Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
297
Application of Liquefaction Processes to Low-Rank Coals' Warrack G. Willson," Curtis L. Knudson, and Gene G. Baker
U S .Department of Energy, Grand Forks Energy Technology Center, Grand Forks, North Dakota 58202
Tom C. Owens and Donald E. Severson Chemical Engineering Department, University of North Dakota, Grand Forks, North Dakota 58202
The Grand Forks Energy Technology Center and the University of North Dakota researchers are conducting research on the liquefaction behavior of low-rank coals necessary to apply major developing processes to these distinctly different coals. In a 5-lb coal/h continuous process unit, synthesis gas, raw lignite, and anthracene oil solvent were reacted at elevated temperatures in single pass tests in a continuous-stirred tank reactor (CSTR). Product yield fractions were correlated with percent coal in the feed slurry, hydrogen donor (tetralin) concentration, and temperature. The molecular weight of the soluble but nondistillable yield fraction was markedly reduced by increasing temperature. In batch autoclave systems, work has been conducted to establish rates and product distributions from several liquefaction solvents. Two solvents were chosen for subsequent tests aimed at determining the catalytic effects of diverse mineral matter in eight different low-rank coals.
Background The lignites of the Northern Great Plains and Gulf Coast Provinces are distinguished from higher-rank coals by high moisture, low sulfur, and an alkaline inorganic content that is predominantly inherent rather than extraneous. The inherent inorganic matter consists of alkali and alkaline earth cations that are bound to exchange sites on the organic coal structure and on very finely divided and dispersed pyrite, clay, and silica. In addition, lignites have a high content of functional groups containing oxygen. 'These various properties are believed to affect the liquefaction behavior of lignites in terms of reactivity, reductant requirement, product yield and quality, solids buildup in reactors, and catalyst life. The effects of the unique properties of lignite are not sufficiently well defined at present to permit direct application of the leading liquefaction processes developed for bituminous coals. The objective of liquefaction research on lignite performed by DOE through the Grand Forks Energy Technology Center (GFETC) is, therefore, to develop the chemical and engineering data base required to liquefy lignites and subbituminous coals using techniques common to the SRC-11, H-Coal, and Donor Solvent processes. Liquefaction research at GFETC was initiated in 1975 to support the development of the CO-Steam process. This process was named for the reaction of carbon monoxide and steam with coal to provide in situ hydrogen; CO also reacts with coal oxygen to form COz. No catalyst is added. The process was principally developed at the Pittsburgh Energy Technology Center of DOE using coldcharge autoclaves and a continuous processing unit. It was found that conversion was particularly high for lignite (Appell et al., 1972; I3el Bel et al., 1975). The goal of the process was to produce a high yield of environmentally acceptable liquid boiler fuel in a one-step reaction process using filtration or centrifugation for solids separation. The principal problems existing when GFETC initiated work 'Presented at the Tenth Biennial Lignite Symposium, Grand Forks, N.D., May 1979, cosponsored by the Grand Forks Energy Technology Center (DOE) and the University of North Dakota.
were an unacceptably high product viscosity, high pressure (4000+ psi), and an excessively long reaction time of 1 to 2 h in continuous experiments. The generic problem of solids separation was being studied on a larger scale in the SRC processes; therefore, research on centrifugation and filtration was not an identified goal in the CO-Steam project. The liquefaction research facilities established a t GFETC since 1975 have included a unique hot-charge and time-sampled batch autoclave system for studying reaction kinetics, a 5-lb coal/ h continuous process unit (CPU) for studying lined-out operation in various reactor flow configurations, and an array of analytical instrumentation for determining elemental and molecular compositions. The batch system and instrumentation (Sondreal et al., 1977) have been described in previous publications. This report presents the design of the continuous unit and preliminary results from an initial run matrix for once-through reaction of a North Dakota lignite with synthesis gas (5050 H,:CO) in anthracene oil/tetralin solvent. The emphasis in work on CO-Steam has been to reduce product viscosity while maintaining high liquid yield and reducing reaction time. Batch kinetic experiments have demonstrated that higher temperature and shorter residence time together are effective in reducing the molecular weight of the heavy ends without an unacceptable production of methane and other light hydrocarbons; however, the required temperatures of 460 to 500 "C can be achieved without repolymerization and coking only if the solvent introduced with the coal has suitable hydrogen donor properties (Sondreal et al., 1977). The time-sampled batch studies have also demonstrated that CO reacts far more rapidly than H2 in the CO-Steam system. Recently, emphasis has been shifted away from viscosity control toward greater interest in distillate products as a means of avoiding the problem of ash-solids separation. Research will, nevertheless, be continued on improving the breakdown of high molecular weight lignite structures by raising temperatures and recycling. The optimization of distillate yield will represent a balance between depolymerization of heavy ends and coincident formation of gaseous products. Determination of the combination of time and temperature which produce this optimum for specific
This article not subject to U.S. Copyright. Published 1979 by the American Chemical Society
298 INPUT
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979 ALTERNATE PREHEATERS
ALTERNATE REACTORS
ws-Lioum KPARATORS
OUTPUT
Figure 1. qchnmatic diagram of the GFETr 5%malih continuous proces
coals will, henceforth, be an important goal of this research. Because of the emphasis on recycling and on raising temperature-which requires a good hydrogen donor solvent-the work at GFETC will offer direct support for application of the SRC-I1 and Exxon Donor Solvent processes to lignite. This support role has recently been expanded to include work in the Chemical Engineering Department at the University of North Dakota. Lignite liquefaction work at UND began in 1965 with cold-charge autoclave studies. It later expanded into the recently completed program of Project Lignite, under which a 50 lb/h Solvent Refined Lignite Process Development Unit was designated, constructed, and operated for four years (Severson et al., 1977). In batch autoclave systems, work has been conducted by UND researchers to establish rates and product distribution for a number of liquefaction solvents. Two solvents will be chosen for subsequent tests aimed at determining the catalytic effects of diverse mineral matter in eight low-rank coals and will form a basis for comparison of previous and future liquefaction solvents. Further tests of the catalytic effects of mineral matter will be made by recycling inorganic reactor residue and by adding selected mineral constituents to the batch charge. Description of the Continuous Process Unit (CPU) The 5-lb coal/h continuous process unit shown in the schematic in Figure 1 has been designed with great flexibility for changing reactor modules and recycle flow patterns to control residence time distribution and turbulence. For data presented in this paper, the unit has been operated in a single configuration involving a sequence of a tubular preheater, a gas/slurry mixing tee, and a continuous stirred tank reactor (CSTR). A slurry of -60 mesh lignite containing about 30% moisture is preblended with solvent or recycle product slurry and pumped to a slurry hold tank, which is stirred by recirculation and suspended from a load cell for accurate feed rate determinations. A high-pressure piston pump delivering up to 25 lb/h of slurry at pressures up to 6000 psi is used to charge the slurry into the system. A single-stage diaphragm compressor is used to compress the reaction gas to 6300 psi, after which it is regulated and metered into the system. Slurry and gas are heated independently and mixed at the entrance of the reactor. The reactor currently installed is a 1-gal magnetically stirred autoclave with two marine-type propellor stirrers circulating downward. The autoclave has a total volume
of 231 in.3, but a dip tube limits slurry volume to about 150 in3. At a normal flow rate of 10 lb slurry/h the mean residence time of the liquid is approximately 30 min. The reactor, constructed of type 316 stainless steel, is rated at 5000 psig at 500 OC. Normal operating pressure is 4000 psi. Slurry can be sampled directly from the reactor through a two-valve system which removes 3 mL per cycle. Reacted slurry and gas leaving the reactor enter a highpressure separation vessel maintained at about 300 "C. The liquid level in this pressure vessel is controlled by a nuclear level detector which actuates the sequential operation of two valves. These pressure letdown valves release 25 mL of slurry every 25 s; the sampling system works very reliably because the small volume removed during each cycle of valve operation results in only a slight fluctuation in pressure and gas flow. The product recovery vessel currently employed acts as a low pressure flash tower, followed by a water-cooled light oil and water condenser. Noncondensable gases are vented thr-ugb n ~ i t : dinlacement ~meter. L ight oils/water are reinovta from t h e .;ash tower through a manual valve. A gas sampling port allows on-line analyses of the dissolved gas. Gases and vapors from the primary separator pass to a condensate accumulator where the light oils and water are condensed by a water-cooled coil. The liquid level in this pressure vessel is monitored by a second nuclear level detector. Light oils are removed from the system through a series of two valves which are operated manually. Tail gas at about 23 "C is let down through a backpressure regulator and metered, and a fraction is analyzed by use of an on-line gas chromatograph. A flare is being installed to burn the off gas. Safety Considerations. All high-pressure equipment is installed in a barricade constructed of 1/2-in.steel boiler plate. The barricade is equipped with blowout windows and an independent ventilation system. The gas supply bottles, compressor, and high pressure accumulators are in a second barricade separate from the main building. The operator and gas supply areas are monitored for CO and explosive gases. During tests, all operating personnel are required to wear CO detector badges. No person is permitted to enter the high-pressure reactor portion of the barricade if a test is in progress or if the pressure is above 3000 psi. No fitting is ever tightened when there is pressure in the system. During leak detection, all electrical power to the heaters is cut off. Efforts are made to minimize exposure of operating personnel to coal liquids and vapors through the use of high rates of ventilation, protective clothing, solvent-resistant gloves, and respirators.
Test Procedures and Measurement Tests performed in runs 1-20, reported in an earlier paper (Sondreal et al., 1978), were made employing a "typical" North Dakota lignite, having about 10% ash on an MF basis, from the Beulah mine in Mercer County (Beulah 2). Subsequent runs were made with a fresh sample of Beulah lignite when it was determined that the first sample had become badly oxidized due to long storage in %-gal drums after pulverizing and that its reactivity had suffered. The new sample (Beulah 3) was atypical, chosen for its high ash content, to be used in the mineral matter catalysis studies. Analyses of Beulah 2 and 3 lignite and their ash are presented in Tables I and 11, respectively. Solvents employed in the once-through experiments were chilled anthracene oil (AO) or an anthracene oil distillate (AOD) fraction (ibp 296 "C at 10 torr) with 0 to 7% added tetralin. Reductant gas was a nominal 50:50 vol % of hydrogen and carbon monoxide (synthesis gas). The
Ind. Eng. Chern. Prod. Res. Dev., Vol. 18, No. 4, 1979
Table I. Analyses of Coal and Coal Ash for t h e Beulah 2, North Dakota, Lignite Used in Liquefaction Experiments
Table 11. Analyses of Coal and Coal Ash for Beulah 3, North Dakota, Lignite Used in Liquefaction Experiments coal analysis (GF-79-2147)
coal analysis (GF-77-712) basis of reported analysis
asreceived
moisturefree
moistureand ash-free
Proximate Analysis, % moisture 27.80 volatile matter 30.45 42.18 fixed carbon 34.45 47.71 10.11 ash . 7.30 total 100.00 100.00
__
hydrogen carbon nitrogen oxygen sulfur ash total
Ultimate Analysis, % 6.09 4.16 46.30 64.14 0.72 1.oo 39.03 19.82 0.56 0.77 10.11 .~7.30 100.00 100.00
__ 46.93 53.07 100.00
4.63 71.35 1.11 22.05 0.86 _100.00
coal ash analysis (GF-75-687)
asdetermined
so
299
3-
free
basis of reported analvsis
asreceived
moisturefree
moistureand ash-free
Proximate Analysis, % moisture 29.48 __ volatile matter 30.21 42.84 fixed carbon 29.58 41.Y4 ash 10.73 15.22 total 100.00 100.00 hydrogen carbon nitrogen oxygen sulfur ash total
Ultimate Analysis, % 6.20 4.15 42.87 60.79 0.48 0.68 16.59 37.9 1 1.81 2.57 1 0.73 15.22 100.00 100.00
-_ 50.53 49.47
__
100.00 4.89 71.71 0.80 19.57 3.03
100.00
coal ash analysis (GF-78-4556) asdetermined
so,free
~~
Oxides in Ash Analysis, % 22.8 silica, SiO, 17.3 14.1 aluminum oxide, AI,O, 10.7 14.3 ferric oxide, Fe,O, 10.9 0.5 titanium oxide, TiO, 0.4 0.9 phosphorus pentoxide, P,O, 0.7 30.8 calcium oxide, CaO 23.4 8.2 magnesium oxide, MgO 6.2 8 .O sodium oxide, Na,O 6.1 0.4 0.3 Dotassium oxide. K .O 24.1 __ sulfur trioxide, SO3_ _ total 100.1 100.0
initial solvent for the first recycle run was AOD, followed by recycling enough of the product slurry, including ash and organic residue, from the first stage, high temperature-pressure separator to make a slurry containing 30% fresh coal. Start-up solvent for the second recycle run was end-of-run material from the previous recycle run, which was nearly solids-free due to coking of the heavy organics with ash inclusion. Test duration was from 12 to 90 h with changes in operating conditions every 4 to 8 h if desired. On-line analyses of the tail gas indicated that relatively steady-state conditions were obtained during the 2-4 h yield periods at the end of each run condition. Yield periods for selected operating conditions were repeated both sequentially and in separate runs, to establish the repeatability of data. Coal, vehicle solvent, and hydrogen donor additive were individually weighed and blended before being fed to the slurry hold tank. Slurry flow was measured by the calibrated displacement of the high-pressure slurry charge pump and by weight changes in the slurry holding tank as measured by a load cell. An “over-the-run average” slurry feed rate was also determined by weightback of slurry. The feed gas of nominally 50:50 H2 and CO is purchased premixed in gas cylinders; it is further compressed, and its flow to the CPU is monitored through both a rotameter and an orifice flow transmitter. The liquids from the heavy-product and light-oil receivers were collected and weighed over part of a 2-4 h yield period. The off-gas rate was determined with a positive displacement flow meter. In current experiments, the dissolved gas released from the slurry upon depressurizing, including some light oil and water, is vented to a low pressure (- 5 psig) flash tower followed by a water-cooled
Oxides in Ash Analysis, % silica, SiO, 27.6 aluminum oxide, Al,O, 14.0 ferric oxide, Fe,O, 14.2 0.6 titanium oxide, TiO, phosphorus pentoxide, P,O, 0.2 calcium oxide, CaO 14.5 magnesium oxide, MgO 3.9 sodium oxide, Na,O 5.5 0.4 potassium oxide, K,O sulfur trioxide, SO, lY.l total 100.0
34.2 17.3 17.6 0.7 0.2 17.9 4.8 6.8 0.5
condenser for recovery of light liquids. Dissolved gas volume is measured by a dry test meter. In earlier runs (no. 1 to 18) this product stream, which could contain up to 20% of the tail gas and 80% of the process water, was vented to the barricade without measurement. While this would not affect the general conclusions based on the percentage of material insoluble in tetrahydrofuran (THF insoluble), overall conversion, or heavy liquids, the reported (Sondreal et al., 1978) net gas, distillate, and water yields are therefore low. Variables Studied. The first series of runs, 1-18, employing Beulah 2 lignite was organized to provide a systematic investigation of the effects of temperature, slurry coal concentration, and slurry hydrogen donor (tetralin) concentration on liquefaction yields and product quality. Additional runs, 26-1 to 12, with a constant coal concentration of 40%, were made at three temperatures, two H-donor concentrations, and in two vehicle solvents with fresh Beulah 3 lignite, to quantify previously reported (Sondreal et al., 1978) results with improved material balances. Conditions for runs reported are shown in Table 111. All runs were a t an operating pressure of approximately 4000 psig, with slurry and gas flow rates held nominally at 10 lb/h and 0.5 scfm, respectively. Process Stream Sampling and Analyses. The following process streams were sampled at least once during each yield period: feed slurry, feed gas, dissolved gas, product gas, reactor slurry, product slurry, high-pressure light oil, and low-pressure light oil. A detailed schedule for analyses has been published previously (Sondreal et al., 1978). Highlights of the scheme included the following: determination of distillable organics (250 “C at 1 torr), nondistillable but T H F soluble material, organic T H F
300
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
Table 111. Matrix of Experimental Conditions for the First Two Series of Runs o n t h e Continuous Process Unit (Pressure -4000 Dsie: Slurrv Flow Rate. 1 0 l b h : Gas Flow Rate. -0.5 scfm)a as received coal in slurry, wt %
400
temp, "C 460
440
0
4.67
9.33
0
3.5
7.0
30% 40%
15-4
15-3
15-1,2 26-2* 26-4
44%
480
500
tetralin in slurry, wt %
12-2
12-3
0
0
3.5
7.0
18-8
18-2 12-1
10-la 18-1
26-1* 26-3
16-4 26-6* 26-8
18-5
12-4
11-3
11-2
3.5
18-9 26-10* 26-12
10-2 18-2 22-1* 26-5* 26-7 12-5
11-1
0
7.0
14-1 18-8 18-7
48%
3.5
7.0
10-1 20-3,4 13-6
16-1 4-3 19-1,4 x2-2* 20-1,2 26-9* 26-1 1 10-3 14-2 10-4
13-3
Run numbers (e.g. 18-3) represent the test number (18)and yield period ( 3 ) . Asterisks indicate that vehicle solvent is anthracene oil distillate (AOD) (ibp 296 " C a t 1 0 torr).
insolubles, ash, moisture, and ultimate analyses for the liquid streams; determination of hydrogen, nitrogen, carbon oxides, CI-CI hydrocarbons, hydrogen sulfide, and ammonia in the gas streams. The major product fractions from the liquefaction of lignite and of interest in these studies were: (1)product gases, (2) distillable organic products, (3) nondistillable but T H F soluble product, and (4) insoluble residue including mineral matter. Effects of Operating Parameters on Liquefaction Yields In the 5 lb coal/h CPU, synthesis gas was reacted with lignite plus inherent moisture in A 0 or AOD, a t elevated temperatures in a once-through reaction employing a CSTR. Product yield fractions, represented by (1)the T H F insoluble residue including ash, (2) soluble but nondistillable liquid, (3) distillate, and (4) gases, are correlated with the percent of coal in the feed slurry, H-donor (tetralin) concentration, solvent, and temperature. Two extended runs were made in the CPU operating as a CSTR employing batchwise recycle of the heavy product slurry, including unreacted organic residue and ash, in an attempt to obtain lined-out product for detailed characterization. Further objectives were to test CPU operability under conditions of extensive slurry recycle with fresh coal addition and to ascertain the function of reaction yields with the degree of line-out or number of recycle passes. Selected solvents were tested a t several temperatures and residence times in the UND batch autoclaves to select two solvents for further work in determining the catalytic effect of diverse mineral matter in eight different low-rank coals on liquefaction and to form a basis for comparison of previous and future liquefaction results utilizing different solvents. Solvent losses are correlated with temperture and residence times in an effort to lend credence to liquid yields obtained from different lignites with vastly different mineral matter constituents. Effects of Coal Concentration. Two sets of runs were made to determine the effect of coal concentration in the feed slurry in the range of 30 to 48%, the first at 480 "C and the second at 500 "C. In both series of runs the other variables were held constant: slurry was fed at 10 lb/h; synthesis gas of 50:50 H2 and CO was fed at 0.5 scfm; the feed tetralin concentration was 7 wt %. Increasing the coal in the feed slurry had essentially no effect on the THF insoluble residue at either temperature (Figure 2). Some reduction in conversion would be ex-
480
"7
0
0
38
42
0
POI
I 30
34
RAW QXL IN FEED SLURRY, w l pct
Figure 2. Net THF insoluble yield as a function of coal concentration when coal slurry contains 7% tetralin (*13-1and 20-3,4 were averaged).
pected as the feed ratio of coal to solvent and reducing gas is increased, simply on the basis of stoichiometry and mass action. However, relatively large net excesses of these reactants probably account for the lack of changes observed in the yields. The increase in coal loading did not result in significant changes in either the soluble and distillable liquids or the gas. Effects of Hydrogen Donor Additive Concentration and Solvent Fractions. Tests to determine the effects of H-donor (tetralin) addition and solvent fractions were run at three temperatures-440, 460, and 480 OC. Coal concentration was held constant at 40 wt 70 while the tetralin concentration of the feed slurry was either zero or 7 wt % while employing A 0 or AOD as the vehicle solvent . The addition of tetralin in these continuous experiments had no significant effect on the overall conversion of lignite at the residence time of 30 min employed in the CSTR, except in A 0 at lower temperatures (Figure 3). In earlier batch autoclave work, tetralin addition increased conversion and decreased the THF-insoluble residue, but only after more extended periods of reaction time of 1 to 2 h (Sondreal et al., 1977). At short residence times, the batch data indicated that the THF-soluble yield with tetralin was not distinguishable from runs made without tetralin. On the other hand, the net liquid (THF soluble) yield obtained from CPU experiments varies dramatically (Figure 4) with tetralin addition to AO, especially at elevated temperatures, but it shows only marginal improvement when added to AOD. The net liquid yield decrease of nearly 90% observed when tetralin
-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18,No. 4, 1979 I
. 7% TETRALIN - 53% A 0 0 0 - 60% AOD - 7%T E T R L I N - 53% A 0 0
301
I
v
A . W% A 0
r
IO1
440
460 TEMPERATURE, 'C
480
Figure 3. Net unreacted coal vs. reaction temperature for 40% coal slurries.
TEMPERATURE, "C
Figure 5. Net soluble vacuum bottom yields vs. reaction temperature for 40% coal slurries.
' "1I
D- - - - - -
'-
---
*\ \
\
\
,
-
i! 0
7 % T E T R L I N - 53% A 0 0
E T : F L l h
- 53%
'
'\
A 0
'\\,,,,,
~
440
460
- 7%
3
60% b 0 0 -7% TETRALIN - 53% b 0 -60% b 0
V A
\
0
0
480
440
TTRbLlN
- 53%
1
AOC
1 480
460
TEMPERATURE, 'C
TEMPERATURE, "C
Figure 4. Net liquid yields vs. reaction temperature for 40% coal slurries.
is not added to A 0 a t 480 "C is consistent with the perceived function of a H-donor (that is, to prevent repolymerization and coking). The relative insensitivity to tetralin addition in the case of AOD probably indicates that the major portion of H-donors present in A 0 are in the lighter fraction and are concentrated by distillation. With both pasting solvents, tetralin addition significantly increases the yield of T H F soluble residue (Figure 5). In addition there is a decrease in the reducing gas consumption (Figure 61, especially at lower temperatures with tetralin addition. Effects of Temperature. Runs were made at 440,460, and 480 " C , with the slurry feed coal concentration held constant at 40 w t % , employing A 0 or AOD as the vehicle solvents with and without tetralin. The feed gas composition was also constant at nominally 0.5 scfm of 50:50 H2 and CO (vol 9%). Increasing the temperature from 440 to 480 " C resulted in a decrease in the T H F insoluble solid residue from 36 to 20 % (Figure 3) in A 0 solvent alone. Only slight changes of around 5% of the nominal 20 wt % of MAF coal T H F insolubles were observed in AOD or in the presence of tetralin. The yield of THF-soluble but nondistillable product varied about 10% and appeared to be somewhat independent of temperature within the tolerance of the data (Figure 5). The molecular weight distribution of this fraction shifts downward with increasing temperature, indicating that the products produced at 480 "C contain lower molecular weight, species than those produced at the
I
60.
8
P
1
I_
E'
.\"
f j20-
, ,
/ - - - -
t J -
L6
0.
+ Y
\ - 7% TETRALIN - 53% A 0 D 0 - 60% A 0 D V . 7% TETRALIN - 53% A 0 A - 60% A 0 0
440
460 TEMPERATURE, 'C
\ \
h 480
Figure 7. Net distillate yields vs. reaction temperature for 40% coal slurries.
lower temperatures of 440 and 460 "C. The general trend in the data indicates that the distillate product yield increases with temperature from 440 to 460 " C with the highest being a 30% increase to over 60% employing the heavier A 0 solvent with tetralin followed by a 10% increase with A 0 alone and 5% in the AOD (Figure 7). The marked decrease in distillate yield (with an actual distillable solvent loss of over 20% employing A 0 alone at 480 "C) illustrates rather dramatically the necessity of having some H-donor present in the solvent at the higher temperature as there is only a slight change in yield when the solvent contains tetralin or has the H-
302
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
0 - 7 % TETRALIN - 53% A 0 D 0 - 60% A O D V - 7 % TETRALIN - 53% A 0 - 60% A 0
,
a
CFU RUNS PLOTTED 0-IA, 2 11-1,2,3
12-3.4 13-1,2,3 14-1,2.3
IS-I,2,3,4
0-REACTOR SAMPLE 0-PRODUCT STREAM
I
-
440
IP I
~~
460
480
TEMPERATURE, O C
Figure 8. Net hydrocarbon gas yield vs. reaction temperature for 40% coal slurries.
donor more concentrated as by distillation (AOD). This prevents loss of distillate due to cracking, gasification, repolymerization, and coking. Yields of hydrocarbon gases (C, to C,) increase nearly linearly with temperature (Figure 8). The hydrocarbon gas make is acceptable up to 460 “C, in the lighter solvents or with tetralin, at less than 13%, but nearly doubles with a 20 “C increase in temperature. Further arguments for cocurrent repolymerization and cracking at 480 “C in A 0 solvent alone are evidenced by the higher hydrocarbon gas production. Reducing-gas consumption increases rapidly with temperature (Figure 6). Addition of tetralin to either of the two-vehicle solvents tested generally reduced the amounts of gaseous reductant required with the highest decrease, roughly 30%, at 440 “C. The effects of tetralin on reducing gas consumption appear to be minimal a t temperatures near or above its critical temperature. Effects of Reactor Slurry Composition. The chemical and physical characteristics of the slurry phase in the reactor are not the same as those of the product. At elevated temperatures in the reactor, a substantial fraction of the product liquid is in the gas phase. This has important ramifications on the design and operation of reactors, particularly as the reactor temperature is increased. Figure 9 presents data on the amount of nondistillable residue, both in reactor samples and in product samples, at temperatures between 400 and 500 “C. These data indicate a sharp increase in the nondistillable fraction in the reactor slurry at temperatures above 464 “C. The amount of nondistillable material in the product samples remained essentially constant over the entire temperature range. This difference increased as coal loading in the feed slurry was raised from 30 to 44 %. Figure 10 depicts the increase in three different heavy fractions in the reactor slurry as a function of coal loading in the feed for operation at 500 “C. Similar linear trends are shown for: (1) the T H F insoluble, (2) the hexane insoluble, and (3) the microdistillation residue. The slope of the line for distillation residue (1.76 vs. 1.19 for the THF-insoluble fraction) indicates that the collisional interaction of high concentrations of coal-derived “particles” preferentially increases the yield of the “soluble but nondistillable” fraction. The significance of the segregation occurring in the reactor is twofold: (1) high concentrations of solids and heavy liquids may plug reactors at high temperature and high coal loading, and (2) the ratio of hydrogen donor materials to coal “particles” is reduced, resulting in lesser conversion and greater repolymerization. In support of the latter, it was observed that the H/C ratio in the reactor
3
420
440
460
480
500
TEMPERATURE, .C
Figure 9. Effects of temperature on the microdistillation residue of reactor and product slurry samples.
1
-1 E
SHEW INS 3213 4259 4 4 W 126 5 5 4 0987 X THF INSOC 2570 3434 3147 I19 0404 09997
0
10
MF COAL, *1 pct
W
30
of tee4 rlurry
Figure 10. Weight percent of THF insoluble, hexane insoluble, and microdistillation residue of reactor slurry samples vs. wt % mf coal in the feed slurry.
liquid was reduced from 0.764 to 0.709 as feed coal loading was increased from 30% to 44%. Effects of Recycle. The objectives of runs 27 and 28 were (1) to test CPU operability under conditions of extensive slurry recycle with fresh coal addition, (2) to produce a quantity of “lined-out’’ coal-derived liquids for analytical characterization, and (3) to ascertain the function of reaction yields with the degree of line-out or number of slurry passes. Nominal reactor temperature and system pressure were 460 “C and 4000 psig, respectively. The slurry feed rate was 5 lb/h and the feed gas rate ‘ I zscfm. A mixture of equal parts hydrogen and carbon monoxide made up the feed gas. The initial feed coal slurry consisted of 30% pulverized and moist Beulah 3 lignite and 70% anthracene distillate. The product slurry was recycled every 4 h as a 70/30 mix with fresh coal. Startup solvent for run 28 consisted of end of run product slurry from run 27, which
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979 --I
- - - - _HIS. -NH,,- CO,-, 4ND - -OTHERS - -!BY- DIFFERENCEi---~
I
901
303
- I
C,-C+ HYDROCARBON GASES
g4CUUM DISTILLATE
I
i
I I B P - 2 W C AT 1 TORR)
IO
1 0
7
,
,
,
,
,
,
2
3
4
5
6
,
,
,
7 8 9 10 NUMWR OF PASSES
I\
.
,
,
.
, - J
12
I3
I4
15
'6
Figure 11. Run 27 water and gas free product distribution vs. number of slurry recycle passes for a 30% cod-70% recycle product feed.
,
0
2
3
5
4
6
8
7
IO
9
II
a
3
I2
15
6
SLURRY R E M L E PASS
Figure 13. Run 27 maf product distribution expressed as % maf coal feed vs. number of slurry recycle passes for a 30% coal-70% recycle product feed.
50
NON-DISTILLABLE,
0
I
2
3
4
5 6 7 8 9 1011 NUMBER OF SLURRY PASSES
1 2 1 3
14
0
1
2
3
4
5
6
7
8
9
a
O
2
3
,
4
5
6
NUMBER OF PASSES
Figure 12. Composition of water/gas free, nonrecycled reaction products VB. number of slurry passes.
Figure 14. Run 28 product distribution vs. number of slurry recycle passes for a 30%-70% recycle product feed.
was virtually solids-free due to coking of the heavy ends with ash inclusion in the previous run. The slurry mix and holding tanks were held at 60 OC which was sufficient to assure reasonable fluid viscosities. The reactor was a l-gal stirred autoclave. Mass balances were made over each 4-h cycle so that yield data as a function of recycle could be obtained. Material balance recoveries for the first 15 data periods during run 27 averaged 93.3% with a low of 87.5 and a high of 102.2%. In general, the majority of the mass balance error can be attributed to water. The water balance poses more than normal difficulty as an undetermined amount was lost by evaporation from the heated slurry preparation and holding tanks. Considering the overall product distribution on a waterand gas-free basis, two approaches are possible. The first would involve all of the product slurry, recycled or not, and would account for concentration changes, while the second would weight only the unrecycled portion of the product slurry and would not account for concentration changes in the recycled material directly. Figure 11 depicts the weight percent of the gas/water free portion of the total product vs. number of passes for run 27. The rather sharp increase in nondistillable but THF-soluble product after pass 6 probably signals the onset of repolymerization and coking while the sudden decrease a t pass 14 is the result of filtering in the reactor caused by coke-like solids. The coking was due to apparent loss of the gas head. From Figure 11, a steady increase in the concentration of inorganic and T H F insolubles is apparent and line-out was not achieved even though the IR ratio of the light oils approaches line-out. The gaslwater free portion of the unrecycled product vs. number of passes for run 27 is graphed in Figure 12.
It is similar in appearance to Figure 11 but differs in the magnitude of the concentration values and the shape of the nondistillable curve. In this case, the product slurry comprises a smaller portion of the total product and concentration changes in the recycled material are neglected. However, a similar interpretation is obtained. Figure 13 depicts the moisture/ash free product distribution of the total product expressed as pct MAF coal feed in run 27 vs. number of slurry recycle passes. This figure gives further evidence of selective repolymerization, coking, and cocurrent gasification as the heavy (nondistillable but THF-soluble) liquids increase abruptly after six passes, proceed to a maximum at pass 10, and then decrease such that at the end of the run only 20% of the vacuum bottoms and almost none of the organic solids remain. Accompanying these changes is an ever-increasing yield of hydrocarbon gases expected during coking which is also evidence that line-out was not reached. Utilization of reducing gases during run 27 fluctuated somewhat from pass to pass but in general averaged about 5.5 wt % of the MAF coal when expressed on a hydrogen equivalent basis. This value is over double that in any of the once through reactions at 460 "C with a slurry feed rate of 10 Ib/h. The yield distribution, on a water- and gas-free basis, for run 28 liquid products as a function of recycle is shown in Figure 14. The curves look similar to those of run 27 with the exception of the line dividing distillable and nondistillable products. In run 28 a gradual increase and leveling is noted with an increase in the number of recycle passes, whereas in run 27 a sharp increase in nondistillables occurs after pass 6 and probably indicates the start of coking due to the loss of the gas head in the autoclave reactor (Figure 11). The apparent leveling-off of these
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05
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v.
/.
420
440
460
480
500
REACTOR TEMPERATURE, *C ,
'9% l6M)
, M4
4w
'
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Figure 16. MWD of the THF-soluble,nonvolatile fraction of reactor slurry samples obtained at 404 and 500 "C.
yields, including ash, is a good indicator that line-out had been achieved. The moisture-ash free product distribution of the total product expressed as pct MAF coal feed vs. number of recycle passes is shown in Figure 15. There was no marked increase in either the T H F soluble or insoluble organics and in fact the yield of these products leveled off after the tenth pass which is a further indication of line-out. It is not surprising that the yield of hydrocarbon gases and high-pressure light oil (HPLO) approached line-out almost immediately since none of the HPLO was recycled and since the bulk of the startup solvent was already process derived. After line-out the approximate MAF coal yields were: organic THF insolubles, 1170;THF soluble vacuum bottoms, 970, vacuum distillate, 36%, high-pressure light oils, 19%, hydrocarbon gases, 19%. While the hydrocarbon gas make is high at about 19%, the total distillable product yield is encouraging at over 50% of the MAF coal feed. Molecular Weight Effects. The determination of molecular weight distribution (MWD) has been a principal analytical method in the GFETC liquefaction project. The procedure used-calibrated gel permeation chromatography-has been published and will not be discussed here (Knudson et al., 1975). The results that are graphed in Figure 16 are the UV absorbance vs. the molecular weight (the latter being equated with an elution time of known molecular weights). Since absorbance is proportional to molar concentration, and since the molar absorptivities were found to be similar, the graph is an estimate of the distribution of mole fraction according to molecular weight. (Note that the MW scale is logarithmic.) Material on which MW determinations are made is the "coal-derived" liquid fraction-that is, THF-soluble but nondistillable (TS/ND)-which under an arbitrary mul-
Figure 17. Ratio of HPLC ultraviolet absorbance at 254 mm of 950 MW to 250 MW material vs. reactor temperature for reaction times under 30 min.
tiple-fragmentation model for depolymerization would be intermediate between coal and the desired distillable product. In previous batch experiments, the amount of this fraction has been found to remain quite constant under almost all reaction conditions (Sondreal et al., 1977), indicating that this fraction of the product is formed rapidly and is stable under the observed conditions. Weak bonds do exist that rupture upon increased thermal agitation, yielding two lower-molecular-weight but still nondistillable fragments. This model is used in interpreting results even though it is unproven. Temperature has the greatest effect on the MWD, with the major peak shifting from 1500 MW to 250 MW as temperature is increased from 404 to 500 "C in the CSTR reactor (Figure 16, reactor samples). The method used to express the molecular weight data is to consider the ratio of UV absorbances at high vs. low molecular weight. The ratio for 950 MW vs. 280 MW (Agm/Am)is correlated with reactor temperature in Figure 17; different curves are shown for batch and CSTR data. It can be observed here that the batch unit is more effective in reducing the ratio at temperatures above 440 "C but that the CSTR is more effective at lower temperatures. The MWD is also shifted downward by increasing the tetralin content in the feed; this effect is more pronounced at 404 "C than at 460 or 500 "C (Figure 17). The level of coal loading in the feed slurry thus far shows little effect on the MWD. Effect of Residence Time Distribution. The interpretation of differences in MWD's requires that we note the residence time distributions (RTD's) for the CSTR and the batch reactor. Under this elementary concept, we recall that the RTD of the batch reactor is a 6 function at the time representing the duration of the batch experiment, i.e., all of the material stays in the reactor for the same length of time. The RTD for the CSTR is represented by an exponential decay curve; Le., for each unit
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
305
Table IV. Measured Mass Balance Run No. 26-3 (Reactor Temperature, 440 ‘C; System Pressure, 3878 psig) rates, g/h wt stream hydrogen nitrogen carbon monoxide methane carbon dioxide ethylene ethane propylene propane butanes hydrogen sulfide ammonia -gas t o taldist . organics non-dist. THF sols organic THF insols ash water total
feed slry
feed gas
dis gas
33.4
33.4
3.0
456.1 0.6
456.7 0.6
42.1 4.9 85.5 0.0 2.9 0.5 2.9 2.4 2.6
490.0 2446.8 278.4 992.0 233.7 593.5 4544.5
total in
490.0
490.9 2446.8 278.4 992.0 233.7 593.5 5035.4
LPLO
prod slry
HPLO
147.3
change
coal
28.4 3.4 218.2 30.0 365.1 0.4 11.0 1.4
31.5 3.4 320.4 34.9 450.6 0.5 14.0 2.0 10.7 6.4 8.7
-1.9 3.4 -136.3 34.3 450.6 0.5 14.0 2 .o 10.7 6.4 8.7
-0.19 0.34 -13.14 3.45 45.42 0.05 1.41 0.20 1.07 0.64 0.88
883.4 2159.1 557.7 205.7 248.2 372.2 5026.5
392.5 312.3 219.3 -786.2 14.5 -221.3 -8.9
39.56 31.48 28.15 -79.26 1.46 -22.30 -0.89
3.9 6.0 736.1 8.8
19.6 28.5
2640.2 557.1 205.7 248.2 109.0 3761.1
109.9
243.5 353.5
of
total out
I .8
147.3
7.5
prod gas
736.1
MAF
%
recovered carbon hydrogen nitrogen sulfur oxygen ( b y diff) ash total
3163.0 328.5 13.4 47.4 158.2 233.7 4544.5
196.2 33.6 261.0 490.0
3359.2 362.1 13.4 47.4 1019.2 233.7 5035.4
52.3 6.0 2.5 86.2
7.8 3.0 0.0 0.0 17.4
147.2
28.5
of entering feed, the increments leaving during equal time segments thereafter are reduced exponentially with time so the largest increment leaves in the first time segment but some small amount remains indefinitely. We should also make note of the fact that depolymerization does progress with time even though it is principally temperature dependent (Knudson et al., 1975). The effect of the RTD on MWD is illustrated in Figure 17 showing UV absorbance ratios vs. temperature. Note that progressive depolymerization causes this ratio to be lowered. At low temperatures where time dependence is greater, the internal backmix of the CSTR causes some of the high-MW material to be internally recycled over a sufficient time to generate the low-MW fractions required to reduce the ratio. At high temperatures where the depolymerization reaction is very rapid and conversion (MWD change) is nearing completion, the predominant effect in the CSTR is the nearly immediate departure of a fraction of the feed material in an uncoverted state; in this case the CSTR results in a lesser reduction in the ratio. A further complication of this argument is introduced by the action of tetralin (or other hydrogen donor materials), which releases hydrogen only in the early stage of a batch reaction but is continuously replenished in a CSTR so that some old reactant is in contact with fresh hydrogen donor. In conclusion, a CSTR reactor should be less effective than a batch or plug-flow reactor, with adequate radial gas-slurry contacting, in achieving a once-through high conversion from high to low molecular weight. This has been generally assumed in liquefaction, and the current results lend support to this assumption. However, the ideal reactor system may be far more complex; for example, we can envision a tubular reactor with a staged side-input of a hydrogen donor and with separation and recycle of the fraction of the coal material that has not depolymerized. We believe that optimization of reactor systems for coal liquefaction has a long way to go.
3093.1 259.8 13.1 51.1 95.5 248.2 3761.1
97.4 38.4 0.0 0.5 211.0
261.4 40.9 3.4 5.6 424.5
353.5
136.0
3512.3 348.4 16.6 59.9 840.8 248.2 5026.4
153.0 -13.7 3.1 12.5 -178.4 14.5 -8.9
104.55 96.21 123.20 126.38 82.49 106.21 99.82
Material Balances Material balances, including total carbon and hydrogen, have shown considerable improvement with the addition of a low-pressure flash system, installed after run 18, for light oil/water recovery as well as the measurement and analysis of the dissolved gas stream. Table IV shows analyses and mass recoveries based on all process streams during run 26-3. Following CPU modifications, mass balance closures were typically within *7% when a slurry containing water was charged, as illustrated by Table V. Generally the hydrogen and oxygen (by neutron activation or difference) recoveries were low, pointing to incomplete water recovery such as through aerosol losses and/or water losses in the heated (60 “C) slurry tank. Further evidence for water losses is the fact that 1 to 2% closures were obtained when a “dry” slurry was fed such as in two catalytic hydrogenation experiments employing moisture-free SRL and anthracene oil. Future CPU Work In the immediate future, the continuous process liquefaction unit at GFETC will be modified by installing a tubular reactor in place of the CSTR. In addition, the system flow pattern will be changed to include recyclepossibly both around the reactor and back to the slurry preparation tank. These modifications will: (1)permit line-out on coal-derived solvent, (2) extend the analysis of reactor design factors, and (3) facilitate continuous experiments on the recycle of mineral residues and heavy organic fractions as an extension of part of the work at the Chemical Engineering Department a t the University of North Dakota. Plans will be developed in the future to study fractionation and selective hydrogenation of recycle streams. Solvent Screening for Batch Autoclave Research As part of the lignite liquefaction program, the University of North Dakota Department of Chemical Engineering
306
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
Table VI.
307
Solvents for Laboratory Autoclave Studies
DCCAO: HDCCAO: HDCCAO2: DPDURS: DFS12O: HPETC:
Distilled Crowley Chilled Anthracene Oil Hydrogenated Distilled Crowley Chilled Anthracene Oil (Hydrogenated in batch autoclave a t UND) Hydrogenated Distilled Crowley Chilled Anthracene Oil (Hydrogenated in CPU at GFETC) Distilled PDU Recycle Solvent Distilled FS-120 (Gulf FS-120 Carbon Black Feedstock-Start-up solvent for PDU) Hydrogenated Pittsburgh Energy Technology Center equilibrium CO-Stream recycle slurry hydrogenated in CPU at GFETC Analyses of Solvents wt % off over the temperature range
DCCAO
0 96.5 2.5 1.0
ibp 1 2 0 "C 120-260 "C 260-280 "C+ 280 "C ash, w t % T H F insol, wt % moisture, wt % ibp, "C 5 vol % off a t temp 10 20 30 40 50 60 70 80 90 95 rnax t e m p "C % off at max temp recovered residue loss
HDCCAO 100.0
HDCCAO2
DPDURS
DFSlPO
5.0 90.4 2.1 2.5
100.0
4.0 94.3 1.7
D-1160 Distillation 81 125 138 152 163 171 180 189 199 211 226 239 2 60 98 98 1.5 0.5
124 157 172 184 193 20 0 206 214 224 2 37 252 264 280 98 98 1 1 SELECTED TIME AT TEMPERATURE
280
240
/
78 140 163 182 189 196 200 206 214 226 252
98 120 130 145 157 167 17 4 17 4 189 201 233 264 27 4 96 96 3 1
HPETC 10.3 5 1 .O 3.0 35.7 4.28 5.67 0.30 39 93 117 155 178 197 217 246
282 69 69 28.5 2.5
i .
a
1
0
,
100 TIME, minutes
A S T Y 0-llB0 DISTILLATIONS AT 5 TORR. 0 DCCAO
1
A
200
HDCCAO2
c, H P E T C
Figure 18. Typical heating and cooling cycles for the 1-gal coldcharged autoclave (time-at-temperature was chosen as 0 or 30 min).
studied several different liquefaction solvents. A 1-gal cold-charged autoclave was used to screen six solvents. The information obtained was used to select two solvents for further study of other process variables. Reaction times in the cold-charged autoclave must be defined in terms of (1)a heating period, (2) the time a t reaction temperature, and (3) the cooling period. A typical cycle of heating and cooling for these batch experiments is shown in Figure 18. Fourteen runs were conducted a t UND. Results are reported in Tables VI, VII, and VI11 and are illustrated, in part, in Figures 18 and 19. Experiments were conducted a t two different times-at-reaction-temperature (0 and 30 min) and two temperature levels (420 and 460 "C) in various solvents.
40
10
40 60 VOLUME X OFF
80
100
Figure 19. Distillation of liquefaction solvents.
A single North Dakota lignite, Beulah 1078, mined by the Knife River Coal Mining Co., and a nominal 5050 mixture of hydrogen/carbon monoxide were used for all the runs. In each run, 200 g of MAF (moisture-ash-free) lignite and 400 g of solvent were charged to the autoclave; then gas was charged to an initial pressure of 1200 psig. The autoclave was heated at an average rate of 2.2 OC/min to the reaction temperature and was held at that temperature for the prescribed period of time. In the cases where the time-at-temperature was zero, the autoclave was heated to the maximum temperature; then it was immediately
308
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
**
Y
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979 309 Table VIII. Composition of Product Slurries U-48
U-50
u-47
u-49
HPETC
HDCCAOZ
solvent HPETC conditions time, min max temp, "C m a x press, psig charge gas 74 H, slurry composition ( g / l O O g of MAF slurry) light oil 1 (from gas) light oil 2 (from slurry) middle oil heavy oil soluble residuum unconverted coal net slurry composition ( g / l O O g of solvent) free MAF slurry) distillable oil soluble residuum unconverted coal slurry composition (g/lOO g of MF slurry) distillable oil soluble residuum unconverted coal
0 460 3975
ash (distillable oil from solvent) (soluble resid. from solvent) (unconverted coal from solvent) (ash from solvent) g of THF soluble slurry/g of solvent fed net g of distillable oil in slurry/per g of solvent fed
cooled. When the autoclave had cooled to 200 "C, the gas was discharged through cold traps into a gas collection bag, analyzed using gas chromatography, and metered through a wet test meter. The slurry remaining in the autoclave was allowed to cool overnight before being discharged under nitrogen pressure through a dip-tube into a preweighed container for later analysis. The slurry was analyzed by various physical methods including vacuum distillation (ASTM D-1160) and dissolution in tetrahydrofuran (THF). Material present in the slurry that was not soluble in THF was considered to be unconverted lignite or ash, and the ash content was determined independently. Three distillate fractions were determined in the vacuum (5 mmHg) distillation. These were the initial boiling point (ibp) to 120 "C fraction (light oil), the 120 to 260 "C (middle oil) fraction, and the fraction distilled above 260 "C (heavy oil). The THF-soluble, but nondistillable, portion of the slurry was called the soluble residuum. Condensable vapors that were discharged with the gas from the reactor at 400 "F were collected in cold traps and are also considered to be light oils. The solvents used in this study consisted primarily of material in the 120 to 260 "C boiling range at 5 mmHg with the exception of the llPETC solvent. In some instances the solvent was catalytically hydrogenated. The designation "distilled" means the solvent is in the "middle oil" boiling range. Nomenclature, description, and analyses of the solvents are given in Table VI, and distillation curves for three of the solvents are shown in Figure 19. A summary of run conditions and the product yields obtained is shown in Table VII. Yields reported are net yields after subtracting the amount of the input solvent. A negative yield indicates a net loss of that fraction as compared to the charge. The objective is to maximize
HDCCAO2
0 460 4175
0 421 37 10
0 42 1 3825
42.2
54.7
51.6
46.5
7.6 5.6 50.9 1.3 27.2 7.4
6.7 10.1 68.1 0 15.1 0
4.1 7.6 44.8 4.5 31.9 7.1
2.7 5.3 7 3.7 0 14.6 3.7
51.5 23.6 24.9
37.3 62.7 0
40.3 38.3 21.5
33.1 52.4 14.5
58.4 24.2 6.7 10.7 (44.8) (18.0) (4.0) (3.0)
78.9 14.1 0 7 .O (71.3) (1.2) 0 0 1.28 0.10
54.7 28.6 6.8 9.9 (42.8) (17.2) (3.8) (2.9) 1.25 0.18
75.4 13.5 4.0 7 .O (67.6) (1.2) 0 0 1.29 0.11
1.18
0.19
distillable liquid products with high conversion of the feed coal without producing excessive yields of product gases. The trends in the data in Table VI1 indicate that liquid yields are reduced and gas yields are increased by increasing either the maximum temperature (420 to 460 "C) or the time-at-temperature (0 to 30 min). The loss in liquids noted at higher temperatures in the cold charge runs can be attributed as follows: high temperature promotes depolymerization of coal and intermediate macromolecules and promotes repolymerization and coking with cocurrent gasification. The greater exposure to time-temperature inherent in cold charge runs relative to hot charge runs a t similar temperature and time-attemperature leads to the observed adverse effect of higher temperature. An optimum time-temperature that would be translated to continuous equipment is felt to be operation at 421 "C and zero to 30 min residence time for a slow heat-up batch autoclave. The data presented in this paper correlate well with earlier hot charge experiments. Lighter more volatile solvents were found to work well at 440 "C and below while heavier solvents were required at high temperatures. The presence of a hydrogen donor (spiked tetralin or a catalytically hydrogenated solvent) was found, again, to increase yields, especially of desirable distillates. Earlier work also indicated that a heavier hydrogen donor (hydrogenated phenanthrene) is more effective in minimizing repolymerization a t 460 "C and above. Effects of Time and Temperature in Cold-Charge Experiments. The first six runs in Table VI1 (U-24, 25, 29, 30, 31, and 35) show the results obtained when using five different solvents a t a nominal maximum reaction temperature of 460 "C and 30 min a t that temperature. Two runs (U-24 and U-25) were duplicates using the distilled anthracene oil, while a third (U-29) was made using
310
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979
that solvent with the addition of tetralin to improve the hydrogen donor properties. The overall results for the six runs are much the same although the yields of specific products vary somewhat. It will be noted that net yield of T H F soluble products varies from about 35% for one of the runs using distilled anthracene oil to a net gain of more than 12% when using either the PDU recycle solvent or the mixed solvent containing a 12% mixture of tetralin in distilled anthracene oil. In general, a relatively high yield of distillable oil has a correspondingly low yield of THF-soluble, but nondistillable, products. The overall conversion of lignite ranges from about 87 to 96% of the MAF coal. Four runs in Table VI1 (U-32, 34, 48, and 50) show results using four different solvents at zero time-at-maximum temperature, while maintaining the maximum temperature at 460 "C. Comparing results for two solvents for the two different times-at-temperature, the total THF soluble and distillable oil yields are increased and the net yield of gas plus water is increased by reducing the timeat-temperature. The yield of hydrocarbon gases (i.e., C1& gases) is lower at the lower time-at-temperature. The overall conversion of MAF coal was not affected. The last four runs in Table VI1 (U-43, 47, 49, and 57) show results at the maximum reaction temperature of 421 "C and at zero time-at-temperature. By lowering temperature, the yield of total T H F soluble products was increased and the gas yield was decreased, while the overall yield was largely unchanged. Comparison of Solvents. Table VI11 compares one composition of the product slurry for two of the liquefaction solvents at two different temperatures (421 and 460 "C). The solvents used were CO-Steam recycle solvent obtained from the Pittsburgh Energy Technology Center (PETC) and anthracene oil distillate hydrogenated over a commercial Co/Mo catalyst a t GFETC. The HPETC solvent was a very heavy solvent that contained nearly 10% ash and THF insoluble organics. The slurry compositions are given on three different bases: moisture-andash-free, moisture- ash-and solvent-free, and moisture-free. Yields and product distribution for the four runs have already been discussed, but the product slurry compositions for the two solvents a t the two temperatures are interesting. At both temperatures, the heavier solvent, HPETC, gave a higher yield of distillable oils than did the lighter hydrogenated anthracene oil distillate solvent. The amount of distillable oil in the slurry increased somewhat with increasing reaction temperature for both solvents. Table VI11 shows, on a moisture-free basis, the portion of the various slurry constituents that can be attributed to the solvent. For example, if one considers 100 g of moisture-free product slurry (for run U-48), 58.4 g is distillable oil, but 44.8 g of that fraction was initially contained in the solvent.
Use of the hydrogenated PETC CO-Steam recycle solvent results in greater yields of THF-soluble products and of distillable oils than do the other solvents tested. It appears that appreciable quantities of prime products result from conversion of the heavier fractions of the solvent rather than from conversion of the coal. Hydrogenation of the anthracene oil resulted in a solvent that produced good conversions and yields, while consuming less carbon monoxide than any of the other solvents. Adding tetralin to distilled anthracene oil increased the yield of distillable oils, particularly light oils that left the reactor with gases at 204 "C, but the total yield of THFsoluble products did not change. The conversion or utilization of the tetralin could not be determined by the usual laboratory analytical procedures. In the case of sulfur addition the sulfur was converted largely to hydrogen sulfide resulting in increased consumption of both hydrogen and carbon monoxide. Addition of tetralin or sulfur to the anthracene oil resulted in solvents that gave increased carbon monoxide consumption. The FS-120 carbon black feed stock performed adequately as a solvent, but it resulted in the highest consumption of carbon monoxide and hydrogen and relatively low solvent recovery. The PDU recycle solvent gave very good performance, though with fairly high carbon monoxide and hydrogen consumption; however, there is only a limited amount of this material available, and there will be no opportunity for producing it in the future. After consideration of the results of the experiments at UND and of the availability of the raw materials, anthracene oil distillate was selected as the base for the two solvents to be used. One will be prepared by simple distillation of the oil to produce a 120-260 "C (at 5 torr boiling range) fraction, and the other by hydrogenation of that fraction. Thus,solvents of two different hydrogen contents that otherwise are quite similar will be used in the experimental program. Literature Cited Appeii, H. R., Miller, R. D., Wender. I., "On the Mechanism of Lignite Liqua faction with Carbon Monoxide and Water", presented at the Dhrision of Fuel Chemistry, 163rd National Meeting, Amerlcan Chemical Society, Boston, Mass., Apr 10-14, 1972. Del Bel, E., Friedman, S., Yavorsky, P. M., Wender, I., "The Liquefaction of Lignite by the CO-Steam Process", AIChE National Meeting, Houston, Texas, Mar 16-20, 1975. Sondreal, E. A,, Knudson, C. L., Schiller, J. E., May, T. H., "Development of the CO-Steam Process for Liquefaction of Lignite and Western SubbitumC nous Coals", Proceedings, 1977 Symposium on Technology and Use of Lignite, Grand Forks, N.D., May 1977. Severson, D. E., Souby, A. M., Harrls, J. J., "Process Development of Lignite Liquefaction", Proceedings, 1977 Symposium on Technology and Use of Lignite, Grand Forks, N.D.. May 1977. Knudson, C. L., Schiller, J. E., Ruud, A. L., ACS Symp. Ser., NO 71 (1975). Sondreal, E. A,, Knudson, C. L., Majkrzak, R . S.,Baker, G. G., Liquefaction of Lignite by the %Steam Process", Preprints, AIChE Meeting, Miami, Fk., Nov 14-18. 1978.
Receiued for review July 19, 1979 Accepted August 14, 1979
