Energy & Fuels 1991,5,102-109
102 100.0 CI
T u
GAS
50.0
OIL SRC OTHERS
n
V."
n
Coal liquid
Shale oil
(1) Without hydrotreatment
Tarsand bitumen
Anthracene oil
(2) Hydrotreatment of solvent and product : 3QO"C,QO min.
Figure 4. Liquefaction of Taiheiyo coal at 450
OC,
60 min.
are higher than the distillable oil vield measured by vacuum distillation (Table IV) for both temperatures and for each solvent. The HS yields obtained at 420 "C were also higher than those at 450 "C, notwithstanding that higher distillable oil yields were observed at 450 "C (Table IV). Therefore, in this study, the HS yield is not a useful index to estimate the yield of distillable oil. The product distribution for liquefaction on a daf coal basis at 450 "C and 90 min, with and without hydrotreatment, were calculated by using results on the reactivity of solvent alone under the same liquefaction and hydrotreatment conditions as shown in Figure 4. The experiment using shale oil without any hydrotreatment showed extremely low conversion of 72 wt % and the oil yield from coal of 9.4 wt % . Conversion and the oil yield were improved with hydrotreatment of solvent and product
up to 47.7 wt %. The oil yield for a coal basis from the liquefaction with tar sand bitumen increased from 21 to 54.5 wt 90with hydrotreatment of solvent and liquefied product. This indicates that the qualities of shale oil and tar sand bitumen for liquefaction were greatly improved with prehydrotreatment. The hydrogenation of aromatic muclei to naphthenes by hydrotreatment plays an important role in liquefaction. Observed oil yields on a coal basis increased with hydrotreatments from 25.5 to 44.6 wt 70for coal liquid and from 28.7 to 52.8 wt % for anthracene oil, respectively. In conclusion, anthracene oil acts as a better solvent for liquefaction in spite of its lower H/C and higher fa value. This is attributed to the higher solubility with coal due to a higher concentration of aromatic compounds and lesser amounts of side chains as reported by Burke et al."J8 On the other hand, shale oil and tar sand bitumen are poor solvents for liquefaction, notwithstanding their higher H/C and lower fa value. As mentioned above in Table 111, their poor affmity for coal due to their paraffmic character gives rise to retrogressive reactions with coal fragments, leading to high molecular weight products. Hydrotreatment of these solvents greatly improved their quality for liquefaction, especially in the case of tar sand bitumen. Acknowledgment. We acknowledge Dr. M. Enomoto of the Fuel Department at the National Research Institute for Pollution and Resources for preparing and supplying Colorado shale oil and Mr. K. Sat0 for performing antoclave experiments. Registry No. Ni, 7440-02-0; Mo, 7439-98-7;A1203,1344-28-1.
Catalytic Hydrotreating of Black Liquor Oils Douglas C. Elliott* Battelle, Pacific Northwest Laboratories, P.O. Box 999, Richland, Washington 99352
Anja Oasmaa Laboratory of Fuel and Process Technology, Technical Research Centre of Finland, Espoo, Finland Received May 7, 1990. Revised Manuscript Received August 27, 1990
Laboratory-scale batch experiments have been completed for testing catalytic hydrotreating as a means of upgrading an oil product produced from kraft black liquor. The experiments show that sulfided cobalt-molybdenum and nickel-molybdenum catalysts can be used to convert the viscous, low-volatility oil into a mixture of hydrocarbons and phenolics at 380 "C and about 1 h residence time. Removal of the sodium in the oil by an acid wash before the experiment results in a more effective reaction and a higher quality product. Addition of water to the hydrotreating experiment can be used as a means to separate the sodium from the upgraded product into a byproduct aqueous stream. The upgraded product contains less oxygen and sodium than the black liquor oil and contains a larger volatile fraction (distillable under vacuum). Comparison of the operating results and the product composition with earlier results in wood-derived oil upgrading shows many similarities.
Introduction The Laboratory of Fuel and Process Technology (POV), Technical Research Centre of Finland (VTT),has developed a thermal process that separates the organics out of the black liquor produced during kraft pulping operations. Under suitable conditions, a large part of the organic material is converted to a hydrophobic oil-like product that separates from the remaining aqueous phase containing
the inorganic material and residual organics. The organic product can be utilized as a fuel in a lime kiln or in an onsite power boiler. By applying the treatment to part of the black liquor stream, it is possible to increase the recovery capacity of an existing kraft mill. This increased capacity is of great interest to the kraft pulping industry because the recovery boiler is often the bottleneck of the mill.
0887-0624/91/2505-0102$02.50/00 1991 American Chemical Society
Energy & Fuels, Vol. 5, No.1, 1991 103
Catalytic Hydrotreating of Black Liquor Oils
no CoMo 90 no lower temp stage
no CoMo
Table I. Summary of Experiments" 9 10 11/19 12 BLFO BLFO acid-washed BLFO BLFO acid-washed BLFO BLFO no no no no yes no CoMo CoMo CoMo CoMo none NiMoI
70
70
70
70
70
70
no lower 350 temp stage 25
280 35
280 35
30 280 40
yes no yes ye? NiMoI NiMoI NiMoII NiMoII 30 30 30 70
280 40
280 40
280 40
280 40
2801300 285 40 35
380 125
405 130
380 65
380 65
380 65
380
380 75
380 70
380 245
380
2 run no. raw material BLFO
extra water catalyst H2,bar T,,"C residence time, min Tr, "C residence time, min
3
BLFO
7
380 40
8
70
13
14
BLFO
BLFO
15 BLFO
70
16
BLFO
380 65
"BLFO = black liquor feed oil. CoMo = cobalt-molybdenum on alumina. NiMo I = nickel-molybdenum on alumina. NiMo I1 = nickel-molybdenum on zeolite.
The quality of the organic product has been under study at V" to determine its value for industrial use. The oil, as produced initially, is too heavy to be analyzed by conventional methods, such as gas chromatography. It also contains much sodium, which affects its volatility and can be a problem in further processing. The aim of the study discussed in this paper was to upgrade the oil to a lower molecular weight product and remove the sodium. Catalytic hydrotreatment was chosen for the upgrading method on the basis of previous successful experiences at Battelle, Pacific Northwest Laboratories, in the upgrading of wood-derived heavy oil products for synthetic liquid fuels production.' This paper describes the processing and analytical methods used for catalytic hydrotreatment of the VTT black liquor oil product and details the results and findings of the experiments. Catalytic hydrotreatment is the use of catalysts in pressurized hydrogen atmospheres to remove unwanted contaminants by reaction with the hydrogen. The process is based on the well-known use of hydrogen in hydrodesulfurization (HDS) of petroleum products. The net effect of such reactions is the breaking of carbon-sulfur bonds to produce hydrogen sulfide gas and clean liquid hydrocarbon fuels. Similar processing also can remove nitrogen (HDN) and metal contaminants (HDM). More recently, oxygen removal (HDO) has also been studied by researchers in the field of synthetic liquid fuels production from coal and biomass. Both HDO and HDS play significant roles in the catalytic hydrotreating of black liquor oils.
Experimental Section Processing Methods. Black liquor feed oil (BLFO) was produced for the experiments in repeated runs using a 1-L, high-pressure autoclave at the Laboratory of Fuel Processing Technology of VTT. In each run, about 500 mL of black liquor (30 wt % solids) and 45 g of NaOH were mixed together and placed in the autoclave equipped with a liner, a stirrer, a gascharging system, and a gas letdown train. After nitrogen was charged (1-2 bar), the reactor was heated to 350 "C at a rate of about 5 "C/min. After a reaction time of 30 min, the autoclave was rapidly cooled to 25 OC. The product gases were released, and the oil phase and aqueous phase were separately recovered from the reactor. This high-pH process has recently been developed in the laboratory by McKeough e t a1.2 The BLFO contained 5.5 wt % sodium (wet basis). T w o types of raw material were used in the experiments to determine the (1) Baker, E. G.; Elliott, D. C. In Pyrolysis Oils from Biomass: Producing, Analyzing and Upgrading;ACS Symposium Series 376; Soltes, E. J., Milne, T. A., Eds.; American Chemical Society: Washington, DC, 1 9 M pp 228-240. (2) McKeough, P. J.; A h , R.; Johanseon, A.; Oasmaa, A. LiquidPhase Thermal Treatment of Black Liquor. Evaluation of Adoanced Process Concepts. Final report for 1988-1988 project. Technical Research Centre of Finland Espoo, Finland, 1989.
role of sodium during upgrading: original BLFO and essentially sodium-free oil prepared by washing the BLFO with acid. In the acid wash, 300 g BLFO was mixed thoroughly with 500 mL of 5 w t % hydrochloric acid solution at about 80 "C for several 6-h periods. A fresh acid solution was used for each period. In the fiial period, the pH remained constant (i.e., all removable sodium had been dissolved into the acid phase). The metal oxide catalysts used in the upgrading experiments were cobalt-molybdenum (CoMo) and nickel-molybdenum (NiMo) hydrotreating and hydrocracking catalysts on alumina and zeolitealumina supports encompassing a range of acidities. The CoMo catalyst was the Katform 499 (Katalco Corp., Oak Brook, IL) containing 4.4% COOand 19.0% MOO, on alumina. The NiMo catalysts were the HDN-60 NiMo (Criterion, Michigan City, IN) on alumina and the M8-85 (BASF Corp., Wyandotte, MI) NiMo on zeolite. The catalysts were sulfided by using carbon disulfide in toluene solution with hydrogen gas. T o assure complete sulfidation, the catalysts were processed in 50% excess of the sulfiding agent at 300-400 "C for 4 h in the same batch reactor used for BLFO production. The upgrading experiments were also performed in the 1-L, stirred, batch reactor. Table I provides a summary of the conditions tested in these experiments. In the experiments, 100 g of BLFO feedstock was treated except in cases of water addition where 50 g each of water and BLFO were treated. Only 70 g of acid-washed BLFO were treated in those experiments where it was used. Catalyst was used in a 7 g/100 g of BLFO proportion. Initial upgrading experiments were performed with the BLFO, as produced, using the CoMo catalyst and a constant reaction temperature (385 or 405 "C) after heatup. The main body of experiments was carried out using a two-stage thermal profile: a stabilization step at 280 "C (35 min) and a hydrotreatment/ hydrocracking step a t 380 "C (65 min). Three catalysts (CoMo on alumina, NiMo on alumina, and NiMo on zeolite) were tested under these conditions. The effect of sodium on the catalytic hydrotreatment was investigated by two types of experiments. In the first type of experiments, water was added, in equal amounts to BLFO, to the reactor. The aim was the transfer of sodium from the oil to the aqueous phase during the treatment so that it would not poison the catalyst. In the second type of experiments, removal of the sodium before the catalytic treatment was tested by using acidwashed BLFO. Analytical Methods. The moisture content of the oils was measured by Karl-Fischer titration and was also determined by collecting the water phase in the vacuum distillation of the oils. The average of these two measurements was used as the moisture content for calculations on the basis of "dry" materials. Thermal gravimetric analysis (TGA) was used in a standard DIN 51719 method to determine ash. In addition to the ash determination by TGA, a temperature level of 500 "C was used to simulate the Conradson carbon analysis described by Noel? The 500 "C plateau was maintained for 30 min to allow for complete volatilization. Elemental analysis was performed using a microelemental analyzer for carbon, hydrogen, and nitrogen (CHN). The sodiuum was determined by an atomic absorption spectrometer and the -(3) N i l , F. Fuel 1 6 4 , 63, 931.
Elliott and Oasmaa
104 Energy & Fuels, Vol. 5, No. 1, 1991 100 90
80 70 Product Gas (
-
H2)
60
0Product wt%
Water
50 D i s t i l l a t e Residue
40
D i s t i 1l a t e O i 1
30 20
10
+
0
+
t
#a
#12
#I 9
#10
Expe r iment s
Figure 1. Material balances for experiments; yields in w t 70 of maf feedstock. sulfur by inductively coupled plasma-atomic emission spectroscopy. No direct measurement of oxygen was attempted in this study; oxygen content is reported only on a difference basis. The oxygen content of the dry oils was calculated from analysis of the wet oils by the following equation: 0% = [ l o o - ( C %
+H
%
+ N % + S % +ash
(16/18
X
%) -
moist %)]/[lo0 - moist % ]
The molecular weight distributions of the product oils were determined by gel permeation chromatography. The eluent feed was 1.00 mL of tetrahydrofuran (THF)/min. A UV detector was used at a wavelength of 254 nm with a 4-nm window. Polystyrenes were used as the standards. For component identification, a mass-selective detector was used with a gas chromatograph containing a 25-m, HP-1 column (0.32 "-0.17 pm). Helium carrier gas a t 80 kPa was used in a split injection mode. The product gas composition was analyzed by gas chromatography using the method described in a previous p~blication.~ The calculation of the hydrogen consumption was based on the difference between the amount of hydrogen added to the reactor and the amount recovered in the product gas.
Results Summary of Experimental Process Results. Washing the BLFO with dilute acid prior to hydrotreatment caused dramatic physical and chemical changes in the oil product. In the washing procedure, the pH of the acid solution (measured after each wash period) dropped from 6.4 to 0.1 by using three acid-wash steps. The final sodium content of the oil was 0.02 wt % (dry basis). The loss of organics during the acid wash was 16 wt % (from dry ash free BLFO). Analysis of the products from the first two upgrading experiments (no. 2 and 3) suggested that either limited reaction or thermal decomposition had occurred. Vacuum distillable oil was recovered but only in limited quantities, 14 and 18 wt % of the dry, ash-free solids in BLFO fed at 405 and 385 "C, respectively. More net gas and water production was noted at 405 "C than at 385 "C. The (4) AlGn, R.;McKeough, P.;Oasmaa, A.; Johansson, A. J. Wood Chem. Technol. 1989, 9 ( 2 ) , 2657276.
nondistillable residue included catalyst (ground to a powder by the agitator during the experiment), sodium salts, and other low-volatility organics. The hydrotreated oil contained some hydrocarbons but for the most part appeared to be phenolic. Sodium content of the oil and distillate oil yield are the primary quality factors evaluated in this research. When water was mixed with the BLFO before the catalytic hydrotreatment, the sodium was removed from the oil during the hydrotreatment and a higher quality product was obtained than without water addition. Of the original sodium in BLFO, 59%-86% was found in the aqueous phase after the experiment, while the corresponding value was less than 7% in experiments without water addition. The distillate yield was 32% (of the dry, ash-free solids fed) when water was added as in experiment no. 10, but it was only 9% (experiment no. 8) without water addition under similar reaction conditions. The sodium contents of the product oils were 2.4% and 7.2% (dry basis), respectively. The sodium effect on the volatility of the product oil was determined by washing the oil product from experiment no. 8 with acid before a second distillation. Oils were washed with dilute hydrochloric acid prior to the distillation in order to reduce the sodium content and release the phenolic components from the sodium salt form to make them distillable. Even for this sodium-freeproduct oil, the yield of distillate from experiment no. 8 (without water addition) remained lower at 23 wt %. When sodium-free BLFO (acid-washed) was used, the product quality was further improved. The yield of distillate reached 42% (of the dry, ash-free solids fed) in experiment no. 12 and 26.5% in experiment no. 9. These experiments were carried out by using the two-stage temperature profile. The only difference between these runs was the catalyst, which was NiMo on alumina in experiment no. 12 and CoMo on alumina in experiment no. 9. Overall material recoveries of 93-97 wt % were obtained in the experiments. The material balances for some of the experiments are shown in Figure 1. Primary oil yields (organic phase minus catalyst added) ranged from 64 to 87 wt %. The highest oil yields, 84-87 wt %, were obtained in catalytic experiments using the
Catalytic Hydrotreating of Black Liquor Oils
Energy & Fuels, Vol. 5, No. 1, 1991 105
Table 11. Black Liquor Feed Oil (BLFO) Analysis water by distilln, wt % water by K-F, wt % av (K-F and distilln) solubility, wt % MF in THF in hexane in toluene carbon, wt % dry basis hydrogen nitrogen sodium sulfur ash by TGA oxygen (calc) H/C Conradson carbon density, g/mL
BLFO 36.7 33.19 34.9
acid-washed BLFO 32.3 34.6 33.5
61 1 20 66.7 5.4 0.15 7.1
99 8 25 82.2 6.4 0.1 0.087 0.83 3.3 7.9 0.93 43.9 1
1.7
18.5 9.2 0.95 63.6 1.17
two-stage hydrotreatment and BLFO as raw material. The lowest oil yield, 64 wt % , was obtained in the high-temperature (400 "C) catalytic experiment no. 3. When the BLFO was upgraded without a catalyst by using the two-stage hydrotreatment, the oil yield was 74 wt %. When water was added at the same conditions before the catalytic treatment, the oil yields were reduced to 67-73 wt % as more organic was lost into the water phase. In catalytic experiments using the acid-washed BLFO without any extra water feed, primary oil yields of 75-79 wt % were obtained; but, as mentioned above the distillate oil yield was higher with the acid-washed feedstock. Gas yields varied from 3.8 to 12.3 wt % (maf feedstock). The gas yield in the noncatalytic experiment was 4.2 wt % (maf feedstock). When a catalyst was used, the gas yield was 2.2-4.4 wt 90 for both CoMo and NiMo. In water addition tests, it ranged from 3.8 to 6.2 wt %. The effect of residence time on gas production can be seen by comparing experiments no. 13 and 14, both NiMo-catalyzed tests. At longer residence time (245 min), the gas yield was 11.1 wt % compared to 6.2 wt % at the normal residence time (70 min). In the only high-temperature experiment (405 "C, no. 3) the gas yield was also high, 12.3 wt %. Details of Oil Analysis Results. The BFLO product used in the upgrading experiments was the combination of products from a large number of black liquor/sodium hydroxide batch runs performed during the spring of 1989. The analytical results of the black liquor product are given in Table 11. The results of the analyses of the products
from the upgrading experiments are given in Table 111. The sodium balances for the experiments ranged from 86 to 109 wt % . In catalytic experiments using BLFO as raw material, 85-100 w t % of the sodium remained in the product oil. In experiments no. 7 and 8, all autoclave liner contents were collected as a single phase. The sodium balance was 100% in no. 7 and slightly high at 107% in no. 8. Only a trace of sodium (0.01 g out of 5.5 g fed to the reactor) was found in a condensate aqueous phase collected outside the autoclave liner. In the water addition experiments, only 18-36 wt % of sodium remained in the oil phase, i.e., by direct measurement, the sodium content (wet basis) of the product oils ranged from 1.7 to 3.5 wt %, compared to 5.5 wt % in BLFO. On a dry basis, in experiment no. 10 the sodium content in the product oil was reduced to 3.4 wt % from the 7.1 wt % in the BLFO feedstock. Therefore, the sodium analysis indicates that 86% of the sodium fed to the reactor was recovered in the aqueous phase following the experiment. However, the removal value is somewhat overstated since the sodium balance was 110% for the experiment. In the noncatalytic experiment (no. ll),45 wt % of the sodium remained in the product oil. The sodium balance was 86 wt %. An initial attempt to distill BLFO resulted in almost no distillate except for the water in the oil. About 1% of the BLFO was recovered as a lighter than water, immiscible yellow oil. The distillation residue was a solid porous material which did not melt at up to 260 "C. Following acid wash of the BLFO, the distillate characteristics were significantly improved. Distillate oil, amounting to 11% of the dry, acid-washed BLFO, was collected under vacuum (17 mmHg). The effect of acid wash was also seen with the upgraded oils. The amount of distillate improved 2-3 times for products from CoMo-catalyzed experiments at 380 "C with the BLFO feedstock. Upgraded products from experiments using the acid-washed BLFO as feedstock could readily be distilled without further treatment. Up to 48% of the dry, ash-free product (42 wt % from maf feedstock) was distillable under vacuum (13 mmHg) at up to 194 "C from the NiMo-catalyzed upgraded product (no. 12). High yields of distillate were also recovered from the product oils of the water addition experiments. Distillate amounting to 47 wt % from the dry, ash-free product oil was obtained for experiment no. 10 (CoMo on alumina as
Table 111. Analyses of Upgraded Products run no. primary oil water by distilln, wt % water by K-F. wt % av (K-F and distilln) solubility, wt % MF (catal-free) in THF in hexane in toluene carbon, wt % dry basis hydrogen nitrogen sodium sulfur ash by TGA oxygen (calc) oxygen removal, % H/C Conradson carbon (catal-free basis) water phase pH, total water phase sodium, w t %
2
3
7
8
9
10
11
12
13
14
15
16
33.5 34.3 33.9
43.5 36.5 40.0
32.0
30.7 33.2 32.0
35.1 31.6 33.4
31.1 25.4 28.3
23.0
33.4 24.8 29.1
28.2
25.0
28.5 26.3 27.4
31.8
65
90
100 70 92
41
53.8 4.7 0.2 7.0 2.8 37.9 3.5 62.6 1.04
36.2 2.9 0.2 11.5 3.5 55.1 5.7 49.5 0.94
10.6 4.30
22
54.4 5.1 0.1 9.3
52.9 4.9 0.1 10.6
79.7 6.7 0.1
32.6 7.8 15.7
37.5 4.6 50.8 1.10 69.1 58.2
12.8 0.7 80.5 1.00 62.7 60.4
31.2 -0.6
17.5
3.2
1.12
61.7 53.6
1.34 60.3 60.3
0.10
2.6 0.06
9.9 3.80
10.3 11.90
1.11
64.7 54.2 10.7 2.40
ao 32 46 63.3 6.0 0.1 3.3
8.2
69 73.5 6.4 0.0 2.9
77.4 6.7 0.0 2.3
53.9 5.0 0.0 4.8
57.8 5.3 0.1
24.1 6.6
17.1 2.9 72.1 1.03 52.2 52.2
14.5 1.3 87.4 1.03 40.1 40.1
35.0 6.0 45.5 1.10 53.2 40.3
33.2 3.6 61.8 1.10 64.7 53.6
3.0 0.07
9.7 3.20
9.7 2.90
9.6 2.70
10.6 3.90
51.4 5.8 0.0 4.5
106 Energy & Fuels, Vol. 5, No.
Elliott and Oasmaa
I, 1991
Table IV. Malor Comwnentr Identified by Marr S m t r o m e t w hvdrotreated black liauor feed oil black liquor feed oil light distillate heavy distillate methyltetralin toluene dimethylcatechol methylnaphthalene methylindan trimethylcatechol ethylmethylcyclopente dimethylphenol ethylguaiacol tetralin methylnaphthalene ethylcatechol tetralin methylguaiacol methylindan dimethylnaphthalene pcresol methylpropylbenzene C8 cyclic hydrocarbon dimethylnaphthalene dimethylphenol ethylmethylcyclohexane methylethylphenol methylnaphthol methylethylphenol dimethylindan dimethylindan indan dimethylphenol ethylphenol methylethylphenol ethylmethylphenol dimethylindan methylcyclohexane ethylnaphthalene methylcatechol dimethylphenol C2-benzothiophene propylcyclohexane dimethylindan methylethylphenol dimethylcyclohexane diethylphenol indanol dimethylindan methylcyclohexanol dimethylindan methylcatechol ethylcyclopentane ethyltetralin alkylbanzbnaphthalene thiophenes" H8-pentalenea methyltetralin C3-C5 tetralinsa dihydroxybenzpropylbenzene C6-C7 benzenes" aldehydea C5-C8 benzenesa chlorophenol" C H I 0 hydrocarbonsn dimethylindenol" (diunsaturated) tolueneb alkylcyclopentaCW10 tolueneb nonesb cyclohydrocarbonsa phenolb cresolsb ethylphenolb C8-C11 methylindansb cyclohydrocarbons" methylindansb C2-C4 benzenesb alkylphenolsb alkylphenolsb alkylphenolsb alkyltetralinsb alkylcatecholsb di- and trimethylindansb methylnaphthalenesb ethylbenzaldehydeb alkyltetralinsb C 6 4 7 benzenesb naphtholb methylnaphthalenesb methylbiphenylb alkylbenzoC3-benzothiopheneb thiophenesb methylnaphtholsb alkylbenzenesb Tentative identification. bMinor peaks.
a catalyst) product and 41.5 wt 5% for experiment no. 15 (NiMo on zeolite) product. The corresponding yields from moisture and ash-free feedstock were 32 and 27 wt %, respectively. Three distillate oil samples were analyzed by gas chromatography-mass spectrometry to identify components and semiquantitatively determine their abundance. The three samples were two distillate fractions from the experiment no. 2 product oil (acid-washed)and the distillate from the acid-washed BLFO. Without the acid wash, only traces of volatile organics were recoverable from the BLFO, and the heavy distillate fraction from the no. 2 product oil was not distillable. Each sample was a complex mixture containing several hundred components. For example, chromatography of the heavy distillate from experiment no. 2 with a massselective detector was used to identify 94 compounds, but 155 other peaks (too small or too broad to integrate) could also be counted in the chromatograph. The 30 largest
peaks (as determined by total ion current) are listed in Table IV for each of the samples. Numerous other peaks were also identified for each of the samples. The solubility of the BLFO in THF is 61 wt % dry basis and 99 wt % for the ash-free (acid-washed) BLFO. The solubility in hexane is 1 wt 9% for BLFO and 8 wt % for the acid-washed BLFO. The thermal treatment (280 OC/380 "C, uncatalyzed) of BLFO changes the product oil so that the solubility in hexane increases to 22 wt % dry basis. The solubility of the product oil in THF is 80 wt % dry basis and 91 wt % for the ash-free (acid-washed)product. Following catalytic (NiMo) experiment no. 12, with acid-washed BLFO, a hexane solubility of 70 wt % was measured for the product. The oil was totally soluble in THF. Following catalytic (NiMo) experiment no. 13 with water addition, a hexane solubility of 41 wt % (dry basis) was measured in the product. In the similar experiment, no. 10, with CoMo as the catalyst, the hexane solubility of the product oil was 32 wt %. Molecular weight distributions for the raw material and some product oils are presented in Table V. The molecular weight distribution was determined for the BLFO and for the acid-washed BLFO. The material loss in acid wash was 16 wt 5% of ash-free oil. This material probably consisted of acid-soluble,low molecular weight compounds. Of the 61 wt % portion of BLFO soluble in THF, 18% was below a molecular weight of 200. Twelve percent of the acid-washed BFLO (99 wt % soluble in THF) was below a molecular weight of 200,60% of the THF-soluble portion of BLFO was below a molecular weight of 500, as was 50% of the acid-washed BFLO. The thermal (uncatalyzed) treatment of BLFO did not change the molecular weight distribution of the oil. Sixty percent of the oil was under a molecular weight of 500. The molecular weight distribution was similar for the acid-washed oil. The lowest molecular weight product was obtained by using sodiumfree BLFO. In the experiment no. 9 product (90 wt % THF solubility), 78% of the oil was under a molecular weight of 500, as was 88% in experiment no. 12 (100% THF solubility). Details of Product Gas Analysis Results. Mass spectrometry was used to identify the numerous components in the gas-phase products. Most of the gas product was residual, unreacted hydrogen; the more numerous components were the easily identified fixed gaseous components including carbon dioxide, methane, hydrogen sulfide, ethane, ethylene, propane, and propylene. However, there were numerous components of higher molecular weight which when considered in total comprised a significant portion of the gaseous product. The components were found to be hydrocarbons for the most part but also included a significant amount of dimethyl sulfide and trace amounts of dimethyl ether, acetone, and methyl ethyl ketone. These higher molecular weight components were quantified by using a gas chromatograph with a flame ionization detector. Dimethyl sulfide was quantified
Table V. Molecular Weight Distribution for Black Liquor Oil and Upgraded Products mol w t distribn, wt %, in each range sodium, T H F solubility, sample wt % MF wt % MF below200 200-500 500-2000 2oo(t5000 black liquor feed oil 7.10 61 17 43 36 4 acid washed 0.09 99 12 31 49 run no. 8, product oil 10.60 65 21 41 36 2 run no. 9, product oil 0.00 90 37 41 21 1 run no. 11, product oil 4.50 80 20 41 37 3 acid washed 1.10 91 17 39 40 4 run no. 12, product oil 0.00 100 48 40 12 0 run no. 16, product oil 69 27 42 30 1
s
5000-9000 0 1 0 0 0 0 0 0
Catalytic Hydrotreating of Black Liquor Oils
separately by using a flame photometric detector. Butane and butylene compounds (five separate peaks) were grouped together for reporting purposes. Components containing five or more carbons were grouped separately. This group included saturated and unsaturated hydrocarbons, both cyclic and noncyclic, including toluene (30 and 60 ppm for catalyzed and uncatalyzed cases, respectively) and benzene (10 and 20 ppm for catalyzed and uncatalyzed cases, respectively). The mass spectral analysis indicated that the components were clearly gasoline range material, containing from five to nine carbon atoms. Over 50 peaks were identified in this group of compounds. Other types of quantitative relationships among different component groups in the gas product provided useful indicators of the reaction tendencies in the system. The saturate to unsaturate mole ratios were higher for the catalyzed cases (suggestingcatalytic hydrogenation), with the acid-washed feedstock tests having the highest ratios of saturates to olefins. The ratios increased from fourcarbon compounds down to two-carbon compounds. The ratio was around 5 for C4sin catalyzed experiments, 25-35 for C3s, and 100 for C2s. The uncatalyzed experiment produced ratios of 3, 9, and 30 for C4s, C3s, and C2s, respectively. An unexpected result was that in the experiment with added water (no. lo), the ratios were even lower than in the uncatalyzed experiment for all C4to C2compounds. Apparently the excess water interfered with the gas-phase hydrogenation mechanism. The shift in products may be a dilution effect resulting from the larger amount of water vapor in the reactor. For five-carbon compounds, the ratio of saturates to olefins ranged from 2 for the uncatalyzed case to 4 for the catalyzed experiment with the BLFO to 5-6 for the experiments with the acidwashed BLFO as feedstock. For the six-carbon compounds, the ratio of saturate to aromatic ranged from 3 for the uncatalyzed case to 4 for the catalyzed experiment with BLFO to 6-8 for the experiments with acid-washed BLFO as the feedstock. In the experiments using acid-washed BLFO as the feedstock, both carbon dioxide and hydrogen sulfide were recovered in the gas product. In these experiments, the amount of sodium in the feedstock, 0.09 wt % , apparently was too small to scrub all the acid gas products. Gas yields based on these experiments show that about 0.017 g of C02/g of maf acid-washed BLFO is produced. Applying this result to a typical experiment with BLFO as the feedstock shows that 0.90 g of C02would have been produced in the experiment. Since none of this carbon dioxide was detected in the product gas, apparently it reacted with sodium to produce sodium carbonate. The required amount of sodium for such reaction would be only 0.94 g out of the 5.5 g included in the feedstock. Discussion Initial upgrading experiments were performed with the BLFO, as-produced, including high levels of water, sodium, and sulfur. Only the sodium was perceived as a potential catalyst poison. High water levels had been present in other biomass oils that had been successfully upgraded by catalytic h y d r ~ t r e a t i n g .The ~ ~ ~sulfur was viewed as potentially helpful in maintaining the catalyst in an active, sulfided state. The first two tests with the CoMo hydrotreating catalyst at 385 and 405 "C (experiments no. 2 and 3) were maintained at these temperatures for 2 h. A strong endothermic reaction was detected during the heatup phase of the experiments, which became noticeable when the reactor heater reached 460 "C and the reactor contents were at
Energy & Fuels, Vol. 5, No. 1, 1991 107 350 "C. This reaction was unusual because hydrotreating and hydrocracking reactions are exothermic, especially hydrodeoxygenation. The pressure and temperature/time profiles showed that during the period of endothermicity there was a large amount of gas production beginning at about 250 "C (indicated by increased pressure in the reactor, higher than that generated from thermal expansion), which essentially ceased by the time the reactor had reached the operating temperature. During the time at operating temperature, there was a small net gas consumption at 385 "C and a small net gas production at 405 "C. Sufficient hydrogen (>85 vol 'YO) remained in the product gas to indicate that the reaction was not hydrogen limited. One interpretation of these results is that thermal decomposition of the BLFO occurred and that the amount of hydrotreating was small especially once the reactor had reached the operating temperature. Since the hydrotreating reactions were believed to be slow (requiring several hours), it appeared that the catalyst did not maintain its activity during the experiment. The lack of pressure drop during the experiment suggested that the reaction may have ceased. This apparent thermal decomposition was addressed in the later experiments performed with a two-stage temperature profile. Such a process has been designed for the thermally unstable wood pyrolysis o i h 7 Low-temperature hydrotreatment was found to stabilize the pyrolyzates and minimize thermal decomposition during subsequent higher temperature hydrotreatment. By this process, catalyst activity was maintained while minimizing coke formation. Product Properties. Carbon, hydrogen, and oxygen were the primary components of the BLFO and were the primary targets of the hydroprocessing. The analytical method used for microelemental analysis effectively measured the carbon, hydrogen, and nitrogen in the BLFO, and hydrotreated BLFO products with the inorganics, primarily sodium, remaining as a carbonate in the residue. The carbon contents of the oils given in Table I1 were assumed to represent the organic portion and would be the material available for the hydroprocessing reactions. As in the microelemental analysis, the sodium was believed to be converted to carbonate in the TGA procedure used for ash determination. Direct determination of the sodium content of the BLFO yielded 7.1 wt %. If all the sodium were converted to carbonate, an additional 7.4 parts of oxygen and 1.9 parts of carbon would be included with the sodium, based on the formula weight. The total of these, 16.4, is a close approximation to the 18.5 wt 5% ash measured in the BLFO. The amount of sodium originally present as the carbonate in the BLFO is not easily measured and is currently only estimated at about 3 / 4 of the total sodium. The hydrogen analysis method used did not distinguish between the hydrogen contained in the organic components and the hydrogen contained in water in the oils. Therefore, the hydrogen analysis had to be corrected to a "dry basis" by subtracting the hydrogen which was determined to be present in the form of water in the oils. The oxygen content of the oils was calculated based on analysis of wet oils and then calculated to a dry basis by (5) Elliott, D. C.; Baker, E. G. In Energy from Biomass & Wastes X, Klass, D. L., Ed.;Institute of Gas Technology: Chicago, 1987; pp 765-784. (6) Baker, E. G.; Elliott, D. C. In Research in Thermochemical Biomass Conuersion;Bridgwater, A. V., Kuester, J. L., Eda.; Elsevier Science: Barking, England, 1988; pp 883-895. (7) Elliott, D. C.; Baker, E. G. 'Process For Upgrading Biomass Pyrolyzates". U S . Patent No. 4,795,841, issued January 3, 1989, filed April 2, 1987.
108 Energy & Fuels, Vol. 5, No. 1, 1991
subtraction of the oxygen contained in the moisture in the oil. Since the ash number is used in the calculation of the oxygen, the oxygen number is effectively the organic oxygen and is that which must be removed in the hydroprocessing reactions. Additional oxygen is found, in the BLFO for example, as inorganic forms such as carbonate. But since such oxygen cannot effectively be removed by hydroprocessing, it is more appropriate to separately quantify such oxygen with the ash and not consider it in the calculations of hydrodeoxygenation or yield. Unfortunately, with this method of oxygen determination, the results are subject to significant error as demonstrated by the analyses of the no. 9 primary oil product, which suggest that the oxygen content is only 0.7%. Reducing the moisture content by only 1.8% (the use of the Karl Fischer result versus the use of the average moisture content), from 33.4% to 31.6%,would increase the oxygen content of the oil product to 2.8%, a fourfold increase in the oxygen content. A t the same time the hydrogen to carbon ratio would increase from 1.00 to 1.06. Such results underscore the need for accurate, direct measurement of oxygen, especially since the oxygen removal is an important consideration of the processing. Nitrogen and sulfur were found at low concentrations in the oils. The amount of nitrogen measured was near the limit of detection of the instrument and was not considered reliable. The presence of nitrogen was not significant relative to oxygen and sulfur, and its removal was not considered in detail. Analysis indicated a significant removal of sulfur during the upgrading experiments. No correction was attempted for sulfur present as inorganic salts as opposed to organic sulfur, which could be removed by hydroprocessing. Solid precipitates were recovered from the first two upgrading experiments at 385 and 405 "C. The precipitates appeared to form at the level of the liquid/gas interface. Samples from experiment no. 2 (385 "C) were analyzed in some detail. The sodium content of the solid was 7.3% compared with 5.7% in the upgraded primary product oil. The sulfur content was 1.7% in the solid compared with 2.3% in the primary oil product. The aluminum, cobalt, and molybdenum contents were 1.7%, 0.14%,and 0.55%,respectively. For each of these three metal concentrations, a representative amount of hydrotreating catalyst in the solid deposit was calculated; 4.2%, 4.0%,and 4.3% by weight of catalyst in the solid deposit was suggested by the amount of Al, Co, and Mo, respectively. These numbers compare with a 6.5 wt % concentration of the catalyst in the ariginal charge to the reactor. From these numbers it appears that the hydrotreating catalyst does not represent a disproportionately large amount of the solid deposits, and therefore it is not a likely cause of the deposit formation but only a component. The sodium, on the other hand, is slightly more concentrated in the deposit than in the feedstock or oil product, suggesting that it is a cause of the formation. However, the major component in the deposits remains organic material. The distillate of the BLFO represented 11% of the dry, acid-washed BLFO and consisted primarily of phenols of varying complexity. Hence, it is not surprising that this material could not be recovered by distillation before the acid wash, which liberated the phenols from the sodium. Traces of cyclopentanones were measured, and these components probably represent the small amount of distillate recoverable before the acid wash. The cyclopentanones were apparently formed by the thermal degradation of cellulosics in an alkaline environment (pH > 4).8 The amount of complexity in the phenolics, as rep-
Elliott and Oasmaa
resented by the degree of oxidation, is as expected for this type of product containing about 20% oxygen. Much of the original methoxyl and carbonyl structure derived from the lignin has been lost, and more condensed aromatic structures have been formed. The phenolic components appear to be similar to those found in the high-temperature (600-650 "C) flash pyrolysis of wood.g However, there appears to be a much larger fraction of higher molecular weight, nondistillable Components. The presence of a significant fraction of benzothiophenes in the BLFO is another major difference from the wood pyrolyzates. The distillate fractions from the upgraded product (6% and 11% of the dry, acid-washed product in the light and heavy distillates, respectively) contained components showing the effects of the catalytic hydrotreating. Aromatic hydrocarbon formation from the BLFO could be derived from either thermal reactions or catalytic hydrodeoxygenation, but the formation of hydroaromatics and saturated cyclic hydrocarbons could only result from catalytic hydrogenation. Little gasoline range hydrocarbon was produced; much of the distillate was higher boiling. The higher boiling components appeared to be a mixture of higher molecular weight hydrocarbons and phenolics. Further hydroprocessing may or may not be necessary to produce useful fuels from these components. If gasoline is the desired product, a more severe, hydrocracking-type catalytic process would be indicated. The quality of the upgraded oil was significantly improved compared with the feedstock since the benzothiophenes were much less evident. The sulfided CoMo catalyst used in this upgrading test is considered the optimum catalyst for the hydrotreatment of this type of sulfur-containing compound. The possible identification of the chlorinated phenolic in the distillate raises the issue of the reaction of the product oil with the acid used to remove the sodium. The presence of chlorinated compounds in the product could have severely negative impacts on the further upgrading or utilization of the oil. Hydrotreating Implications. The oxygen content in the BLFO was determined to be 9.2 wt % on a dry, ashfree basis. The upgraded product oils contained varying levels of oxygen depending on catalyst and the feedstock form (as-received BLFO or acid-washed BLFO). The typical result from CoMo-catalyzed hydrotreatment of BLFO was a product containing 4% oxygen representing 58% HDO. A shortened reaction time h vs 4 h) resulted in only 16% HDO. When the acid-washed BLFO was used as the feedstock, 80% HDO was achieved with the CoMo or NiMo (on alumina) catalysts at 380 "C. About the same HDO level was achieved also in water addition runs with CoMo or NiMo (on alumina) as a catalyst. The sulfur content of the BLFO was measured at 1.3 wt % (1.7 wt % on a dry, ash-free basis). Analyses of sulfur content in two upgraded primary oil products showed 2.3 w t % and 2.5 wt % for experiment no. 2 and experiment no. 3 products, respectively. The primary oil products contained catalyst particles, which included metal sulfides. On the basis of mass recoveries of these two products and correcting for the amount of sulfided catalyst, the amount (8)Nelson, D. A.; Hallen, R. T.; Theander, 0. In Pyrolysis Oils from Biomass: Producing, Analyzing and Upgrading; ACS Symposium Series 376; Soltea, E. J., Milne, T. A., Eds.; American Chemical Society: Washinen, DC,1988;pp 113-118: (9) Elliott, D. C.. In Pyrolysis 0th from Biomass: Producing, A M lyring and Upgrading; ACS Symposium Series 376; Soltea, E. J., Milne, T. A., Eds.; American Chemical Society: Washington, DC, 1989; pp 55-65.
Catalytic Hydrotreating of Black Liquor Oils
of sulfur was reduced in the catalyst-free product by 60% and 14% in the 405 and 385 "C tests, respectively. Additional HDS may actually have occurred as the hydrogen sulfide gas produced in the reaction was apparently scrubbed by the alkali in the aqueous phase in the reactor, and some of this aqueous phase remained in the primary oil product. No hydrogen sulfide was measured in the gas product except in the tests performed with the alkali-free (acid-washed) BLFO. The experimental product gas consisted primarily of hydrogen residual gas from the initial feed to the reactor. Usually the hydrogen concentration was in excess of 95 mol 7%. The high concentration of residual hydrogen indicated that a large excess of hydrogen was added to the reactor initially. Excess hydrogen feed is appropriate for this type of reaction. The primary gaseous products from the hydroprocessing reactions were saturated hydrocarbons. Methane was present, typically, at 1-2%, ethane at less than 1% ,and higher hydrocarbons in lower concentrations. Carbon dioxide was believed to be a major product gas from these reactions based on previous work in hydrotreating wood-derived oils. However, it was only recovered in tests where the sodium had been removed from the feedstock oil and in the longer term test of NiMo catalyst with BLFO. In the latter test, the gas production became large enough that, apparently, all sodium had reacted and excess COz was recovered in the gas product. The most important acid gases produced in these experiments were carbon dioxide and hydrogen sulfide. In a typical experiment in which BLFO was used as the feedstock, these two product gases were not present in the gas-phase product. Apparently, the sodium in the aqueous phase product effectively "scrubbed" these two gases from the gas product by an acid/base neutralization to form sodium carbonate and sodium sulfide. The actual form of the reactive sodium was uncertain but could either be sodium hydroxide existing in the BLFO or sodium liberated by hydrotreatment of organosodium salts, or the sodium salts themselves might have acted as weak bases which reacted with the acid gases to generate free organic acids or phenolates. Hydrogen consumption is an important consideration in the hydrotreating not only as a primary indicator of the extent of reaction but also because hydrogen consumption is the major cost consideration for the process. In these experiments hydrogen is consumed primarily for heteroatom (oxygen, sulfur, and nitrogen) removal [hydrotreating], with some molecular weight reduction [hydrocracking] and aromatic saturation [hydrogenation]to lesser degrees. Hydrogen consumption of about 1 g/100 g of BLFO is typical for these experiments. A calculation shows that for a typical experiment wherein 65.1 g of BLFO (dry basis) is fed to the reactor, 6.02 g of oxygen, 1.11 g of sulfur, and 0.10 g of nitrogen are believed to be bound organically in the oil feedstock and must be removed by hydrotreatment. These amounts of heteroatoms are equivalent to 0.42 mol of hydrogen gas or 0.85 g. This calculation shows that most of the amount of hydrogen consumption measured in the experiments is reasonably represented by the hydrotreating requirement. The balance of the hydrogen consumption is apparently utilized in the other hydrocracking and hydrogenation reactions.
Energy 13Fuels, Vol. 5, No. 1, 1991 109
Hydrogen consumption in these experiments was calculated because of its importance in the cost of the process. Hydrogen consumption was determined to be low to moderate relative to typical petroleum processing requirements. For example, in experiment no. 12, with NiMo catalyst and acid-washed feedstock, the hydrogen consumption was 1-01g/44.2 g of maf feedstock or 297 L/L of maf oil processed. These results can be compared with wood-oil hydrotreating results.' The comparison suggests that similar amounts of reaction are achieved at the given equivalent reaction time, based on hydrogen consumption and product properties. In hydrotreating, the source of hydrogen must be considered. An obvious route to hydrogen in this reaction system is the steam-reforming of the byproduct hydrocarbon gases (C, to C4).One way to balance the hydrocarbon gases with the hydrogen requirement is to vary the operating temperature because the gas production by pyrolysis is strongly a function of temperature and increases more quickly than the rate of the hydrogenation reactions. Summary of Issues The following important issues were addressed by this research: The organics in kraft black liquor were successfully concentrated by a high-pressure liquefaction process and then catalytically hydrotreated to produce high yields of useful hydrocarbon oil. The catalytic hydrotreating process was used to increase the hydrogen to carbon ratio; reduce the oxygen, sulfur, and nitrogen contents; remove the sodium by phase separation or acid wash; lower the temperature of the boiling range of the product oil; and produce a thermally stable, low-viscosity, low-ash-content product oil. The hydrogen consumption was determined to be low. The negative effect of the sodium on the product recovery was demonstrated, including water solubility of the oil, reduction of volatility of the sodium salts, and the formation of solid byproducts. Removing the sodium from the BLFO before hydrotreating was found to be highly beneficial. As an alternative, adding extra water with the BLFO resulted in a cleaner hydrotreated product with sodium dissolved primarily in the aqueous phase after hydrotreating. Effects of processing parameters, including the catalyst composition and operating temperature, were assessed. The NiMo and CoMo catalysts exhibited similar activities. The temperature limitation to 380 "C resulted in less gas and water production while allowing sufficient hydrotreating activity. Acknowledgment. The financial support from the JALO Research Program is gratefully acknowledged. Thanks are expressed to Dr. Paterson McKeough, Project Leader, and to technicians Mr. Reijo Hakkinen, Mr. Olavi Ruotsalainen, Ms. Mirva Mattila, and Ms. Outi Kovanen for their assistance with the experimental work. Thanks are also due to the personnel of the analytical department of the Laboratory of Fuel and Process Technology. Registry No. COO,1307-96-6; Moo3, 1313-27-5;Ni,7440-02-0; Mo, 7439-98-7.
