Method and system for catalytically converting oxygenates and regenerating and stripping catalyst

A method of converting oxygenate-containing feedstock to light olefins comprises charging a reactor with catalyst, feeding the feedstock into the reactor, contacting the feedstock with the catalyst and converting the feedstock to olefins while depositing byproducts on catalyst resulting in spent catalyst, regenerating the spent catalyst by combustion gases, and stripping the regenerated catalyst of gases entrained in the regenerating step. The stripping step is accomplished using nitrogen gas to strip the entrained gases from the regenerate catalyst. In one embodiment, regenerated catalyst is passed through a regenerated catalyst stripper before it is returned to the reactor.

BACKGROUND OF THE INVENTION

This invention relates to a method and system for treating regenerated catalyst in an oxygenate to olefin conversion process.

DESCRIPTION OF THE PRIOR ART

Light olefins have traditionally been produced through the process of steam or catalytic cracking. Because of the limited availability and high cost of petroleum sources, the cost of producing light olefins from such petroleum sources has been steadily increasing. Light olefins serve as feeds for the production of numerous chemicals.

The search for alternative materials for light olefin production has led to the use of oxygenates such as alcohols and, more particularly, to the use of methanol, ethanol, and higher alcohols or their derivatives. Molecular sieves such as microporous crystalline zeolite and non-zeolitic catalysts, particularly silicoaluminophosphates (SAPO), are known to promote the conversion of oxygenates to hydrocarbon mixtures in a reactor. Numerous patents describe this process for various types of these catalysts: U.S. Pat. Nos. 3,928,483; 4,025,575; 4,052,479; 4,496,786; 4,547,616; 4,677,242; 4,843,183; 4,499,314; 4,447,669; 5,095,163; 5,191,141; 5,126,308; 4,973,792; and 4,861,938.

When a catalyst is exposed to oxygenates, such as methanol, to promote the reaction to olefins, carbonaceous material (coke) is generated and deposited on the catalyst. Accumulation of coke deposits interferes with the catalyst's ability to promote the reaction. As the amount of coke deposit increases, the catalyst loses activity and less of the feedstock is converted to the desired olefin product. The step of regeneration removes the coke from the catalyst by combustion with oxygen, restoring the catalytic activity of the catalyst. The regenerated catalyst may then be exposed again to oxygenates to promote the conversion to olefins.

The exposed catalyst with coke deposit is continuously withdrawn from the reactor and regenerated in a regenerator and then returned to the reactor. The catalyst is then directed to the regenerator where combustion with oxygen-containing air burs off the coke deposit on the catalyst. The combustion air used in regenerating the catalyst leaves carbon monoxide, oxygen, and carbon dioxide gases entrained in the catalyst. Oxygen is not a natural byproduct of the oxygenate-to-olefin reaction and when introduced through entrainment with regenerated catalyst creates processing difficulties downstream. The presence of oxygen will increase the amount of contaminant carbon dioxide and carbon monoxide formed among the desired product. Carbon dioxide and carbon monoxide entrained from the regenerator also significantly increase the concentration of these contaminants in the olefin product.

Carbon dioxide is a contaminant in polymer grade ethylene and propylene and is removed using caustic scrubbing. By eliminating the entrained carbon dioxide from the regenerated catalyst, the caustic consumption in the downstream caustic scrubber is significantly reduced. Eliminating the entrained oxygen has the benefit of reducing the potential for fouling in the downstream caustic scrubber and reducing operational problems in the downstream acetylene converter. Increased carbon monoxide increases the operating temperature of the acetylene converter and therefore narrows the operating range between normal operation and the maximum allowable temperature to avoid thermal runway.

SUMMARY OF THE INVENTION

A method is disclosed for stripping entrained gases from regenerated catalyst used in converting an oxygenate-containing feedstock to olefins comprising regenerating a catalyst and then stripping the regenerated catalyst. In another aspect, a method is also disclosed for converting oxygenate-containing feedstock to olefins comprising charging a reactor with catalyst, feeding the oxygenate-containing feedstock into the reactor, contacting the oxygenate-containing feedstock with the catalyst in the reactor and converting the oxygenate-containing feedstock to olefins while spending the catalyst, regenerating the spent catalyst, and stripping the regenerated catalyst of gases entrained during the regeneration step. In another aspect of the method, nitrogen gas strips the regenerated catalyst. In a further aspect, hydrocarbons are stripped from the catalyst before the regenerating step.

A system also is disclosed for regenerating catalyst used in converting oxygenate-containing feedstock to olefins comprising a regenerator and a regenerated catalyst stripper having a regenerated catalyst inlet, a stripping gas distributor and a stripped regenerated catalyst outlet. In one embodiment, the stripper has a plurality of baffles to enhance contact between the stripping gas and the catalyst. The catalyst inlet is preferably near the top of the stripper.

DETAILED DESCRIPTION OF THE INVENTION

Oxygenate-containing feedstock may be converted to light olefins in a catalytic reaction and the catalyst may be regenerated and stripped of entrained gases before being returned to catalyze further reactions. Feedstock comprising oxygenate, such as methanol, may be contacted with a catalyst containing a molecular sieve in reactor10. Catalytic activity should be maintained at a predetermined level for oxygenate-containing feedstock to be continuously converted to olefins. Deposits on catalyst that impair catalytic activity should be removed without disrupting conditions for the reaction of oxygenate-containing feedstock to olefins. Fluidization of catalyst particles by various gaseous streams allows transport of catalyst between reactor10, stripper30, regenerator50, and stripper70. One oxygenate-containing feedstock may be methanol. The methanol to olefin (MTO) conversion process may be a vapor phase, fluid catalytic process that converts methanol to olefins, primarily ethylene and propylene. Feedstock may be commercial grade methanol, crude methanol or any combination of the two. Crude methanol may be an unrefined product from a methanol synthesis unit. Feed comprising methanol and water blend may have methanol between about 65% and about 90% by weight. More preferably, feed comprising methanol and water blend may have methanol between about 78% and about 82% by weight. Most preferably, feed comprising methanol and water blend may be about 80% methanol by weight. As seen inFIG. 1, MTO reactor10may be a fluid catalytic design. Coke may be a byproduct of the MTO process that accumulates on catalyst during contact with oxygenate-containing feedstock. Catalyst becomes spent as coke deposits accumulate on the catalyst and decrease its ability to convert oxygenate-feedstock to olefins. Therefore, spent catalyst from reactor10may be continuously regenerated to maintain the desired activity. The catalyst may be silicoaluminophosphate (SAPO), having a tetrahedral unit framework forming numerous pores to best contact methanol feed during conversion to olefins.

At least a portion of the spent catalyst may be continuously drawn out of reactor10for regeneration. Before the spent catalyst may be regenerated, hydrocarbons may be stripped from the spent catalyst in reactor stripper30using steam. The spent catalyst may be transferred to regenerator50where the coke may be removed from the catalyst, resulting in a regenerated catalyst. Gases from regeneration may be entrained in the regenerated catalyst when the catalyst is removed from regenerator50and the entrained gases may be disruptive to reactor10conditions for converting methanol to olefins.

The catalyst may preferably have a void fraction between about 0.2 and about 0.5 and more preferably be between about 0.3 and about 0.4. The coke on the spent catalyst may typically be between about 2 wt-% and about 20 wt-% and more typically be between about 3 wt-% and about 10 wt-% The coke on the regenerated catalyst may preferably be between about 0.1 wt-% and about 0.5 wt-% and more preferably be between about 0.2 wt-% and about 0.4 wt-% The percentage of coke oxidized may preferably be at least 80 wt-%, more preferably be at least about 85 wt-%, and most preferably be at least about 90 wt-%.

Much of entrained gases used during regeneration may be removed from the regenerated catalyst after regeneration by cyclone55within regenerator50, but as seen in example below, a significant amount of entrained gases remain with regenerated catalyst when it leaves regenerator50. These entrained gases include gases which may be adsorbed onto the catalyst, located within its pore structure or simply carried in interstitial volume between catalyst particles. Regenerated catalyst may be transferred to regenerated catalyst stripper70before being returned to reactor10where entrained gases remaining with the catalyst from the regenerating step are stripped by a stripping gas. In one embodiment, the entrained gases which may comprise carbon dioxide (CO2), carbon monoxide (CO), and oxygen (02) are removed by nitrogen (N2) gas. Use of a CO oxidation promoter additive in the catalyst will reduce the relative ratio of CO to C02in the entrained gases.

Oxides of nitrogen may be formed in regenerator50and can be entrained with catalyst delivered into reactor10. Oxides of nitrogen (NOx) can create a hazard in the downstream cryogenic recovery section. NOx may include nitric oxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O2), and dinitrogen tetroxide (N2O4). Trace amounts of NOx can react with hydrocarbons to form unstable compounds. These compounds are known as NOx gums, nitrogenous gums, vapor phase gums, etc., and accumulate over time in cryogenic equipment used for purifying ethylene. NOx gums can lead to a risk of explosion when equipment is taken offline and warmed to ambient conditions for maintenance.

Stripped regenerated catalyst may be returned to reactor10for further conversion of methanol to olefins. An absence or reduction of entrained gases with regenerated catalyst helps conversion of methanol to olefins by reducing byproducts formed within reactor10and cutting down costs on downstream product recovery.

Nitrogen directed to the regenerated catalyst stripper to strip the entrained gases from the regeneration step may be between about 0.5 and about 8.0 kg of nitrogen per 1000 kg of catalyst. Preferably, the nitrogen directed to the regenerated catalyst stripper may be between about 2.0 and about 5.0 kg of nitrogen per 1000 kg of catalyst. More preferably, the nitrogen directed to the regenerated catalyst stripper may be between about 3.0 and about 4.0 kg of nitrogen per 1000 kg of catalyst.

A method of processing catalyst exposed in a conversion reaction of oxygenate-containing feedstock to olefins includes regenerating the exposed catalyst and stripping the regenerated catalyst of gases entrained during the regenerating step.

In one embodiment of a method for processing oxygenate-containing feedstock to olefins, the catalyst for converting oxygenate-containing feedstock to olefins travels a route from reactor10to a hydrocarbon stripper30to regenerator50to regenerated catalyst stripper70back to reactor10. A method of regenerating a catalyst used in converting oxygenate-containing feedstock to olefins includes: charging reactor10with catalyst, feeding the oxygenate-containing feedstock into reactor10, contacting the oxygenate-containing feedstock with the catalyst in reactor10and converting the feedstock to olefins while spending the catalyst, regenerating the spent catalyst, and stripping the regenerated catalyst of gases entrained during the regenerating step. In another aspect of the method, hydrocarbon vapors are stripped from the spent catalyst before the regeneration step. Entrained gases may include oxygen, carbon dioxide, and carbon monoxide. The method may also comprise the regenerated catalyst stripping gases combining with the combustion gases and discharging from a common outlet.

As seen inFIG. 1, reactor10comprises a lower reactor section11and an upper reactor section12. Lower reactor section11, where the process reaction actually occurs, consists of a feed distributor20, a fluidized bed of catalyst and an outlet riser13. Upper reactor section12may be primarily the vapor/catalyst separation zone. After the preliminary disengagement at the top of the outlet riser13, cyclone14carries the separation to a greater degree. Separated catalyst may be continually recycled from upper reactor section12back down to lower reactor section11, via recirculation conduit16through a slide valve, to maintain the desired catalyst density in lower reactor11. The olefins and byproducts produced by the reaction are discharged from reactor10by conduit15which directs the olefins and byproducts to a product recovery process. The temperature range in the reactor is preferred to be between about 440° C. and about 520° C. The more preferred range of the temperature in the reactor is between about 450° C. and about 500° C. The preferred feed temperature range is between about 120° C. and about 200° C. More preferably the feed temperature range is between about 180° C. and 200° C.

Hydrocarbons also may be included as part of the oxygenate-containing feedstock. Hydrocarbons included in the feedstock are adsorbed or trapped in the molecular sieve structure of the catalyst during conversion of oxygenate feed to olefins. Hydrocarbons may include olefins, reactive paraffins, reactive aromatics, or mixtures thereof. The catalyst exposed to the oxygenate-containing feedstock in the reactor becomes spent catalyst, and may be withdrawn from reactor10and directed to reactor stripper30situated adjacent to reactor10.

The stripping process removes the volatile organic components which may be entrained with the catalyst prior to entering regenerator50. A stripping gas may be passed over the catalyst in reactor stripper30. In one embodiment the stripping gas comprises steam directed into the reactor stripper30via lines40.

The entrained steam traveling from reactor stripper30to regenerator50may typically be between about 0.40 m3/m3catalyst and 0.80 m3/m3catalyst measured at operating conditions. More typically the range for entrained steam is between about 0.60 m3/m3catalyst and 0.70 m3/m3catalyst.

MTO regenerator50may be a bubbling (turbulent) bed type of design. Regenerator50may comprise a vessel containing a distributor49fed by combustion gas line52, a fluidized bed of catalyst and cyclones55. Main air blower on line52or pressurized air supplies combustion gas to regenerator50. Catalyst regeneration may be exothermic. The heat of combustion of the coke may be combusted from regenerator50by vaporizing water circulated through tubes in regenerator catalyst coolers60.

Combustion air may contain oxygen (O2) or other oxidants. It may be preferred to supply oxygen in the form of air. The air can be diluted with nitrogen, C02, flue gas, or steam. Coke deposits are removed from the catalyst during regeneration, forming a regenerated catalyst.

In one embodiment, regenerator50comprises a fluid bed section53, an upper disengaging section54and a lower section56comprising regenerated catalyst stripping section70and regenerator catalyst coolers60. In operation, regenerator50contacts spent catalyst transferred from reactor10by an exposed catalyst standpipe22with compressed air from distributor51. Contact with oxygen combusts coke from the catalyst as it passes upwardly through fluid bed section53. A small portion of the catalyst remains entrained with the combustion gases and enters inlet57of cyclones55which separate much of the entrained catalyst from the combustion gases. Catalyst travels to the lower section56via the dip legs on cyclones55.

In one embodiment, typical flue gas in regenerator50may comprise as follows by percentage volume: 2-11% H2O, 3-7% O2,75-80% N2, and 10-15% CO2. There may be residual CO in the flue gas within regenerator50. Combustion flue gases are discharged from regenerator50via conduit58.

Catalyst enters regenerator catalyst cooler60through an opening61. Catalyst entering cooler60contacts the outer surface of a heat exchange tube as it passes downwardly through the cooler and returns to fluid bed section53via a conduit. Cooler60may be cooled by vaporizing water into steam. Heat exchange tubes are bayonet style tubes having an outer tube that contacts the catalyst and an inner tube for circulating a cooling fluid. Fluidizing gas comprising air and distributed by a plurality of conduits enter cooler60. Fluidizing gas passes upwardly through the cooler60and through opening61into upper disengaging section54. The fluidizing gas requirement will depend on the amount of coke being combusted from the catalyst.

To prevent the entry of large objects such as agglomerated masses of catalyst from entering the cooler60, a sheet of screen material covers opening61. The screen can be secured to opening61using a suitable method.

After catalyst is regenerated, there are gases entrained in the regenerated catalyst that increase amounts of byproducts in oxygenate to olefin conversion processes and make downstream product recovery more difficult. Entrained gases should be removed prior to the regenerated catalyst returning to reactor10to cut down on downstream product recovery resources. When the regenerated catalyst travels directly from regenerator50back to reactor10, the range of flue gas entrained may preferably be between about 0.5 m3/m3catalyst and about 0.8 m3/m3catalyst.

The entrained gases from regenerator50may include O2, CO, CO2, N2, NOx, and H2O. If the catalyst travels directly from regenerator50back to reactor10, the entrained flue gases may comprise between about 8 and about 24 kg H2O, between about 32 and about 40 kg O2, between about 440 and about 530 kg N2, between about 100 and about 150 kg CO2per 1000 m3of catalyst. The entrained flue gas may also comprise residual amounts of CO, NOx, and sulfur compounds.

Preferably at least about 97 wt-% of the entrained gases are removed before returning to reactor10. More preferably at least about 99 wt-% of the entrained gases are removed before returning to reactor10.

After catalyst is regenerated, the catalyst enters a regenerated catalyst stripper70. The regenerated catalyst stripper70may be contiguous with the lower section56of the regenerator50along with cooler60such that the regenerated catalyst stripper70is within the vessel of the catalyst regenerator50. As seen inFIG. 2, in one embodiment, regenerated catalyst stripper70may be a cylindrical chamber connected to the lower section56of regenerator50such that regenerator50and regenerated catalyst stripper70form a single vessel with a common shell. Regenerator50and stripper70may also be two separate vessels connected by conduits directing the regenerated catalyst from the regenerator to the stripper. Regenerated catalyst is withdrawn from regenerator50and directed into stripper70where the catalyst may be fluidized by a gas and stripped by a stripping gas. The regenerated catalyst may be fluidized by fluidizing gas and may be impeded from direct downward flow by packing, or baffles. The assumed fraction of stripping gas entrained in the catalyst when it leaves the stripper70may be between about 0.3 and 0.7 and typically between about 0.4 and 0.6. In one embodiment, the stripping gas comprises nitrogen. The total stripping nitrogen used may preferably be between approximately 1 kg nitrogen/m3catalyst and about 4 kg nitrogen/m3of catalyst.

The regenerated catalyst may be fluidized in stripper70by an inert gas such as nitrogen for the stripping of the entrained gases from regeneration. The fluidizing gas may be introduced via distributor80. The stripping gas may be introduced via distributor90. In one embodiment the stripping gas comprises nitrogen. Alternatively, the catalyst may be dispersed in stripper70by packing. The stripping gas may be introduced via distributor90to the underside of the packing to strip the suspended regenerated catalyst. The assumed fraction of entrained stripping gas is between about 0.3 and 0.7.

In a preferred embodiment, the regenerated catalyst may be impeded from direct downward flow in the stripper70by baffles75as shown inFIG. 2. The baffles75comprise a perforated section76with openings that allow upward passage for the fluidizing gas to strip entrained gases from the regenerated catalyst. The baffles75may be sloped at an angle for drainage, especially during shutdown. Baffle75preferably does not extend across the entire cross-section of the stripping section70. A downcomer area77is provided between an edge of each baffle75and an inside wall of stripper70for the catalyst to cascade from one baffle to the subjacent baffle in the stripper70. An end plate78may be attached to the ends of each baffle75and extend downwardly from the edge of baffle to define a skirt which may serve to regulate the amount of any gas which may accumulate under each baffle. Arranging downcomer77area on opposite sides of stripper70on adjacent baffles75assures that the catalyst cascades downwardly through the stripping section from side to side. The spacing of perforations over the perforated section may be arranged in any manner that eliminates wide bands or areas that do not contain holes for delivery of the fluidization medium. It may be important that at least about 30% of the area of the perforated section76comprise openings to allow the passage of stripping medium therethrough and break up gas bubbles beneath the baffle. It is preferred that between about 50% and about 80% of the perforated section76comprise openings. It is further preferred that between about 65% and about 75% of perforated section76comprise openings. The baffles75are held in place by a support79which may be connected to the wall of stripping section70supporting the baffles from underneath. In one embodiment, support79may be wedge-shaped with one side of the wedge flush against the underside of each baffle75.

Perforated section76, with a high percentage of open area, allows the stripping medium to rise vertically upwardly through the stripper70through succeeding baffles75to engage the downwardly flowing catalyst wending through the stripping section in a transverse manner to promote better mixing between the stripping medium and the catalyst. In a preferred embodiment, the height of separation between succeeding baffles may be 61 cm (24 inches) apart but it may also be preferred to reduce the height to 46 cm (18 inches). In one embodiment, there may be an imperforate section73on the baffle75. Imperforate section73may comprise a blank-off plate which rests on top of perforated section76of each baffle75below the downcomer77of the superjacent baffle75. Preferably, blank-off plate comprising part or all of imperforate section73may be secured to perforated section76. The imperforate section further promotes the horizontal movement of the catalyst by forcing it to change direction after coming through a superjacent downcomer area77. The imperforate section73comprises between about 10% and about 30% of the cross-sectional area of the stripper with about 20% being preferred. Perforated section76of the baffle75not covered by the imperforate section73in an embodiment comprises about 40% to about 80% of the cross-sectional area of the stripper70with about 60% being preferred. The downcomer section77comprises between about 10% and about 30% of the cross-sectional area of the stripper70with about 20% being preferred.

Stripping gas and fluidizing gas may be introduced into stripper70via different distributors or stripping gas introduced via only one distributor may also act as fluidizing gas. Stripping gas may be introduced via distributor80and fluidizing gas may be introduced via distributor90. In a preferred embodiment, stripping gas and fluidizing gas for stripper70comprise nitrogen gas. Both stripping gas distributor80and fluidizing gas distributor90introduce nitrogen to the underside of baffles75by jets. In an embodiment, slightly, downwardly sloped jets82on stripping gas distributor80direct nitrogen toward a centerline of the stripper70to produce upward gas flow under the perforated sections76of the baffles75. Jets91on fluidizing gas distributor90direct nitrogen upwardly toward baffles75to fluidize regenerated catalyst. Jets92on the fluidizing gas distributor90direct nitrogen downwardly toward a regenerated catalyst standpipe100to keep catalyst fluidized in standpipe100. Having two distributors of nitrogen keeps the flow density constant throughout stripper70to optimize stripping. The flow rate of stripping gas from stripping gas distributor80may be between about three times and about five times greater than the flow rate of fluidizing gas from fluidizing gas distributor90. The stripping gas flow rate may more preferably be about four times greater than fluidizing gas flow rate.

Stripping gas and fluidizing gas introduced into stripper70for the stripping of the regenerated catalyst travel upwardly through the stripper70and regenerator50and exit along with regenerator flue gases through conduit58. After the regenerated catalyst travels downwardly through stripper70by means of transverse movement along the baffles75, the catalyst may be funneled into the regenerated catalyst standpipe100through funnel95at the bottom of the stripping section70. The regenerated catalyst that has been stripped of gases entrained during regeneration may be returned back to reactor10by the regenerated catalyst standpipe100.

A system for regenerating spent catalyst used in converting oxygenate-containing feedstock to olefins comprises a catalyst regenerator, a regenerated catalyst stripper having a regenerated catalyst inlet, a stripping gas distributor, and a stripped regenerated catalyst outlet. In one embodiment of the invention, a regenerated catalyst stripper contains a plurality of baffles. Each one of the baffles may have a perforated section. Each baffle may be mounted and spaced apart overlappingly on alternate sides of the regenerated catalyst stripper. The stripping gas in the stripper may comprise nitrogen. There may be a common outlet for gases from said regenerated catalyst stripping section and the regenerator, where the regenerated catalyst stripping section is in fluid communication with the regenerator. The stripping distributor may be below said regenerated catalyst inlet. The catalyst inlet may be near the top of the stripper. The catalyst outlet may be near the bottom of the stripper.

EXAMPLE

An experimental simulation was conducted for an MTO unit to more fully demonstrate the stripping of entrained gases from regenerated catalyst before returning the catalyst to the reactor. The catalyst comprises a silicoaluminophosphate (SAPO) molecular sieve. SAPO molecular sieves comprise a molecular framework of SiO2, Al2O3, and P2O5tetrahedral units. The catalyst may be circulated at a rate of about 136,077 kg/hr (300,000 lb/hr). The coke on the spent catalyst is about 3.0 wt-% and the coke on the regenerated catalyst is about 0.3 wt-%. The amount of coke oxidized in the regenerator is 3,659 kg/hr (8,066 lb/hr).

MTO unit is designed to convert methanol to light olefins. In this particular example, we have assumed the following composition: 5.0 wt-% water, 95 wt-% methanol, 0.2 wt-% ethanol, 1000 wppm higher alcohols and 30 wppm dimethyl ether.

The above example is only intended to illustrate certain aspects of the present invention and is not meant to be limiting.