GB2114595A - Coal-solvent slurries - Google Patents

Abstract

A method for controlling coal liquefaction feed slurry viscosity comprising pulverizing the coal, so that the particle size distribution is such that at least 80% of the coal will pass through a 30 mesh screen and not more than 30% will pass through a 400 mesh screen.

Description

SPECIFICATION Method of controlling coal liquefaction feed slurry viscosity BACKGROUND OF THE INVENTION The government of the United States of America has rights in this invention pursuant to contract number DE-AC01-79ET10104 awarded by the U.S. Department of Energy to the Pittsburg and Midway Coal Mining Company, a subsidiary of Gulf Oil Corporation.

The present invention is directed to the method of controlling the viscosity of coal-solvent slurries. These slurries are used as feed in coal liquefaction processes.

The declining reserves and high cost of petroleum coupled with the need for low sulfur clean burning fuels has led to a variety of research efforts seeking to obtain alternate fuel sources. The abundance of domestic coal has prompted extensive research into methods of replacing petroleum with coal products in an environmentally acceptable manner. Among the more promising of these methods are solvent refined coal processes.

Among the problems encountered in the solvent refined coal process is control of the viscosity of the initial coal-solvent slurry. Slurries of too high viscosity are difficult to pump and handle downstream in the process. Slurries having a low viscosity present the problems of particles settling out of the slurry and erosion of the process equipment.

While the viscosity of the feed slurry may be controlled by lowering the ratio of solids to solvents, this leads to increased equipment cost as the downstream processing system must be of greater capacity to handle the increased volume of the feed stream. U.S. Patent No.

3,341,447 to Bull discloses the relationship between slurry concentration and viscosity.

U.S. Patent No. 3,856,658 to Wolk discloses a system for dealing with coal-solvent slurry viscosity problems. In this method, a dilute slurry stream is first pressured to a pressure slightly above the desired reactor pressure. The slurry is then concentrated downstream from the slurry pump by means of a liquid cyclone and then passed into the reactor. This method requires an additional process step between the slurry pump and the reactor and therefore necessitates additional capital expenditures as well as higher operating costs.

SUMMARY OF THE INVENTION The method of the present invention allows the feed slurry viscosity to be varied while holding other variables such as slurry concentration, temperature and shear rate constant. Thus, the coal concentration temperature and other variables can be chosen based on downstream processing equipment parameters and the viscosity still maintained in the desired range. In addition, there is no need for concentration of the slurry after it passes through the slurry pump as in the prior art method noted previously.

The coal-solvent slurry having a controllable viscosity is formed by first pulverizing the coal. A slurry of the proper viscosity can be formed by grinding the coal in such manner as to produce a particle size distribution such that at least 80% of said coal will pass through a 30 mesh screen and not more than 30% of said coal will pass through a 400 mesh screen. The ground coal is then mixed with the solvent. Alternatively the pulverized coal is then classified so that the largest particles will pass through a screen of about 8 mesh and so that at least 80% will pass through a 30 mesh screen (all references to particle size in both the claims and specification of this application are to the Tyler Standard Sieve Series). During this classification particles which will pass through a screen smaller than 400 mesh are partially removed.These particles, hereinafter referred to as fines, are then collected. The pulverized coal, thus classified, is then mixed with a hydrocarbon solvent to form a slurry. A portion of the fines previously removed may then be returned to the slurry to further regulate the slurry viscosity.

DETAILED DESCRIPTION Slurry Properties Coal slurries are non-Newtonian fluids. The viscosity of coal slurries is affected by coal concentration, temperature, shear rate, and elapsed time. To a lesser extent, the type of coal used may have an effect on slurry viscosity. The inventors of the present invention have discovered that the particle size of the pulverized coal also has a major impact on the viscosity of the slurry. In particular, they have discovered that fines produced in the coal pulverization process greatly increase the slurry viscosity. Based on this discovery of a relationship between coal particle size and coal feed slurry viscosity, the method of the present invention allows the elimination of the possibility of erosive low viscosity slurries and inoperably high slurry viscosities.In addition, the present invention allows use of higher temperatures in the slurry mix tank without exceeding the allowable viscosity. These advantages in turn allows improved thermal and mechanical efficiency in the operation of the coal liquefaction plant.

Experimental Results A series of tests were performed on Powhatan No. 5 coal which was coarsely pulverized to - 30 mesh. A portion of this pulverized coal was reground in a laboratory ball mill consisting of a thick walled ceramic container and 2 one inch steel balls.

Both grinds were mixed with a recycle slurry from the SRC process at 325"F, (163"C). The resulting slurries each contained 30 percent by weight of the two respective grinds. The slurry recycle solvent was composed of hydrocarbons, ash, and insoluble organic matter.The composition of the recycle solvent as hydrocarbon fractions within the stated boiling ranges and of the ash and insoluble organic matter (IOM) are given below: Component Weight % water and hydrocarbons boiling at less than 150 F (65 ) 0.04 hydrocarbons boiling at 150"-250"F (65o-121 C) 0.05 250"-350"F (121 '-1 76'C) 0.25 350"-450"F (176 -232 C) 1.31 450'-550'F (232'-287'C) 4.90 550'-650'F (287 -343 C) 8.01 650"-750"F (343 -398 C) 8.62 750'-850'F (398'-454'C) 7.42 850"-900"F (454 -482 C) 1.30 greater than 900'F (482 C) 39.17 IOM 7.55 ash 21.38 The viscosity of the two slurries was measured on a Brookfield Model HA viscometer at a shear rate of 18.6 sec. -' at regular time intervals. The resulting data is shown as a plot of viscosity versus time in Fig. 1. The size distribution of the two grinds is as follows: Fine Grind Coarse Grind Mesh % Passed Mesh % Passed 60 100 60 96.2 100 94.2 100 85.0 140 87.2 140 68.5 200 76.8 200 48.1 270 59.3 270 44.2 325 48.6 325 39.3 400 41.1 400 29.5 As can be readily seen from this data, the particle size of the coal solids in the slurry has a dramatic effect on the slurry viscosity and this effect becomes even more pronounced with time.

All percentages as used through the specification and claims of this application are weight percentages.

Another sample of Powhatan No. 5 coal was ground and separated into fractions of the following particle size. For ease of expression a particle, for example that will pass through a 200 mesh screen and will not pass a 270 mesh screen will be refered to as less than 200 mesh.

Notation Particle Size (Mesh) less than 400 400 + less than 325 325-400 less than 270 270-325 less than 200 200-270 less than 1 50 150-200 less than 100 100-150 less than 60 60-100 Each fraction was then mixed with the recycle slurry as described above to yield a slurry containing 30 percent by weight of the pulverized coal. The viscosity of each feed slurry thus prepared was measured with a Brookfield Model 5X HBT viscometer and plotted as a function of time at several temperatures and shear rates.

Figs. 2 and 3 depict the variation in the time-viscosity plots when measured at different shear rates and temperatures for a less than 200 mesh particle size slurry. As seen in each of these figures, the viscosity increases inversely to the shear rate.

The relationship between slurry viscosity and temperature for a slurry containing particles less than one hundred mesh is shown in Fig. 4. Feed slurry viscosity increases as the slurry temperature increases.

Fig. 5 depicts the time-viscosity plots of the fractions as measured at a temperature of 163"C (325OF) and at a shear rate of 18.6 sec.-1. Similarly, Fig. 6 shows the viscosity data for a temperature of 177"C (350OF) and a shear rate of 18.6 sec.-1. The viscosity data for each fraction at a shear rate of 37.2 sec. -' is shown in Figs. 7 and 8 at temperatures of 163"C (325OF) and 177 C (350 F) respectively. Finally, the data for each fraction at a temperature of 177"C (350OF) and a shear rate of 93 sec.-1 is depicted in Fig. 9.These figures demonstrate that the use of larger particle sizes permits operation at higher temperature levels for the same slurry viscosities. This substantially improves overall thermal efficiency of the process by decreasing the extent of cooling required for the slurry recycle stream.

As can be seen from the data graphically shown in Figs. 4 through 9, the coal particle size of the slurry is the dominant variable affecting slurry viscosity. Those slurries having particles that will pass a 270 mesh screen have low initial viscosities and exhibit comparatively small increases in viscosity with time. In contrast, those slurries with particles that will pass a 270 mesh screen have higher initial viscosities and their viscosities increase markedly with time.

Slurry Formation Although applicable to the control of slurry viscosities in any process requiring a slurry of pulverized coal in a solvent, the invention will be illustrated as applied to a coal liquefaction process. The initial step in a coal liquefaction process is reducing the raw lump coal to the proper particle size. The raw lump coal as received may range from 0.25 inches (.63 cm.) to 2 inches (5.08 cm.) in diameter. This coal must be reduced in size to at least 8 mesh. Preferably the average particle size of the pulverized coal would be such that at least 80% of the coal will pass through a 30 mesh screen.

One mode in which the present invention may be carried out is by grinding the coal in a manner such that the desired particle size distribution is achieved as result of the grinding operation itself without the need for the additional step of removing fines.

Generally the particle size distribution desired would be one where at least 80% of the coal would pass through a 30 mesh screen and not more than 30% of the coal would pass through a 400 mesh screen. Often it will be desirable to operate using a coarser grind such as one where not more than 80% of the coal will pass through a 100 mesh screen, not more than 30% will pass through a 270 mesh screen, and not more than 5% will pass through a 400 mesh screen. Other times an even coarser grind will be indicated, for example a distribution where not more than 80% of the coal will pass through a 100 mesh screen, not more than 5% will pass through a 270 mesh screen and not more than 3% through a 400 mesh screen. The exact particle size distribution for a given process will of course have to be chosen based on the other variables affecting slurry viscosity.The ability to control feed slurry viscosity based on particle size and thus have more leeway in choosing values of other viscosity related variables based on other process parameters is one of the unique advantages of the instant invention.

The pulverization can be carried out using conventional equipment well known in the art. For example, a ball and race mill may be used with satisfactory results. The pulverizing equipment may be operated so that oversized material, i.e. coal particles larger than 8 mesh are returned to the equipment for further pulverization.

In a second method of carrying out the present invention, following the pulverization step, at least a portion of the fines created during the pulverization are removed. Depending upon the other variables which affect the slurry viscosity generally particles that will pass a 400 mesh screen are partially removed to the extent that not more than 30% of the coal will pass through a 400 mesh screen. In some instances, for example if an especially concentrated slurry was desired, particles that will pass a 200. mesh screen could be partially removed. The removal of the fines can be carried out in standard equipment known in the art. For example, the pulverized coal can be air classified and the fines then collected by means of a cyclone.In addition, to being used to control the viscosity of the slurry these fines are also of the proper size to be used as fuel for providing process heat or alternatively may be used as feed in a coal gasifier unit.

The classified pulverized coal is then mixed with the process solvent. The solvent is a petroleum or coal derived distillate. Generally, the solvent would be a fraction of the liquid product produced in the coal liquefaction process. A portion of the solvent may also be recycled from the downstream liquefaction process and may contain dissolved coal and coal solids.

Selection of the boiling range of the distillate used, is based on operating conditions in the coal refining process. The solvent will generally have a boiling range of about 150"C (302OF) to about 750"C (1382OF). Preferably a heavy distillate having a boiling range of about 230"C (446OF) to 480"C (896OF) will be used.

The slurry may be prepared by mixing the pulverized coal and the solvent (including recycle slurry if used) in a vessel equipped with a paddle or other mixing means. The solvent to coal ratio generally used in solvent refined coal processes is generally between about 1 to 1 and about 2.5 to 1.

During the slurry preparation a portion of the fines previously removed are added back to the slurry to bring the final slurry viscosity into the desired range. Preferably the slurry viscosity will be in the range of about 500-1000 centipoise. This provides a slurry which is viscous enough to avoid problems of settling and erosion of process equipment and not so viscous as to be unpumpable.

The method of the invention could be carried out without the final step of adding fines back to the slurry. This could be accomplished by varying the average particle size of the pulverized coal or by incompletely removing the fines from the pulverized coal. However, due to the flexibility of control, the preferred method includes returning a portion of the separated fines to the slurry to tailor the viscosity to the desired range. Regardless of whether the fines are added back or not, the particle size distribution of the final slurry should fall into the ranges previously stated.

The slurry thus prepared is then fed into processing equipment of the solvent refined coal process. By optimizing the feed slurry viscosity, it is possible to operate with a higher solids percentage in the slurry. This in turn allows greater thermal and mechanical efficiency in the utilization of downstream process equipment as a smaller volume of the solvent is passed through the process. In addition, the life of equipment subject to erosion, such as pumps and valves, is increased by processing a slurry having the desired viscosity.

Modifications and variations of the invention will be apparent to those skilled in the art. It is the applicant's intention in the following claims to cover all such equivalent modifications and variations as fall within the true spirit and scope of the invention.

Claims (9)

1. A method for producing a coal-solvent slurry for use as a feed in a coal liquefaction process characterized in that is comprises: grinding said coal so that the particle size distribution is such that at least 80% of said coal will pass through a 30 mesh screen and not more than 30% of said coal will pass through a 400 mesh screen; and mixing said ground coal with a solvent to form a slurry.

2. The method of claim 1 further characterized in that said particle size distribution is further defined so that not more than 85% of said coal will pass through a 100 mesh screen, not more than 50% of said coal will pass through a 270 mesh screen, and not more than 30% of said coal will pass through a 400 mesh screen.

3. The method of claim 2 further characterized in that said particle size distribution is further defined so that not more than 80% of said coal will pass through a 100 mesh screen, not more than 30% of said coal will pass through a 270 mesh screen, and not more than 5% of said coal will pass through a 400 mesh screen.

4. The method of claim 3 further characterized in that said particle size distribution is further defined so that not more than 80% of said coal will pass through a 100 mesh screen, not more than 5% will pass through a 270 mesh screen, and not more than 3% will pass through a 400 mesh screen.

5. A method for producing a coal-solvent slurry for use as feed in a coal liquefaction process having a controllable viscosity characterized in that it comprises: pulverizing the coal; classifying the coal so that the maximum size particles will pass through an 8 mesh screen and so that at least 80% of said particles will pass through a 30 mesh screen and so that the fines that will pass through a 400 mesh screen are partially removed to the extent that not more than 30% of the remaining particles will pass though a 400 mesh screen.

collecting said fines following the classification step; mixing said classified pulverized coal with a hydrocarbon solvent to form a slurry.

6. The method of claim 5 further characterized in that the ratio of solvent to coal is between about 1 to 1 and 2.5 to 1.

7. The method of claim 6 further characterized in that a portion of the fines removed from the pulverized coal are added back to the slurry mixture to further control the slurry viscosity.

8. A method for controlling the viscosity of a coal-solvent slurry for use as feed in a coal liquefaction process characterized in that it comprises: pulverizing the coal; classifying the coal so that the maximum size particles will pass through an 8 mesh screen and so that at least 80% of said particles will pass through a 30 mesh screen and so that fines that will pass through a 400 mesh screen are partially removed to the extent that not more than 30% of the remaining particles will pass though a 400 mesh screen; collecting said fines following the classification step; mixing said classified pulverized coal with a hydrocarbon solvent to form a slurry; and adding a portion of the fines previously removed from the pulverized coal back to the slurry mixture to control the slurry viscosity.

9. The method of claim 8 further characterized in that the ratio of solvent to coal is between about 1 to 1 and about 2.5 to 1.