run with the thermal shock is contrasted with a run in which the catalyst was not thermally shocked. As can be seen from Table 2 the HOS numbers for the two samples of Catalyst A are identical and thus there was no effect of the thermal shock step on the activity-stability of the SAPO-34- containing MTO catalyst.
TABLE 2 Effect of Thermal Shock on a Wet Scrubbing MTO Catalyst
Both of the catalysts that were tested for purposes of preparing Table 2 were also sent out for a scanning electron microscope check to ascertain whether or not there had been any significant change in their morphology. The results of this SEM analysis showed that approximately 1 to 2% of the catalyst particles in the sample that had been exposed to thermal shock step were fractured or cracked by this procedure. This low rate of fractionating of the SAPO-34 catalyst particles was viewed as a result extremely favorable to the practice of the present invention.
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