US3844929A

US3844929A - Retorting oil shale with special pellets

Info

Publication number: US3844929A
Application number: US00410200A
Authority: US
Inventors: D Wunderlich; J Skinner
Original assignee: Atlantic Richfield Co
Current assignee: Atlantic Richfield Co
Priority date: 1972-08-28
Filing date: 1973-10-26
Publication date: 1974-10-29
Anticipated expiration: 1991-10-29

Abstract

Crushed oil shale is retorted by tumbling the shale with hot special heat-carrying pellets under conditions which improve regulation over a combustible, carbon-containing deposition formed on the pellets and which increase the utility of the retorted kerogen products and in an overall operation eventually provide a way of increasing liquid oil yield. After retorting, the pellets are separated from substantially all of the spent shale smaller than the pellets. The pellets are then lifted to a combustion zone where the combustible deposition on the pellets is burned to provide a main source of retorting heat. Separation of the pellets prior to burning increases burning efficiency, and decreases handling and other operational difficulties. The special pellets are characterized primarily by their effective surface area, size, and quantity relative to the oil shale.

Description

United States Patent 11 91 Wanderlich et al.

1111 3,844,929 1 51 Oct. 29, 1974 [541 RETORTING OIL SHALE WITHSPECIAL 3,573,197 3/1971 Gcssner 208/11 PELLETS 3,303,021 4/1974 Abdul-Rahman.

3,803,022 4/1974 Abdul-Rahman 208/11 [75] Inventors: Donald K. Wunderlich; James L.

I Skinner, both of Richardson, Tex. Primary c Davis [73] Assignee: Atlantic Richfield Company, Los

Angeles, Calif. ABSTRACT [22] Filed: Oct 26 1973 Crushed oil shale is retorted by tumbling the shale with hot special heat-carrying pellets under conditions PP 410,200 which improve regulation over a combustible, carbon- Related Application Data containing deposition formed on the pellets and which [63] c f S N 84 288 increase the utility of the retorted kerogen products gy gg k glgg 2 and in an overall operation eventually provide a way of increasing liquid oil'yield. After retorting, the pellets are separated from substantially all of the spent CCll. Shale-Smaller than the pellets; The pellets are then [58] Fie'ld 208/1 I lifted to a combustion zone where the combustible deposition on the pellets is burned to provide a main 5 6] References Cited source of retorting heat. Separation of the pellets prior to burning increases burning efficiency, and decreases UNITED STATES PATENTS handling and other operational difficulties. The special 3,008,894 11/1961 Culbertson 208/11 pellets are characterized primarily by their effective g -tl surface area, size, and quantity relative to the oil evens e a. 3,058,903 10/1962 Otis 208/11 Shale 3,252,886 5/1966 Crawford 208/11 Claims, 2 Drawing Figures DUST CYCLONE {2| SURGE HOPPER PELLET 49 HEATING ZONE HEATER APOR RECOVERY CRUSHED SECTION OIL SHALE ROTATING T RETORT ZONE 43 PRIMARY PELLET I E SEPARATION ZONE 1-- I. 1 233cm: i I

; NONCOMBUSTING 21 1 ELUTRIATING GAS SECONDARY PELLET SEPARATION ZONE PELLET 1 LIFT POT 39 41 LIFT GAS PAIENIEBum 29 m4 FLUE GAS DEP BURNING gl COMBUSTION GAS I ZONE l5 HOT SPECIAL PELLETS 13 Ems? FEED CONTROLLED VAPORS ROTATING 29 PELLET SHALE SEPARATION PELLET zouz LIFTING PRIMARILY PELLETS 39 PATENTEMBT 29 1874 PELLET HEATING ZONE PELLET 5 LIFT POT 5 LIFT GAS SNEEIZBF 2 DUST CYCLON E SURGE HOPPER HEATER CRUSHED OIL SHALE FEED ROTATING RETORT ZONE DUST CYCLONE NONCOMBUSTING ELUTRIATING GAS FIGZ COM BU STION GAS VAPOR RECOVERY SECTION PRIMARY PELLET/ 23 SEPARATION zone PELLET ZONE SECONDARY SE PA RATION RETORTING OIL SHALE WITH SPECIAL PELLETS CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of copending application Ser. No. 284,288, filed Aug. 2S, 1972, entitled Retorting Oil Shale with Special Pellets," now abandoned. Both applications have been filed by the same inventors and are owned by a common assignee.

BACKGROUND OF THE INVENTION This invention relates to a process for retorting the solid carbonaceous organic matter in crushed oil shale.

In the process, special heat carriers comprised of low internal surface area bodies and high internal surface pellets are cycled through a retort system.

As a preliminary stage in the production of petroleum oils and gases, the solid carbonaceous organic solid matter or kerogen in oil shale is pyrolyzed or retorted. In an overall commercial operation, the products of retorting are processed'in additional stages, for example, solids separation, condensation, fractionation, coking, hydrogenation, and the like, depending on the types of marketable products being produced. Many processes have been suggested for the retorting stage of a commercial operation. The term retorting denotes thermal conversion of kerogen or organic matter to oil vapors and gas thereby leaving solid particulate spent shale and includes separation of the oil vapors and gas from the spent shale. The spent shale is comprised mainly of matrix mineral matter and residual carbonaceous organic matter.

When the kerogen is retorted, a normally gaseous fraction. a normally liquefiable vaporous fraction, and an organic residue are formed. The product distribu-. tion between gas. liquid, and residue is indicative ofthe distribution of the various boiling point fractions in the liquid product. It is highlydesirable to obtain a liquid product that is directly adaptable to prereflning and avoids or lessens the amount of residue or 975F. plus fraction that must be subjected to coking or other similar treatments. In many retorting processes control over the product distribution is virtually absent, and in others. any attempt to reduce the need for coking and the like by altering the boiling point distribution in the liquids either results in too much unusable material, or too much gas product, or too much organic residue, or both, which in turn eventually results in a loss of liquid oil yield. in addition, the kerogen content of the oil shale inherently or naturally fluctuates between rich and lean, and many processes are not sufficiently flexible to control product distribution when the kerogen content varies.

Excess gaseous product is undesirable. The normally gaseous fraction is useful as fuel for retorting and closely related processes; however, unless the process is directed essentially toward gasification of the kerogen, any gaseous fraction exceeding fuel requirements is not as valuable as the liquid fraction. Processing the excess gas to pipeline gas is expensive. In many pro cesses, any advantage obtained by attempting to reduce conversion to gaseous product is offset by an undesirable increase in the high boiling point 975F. plus materials which need to be coked. It would, therefore, be highly desirable to provide a process which allows more control over product distribution and which, at the same time, is efficient and flexible and reduces the effects of other process difficulties.

Frequently, the yields of various processes are compared with Fischer Assay yields. For a description of the Fischer Assay refer to Method of Assaying Oil Shale by a Modified Fischer Retort" by K. E. Stanfield and l. S. Frost, R. l. 4477, June 1949, US. Department of interior.

Processes for pyrolysis or retorting of oil shale have used direct combustion retorts and retorts heated by a retorting medium with fixed beds, moving beds, and fluidized beds. Gas has frequently been used as a retorting medium; however, gas retorts have many undesirable characteristics. Retorting with gas is objectionable since the gas dilutes the product vapors and has other undesirable features. For example, temperature control within a gas heating zone is difficult. Because of the small heat-carrying capacity of gases, high retorting gas temperatures are used and are difficult to control. High gasflow rates also tend to cause dust and to undesirably carry waste solids into the product streams.

Some of the problems associated with gaseous retorting mediums have been overcome by using solid heatcarrying bodies which exhibit better heat transfer properties than gases and supply the heat needed for retorting with a reduction in dust emission. Hot solid heatcarrying bodies, such asalumina, metal, sand, ceramic balls, pellets, or particulate solid partially spent shale, are fed into a retorting zone with the crushed carbonaceous feedstock. ln the retorting zone, the heatcarrying bodies and the feedstock intermix thereby retorting oil vapors and gases from the feedstock. The heat-carrying bodies are usually heated in a separate heating zone by burning combustible fuel material, such as heavy residor natural gas. in general, this method of heating necessitates additional equipment and creates additional handling problems.

Others have proposed cycling the partially spent shale and supplying some of the heat by burning the residual carbonaceous organic matter or solid organic char developed in the retort, or cycling catalyst particles and supplying some of the heat by burning carbon deposited on the catalyst (for example, US. Pat. No. 3,281,349). In this latter process, the surface area of the catalyst particles is not specified. Some types of catalyst particlesfrequently have a high surface area on the order of 200 square meters per gram and higher. Excessive surface area results in loss of valuable liquid product and excessive gaseous product, excessive residue, excessive heatingof the catalyst during burning, loss of valuable'heat values, higher oxygen demands, and other disadvantages including an undesirable shift in product distribution.

Oil shale yields fluctuate as streaks or veins of different richness are encountered. The yield varies for the most part between about 20 to 50 gallons per ton of raw oil shale. A large amount of particulate spent shale' is, therefore, generated. When solid heat-carrying bodies are used, about 65 percent or more of the spent shale passes through a No. 14 US. Standard Sieve size. As a result, both of the above-mentioned proposals have a major disadvantage in that a large amount of fine spent shale is combined or entrained with the cycled heat-carrying bodies especially during burning. This fine spent shale typically contains more than 2 percent by weight of organic carbon. Entrainment of this spent shale increases oxygen demands, causes loss of useful heat values, and adversely enlarges the size of equipment. Entrained tine spent shale smaller than the heat-carrying bodies also interferes with Control of the process and creates many other problems. Moreover, the presence of appreciable amounts of entrained fine material severely limits of the type of equipment which can be used for burning retort residue. Generally, burning in the presence of fine material requires the use of lift pipes. If air is used for lifting, the burning could entail a large excess of oxygen which would rapidly burn the organic matter and create disadvantages in the process of this invention. In a process like that described in U.S. Pat. No. 3,281,349, the crushed shale is originally larger than the catalyst particles, and spent shale particles larger than the catalyst particles are screened out. But it is practically infeasible to attempt to maintain this size relation so that most of the spent shale particles can be separated by retention on a screen or some similar means, and any such attempt will result in a decrease in yield and a loss of efficiency.

Briefly, therefore, known processes suffer certain disadvantages which chiefly reside in poor product distribution and utility and in process handling difficulties.

A principal object of this invention is to increase the utility of the retort products in a way which reduces the amount of organic residue left on the spent shale, improves liquid product quality, and eventually, in an overall operation, leads to increased liquidoil yields while at the same time overcoming many of the problems encountered in other processes.

A further object of this invention is to provide an improved method for retorting of crushed oil shale with improved control over retort product distribution.

Another object of this invention is to simultaneously retort oil shale with special heat-carrying pellets and develop a controlled amount of fuel for directly reheating the pellets, thereby reducing the amount of fuel required to carry out the process, improving yield, reducing costs and equipment requirements, and lessening heat loss and avoiding waste of valuable heat values.

SUMMARY OF THE INVENTION In a retorting process, crushed carbonaceous solid organic matter is retorted to gases, oil vapors, and combustible residue with special heat-carrying pellets in a manner which improves the quality of the liquid oil products and the usefulness of the gas and residue, thereby conserving the liquid product. The product gases are not diluted by extraneous retorting gases and, because the oil product is improved during retorting, are produced at a time which renders them more useful for fuel and other operational requirements. The combustible residue is deposited on the pellets and this deposition is then burned in ways which render the deposition more useful as fuel for heating the pellets. Some of this deposition is comprised of organic carbon that would otherwise be left on the spent shale, thereby improving the location and usefulnessof such combustible deposition. The remaining portion of the combustible deposition on the pellets is comprised of the cokelike residue that is expected to be produced in a process of equal liquid product quality, but which would normally have been produced in such a way as to not be nearly so useful in heating the pellets. The retorting process has the further advantage of reducing the need for liquid or gaseous fuels which are normally required in the production of syncrude from solid carbonaceous materials. In addition, therefore, the retorting process lends itself with great success to treatment of raw mined oil shale in a way which is highly compatible with other syncrude processing stages being conducted at the site of the retorting stage of an overall commercial operation which eventually results in increased liquid product yield.

More particularly, in the retorting process mined oil shale which has been crushed and which contains said carbonaceous organic matter and other mineral matter is fed to a rotating retorting zone. This type of retort is especially suited to this process and the variables pre scribed for the process. The crushed oil shale feedstock may have been preheated. At the same time special hot heat-carrying pellets at a temperature hot enough to retort the raw crushed oil shale are fed into the retorting zone, thereby causing pyrolysis of gases and oil vapors from the oil shale and forming particulate spent shale and a combustible deposition onthe special pellets. The special pellets are comprised of particulate or divided solid heat carriers whose physical properties and characteristics, especially size, effective surface area, temperature, and amount, coact with other process variables to control the amount of combustible deposition formed on the pellets during retorting and to accomplish the other objectives and advantages of the process. More specifically, the size and preferably the shape, of the pellets assist in separation of the pellets from spent shale and in controlling the amount of deposition formed on the pellets which in turn relate to the product yields of the process. The amount of deposition formed on the pellets relates to the yield of various fractions in the products, provides combustible fuel for heating the pellets, and affects the deposition burning time, combustion temperature of the pellets, and oxygen requirements. The combustion temperature is controlled to prevent inefficient heating and excessive loss of pellets surface area. The effective surface area, retorting inlet zone temperature, and ratio of pellets to oil shale also control the amount of deposition formed on the pellets, the retort temperature of the oil shale, and the sensible heat applied to the process by way of the pellets. .Preferably, the pellets will not include active acid sites which could cause excessive deposition formation and excessive gas production.

The preferred amount of deposition formed on the pellets is less than 1.5 percent by pellet weight per pass through the retort. The pellets may be derived from suitable particles originally exhibiting too high a surface area particles, forexample some types of catalyst particles, but which have been subjected to conditions which have reduced their surface area to 150 square meters per gram or less. Catalyst particles bearing active acid catalytic sites may also be treated to deactivate such catalytic sites.

The gases and oil vapors are recovered, and after retorting of the oil shale, at least percent of the total particulate spent shale and at leastpercent of the ment, the degree of total separation is enhanced by controlling the sphericity factor of the pellets to at least 0.9, or by crushing the raw oil shale to a smaller than normal size, that is, to minus 6 US. Sieve Series size.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIG. 1 is a schematic flowsheet of the process of this invention; and

FIG. 2 is a'partly schematical, partly diagrammatical 5 flow illustration of a system for carrying out a preferred sequence of the process of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION The process for retorting of crushed oil shale containing carbonaceous organic matter and other mineral matter is described in general terms having reference to FIG. 1 and in more particular terms by reference to FIG. 2.

Raw or fresh oil shale which has been mined and pulverized, crushed or ground for the most part to a predetermined maximum size for handling in a retorting system by any suitable particle diminution process is fed directly from a crusher or from a hopper or accumulator by way of shale inlet line 11 into rotating zone 13. At the same time. special heat-carrying pellets substantially hotter than the shale feed are fed by gravity or other mechanical means to the retort zone by way of pellet inlet pipe 15. The pellets and shale feedstock could be fed to the retort zone by way of a common retort zone inlet.

Crushing of the raw mined shale expedites more uniform contact and heat transfer between the shale feedstock and hot pellets and more uniform deposition of a carbon-containing deposition on the pellets. In normal practice, the degree of crushing is simply dictated by an economic balance between the cost of crushing and the advantages to be gained by crushing when retorting the kerogen from the shale. Generally the shale feedstock is crushed to about minus three-fourths inch and no particular care is taken to produce or restrict production of finer material. In this process. crushing has a special purpose and aids in a preburn separation step. In one embodiment for reasons which will be hereinafter shown and despite the added costs, and standard practice, the mined shale is crushed to a substantially finer size wherein at least 95 percent by weight of the crushed oil shale will pass through a US. Sieve Series size 6 screen.

The crushed oil shale may or may not be preheated by direct or indirect heat from any source including indirect heat exchange with pellets or flue gases generated during this retorting process. lfthe scale feedstock is preheated, the temperature of the feedstock will not exceed 600F. The shale feedstock will usually be fed by way of a metered weight controller system for reasons hereinafter made apparent and which may include a preheat and/or gas lift system. The preferred system for preheating the raw shale is to lift the shale in lift pipes with the hot flue gases generated in the combustion phase of the process.

The hot special heat-carrying pellets are especially characterized by having a size during use of between approximately 0.055 and 0.5 inch and a surface area during use of between l0 and 150 square meters per gram. The surface area is the average effective surface area upon entry into the retort zone. In one embodiment of the process of this invention. the surface area of the pellets on a gram basis is between 10 and square meters. The importance of surface area is here inafter discussed in detail. The heat-carrying pellets are at a temperature ranging between l,0O0F. and l,400F. which is about 100F. to 500F. higher than the designed retort temperature within the retort zone. The most favorable practical temperature range depends on the process variables and more particularly on the specific advantages and characteristics of this process. The quantity of pellet heat carriers is controlled to cooperate with other process variables so that the pellet-to-shale feedstock ratio on a weight basis is between 1 and 3 with a ratio between 1.5 to 2.5 being preferred. This ratio is, moreover, such that the sensible heat in the pellets is sufficient to provide at least 50 percent of the heat required to heat the shale feedstock from its feed temperature to the designed retort temperature. The feedstock feed temperature is the temperature of the oil shale after preheating, that is, the temperature of the shale upon entry into the rotating retort. The average retort temperature ranges between about 850F. and l,200F. depending on the nature of the shale feedstock, the pellet-to-shale ratio, the type of product distribution desired, and heat losses. The relative mass and size of the pellets are selected in a manner hereinafter set forth which facilitates separation of the pellets from spent shale, controls the amount of combustible residue deposited on the pellets, optimizes other facets of the retorting process, and makes allowance for wear or size reduction of the pellets as they are cycled and recycled through the retorting process.

The term pellets refers to subdivided or particulate bodies a majority of which have the characteristics and properties herein required and which are composed of the same or dissimilar materials having the specified surface area and strength and of irregular shape, cylindrical shape, approximately oval or spherical shape, or purely spherical shape. The preferred pellets have a sphericity factor ofat least 0.9 which, in addition to the usual advantages of facilitating movement of the pellets through the retorting process and of providing optimum solid-to-solid heat transfer and contact between the pellets and oil shale feedstock, has an advantage particularly useful in separating the pellets from other solids produced in the process as hereinafter set forth. The sphericity factor is the external or geometric surface area of a sphere having the same volume as the pellet divided by the external surface area of the pellet.

The pellets are made up of materials, such as, alumina or silica alumina, which are not consumed in the process and which are subdivided or particulate matter having significantly high internal surface area but not excessively high. The pellets are sufficiently wear or breakage resistant and heat resistant to maintain their physical characteristics under the conditions employed in the process,to effect retorting of the oil shale, and to permit controlled burning of acarbon-containing deposition formed on the pellets during such retorting. More specifically, the pellets do not disintegrate or decompose, melt or fuse, or undergo appreciable surface area reduction at the temperatures encountered during such burning and the thermal stresses inherent in the process. The pellets will,'o'f course, undergo some gradual wear or size reduction.

As will be shown, the size of the pelletsis related to the other process variables and to the preburn separation step of this process. The original or fresh pellets will be comprised of particulate sensible heat carriers in a size range between about 0.1 inch and 0.5 inch, with a size range between about 0.1 inch and 0.375 inch being preferred, and will for the most part be maintained during use at a plus 14 US. Sieve Series Screen size, that is, approximately 0.055 inch or greater. Finer pellet grain sizes are undesirable in the process of this invention.

Suitable pellet materials are found in cracking catalyst; however, the retorting process of this invention is not to be considered as relying on active catalytic sites. Many catalysts have surface areas far in excess of the maximum surface area of 150 square meters per gram provided in this process. For example, some silica alumina catalysts have a surface area ranging between 180 and 700 square meters per gram. As will hereinafter be discussed and as indicated by the trend shown in TABLE I, this high surface area results in too much carbon-containing residue being deposited and, as further indicated in TABLES 2 and 3, in an excessive shift in product distribution and gas formation which is just as undesirable as producing too much high boiling point products. Excessive gas and coke production cuts into the final yield of liquid product which this process seeks to maintain.

Active catalytic sites have effects similar to excessive surface areas and the process of this invention seeks to maintain the deposition formed per pass through the retort zone to less than 1.5 percent by weight. As a result. in this process, no active acid cracking catalyst sites or the like are intentionally supplied to the retort zone. What is used is pellets that have the size and surface area limitations herein set forth. Of course, the retorting phase ofthis process and the subsequent deposition combustion phase could be conducted with a catalyst and carried outwith some loss of flexibility in such a manner as to kill active catalyst sites and limit or destroy excessive available pellet surface area; but it is preferred that the pellets not bear such sites and have or rapidly develop the prescribed surface area naturally. Thus, in one embodiment of this invention the pellets are comprised of particulate or subdivided matter, for example, catalyst particles. composed or manufactured of materials which can be treated to reduce their surface area and which are of appropriate size, but which originally had a surface area in excess of 150 square meters per gram, and which have been treated to reduce the effective surface area to less than 150 square meters per gram. An originally high surface area can be reduced by methods similar to the way that catalyst particles lose their effective surface area as they age when used in catalytic cracking or hydrogenation units, or by subjecting the particles to rapid or prolonged aging at temperatures and fluid pressures suffi cient to reduce the surface of the particles. A preferred way to cause this reduction in surface area is to subject the particles to temperatures above l,400F. in the presence ofsteam at pressures between-0.5 and 7 atmospheres until the surface area is reduced to the desired level. By way of illustration, it has previously been reported in accelerated aging experiments that by subjecting a silica alumina catalyst to one atmosphere of steam for l hour at l,585F. the surface area was reduced from about 180 square meters per gram to about square meters per gram, and in a similar experiment at l,432F. a surface area of 400 square meters per gram was reduced to square meters per gram in about 20 hours. The high surface area particulate matter thus treated may originally have been comprised of high surface area particles with active acid catalytic sites. In such case, the particles could also be treated to deactivate their active acid catalytic sites by subjecting them to conditions and chemicals known to poison or kill such active catalytic sites, for example, by treatment with sodium bicarbonate, sodium hydroxide, or sodium carbonate.

The retort zone is any sort of horizontal or inclined retort or drum which causes an intimate mixing and tumbling action of crushed oil shale and pellets. For purposes of this invention, this sort of retort is called a rotating retort zone. This type of retort zone is quite flexible over a wide range of conditions and more specifically has the advantages of causing rapid solid-tosolid heat exchange between the pellets and shale feedstock thereby flashing and pyrolyzing the oil and gas vapors from the shale in a way which allows the vapors to separate from the solids without passing up through a long bed of solids and which minimizes dilution of the product vapors by extraneous undesirable retorting gases; of allowing for a high scale throughput rate at high yields for a given retort volume; of providing for greater control over residence time; of aiding in preventing overcoking and agglomeration of the pellets and shale; of facilitating formation of a more uniform controlled amount of combustible carbon-containing residue on the surface area of the pellets; and of causing flow of the pellets and shale through the retort zone in a manner which aids in eventual separation of the pellets from the spent shale. The amount of deposition deposited on the pellets is an important feature of this process and will be discussed later in more detail. The retorting process is carried out in concurrent or parallel flow fashion with the hot pellets and the raw shale feedstock being fed into the same end of the retort. The retort zone may be maintained under any pressure which does not hamper efficient operation of the retort, interfere with production of valuable retort vapors, or cause excessive deposition of residue on the pellets. Generally, pressurization of the pyrolysis or retort zone causes considerable difficulties because of the rotating nature of the retort zone. The pressure employed is, therefore, generally the autogenous pressure.

As'the retort zone is rotated, the hotter pellets and cooler crushed shale feedstock are admixed and intimately contacted almost immediately upon being charged into retort zone. The shale particles are rapidly heated by sensible heat transfer from the pellets to the shale. Any water in the shale is distilled and the kerogen or carbonaceous matter in the shale is decomposed, distilled, and cracked into gaseous and condensable oil fractions, thereby forming a valuable vaporous effluents including gas, oil vapors, and superheated steam. Pyrolysis and vaporization of the carbonaceous matter in the oil shale leaves a particulate spent shale in the form of the spent mineral matrix matter of the oil shale and relatively small amount of unvaporized or ing process produces more than 100 percent of Fischer Assay if the total usable products are added, that is, the gas, the oil, and deposition on the pellets.

As the aforementioned vaporous effluents are formed, a combustible carbon-containing deposition or residue is formed or deposited on the controlled surface area of the special heat-carrying pellets. It has been found that the variables of this process as herein set forth may be related in a manner which aids in regulation of the amount of carbon-containing deposition thus deposited and at the same time make allowance for the fact that the original kerogen content of the raw shale feedstock will intrinsically and periodically vary. The amount-of deposition formed or deposited on the pellets upon passage through the retort zone is sufficient upon combustion to provide at least 50 percent of the heat required to reheat the pellets and is on an average less than l.5 percent by weight of the pellets per pass through the retorting zone. The preferred pellet residue range is between 0.8 and 1.5 percent. Basically, this control is critical in two respects. First, the amount of deposition on the pellet is important since as will hereinafter be shown this deposition is burned in a controlled manner to generate a major portion of the heat necessary for heating the pellets to carry out the retorting phase of the process. In addition to decreasing efficiency, excessive deposition increasesgreatly the possibility of overheating the pellets and destroying or altering their controlled surface area. Second, the total amount of deposition affects the ultimate relative yields of gas and condensable or final liquefied products. This in turn affects the distribution of various boiling point fractions in the liquefied products. The amount of deposition deposited is basically controlled in this process by the interrelation of several variables, such as, pelletto-shale ratio, pellet size and surface area, pellet inlet temperature, retort temperature, and the type of retort or pyrolysis zone. Additional control over both the total amount of deposition deposited on the pellets and the amount of deposition deposited on each pellet may be obtained by residence time or throughput rate, partial or complete combustion of the deposition, controlled residue combustion time or amount of oxidizing gas used during burning, the noncatalytic characteristic of the pellets, and the size of the pores at the surface sible heat in the pellets may not be sufficient to carry out the retorting phase of this process. This cannot be offset by heating the pellets to higher temperature beofthe pellets. As can be readily seen by this description of the process, the degree of control provided by a single variable is never independent and the flexibility of control varies with the type of variable.

The pellet surface area is considered one of the most important variables. The effect of pellet surface area is illustrated by the test results set forth in TABLES 1,2, and 3. The effect of pellet surface area on the amount of carbon-containing deposition formed on the pellets and on the distribution of carbon deposition between the pellets and spent shale is illustrated in TABLE I. TABLE 2 illustrates the effect of pellet surface area on liquid product distribution when a modified Fischer retort was used. The effect of pellet-to-shale ratio and, therefore, total surface area of the pellets is illustrated in TABLE 3. The results illustrated in these tables lead to several conclusions. First, if the surface area-exceeds I square meters per gram, either too much total deposition or too much deposition per pellet. or both, will result. If, in the event of excessively high surface area pellets, an attempt is made to control the total deposition by employing a lower pellet-to-shale ratio, the sencause at pellet temperatures above I,400F., the surface area is likely to be destroyed; moreover, excessively high temperatures increase problems in the retort zone and undesirably change the product distribution and the amount of noncondensable gases produced. Conversely, if the surface area is less than 10 square meters per gram, either too little total deposition will be formed or the burning of the deposition will not be sufficient to provide a major portion of the heat required to heat the pellets to the desired temperature and to carry out the retorting phase of this process. This would necessitate the use of supplementary fuels and as indicated previously, this has significant disadvantages to the objects of this process.

TABLE 1 EFFECT OF PELLET SURFACE AREA ON CARBON DEPOSITION WT. PERCENTAGE OF CARBON PELLET AREA ON ON RESIDUAL m /g PELLETS SHALE No Pellets 4.30

TABLE 2 EFFECT OF PELLET SURFACE'AREA ON LIQUID PRODUCT DISTRIBUTION PRODUCT NO PELLET AREA" BOILING RANGE PELLETS 47m"'/g 96ni"/g 400F. I27 2771 3471 400 700F. 3771 4671 48% 700 900F. 3271 22 7! I471 )00"F.+ I971 571 471 Pellet Shale Ratio 2:l

TABLE 3 EFFECT OF PELLET SHALE RATIO ON LIQUID PRODUCT DISTRIBUTION PRODUCT NO PELLET SHALE RATIO BOILING RANGE PELLETS l:l l.5:l 2:1

l50 400F. I471 257? 307i 347: 400 700F. 3871 4571 4771 487: 700 900F. 3l71 227: I77: I47: 900F.+ I771 871 671 47:

ables including surface area, pellet-to-shale ratio, pellet size, and richness of the oil shale being considered, it

has been found that pellet surface areas'between l and 150 square meters per gram are acceptable with surface areas between and 100 square meters per gram being preferred and that pellet-to-shale ratios between one to three are acceptable with pellet-to-shale ratios between 1.5 and 2.5 being preferred.

As illustrated in TABLE 1, of particular additional significance is the fact that a substantial portion of the combustible deposition on the pellets comes from the residual carbonaceous material that would normally be left on the spent shale. In other words, the organic carbon material that is normally left on the spent shale divides itself between the pellets and the spent shale. This process, thereby, recovers residue that would normally be lost with the spent shale. The recovered residue is then made useful as fuel for heating the pellets. At first glance. this recovery may seem small, but when it is remembered that the organic content of the oil shale is small, it can be seen that this increased recovery and utility of the residue is quite significant.

As the retort zone is rotated, the mixture of pellets and shale moves toward retort exit 17 and the gaseous and vaporous effluentscontaining the desired hydrocarbon values separate from the mixture. Since in this type of retorting system there is no need to use carrier, fluidizing, or retorting gases in the retort zone, the vaporous effluent is able to leave the retort essentially undiluted by extraneous fluids except for any water or steam vapor added to prevent or retard carbonization, or to sweep product vapors from the solids, or for other reasons to the retort or the effluent collection chamber. The mixture movement is continuous and is aided by the action or design of this type of retort and by continuous withdrawing of pellets and spent shale from the exit end of the retort zone. Any caking or coking together of the heat-carrying pellets or spent shale is kept low. Moreover, this type of retort zone is especially suited to varying the residence time, that is, the length of time'that the shale and pellets remain in the retort zone by allowing variations in pellet-to-shale ratio and volume of shale throughput. As mentioned previously, greater than normal leeway in control over these variables is especially advantageous to regulation of the amount ofdeposition deposited on the pellets. The residence time required to effect retorting and deposition of the pellet residue is on the order of about 3 to about minutes with residence times of less than 12 minutes for the pellets being preferred. The shale residence time depends on its flow or movement characteristics and since the shale is not uniform in size and shape, the shale residence time varies.

The mixture of pellets and spent shale exits from retort zone 13 at a temperature between 800F. and l,050F. by way of retort exit 17 into separation zone 19 for separation of the vapor, pellets, and spent shale. The separation zone may be any sort of exiting and separation system accomplishing the functions hereinafter mentioned and may be comprised of any number of units of equipment for separating and recovering one or more of these three classes of retort zone effluents either simultaneously, or in combination, or individually. In the process of this invention, it is critical that at least percent of the total spent shale be separated from the pellets in the separation zone to eventually be collected in separation zone exit line 21. In addition, at least percent of the spent shale smaller than the pellets, that is, smaller than 0.055 inch, are separated. As

shown in FIG. 2,v the retort zone mixture is first passed through revolving screen or trommel 23 which has openings or apertures sized to pass the pellets and spent shale of about the same or smaller size than the pellets. The trommel extends into product recovery chamber 25. In the trommel, the gaseous and vaporous products separate from the mixture of pellets and spent shale and, at the same time, at least a portion of the larger spent shale particles or agglomerates are separated from the pellets and spent shale. Spent shale and pellets flow through the openings in trommel 23 and drop to the bottom of recovery chamber 25 to exit via retort exit line 27. Any rocks or spent shale too large to pass through the openings in the trommel pass outward through exit 29. The product vapors and gases resulting from retorting the oil shale collect overhead in recovery chamber 25 and rapidly pass to overhead retort products line 31 atan exit temperature between about 750F. and 950F. where the product vapors are subjected either in their vaporous or condensed state to hot dust separation (not shown) and passed to other stages (not shown) of the overall operation. The hot dust separation may be interior or exterior, or both, of recovery chamber 25 and the dust thus collected may be combined and handled with other spent shale. Hot dust or fines separation may be accomplished by hot gas cyclones, quenching and washing, agglomeration with sludge or a separately condensed heavy'product fraction, centrifuging, filtration, or the like.

As mentioned previously, the gases produced in the retort zone are not diluted by extraneous retort gases and are, therefore, readily used in the overall shale operation. Some gas may be needed for supplementary fuel and some for production in the usual manner of hydrogen if hydrogenation is used in the overall shale operation. The optimum amount of gas production is enough to satisfy these requirements as this process stresses liquidoil products.

The spent shale and pellets in recovery chamber 25 are discharged via exit line 27 at a temperature between about 750F. and 950F. where these particulate solids are passed or conducted by gravity or other means of conveyance to gas elutriation system 33 which is a part of separation zone 19. In the elutriation system, a major portion, and more. preferably substantially all, of the remaining spent shale is separated from the pellets. it is essential that elutriation be accomplished in a way which retains the desired amount of combustible deposition on the pellets; consequently, the elutriating gas is a noncombustion supporting gas. By conducting the process with pellets in the size range between 0.055 and 0.5 inch, at least 75 percent of the total spent shale may be separated by action of the trommel and subsequent gas elutriation at a velocity of between 18 and 25 feet per second if most of the raw shale feedstock was crushed to minus three-fourths inch. Based on an average of'six sieve analyses of the spent shale produced by retortingminus three-fourths inch shale feedstock in a rotating retort using ceramic /2 inch balls, about l6 percent byweightianalyses range 8 to 27 percent) of the spent shale is retained on a U.S. Sieve Series'size 14 screen which is in a size range similar to the pellets. Gas elutriation with irregular or cylindrical shaped pellets only separatesabout 2.0 to 4.0 percent of this portion of the spent shale from the pellets. Therefore, on an average between 12 and 13 percent of the spent shale is difficult to separate by screening and elutriation depending on whether the pellets cover the entire size range of this part of the spent shale. As mentioned previously, retention of more than percent of the spent shale interferes with proper operation of the pellet deposition burning zone even if most of the spent shale entering the burning zone is originally in the same size range as the pellets. Upon combustion, this spent shale would disintegrate further to fine ash and cause erratic operation of the combustion zone and other operating difficulties. In addition, some allowance is made for spent shale and ash buildup as the pellets are cycled and recycled through the process.

Since the spent shale having a size similar to the pellets is difficult to elutriate while the spent shale smaller than the pellets is readily separated by elutriation, and practically complete, it is desirable to alter the characteristics of the spent shale or of the pellets to accomplish a greater degree of separation while holding heat losses in the pellets to a reasonable level. One way to accomplish this objective is to crush at least 95 percent by weight of the shale feedstock to a minus 6 screen size. This results in a separation of at least 95 percent by weight of the total spent shale from the pellets and the trommel may also be eliminated. As mentioned previously, crushing to this size is costly and normally not donerhowever, in view of the fact that in this invention it is essential that the bulk of the spent shale be separated from the pellets prior to reheating of the pellets, the cost of additional crushing may be justified. Another way to accomplish the objective of this separation prior to reheating the pellets has been discovered. It has been found that if the pellets are essentially spherical, that is, have a sphericity factor of at least 0.9, the efficiency of separation by gas elutriation is greatly increased even if the raw shale is not crushed to a finer size. Spherical pellets have improved flow properties over the spent shale and for a given screen size particle exhibit greater weight per particle. Gas elutriation with spherical pellets will separate about 97 percent or more of the total spent shale retained on a U.S. Sieve Series size 14 screen and will provide almost complete separation of the smaller spent shale. Thus, if spherical pellets are used, gas elutriation will separate at least 95 percent of the total spent shale. As mentioned previously, therefore, the preferred shape of the pellets is spherical, that is, the preferred pellet should have a sphericity factor of at least 0.9.

The separated spent shale is carried out of the elutriating chamber overhead through line to dust cyclone 37 where the spent shale is collected and the spent shale thus collected may be combined and handled with other spent shale for eventual compaction and waste disposal or sale for use in manufacturing other products. Y

The separated pellets with their combustible deposition are then passed from the separation zone to a pellet deposition burning zone'via pellet return line 39 to pellet lifting system 41 where the pellets are lifted preferably to an elevation which allows gravity feed to retort zone 13 by way of lift line 43 to pellet deposition burning zone 45, which as shown in FIG. 2 has surge hopper 47 for collecting the lifted pellets and leveling out fluctuations and from which the pellets fall into combustion zone 49. While any conveying and lifting system holding heat losses to a reasonable value may be used, it is preferred as shown in H6. 2 that the pellet lifting system be a pneumatic conveying system which will operate in the conventional manner to lift the pelbustible deposition is burned in pellet deposition burning zone to provide at least 50 percent or more of the heat required to reheat the pellets to the temperature required to effect retorting of the shale. The combustible deposition is burned in a manner similar to the way that cracking catalysts particles are regenerated and which would excessively reduce the effective surface area of the pellets to less than ten square meters per gram. A progressive bed burner with a gas flow of about one to two feet per second is preferred. A combustion supporting gas, for example air, a mixture of air and fuel gas generated in the process, flue gas with the desired amount of free oxygen, is blown into the pellet deposition burning zone at a temperature at which the deposition on the pellets is ignited by way of combustion gas inlet 51 which in FIG. 2 includes a blower. Steam may also be used to control burning provided that the steam does not excessively reduce the surface area of the pellets. The combustion supporting gas may be preheated in heaters 53 by burning some of the gases produced in the process to reheat the pellets to the minimum ignition temperature. The quantity of combustion supporting gas, e.g., about 10 to 15 pounds of air per pound of deposition, affects the total amount of deposition burned and the heat generated by such burning and in turn the temperature of the pellets. The bulk density of the pellets is about 40 to 50 pounds per cubic foot. The specific heat of the pellets varies between about 0.2 and 0.3 British Thermal Units per pound per degree Fahrenheit. The gross heating value of the carbon-containing deposition is estimated to be about 15,000 to 18,000 Btu per pound. The amounts of carbon dioxide and carbon monoxide produced in the flue gases created by burningthe pellet deposition indicate the amount of combustion supporting gas required or used and the amount of carbon-containing deposition not burned. Generally, it is desirable to attempt to free the pellets of deposition. in any case, as a general rule, at least 50 percent of the deposition is burned. The unburned deposition is returned to the retort zone with the pellets. In this manner, the total amount of carbon-containing deposition deposited per cycle on the pellets is also regulated to some degree. It should be noted that this type ofcontrolled burning does not selectively burn the same amount of deposition from every pellet. Other factors taken into consideration during burning of the pellet deposition are the pellet porosity, density, and size, the burner chamber size and pellet bed size, residence burning time, the desired temperature forthe pellets, heat losses and inputs, the pellet and shale-feed rates to the retort zone and the like. The residence burning time will usually be rather long and up to about 30 to 40 minutes. Combustion of the deposition should be controlled in'a manner which does not heat the pellets to above 1,400F. The hot flue gases generated in the pellet deposition burning zone may be removed by burning zone exit line 55 and used to preheat cool raw shale feedstock or for heat transfer to any other phase or partof the shale operation. For example. this stream could be fed to a carbon monoxide boiler, and the heat available from the boiler could be used for processing product vapors or to drive turbines. Of course, additional fuel material or gases may be used to supplement burning of the pellet deposition if this is necessary, but it is to be understood that the pellet deposition supplies the major portion of the sensible heat required for retorting of the shale and that the process variables are set to accomplish this objective along with the other advantages and objectives of this process.

A continuous stream of hot pellets having a temperature between l,000F. and 1,400F. is thereby produced for return and introduction back through pellet inlet pipe 15 into retort zone 13. As previously indicated, the rate of return of the pellets will be metered or controlled in conventional manners to correspond to the crushed raw oil shale feed rate, the organic content of the raw oil shale, the optimum pellet-to-oil' shale feedstock ratio, the desired distribution of products, and to the other process variables previously described.

Although the retorting process is carried out in a manner to hold loss of pellets to a minimum, some pellets will be lost in the process and a relatively small quantity of pellets may be added continuously to maintain the pellet quantity.

The foregoing description of the conditions and variables of the process illustrates a preferred method of conducting a retorting process. Reasonable variations and modifications are practical with the scope of this disclosure without departing from the spirit and scope of the claims of this invention. For example, while the disclosure of this process and the variables have been limited to oil shale, the process concepts lend themselves readily to retorting of any solid organic carbonaceous material containing hydrocarbon values which can be recovered by thermal vaporization of the solid carbonaceous material. such as, for example. coal peat, and tar sands. By way of further example, while onlya single train of units and stages have been described, it is to be understood that any stage or zone could be comprised of more than one stage or zone, each of which could be operated under different conditions to provide the overall combined effect set forth.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A method for retorting crushed oil shale containing carbonaceous organic matter and mineral matter comprising a. feeding crushed oil shale and pellets'to a rotating retort zone, said pellets being comprised of particulate heat carriers being in a size range between 0.5 inch and approximately 0.055 inch and having a surface area of between and 150 square meters per gram of pellets. said pellets being at a retort zone inlet temperature between l,000F. and l,400F. and in a quantity such that the ratio of said heat-carrying pellets to said crushed oil shale in said retort zone on a weight basis is between one and three. said ratio also being such that the sensible heat in said pellets is sufficient to provide at least fifty percent of the heat required to heat said crushed oil shale from its retorting zone feed temperature to a retort zone outlet temperature of between 800F. and l,050F.;

b. tumbling said crushed oil shale and said pellets in said retort zone to retort oil vapors frm said crushed oil shale and form particulate spent shale and a combustible carbon-containing deposition on said pellets;-

c. causing said pellets and said spent shale to pass from said retort zone to a particle separation zone and separating from said pellets at least percent by weight of the total spent shale and at least percent by weight of the portion of said spent shale that is smaller in size than said pellets prior to burning said combustible carbon-containing deposition on said pellets;

d. recovering said oil vapors generated by retorting said crushed oil shale;

e. lifting said pellets from said separation zone to a pellet deposition burning zone;

f. heating said pellets in said pellet deposition burning zone to an outlet temperature of between l,000F. and l,400F. by burning at least a portion of said combustible carbon-containing deposition with a combustion supporting gas; and

g. returning said heated pellets to said retort zone in step (a) for retorting crushed oil shale.

2. The method according to claim 1 wherein the particulate carriers are in a size range between 0.375 inch and approximately 0.055 inch.

3. The method according to claim 1 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than l.5 percent by weight of said pellets.

4. The method according to claim 3 wherein the pellets have a sphericity factor of at least 0.9.

5. The method according to claim 1 wherein the separation of step (c) is comprised of first passing said pellets and said spent shale through apertures in a trommel to screen out at least a portion of the spent shale and any agglomerates larger than said pellets. and thereafter subjecting the remaining pellets and spent shale to gas elutriation with a noncombustion supporting gas to effect further separation of the spent shale from the pellets.

6. The method according to claim 5 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

7. The method according to claim 1 wherein the pellets have a sphericity factor of at least 0.9 and at least 95 percent by weight of the total spent shale is separated from said pellets in step (c).

8. The method according to claim 7 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets. I

9. The method according to claim 1 wherein at least 95 percent by weight of the crushed oil shale of step (a) has been crushed to a size to pass through a U.S. Sieve Series size 6 screen and at least 95 percent by weight of the total spent shale is separated from said pellets in step (c).

10. The method according to claim 9 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

11. The method according to claim 1 wherein the pellets of step (a) are comprised of particulate matter which originally had a surface area in excess of 150 square meters per gram and were treated to reduce the surface area to less than 150 square meters per gram.

12. The method according to claim 11 wherein the particulate matter was originally comprised of cracking catalyst'particles with active acid catalytic sites and which were also treated to reduce their active acid catalytic activity.

13. The method according to claim wherein the particulate heat carriers are in a size range between 0.375 inch and approximately 0.055 inch.

14. The method according to claim 1 wherein said pellets have a surface area of between 10 and 100 square meters per gram of pellets and said ratio of said heat-carrying pellets to said crushed oil shale in said retort zone on a weight basis is between 1.5 and 2.5.

15. The method according to claim 14 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

16. The method according to claim 15 wherein the pellets have a sphericity factor of at least 0.9.

17. The method according to claim 14 wherein the particulate heat carriers are in a size range between 0.375 inch and approximately 0.055 inch.

18. The method according to claim 14 wherein the separation of step (c) is comprised of first passing said pellets and said spent shale through apertures in a trommel to screen out at least a portion of the spent shale and any agglomerates larger than said pellets. and thereafter subjecting the remaining pellets and spent shale to gas elutriation with a noncombustion supporting gas to effect further separation of the spent shale from the pellets.

19. The method according to claim 18 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

20. The method according to claim 14 wherein the pellets have a sphericity factor of at least 0.9 and at least percent by weight of the total spent shale is separated from said pellets in step (c).

21. The method according to claim 20 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

22. The method according to claim 14 wherein at least 95 percent by weight of the crushed oil shale of step (a) has been crushed to a size to pass through a US. Sieve Series size 6 screen and at least 95 percent by weight of the total spent shale is separated from said pellets in step (c).

23. The method according to claim 22 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

24. The method according to claim 14 wherein the pellets of step (a) are comprised of particulate matter which originally had a surface area in excess of square meters per gram and were treated to reduce the surface area to less than 150 square meters per gram.

25. The method according to claim 24 wherein the particulate matter was originally comprised of cracking catalyst particles with active acid catalytic sites and which were also'treated to reduce their active acid catalytic activity.

Claims

1. A METHOD FOR RETORTING CRUSHED OIL SHALE CONTAINING CARBONACEOUS ORGANIC MATTER AND MINERAL COMPRISING A. FEEDING CURSHED OIL SHALE AND PELLETS TO A ROTATING RETORT ZONE, SAID PELLETS BEING COMPRISED OF PARTICULATE HEAT CARRIERS BEING IN A SIZE RANGE BETWEEN 0.5 INCH AND APPROXIMATELY 0.055 INCH AND HAVING A SURFACE AREA OF BETWEEN 10 AND 150 SQUARE METERS PER GRAMS OF PELLETS, SAID PELLETS BEING AT A RETORT ZONE INLET TEMPERATURE BETWEEN 1,000*F. AND 1,400*F. AND IN A QUANTITY SUCH THAT THE RATIO OF SAID HEAT-CARRYING PELLETS TO SAID CRUSHED OIL SHALE IN SAID RETORT ZONE ON A WEIGHT BASIS BETWEEN ONE AND THREE, SAID RATIO ALSO BEING SUCH THAT THE SENSIBLE HEAT IN SAID PELLETS IS SUFFICIENT TO PROVIDE AT LEAST FIFTY PERCENT OF THE HEAT REQUIRED TO HEAT SAID CRUSHED OIL SHALE FROM ITS RETORTING ZONE FEED TEMPERATURE TO A RETORT ZONE OUTLET TEMPERATURE OF BETWEEN 800*F. AND 1,050*F.; B. TUMBLING SAID CRUSHED OIL SHALE AND SAID PELLETS IN SAID RETORT ZONE TO RETORT OIL VAPORS FROM SAID CRUSHED OIL SHALE AND FORM PARTICULATE SPENT SHALE AND A COMBUSTIBLE CARBON-CONTAINING DEPOSITION ON SAID PELLETS; C. CAUSING SAID PELLETS AND SAID SPENT SHALE TO PASS FROM SAID RETORT ZONE TO A PARTICLE SEPARATION ZONE AND SEPARATING FROM SAID PELLETS AT LEAST 75 PERCENT BY WEIGHT OF THE TOTAL SPENT SHALE AND AT LEAST 95 PERCENT BY WEIGHT OF THE PORTION OF SAID SPENT SHALE THAT IS SMALLER IN SIZE THAN SAID PELLETS PRIOR TO BURNING SAID COMBUSTIBLE CARBONCONTAINING DEPOSITION ON SAID PELLETS; D. RECOVERING SAID OIL VAPORS GENERATED BY RETORTING SAID CRUSHED OIL SHALE; E. LIFTING SAID PELLETS FROM SAID SEPARATION ZONE TO A PELLET DEPOSITION BURNING ZONE; F. HEATING SAID PELLETS IN SAID PELLET DEPOSITION BURNING ZONE TO AN OUTLET TEMPERATURE OF BETWEEN 1,000*F. AND 1,400*F. BY BURNING AT LEAST A PORTION OF SAID COMBUSTIBLE CARBON-CONTAINING DEPOSITION WITH A COMBUSTION SUPPORTING GAS; AND G. RETURNING SAID HEATED PELLETS TO SAID RETORT ZONE IN STEP (A) FOR RETORTING CRUSHED OIL SHALE.

3. The method according to claim 1 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

5. The method according to claim 1 wherein the separation of step (c) is comprised of first passing said pellets and said spent shale through apertures in a trommel to screen out at least a portion of the spent shale and any agglomerates larger than said pellets, and thereafter subjecting the remaining pellets and spent shale to gas elutriation with a noncombustion supporting gas to effect further separation of the spent shale from the pellets.

8. The method according to claim 7 wherein the average amount of said combustible carbon-containing deposition formed on said pellets upon passage through said retort zone is on said average less than 1.5 percent by weight of said pellets.

12. The method according to claim 11 wherein the particulate matter was originally comprised of cracking catalyst particles with active acid catalytic sites and which were also treated to reduce their active acid catalytic activity.

13. The method according to claim 10 wherein the particulate heat carriers are in a size range between 0.375 inch and approximately 0.055 inch.

18. The method according to claim 14 wherein the separation of step (c) is comprised of first passing said pellets and said spent shale tHrough apertures in a trommel to screen out at least a portion of the spent shale and any agglomerates larger than said pellets, and thereafter subjecting the remaining pellets and spent shale to gas elutriation with a noncombustion supporting gas to effect further separation of the spent shale from the pellets.

20. The method according to claim 14 wherein the pellets have a sphericity factor of at least 0.9 and at least 95 percent by weight of the total spent shale is separated from said pellets in step (c).

22. The method according to claim 14 wherein at least 95 percent by weight of the crushed oil shale of step (a) has been crushed to a size to pass through a U.S. Sieve Series size 6 screen and at least 95 percent by weight of the total spent shale is separated from said pellets in step (c).

24. The method according to claim 14 wherein the pellets of step (a) are comprised of particulate matter which originally had a surface area in excess of 150 square meters per gram and were treated to reduce the surface area to less than 150 square meters per gram.

25. The method according to claim 24 wherein the particulate matter was originally comprised of cracking catalyst particles with active acid catalytic sites and which were also treated to reduce their active acid catalytic activity.