GB1584585A - Process for liquefying coal - Google Patents

Description

(54) PROCESS FOR LIQUEFYING COAL (71) We, GULF RESEARCH & DEVELOPMENT COMPANY, a corporation organized under the laws of the State of Delaware, of P.O. Box 2038, Pittsburgh, Pennsylvania 15230, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for producing a coal liquid product.

According to the present invention there is provided a non-catalytic process for producing a coal liquid product comprising contacting a slurry of coal and a solvent with hydrogen in a tubular preheater zone, heating the slurry in the preheater zone to a maximum temperature of from 710 to 8000F, passing effluent from the preheater zone to a dissolver zone maintained at a temperature at least 10 F higher than the maximum in the preheater zone and within the range of from 750 to 9000F, the residence time in the dissolver zone being greater than in the preheater zone, a hydrogen pressure of greater than 3,100 psi being maintained in the dissolver zone.

Preferably the slurry contains water and is contacted with carbon monoxide, whereby water and carbon monoxide react to form hydrogen in situ.

More preferably the hydrogen pressure is above 3,500 psi.

Advantageously an effluent stream is removed from the dissolver zone and is cooled by passage in heat exchange relationship with the slurry to heat the slurry.

In the prdcess according to the invention the dissolver zone may be operated adiabatically in which case no heat is transferred to the slurry other than in the preheater zone.

This invention relates to a process for converting ash-containing raw coal to deashed coal.

The present invention relates to a coal liquefaction process employing a' relatively non-backmixed tubular preheater supplied with a slurry of pulverized feed coal and solvent.

The temperature of each plug or increment of slurry flowing through the preheater increases to a maximum at the preheater outlet. The preheater zone is followed by a dissolver zone operated under conditions tending to approach backmixing in order to maintain as uniform a temperature throughout as possible, which temperature is higher than the maximum temperature in the preheater zone. The temperature of the dissolver zone is at least 10 F. (5.5"C.), generally, or at least about 50"F (27.8 or 55.5"C.), preferably, higher than the' maximum preheater temperature.

It is critical that the preheater exit temperature be maintained within the range from 710 to below 800"F (377 to below 427"C.), generally, or from 750 to 790"F. (399 and 421"C.), preferably. During the preheating step, the viscosity of each increment of feed slurry initially increases, then decreases and would finally tend to increase again. However a significant final increase in viscosity is avoided by terminating the preheating step within the temperature range from 710 to below 800"F. (377 to below 427"C.). If the preheater temperature exceeds this range, a substantial increase in viscosity will occur caused by polymerization of the dissolved coal.Such polymerization should be avoided since its result would be a product comprising an increased amount of relatively low value high molecular weight solid deashed coal as well as coal insolubles, at the expense of more valuable coal liquids. These viscosity effects are described in U.S. 3,341,447 to Bull et al.

A final increase in viscosity in the preheater is avoided by passing the plug flow preheater effluent directly into a dissolver zone maintained at an essentially uniform temperature which is higher than the maximum preheater temperature. The dissolver temperature is from 750 to 9000F. (399 and 482"C.), generally, and from 800 to 9000F. (427 and 482"C.), preferably. The temperature hiatus between the preheater and dissolver zones can be the temperature range in which undesired coal polymerization can occur. At the elevated dissolver temperature, instead of the aforementioned coal polymerization and viscosity increase, there is a viscosity decrease due to a molecular weight reduction via hydrocracking reactions.However, in order for the hydrocracking reactions to proceed effectively in the dissolver, we have found that a process hydrogen process hydrogen pressure of at least 3,100 psi or, preferably, at least 3,500 psi (217 or 245 Kg/cm2) is required. If carbon monoxide is utilized to generate hydrogen in the process as described below, the term "hydrogen pressure" as used herein and in the claims means the pressure of hydrogen plus carbon monoxide, since each mol of carbon monoxide is potentially one mol of hydrogen.

At lower process hydrogen pressures, the elevated dissolver temperature of this invention in combination with the extended residence times indicated below was found to induce excessive coking and thereby encourage production of carbonaceous insolubles at the expense of coal liquids. Therefore, in the dissolver stage of this invention, the use of an elevated temperature within the range from 750 to 9000F. (399 and 482"C.) is accompanied by a process hydrogen pressure between 3,100 and 5,000 psi (217 and 350 Kg/cm2), generally and between 3,500 and 5,000 psi (245 and 350 Kg/cm2), preferably.

The residence time in the preheater zone is between about 2 and 20 minutes, generally, and between about 3 and 10 minutes, preferably. The residence time in the dissolver zone is longer than in the preheater zone in order to provide adequate time for thermal hydrocracking reactions to occur. A dissolver residence time between about 5 and 60 minutes, generally, and between about 10 and 45 minutes, preferably, is required. The use of an external preheater avoids a preheating function in the dissolver zone and thereby tends to reduce the residence time in the dissolver zone, thereby reducing the amount of coking occurring in the dissolver zone. Hydrocracking and coking are concurrent reactions in the dissolver zone.Hydrocracking is the more rapid of the two reactions, and any unnecessary extension of dissolver residence time will relatively favor the slower coking reactions over the more rapid hydrocracking reactions.

The primary solvation reactions in the preheater zone occur between the solvent and the feed coal and are considered to be endothermic. In contrast, the hydrocracking reactions occurring in the dissolver zone are known to be exothermic. Therefore, the preheater zone requires heat input for the solvation reactions and to heat the mass of the feed material while the dissolver zone not only sustains its own heat requirements but also produces excess heat which is available for transfer to the preheater zone. By maintaining the indicated temperature differential between the preheater and dissolver zones the excess heat available at the dissolver zone is at a sufficiently elevated temperature level that it can advantageously supply at least a portion of the heat requirement of the preheater, providing a heat-balanced system.Cooperative functioning of the relatively low temperature preheater and the relatively high temperature dissolver zones in this manner can provide process heat economies in a coal liquefaction process which were not possible under prior art processes.

The dissolver effluent is ultimately reduced in pressure and passed to a distillation zone, preferably a vacuum distillation zone, to remove overhead separate fractions comprising product coal liquid, recycle solvent and deashed solid coal. The bottoms fraction can comprise a mixture of ash and non-distillable hydrocarbonaceous residue.

Table 1 shows the results of tests performed to illustrate the advantageous effect of elevated dissolver temperatures. In these tests, a slurry of pulverized Big Horn coal and anthracene oil was passed through a tubular preheater in series with a dissolver. Some vertical sections of the dissolver were packed with inert solids enclosed by porous partitions as shown in U.S. 3,957,619 to Chum et al. No external catalyst was added to the dissolver.

Heat was added to the preheater but the dissolver was operated adiabatically. No net heat was added between the preheater and the dissolver. Elevated dissolver temperatures were achieved by exothermic dissolver hydrocracking reactions.

The Big Horn coal had the following analysis: Feed Coal (Moisture Free) Carbon, Wt. % 70,86 Hydrogen, Wt. % 5.26 Nitrogen, Wt. % 1.26 Oxygen, Wt. % 19.00 Sulfur, Wt. % 0.56 Metals, Wt. % 3.06 Ash, Wt. % 6.51 Suflur, Wt. % 0.32 Oxygen, Wt. % 3.12 Metals, Wt. % 3.06 Moisture, Wt. % 21.00 Following are the data obtained in the tests:: TABLE 1 Run time (days) 3.88 5.00 11.38 MAG* Coal in Slurry, Wt. % 29.53 29.53 29.53 MAF* Coal Rate, gm/hr 1225.71 1101.42 1035.20 Preheater Outlet Temp., F. ( C.) 713(378) 715(379) 729(387) Dissolver Temp., F. ( C.) 750(399) 775(413) 800(427) Total Pressure, psi (Kg/cm) 4100(287) 4100(287) 4100(287) H2 pp, psi (Kg/cm) 3785(265) 2842(269) 3828(268) Unconverted Coal, Wt. % of MAF* Coal 32.48 24.67 12.20 Chemical H2 Consumption decimeters /kg MAF* Coal 341.96 468.42 749.10 Conversions, Wt. % MAF* Coal Solvation 67.52 75.36 87.80 Hydrocracking (fraction of MAF* coal converted to product boiling below 415 C.) 17.31 31.65 54.33 Denitrogenation, Wt. % 4.78 6.31 21.32 Oxygen Removal, Wt. % 42.98 47.89 51.53 *MAF means moisture-and ash-free The data of Table 1 show that as the dissolver temperature was increased in steps from '750 to 775 and 800"F (399 to 413 and 427"C.), so that the temperature differential between the preheater and dissolver was increased from 37 to 60 and 71"F. (20 to 33 to 390C.), respectively, the amount of coal dissolved increased from 67.52 to 75.36 and 87.80 weight percent of MAF coal, respectively, while the fraction of MAF coal converted to product boiling below 415"C. (783"F.) increased from 17.31 to 31.65 and 54.33 weight percent of MAF coal, respectively.These results illustrate the substantial advantage in terms of both quantity and quality of product obtained by autogenously increasing the temperature differential between the preheater and the dissolver stages by means of exothermic dissolver hydrocracking reactions. Not only is the product quantity and quality advantageously increased as the dissolver temperature and the temperature differential between the stages are increased, but also the process advantageously can become increasingly self-sufficient in heat requirements by transferring the increasingly high level sensible heat autogenously generated at the dissolver to the preheater. One means of accomplishing this heat transfer is by cooling the dissolver effluent by heat exchange with the preheater feed stream.A noteworthy feature of the tests is that the increasing temperatures were achieved in the dissolver with no net addition of heat to the process downstream from the preheater exit.

The extent of hydrocracking occurring in the dissolver can be adjusted by control of dissolver temperature. The extent of hydrocracking can be diminished by lowering the dissolver temperature and such a reduction may be required if the yield of low boiling products is so great that there is an insufficient yield of higher boiling solvent boiling range liquid to satisfy full process solvent requirements. However, if an inexpensive solvent boiling range oil is available from outside of the process, it may be economical to operate the dissolver at a high temperature to obtain a high yield of valuable low boiling liquid products, even though the yield of solvent boiling range liquid is insufficient to satisfy process solvent requirements.If desired, the dissolver effluent can provide a portion of process solvent requirements, with the remainder of the solvent comprising a liquid supplied from outside of the process. In the above tests, the entire process solvent requirement was supplied from outside of the process.

The use of dissolver temperatures above the maximum temperature of the preheater stage in accordance with this invention is feasible only in conjunction with the elevated hydrogen pressure of this invention. U.S. 3,892,654 to Wright et al teaches that the dissolver temperature should be lower than the preheater temperature because higher temperatures tend to lower the hydro-aromatic content of the system, and thereby fail to produce an acceptable hydrogen donor solvent for recycle. However, while the tests of that patent were performed at a relatively low pressure the high hydrogen pressure of this invention are sufficient to counteract the effect of the elevated temperatures in this regard so that the process can produce an acceptable hydrogen donor solvent in spite of the elevated temperatures.

While elevated temperatures and elevated hydrogen pressures have an opposite effect upon production of hydroaromatic material in the dissolver, they tend to have a similar effect in regard to hydrocracking. However, while increases in temperature have an exponential effect upon hydrocracking rate, increases in hydrogen pressure have only a linear effect, or less, upon hydrocracking rate. As hydrocracking temperatures are increased, the coking rate increases exponentially. It is in regard to the coking rate that high hydrogen pressures exert a significant effect. High hydrogen pressures tend to inhibit coking by forcing the contribution of large amounts of hydrogen to the cracking reaction and thereby enhancing the production of liquid rather than solid products at any given temperature.Production of a high yield of solid products would be tantamount to operation of the dissolver as a delayed coker, rather than as a hydrocracker. Therefore, while elevated hyrogen pressures do not have a great direct effect upon hydrocracking rate itself, they have a significant effect upon hydrocracking selectivity at any given temperature by enhancing the production of valuable liquid products rather than coke.

The high hydrogen pressure of this invention and the consequent avoidance of substantial coking is especially important where the dissolver effluent is employed to transfer heat to the feed coal slurry entering the preheater. Not only do high hydrogen pressures enhance production of liquid products, and liquid is an effective heat transfer material, in preference to the production of coke which is a solid product and therefore not an effective heat transfer material, but also hyrocracking reactions are highly exothermic so that the liquid product contains considerable sensible heat. In contrast, a low hydrogen pressure would not only permit enhanced coking, which is an endothermic reaction and which therefore substitutes an endothermic for an exothermic reaction, depriving the dissolver effluent of sensible heat which would otherwise be available for heat transfer, but it would also deprive the dissolver effluent of heat transfer fluid since a portion of the heat transfer liquid which would be produced via hydrocracking is converted into coke which is not amenable to pumping to a heat exchanger but which deposits inertly within the dissolver vessel.

In a process wherein it is primarily desired to produce a high grade of hydroaromatic solvent without regard to the upgrading of dissolved solid coal to coal liquids, low dissolver temperatures can be employed. However, where it is desired to upgrade dissolved solid coal to more valuable coal liquids, a high dissolver temperature coupled with an elevated hydrogen pressure must be employed. A high dissolver temperature will be especially valuable where a high grade solvent is available at least in part from a source outside of the process. An advantage of a high dissolver temperature, in addition to the production of valuable coal liquids, is the aforementioned availability of high temperature sensible heat for preheating the stream flowing through the preheater.

The process of this invention is particularly suited to the use of inexpensive carbon monoxide and water as a substitute for hydrogen, which is considerably more expensive.

Carbon monoxide and water are converted to carbon dioxide and hydrogen in the process via the water gas shift reaction: CO+H2O > CO2+H2. This reaction is exothermic and the equilibrium is unaffected by pressure. However, low temperatures favor completion of the reaction. Following are equilibrium constants Kp at various temperatures, where Kp = Pco2PH2/PcoPH2o.

Temperature oF 0C.) Kp, Atmospheres 500 260 78.0 600 316 36.0 700 371 17.5 800(427) 9.2 900(482) 5.6 1,000(538) 3.75 The above data show that the production of hydrogen via the shift reaction is enhanced at relatively low temperatures. On the other hand, Figure 1 illustrates hydrogen consumption data in a coal dissolver at various space times and shows that in a coal dissolver hydrogen is consumed most rapidly at high temperatures. The above data and Figure 1 illustrate the particular effectiveness derived from employing in combination a relatively low temperature preheater and a relatively high temperature dissolver when producing hydrogen via the shift reaction.The data show that the equilibrium favors the formation of hydrogen at the relatively low temperature of the dissolver. However, Figure 1 shows that the hydrogen produced is most rapidly consumed (via hydrocracking reactions) at a relativaly high temperature. The indicated high rate of hydrogen consumption obtained by employing a high temperature in the dissolver tends to prevent reversal of the shift reaction, which the above data for Kp show would otherwise tend to occur at high temperatures if the hydrogen were not being consumed.

Therefore, when employing the shift reaction for in situ production of hydrogen, the relatively low temperature of the preheater zone and the relatively high temperature of the dissolver zone function interdependently. The lower temperature in the preheater zone favors the conversion of carbon monoxide and water to hydrogen while the subsequent higher temperature in the dissolver zone tends to prevent reversal of the reaction in the dissolving process by increasing the rate of consumption of the hydrogen produced via hydrocracking reactions.

The use of a dissolver vent for independent removal of gases from the dissolver zone would also exert an inter-dependent function in inhibiting reversal of the shift reaction. The vented gases can include the shift reaction gases and light hydrocarbons boiling up to about 450"F (232"C.). A continuous venting of dissolver gases so that gases are removed more rapidly than liquid from the dissolver zone accomplishes rapid cooling of excess gases not required for hydrocracking reactions to temperatures more favorable to the preservation of the hydrogen product. In this manner, reversal of the equilibrium of the shift reaction tends to be prevented by physical removal of the shift reaction product.

A process scheme of this invention is shown in Figure 2. As shown in Figure 2, a slurry of pulverized feed coal with water and recycle or make-up solvent in line 10 is mixed with hydrogen and/or carbon monoxide entering through line 12 and is passed through heat exchanger 14 in transit to plug flow preheater 16. Heat is supplied to heat exchanger 14 from hot dissolver effluent liquid entering through line 36. Heat is supplied to preheater 16 by any suitable means, such as by means of a flame from oil burner nozzle 18.The slurry residence time in preheater 16 is between about 2 and 20 minutes and preheater effluent at a temperature between about 710 and 800"F. (377 and 427"C.) in line 20 is passed to the bottom of a high temperature dissolver chamber 22 which can operate adiabatically under the influence of exothermic hydrocracking reactions so as to maintain a dissolver temperature of between about 750 and 900"F. (404 and 482"C.). The residence time in dissolver 22 is between about 5 and 60 minutes. A slurry of ash-containing solids tends to concentrate at the bottom of dissolver 22 and is passed through line 26 to hydroclone 28, in which ash is separated from liquid. The ash is discharged through line 30 while liquid is returned to the dissolver through line 32.If desired, the dissolver can be heated or cooled by means of hot or cold hydrogen injected through line 34. Gases can be independently vented from the dissolver zone through line 40 by opening valve 42. The dissolver effluent is passed through line 36 in indirect heat exchange relationship with respect to cool slurry and hydrogen in heat exchanger 14 to supply a portion of the slurry heating requirements, before being discharged through line 38. The stream in line 38 contains hydrogen which can be purified and recycled to line 12 and solvent boiling range liquid which can be recycled to line 10.

We draw attention to our copending Patent Applications numbered 24921/77; 24922/77 (Serial Nos. 1584582 & 1584538), 24928/77 and 24930/77 (Serial Nos. 1584584 & 1584586) which also relate to the production of coal liquid products by processes generally similar to that of the present invention.

WHAT WE CLAIM IS: 1. A non-catalytic process for producing a coal liquid product comprising contacting a slurry of coal and a solvent with hydrogen in a tubular preheater zone, heating the slurry in the preheater zone to a maximum temperature of from 710 to 800"F, passing effluent from the preheater zone to a dissolver zone maintained at a temperature at least 10 F higher than the maximum in the preheater zone and within the range of from 750 to 900OF, the residence time in the dissolver zone being greater than in the preheater zone, a hydrogen pressure of greater than 3,100 psi being maintained in the dissolver zone.

2. A process according to claim 1, in which the slurry contains water and is contacted with carbon monoxide, whereby water and carbon monoxide react to form hydrogen in situ.

3. A process according to claim 1 or claim 2, wherein the hydrogen pressure is above 3,500 psi.

4. A process according to claim 1 or 2 or 3, wherein an effluent stream is removed from the dissolver zone and is cooled by passage in heat exchange relationship with the slurry to heat the slurry.

5. A process according to claim 1 or 2 or 3, wherein the dissolver zone is operated adiabatically and no heat it transferred to the slurry other than in the preheater zone.

6. A process according to any preceding claim, wherein the dissolver temperature is adjusted to control the amount of solvent boiling range liquid produced.

7. A process according to any preceding claim, wherein the solvent is a liquid supplied from outside the process.

8. A process according to any preceding claim wherein the residence time in said preheater zone is 2 to 20 minutes.

9. A process according to any preceding claim, wherein the residence time in the dissolver zone is 5 to 60 minutes.

10. A process according to any preceding claim, wherein gases are independently vented from the dissolver zone.

11. A process according to any preceding claim, wherein the temperature in the dissolver zone is at leat 50"F higher than the maximum temperature in the preheater zone.

12. A process according to claim 11, wherein the temperature in the dissolver zone is at least 100"F higher than the maximum temperature in the preheater zone.

13. A process according to any preceding claim, wherein the maximum preheater zone temperature is in the range 750 to 7900F.

14. A process according to claim 1, substantially as hereinbefore described.

15. Coal liquids whenever produced by the process claimed in any one of claims 1 to 14.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. bottom of a high temperature dissolver chamber 22 which can operate adiabatically under the influence of exothermic hydrocracking reactions so as to maintain a dissolver temperature of between about 750 and 900"F. (404 and 482"C.). The residence time in dissolver 22 is between about 5 and 60 minutes. A slurry of ash-containing solids tends to concentrate at the bottom of dissolver 22 and is passed through line 26 to hydroclone 28, in which ash is separated from liquid. The ash is discharged through line 30 while liquid is returned to the dissolver through line 32. If desired, the dissolver can be heated or cooled by means of hot or cold hydrogen injected through line 34.Gases can be independently vented from the dissolver zone through line 40 by opening valve 42. The dissolver effluent is passed through line 36 in indirect heat exchange relationship with respect to cool slurry and hydrogen in heat exchanger 14 to supply a portion of the slurry heating requirements, before being discharged through line 38. The stream in line 38 contains hydrogen which can be purified and recycled to line 12 and solvent boiling range liquid which can be recycled to line 10. We draw attention to our copending Patent Applications numbered 24921/77; 24922/77 (Serial Nos. 1584582 & 1584538), 24928/77 and 24930/77 (Serial Nos. 1584584 & 1584586) which also relate to the production of coal liquid products by processes generally similar to that of the present invention. WHAT WE CLAIM IS:

1. A non-catalytic process for producing a coal liquid product comprising contacting a slurry of coal and a solvent with hydrogen in a tubular preheater zone, heating the slurry in the preheater zone to a maximum temperature of from 710 to 800"F, passing effluent from the preheater zone to a dissolver zone maintained at a temperature at least 10 F higher than the maximum in the preheater zone and within the range of from 750 to 900OF, the residence time in the dissolver zone being greater than in the preheater zone, a hydrogen pressure of greater than 3,100 psi being maintained in the dissolver zone.

2. A process according to claim 1, in which the slurry contains water and is contacted with carbon monoxide, whereby water and carbon monoxide react to form hydrogen in situ.

3. A process according to claim 1 or claim 2, wherein the hydrogen pressure is above 3,500 psi.

4. A process according to claim 1 or 2 or 3, wherein an effluent stream is removed from the dissolver zone and is cooled by passage in heat exchange relationship with the slurry to heat the slurry.

5. A process according to claim 1 or 2 or 3, wherein the dissolver zone is operated adiabatically and no heat it transferred to the slurry other than in the preheater zone.

6. A process according to any preceding claim, wherein the dissolver temperature is adjusted to control the amount of solvent boiling range liquid produced.

7. A process according to any preceding claim, wherein the solvent is a liquid supplied from outside the process.

8. A process according to any preceding claim wherein the residence time in said preheater zone is 2 to 20 minutes.

9. A process according to any preceding claim, wherein the residence time in the dissolver zone is 5 to 60 minutes.

10. A process according to any preceding claim, wherein gases are independently vented from the dissolver zone.

11. A process according to any preceding claim, wherein the temperature in the dissolver zone is at leat 50"F higher than the maximum temperature in the preheater zone.

12. A process according to claim 11, wherein the temperature in the dissolver zone is at least 100"F higher than the maximum temperature in the preheater zone.

13. A process according to any preceding claim, wherein the maximum preheater zone temperature is in the range 750 to 7900F.

14. A process according to claim 1, substantially as hereinbefore described.

15. Coal liquids whenever produced by the process claimed in any one of claims 1 to 14.