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Liquefaction process with bottom recycling for complete conversion of brown coal
JFUE 1259
Fuel 79 (2000) 355–364 www.elsevier.com/locate/fuel
Liquefaction process with bottom recycling for complete conversion of brown coal Osuma Okuma 1,* Chemical and Environmental Technology Laboratory, Kobe Steel Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe 651-2271, Japan
Abstract The liquefaction conditions of brown coal with bottom recycling were investigated in the presence of an iron–sulfur catalyst using a continuous reactor system for the complete conversion of the coal into distillate and the long-term stable operation of a plant. Two kinds of heavy fractions were used as recycled bottoms: CLB (coal liquid bottom, b:p: . 4208C) and HDAO (hydrogenated–deashed oil, b:p: . 2508C), where they were produced in primary hydrogenation and secondary hydrogenation, respectively. HDAO was hydrogenated over Ni–Mo/Al2O3 catalyst and included HDB (hydrogenated bottom, b:p: . 4208C). The distillate yield increased in proportion to an increase in the amount of the recycled bottom (CLB and/or HDB) in the feed solvent. Fe12x S contained in the CLB had almost the same catalytic activity as that of Fe2O3 –S catalyst, and HDAO had a higher ability of hydrogen donation. They played important roles in the enhanced conversion of the coal and recycled bottom into distillate. The increased gas flow rate (GFR) through the reactors by gas circulation from the gas–liquid separator to the first reactor markedly increased the distillate yield at conditions with bottom recycling. The prolonged nominal residence time (u NT: defined by the ratio of slurryfeed rate to reactor volume) also markedly enhanced the conversion of heavy fraction at higher GFR condition with bottom recycling. Accordingly, the complete conversion of the coal, which provides CLB yield of zero, is confirmed as being achieved at conditions of 4508C and 14.7 MPa by cooperative effects of bottom recycling and increases in GFR and u NT. At such conditions, a liquefaction plant is confirmed to be ratably operated for a long time because the reactor liquid still maintains its fluidity. The yields of all products at the conditions for the complete conversion of the coal (CLB yield of zero) were determined by using their correlation to the CLB yield. The distillate yield correlated very well to CLB yield regardless of factors affecting the conversion such as bottom recycling, GFR and u NT, and attained 66.3 wt% on daf coal at the conditions of CLB yield of zero. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Liquefaction; Bottom recycling; Brown coal
1. Introduction Although a number of direct coal liquefaction processes have been developed to produce liquid fuel [1–4], their coal-derived liquids are still expensive compared with petroleum [5]. Therefore, these processes must be improved to reduce the production cost if coal-derived liquid fuel is to be a viable substitute for petroleum. Since distillate yield is a crucial factor in reducing the cost, it should be maximized in the liquefaction process. The distillate yield generally increases along with the severity of liquefaction conditions such as temperature and pressure, while it decreases beyond a specific temperature due to marked increases in hydrocarbon gases (C1 –C4) yield and hydrogen gas consumption * Tel.: 181-78-306-6801; fax: 181-78-306-6812. E-mail address: [email protected] (O. Okuma). 1 Present address: The New Industry Research Organization, 1-5-2 Minatojima-minamimachi, Chuo-ku, Kobe 650-0047, Japan.
(DH2) [6]. In addition, the increased severity of liquefaction conditions raises the plant costs of construction and operation. Therefore, it is desirable to maximize the distillate yield under milder liquefaction conditions. The author has already reported that Victorian brown coal can be completely converted into distillate (b:p: , 4208C) using a cheaper catalyst of iron–sulfur at conditions of 4508C and 18.6 MPa by increasing the gas flow rate through the reactors and prolonging the nominal residence time of the feed slurry (u NT: defined by the ratio of slurry-feed rate to reactor volume) [7]. However, at such conditions, stable operation of a liquefaction plant is considered to be difficult due to drying up of the liquid in the reactors (reactor liquid) by excessive vaporization of the feed solvent (b.p. 180– 4208C). This suggests that the plant can be operated stably for a long time by using a heavier feed solvent [7]. On the other hand, feed solvent containing heavy fraction (bottom recycling) is known to improve the distillate yield in the liquefaction process [8–14]. Therefore, the
0016-2361/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00170-2
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Osuma Okuma / Fuel 79 (2000) 355–364
Table 1 Compositions of feed slurry and liquefaction conditions (other conditions: temp. 4508C, solvent/coal (daf-C) ratio 2.5 (w/w), cat. Fe2O3 3.0 on daf-C as Fe, S/Fe atomic ratio 1.2, H2 feed 10 wt% on daf-C) Run
Coal
Slurry composition
Pressure (MPa)
u NT a(h)
Coal b/Solv. Fr. c/CLB d/HDAOd (wt/wt/wt/wt) A-1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3 C-1 C-2 C-3 D-1 D-2 D-3 D-4 D-5 E-1 E-2 E-3 E-4 E-5 E-6 E-7 F-1 F-2 F-3 F-4 F-5 G-1 G-2 G-3 G-4 G-5 G-6 G-7
Mg M M M M M M M M M M M Yh Y Y Y Y Y Y Y Y Y Y Y M M M M M M M M M M M M
1.0/2.5/0/0 1.0/2.25/0.25/0 1.0/2.0/0.5/0 1.0/1.5/1.0/0 0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/1.5/1.0/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/1.5/1.0/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.5/0/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/1.0/0.5/1.0 1.0/2.0/0.5/0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0
18.6 18.6 18.6 18.6 18.6 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 18.6 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 18.6 18.6 18.6 18.6 18.6 18.6 18.6 18.6 18.6
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0 2.0 3.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0
Gas flow rate H2 e m3s =h
R.G. f m3s =h
4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 2.1 4.2 2.1 1.4 4.2 2.1 4.2 4.2 4.2 4.2 4.2 4.2 2.1 1.4 4.2 2.1 1.4 4.2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.0 8.0 8.0 0.0 0.0 8.0 16.0 8.0 8.0 8.0 0.0 0.0 0.0 8.0 8.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.0 8.0 8.0 12.0
a
Nominal residence time defined by the ratio of slurry-feed rate (volume) to reactor volume. daf-C basis. c Solvent fraction (b.p. 180–4208C). d Bottom fraction (see Table 2). e Pure H2 gas fed before the slurry preheater. f Gas flow rate circulated from the gas–liquid separator to the first reactor (see Ref. [17]). g Morwell coal (see Table 2). h Yallourn coal (see Table 2). b
liquefaction process for complete conversion of low rank coals is considered to be best achieved by the increases in the gas flow rate and slurry residence time under the conditions with bottom recycling. This paper discusses the liquefaction conditions for the complete conversion of brown coals into distillate and longterm stable operation of the plant based on the results obtained using many kinds of feed solvents with a continuous reactor system.
2. Experimental 2.1. Apparatus and conditions for liquefaction A process development unit (PDU) of 0.1 t/d dry coal throughput, with three stirred tank reactors in series, was used for liquefaction [15,16]. The inner volume of each reactor was 5.2 l, and 4.0 l of it was occupied by reactor liquid and 1.2 l above the liquid surface was gas phase. The
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Table 2 Properties of coals and bottom fractions in feed solvent Coal Mine
Moist. Morwell Yallourn
Ultimate analysis (wt% on daf-C a)
Proximate analysis (wt%)
11.9 9.5
Ash 2.4 1.7
CLB ( b:p: . 4208C) produced in primary hydrogenation Run b CLB Ash c (wt%)
V.M.
C
42.2 45.8
69.0 66.4
Atomic ratio
BM-A BM-B BM-B BM-D BM-E BM-F BM-G
10.3 14.1 14.1 12.6 12.5 13.3 10.7
O/C
0.79 0.79 0.79 0.91 0.81 0.84 0.86
0.04 0.04 0.04 0.06 0.03 0.04 0.04
HDAO (b:p: . 2508C) produced in secondary hydrogenation over Ni–Mo catalyst Run b HDAO HDB h content (wt%) Atomic ratio H/C F G
HDAO-F HDAO-G
21.48 32.07
1.083 1.190
4.6 4.7
N 0.6 0.5
S
O (diff.)
0.3 0.2
25.6 28.1
Solvent extraction (wt%)
H/C A B C D E F G
H
HS d
HI-BS e
BI-PS f
PI g
32.0 8.7 8.7 21.9 21.5 26.7 24.1
34.8 33.9 33.9 44.8 48.3 35.0 49.1
20.8 40.5 40.5 17.7 14.6 21.7 13.2
12.5 16.9 16.9 15.6 15.6 16.6 13.6
Solvent extraction of HDB (wt%) O/C 0.004 0.019
HS d
HI-BS e
83.2 42.6
15.0 53.7
BI-PS f 1.7 3.4
PI g 0.2 0.3
a
Dry and ash free coal. See Table 1. c Proximate analysis. d n-Hexane-solubles. e n-Hexane-insoluble/benzene-solubles. f Benzene-insoluble/pyridine-solubles. g Pyridine-insolubles including ash. h Bottom fraction (b:p: . 4208C). b
configuration of the PDU was shown in a previous paper [15]. The feed solvent/daf-C (dry and ash free coal) ratio in feed slurry was 2.5 by weight. As a catalyst, iron oxide (Fe2O3, particle size of 50 wt%: ca. 2 mm) of 3.0 wt% on daf-C as Fe and sulfur (S/Fe atomic ratio 1.2) were added to the feed slurry. Pure hydrogen gas of 10 wt% on daf-C was fed before the slurry preheater. The liquefaction temperature and pressure were 4508C and 14.7 or 18.6 MPa, respectively. The gas flow rate (GFR) through the reactors was controlled by gas circulation from the gas–liquid separator (V-04) to the first reactor (R-01) and the nominal residence time (u NT) was changed from 1.0 to 3.0 h depending on the slurry-feed rate, as described in previous papers [7,16]. For all experiments, the slurry was preheated up to about 3008C within 20 s in the preheater and stirred at 1000 rev/min in each reactor maintained at 4508C with a magnetic-driven agitator. Table 1 summarizes the compositions of feed slurry and liquefaction conditions for all experiments. In addition to the PDU experiments, experiments using a 5 l autoclave was carried out in order to examine the
catalytic effects of inorganic matters in the pyridineinsolubles (PI) extracted from CLB on coal liquefaction reaction. 2.2. Materials The brown coals used for liquefaction were powdered Victorian brown coals, Morwell and Yallourn, dried with a tubular dryer. More than 80 wt% of them passed through a 200 mesh sieve. Three types of feed solvents were used: a solvent fraction alone (solvent fraction mode) and mixtures of solvent fraction with CLB or CLB 1 HDAO (bottom recycle mode), where solvent fraction, CLB (coal liquid bottom) and HDAO (hydrogenated de-ashed oil) were middle distillate with b.p.180–4208C, heavy fraction with b:p: . 4208C produced in primary hydrogenation and liquid product with b:p: . 2508C produced in secondary hydrogenation, respectively [6,14]. HDAO was a further hydrogenated product consisting of de-ashed CLB and heavy solvent fraction over Ni–Mo/Al2O3 catalyst [17]. These components of the feed solvents used were seven kinds of solvent fraction, six kinds of CLB and two kinds of HDAO.
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Fig. 1. Procedure for calculation of product yields in bottom recycle mode.
The solvent fractions were recycled for the liquefaction of the brown coal in the PDU and a 50 t/d pilot plant of the BCL (brown coal liquefaction) process [18]. Their atomic ratios of H/C and O/C were 0:96 , 1:29 and 0:017 , 0:036, respectively. The CLBs and HDAOs were produced in the pilot plant. Table 2 shows the properties of the coals and bottom fractions used as the feed solvents (CLBs and HDAOs).
2.3. Product yield and analysis The PDU was operated for longer than 30 h (up to 100 h) in each run. The liquid product was sampled at every 3 h interval. The yield of gaseous products was calculated from the flow rate of the tail gas and its composition determined by g.c. analysis. Hydrogen gas consumption (DH2) was calculated by subtracting the H2 flow rate in the tail gas
Table 3 Effects of bottom recycling on liquefaction yields (liquefaction conditions: see Table 1, p : the feed solvent alone was hydrogenated under the same conditions as those of Run A-3) Run
B.R. a
Product yield (wt% on daf-C) CLB
A-1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3 C-1 C-2 C-3 F-1 F-2 F-3
0 25 50 100 p 50 0 50 100 0 50 100 0 50 71
b
46.4 43.9 40.9 35.9 2 14.0 39.4 42.3 27.9 14.5 28.4 21.4 14.3 47.4 39.1 32.0
Solv. Fr. 11.7 14.7 15.2 16.6 2.6 14.7 13.6 28.9 40.8 22.2 27.0 47.3 10.4 12.8 21.9
c
Naph. Fr. 12.4 13.0 15.4 16.6 5.8 14.5 15.8 14.4 15.3 17.2 17.3 15.2 10.8 13.5 16.6
Dist./DH2 d
H2O
C1-C4
CO 1 CO2
2 DH2
Dist.
11.7 11.7 10.1 11.7 2.9 11.2 9.5 12.0 12.3 15.7 17.1 7.5 11.7 14.4 9.8
7.2 6.9 7.8 8.5 4.7 8.4 10.3 11.0 11.8 8.8 9.5 9.1 8.4 9.1 8.6
15.1 14.5 15.3 15.8 1.7 15.7 12.5 11.4 12.0 13.1 13.6 12.5 15.0 15.3 15.0
2 4.4 2 4.5 2 4.8 2 5.3 2 3.7 2 4.5 2 4.1 2 5.5 2 6.8 2 5.4 2 5.9 2 5.8 2 3.9 2 4.3 2 3.8
24.1 27.7 30.7 33.4 8.5 29.2 29.4 43.3 56.1 39.4 44.3 62.5 21.2 26.3 38.5
e
Amount of recycled bottom (CLB and/or HDB, b:p: . 4208C) in the feed solvent (wt% on daf-C). b:p: . 4208C: c b.p. 180–4208C. d b:p: , 1808C. e b:p: , 1808C Solv:Fr: 1 NaphthaFr:: f Here n-Hexane-solubles. g Here n-Hexane-insoluble/benzene-solubles. h Benzene-insoluble/pyridine-solubles. i Organic pyridine-insolubles (ash free base). a
b
5.5 6.2 6.4 6.3 2.3 6.6 7.2 7.8 8.3 7.3 7.6 10.7 5.4 6.1 10.2
Solvent extract yield (wt% on daf-C) HS f
HI-BS g
BI-PS h
PI org. i
5.0 0.0 7.3 7.2 0.0 6.3 6.5 2.8 6.1 11.6 11.3 13.1 11.3 10.1 0.0
25.0 26.4 20.6 22.1 0.9 20.9 23.4 19.3 19.6 13.8 14.3 14.3 18.3 18.4 22.5
16.4 15.7 11.2 7.2 2 12.8 12.3 11.3 1.7 2 10.6 3.8 2 4.3 2 7.1 13.9 8.1 8.2
0.0 1.8 1.5 0.6 2 2.1 0.2 1.1 4.2 2 0.6 2 0.8 1.7 2 6.0 3.9 2.5 1.3
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each fraction, moisture and ash in the feed slurry from those of the liquid product. This is because the bottom contained in the feed solvent should be recycled at a constant ratio in the bottom recycle mode for the liquefaction process. The detailed procedure of calculation is shown in Fig. 1. Accordingly, the yields of all products include products derived from the heavy fraction (recycled bottom) in the feed solvent: CLB and heavy fraction with b:p: . 4208C contained in HDAO (hydrogenated–deashed bottom, HDB). The yields are reported on the daf-C basis as the averages of those of three samples. Both CLBs in the feed and product were sequentially fractionated by solvent extraction with n-hexane, benzene and pyridine [6]. Then, the yields of these solvent extracts were calculated by the same procedure as those for the liquid product.
Fig. 2. Correlation of experimental yields in bottom recycle mode to those calculated by using the results in solvent fraction mode and bottom recycle mode for CLB alone (Run A series). Liquefaction conditions: see Table 1. Keys: (V) CLB, (S) Solv. Fr., (K) Naph. Fr., ( × ) H2O, ( p ) C1 –C4, (X) CO 1 CO2 , (W) 2DH2, (A) Dist., (B) Dist/DH2 ratio. Calculated yield, (wt% on daf-C) (yield of Run A-1) 1 (Yield of Run A-5) × (amount of ash free CLB in the feed solvent).
2.4. Analysis of the state in the reactor The liquid in the reactor (reactor liquid) was sampled from its bottom through a dip-tube and was analyzed by the same procedure as those for the liquid product [15,16]. As the reactor liquid in the present PDU is in a complete mixing state, using the distillation results of the reactor liquid in the third reactor (R-03) and product liquid, their actual residence time (u RT) in R-03 was estimated by the equations introduced in the previous papers [15,16].
from that of the feed. The feed solvent and liquid product were fractionated by vacuum distillation (ASTM D1160) into H2O, naphtha fraction (b:p: , 1808C), solvent fraction (b.p. 180–4208C) and residue (b:p: . 4208C, CLB). Then, the yield of liquid products was calculated by subtracting
Table 4 Effects of gas flow rate and nominal residence time on liquefaction yields (liquefaction conditions, see Table 1) Run B.R. a GFR b m3s =h D-2 D-3 D-4 E-1 E-2 E-6 E-7 G-1 G-2 G-3 G-4 G-5
50 50 50 0 0 50 50 82 82 82 82 82
0 8 16 8 8 8 8 0 0 0 8 8
u NT c Product yield (wt% on daf-C) (h) 1 1 1 1 2 1 2 1 2 3 1 2
Dist./DH2 Solvent extract yield (wt% on daf-C)
CLB d
Solv. Fr. e Naph. Fr. f H2O C1 –C4 CO 1 CO2
2 DH2 Dist. g
31.9 20.0 11.1 30.5 9.3 21.0 1.4 22.3 14.4 7.1 12.5 2 9.5
22.1 36.9 44.0 25.1 45.2 35.7 48.1 25.6 32.0 32.8 41.7 63.7
2 5.0 2 6.1 2 6.5 2 5.8 2 7.7 2 6.2 2 7.7 2 5.4 2 6.6 2 7.3 2 6.7 2 8.0
10.8 10.2 10.7 11.7 15.5 10.9 14.0 16.0 20.2 25.0 12.3 14.4
13.0 16.0 19.6 15.9 16.7 15.2 19.6 14.4 14.1 14.7 18.6 20.4
10.6 10.2 9.6 9.5 10.6 10.1 12.7 11.2 12.1 13.9 9.6 9.8
16.6 12.8 11.5 13.1 10.4 13.3 11.9 15.9 13.9 13.8 9.6 9.8
32.9 47.1 54.7 36.7 60.8 46.6 62.1 41.6 52.2 57.8 54.0 78.1
Amount of recycled bottom (CLB and/or HDB, b:p: . 4208C) in the feed solvent (wt% on daf-C). Gas flow rate (standard state) through the reactors. c Nominal residence time of the feed slurry. d b:p: . 4208C: e b.p. 180–4208C. f b:p: , 1808C. g b:p: , 4208C Solv:Fr: 1 Naph:Fr: h n-Hexane-solubles. i n-Hexane-insoluble/benzene-solubles. j Benzene-insoluble/pyridine solubles. k Organic pyridine-insolubles (ash free base). a
b
6.6 7.7 8.4 6.4 7.9 7.5 8.1 7.7 7.9 7.9 8.0 9.7
HS h
HI-BS i BI-PS j PIorg. k
2 0.5 4.5 8.9 10.1 2.4 0.5 2 2.7 2 2.2 0.6 2 5.0 2 4.9 2 2.2
20.2 11.7 2.5 14.4 4.4 17.3 3.3 12.3 6.1 8.2 13.9 2 5.3
11.8 2.3 2.8 6.1 1.2 2.6 2 2.3 11.5 6.3 4.0 4.8 2 3.2
0.4 1.5 2 3.1 2 0.2 1.2 0.7 3.1 0.7 1.3 0.0 2 1.3 1.2
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Osuma Okuma / Fuel 79 (2000) 355–364
Fig. 3. Correlation of CLB yield to other products, hydrogen consumption (DH2) and hydrogen efficiency (Dist/DH2). Open keys: without gas circulation, closed keys: with gas circulation. Mode: solvent fraction mode (V S), bottom recycle mode (CLB) (W X), (CLB 1 HDAO) (K O).
Osuma Okuma / Fuel 79 (2000) 355–364
3. Results 3.1. Effects of bottom recycling on liquefaction reaction Table 3 shows the effects of the amount of the recycled bottom (CLB and HDB) in the feed solvent at conditions with or without gas circulation on the liquefaction yields. Most of the coal was converted into pyridine-solubles for all runs. The CLB yield decreased and the distillate (b:p: , 4208C) yield increased in proportion to an increase in the amount of the recycled bottom at both conditions with and without gas circulation, while they depended on the properties of CLB in the feed. The yield of preasphaltenes (benzene-insoluble/pyridine-solubles, BI-PS) also decreased as the amount of recycled bottom increased. In particular, the recycled bottom used for Run B series, which contained much BI-PS, provided higher distillate yield than that used for Run A series. Hydrogen efficiency, ratio of distillate yield (Dist) to H2 consumption (DH2), increased with the amount of the recycled bottom, while DH2 also increased. The bottom recycling effect on the yields of gaseous products (C1 –C4 and CO 1 CO2 ) and H2O was not clear from the results shown in Table 3. Fig. 2 shows a correlation of the experimental yields for Run A series to those calculated by using the results in solvent fraction mode (Run A-1) and in bottom recycle mode for CLB alone (Run A-5) at conditions without gas circulation. These results confirm that the bottom recycling effects are brought about by the conversion of the recycled bottom into lighter fraction, mainly solvent fraction. In addition, since the yields of these gaseous products and H2O are smaller than those calculated by using the results from Runs A-1 and A-5, it is suggested that they are suppressed in the bottom recycle mode [19]. This is considered to be other effect of bottom recycling on the liquefaction of the coal. No pressure effect on the yields was found at the conditions between 14.7 and 18.6 MPa because the yields of CLB and distillate were almost the same (Runs A-3 and A-6). For other runs (Runs D and F series), the effect of pressure difference between 14.7 and 18.6 MPa was only slight. These results indicate that the liquefaction pressure of 14.7 MPa is enough to convert the coal and recycled bottom into distillate in the bottom recycle mode. The feed solvent containing CLB and HDAO provided still higher distillate yield and lower DH2 (Run F series). This is because, as shown in Table 2, the HDAO-F also contained 21.5 wt% of HDB and had a higher ability of hydrogen donation compared with both solvent fraction and CLB, resulting in increasing hydrogen efficiency (Dist/DH2 ratio) [20]. 3.2. Effects of gas flow rate (GFR) and nominal residence time (u NT) Table 4 shows the effects of the gas circulation from V-04
361
to R-01 and the prolongation of u NT in the bottom recycle mode on the yields. The decreased CLB and increased distillate yields indicate that the liquefaction reaction is enhanced markedly by the gas circulation. In addition, the increase in the GFR also provided the lighter composition of solvent extract in the CLB. These effects of GFR and u NT in the bottom recycle mode are similar to those in the solvent recycle mode [7,15,16] and cooperatively affect the yields, resulting in a marked increase in the distillate yield, especially in the solvent fraction. As shown in Table 4, at uNT 2:0 h and higher GFR with bottom recycling (Run G-5), the yields of CLB and distillate were 29.5 and 78.1 wt% on daf-C, respectively. This means that the CLB produced in this experiment is smaller than total amount of the recycled bottom (CLB and HDB) and that the coal can be completely converted into distillate. The results discussed above confirm that a liquefaction process for complete conversion of the coal, which yields no heavy fraction (CLB), is possible at the conditions of increased u NT and GFR with bottom recycling. Under such conditions, the product yields derived from the coal are represented by the yields at the CLB yield of zero. Fig. 3(a)–(h) show the correlation of the CLB yield to the yields of other product, DH2 and Dist/DH2 ratio for the results of all experiments listed in Table 1. As shown in Fig. 3(a), since the distillate yield is correlated very well to the CLB yield by a linear equation R2 0:971, the CLB is considered to be mainly converted into distillate regardless of the factors enhancing the liquefaction reaction such as GFR, u NT and bottom recycling. When the CLB yield is zero, the distillate yield is 66.3 wt% on daf-C, which is the same as that at 4508C and 14.7 MPa for the complete conversion of the coal. Fig. 3(d) and (f) show that H2O yield increases and (CO 1 CO2 ) yield decreases with the decrease in CLB yield at the condition with gas circulation, while they do not depend on the CLB yield without gas circulation. These are explained by the enhanced conversion of CLB with de-oxygenation reaction and by the suppressed CO shift reaction in the gas phase at higher GFR [15]. At higher GFR, solvent fraction yield markedly increases due to the conversion of the heavy fraction enhanced by the prolongation of their u RT with the increased vaporization of the solvent fraction. The naphtha and C1 –C4 yields decrease due to the decrease in the residence time of the vaporized solvent fraction in the gas phase [15,16]. DH2 and Dist/DH2 ratio increased as the CLB yield decreased regardless of the reaction conditions. When the feed solvent contained HDAO having a higher ability of hydrogen donation, they decrease at the conditions providing higher CLB yield [20]. Table 5 summarizes the product yields at the CLB yield of zero which are estimated from the results shown in Fig. 3(a)–(h). Modes A and B represent the yields at the conditions of 4508C and 14.7 MPa with and without the gas circulation for the complete conversion of the coal (CLB yield of zero), respectively.
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Table 5 Product yields derived from Victorian brown coal in the complete conversion process (liquefaction conditions: temperature 4508C, pressure 14.7 MPa. Solv./ daf-C 2.5 (w/w), cat. 3.0 wt% on daf-C as Fe, S/Fe atomic ratio 1.2) Mode
Mode-A d Mode-B e
Product yield (wt% on daf-C)
Dist./DH2
Solv. Fr. a
Naph. Fr. b
H2O
C1 –C4
CO 1 CO2
2 DH2
Dist. c
52.8 42.4
13.5 23.6
19.2 14.2
10.4 15.2
11.8 12.3
2 7.7 2 7.7
66.3 66.0
8.6 8.6
a
b.p. 180–4208C. b:p: , 1808C. c b:p: , 4208C Solv:Fr: 1 Naph:Fr:: d High GFR conditions with gas circulation. e Low GFR conditions without gas circulation. b
3.3. Catalytic effects of inorganic matters in CLB
4. Discussion
The results described earlier indicate that the bottom recycling effects also depend on the reactivity of the added bottom because the increase in the distillate yield is mainly due to their increased conversion rate as shown in Table 3 (Runs A and B). As described in the previous papers [15,16], the ratio of catalyst to heavy fraction in the reactor liquid has a very important role in enhancing the conversion of the heavy fraction. The recycled bottom contains the inorganic matters, a major component of which is mainly iron sulfide (Fe12x S: pyrrhotite). Table 6 shows the results of the liquefaction of Morwell coal using a 5 l autoclave to examine catalytic effects of the pyridine-insolubles (PI) in the CLB. The PI containing Fe12x S provided almost the same liquefaction yields as those with a Fe2O3 –sulfur catalytic system (S/Fe atomic ratio: 1.2) when the Fe content was the same for both systems. In addition, the effect in PI– sulfur system (S=Fe . 1:2) was larger than that in Fe2O3 – sulfur system (S/Fe:1.2). These results indicate that Fe12x S in the recycled CLB has almost the same catalytic effects as that of Fe2O3 –sulfur catalyst, and that the ratio of the catalyst to the heavy fraction in the reactor liquid is kept high in bottom recycle mode if S/Fe ratio is $ 1.2.
As described earlier, the prolonged u NT at higher GFR is found to markedly enhance the conversion of the coal into distillate in the bottom recycle mode as well as in the solvent fraction mode [15,16]. Fig. 4 shows the relationship between the CLB yield and actual residence time (u RT) of the reactor liquid estimated by the procedure introduced in previous papers [15,16]. The CLB yield in the bottom recycle mode using CLB becomes zero at a shorter u RT than that in the solvent fraction mode because the CLB in the feed solvent also converts into the lighter fractions. u RT at CLB yield of zero becomes still shorter when the feed solvent containing CLB and HDAO is used. The previous papers [15,16] show that the effects of GFR are due to the prolongation of u RT and the increased concentration of heavy fraction and catalyst. Since the recycled CLB contains Fe12x S having a catalytic activity, the mechanisms of the enhancement by the bottom recycling are similar to that by the increased GFR, and they work cooperatively to markedly increase the distillate yield. The feed solvent containing CLB and HDAO is more effective on enhancing it at such conditions. However, since the effects of GFR described above depend on the degree of
Table 6 Effects of inorganic matters in pyridine-insolubles on liquefaction yields (A.C. tests) (liquefaction conditions: 4308C, H2 int. press. 6 MPa, Solv./daf-C 2.5 (w/ w), properties of feed solvent: H/C 1.012, b.p. 180–4208C) Run
AC-0 AC-1 AC-2 AC-3
Additive Cat.
Fe b
S/Fe c
CLB d
Dist. e
Oth. f
2DH2 g
(wt%)
No Fe2O3 1 S PI h PI h 1 S
No 3.0 3.0 3.0
No 1.2 1.0 2.2
75.4 47.6 49.3 41.6
2 7.4 26.9 24.3 30.5
33.9 29.4 30.1 31.9
2 1.9 2 3.9 2 3.7 2 4.0
92.6 99.5 , 100 , 100
PI conversion ((daf-C) 2 (PIorg))/daf-C × 100. Amount of Fe in additives (wt% on daf-C). c Atomic ratio. d Heavy fraction (b:p: . 4208C). e Distillate (b:p: , 4208C; Solv:Fr: 1 Naph:Fr:). f S (C1 –C4, CO 1 CO2 , H2O). g Hydrogen gas consumption. h Pyridine-insolubles containing Fe 33.5 and Ca 4.1 wt% on PI. a
b
PI conv. a
Yield (wt% on daf-C)
Osuma Okuma / Fuel 79 (2000) 355–364
Fig. 4. Relationship between actual residence time (u RT) of reactor liquid and CLB yield. Open keys: without gas circulation, closed keys: with gas circulation. Mode: solvent fraction mode (V S), bottom recycle mode (CLB) (W X), (CLB 1 HDAO) (K O).
vaporization of the feed solvent fraction, they decrease with a decrease in the amount of the solvent fraction in the feed slurry in bottom recycle mode, as shown in Table 3 (Runs B and C series). In addition, the bottom recycling effects also depend on the reactivity of the heavy fractions (CLB and HDB) and on the content of active Fe12x S as a catalyst. Therefore, the most suitable GFR and u NT should be determined as a function of the amount and properties of recycled CLB and/or HDB which depend on the properties of parent coal and the liquefaction conditions such as temperature and pressure. As reported in the previous papers [15,16], brown coal is completely converted into distillate by increasing the GFR and u NT in solvent fraction mode. However, under such conditions, the heavy fraction and inorganic matters in the reactor liquid are too concentrated to maintain its fluidity due to excessive vaporization of the feed solvent fraction and liquid product derived from the coal, resulting in drying up of the reactor liquid. Thus, the plant cannot be stably operated for a long time. On the other hand, in bottom recycle mode, the heavy fraction still remains in the reactor liquid at higher GFR and longer u NT which provide the CLB
363
yield of zero. It is considered that the CLB yield becomes zero at lower GFR when the feed solvent contains significantly large amount of the heavy fractions (CLB and/or HDB), as shown in Mode-B in Table 5. However, the CLB of more than 100 wt% on daf-C must be added to the feed slurry in such a case, as seen from the results of Runs A-4, B-3 and C-3 in Table 3. The coal slurry made with such a feed solvent is too viscous to feed using a pump into a high-pressure reactor system. Such large bottom recycling cannot be adopted in a commercial process. Therefore, the cooperative effects of GFR and bottom recycling are very important for determination of the liquefaction conditions that guarantee both the stable plant operation for a long time and the complete coal conversion into distillate. Thus, the product yields under such conditions are given by ModeA in Table 5. HDAO derived from the secondary hydrogenation of deashed CLB [6,18] raises the cost of the liquid product. There is no difference in the bottom recycling effects between CLB and HDB at the conditions, which completely convert the coal, as shown in Fig. 3(a). Therefore, only CLB should be recycled in a commercial process. On the other hand, since the ash in the coal should be removed from the liquid products, a number of de-ashing processes have been developed to remove the ash included in the coal and the spent catalyst [21–24]. For the complete coal conversion in the commercial process, all of the heavy fraction (CLB) except that lost in the de-ashing process should be recycled as a part of the feed solvent for liquefaction. Although the amount of lost CLB depends on the de-ashing process and on the amount of inorganic matters comprising the spent catalyst (Fe12x S), the latter should be kept constant in the liquefaction process. The amount and properties of the recycled bottom (CLB) are considered to be spontaneously fixed by other conditions such as temperature, pressure, GFR, u NT and fresh catalyst. Therefore, the amount of the recycled CLB can be selected from the viewpoint of long-term stable operation of the plant by controlling other factors. The distillate yield in such a process is considered to be slightly lower than 66.3 wt% on daf-C as shown in Table 5 because of the loss in the de-ashing process. These results also suggest that the amounts of fresh catalyst, especially
Fig. 5. Conceptual flow diagram of liquefaction process for complete conversion of brown coal.
364
Osuma Okuma / Fuel 79 (2000) 355–364
Fe2O3, and loss of CLB can be decreased by optimizing the conditions of liquefaction and de-ashing processes. The conceptual flow diagram of such a liquefaction process is shown in Fig. 5.
process with bottom recycling is considered to be suitable for long-term operation at higher GFR and longer u NT. Acknowledgements
5. Conclusion The results obtained are as follows: 1. The feed solvent containing bottom (CLB and/or HDB, b:p: . 4208C) provided a higher distillate yield than that consisting of solvent fraction (b:p: . 4208C) alone. This is explained by the conversion of the recycled bottom into lighter fraction because the increment of distillate was proportional to the amount of the recycled bottom. 2. The increases in GFR and u NT in bottom recycle mode markedly increased the distillate yield by the prolonged u RT and the increased concentration of the heavy fraction and catalyst in the reactor liquid. These effects were the same as those in solvent fraction mode. 3. The inorganic matters in the recycled CLB promoted the conversion of the coal and heavy fraction, because the Fe12x S in the CLB had almost the same catalytic activity as the fresh catalyst (Fe2O3 –S, S/Fe 1.2). 4. HDAO also increased the distillate yield and decreased DH2 at the conditions providing higher CLB yield because of its high ability of hydrogen donation. However, only CLB should be recycled in the commercial process to reduce the cost of liquid product because HDAO raises the cost of liquid product. 5. No pressure effects were found between 14.7 and 18.6 MPa and the coal was completely converted into distillate and gases at 4508C and 14.7 MPa. 6. The CLB yield correlated well to the yields of other products, DH2 and hydrogen efficiency. In particular, the relationship between CLB and distillate was expressed very well regardless of the conditions of bottom recycling, GFR and u NT, and the distillate yield at CLB yield of zero attained 66.3 wt% on daf-C. Using these correlations, the yields of all the products were estimated for the complete coal conversion at 4508C and 14.7 MPa. 7. Based on the results described above, the complete coal conversion at 4508C and 14.7 MPa is confirmed to be achieved by selecting the conditions of the amount of recycled CLB, GFR and u NT. In addition, the liquefaction
This study was carried out in the collaboration of Kobe Steel Ltd. and Nippon Brown Coal Liquefaction Co. Ltd. (NBCL) with the financial support of New Energy and Industrial Technology Development Organization (NEDO). The author thanks both NBCL and NEDO for their permission to publish this paper. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20]
[21] [22] [23] [24]
Epperly WR, Wade DT. Chem Engng Prog 1981;77:73. Ecceles RM, De Vaux GR. Chem Engng Prog 1981;77:80. Freel J, Jackson DM, Schmid BK. Chem Engng Prog 1981;77:86. MacArther JB. Proceedings of the 10th Annual International Pittsburgh Coal Conference, 1993:283. Gray D, Tomlinson G, WlSawy A. Proceedings of the 10th Annual International Pittsburgh Coal Conference, 1993:277. Okuma O, Saito K, Kawashima A, Okazaki K, Nakako Y. Fuel Process Technol 1986;14:23. Okuma O, Yasumuro M. Fuel 1998;77:1755. Burke FP, Winschel RA, Pochapsky TC. Fuel 1981;60:562. Silver HF, Corry RG, Miller RL, Hurtubise RJ. Fuel 1982;61:111. Boduszynski MM, Hurtubise RJ, Silver HF. Fuel 1984;63:93. Silver HF, Corry RG, Miller RL, Hurtubise RJ. Fuel 1984;63:872. Kang D, Givens EN. Chem Engng Prog 1984;(November):38. Longanbach JR. Chem Engng Prog 1984;(November):29. Okuma O, Yasumuro M, Kageyama Y, Matsumura T. Proceedings of the 10th Annual International Pittsburgh Coal Conference, 1993:235. Okuma O, Yasumuro M, Matsumura T. Fuel 1996;75:313. Okuma O, Yasumuro M, Yanai S. Fuel 1998;77:797. Nagae S, Mito Y, Okuma O, Saito K, Matsumura T. In: International Conference on Coal Science. NEDO, Tokyo, 1989:931. Nakako Y, Ohzawa T, Narita H. International Conference on Coal Science, Oxford: Butterworth-Heinemann, 1991. p. 652. Yasumuro M, Yanai S, Ida T, Hirano T, Okuma O, Matsumura T. Proceedings of the 27th Sekitan-kagaku-kaigi (Conference of Coal Science, Japan), Tokyo, 1990:75. Yasumuro M, Ida T, Hirano T, Okuma O, Matsumura T. Proceedings of the 28th Sekitan-kagaku-kaigi (Conference of Coal Science, Japan), Osaka, 1991:337. Leu WF, Tiller FM. Powder Technol 1984;40:65. Romey I. Fuel 1982;61:988. Okuma O, Masuda K, Okuyama N, Hirano T. Fuel Process Technol 1997;51:177. Okuma O, Masuda K, Okuyama N, Hirano T. Fuel Process Technol 1998;56:229.
Fuel 79 (2000) 355–364 www.elsevier.com/locate/fuel
Liquefaction process with bottom recycling for complete conversion of brown coal Osuma Okuma 1,* Chemical and Environmental Technology Laboratory, Kobe Steel Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe 651-2271, Japan
Abstract The liquefaction conditions of brown coal with bottom recycling were investigated in the presence of an iron–sulfur catalyst using a continuous reactor system for the complete conversion of the coal into distillate and the long-term stable operation of a plant. Two kinds of heavy fractions were used as recycled bottoms: CLB (coal liquid bottom, b:p: . 4208C) and HDAO (hydrogenated–deashed oil, b:p: . 2508C), where they were produced in primary hydrogenation and secondary hydrogenation, respectively. HDAO was hydrogenated over Ni–Mo/Al2O3 catalyst and included HDB (hydrogenated bottom, b:p: . 4208C). The distillate yield increased in proportion to an increase in the amount of the recycled bottom (CLB and/or HDB) in the feed solvent. Fe12x S contained in the CLB had almost the same catalytic activity as that of Fe2O3 –S catalyst, and HDAO had a higher ability of hydrogen donation. They played important roles in the enhanced conversion of the coal and recycled bottom into distillate. The increased gas flow rate (GFR) through the reactors by gas circulation from the gas–liquid separator to the first reactor markedly increased the distillate yield at conditions with bottom recycling. The prolonged nominal residence time (u NT: defined by the ratio of slurryfeed rate to reactor volume) also markedly enhanced the conversion of heavy fraction at higher GFR condition with bottom recycling. Accordingly, the complete conversion of the coal, which provides CLB yield of zero, is confirmed as being achieved at conditions of 4508C and 14.7 MPa by cooperative effects of bottom recycling and increases in GFR and u NT. At such conditions, a liquefaction plant is confirmed to be ratably operated for a long time because the reactor liquid still maintains its fluidity. The yields of all products at the conditions for the complete conversion of the coal (CLB yield of zero) were determined by using their correlation to the CLB yield. The distillate yield correlated very well to CLB yield regardless of factors affecting the conversion such as bottom recycling, GFR and u NT, and attained 66.3 wt% on daf coal at the conditions of CLB yield of zero. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Liquefaction; Bottom recycling; Brown coal
1. Introduction Although a number of direct coal liquefaction processes have been developed to produce liquid fuel [1–4], their coal-derived liquids are still expensive compared with petroleum [5]. Therefore, these processes must be improved to reduce the production cost if coal-derived liquid fuel is to be a viable substitute for petroleum. Since distillate yield is a crucial factor in reducing the cost, it should be maximized in the liquefaction process. The distillate yield generally increases along with the severity of liquefaction conditions such as temperature and pressure, while it decreases beyond a specific temperature due to marked increases in hydrocarbon gases (C1 –C4) yield and hydrogen gas consumption * Tel.: 181-78-306-6801; fax: 181-78-306-6812. E-mail address: [email protected] (O. Okuma). 1 Present address: The New Industry Research Organization, 1-5-2 Minatojima-minamimachi, Chuo-ku, Kobe 650-0047, Japan.
(DH2) [6]. In addition, the increased severity of liquefaction conditions raises the plant costs of construction and operation. Therefore, it is desirable to maximize the distillate yield under milder liquefaction conditions. The author has already reported that Victorian brown coal can be completely converted into distillate (b:p: , 4208C) using a cheaper catalyst of iron–sulfur at conditions of 4508C and 18.6 MPa by increasing the gas flow rate through the reactors and prolonging the nominal residence time of the feed slurry (u NT: defined by the ratio of slurry-feed rate to reactor volume) [7]. However, at such conditions, stable operation of a liquefaction plant is considered to be difficult due to drying up of the liquid in the reactors (reactor liquid) by excessive vaporization of the feed solvent (b.p. 180– 4208C). This suggests that the plant can be operated stably for a long time by using a heavier feed solvent [7]. On the other hand, feed solvent containing heavy fraction (bottom recycling) is known to improve the distillate yield in the liquefaction process [8–14]. Therefore, the
0016-2361/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S0016-236 1(99)00170-2
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Osuma Okuma / Fuel 79 (2000) 355–364
Table 1 Compositions of feed slurry and liquefaction conditions (other conditions: temp. 4508C, solvent/coal (daf-C) ratio 2.5 (w/w), cat. Fe2O3 3.0 on daf-C as Fe, S/Fe atomic ratio 1.2, H2 feed 10 wt% on daf-C) Run
Coal
Slurry composition
Pressure (MPa)
u NT a(h)
Coal b/Solv. Fr. c/CLB d/HDAOd (wt/wt/wt/wt) A-1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3 C-1 C-2 C-3 D-1 D-2 D-3 D-4 D-5 E-1 E-2 E-3 E-4 E-5 E-6 E-7 F-1 F-2 F-3 F-4 F-5 G-1 G-2 G-3 G-4 G-5 G-6 G-7
Mg M M M M M M M M M M M Yh Y Y Y Y Y Y Y Y Y Y Y M M M M M M M M M M M M
1.0/2.5/0/0 1.0/2.25/0.25/0 1.0/2.0/0.5/0 1.0/1.5/1.0/0 0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/1.5/1.0/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/1.5/1.0/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.5/0/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.0/0.5/0 1.0/2.5/0/0 1.0/2.0/0.5/0 1.0/1.0/0.5/1.0 1.0/2.0/0.5/0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0 1.0/1.0/0.5/1.0
18.6 18.6 18.6 18.6 18.6 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 18.6 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 14.7 18.6 18.6 18.6 18.6 18.6 18.6 18.6 18.6 18.6
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0 2.0 3.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0
Gas flow rate H2 e m3s =h
R.G. f m3s =h
4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 4.2 2.1 4.2 2.1 1.4 4.2 2.1 4.2 4.2 4.2 4.2 4.2 4.2 2.1 1.4 4.2 2.1 1.4 4.2
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.0 8.0 8.0 0.0 0.0 8.0 16.0 8.0 8.0 8.0 0.0 0.0 0.0 8.0 8.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 8.0 8.0 8.0 12.0
a
Nominal residence time defined by the ratio of slurry-feed rate (volume) to reactor volume. daf-C basis. c Solvent fraction (b.p. 180–4208C). d Bottom fraction (see Table 2). e Pure H2 gas fed before the slurry preheater. f Gas flow rate circulated from the gas–liquid separator to the first reactor (see Ref. [17]). g Morwell coal (see Table 2). h Yallourn coal (see Table 2). b
liquefaction process for complete conversion of low rank coals is considered to be best achieved by the increases in the gas flow rate and slurry residence time under the conditions with bottom recycling. This paper discusses the liquefaction conditions for the complete conversion of brown coals into distillate and longterm stable operation of the plant based on the results obtained using many kinds of feed solvents with a continuous reactor system.
2. Experimental 2.1. Apparatus and conditions for liquefaction A process development unit (PDU) of 0.1 t/d dry coal throughput, with three stirred tank reactors in series, was used for liquefaction [15,16]. The inner volume of each reactor was 5.2 l, and 4.0 l of it was occupied by reactor liquid and 1.2 l above the liquid surface was gas phase. The
Osuma Okuma / Fuel 79 (2000) 355–364
357
Table 2 Properties of coals and bottom fractions in feed solvent Coal Mine
Moist. Morwell Yallourn
Ultimate analysis (wt% on daf-C a)
Proximate analysis (wt%)
11.9 9.5
Ash 2.4 1.7
CLB ( b:p: . 4208C) produced in primary hydrogenation Run b CLB Ash c (wt%)
V.M.
C
42.2 45.8
69.0 66.4
Atomic ratio
BM-A BM-B BM-B BM-D BM-E BM-F BM-G
10.3 14.1 14.1 12.6 12.5 13.3 10.7
O/C
0.79 0.79 0.79 0.91 0.81 0.84 0.86
0.04 0.04 0.04 0.06 0.03 0.04 0.04
HDAO (b:p: . 2508C) produced in secondary hydrogenation over Ni–Mo catalyst Run b HDAO HDB h content (wt%) Atomic ratio H/C F G
HDAO-F HDAO-G
21.48 32.07
1.083 1.190
4.6 4.7
N 0.6 0.5
S
O (diff.)
0.3 0.2
25.6 28.1
Solvent extraction (wt%)
H/C A B C D E F G
H
HS d
HI-BS e
BI-PS f
PI g
32.0 8.7 8.7 21.9 21.5 26.7 24.1
34.8 33.9 33.9 44.8 48.3 35.0 49.1
20.8 40.5 40.5 17.7 14.6 21.7 13.2
12.5 16.9 16.9 15.6 15.6 16.6 13.6
Solvent extraction of HDB (wt%) O/C 0.004 0.019
HS d
HI-BS e
83.2 42.6
15.0 53.7
BI-PS f 1.7 3.4
PI g 0.2 0.3
a
Dry and ash free coal. See Table 1. c Proximate analysis. d n-Hexane-solubles. e n-Hexane-insoluble/benzene-solubles. f Benzene-insoluble/pyridine-solubles. g Pyridine-insolubles including ash. h Bottom fraction (b:p: . 4208C). b
configuration of the PDU was shown in a previous paper [15]. The feed solvent/daf-C (dry and ash free coal) ratio in feed slurry was 2.5 by weight. As a catalyst, iron oxide (Fe2O3, particle size of 50 wt%: ca. 2 mm) of 3.0 wt% on daf-C as Fe and sulfur (S/Fe atomic ratio 1.2) were added to the feed slurry. Pure hydrogen gas of 10 wt% on daf-C was fed before the slurry preheater. The liquefaction temperature and pressure were 4508C and 14.7 or 18.6 MPa, respectively. The gas flow rate (GFR) through the reactors was controlled by gas circulation from the gas–liquid separator (V-04) to the first reactor (R-01) and the nominal residence time (u NT) was changed from 1.0 to 3.0 h depending on the slurry-feed rate, as described in previous papers [7,16]. For all experiments, the slurry was preheated up to about 3008C within 20 s in the preheater and stirred at 1000 rev/min in each reactor maintained at 4508C with a magnetic-driven agitator. Table 1 summarizes the compositions of feed slurry and liquefaction conditions for all experiments. In addition to the PDU experiments, experiments using a 5 l autoclave was carried out in order to examine the
catalytic effects of inorganic matters in the pyridineinsolubles (PI) extracted from CLB on coal liquefaction reaction. 2.2. Materials The brown coals used for liquefaction were powdered Victorian brown coals, Morwell and Yallourn, dried with a tubular dryer. More than 80 wt% of them passed through a 200 mesh sieve. Three types of feed solvents were used: a solvent fraction alone (solvent fraction mode) and mixtures of solvent fraction with CLB or CLB 1 HDAO (bottom recycle mode), where solvent fraction, CLB (coal liquid bottom) and HDAO (hydrogenated de-ashed oil) were middle distillate with b.p.180–4208C, heavy fraction with b:p: . 4208C produced in primary hydrogenation and liquid product with b:p: . 2508C produced in secondary hydrogenation, respectively [6,14]. HDAO was a further hydrogenated product consisting of de-ashed CLB and heavy solvent fraction over Ni–Mo/Al2O3 catalyst [17]. These components of the feed solvents used were seven kinds of solvent fraction, six kinds of CLB and two kinds of HDAO.
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Osuma Okuma / Fuel 79 (2000) 355–364
Fig. 1. Procedure for calculation of product yields in bottom recycle mode.
The solvent fractions were recycled for the liquefaction of the brown coal in the PDU and a 50 t/d pilot plant of the BCL (brown coal liquefaction) process [18]. Their atomic ratios of H/C and O/C were 0:96 , 1:29 and 0:017 , 0:036, respectively. The CLBs and HDAOs were produced in the pilot plant. Table 2 shows the properties of the coals and bottom fractions used as the feed solvents (CLBs and HDAOs).
2.3. Product yield and analysis The PDU was operated for longer than 30 h (up to 100 h) in each run. The liquid product was sampled at every 3 h interval. The yield of gaseous products was calculated from the flow rate of the tail gas and its composition determined by g.c. analysis. Hydrogen gas consumption (DH2) was calculated by subtracting the H2 flow rate in the tail gas
Table 3 Effects of bottom recycling on liquefaction yields (liquefaction conditions: see Table 1, p : the feed solvent alone was hydrogenated under the same conditions as those of Run A-3) Run
B.R. a
Product yield (wt% on daf-C) CLB
A-1 A-2 A-3 A-4 A-5 A-6 B-1 B-2 B-3 C-1 C-2 C-3 F-1 F-2 F-3
0 25 50 100 p 50 0 50 100 0 50 100 0 50 71
b
46.4 43.9 40.9 35.9 2 14.0 39.4 42.3 27.9 14.5 28.4 21.4 14.3 47.4 39.1 32.0
Solv. Fr. 11.7 14.7 15.2 16.6 2.6 14.7 13.6 28.9 40.8 22.2 27.0 47.3 10.4 12.8 21.9
c
Naph. Fr. 12.4 13.0 15.4 16.6 5.8 14.5 15.8 14.4 15.3 17.2 17.3 15.2 10.8 13.5 16.6
Dist./DH2 d
H2O
C1-C4
CO 1 CO2
2 DH2
Dist.
11.7 11.7 10.1 11.7 2.9 11.2 9.5 12.0 12.3 15.7 17.1 7.5 11.7 14.4 9.8
7.2 6.9 7.8 8.5 4.7 8.4 10.3 11.0 11.8 8.8 9.5 9.1 8.4 9.1 8.6
15.1 14.5 15.3 15.8 1.7 15.7 12.5 11.4 12.0 13.1 13.6 12.5 15.0 15.3 15.0
2 4.4 2 4.5 2 4.8 2 5.3 2 3.7 2 4.5 2 4.1 2 5.5 2 6.8 2 5.4 2 5.9 2 5.8 2 3.9 2 4.3 2 3.8
24.1 27.7 30.7 33.4 8.5 29.2 29.4 43.3 56.1 39.4 44.3 62.5 21.2 26.3 38.5
e
Amount of recycled bottom (CLB and/or HDB, b:p: . 4208C) in the feed solvent (wt% on daf-C). b:p: . 4208C: c b.p. 180–4208C. d b:p: , 1808C. e b:p: , 1808C Solv:Fr: 1 NaphthaFr:: f Here n-Hexane-solubles. g Here n-Hexane-insoluble/benzene-solubles. h Benzene-insoluble/pyridine-solubles. i Organic pyridine-insolubles (ash free base). a
b
5.5 6.2 6.4 6.3 2.3 6.6 7.2 7.8 8.3 7.3 7.6 10.7 5.4 6.1 10.2
Solvent extract yield (wt% on daf-C) HS f
HI-BS g
BI-PS h
PI org. i
5.0 0.0 7.3 7.2 0.0 6.3 6.5 2.8 6.1 11.6 11.3 13.1 11.3 10.1 0.0
25.0 26.4 20.6 22.1 0.9 20.9 23.4 19.3 19.6 13.8 14.3 14.3 18.3 18.4 22.5
16.4 15.7 11.2 7.2 2 12.8 12.3 11.3 1.7 2 10.6 3.8 2 4.3 2 7.1 13.9 8.1 8.2
0.0 1.8 1.5 0.6 2 2.1 0.2 1.1 4.2 2 0.6 2 0.8 1.7 2 6.0 3.9 2.5 1.3
Osuma Okuma / Fuel 79 (2000) 355–364
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each fraction, moisture and ash in the feed slurry from those of the liquid product. This is because the bottom contained in the feed solvent should be recycled at a constant ratio in the bottom recycle mode for the liquefaction process. The detailed procedure of calculation is shown in Fig. 1. Accordingly, the yields of all products include products derived from the heavy fraction (recycled bottom) in the feed solvent: CLB and heavy fraction with b:p: . 4208C contained in HDAO (hydrogenated–deashed bottom, HDB). The yields are reported on the daf-C basis as the averages of those of three samples. Both CLBs in the feed and product were sequentially fractionated by solvent extraction with n-hexane, benzene and pyridine [6]. Then, the yields of these solvent extracts were calculated by the same procedure as those for the liquid product.
Fig. 2. Correlation of experimental yields in bottom recycle mode to those calculated by using the results in solvent fraction mode and bottom recycle mode for CLB alone (Run A series). Liquefaction conditions: see Table 1. Keys: (V) CLB, (S) Solv. Fr., (K) Naph. Fr., ( × ) H2O, ( p ) C1 –C4, (X) CO 1 CO2 , (W) 2DH2, (A) Dist., (B) Dist/DH2 ratio. Calculated yield, (wt% on daf-C) (yield of Run A-1) 1 (Yield of Run A-5) × (amount of ash free CLB in the feed solvent).
2.4. Analysis of the state in the reactor The liquid in the reactor (reactor liquid) was sampled from its bottom through a dip-tube and was analyzed by the same procedure as those for the liquid product [15,16]. As the reactor liquid in the present PDU is in a complete mixing state, using the distillation results of the reactor liquid in the third reactor (R-03) and product liquid, their actual residence time (u RT) in R-03 was estimated by the equations introduced in the previous papers [15,16].
from that of the feed. The feed solvent and liquid product were fractionated by vacuum distillation (ASTM D1160) into H2O, naphtha fraction (b:p: , 1808C), solvent fraction (b.p. 180–4208C) and residue (b:p: . 4208C, CLB). Then, the yield of liquid products was calculated by subtracting
Table 4 Effects of gas flow rate and nominal residence time on liquefaction yields (liquefaction conditions, see Table 1) Run B.R. a GFR b m3s =h D-2 D-3 D-4 E-1 E-2 E-6 E-7 G-1 G-2 G-3 G-4 G-5
50 50 50 0 0 50 50 82 82 82 82 82
0 8 16 8 8 8 8 0 0 0 8 8
u NT c Product yield (wt% on daf-C) (h) 1 1 1 1 2 1 2 1 2 3 1 2
Dist./DH2 Solvent extract yield (wt% on daf-C)
CLB d
Solv. Fr. e Naph. Fr. f H2O C1 –C4 CO 1 CO2
2 DH2 Dist. g
31.9 20.0 11.1 30.5 9.3 21.0 1.4 22.3 14.4 7.1 12.5 2 9.5
22.1 36.9 44.0 25.1 45.2 35.7 48.1 25.6 32.0 32.8 41.7 63.7
2 5.0 2 6.1 2 6.5 2 5.8 2 7.7 2 6.2 2 7.7 2 5.4 2 6.6 2 7.3 2 6.7 2 8.0
10.8 10.2 10.7 11.7 15.5 10.9 14.0 16.0 20.2 25.0 12.3 14.4
13.0 16.0 19.6 15.9 16.7 15.2 19.6 14.4 14.1 14.7 18.6 20.4
10.6 10.2 9.6 9.5 10.6 10.1 12.7 11.2 12.1 13.9 9.6 9.8
16.6 12.8 11.5 13.1 10.4 13.3 11.9 15.9 13.9 13.8 9.6 9.8
32.9 47.1 54.7 36.7 60.8 46.6 62.1 41.6 52.2 57.8 54.0 78.1
Amount of recycled bottom (CLB and/or HDB, b:p: . 4208C) in the feed solvent (wt% on daf-C). Gas flow rate (standard state) through the reactors. c Nominal residence time of the feed slurry. d b:p: . 4208C: e b.p. 180–4208C. f b:p: , 1808C. g b:p: , 4208C Solv:Fr: 1 Naph:Fr: h n-Hexane-solubles. i n-Hexane-insoluble/benzene-solubles. j Benzene-insoluble/pyridine solubles. k Organic pyridine-insolubles (ash free base). a
b
6.6 7.7 8.4 6.4 7.9 7.5 8.1 7.7 7.9 7.9 8.0 9.7
HS h
HI-BS i BI-PS j PIorg. k
2 0.5 4.5 8.9 10.1 2.4 0.5 2 2.7 2 2.2 0.6 2 5.0 2 4.9 2 2.2
20.2 11.7 2.5 14.4 4.4 17.3 3.3 12.3 6.1 8.2 13.9 2 5.3
11.8 2.3 2.8 6.1 1.2 2.6 2 2.3 11.5 6.3 4.0 4.8 2 3.2
0.4 1.5 2 3.1 2 0.2 1.2 0.7 3.1 0.7 1.3 0.0 2 1.3 1.2
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Fig. 3. Correlation of CLB yield to other products, hydrogen consumption (DH2) and hydrogen efficiency (Dist/DH2). Open keys: without gas circulation, closed keys: with gas circulation. Mode: solvent fraction mode (V S), bottom recycle mode (CLB) (W X), (CLB 1 HDAO) (K O).
Osuma Okuma / Fuel 79 (2000) 355–364
3. Results 3.1. Effects of bottom recycling on liquefaction reaction Table 3 shows the effects of the amount of the recycled bottom (CLB and HDB) in the feed solvent at conditions with or without gas circulation on the liquefaction yields. Most of the coal was converted into pyridine-solubles for all runs. The CLB yield decreased and the distillate (b:p: , 4208C) yield increased in proportion to an increase in the amount of the recycled bottom at both conditions with and without gas circulation, while they depended on the properties of CLB in the feed. The yield of preasphaltenes (benzene-insoluble/pyridine-solubles, BI-PS) also decreased as the amount of recycled bottom increased. In particular, the recycled bottom used for Run B series, which contained much BI-PS, provided higher distillate yield than that used for Run A series. Hydrogen efficiency, ratio of distillate yield (Dist) to H2 consumption (DH2), increased with the amount of the recycled bottom, while DH2 also increased. The bottom recycling effect on the yields of gaseous products (C1 –C4 and CO 1 CO2 ) and H2O was not clear from the results shown in Table 3. Fig. 2 shows a correlation of the experimental yields for Run A series to those calculated by using the results in solvent fraction mode (Run A-1) and in bottom recycle mode for CLB alone (Run A-5) at conditions without gas circulation. These results confirm that the bottom recycling effects are brought about by the conversion of the recycled bottom into lighter fraction, mainly solvent fraction. In addition, since the yields of these gaseous products and H2O are smaller than those calculated by using the results from Runs A-1 and A-5, it is suggested that they are suppressed in the bottom recycle mode [19]. This is considered to be other effect of bottom recycling on the liquefaction of the coal. No pressure effect on the yields was found at the conditions between 14.7 and 18.6 MPa because the yields of CLB and distillate were almost the same (Runs A-3 and A-6). For other runs (Runs D and F series), the effect of pressure difference between 14.7 and 18.6 MPa was only slight. These results indicate that the liquefaction pressure of 14.7 MPa is enough to convert the coal and recycled bottom into distillate in the bottom recycle mode. The feed solvent containing CLB and HDAO provided still higher distillate yield and lower DH2 (Run F series). This is because, as shown in Table 2, the HDAO-F also contained 21.5 wt% of HDB and had a higher ability of hydrogen donation compared with both solvent fraction and CLB, resulting in increasing hydrogen efficiency (Dist/DH2 ratio) [20]. 3.2. Effects of gas flow rate (GFR) and nominal residence time (u NT) Table 4 shows the effects of the gas circulation from V-04
361
to R-01 and the prolongation of u NT in the bottom recycle mode on the yields. The decreased CLB and increased distillate yields indicate that the liquefaction reaction is enhanced markedly by the gas circulation. In addition, the increase in the GFR also provided the lighter composition of solvent extract in the CLB. These effects of GFR and u NT in the bottom recycle mode are similar to those in the solvent recycle mode [7,15,16] and cooperatively affect the yields, resulting in a marked increase in the distillate yield, especially in the solvent fraction. As shown in Table 4, at uNT 2:0 h and higher GFR with bottom recycling (Run G-5), the yields of CLB and distillate were 29.5 and 78.1 wt% on daf-C, respectively. This means that the CLB produced in this experiment is smaller than total amount of the recycled bottom (CLB and HDB) and that the coal can be completely converted into distillate. The results discussed above confirm that a liquefaction process for complete conversion of the coal, which yields no heavy fraction (CLB), is possible at the conditions of increased u NT and GFR with bottom recycling. Under such conditions, the product yields derived from the coal are represented by the yields at the CLB yield of zero. Fig. 3(a)–(h) show the correlation of the CLB yield to the yields of other product, DH2 and Dist/DH2 ratio for the results of all experiments listed in Table 1. As shown in Fig. 3(a), since the distillate yield is correlated very well to the CLB yield by a linear equation R2 0:971, the CLB is considered to be mainly converted into distillate regardless of the factors enhancing the liquefaction reaction such as GFR, u NT and bottom recycling. When the CLB yield is zero, the distillate yield is 66.3 wt% on daf-C, which is the same as that at 4508C and 14.7 MPa for the complete conversion of the coal. Fig. 3(d) and (f) show that H2O yield increases and (CO 1 CO2 ) yield decreases with the decrease in CLB yield at the condition with gas circulation, while they do not depend on the CLB yield without gas circulation. These are explained by the enhanced conversion of CLB with de-oxygenation reaction and by the suppressed CO shift reaction in the gas phase at higher GFR [15]. At higher GFR, solvent fraction yield markedly increases due to the conversion of the heavy fraction enhanced by the prolongation of their u RT with the increased vaporization of the solvent fraction. The naphtha and C1 –C4 yields decrease due to the decrease in the residence time of the vaporized solvent fraction in the gas phase [15,16]. DH2 and Dist/DH2 ratio increased as the CLB yield decreased regardless of the reaction conditions. When the feed solvent contained HDAO having a higher ability of hydrogen donation, they decrease at the conditions providing higher CLB yield [20]. Table 5 summarizes the product yields at the CLB yield of zero which are estimated from the results shown in Fig. 3(a)–(h). Modes A and B represent the yields at the conditions of 4508C and 14.7 MPa with and without the gas circulation for the complete conversion of the coal (CLB yield of zero), respectively.
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Osuma Okuma / Fuel 79 (2000) 355–364
Table 5 Product yields derived from Victorian brown coal in the complete conversion process (liquefaction conditions: temperature 4508C, pressure 14.7 MPa. Solv./ daf-C 2.5 (w/w), cat. 3.0 wt% on daf-C as Fe, S/Fe atomic ratio 1.2) Mode
Mode-A d Mode-B e
Product yield (wt% on daf-C)
Dist./DH2
Solv. Fr. a
Naph. Fr. b
H2O
C1 –C4
CO 1 CO2
2 DH2
Dist. c
52.8 42.4
13.5 23.6
19.2 14.2
10.4 15.2
11.8 12.3
2 7.7 2 7.7
66.3 66.0
8.6 8.6
a
b.p. 180–4208C. b:p: , 1808C. c b:p: , 4208C Solv:Fr: 1 Naph:Fr:: d High GFR conditions with gas circulation. e Low GFR conditions without gas circulation. b
3.3. Catalytic effects of inorganic matters in CLB
4. Discussion
The results described earlier indicate that the bottom recycling effects also depend on the reactivity of the added bottom because the increase in the distillate yield is mainly due to their increased conversion rate as shown in Table 3 (Runs A and B). As described in the previous papers [15,16], the ratio of catalyst to heavy fraction in the reactor liquid has a very important role in enhancing the conversion of the heavy fraction. The recycled bottom contains the inorganic matters, a major component of which is mainly iron sulfide (Fe12x S: pyrrhotite). Table 6 shows the results of the liquefaction of Morwell coal using a 5 l autoclave to examine catalytic effects of the pyridine-insolubles (PI) in the CLB. The PI containing Fe12x S provided almost the same liquefaction yields as those with a Fe2O3 –sulfur catalytic system (S/Fe atomic ratio: 1.2) when the Fe content was the same for both systems. In addition, the effect in PI– sulfur system (S=Fe . 1:2) was larger than that in Fe2O3 – sulfur system (S/Fe:1.2). These results indicate that Fe12x S in the recycled CLB has almost the same catalytic effects as that of Fe2O3 –sulfur catalyst, and that the ratio of the catalyst to the heavy fraction in the reactor liquid is kept high in bottom recycle mode if S/Fe ratio is $ 1.2.
As described earlier, the prolonged u NT at higher GFR is found to markedly enhance the conversion of the coal into distillate in the bottom recycle mode as well as in the solvent fraction mode [15,16]. Fig. 4 shows the relationship between the CLB yield and actual residence time (u RT) of the reactor liquid estimated by the procedure introduced in previous papers [15,16]. The CLB yield in the bottom recycle mode using CLB becomes zero at a shorter u RT than that in the solvent fraction mode because the CLB in the feed solvent also converts into the lighter fractions. u RT at CLB yield of zero becomes still shorter when the feed solvent containing CLB and HDAO is used. The previous papers [15,16] show that the effects of GFR are due to the prolongation of u RT and the increased concentration of heavy fraction and catalyst. Since the recycled CLB contains Fe12x S having a catalytic activity, the mechanisms of the enhancement by the bottom recycling are similar to that by the increased GFR, and they work cooperatively to markedly increase the distillate yield. The feed solvent containing CLB and HDAO is more effective on enhancing it at such conditions. However, since the effects of GFR described above depend on the degree of
Table 6 Effects of inorganic matters in pyridine-insolubles on liquefaction yields (A.C. tests) (liquefaction conditions: 4308C, H2 int. press. 6 MPa, Solv./daf-C 2.5 (w/ w), properties of feed solvent: H/C 1.012, b.p. 180–4208C) Run
AC-0 AC-1 AC-2 AC-3
Additive Cat.
Fe b
S/Fe c
CLB d
Dist. e
Oth. f
2DH2 g
(wt%)
No Fe2O3 1 S PI h PI h 1 S
No 3.0 3.0 3.0
No 1.2 1.0 2.2
75.4 47.6 49.3 41.6
2 7.4 26.9 24.3 30.5
33.9 29.4 30.1 31.9
2 1.9 2 3.9 2 3.7 2 4.0
92.6 99.5 , 100 , 100
PI conversion ((daf-C) 2 (PIorg))/daf-C × 100. Amount of Fe in additives (wt% on daf-C). c Atomic ratio. d Heavy fraction (b:p: . 4208C). e Distillate (b:p: , 4208C; Solv:Fr: 1 Naph:Fr:). f S (C1 –C4, CO 1 CO2 , H2O). g Hydrogen gas consumption. h Pyridine-insolubles containing Fe 33.5 and Ca 4.1 wt% on PI. a
b
PI conv. a
Yield (wt% on daf-C)
Osuma Okuma / Fuel 79 (2000) 355–364
Fig. 4. Relationship between actual residence time (u RT) of reactor liquid and CLB yield. Open keys: without gas circulation, closed keys: with gas circulation. Mode: solvent fraction mode (V S), bottom recycle mode (CLB) (W X), (CLB 1 HDAO) (K O).
vaporization of the feed solvent fraction, they decrease with a decrease in the amount of the solvent fraction in the feed slurry in bottom recycle mode, as shown in Table 3 (Runs B and C series). In addition, the bottom recycling effects also depend on the reactivity of the heavy fractions (CLB and HDB) and on the content of active Fe12x S as a catalyst. Therefore, the most suitable GFR and u NT should be determined as a function of the amount and properties of recycled CLB and/or HDB which depend on the properties of parent coal and the liquefaction conditions such as temperature and pressure. As reported in the previous papers [15,16], brown coal is completely converted into distillate by increasing the GFR and u NT in solvent fraction mode. However, under such conditions, the heavy fraction and inorganic matters in the reactor liquid are too concentrated to maintain its fluidity due to excessive vaporization of the feed solvent fraction and liquid product derived from the coal, resulting in drying up of the reactor liquid. Thus, the plant cannot be stably operated for a long time. On the other hand, in bottom recycle mode, the heavy fraction still remains in the reactor liquid at higher GFR and longer u NT which provide the CLB
363
yield of zero. It is considered that the CLB yield becomes zero at lower GFR when the feed solvent contains significantly large amount of the heavy fractions (CLB and/or HDB), as shown in Mode-B in Table 5. However, the CLB of more than 100 wt% on daf-C must be added to the feed slurry in such a case, as seen from the results of Runs A-4, B-3 and C-3 in Table 3. The coal slurry made with such a feed solvent is too viscous to feed using a pump into a high-pressure reactor system. Such large bottom recycling cannot be adopted in a commercial process. Therefore, the cooperative effects of GFR and bottom recycling are very important for determination of the liquefaction conditions that guarantee both the stable plant operation for a long time and the complete coal conversion into distillate. Thus, the product yields under such conditions are given by ModeA in Table 5. HDAO derived from the secondary hydrogenation of deashed CLB [6,18] raises the cost of the liquid product. There is no difference in the bottom recycling effects between CLB and HDB at the conditions, which completely convert the coal, as shown in Fig. 3(a). Therefore, only CLB should be recycled in a commercial process. On the other hand, since the ash in the coal should be removed from the liquid products, a number of de-ashing processes have been developed to remove the ash included in the coal and the spent catalyst [21–24]. For the complete coal conversion in the commercial process, all of the heavy fraction (CLB) except that lost in the de-ashing process should be recycled as a part of the feed solvent for liquefaction. Although the amount of lost CLB depends on the de-ashing process and on the amount of inorganic matters comprising the spent catalyst (Fe12x S), the latter should be kept constant in the liquefaction process. The amount and properties of the recycled bottom (CLB) are considered to be spontaneously fixed by other conditions such as temperature, pressure, GFR, u NT and fresh catalyst. Therefore, the amount of the recycled CLB can be selected from the viewpoint of long-term stable operation of the plant by controlling other factors. The distillate yield in such a process is considered to be slightly lower than 66.3 wt% on daf-C as shown in Table 5 because of the loss in the de-ashing process. These results also suggest that the amounts of fresh catalyst, especially
Fig. 5. Conceptual flow diagram of liquefaction process for complete conversion of brown coal.
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Osuma Okuma / Fuel 79 (2000) 355–364
Fe2O3, and loss of CLB can be decreased by optimizing the conditions of liquefaction and de-ashing processes. The conceptual flow diagram of such a liquefaction process is shown in Fig. 5.
process with bottom recycling is considered to be suitable for long-term operation at higher GFR and longer u NT. Acknowledgements
5. Conclusion The results obtained are as follows: 1. The feed solvent containing bottom (CLB and/or HDB, b:p: . 4208C) provided a higher distillate yield than that consisting of solvent fraction (b:p: . 4208C) alone. This is explained by the conversion of the recycled bottom into lighter fraction because the increment of distillate was proportional to the amount of the recycled bottom. 2. The increases in GFR and u NT in bottom recycle mode markedly increased the distillate yield by the prolonged u RT and the increased concentration of the heavy fraction and catalyst in the reactor liquid. These effects were the same as those in solvent fraction mode. 3. The inorganic matters in the recycled CLB promoted the conversion of the coal and heavy fraction, because the Fe12x S in the CLB had almost the same catalytic activity as the fresh catalyst (Fe2O3 –S, S/Fe 1.2). 4. HDAO also increased the distillate yield and decreased DH2 at the conditions providing higher CLB yield because of its high ability of hydrogen donation. However, only CLB should be recycled in the commercial process to reduce the cost of liquid product because HDAO raises the cost of liquid product. 5. No pressure effects were found between 14.7 and 18.6 MPa and the coal was completely converted into distillate and gases at 4508C and 14.7 MPa. 6. The CLB yield correlated well to the yields of other products, DH2 and hydrogen efficiency. In particular, the relationship between CLB and distillate was expressed very well regardless of the conditions of bottom recycling, GFR and u NT, and the distillate yield at CLB yield of zero attained 66.3 wt% on daf-C. Using these correlations, the yields of all the products were estimated for the complete coal conversion at 4508C and 14.7 MPa. 7. Based on the results described above, the complete coal conversion at 4508C and 14.7 MPa is confirmed to be achieved by selecting the conditions of the amount of recycled CLB, GFR and u NT. In addition, the liquefaction
This study was carried out in the collaboration of Kobe Steel Ltd. and Nippon Brown Coal Liquefaction Co. Ltd. (NBCL) with the financial support of New Energy and Industrial Technology Development Organization (NEDO). The author thanks both NBCL and NEDO for their permission to publish this paper. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
[20]
[21] [22] [23] [24]
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