Abstract:
This invention relates to an improved two-stage process for the production of liquid carbonaceous fuels and solvents from carbonaceous solid fuels, especially coal.

Description:
The Government of the United States of America has rights in this invention pursuant to contract No. DE-AC05-780R03054 (as modified), awarded by the U.S. Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a process for the liquefaction of carbonaceous solid fuels, particularly coals enhanced with respect to production of liquid carbonaceous fuels and solvents. 
     Many processes have been proposed for the production of low-sulfur, low-ash, carbonaceous fuels and distillate hydrocarbon fuels by solvent refining of coal in the presence of a hydrogen donor solvent. Typically, such a process includes the heating and liquefaction of the coal yielding light gases and a slurry which is further processed by vacuum distillation to produce a light distillate product, a recycle solvent, and a heavy fraction, including residual solvent, dissolved coal products, undissolved coal, minerals or ash materials, and unconverted coal macerals. 
     It is well known that further products may be produced by subjecting the vacuum still bottoms to a solvent deashing process which is sometimes referred to as &#34;critical solvent deashing.&#34; Such a process is disclosed in U.S. Pat. No. 4,070,268. As indicated in that patent, the products of the critical solvent deashing process include a stream (HSRC) which is rich in coal products soluble in pyridine, but which is essentially free of ash and unconverted particulate coal. A bottom stream is also produced which includes insoluble coal products and ash. Finally, an underflow stream of LSRC rich in products soluble in benzene or toluene is produced which is either recycled as solvent in the SRC process or removed as a product. 
     As shown by U.S. Pat. No. 4,164,466, the solvent deashing stage often comprises several separation zones, each maintained at successively higher temperatures and at high pressure. This patent also discloses a process wherein the underflow stream of the second zone in the deashing stage is recycled to the entry mixing zone in the deashing stage. 
     In the process disclosed in U.S. Pat. No. 4,189,372, a portion of the underflow from the third and fourth separators is hydrogenated and recycled to the coal liquefaction slurry tank. Substantially all other intermediate streams from the second through the fourth separators are recycled to the entry mixing zone of the SRC process stage as in the U.S. Pat. No. 4,164,466. 
     In U.S. Pat. No. 4,119,523, the underflow from the first separator in the solvent deashing stage is extracted to separate the resulting ash and undissolved coal, and the remaining extract recycled to the coal liquefaction stage. 
     U.S. Pat. No. 4,298,451 teaches the catalytic hydrocracking of a clean coal extract 500° F.+ (260° C.+). The process disclosed uses a catalytic ebullated bed hydrocracker maintained at a temperature of 750°-825° F. (399°-441° C.) and a hydrogen pressure of 2000-3000 psi (13793-20689 Kpa). The preferred catalyst is NiMo. 
     U.S. Pat. No. 4,111,788 teaches the hydrogenation of the total effluent of a non-catalytic first stage reaction in an ebullated bed catalytic reaction zone which consists of two reactors. The first reactor may comprise an ebullated bed of non-catalytic material while the second zone is an ebullated bed of catalyst. 
     U.S. Pat. No. 4,255,248 discloses a two-stage process for the catalytic hydrocracking of coal in which the first stage comprises a catalytic reactor operating under hydrocracking conditions. 
     In view of this prior art there remains a need for further varieties of products and enhancements to an integrated two-stage coal liquefaction process. 
     It is, therefore, the general object of the present invention to provide such products and improved processes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention involves a solvent coal refining process in which, following liquefaction and light gas separation, the coal slurry is subjected to vacuum distillation, the bottom stream of which is solvent deashed. This solvent deashing includes a sequence of separation steps at elevated temperature and pressure. 
     Solvent and the &#34;Light SRC&#34; and &#34;Heavy SRC&#34; are hydrocracked on a second stage catalyst bed to yield commercially useful liquid fuels, solvents, and gases. The present invention involves an improvement in the process wherein substantially all the 500° F.+ hydrocracker flash bottoms are recycled as first stage dissolver solvent, and the hydrocracker solvent is substantially comprised of 500° F. to EP distillate from the first state fractionator. 
     The term &#34;Light SRC&#34; or &#34;LSRC&#34; refers to and defines that SRC material which is comprised of approximately one-third (1/3) oils, which are pentane-soluble, and two-thirds (2/3) asphaltenes, which are pentane insoluble, benzene soluble. LSRC has a softening point of about 180° F. (82.2° C.). 
     The term &#34;Heavy SRC&#34; or &#34;HSRC&#34; refers to and defines that SRC material which is comprised of approximately equal amounts of asphaltenes which are pentane insoluble, benzene soluble and preasphaltenes which are benzene insoluble, pyridine soluble with only a trace amount (about 1%) soluble in pentane. HSRC has a softening point of about 380° F. (193.3° C.). 
     In the present invention, coal, recycle solvent, and hydrogen are mixed, preheated, and reacted in a first stage dissolver vessel of a type which is well-known in the coal liquefaction art. The dissolver effluent, comprising a mixture of hydrogen, water vapor, light hydrocarbon gases, light oil, solvent, solvent refined coal (SRC), insoluble carbon, and ash, is sent to a high-pressure, high-temperature separator to remove most or all of the vapor-phase material for recovery as recycle hydrogen and condensate products. 
     The underflow from the separator is directed to a distillation system for recovery of the process solvent and then to a critical solvent deashing system for separation of oils and asphaltenes from solids and preasphaltenes. The residue stream consisting of unconverted coal, minerals, and preasphaltenes is sent to a gasifier system. The Heavy SRC product is partially removed as product and partially combined with the Light SRC and directed to an ebullated bed hydrocracker and then to a separator which allows recovery of process solvents, products, and gases. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic flow diagram of a coal liquefaction process with a fixed bed hydrocracker stage which is the process improvement of the present invention. 
     FIG. 2 is a graph depicting yields for a variety of process solvents. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For a better understanding of the present invention, reference may be made to the detailed description which follows, taken in conjunction with the accompanying Figures and the claims. 
     Feed coal 4, SRC solvent 79, and approximately 500° F.+ hydrocracker flash bottoms 77 are combined to form a slurry in mix tank 5 at temperatures from 250° F. to 450° F. (232° C.) in ratio of from 2:0:1 to 0.2:1.5:1.0 hydrocracker flash bottoms to SRC solvent to MF coal. 
     The slurry is then passed via line 8 to preheater 10, where it is heated at a pressure of from 500 to 3000 psig (3448 to 20690 Kpa) to a temperature of from 600° to 850° F. (316° to 454° C.). Hydrogen-rich gas is mixed with the slurry prior to its introduction into the preheater via feed line 9. 
     The heated and pressurized slurry is then passed via line 15 to dissolver 18, which may consist of one or more reactor vessels operated in series or in parallel. Hydrogen-rich gas may be added to the dissolver via line 17 if desired. 
     The superficial flow through dissolver 18 is generally from 0.003 to 0.1 feet per second for the gas phase. These rates are selected to ensure adequate mixing in the reactor. Hydrogen feed rates are maintained at 10-40K SCF/TON coal. Residence time in dissolver 18 is greater than 40 minutes. 
     The effluent from dissolver 18 is passed to a gas separation system 26 via line 20. Light gases including hydrogen H 2  S, CO 2 , NH 3 , H 2  O, and C 1  -C 4  hydrocarbons are removed via line 24, to hydrogen purification system 110. The underflow from gas separator 26 is passed via transfer line 27 to distillation system 37. 
     Distillation system 37 yields four effluent streams 79, 81, 39, and 40. Stream 81 is composed of 400° F.- material (204° C.-). Effluent streams 39 and 79 are composed of 400° to 850° F. material (204° to 454° C.). Effluent stream 40 contains SRC bottoms which consist primarily of 850° F.+ material (454° C.+) including SRC, unconverted coal, and ash. Stream 40 is routed to critical solvent deashing system 50 for subsequent processing. Stream 79 is recycled to the mix tank 5 as solvent. 
     The critical solvent deashing process is described in U.S. Pat. No. 4,119,523. Critical solvent deashing unit 50 yields an ash concentrate stream 51 which is removed from the system and may be passed to equipment for hydrogen generation, preferably a gasifier 100. 
     Critical solvent deashing unit 50 also yields effluent streams 52 and 53. Effluent stream 52 consists of Light SRC and is directed to hydrocracker 60 via line 59. Effluent stream 53 consists of Heavy SRC and is partially directed to hydrocracker 60 via line 59 and partially removed as product via line 58. Distillation system effluent 39 is also sent to hydrocracker 60 via line 59, comprising less than 50% of the hydrocracker feed. 
     Hydrocracker 60 is operated as an ebullated catalyst bed at 1500° to 3500 psig (10345 to 24139 Kpa) at 700° to 850° F. (371° to 454° C.). The effluent stream from hydrocracker 60 is sent via line 62 to hydrocracker flash unit 70 where recycle hydrogen and other light gases are transferred via line 73 to hydrogen purification system 110. The liquid product is flashed to separate streams boiling above and below 500° F. (260° C.), substantially all of the former being directed via line 77 to the first stage of the process where it serves as process solvent. The 500° F.- stream (260° C.-) is combined with the light distillate stream 81 and sent via line 75 to distillation system 80. 
     Distillation system 80 produces three product streams 85, 86, and 87. Streams 85, 86, and 87 are typically a 350° F.- stream (177° C.-), a 350° to 450° F. stream (177° to 232° C.), and a 450°+ stream (232° C.+) or any combination thereof. 
     The following example is an illustration of the integrated two-stage liquefaction process of this invention: 
     Illinois No. 6 coal is slurried with hydrocracker flash bottoms and first stage distillation system effluent, pressurized, and pumped through the liquefaction reactor. The liquefaction effluent is flashed to remove light gases which are subsequently scrubbed to remove acidic and alkaline components. Hydrogen and lower hydrocarbons are recovered and recycled after purification to various process stages. Alternatively, these gases may be burned for fuel. The flash bottoms are then distilled at atmospheric and subatmospheric pressure. A portion of the distillation overhead is recovered as net product while the rest of such distillation overhead is used as solvent for the hydrocracker stage. 
     Vacuum tower bottoms are routed to a Kerr-McGee critical solvent deashing unit which characteristically rejects the highest molecular weight refractory preasphaltenes along with unconverted coal and ash. Portions of the HSRC and LSRC products of the critical solvent deashing unit are blended together, mixed with process solvent, pressurized, and preheated before being sent to the hydrocracker. The products from the hydrocracking reactor are flashed to recover recycle hydrogen and gases which are fractionated and purified in the same manner as for the first stage. Liquid product from the flash stage is flashed again to separate streams boiling nominally above 500° F. and below 500° F. The 500° F.- stream is collected as product while the 500° F.+ stream is recycled to the first stage to be used as process solvent. 
     Table I details the process conditions for the calculated example. Table II details the yield structure for the calculated example. 
     
                       TABLE I______________________________________              PDU     Com-              Process mercial              Conditions                      Range______________________________________SRC UnitCoal (MF):1st stage distillate solvent:                1:1.1:.55 1:1.5:0.2 toHydrocraker flash bottoms, wt. ratio                          1:0.0:2Slurry concentration, wt. % MF coal                37.8      35-40Feed gas, scf/lb MF coal                 20       15-30Hydrogen purity, mol %                100        80-100Reactor nominal residence time, min                 60        30-120Reactor pressure, psig                2000      1500-2500Hydrogen partial pressure, inlet psia                2000      1000-2000Reactor temperature, outlet °F.                810       750-840HTR UnitFeed slurry concentration, wt. % SRC                70.0      50-80Space velocity (lb feed/hr) lb cat                0.25      0.1-4.0Recycle gas rate, SCF/lb SRC                 30       20-40Hydrogen purity, mol %                100        80-100Hydrogen partial pressure, inlet psia                2000      2000-2500Temperature, °F.                805       700-840______________________________________ 
    
     
                       TABLE II______________________________________Yield Structure______________________________________Yields, wt. % MAF CoalHydrogen consumption               (3.9)Total gasesC.sub.1 /C.sub.4    13.7H.sub.2 S           2.5CO.sub.x            1.6NH.sub.3            0.6H.sub.2 O           6.3Net Usable ProductIBP-400° F.  16.2400-500° F.  8.4500-650° F.  9.1650-EP              12.4CSD SRC             11.9Ash ConcentrateSRC                 14.6Unconverted coal    6.6Total               100.0______________________________________ 
    
     A necessary feature of the process integration scheme of the present invention is that the process solvent for the first stage includes substantially all of the 500° F.+ hydrocracker flash bottoms. These hydrocracker flash bottoms are rich in asphaltenes and preasphaltenes and provide a substantially improved solvent quality as compared to the prior art. Comparative solvent qualities are illustrated in Table III. 
     
         TABLE 3  Summary of Yield Distribution Data for Kentucky #9 Mulford Coal   Temperature, (°F.) 800 800 800 800 840 840 840 760 810 810 810 840 840 840 700 Residence Time (min)* 20 20 + 20 30 + 30 40 + 40 20 20 + 20 30 + 30 30 + 30 20 40 30 + 30 20 20 + 20 30 + 30 30 + 30 Solvent ARS ARS ARS ARS ARS ARS ARS ARS BASE BASE BASE BASE BASE BASE BASE Conversion  (% MAF Coal) 87.5 91.3 91.4 93.6 78.2 80.1 81.9 92.0 88.4 89.6 91.8 88.0 89.5 88.2 91.5 H.sub.2 Consumption 1.4 2.1 2.9 3.1 2.0 3.4 4.1 2.0 1.5 2.0 2.4 1.5 2.6 3.1 2.1 Yields C.sub.1 -C.sub.4 4.9 8.5 9.1 10.2 10.3 16.2 19.1 3.2 4.4 7.2 9.3 6.3 12.3 14.9 4.9 CO + CO.sub.2 1.4 1.9 1.7 1.6 1.8 2.3 2.2 1.4 1.5 1.8 2.0 1.6 1.7 2.2 0.5 H.sub.2 S + NH.sub.3 1.2 1.7 1.7 1.5 1.5 2.1 2.0 1.3 1.6 2.0 2.3 1.4 2.8 2.7 1.0 H.sub.2 O 2.9 3.1 5.0 4.7 4.5 5.0 5.9 3.8 3.2 3.6 3.0 2.2 4.0 4.9 4.4 Distillate 12.3 23.9 33.7 39.1 12.5 16.8 25.4 28.5 17.9 20.0 26.9 15.5 20.2 22.6 25.0 SRC 64.2 54.7 42.7 39.7 49.3 39.0 29.5 55.8 58.4 53.1 48.9 60.3 49.0 40.9 54.5 Asphaltene 16.2 21.0 19.1 18.2 18.8 11.8 8.7 27.1 31.0 29.1 25.3 31.0 24.6 23.0 31.2 Preasphaltene 48.0 33.7 23.5 21.5 30.5 27.2 20.8 28.7 27.4 24.0 23.7 29.2 24.4 18.0 23.3   Temperature, (°F.) 800 840 760 800 840 760 800 840 760 800 840 Time (min.) 20 20 20 + 20 20 + 20 20 + 20 30 + 30 30 + 30 30 + 30 70 + 70 70 + 70 70 + 70 Solvent PRS PRS PRS PRS PRS PRS PRS PRS PRS PRS PRS Distillate 35.9 35.1 29.0 42.7 44.3 33.8 47.5 51.6 46.9 62.4 57.0 SRC 41.9 37.8 53.6 35.0 23.1 45.4 28.8 13.2 32.3 10.8 7.2 Asphaltene 15.1 11.09 20.4 15.3 10.9 21.4 16.0 8.1 22.5 6.4 7.4 Preasphaltene 26.8 25.9 33.2 19.7 12.2 24.0 12.8 5.1 9.8 4.3 -0.3 HC Gases 3.0 6.3 2.1 6.0 12.5 3.5 7.0 16.3 4.7 10.8 20.5 CO + CO.sub.2 1.5 2.0 1.3 2.1 2.5 1.6 2.1 2.7 1.5 2.1 2.1 H.sub.2 S + NH.sub.3 1.6 1.8 1.4 2.1 2.5 1.6 1.5 2.6 1.9 2.6 2.5 H.sub.2  O 3.5 4.9 3.0 3.8 5.7 5.2 5.7 6.0 5.5 6.7 6.1 Coal Conversion 86.2 86.5 89.2 89.7 87.9 89.2 90.1 88.9 90.5 91.8 90.2 H.sub.2 Consumption 1.4 1.7 1.4 2.3 3.0 1..7 2.6 4.0 2.4 3.3 5.6 *Addition of two numbers indicates two reactors in series. 
    
     An asphaltene rich solvent (ARS) is defined as a non-integrated SRC-I process solvent to which 30 wt% LSRC gas been added. A presaphaltene rich solvent (PRS) is defined as the non-integrated SRC-I process solvent to which 30% HSRC has been added. The compositions of the non-integrated base solvent, ARS and PRS, are illustrated in Table IV. 
     
                       TABLE IV______________________________________SolventComposition, wt. %          Base       ARS    PRS______________________________________Oils           96         85     74Asphaltenes    4          13     16Preasphaltenes 0           2     10______________________________________ 
    
     In each case, Kentucky No. 9 Mulford coal was slurried with each solvent and reacted at a range of dissolver operating conditions. These conditions and the product yields achieved for each are reported in Table III. The yields are graphically represented as FIG. 2. Clearly, FIG. 2 demonstrates that recycle of asphaltenes and preasphaltenes to the dissolver stage improves distillate yield substantially. 
     Several important facets of the integrated two stage liquefaction process of this invention are demonstrated by the results achieved in Wilsonville Run No. 242 which was reported in &#34;Technical Progress Report Run 242, with Illinois #6 Coal,&#34; DOE/PC/50041-19. 
     Illinois No. 6 coal was slurried with hydrotreated flash bottoms, pressurized, and pumped through the liquefaction reactor. The liquefaction effluent was flashed to remove light gases which were scrubbed to remove acidic and alkaline components. The hydrogen and lighter hydrocarbons were recovered and recycled to various process stages or burned for fuel. The flash bottoms were then distilled at atmospheric and reduced pressure. Some of the distillation overhead was recovered as net product while the rest was used as solvent in the hydrocracker. The vacuum tower bottoms were routed to the Kerr-McGee critical solvent deashing unit where ash and unconverted coal were removed. The HSRC and LSRC products of the CSD were blended, mixed with process solvent from the SRC unit, pressurized, and preheated before being sent to the ebullated bed hydrocracker. The products from the hydrocracking reactor were flashed to recover recycle hydrogen and processed gas which were fractionated and purified in the same manner as in the SRC area. The liquid product from the flash stages was flashed again to separate a stream boiling nominally above 500° F. from a stream boiling below 500° F. The 500°  F.- stream was collected as product, and the 500° F.+ stream was recycled to the first stage to be used as process solvent. The process conditions for the example are presented in Table V and the yield structure is given in Table VI. 
     
                       TABLE V______________________________________Process Conditions for Wilsonville Run 242______________________________________Material Balance 242A - 12/10/82SRC UnitCoal (MF):first stage distillate solvent:                  1:0:0.0:2.0Hydrocracker flash bottoms ratioSlurry concentration, wt. % MF coal                  36.4Feed gas, scfh         3,650Hydrogen purity, mol % 90.1Reactor coal space, rate, lb/hr-ft.sup.3                  38.5Reactor pressure, psig 2,410Hydrogen partial pressure, inlet psia                  2,150Reactor temperature, outlet °F.                    859HTR UnitFeed slurry concentration, wt. % SRC                  50.1Space velocity (lb feed/hr) lb cat                  1.08Recycle gas rate, MSCF/ton SRC                  62.6Hydrogen purity, mol % 95.9Hydrogen partial pressure, inlet psia                  2,721Temperature, °F.                    680______________________________________ 
    
     
                       TABLE VI______________________________________Yield Structure for Wilsonville Run 242______________________________________Material Balance 242A - 12/10/82Yields, wt. % MAF CoalHydrogen consumption  (3.85)Total gasesC.sub.1 /C.sub.5      4.72H.sub.2 S             2.11CO.sub.x              1.20NH.sub.3              0.76H.sub.2 O             8.22Net Usable Productibp-350° F.    5.85350°-450° F.                 5.70450° F.-EP     43.38HTR SRC               9.60Ash Concentrate450° F.- EP    0.72SRC                   8.36Unconverted coal      13.22Total                 100.01______________________________________