US2758979A - Method for regenerating catalyst by combustion - Google Patents

Description

- `Au@ 14, 1956 J. GUTHRIE 2351x979 1 METHOD FOR REGENERATING CATALYST BY COMBUSTION Filed March 28,' 1952 2 sheets-sheet 1 WATE Tumbma J. GuTHRn-z 2,758,979

METHOD FOR REGENERATING CATALYST BY COMBUSTION Aug. 14, 1956 @e335 EJE; Clbbor'rze United States Patent METHOD FOR REGENERA'I'JNG CATALYST BY CONIBUSTION `lames Guthrie, Sarnia, Ontario, Canada, assigner to Esso Research and Engineering Company, a corporation of Delaware Application March 23, 1952, Serial No. 279,061

1 Claim. (Cl. 252-417) This invention relates to catalytic processes and relates particularly to the use of gas turbines for supplying air or the like to a catalyst regenerator. Most specifically, it relates to a process and apparatus wherein a gas turbine is so integrated in a petroleum cracking system that the turbine exhaust gas is used as the air required for combustion of the carbonaceous catalyst deposit in a iiuid catalyst regenerator.

ln recent years catalytic cracking processes have gained prominence in the conversion of crude petroleum, and fluid catalytic cracking has assumed a particularly important position in the petroleum industry. ln all catalytic cracking processes, whether in a fixed bed, a moving bed, or turbulent uidized bed, hydrocarbon vapors are contacted with catalyst at suitable temperatures whereby heavy molecules are cracked to yield more valuable lighter products, especially gasoline. However, during such cracking the catalyst gradually becomes covered by a carbonaceous deposit which reduces the catalytic activity of the catalyst. As a result, after a certain residence time in the cracking zone the catalyst must be regenerated by combustion of the carbonaceous deposit.

Such regeneration represents one of the major steps in modern catalytic cracking processes and especially in uid operations usually requires a larger vessel than the principal cracking reaction. The cost of the required air compressors, usually driven by electric motors or by steam turbines, as well as the necessary boilers or electrical power system, constitute one of the iarge cost items in the operation of such catalytic cracking systems. Consequently the regeneration stage often limits the overall cracking capacity of a given commercial unit. Furthermore, conventional units usually require auxiliary burners which are used for heating the air whenever the regeneration step is tirst started.

Heretofore some improvement in economy has been obtained by using the het exhaust gas from catalyst regenerators to drive a dow-type gas turbine. However, this modication did not in any way affect the major investment required for compressors and incidental prime movers needed to feed air into the regenerator initially, nor did it eliminate the need for auxiliary burners required in starting up the operation of a conventional catalyst regenerator.

Accordingly, it is an object of this invention to improve the operation ol catalytic cracking processes, and particularly to reduce capital expenditure. Another object is to provide a cracking plant which can be operated without requiring the conventional air compressors and prime movers needed to drive such compressors. Still other objects as well as the nature of the present invention will become apparent from the subsequent description as well as the attached drawing wherein:

Fig. l is a diagrammatic illustration of a system comprising a combustion gas turbine which can be used according to this invention for supplying the air requirements of a catalyst regenerator;

Fig. 2 is a diagrammatic illustration of a similar but more elaborate system containing a heat exchanger for extracting heat from the regenerator combustion gases as well as an electric power generator, as a result of which a uid catalytic cracking unit may be made still more economical both with respect to air requirements and power for auxiliaries.

ln the subsequent description it will be understood that, unless otherwise stated, whenever the expressions gas turbine or turbine are used, a gas turbine of the combustion type rather than of the ow or steam type is meant. In the latter case, for instance, steam must first be brought up to the desired pressure and temperature in an auxiliary power plant before it is fed to and expanded through the turbine Where power is generated. In contrast, in a gas turbine air is usually compressed by an axial dow compressor, fuel is injected into the compressed air and burned in a combustion chamber, the hot combustion gases are expanded through the turbine proper, and discharged to the atmosphere. Where air is expensive because of required purification, a so-called closed cycle may be used wherein the exhausted air may be used over again.

The gas turbine itself drives its own compressor and any excess power may be used to drive a useful load. While the combustion gas turbine requires some external means such as an electric motor to start it and get it up to partial speed, it is otherwise a completely self-contained power plant. lt is .not necessary to make any steam or condense it.

Thermal eciency of a gas turbine to its output shaft may range from about l5 or 17 percent upward, depending on permissible maximum temperature and on various modifications of its operation. For instance, when some ot' the heat of the exhaust gases is utilized for pre-heating the compressed air, the thermal etciency may be raised to some 20 to 30 percent. Thermal el'liciency may also be increased by compressing the air in two stages, with water cooling between stages. This arrangement makes the second-stage compressor smaller and more eicient, more than compensating for the heat lost.

For flexibility and economy, the turbine may be a twoshaft or multipleshaft unit, rather than a singie-shaft unit. For instance, in a two-shaft unit the turbine may be split into a high-pressure and a low-pressure turbine unit in such a fashion that the exhaust from the high-pressure unit drives the low-pressure um't. ln such a case the high-pressure turbine may be used entirely to drive the compressor and the mechanically independent low-pressure turbine may be connected to the output shaft; or when a two-Stage compressor and intercooler are used, the high-pressure turbine may drive both the high-pressure compressor and the output shaft, while the low-pressure turbine may drive the low-pressure compressor. The main advantage of the multi-shaft turbine is its ability to have its compressor speed adjusted to give the needed air iiow, independently of the output speed.

Still other modifications and variations in the mechanical arrangement and operation of the gas turbine, as such, are well known in the art. They have been summarized above only incidentally to the description of the present invention, the essence of which lies in the combination of the gas turbine with a catalyst regenerator in such a fashion that the turbine exhaust serves to supply air to the catalyst regeneration zone as described hereafter.

Referring specically to Figure l of the drawing, atmospheric air or other combustion supporting gas from line l may enter axial iiow compressor 2 where it is compressed to about 60 to 200 p. s. i. gauge, or preferably to 150 p. s. i. gauge, or any other pressure required for the particular power output and desired conditions of the turbine exhaust gases. The compressed air. may pass. via line.. 3. through heatl exchangerY 4 where it may be heated to about 300 to 600 F. by the hot turbine exhaust gases passing through line S. The hot compressed lair is thenV passedy to turbine` combustion chamber vwhere it is mixed with a fuel such as diesel oil, burner fuel, hydrocarbon -gas or powdered coal injected through line 7. According to common practice the air is used in at least about 3 to 6 times the theoretical amount required to burn the fuel inthe combustion chamber. The hot excess air and combustion products pass from combustor 6 through line S to turbine 9 at a temperature of about 800 to 2000 F., preferably 1200 to 1500 F., depending on the inherent limitations of themetal used in the turbine. On expansion through turbine 9 enough power is produced to drive both air compressor 2 by means of shaft 10 and electrical generator 11l or any other load attached to output shaft 12. In normal operation the turbine may be operated at a temperature to just operate the compressor 2 and have no net power output or conversely, only a fraction of the total power produced in turbine 9 may be used for driving the compressor 2, the balance of power 4generated being available to drive any other load. Shaft speeds may be of the order of about 4000 to 10,000 or more revolutions per minute.

The expanded oxygen-rich exhaust gases pass from turbine 9 through line 5 at a temperature of about 400 to 900 F. and preferably at only slightly superatmospheric pressure, e. g. l to 25 p. s. i. gauge, as may be 'required for catalyst regeneration. Finally the gas may be cooled to about 400 to 600 F. in heat exchanger 4 to obtain optimum conditions of temperature and pressure for feeding to a conventional catalyst regenerator 17 wherein itv serves to oxidize carbonaceous deposits off spent catalyst particles. For instance, spent catalyst particles may be withdrawn from a conventional fluid catalytic cracking reactor 26 via standpipe 27 and mixed into the turbine exhaust gases passing through line 15. The resulting mixture of catalyst particles and air then may pass in the form of a suspension through riser 16 into regenerator 17, where carbonaceous deposits are burned off the catalyst at temperatures between about 850 and 1200o F. Because of the relatively large diameter of vessel 17, the mixture of catalyst and gas forms a dense turbulent liquid simulating bed of solids in the lower part of vessel 17 with a more dilute phase thereabove, as is well known per se. 'Regenerated catalyst particles may be withdrawn from vessel 17 via standpipe 18 and returned to reactor 26 together with fresh cracking feed such as gas oil or the like. Hot ue gas may be withdrawn overhead, preferably after passage through a cyclonic dust separator 19, and passed via line 20 to a.

waste heat boiler 21 for recovering additional heat.

From the foregoing it will be seen that a plant designed as described above avoids any need for separate air compressors to supply air to the catalyst regenerator, since the oxygen-rich exhaust from the turbine is well suited forV use in the regenerator.

Furthermore, a plant designed according to the present invention has the advantage of eliminating any need for an auxiliary burner such as is otherwise required in' conventional catalytic cracking units. Specifically, when a fluid catalytic cracking plantis first started, it is essen-V tial that the air fed to the regenerator be heated to about 650 to 700 F. before any oil can be added to the reactor. This heat during start-up has been heretofore obtained through they use of auxiliary burners which have represented a substantial portion of the totall investment required, although the actual need for such burners is limitedito a very small percentage of the operatingv lifel of the principal vr:cracking unit. The present invention avoids the' need for any suchauxiliary burners since the turbine exhaust gases in line 15-of` Fig. 1 may themselves be at'suiciently'high temperatures, especially if at least a portion of Athe-'gases is allowedV to byipass -heat exchanger 4 =viailine14^duringthe start-up period.

Another still more efficient embodiment of the invention is illustrated in Fig. 2. Referring to Fig. 2, atmospheric air at about F.. may be introduced into the system via line 201 which leads to low-pressure compressor 202. In compressor 202 the air may be compressed and its temperature raised thereby to around 300 to 350 F. This air may then be passed via line 203 through intercooler 204 whereinv it may be cooled to about er 100 F. byheat exchange with cooling water passing through line 205. The cooled compressed air may then be passed to-highepressure compressory 206 Where it is compressed to a pressure preferably in excess of 90 p. s. i. gauge and its temperature raised to about 350 F. or more.

This hot compressed air may then be passed Via line consecutively through heat exchangers 208 and 209 where it acquires heat from other gas streams later to be described. Thus the compressed gas may be heated-to at least 600 or 800 F. before introductionv into combustor 210. ln combustor 210 the compressed air is used to burn fuel injected through line 211. Again, a large excess of air is used as described earlier in connection with Fig. l andthe resulting oxygen-containing combustion gas may be at a temperature of about 1300 F. or higher if'the turbine metal permits. This hot compressed gas is then passed via line 212 to high-pressure turbine v213 through' which it is expanded to an intermediate pressure, e. g. 10 to 50 p. s. i. gauge or such` other pressure as may be desired for catalyst regeneration. The power generated in turbine 213 may be used' to drive 'high-pressure compressor 206 by means of shaft 214 as well as a useful load such as electric power generatorv 215. The shaft speed in this particular instance may be between about 8500 and 9000 R. P. M. From turbine 213`the amount of partially expanded gas required for catalyst regeneration is preferably withdrawn via line 240 or from an analogous intermediate stage passed through cooler 241 before continuing into regenerator vessel 220 where it is usedto burn coke-like deposits off spent catalyst particles in the same manner as described earlier in connection with` regenerator 17 of Fig. 1. However, whereas the hot ue gas was used for steam generation in Fig. l, in the presently illustrated preferred embodimentthe hot ilue. gas from regenerator 220k is'passed via line 221 through heat exchanger 209 where it transfers heat'to the incoming compressed air, thus contributing towards a further fuel saving in the operation of the process. Finally, the ue gas from exchanger 209 is exhausted to the atmosphere, or may still be passed through a waste heat boiler for recovery of additional heat therefrom.

The amount of partially expanded gas not required for regeneration may be passed fromhighpressurev turbine 213 to low-pressure turbine 217 via line 216. In turbine 217 it' is further expanded preferably to atmospheric pressure, yielding additional power and high eiciency. The power thus produced in low-pressure turbine 217 may be used to drive low-pressure compressor or any other load via shaft 218 which may have a speed of about 7000 to 7500 R. P. M.

The atmospheric gas exhausted from turbine 217 may be passed at about 800 to 900 F. via line 219 through heat exchanger 208 land nally vented through line 250, or at least a portion lof it may be recycled to low-pressure compressor 202 via line'2'51 and further cooler 252. In exchanger 208 some of the heat contained in the exhaust gas may be used to preheat the freshly compressed air in lineA 207.

On theother hand, if -no net power `output is con' templated in theV system, or at any rate if all the air passing through the turbines is required for catalyst regeneration, then lthe `exhaust pressure of the low-pressure turbine Vis set at 15 -to 20 p. s. i. gauge or whatever pres-` 'sure 'may' vbe 'desired :in catalyst regenerator 220.r In such a case no gas is drawn off'atanyaintermediateH'turbine stage "sueltas line-F240.' Instead, Vall the'air exhausted at the desired pressure from low-pressure turbine 217 is passed through line 219 and heat exchanger 208 into catalyst regenerator 220. The intermediate pressure between turbine stages may of course be higher here than in the alternative design wherein the terminal exhaust gas is at substantially atmospheric pressure. For instance, Where the exhaust from the low-pressure turbine is at about 15 p. s. i. gauge, the intermediate turbine pressure may be between about 35 and 70 p. s. i. gauge, depending on the pressure of the high-pressure stage and other factors well known in turbine design. Also, it will be understood that where the turbine exhaust is at a higher temperature than the regeneration flue gas, the position of heat exchangers 208 and 209 may be reversed so that the compressed air feed is preheated first by the flue gas and then by the turbine exhaust.

Having described the general nature of the invention as well as several specific embodiments thereof, it will be understood that still other embodiments and modifications are possible without departing from the scope hereof. For instance, it is entirely feasible to pass a portion of the compressed air from the compressor directly to the turbine, by-passing the combustor as shown at 230 in Fig. 2, instead of feeding all the air through the com buston Likewise, it may often be advantageous to have a multiple-stage combustor instead of the single stage shown, particularly Where complete fuel clean-up is important.

The invention is particularly pointed out and claimed as follows.

I claim:

In a process for regenerating spent solid catalyst containing a carbonaceous deposit thereon, the improvement which comprises compressing atmospheric air in at least one compression stage to a pressure of from about 60 to about pounds per square inch gauge, passing a stream of said compressed atmospheric air through at least one indirect heat exchange zone as a heat exchange medium and producing a final temperature in said air stream of from about 300 F. to about 800 F., mixing said air with fuel in a combustion zone in an air/ fuel ratio wherein said air is present in an amount from about 3 to about 6 times the theoretical requirement for complete combustion of said fuel, thereby to produce a hot combustion product rich in oxygen and at a temperature of at least 1300 F., passing said hot combustion product through at least one gas turbine, expanding said product therein to a pressure of from about 10 to about 25 pounds per square inch gauge, generating motive power through said turbine and utilizing at least a portion of said motive power initially to compress said atmospheric air prior to mixing with said fuel in said combustion zone, passing a stream of said expanded combustion product into indirect heat exchange relation through a heat exchange zone and thence, at a temperature of at least 600 F., passing said combustion product rich in oxygen into a regeneration zone for said spent solid catalyst wherein the oxygen present in said combustion product supports combustion of carbonaceous deposits on said solid catalyst as supplied to said zone.

References Cited in the le of this patent UNITED STATES PATENTS 2,346,750 Guyer Apr. 18, 1944 2,405,922 Wyman Aug. 13, 1946 2,420,534 Gohr et al. May 13, 1947 2,443,402 Schulze June 15, 1948 2,449,096 Wheeler Sept. 14, 1948 2,605,610 Hermitte et al. Aug. 5, 1952 2,608,055 Welsh Aug. 26, 1952

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