US3617003A

US3617003A - Regulating coal slurry settling rates

Info

Publication number: US3617003A
Application number: US34233A
Authority: US
Inventors: Edgar C Winegartner
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1970-05-04
Filing date: 1970-05-04
Publication date: 1971-11-02
Anticipated expiration: 1988-11-02

Abstract

The sizes and specific gravities of coal solids in a slurry are limited on the basis of particle settling rates to prevent undesirable settling of the solids in a coal slurry handling system, especially in a liquefaction reactor for liquefying slurried coal solids. After makeup with crushed coal particles in a suitable liquid carrier, a coal slurry is charged in a continuous process to a settling rate separation zone, preferably embodied in a hydrocyclone, where the slurried coal solids are segregated according to whether they exceed a predetermined settling rate limit, one which preferably is within the range from about 0.5 to about 300 ft./min., coal solids settling faster than the limit rate being separated for constitution in an unfinished heavy slurry stream from the slurry particles which do not settle, at maximum settling velocity, in excess of the settling rate limit, such particles issuing in a finished slurry stream as product from the settling rate separation zone. The unfinished heavy slurry stream is passed to a comminution zone, preferably a ball mill, where the coal particles in the stream are milled to reduce their sizes, at least a portion of the particles being reduced to sizes, which for the specific gravities of such particles, will permit the particles to settle in the separation zone at a rate not greater than the predetermined limit. The comminuted heavy slurry stream is then mixed with new slurry feed and recycled to the settling rate separation zone where the comminuted coal particles and the new feed coal solids which do not settle at a maximum velocity in excess of the predetermined limit are constituted as finished slurry product, the particles exceeding the limiting rate being cycled to the comminution zone for further treatment.

Description

a United States Patent [72] Inventor Edgar (I. Winegartner ABSTRACT: The sizes and specific gravities of coal solids in a Baytown, Tex. slurry are limited on the basis of particle settling rates to [21] Appl. No. 34,233 prevent undesirable settling of the solids in a coal slurry han- [22] Filed May 4, 1970 dling system, especially in a liquefaction reactor for liquefying' [45] Patented Nov. 2,1971 slurried coal solids. After makeup with crushed coal particles [73] Assignee Esso Research and Engineering Company in a suitable liquid carrier, a coal slurry is charged in a continuous process to a settling rate separation zone, preferably embodied in a hydrocyclone, where the slurried coal solids are segregated according to whether they exceed a predetermined settling rate limit, one which preferably is within the range from about 0.5 to about 300 ft./min., coal solids settling faster than the limit rate being separated for constitution in an un- [54] REGULATING COAL SLURRY SETTLING RATES- 6 Claims, 1 Drawing 8- finished heavy slurry stream from the slurry particles which do [52] US. Cl 241/16, Settle at maximum Sewing velocity in excess of 241/29 tling rate limit, such particles issuing in a finished slurry [51] Int. Cl. B02c 23/00 stream as Product fmm the settling rate separation zone The 501 Field of Search 241/15, 16, unfinished heavy is Passed a cmminufin 19 20, 21,29 zone, preferably a ball mill, where the coal particles in the stream are milled to reduce their sizes, at least a portion of the [56] Ref ren e Cited particles being reduced to sizes, which for the specific gravi- UNITED STATES PATENTS ties of such particles, will permit the pfiirticllfs to sgttle in th;

se aration zone at a rate not greater t an t e re etermine gigs g: lil'f l il. The comminuted heavy slurry stream is tlFen mixed with 3 524:682 8/1970 35mm 241/15 x new feed and "cycled the 8mm; me sepmm Primary Examiner-Granville Y. Custer, Jr. Attorneys-Timothy L. Burgess, Thomas B. McCulloch,

Melvin F. Fincke, John S. Schneider, Sylvester W. Brock,

zone where the comminuted coal particles and the new feed coal solids which do not settle at a maximum velocity in excess of the predetermined limit are constituted as finished slurry product, the particles exceeding the limiting rate being cycled Jr. and Kurt S. Myers to the comminution zone for further treatment.

COARSE COAL CRUSHER CONDENSER CYCLONE WATER l5 l2 E LIQUEFACTION ZONE SURGE sit as}; 3:

SLURRY HEATER coulieron DMER 3-3 24 3O FINISHED 23 SLURRY 20 2:3

SEPARATION zone 2s 1 l BALL MILL PATENTEDuu'v 2 ISYI COAR SE COAL CRUSHER '/'CYCLONE SURGE B IN CONVEYOR CONDENSER SLURRY DRYER BALL MILL WATER LIQUEFA ZONE ' HEATER FINISHED CTION SLURRY i SEPARATION ZON I NVENTOR.

Edgar C. Wlnegortner,

BY$WXWM ATTORNEY.

1 REGULATING COAL SLURRY SETTLING RATES BACKGROUND OF THE INVENTION This invention is directed to the processing of coal slurries to control excessive particle settling and plugging in slurryhandling systems. More particularly, it involves the processing of coal slurries to restrict the particle makeup of the slurries to solids which do not settle at rates exceeding a predetermined limit.

When handling coal slurries in systems such as slurry transport pipelines and slurry-treating reactors, it is necessary to constitute the slurries so that the slurried coal solids do not settle out of suspension and plug up the slurry-handling system. Particle settling in a slurry is a function of the size and specific gravity of the particle. On the assumption that the specific gravity of the particle material is constant, i.e., that the differences in specific gravity of solids in the slurry are negligible, efforts heretofore to control settling in slurry handling systems have been based on sizing operations. Thus, the design of slurry-handling systems has specified that slurries handled by the system be constituted of particles smaller than a given size. (See e.g. Manno, P. .I., Slurry Pipelines, Stanford Research Institute Report, 1967; Wicks, M., Optimization of Solids Concentration and Particle Size Distribution," 22 nd Technical Meeting of South Texas Section of AIChE, Oct. 13, 1967; and Wasp, E. J., et al., A Quick Method of Making Initial Cost Estimates of Slurry Pipelines," AIME Paper 69865, AIME, Feb. 16-28, 1969, Washington D. C.) The assumption that differences in specific gravities of materials in a slurry are negligible can have serious consequences. A well-documented example is the plugging of the 1.5 million tons per year Cadiz, Ohio to East Lake, Ohio coal slurry pipeline in 1957 because 0.01 weight percent of the coal solids in the slurry were of higher specific gravity than the other solids in the slurry, which was made up in accordance with design specifications of solids finer than a specified size. In reactors such as coal liquefaction reactors which are used to liquefy slurried coal particles in processes for producing liquid hydrocarbon products from solid coal, neglect of the differences between specific gravities of coal particles in a slurry can produce excessive settling of nonliquefiable solids and resultant plugging of the reactor, requiring frequent and costly shutdowns for cleanout.

The specific gravity of coal particles in a slurry can be made uniform by sorting of the solids used to make up the slurry, but sorting operations offer no size control of the solids, and consequently, do not solve the problem of excessive settling in coal slurry handling systems. What is needed is a process which makes up coal slurries with regard both to the size and the specific gravity of the constituent particles so that undesired settling of the particles in a coal slurry handling system is prevented. That is what this invention provides.

SUMMARY OF THE INVENTION In accordance with this invention, prevention of undesired settling of slurried coal particles in a coal slurry handling system is achieved in a continuous process which entails passing a slurry of the coal particles into a settling-rate separation zone where slurried coal particles are segregated according to whether or not they settle at velocities which exceed a predetermined settling-rate limit, slurry solids exceeding the limit being separated in an unfinished heavy slurry stream from slurry solids which at maximum settling velocity do not exceed the limit, such solids being constituted in a finished slurry stream issued as product from the separation zone. The unfinished heavy slurry stream is transferred to a comminution zone where slurried solids in the stream are milled or otherwise comminuted to reduce the sizes of at least a portion of the solids to sizes which, for the specific gravities of such solids, will provide the solids with a maximum settling rate less than the limiting settling rate of the separation zone. The comminuted heavy slurry stream is then mixed with new slurry feed for the separation zone, and the slurry mixture is passed to the separation zone where new feed-slurried solids and comminuted-slurried solids which do not settle at maximum settling velocities in excess of the predetermined maximum settling rate are segregated from the remainder of the slurry solids in the zone and are discharged from the zone as a finished slurry product, the particles exceeding the limit being cycled in the unfinished heavy slurry stream to the comminution zone for further processing.

Aspects of the Invention The slurry which is processed according to this invention is made up of solid, particulate coal suspended in a suitable liquid carrier. The liquid carrier utilized will normally depend on what is done with the slurry. If handling of the slurry involves only its transport in a pipeline, the carrier liquid suitably may be water or oil. If handling of the slurry involves transport in a reactive system, such as when the coal slurry is treated in a liquefaction process to convert the dissolvable portions of the coal into liquid form, the liquid carrier suitably may be a coal-derived oil, but preferablyis a hydrogen-donor solvent which boils withinthe range from about 300 F. to about 900 F. at atmospheric pressure. Preferably, the liquid carrier has a specific gravity within the range from about 0.90 to about 1.10.

The solid, particulate coal suspended in the liquid carrier may be of any type or rank, depending on the purpose for which it is handled, but for liquefactionprocesses, it preferably is bituminous coal, subbituminous coal, lignite, brown coal, or a mixture thereof. The coal particles are suitably ground to a particle size distribution from about 8 mesh (Tyler and finer, but there may be particles as large as one-quarter inch on the major dimension present in the slurry. In a liquefaction process, adequate conversion is obtained even with the larger sized particles.

The specific gravity of a coal particle of given size will vary for any rank of coal with the different maceral group constituents of the coal (vitrinites; exinites, including sporinites, cutinites, and resinites; and inertites, including micrinites, semifusinites, and fusinites) and their relative proportions in the coal, together with the kind and relative proportions of mineral constituents in the coal. Common mineral constituents include clay, felspar, kaolinite, calcite and pyrite. In a liquefaction process, it is particularly important to control the settling of coal particles primarily consisting of nonliquefiable constituents such as clay, pyrite, fusinite etc., for these are the constituents which otherwise will tend to settle excessively in the liquefaction reactor and cause plugging. The specific gravity of a coal particle is also affected by the extent to which it contains absorbed water, and this depends on the constituency of the particular particle. In a liquefaction process. it is preferable to use coal which has dried to remove excess moisture, although it is feasible to employ coal which is not moisture free if the liquefaction facilities have been sized to allow the withdrawal of evolved steam. In general, the specific gravity of coal particles will vary over the range from about 1.25 to about 1.4 with mineral particles as heavy as 5.2 specific gravity also being present.

The bulk viscosity and the bulk specific gravity of a slurry will vary with the kinds, sizes, specific gravity and concentration of coal in a slurry as well as the nature of the liquid carrier, i.e. its specific gravity, whether it be solvent or nonsolvent, hydrogenated or not etc. However, for handling in a liquefaction process, the slurry charged to the separation zone, including recycled coal particles from the comminution zone, will suitably have a bulk viscosity within the range from about 1 to about 1,000 centipoise at 60 F., and also, a bulk specific gravity within the range from about 0.95 to about 1.4 at 60 F. The solvent-to-coal ratio of the slurry charged to the separation zone will suitably be within the range from about 0.8:! to about 2: l.

Settling-Rate Separation Zone.

The coal particles of the slurry charged to the separation zone are separated according to whether their settling rate exceeds a predetermined limit in the settling-rate separation zone. In accordance with Stokes law, the settling rate of individual particles in the settling zone varies directly with the sizes of the particles, the difference between the specific gravity of the particles and the bulk specific gravity of the slurry and inversely with the bulk viscosity of the slurry. The limiting settling rate which is established to segregate the particles of a particular coal slurry will be determined by the particular handling system in which settling is to be controlled, the extent to which settling is to be avoided in that system, the velocity and throughput of a slurry in a time period in the handling system, the bulk viscosity and bulk specific gravity of the coal slurry charged to the separation zone, and other factors. For a coal liquefaction process utilizing an unstirred upflow reactor, in which it is specified that all settling is to be prevented in the reactor, and in which the coal slurry charged to the separation zone is a 33 weight percent slurry of -20 mesh bituminous coal in a hydrogenated creosote oil, and wherein the slurry viscosity is within the range from about 5 to about centipoise and the slurry specific gravity is within the range from about l.l5 to about 1.2 at temperatures within the range from about 650 F. to about 950 F., the limiting settling rate in the separation zone will suitably be within the range from about 5 to about 20 ft./minute. For a settling rate of about per minute in the same reactor under like conditions, one would use a slurry of l00 mesh coal to feed to settling-rate separation zone. For a settling rate of about 166 ft./min., the feed to the separation zone would be made up with -8 mesh coal. However, if the upflow liquefaction reactor is a well-mixed reactor, a 8 mesh coal may be used, for settling rates of about 300 ft./second are suitable. Accordingly, settling rates in the separation zone may suitably be within the range from about 0.5 ft./minute to about 300 ft./minute, as desired for the system in which the slurry will be handled.

Preferably the settling rate separation zone is embodied in a liquid cyclone, or as it is more commonly called, a hydrocyclone. In this unit, commonly used for mineral dressing, washing, concentration, etc. settling velocities are increased by increasing the acceleration of gravity such that the higher settling rate particles can be separated in a relatively short period of time. The use of a hydrocyclone to prepare coal slurries for handling in a transport system or a slurry-treating reactor to avoid undesired settling is, to applicant's knowledge, wholly novel.

Comminution Zone.

As above described, particles settling in excess of the limiting rate in the separation zone are constituted in an unfinished heavy slurry stream and transferred to a comminution zone. The comminution zone is suitably a ball, rod, or other suitable mill which will pulverizc, grind, mill or otherwise act upon the particles received from the separation zone to reduce the size of at least a portion of the particles to an extent which, for the specific gravity of such particles, will permit them to settle in the settling zone at a maximum settling velocity not greater than the settling-rate limit when recycled from the comminution zone to the settling zone.

In this closed circuit continuous size reduction and solids separation system, a circulating load is maintained between the separation zone and the comminution zone, the load varying with the feed rate of new feed to the separation zone and the rate at which a finished slurry is discharged from the separation zone. Normally, the circulating load will be in the range of IO percent to 300 percent ofthe fresh feed.

The invention will be better understood by considering the drawing in connection with a description of a preferred manner ofcarrying out the invention.

DESCRIPTION OF THE DRAWING The drawing is a schematic flow scheme illustrating the preparation of a slurry of coal particles, in accordance with this invention, for use in a liquefaction reactor in a coal liquefaction process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Coarse coal is introduced by line 10 into a sizing crusher ll, suitably a conventional hammer or impact mill, in which the coarse coal is reduced in size to a desired level, suitably percent through 8 mesh. The crusher 11 is located directly above a receptacle or surge bin 12 which receives the crushed coal from crusher 11. Where the crusher is a hammer or impact mill, the crusher acts as a blower and transports considerable quantities of air, into the Surge bin 12. This air is conveniently removed from the surge bin by exhausting it through line 13 into a small conventional gas cyclone 14, which drops out most of the particulate solids entrained in the exhausted air and returns the solids to the surge bin by way of conduit 15. The unladen exhaust is recycled to the inlet side of crusher 11 by recycle line 16. In this manner, the quantity of fresh air entering the system is reduced and oxidation of coal during the crushing operation is decreased.

Crushed coal from the surge bin 12 is fed to a slurry drier 17 by way of a conveyor 18 for drying, preferably in accordance with the method described in my copending U.S. Pat. application Ser. No. 770,281, filed Oct. 24, 1968, entitled Coal Drying. The slurry drier 17 contains a pool of an inert heat transfer liquid maintained at a temperature from about 220 F. to about 600 F. The crushed coal is introduced into slurry drier 17 and into the pool of heat transfer liquid at a coal inlet point adjacent but below the surface of the heat transfer liquid. The heat transfer liquid is introduced into the slurry drier zone at a point near the coal inlet point. Moisture vaporized from the coal is vented from slurry drier 17 by way of exhaust line 19, condensed, collected and ultimately discharged. The crushed coal introduced into the slurry drier forms a slurry with the inert heat transfer liquid, preferably a hot hydrogenated creosote oil or the like, and the slurry is withdrawn by line 20 from the bottom of the slurry drier, preferably as a 30 to 40 weight percent solids slurry.

In accordance with this invention, the slurry from slurry drier 20 is charged, as by slurry drier pump 21, into a maximum settling rate separation zone 22, where the slurried coal particles are segregated according to whether they exceed a settling-rate limit or not, slurried solids settling at rates exceeding the limit being separated in an unfinished heavy slurry stream from the slurry particles which at maximum settling velocity do not exceed the limit, these particles being constituted in a finished slurry product. Preferably, the maximum settling rate separation zone is embodied in a hydrocyclone. Thus, the slurry feed is introduced into a hydrocyclone 22 by way of a tangential feed inlet 23, the finished slurry product constituted of particles having a settling rate less than the predetermined limit is discharged from the cyclone 22 by way of main fluid outlet 24, and the unfinished heavy slurry stream consisting of particles exceeding the settling rate limit is issued from cyclone 22 by peripheral fluid outlet 25.

The unfinished heavy slurry stream includes all particles larger than 8 mesh plus some high specific gravity particles finer than 8 mesh in the case where the crusher 11 is operated to reduce the coarse coal to 90 percent through 8 mesh. The unfinished heavy slurry is transferred by line 26 to a comminution zone 27, preferably a ball mill, where the slurried solids in the heavy stream are milled to reduce the sizes of at least a portion of the solids to sizes which, for the specific gravities of the solids, will provide the solids with a maximum settling rate less than the predetermined settling rate limit of the settling zone. The discharge from the comminution zone is withdrawn by line 28 and recycled back to the suction side of the slurry drier pump 21, mixing the comminuted heavy slurry stream with new slurry feed from the slurry drier. A slurry mixture of new feed and comminuted slurry solids is thus charged back through the hydrocyclone 22, which issues the comminuted slurry solids and new feedslurried solids which do not settle at a maximum velocity in excess of the predetermined settling rate limit from main fluid outlet 24 as finished slurry product. Line 30 conducts the finished slurry product at a velocity in excess of saltation velocity to a liquefaction reactor 31 for further processing to produce liquid hydrocarbon products in a coal conversion operation. In the preferred embodiment illustrated, a portion of the liquid carrier in the slurry product is withdrawn from the slurry in line 30 and recycled by line 32 back to the slurry drier 17 through a heater 33 to assist in the maintenance of the hot heat transfer pool.

Having now described the invention in detail, and having disclosed the preferred manner in which the invention may be employed, what is desired to be obtained in Letters Patent is described in the appended claims.

I claim:

1. A process of preparing a slurry of coal particles for plug free handling in a slurry-handling system, which comprises, continuously:

charging said slurry as new feed to a settling-rate separation zone where coal particles settling at a velocity greater than a predetermined settling-rate limit are separated and constituted in an unfinished heavy slurry stream from other coal particles which at maximum settling velocity do not exceed the predetermined settling-rate limit, such other coal particles being constituted in a finished slurry stream product for issuance from said separation zone, transferring said unfinished heavy slurry stream to a comminution zone where at least a portion of the coal particles in the heavy slurry stream are reduced to a size, which for the specific gravity of such particles, provides such particles with a maximum settling velocity no greater than said predetermined limit when in said separation zone,

passing the comminuted heavy slurry stream to a mixing zone where the coal particles in the comminuted heavy slurry stream are admixed with coal particles in new feed slurry, and

charging the admixed slurry to said separation zone where the comminuted coal particles which do not settle at a maximum velocity in excess of said predetermined limit are constituted in said finished slurry stream product, and the comminuted coal particles and new feed coal particles which settle at rates in excess of said predetermined limit are constituted in said unfinished heavy slurry stream,

recycling said unfinished heavy slurry stream to said comminution zone for comminution, and discharging said finished slurry stream product into said slurry handling system. 2. The process of claim 1 in which said predetermined settling rate limit is within the range from about 0.5 to about 300 ftJminute.

3. The process of claim 1 in which the slurry charged to said separation zone has a slurry specific gravity within the range from about 0.95 to about 1.4 at 60 F.

4. The process of claim 1 in which said separation zone is a hydrocyclone.

5. The process of claim 1 in which said comminution zone is a ball mill.

6. The process of claim 1 in which the new slurry feed is prepared by crushing coarse coal to a desired size in a sizing crusher which blows the crushed coal into a surge bin therebelow,

exhausting air transported into the surge bin with the crushed coal into a gas cyclone above said surge bin to separate any particulate coal solids entrained in the air,

recycling the air from the gas cyclone to the crusher whereby the quantity of fresh air in the crusher is reduced and oxidation of coal during the crushing operation is reduced.

Claims

2. The process of claim 1 in which said predetermined settling rate limit is within the range from about 0.5 to about 300 ft./minute.
3. The process of claim 1 in which the slurry charged to said separation zone has a slurry specific gravity within the range from about 0.95 to about 1.4 at 60* F.
4. The process of claim 1 in which said separation zone is a hydrocyclone.
5. The process of claim 1 in which said comminution zone is a ball mill.
6. The process of claim 1 in which the new slurry feed is prepared by crushing coarse coal to a desired size in a sizing crusher which blows the crushed coal into a surge bin therebelow, exhausting air transported into the surge bin with the crushed coal into a gas cyclone above said surge bin to separate any particulate coal solids entrained in the air, recycling the air from the gas cyclone to the crusher whereby the quantity of fresh air in the crusher is reduced and oxidation of coal during the crushing operation is reduced.