US3503869A

US3503869A - Process for improving thermal efficiency of gas combustion shale retorting

Info

Publication number: US3503869A
Application number: US618195A
Authority: US
Inventors: James H Haddad; John G Mitchell
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1967-02-23
Filing date: 1967-02-23
Publication date: 1970-03-31
Anticipated expiration: 1987-03-31

Description

United States Patent US. Cl. 20811 4 Claims ABSTRACT OF THE DISCLOSURE Gas temperature in a shale combustion zone is regulated by passing combustion air concurrently with downwardly moving shale and removing gaseous combustion products prior to contracting the spent shale with upwardly moving recycle gas. Recycle gas is heated by the pent shale from the combustion zone and is directed from the shale cooling zone to the retorting zone without passing through the combustion zone.

BACKGROUND OF THE INVENTION Field of the invention Oil shale is a sedimentary rock which contains a solid organic material known as kerogen. When this oil shale is heated to an elevated temperature, the kerogen is decomposed by pyrolysis to shale oil, gas and a carbonaceous residue commonly identified as coke. A common process for eifecting kerogen pyrolysis is by retorting in a gas combustion shale oil process. A gas combustion oil shale process is carried out in a retort comprising four zones known as:

1) A shale preheating zone forming the upper part of the retort wherein raw crushed shale of a granular size is introduced and brought up to retorting temperatures by direct heat exchange with a heat yielding fluid;

(2) A retorting zone wherein the kerogen component of the shale is decomposed to shale oil vapors and gas;

(3) A combustion zone wherein controlled combustion of the available combustible material is effected to provide at least a portion of the heat energy required in the retorting operation, and

(4) A shale cooling zone wherein the spent shale particles are cooled to a desired low temperature suitable for handling, while preheating at least a portion of the recycled gases separated from the shale decomposition products of the retorting operation.

In gas-combustion type retorting processes, crushed raw shale material having a particle size of from about A to about 3 inch particles and hereinafter referred to as granular particle material, is passed downwardly through the zones comprising the retort as a relatively dense moving bed of granular material while recycle and other gases passed thereto move generally upwardly through the retort and countercurrent to the downwardly moving shale.

Some advantages of the above-discussed system are attributable to its simplicity, potentially large capacity and relatively efficient heat utilization. Accordingly, in an acceptable operation, the spent shale particles are cooled sufficiently in the retort to exit therefrom at a relatively low temperature, preferably below about 400 F., suitable for handling and disposal without resorting to expensive heat exchange equipment. The shale oil exits from the preheating zone in the form of a fog or mist after having released its heat to the incoming raw shale. The shale oil mist is separated from the gas mixed therewith, and recovered.

3,503,869 Patented Mar. 31, 1970 Description of the prior art In prior art oil shale gas combustion processes, gas is passed upwardly through a vertical retort and exits from the top portion thereof together with shale oil mist. In the combustion zone, high temperatures develop which cause shale weakening with resultant shale fines forma tion. The fines are carried upwardly with the gaseous combustion products into the retorting zone. In the retorting and shale preheating zones, the oil formed by kerogen decomposition can be adsorbed on the fines and carried downwardly into the combustion zone. In the combustion zone, the shale oil is partially converted to coke which results in oil yield loss and possible stoppage of shale flow. In addition, when operating in the manner shown by the prior art, the hot gas from the combustion Zone may have such a high temperature as to cause shale oil conversion to coke in the retorting zone. In addition, too high a combustion gas temperature can result in the shale oil product from the retort having too high a temperature. This results in a need for condensing vaporous product to permit recovery of the lower boiling portions of the oil product. In areas where oil shale is normally found, water is scarce and condensation as a means to recover oil product is often uneconomical.

In addition, in prior art processes, hot recycle gas from the shale cooling zone is passed directly into the combustion zone. Vaporous hydrocarbons comprise a large portion of this recycle gas and a portion of these hydrocarbons are burned in the combustion zone. Operating the combustion zone in this manner results in relatively incomplete combustion of the coke on the shale. This is due to the constant introduction of hydrocarbon recycle gas into the combustion zone. Since a limited amount of free oxygen is introduced into the combustion zone sufficient to supply the heat requirements of the system, a large portion of the hydrocarbon vapors will be burned while a corresponding portion of the coke required for heat release will remain on the shale. This is undesirable since spent shale usually leaves the retort at a temperature of from about 300 F. to about 400 F. and upon contact with outside air the residual coke thereon may burn. The constant introduction of recycle gas containing shale oil vapor into the combustion zone causes a loss of shale oil yield and high temperatures to occur in a relatively small volume of the shale bed. These high temperatures promote undesirable carbonate decomposition and fines formation.

It is known in the prior art to recover shale'oil from a gas combustion retorting process by withdrawing shale oil vapors from a vertically intermediate stage in the retort; as for example, US. Patent 2,710,828, issued June 14, 1955. The vapors are withdrawn from the retorting zone or above the retorting zone and are processed to condense a portion of the shale oil therein. The remaining vapors are directed to the shale preheating zone. However, in these processes, there is no provision for minimizing direct contact between combustion supporting gas, and fines with kerogen decomposition products and oil yield losses result. In the prior art processes, the means for regulating retorting temperatures are severely limited and there is no provision for minimizing contact of combustion gas with recycle gas.

SUMMARY OF "II-IE INVENTION By the present invention, a modified gas combustion shale retorting process is provided wherein direct contact is minimized between gas combustion products from the combustion zone and both kerogen decomposition products in the retorting zone and recycle gas from the shale cooling zone. Shale having coke thereon from a retorting zone is passed downwardly to a combustion zone. In the combustion zone, the shale is contacted with free-oxygen containing gas to promote combustion of the coke and heat the shale and gas. The combustion gas and shale in the combustion zone are passed concurrently downward through the combustion zone. The shale from the combustion zone is passed over gas disengagement means located within the retort to separate gaseous combustion products from the spent shale prior to contacting recycle gas from the shale cooling zone. The gaseous combustion products are removed from the retort while the separated spent shale is passed downwardly through a shale cooling zone. In the shale cooling zone, the spent shale is contacted with upwardly moving recycle gas whereby heat is transferred from the spent shale to the recycle gas. The recycle gas is removed from the retort prior to entering the combustion zone by gas disengaging means located within the retort. The spent shale is removed from the retort after passing through the shale cooling zone. The heated recycle gas can contain fines and after being removed from the retort is directed to a fines separation zone wherein fines are separated from the heated recycle gas. The recycle gas from the fines separation zone is directed to the retorting zone located above the combustion zone and is therein passed upwardly through the downwardly moving shale. In the retorting zone, kerogen in the raw shale is thermally decomposed to oil vapors. The resultant gasiform material is passed upwardly through incoming raw shale to transfer heat from the gasiform material to the raw shale. This results in the formation of an oil mist or fog which is removed from the shale preheating zone and directed to an oil separation zone. In the oil separation zone, liquid oil product is separated from gasiform material. A portion of the gasiform material is recycled to the lower portion of the shale cooling zone in a manner described above.

Thus by the process of this invention, transfer of gaseous products from the combustion zone to either the retorting zone or the shale cooling zone is minimized. This results in substantial advantages for the overall gas combustion process. By minimizing contact between gaseous combustion products and oil formed by kerogen decomposition in the retorting zone, the chances of burning shale oil product are minimized. In addition, because the large proportion of fines found in a retorting process are formed in the combustion zone, the chances for contacting fines and shale oil product with the attendant undesirable results are minimized. The process of this invention also provides substantial advantages by removing the recycle gas from the retort prior to contacting gaseous products from the combustion zone. In this manner, selective burning of coke on the shale is accomplished while minimizing burning of gaseous hydrocarbons.

This results in more uniform coke burning and prevents substantial loss of shale oil vapors in the combustion zone. Accordingly, higher retort shale oil yields are obtained since condensible oil not previously separated will not be consumed as fuel in the combustion zone.

The present invention also provides substantial advantage in that the vent gas (i.e., excess gas from the oil product separation step over that needed for recycle gas) will be characterized by a higher heat content per unit volume and hence more valuable. This is a result of separately removing gaseous combustion products and recycle gas from the retort prior to their being substantially mixed. Thus, the gaseous products subsequently recovered from the oil separation step will contain substantially reduced concentrations of nitrogenous gases and gaseous carbon oxide as compared to the prior art processes. The excess vent gas can be employed as a fuel gas in outside processes. In addition, the heat from the gaseous combustion products can be recovered by heat exchange. Thus one net result of the present invention is that the heating potential of the coke produced is recovered for use in the retorting process and carbonaceous gaseous by-product is freed for use in other processes. This promotes a substantially improved thermal efiiciency of the overall process.

The concurrent fiow of combustion supporting gas and shale in the combustion zone of this invention is desirable over the countercurrent flow of gas in the prior art. In countercurrent flow of prior art it has been found that fines tend to separate out and accumulate in the bed causing uneven gas flow, shale flow stoppage and poor unit operations. In concurrent flow of the shale and gas in the combustion zone of this invention fines will not separate out but will be swept along with the downwardly flowing gas. Thus the undesirable effect of fines separation and accumulation in the bed above is avoided in this invention.

In the process of this invention, pressures within various portions of the retort are maintained at levels to insure desired gas flow. The pressures at the lower portion of the combustion zone where combustion products are removed from the retort and the higher portion of the shale cooling zone where hot recycle gas is removed, are maintained substantially equal to minimize gas flow therebetween, This can be accomplished by means well known in the art as for example by controlling valves at each of the gas outlets, said valves being responsive to a pressure differential between the two zones. The pressure at the upper portion of the combustion zone and the lower portion of the shale cooling zone at the respective gas inlets are maintained somewhat above the pressures at the above mentioned gas outlets to eifect gas flow from the respective gas inlets to the gas outlets. The pressure in the retorting zone is maintained somewhat below the pressure in the upper combustion zone to minimize flow of shale oil product to the combustion zone. Flow of combustion gas to the retorting zone is minimized by the use of standpipes for shale flow therebetween having a suflicient vertical height to absorb the difference in pressure between the top of the combustion zone and the bottom of the retorting zone while minimizing gas flow from the combustion zone to the retorting zone. The standpipes are at least long enough so the head of solids therein is greater than the pressure difference between zones.

BRIEF DESCRIPTION OF THE DRAWING The attached figure shows one interior arrangement for a retort wherein gases in the combustion zone are retained separate from both the retorting zone gas and the shale cooling zone gas.

DESCRIPTION OF SPECIFIC EMBODIMENTS Referring now to the attached drawing, crushed raw shale particles having a size of from about A to about 4 inches mean diameter is introduced into retort 1 through closed conduit 2. The shale particles are directed through a plurality of conduits 3 to the top of a downwardly moving compact bed of shale particles in preheating and retorting zone 4. In this manner, relatively even particle distribution over the bed surface area is effected. The top of the particle bed in preheating and retorting zone 4 is maintained at a desired distance below the top of the retort 1 to form a plenum chamber 5 which facilitates separation of particles and oil mist. In preheating and retorting zone 4, the incoming particles are heated by upwardly moving vapors and reach retorting temperature as the particles move downwardly. The heat supplying gas is introduced through conduit 6 into header 7 and is introduced into the bed through distributors 8. When retorting temperature is reached in zone 4, the kerogen decomposes to form gas, oil vapors, and coke residue. The spent shale having coke thereon is passed downwardly through standpipes 9 into combustion zone 10. The standpipes 9 extend from baffle 11 to form a plenum chamber 12 and to provide back pressure means to prevent incoming combustion gas from entering preheating and retorting zone 4. Provision can be made to introduce seal gas into each standpipe 9.

Combustion gas is introduced into plenum chamber 12 through conduit 13. The combustion gas is passed downwardly from plenum chamber 12 into the combustion zone to eifect burning of coke on the shale particles. In combustion zone 10, gaseous material and shale particles are passed concurrently downwardly to contact a gas-solids disengager comprising a plurality of collectors 14 and a header 15. The gaseous combustion products and spent shale fines are removed from the retort through header 15 and conduit 16. Due to the burning effected in combustion zone 10, the shale particles become heated. The heated shale particles pass downwardly from the combustion zone 10 to shale cooling zone 17 wherein contact is made with relatively cool recycle gas. The recycle gas is introduced into the retort 1 through conduit 18, header 19 and distributors 20. The recycle gas passes upwardly through the shale cool ing zone 17 to contact downwardly moving shale particles. In this manner, heat is transferred from the shale particles to the recycle gas. The cool shale particles are removed from retort 1 through conduit 21 and discarded. The heated recycle gas is removed from retort 1 by way of gas-solids disengager comprising header 22 and collectors 23 and conduit 24. Means now shown are provided in conduit 21 for effecting relatively uniform shale flow through the retort 1.

The oil mist or fog separated from the shale preheating zone 4 is removed from plenum chamber 5 through conduit 25. The oil mist is directed to a separation step 26 wherein liquid oil product is separated from vaporous material. An electrostatic precipitator and/or cyclone or the like can be employed in the separation step 26. Oil product is obtained and recovered through conduit 27. Gaseous material is recovered from separation step 26 and recycled in a manner whereby a portion is directed to the shale cooling zone 17, another portion can be mixed with the gaseous efliuent from the shale cooling zone, and the remainder is vented to be employed in processses not shown. The gas recycled to the shale cooling zone 17 is pumped by pump 28 through

conduits

29 and 18 into header 19. The gas to be mixed with hot recycle gas is pumped through

conduits

18, 30 and 6 and mixed with hot recycle gas from conduit 31. Alternately, this cooling gas may pass to conduit 24 through conduit 32. Excess recycle gas is vented through conduit 33. Hot recycle gas is removed from the retort through conduit 24 and directed to a fines separation step 35 wherein fines carried over with the gas are separated and removed through conduit 36. The hot recycle gas is directed to the preheating and retorting zone 4 through

conduits

31 and 6.

Combustion air is introduced into plenum chamber 12 through conduit 13. The air can be mixed with combustion products obtained from the retort and directed through

conduits

37 and 38 prior to being introduced into the retort. This is an effective means for diluting the combustion air to control burning in the combustion zone 10 and minimize the development of high localized peak temperatures. The gaseous combustion products are withdrawn from the retort through conduit 16 and directed to a separation zone 39 wherein carried over fines are separated from the gases and removed through conduit 40. The combustion gas exits from the retort at relatively high temperatures and the heat therein can be extracted by heat exchange means not shown. The heat from the gaseous combustion products can \be converted to steam, for example, which can then be used to drive the compressors which pump recycle gas and combustion air to and from the retorting process.

The amount of free oxygen which is introduced into the combustion zone is regulated so as to maintain the 6 shale temperature in the combustion zone between about 900 F. and about 1400 F., preferably between about 950 F. and about 1100 F. By operating in this manner, sufficient heat can be transferred to the recycle gas in the shale cooling zone to support kerogen decomposition in the retorting zone without causing excessive shale disintegration in the combustion zone. The amount of free oxygen in the combustion zone can be regulated by diluting combustion air with flue gas to obtain a free-oxygen concentration in the resultant mixture usually between about 5 and about 15 volume percent. Since carbon burning rate is proportional to oxygen partial pressure, the diluted gas will minimize the attainment of high peak temperatures and carbonate decomposition in the combustion zone while the combustion gas is being difiused into the bed. The amount of recycle gas directed to the shale cooling zone is sulficient to reduce the shale temperature to below about 400 F. and preferably below about 300 F. By operating in this manner, the hot recycle gas from the fines separation step associated with the shale cooling step can have a temperature as high as about 1400 F.

It therefore may become desirable to reduce the hot recycle gas temperature prior to introducing it into the retorting zone. Thus the present invention provides for mixing the hot gas with a portion of the cold recycle gas prior to introducing gas into the retorting zone. The relative amounts of cold and hot gas mixed is that which will maintain the maximum retorting temperature between about 700 F. and about 1100 F., preferably between about 850 F. and about 1000 F. In this manner, increased retorting times result which increases oil yield. The oil mist exits from the top of the shale retorting and preheating zone at a temperature below about 300 F. and preferably between about F. and about 200 F. In the process of the present invention, the hot recycle gas can be introduced thereto by means of a plurality of inlets at different vertical heights. This permits more effective heating in the retorting zone and permits heat soaking of the shale in the retorting zone. This heat soaking effects increased vaporization of the liquid decomposition products and this provides for increased shale oil yields and flowability of the solids.

Having thus presented a general description of the process of this invention, it is to be understood that minor modifications can be made thereto without departing from the scope thereof and no undue restrictions are to be imposed by reason of the specific examples presented.

We claim:

1. A method for retorting oil shale particulate material which comprises (a) passing oil shale particulate sequentially downwardly through a gas combustion retort comprising a shale preheating-kerogen decomposition retorting zone, a combustion zone and a shale cooling zone,

(b) passing hot recycle gas obtained from the upper end of said shale cooling zone by countercurrent passage therethrough into a fines-removal zone and thence into the lower portion of the shale retorting zone for upward flow therethrough under kerogen decomposition conditions,

(0) effecting decomposition of kerogen in said oil shale particulate with said hot recycle gas and recovering gasiform products of decomposition including gaseous hydrocarbons and liquid hydrocarbons from the upper portion of said preheating zone,

(d) passing an oxygen-containing combustion gas concurrently with retorted oil shale particulate downwardly through said combustion zone under conditions to further heat said shale particulate tov an elevated temperature by burning combustible materials associated therewith,

(e) removing hot gaseous combustion products from the lower portion of said combustion zone prior to hot shale particulate coming in contact with preheated recycle gas removed from the upper portion of said shale cooling zone, and

(f) passing recycle gaseous hydrocarbons separated from liquid products of retorting countercurrent to and upwardly through said shale moving downwardly through said shale cooling zone from said combustion zone.

2. The method of claim 1 wherein a portion of the recycle gas separated from the liquid oil product is mixed with hot recycle gas from the shale cooling step prior to introducing the hot recycle gas into the retorting zone.

3. The method of claim 1 wherein the gaseous combustion products are freed of entrained fines and thereafter heat is recovered therefrom as a source of power for pumping recycle gas and combustion gas.

' 4. The method of claim 1 wherein combustion gas freed of entrained fines is employed to dilute oxygen-conintroduction-into the com- References (lited I UNITED STATES PATENTS 11/1955 Leffer 20811 11/1957 Van Dijck 208-11 5/1968- Peet 208-11 FOREIGN PATENTS 10/1960 Canada.

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