GB2024033A - Improved power recovery from catalyst regeneration gas - Google Patents

Abstract

A catalyst regeneration process with improved recovery of energy via an expander turbine-axial gas compressor set which utilizes hot regeneration gases recovered from the regeneration of fluidized catalytic cracking catalyst includes the steps of regenerating spent catalyst under conditions to combust substantially all carbon monoxide produced in the burning of carbonaceous material deposited on the spent catalyst to carbon dioxide, compressing sufficient oxygen containing gas in the axial gas compressor to prevent surging of the compressor, and introducing substantially all of the compressed gas into the regeneration zone as regeneration gas.

Description

SPECIFICATION Improved power recovery from regeneration gas The present invention relates to a process for the regeneration of spent catalyst from a fluid catalytic cracking unit (FCCU). More particularly, the invention relates to the recovery of power from hot regeneration gases recovered from such regeneration process.

In a typical FCCU, spent catalyst is continuously removed from the cracking unit, sent to a regenerator, and then returned to the cracking unit. In the regenerator, the fouled catalyst is contacted with an oxidative regeneration gas, at elevated temperatures and pressures, to remove coke or other carbonaceous deposits from the catalyst by combustion.

Such combustion of carbonaceous deposits can be effected in a fluidization chamber containing the solid catalyst particles through which a fluidizing gas is passed upwards at a rate to maintain the particles as a fluidized bed, i.e. in a turbulent state with quasi liquid properties, including a recognizable upper level. Typically, the fluidizing gas is, or at least contains, the oxidative regeneration gas. The combustion or regeneration gases produced by the burning of the carbonaceous deposits are typically at high temperatures and elevated pressures. For example, it is not uncommon for regeneration gases to have a temperature in excess of 1000"F and range up to 1 500=F, or even higher, while pressures may range from about 10 p.s.i.g. up to about 35 p.s.i.g. and greater.Thus the gases, commonly referred to as flue gas, emerging from the regeneration zone represent a large energy potential which may be utilized to recoup a part of the power invested in the system in compressing the air used as the oxidative regeneration gas. In some cases, enough energy is released in the regeneration process that, if properly recovered, a net gain may be realized in the regeneration, thus supplying a surplus of power for utilization in other operations, e.g. generation of electric power.

It is common practice to utilize expansion turbines or turboexpanders to recover energy from hot flue gases from regenerators. In the usual case, the flue gas, at high temperature and elevated pressure, is passed to an expansion turbine which then supplies shaft power to an air compressor used to generate compressed air for the regeneration process.

There have recently been introduced fluid catalytic cracking catalysts which allow essentially complete combustion of the carbonaceous material on the spent catalyst to carbon dioxide in the dense phase zone of the regenerator, essentially no carbon monoxide being produced. The use of such catalysts is highly desirable as it prevents undesirable "afterburn" in the regenerator dilute phase zone, a condition brought on by the presence of carbon monoxide and oxygen in the regeneration gas. Additionally, all heat of combustion is utilized within the fluid catalytic cracking process rather than being lost completely or recovered externally of the regenerator in a carbon monoxide boiler. The latter process is disclosed, for example, in U.S. Patents 3,137,133 and 3,139,726.The use of complete combustion catalysts, which give increased recovery of heat of combustion, provides a higher regenerative dense phase zone temperature and thereby allows lower catalyst to oil ratios in the cracking zone and hence improved yields. As noted, the hot flue gases from the regeneration zone are typically expanded through the expansion turbine of an expander turbine-compressor set to recover energy from the flue gas. While centrifugal compressors or turbo blowers may be used in such a power recovery set, axial compressors, because of their high efficiency and higher capacity, offer certain advantages. There is, however, a distinct problem with the use of axial compressors in such systems. Because of the relatively steep head-capacity-characteristics of axial compressors, the pumping point may be close to, i.e.

within 10% of the design flow. This characteristic of axial compressors makes them susceptible to surging or pumping, i.e. unless the axial compressor is operated under conditions where it is required to compress more air than needed on the discharge side, the compressor will begin to surge. There is a minimum capacity for axial blowers below which operation becomes unstable, i.e. surging occurs. Surging results when the line pressure on the exhaust side of the compressor exceeds the exhaust pressure which the machine is capable of producing. Since the compressed gas cannot get into the outlet or exhaust line, it "rushes" back into the compressor.

This lowers the outlet line pressure momentarily and the compressor begins to, once again, discharge into the outlet line. However, the pressure immediately gets too high in the outlet line and, again, the air cannot be discharged from the compressor. The gas then rushes back into the compressor and the entire cycle is repeated. Continued operation of a compressor under surging conditions will eventually cause the compressor to literally tear itself apart.

Surging in axial compressors can become a serious problem in FCC processes because of the fact that, at times, it is desirable that the unit be operated under "turndown conditions." In turndown conditions, the amount of feed stock to the FCCU is reduced to below design capacity. This means that the amount of catalyst being regenerated, and accordingly the amount of compressed regeneration gas being used in the regenerator, is reduced. It also means that less compressor capacity is required. It has been customary practice, in turndown conditions, and when using axial compressors, to require the compressor to compress more gas than needed for the regeneration process and vent the excess at the blower discharge. While this prevents surging of the compressor, it represents a loss of recoverable energy in the system.

The present invention represents an improvement in the field of regeneration of spent catalyst(s) from fluidized catalytic cracking processes, and more specifically, an improvement in the recovery of energy from the hot regeneration gases derived therefrom. In the conventional process, the spent catalyst is contacted with a regeneration gas, generally oxygen containing, in a regeneration zone, at elevated pressures and temperatures, to effect burning of carbonaceous material deposited on the spent catalyst. Hot regeneration gases and regenerated catalyst are separated and a hot flue gas is recovered. The flue gas is expanded through the turbine of an expander turbine-axial gas compressor set, the energy recovered by the expander being used to drive the compressor which is turn compresses oxygen containing gas which is fed to the regenerator as regeneration gas.The flue gas production rate in the regeneration zone generally ranges from about 50 to about 100% of the design capacity of the expander turbine-axial gas compressor set to ensure that sufficient expander inlet volume is available.

In the improved process, the spent catalyst is regenerated under conditions which ensure that substantially all of the carbon monoxide produced in the burning of the carbonaceous material is converted to carbon dioxide, preferably in the dense phase zone of the regenerator. Additionally, there is sufficient oxygen gas compressed in the axial compressor to prevent surging and substantially all of the compressed gas from the axial compressor is introduced into the regeneration zone as regeneration gas.

It is an advantage of the present invention that it makes it possible to provide an improved regeneration process for spent catalyst from a fluidized catalytic cracking process.

A further advantage of the present invention is it makes it possible to provide an improved process for recovering power from hot regeneration gases derived from the regeneration of spent, fluid catalytic cracking catalyst.

A process according to the invention will now be described by way of example and with reference to the single figure of the drawing which is a schematic flow sheet depicting a process according to the present invention.

Referring to the accompanying drawing, fouled or spent catalyst from a fluidized catalytic cracker (not shown) is introduced, via line 10, into regenerator 1 2. As is well known, spent catalyst from a typical FCCU contains deposits of coke and tarry residues, i.e. carbonaceous material, which inhibit the cracking activity of the catalyst.

Oxidative regeneration gas, such as compressed air or other oxygen containing gas, is intoduced into regenerator 12 via line 16. The oxidative regeneration gas is passed upwardly through regenerator 1 2 at a rate sufficient to maintain the catalyst particles in a fluidized or turbulent state with quasi liquid properties, a recognizable upper level defining the dense phase zone in regenerator 1 2. In regenerator 1 2, conditions are such that substantially all of the carbon monoxide produced by combustion of the carbonaceous material is coverted to carbon dioxide. As previously noted, this can be accomplished, in one manner, by the use of so-called complete combustion catalysts.Such catalysts allow essentially complete combustion of the carbonaceous material deposited on the spent catalyst to carbon dioxide in the regenerator dense phase zone. Thus, the hot regeneration gases leaving the dense phase zone of regenerator 1 2 contain little or not carbon monoxide which can cause afterburn in the dilute phase zone of the regenerator, or in the gas solid separator used to remove entrained catalyst from the hot regeneration gases. Generally speaking, temperatures in the regenerator will range from about 1100 to about 1 500 F. Additionally, the oxidative regeneration gas supplied usually contains oxygen in excess of the stoichiometric amount necessary to convert the carbonaceous material to carbon dioxide.The hot regeneration gases produced by the combustion in regenerator 1 2 pass through a separating system 18, which may consist of one or more cyclones which serve to remove entrained catalyst particles from the hot regeneration gases. The regenerated catalyst, enhanced in cracking activity, is then returned to the cracking reactor by way of line 14.

The hot flue gases, i.e. the gases generated in the combustion and which have been substantially freed of solid catalyst particles leave regenerator 1 2 via line 20 and are introduced into expander turbine 22 which exhausts the expander gases, via line 24, to the atmosphere or other energy recovery means, e.g. a steam generator or the like. Expander turbine 22 serves to produce available rotative horsepower in proportion to the pressure level of the overall system.

In the case shown, expander turbine 22 forms a part of the expander-compressor set, and is linked, via a direct drive connection, to a corresponding axial gas compressor 26. Gas compressor 26 takes in atmospheric air or other oxygen containing gas via line 28 and compresses it to the pressure required in regenerator 1 2. Sufficient oxygen containing gas, e.g.

air, is compressed in compressor 26 to prevent surging. Generally speaking, the amount of oxygen containing gas or air compressed is sufficient to maintain the compressed gas rate discharged from compressor 26 to at least about 10% above the surge line of the compressor.

The compressed gas is then discharged from compressor 26 into line 16. A vent line 32 provided with a valve 34 leads from discharge line 1 6. In the process herein, valve 34 is closed.

Power developed in expander 22, in excess of that necessary to drive compressor 26, is used to ge'nerate electricity via a motor/generator auxiliary system 30 linked to expander 22. It will be appreciated that when insufficient energy is developed in expander 22 to drive compressor 26 at the desired capacity, motor/generator 30 serves as an auxiliary driver to supply the power deficit.

As can be seen, all of the oxygen containing gas (air) compressed in compressor 26 is introduced into regenerator 12, no compressed air being vented from the discharge of compressor 26 via line 32 and valve 34. This condition prevails regardless of whether or not regenerator 1 2 is being operated under turndown conditions. Under such turndown conditions, air supplied by compressor 26 to regenerator 1 2 will be in excess of that required to effect combustion of the carbonaceous material on the spent catalyst in regenerator 1 2. In prior art processes employing axial gas compressors, it was common to permit the compressor to operate at a capacity which would prevent surging and simply vent the excess air rather than passing it through the regenerator.This resulted in a loss of energy from the system, to wit, the energy of the vented excess air. In the process herein, since the excess air is not vented, the energy is recovered in the expander-compressor set. As noted, and in the preferred case, the catalyst employed is the complete combustion type which ensures that the carbonaceous material is combusted to carbon dioxide in the dense phase zone of regenerator 1 2. Accordingly, the presence of excess oxygen in regenerator 1 2 under turndown conditions will not result in afterburn in the dilute phase zone of the regenerator, or in the cyclone, due to the fact that little or no carbon monoxide is present in the regeneration gases leaving the dense phase zone.It is to be understood however that the process of the present invention is applicable to any regeneration process wherein substantially all of the carbon monoxide produced in the burning of the carbonaceous material is converted to carbon dioxide in the regeneration zone. Under such conditions, the presence of excess oxygen will not result in afterburn occurring in the separators or, indeed, in downstream equipment.

The advantages of the present invention are seen from the tables below which set forth pertinent operating data for a typical FCCU power recovery system. Table I shows reactor-side conditions during design and turndown modes while Tables II and III show power recovery system conditions during design and turndown modes: TABLE I CASE DESIGN A/B(75% Turndown) C/D(50% Turndown) Feed Heater Outlet Temp., 'F 525 525/563(') 498/659(' Riser Outlet Temp., "F 970 955(2) 935(2) Throughput Ratio 1.14 1.14 1.14 Cat.Circulation Rate, TPM 48.0 34.4 21.8 Reactor Top Pressure, psia 39.7 39.7 39.7 Coke Yield, WT.%, FF(2) 5.30 5.30 5.30 Gas Oil Conversion, Vol%, FF 75.0 75.0 75.0 Feed preheat temperature increased in B to maintain catalyst/oil weight ratio constant while regenerator bed temperature drops due to presence of excess air.

Fresh Feed.

(3) Riser outlet temperature lowered to maintain constant conversion while reducing FF rate below design.

(4) 400'FASTM.

TABLE IT CASE DESIGN A(75% Turndown) B(75% Turndown) Air Rate to Regen. Mlbs/hr 604.67 453.5 521.8(' Air Rate From Blower Mlbs/hr 604.67 521.8 521.8 Air Required for Combustion Mlbs/hr 604.67 453.5 453.5 Air Vented Mlbs/hr 0 68.26 0 Regen. Bed Temp., "F 1325 1325 1312 Regen. Top Temp., "F 1 340 1340 1327 Regen.Top Pressure, psia 44.7 44.7 44.7 Air Blower Discharge Pressure, psia 50.53 49.66 50.02 Expander Inlet Pressure, psia 38.53 30.55 33.77 Expander Inlet Temperature, "F 1 285 1 285 1 285 Expander HP 18,400 11,500 14,260 Blower HP 16,400 13,893 13,994 HP Generated 2000 (2393)(2) 266 2 in Flue Gas, Vol % 1.0 1.0 3.7 Blower performance map indicates that air rate from blower under these conditions must be 86.2% of design rate in order to be 10% away from surge line. This number is slightly off due to its being calculated in heat and material balance program basis input value for O2 in flue gas.

(2) Value in parentheses indicates HP deficit made up by motor.

TABLE Ill CASE DESIGN C(50% Turndown) D(50% Turndown) Air Rate to Regen., Mlbs/hr 604.67 302.33 524.9(' Air Rate from Blower, Mlbs/hr 604.67 524.9 524.9 Air Required for Combustion, Mlbs/hr 604.67 302.33 302.33 Air Vented Mlbs/hr 0 222.6 0 Regen. Bed Temp., "F 1 325 1325 1 258 Regen.Top Temp., "F 1 340 1340 1 273 egen. Top Pressure, psia 44.7 44.7 44.7 Air Blower Discharge Pressure, psia 50.53 49.03 50.04 Expander Inlet Pressure, psia 38.53 (2) 33.03 Expander Inlet Temp., "F 1 285 (2) 1273 Expander HP 18,400 (2) 13,524 Blower HP 16,400 (2) 14,097 HP Generated 2000 (2) (573)(3) 2 in Flue Gas, Vol% 1.0 1.0 9.6 Blower performance map indicates that air rate from blower under these conditions must be 86.8% of design rate in order to be 10% away from surge line.

(2) Expander inlet volume so low that these conditions are far below range covered on expander performance map. Thus, this case not workable.

(3) Value in parentheses indicates HP deficit made up by motor.

As seen with reference to Table II, Case A, a 75% turndown fresh feed rate, the excess air required by the compressor to keep it at a safe distance from the surge point is vented, e.g.

valve 34 is open, rather than being passed into the regenerator. Under such circumstances there is a significant deficiency between the expander horsepower generated (11,500) and the blower horsepower requirements (13,893). The deficiency requires that an auxiliary driver, such as for example motor generator 30, be employed. However, with reference to Case B in Table II, also a 75% turndown fresh feed rate, it can be seen that when the excess compressed air is put through the regenerator, the recovered horsepower (14,260) exceeds the blower horsepower requirements (13,994).

With reference to Table III, Case D represents a situation, at a 50% turndown fresh feed rate, wherein the excess air required for combustion is put through the regenerator. While there is a deficit of 573 horsepower which must be supplied by an auxiliary driver, it should be noted that even under such severe turndown conditions very little auxiliary power is required. In Case C, while also depicts 50% turndown conditions, the excess air required for combustion is vented.

As can be seen under such circumstances, the expander inlet volume is so low that the system is operable.

As the above data shows, the present invention provides a process in which maximum recovery of energy from a FCCU can be achieved. Furthermore, the process permits the use of high efficiency axial gas compressors without resorting to venting of the compressor discharge, in turndown conditions, to prevent surging or pumping of the compressor. An additional advantage of the process of the present invention is that by putting the excess air through the regenerator rather than venting, improved air distribution in the regenerator dense phase bed is achieved.

While the invention has been described with considerable particularity, it is to be understood that many changes and modifications may be made in the process without departing from the spirit and scope of the invention. Accordingly, it is intended that the scope of the invention be limited only by the claims which follow.

Claims (9)

1. A process for the regeneration of spent catalyst from a fluidized catalytic cracking process wherein the spent catalyst is contacted with an oxidative regeneration gas in a regeneration zone at elevated pressures and temperatures to effect burning of carbonaceous material deposited on said spent catalyst, and there are produced hot regeneration gases and regenerated catalyst, said hot regeneration gases and said regenerated catalyst being separated to provide a hot flue gas, and wherein said hot flue gas is expanded through the turbine of an expander turbine-axial gas compressor set, and compressed oxygen containing gas from said compressor is employed as at least a part of said regeneration gas in said regeneration zone, the flue gas production rate in said regeneration zone ranging from about 50 to about 100% of the design capacity of said expander turbine-axial gas compressor set, the process comprising: a. regenerating said spent catalyst in said regeneration zone under conditions such that substantially all of the carbon monoxide produced in the burning of said carbonaceous material is converted to carbon dioxide; b. compressing sufficient oxygen containing gas in said axial gas compressor to prevent surging of said axial gas compressor; and c. introducing substantially all of said compressed gas from said axial gas compressor into said regeneration zone as regeneration gas.

2. A process according to Claim 1, wherein said oxidative regeneration gas comprises air.

3. A process according to Claim 1 or Claim 2, wherein said oxygen containing gas comprises air.

4. A process according to any of Claim 1 to 3, wherein said regeneration of said spent catalyst is conducted at a temperature in the range of from about 1100 to about 1500"F.

5. A process according to any of Claims 1 to 4, wherein said regeneration of said spent catalyst is conducted in the presence of excess oxygen.

6. A process according to any of Claims 1 to 5, wherein excess power generated by said flue gas expanding through said expander turbine is employed to generate electricity.

7. A process according to any of Claim 1 to 6, wherein power required for compressing air in said axial compressor in excess of that provided by flue gas expanding through said expander turbine is provided by an auxiliary driver connected to said expander turbine-axial air compressor set.

8. A process according to any of Claims 1 to 7, wherein the amount of oxygen containing gas compressed is sufficient to maintain the compressed gas rate discharged to at least about 10% above the surge line of said compressor.

9. A process for the regeneration of spent catalyst from a fluidized catalytic cracking process, substantially as herein before described with reference to the drawing.