Catalytic Hydrogenation and Hydrocracking of Oxygenated Compounds to Liquid and Gaseous Fuels Dharam V. Gupta,' Wilmer L. Kranlch, and Alvin H. Welss' Department of ChemicalEngineering, Worcester Polytechnic Institute, Worcester, Massachusetts 0 1609
In the presence of hydrogen and a paraffin oil medium both cy-cellulose and powdered newspaper react catalytically over nickel crystallites (prepared in situ from nickel hydroxide) through a set of first order unidirectiongases. The rate law followed between 343 and 482 OC liquid oils al steps: cellulose pyrobitumen when hydrogen partial pressure is above 54 atm is dxldt = -klCat(l x) and dLldt = klC,,t(l x) k2CCat.Lwhere x = fraction of carbon in cellulose converted to liquid and gases, L = fraction of carbon as liquid fuels, and C,,, = wt of nickel/wt of cellulose. The value of the frequency factor for kl is 1.55 X lo7 (wt of cell/wt of cat.) (l/h) and the activation energy is 17.2 kcal/mol. The ratio &*/&I is approximately 0.7. The reaction mechanism is postulated to be dissociation of hydrogen at the nickel metal sites followed by reaction with partially pyrolyzed cellulose fragments which have diffused through the liquid reaction medium. The zeroorder behavior in hydrogen is lost below 54 atm where the limited availability of hydrogen becomes significant. The liquid phase hydrogenation reaction can be optimized to produce liquid or high Btu gaseous fuels from cellulose or pyrobitumens from other sources. It is believed that the mechanisms and kinetics of hydrogenation of pyrobitumens formed from cellulose are directly applicable to the hydrogasiflcation/hydroliquefactionof pyrobitumens containing lesser amounts of oxygen, e.g., coal, shale oil, peat, lignite, etc. +
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Introduction Research that has been in progress for a number of years a t Worcester Polytechnic Institute for the purpose of developing a process to hydrogenate municipal solid waste to a usable liquid fuel has been reported previously by Weiss (1974) and Kaufman et al. (1974). Both powdered newspaper and pure a-cellulose were used as solid waste simulants, and detailed experimental procedures are given by Kaufman et al. (1974). The organic portions of solid waste are quite analogous to the two materials used. In the course of the research it was found that no differences in behavior could be assessed between cellulose and newsprint, and that data from batch or continuous systems could be interchanged. A background in the technology and chemistry in this,area can be obtained from review papers,by Reese et al. (1972) and Weiss (1972). In the process research, powdered newspaper was slurried in paraffin oil and mixed with 0.2 wt % nickel hydroxide catalyst. This material was then pumped at about 425 "C and 1000 psig into a continuous stirred tank reactor. Since reaction conditions were effectively those of the water-gas generation and shift reaction, about 50% of the hydrogen requirements of the process were met by in situ hydrogen generation. It was found by Kaufman, et al. that the reactions that occur were actually consecutive cellclose
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pyrobitumens
-oil
gas
The maximum liquid yield was found to be 45 wt %. By increasing the reaction severity, the process can, in principle, be adapted to make high Btu gas (e.g., SNG) rather than liquid, by recycling to extinction the liquid that is the slurrying carrier. Thermodynamic Limitations Since one of the key economic points of any such process is to generate hydrogen in situ and to minimize the size of a separate water-gas generation and shift reaction system, it Present address: American Cyanamid Corp., Bound Brook, N.J. 256
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 2, 1976
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is interesting to consider equilibrium effects. Free energy minimization techniques were used to calculate the equilibrium composition of mixtures using 6C 5&0 as a cellulose simulant. Water must be present for in situ hydrogen production; when it is present in excess it eliminates the presence of solid carbon. Figure 1 shows that in the absence of excess water, elemental carbon is present at thermodynamic equilibrium in a thermal pyrolysis process. The only hydrocarbon that appears in anything more than trace quantities is methane. Figure 1 shows that equilibrium highly favors the gasification of carbohydrate in the presence of hydrogen and water. In Figure 2 it is shown that pressure per se is not a necessity for obtaining high yield of methane from carbohydrate a t equilibrium. When water and hydrogen in excess are present in the system, practically complete equilibrium conversion of solid carbohydrate to methane (rather than carbon) is possible. The problem, of course, is to catalyze the rate a t which the process proceeds toward equilibrium.
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Catalyst N a t u r e Figure 3 shows results obtained in catalyzing such a process. There are really two types of catalyst: those which a t the reaction conditions reduce to the metallic state, and those that do not. The process of cellulose hydrogenation is the well-known conventional type of catalytic hydrogenation using a metal catalyst, in this case nickel. Nickel was chosen because in an ultimate process it could be recovered magnetically and recycled. To be certain that it was indeed nickel metal (since a slurry of dried nickel hydroxide was actually charged to the reactors) x-ray diffractograms were made of both the charge material in the reactor slurry and the sludge from reactor products. Peaks on the diffractograms corresponded exactly to those expected for nickel hydroxide in the charge and pure nickel in the product sludge. Neutron activation analysis showed that the nickel hydroxide used contained the expected 58 wt % nickel. A differential scanning calorimeter thermogram at 54.4 atm of H2 showed that Ni(OH)2 reduced to Ni at 325 "C, well below the temperatures used in our reaction studies (Figure 4).
Catalyst S c r e e n i n g At L o w S e v e r i t y 3862
16‘C,44Of55Atm
H,
5Wi0OnNewspopOr
9
Carbon Conv
8
7
c 6 v)
> J
5
2
4
a V
3
2 I
b
I
I
I
20
40
CONVERSION
1
I
300
400
01
I 500
600
TEMP.
C
I IO
%
Figure 3. Catalyst screening a t low severity shows high carbon conversions to be possible with reduced metals.
I
I
700
800
D S C T h e r m o g r a m Showing R e d u c t i o n O f Nickel Hydroxide
Figure 1. Equilibrium conversion as function of temperature of solid carbon in 6C
I
80
60
+ 5H20 to gases. -0
O R 16C+17Hp01 O R ( 6 C t 5 H 2 0 t 1 2 H 2 1 0
I
I
H 2 Pressure
!I
54.42 A i m .
I
I
I
-
EXO
q 0
70
ENDO
I
I
I
E m p t y Pane
7
100
200
3 00
TEMP.
400
:
‘C
Figure 4. DSC thermogram of nickel hydroxide in hydrogen shows metal to be in reduced form at the conditions of this study.
01 0
50
TOTAL
75
IO0
PRESSURE (Atm. )
Figure 2. Equilibrium conversion as function of pressure of solid carbon in 6C HzO to gases.
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Reaction Kinetics
Before proceeding to provide a background in the concepts developed on the conversion of oxygenated materials t o synthetic natural gas and liquid, it is necessary to clearly define terms to be used. Conversion can be measured by conventional analytical techniques in two ways: by analyzing the carbon in the material that is charged to a reactor and the carbon remaining in the solid product from that reactor after benzene treatment. The fraction of carbon removed based on this elemental analysis is defined as carbon conversion, X. The second technique for measuring conversion is DAFB (dry ash free basis) conversion, XD.The material that is charged to the reactor is treated with benzene and the product is also treated with benzene. Any ash-free material that is not benzene soluble then is called pyrobitumen [In
the strict definition, asphaltic (e.g., oil shale) and non-asphaltic (e.g., peat, lignite, and coal) pyrobitumens are infusible solids, insoluble in carbon disulfide. On heating, pyrobitumens generate cokes and fusible bitumens (e.g., petroleum and asphalt) that are soluble.] and is used as the basis to calculate dry ash-free basis conversion. We then define (1 - XD)as the fraction of pyrobitumens remaining, that is organic material which is not benzene soluble. Under this broader definition, the term pyrobitumens includes cellulose that was thermally converted to other solid carbonaceous material. Any such material remaining when reaction has been completed represents solid unwanted byproduct. Oil and gas combined are products that are wanted. The ratio of X to (1 - XD) at the end of reaction then becomes a “selectivity” figure that is desirable to maximize, namely, the ratio of oil and gas produced in the process to pyrobitumens remaining. In other words, the goal of the process is not to make a benzene-insoluble char, but rather a hydrocarbon liquid or gas. Note the precise definition here of pyrobitumen as material that is not soluble in benzene and which contains carbon, hydrogen, and oxygen, There is no intent to limit the relative proportions of these elements in the definition. When the term pyrobitumen is used, it can be any composition of C, H, and 0 and, in fact, any varying composition. This key concept then allows development of a fundamenInd. Eng. Chem.. Process Des. Dev., Vol. 15,No. 2, 1976
257
Table I. Low Conversiona Conversions % Solid feed
Carbon X
DAFB X D
X / ( 1- XD )
a-Cellulose Powdered newspaper
32.4 33.1
64.5 60.9
0.91 0.85
a
H, flow batch reactor, 427 "C; H, partial pressure, 53.7
+ 1.0 atm; holding time, 30 min; cat. loading, 0.5 wt
% Ni
on celhlose. Table 11. High Conversiona Conversions % Solid feed
Carbon X
DAFB XD
X/(1 - X D )
a-Cellulose Powdered newspaper
62.7 65.6
79.8 84.5
3.10 4.23
a Rocking autoclave, 427 "C; initial H, pressure, 47.6 atm; holding time, 60 min; cat. loading, 3.0 wt % Ni on cellulose.
tion predominates over catalytic reaction, and little gas and oil are formed. This is shown in Table V. It is possible to separate the thermal, hydrogen, and catalytic effects by comparing the data of Tables VI and VII. In each of the experiments described, the batches were heated over a period of 100 min from room temperature to 427 "C, then immediately cooled. In some of the experiments the heating and cooling step was then repeated once with addition of catalyst or change of gas. There was very low carbon conversion and no significant difference between heating in NZand H2 in the absence of a catalyst as shown by the experiments of Table VI. Thus there is no significant thermal hydrogenation. Table VI also shows that in the absence of hydrogen the catalyst has a negligible effect on conversion (i.e., nickel does not behave importantly as a cracking catalyst at the indicated conditions). The fact that thermal reactions are effectively completed within a single 100-min heat-up period is shown in Table VII. The results of a single 100-min heat-up in hydrogen with catalyst are essentially the same as those obtained when the catalyzed heat-up is preceded by a second, but
Table 111. Intimate Catalyst-Cellulose Contact Is Not Advantageousa Conversions % Form of catalyst
Carbon X
Nickel acetate impregnated Nickel hydroxide gel
41.2 62.2
Stirred autoclave DAFB XD X / ( 1 - X D ) 67.3 80.6
Carbon X
1.25 3.21
43.2 62.7
Rocking autoclave DAFB X D X / ( l - X D ) 70.4 79.8
1.45 3.10
a427 'C; 60 min. hold; 3.0% Ni on cellulose; initial H, pressure 47.6 atm. tal understanding of pyrobitumen hydrogenation and permits relating our findings with cellulose and powdered newspaper to other pyrobitumens such as coal, peat, and lignite. Most of the kinetic data presented in this paper were obtained in batch reactors-either a 1-1. stirred autoclave or a 750-ml rocking autoclave. The ground cellulose was dispersed as a 30% slurry in a white paraffinic mineral oil carrier. In most of the experiments the reactor and slurry were heated to 427 OC over a period of about 100 min. During this period no catalytic hydrogenation occurred. The reactor was then quickly pressurized with hydrogen (to about 50 atm) and retained at the desired temperature for the stated "holding time". Under these conditions about 8% of the carrier oil decomposes-principally to hydrocarbon gases. Details are provided by Kaufman et al. (1974). Tables I and I1 present data obtained a t both low and high conversions which show the equivalency of newspaper and a-cellulose as reactant. Table I11 shows the results of experiments in which acellulose was first impregnated with nickel acetate and then mixed into oil for hydrogenation as compared with experiments in which the a-cellulose was mixed with nickel hydroxide and then slurried in the oil. A far higher conversion results in the latter case a t the same operating conditions, even though the catalyst to cellulose physical intimacy is much poorer. The mechanism of the reaction is probably conventional dissociation of hydrogen on nickel crystallites followed by reaction of the adsorbed hydrogen atoms with partially pyrolyzed cellulose fragments which have diffused through the oil medium. The presence of nickel as crystallites rather than monodispersed appears essential. Table IV shows the effect of catalyst loading on conversion. Catalyst is required to produce oil and gas rather than char. When temperatures are too low, pyrobitumen forma258
Ind. Eng. Chem., Process Des. Dev., Vol. 15, No. 2, 1976
Table IV. Increased Catalyst Loading Increases Conversiona Cat. loading wt % Ni on cellulose
Conversions % Carbon X
DAFB X D
X / (1- X D )
0.2 0.5
18.9 60.3 0.48 34.6 66.7 1.06 1.0 54.6 75.0 2.18 3.0 62.2 80.6 3.20 a 427 "C; 60 min. hold; initial H, pressure, 47.6 atm. Table V. Low Temperature Produces Pyrobitumens and Little Oil and Gasa Cat. loading wt % Ni on cellulose
Conversions % Carbon X
0.25 0.50 0.75 a 382 "C; 30 min.
DAFB XD
X/(l -X D )
51.1 0.14 7.2 13.7 53.9 0.30 57.0 0.46 19.8 hold; H, pressure, 54.42 atm.
Table VI. Neither H, Nor Catalyst Alone Affects Bitumen Formation or Selectivitya ~
~~
Gas
Cat. loading Conversion % wt % Ni on cellulose Carbon X DAFB XD X / ( l - XD)
0.0 8.0 49.9 0.16 N* 10.5 49.2 0.21 0.5 N, 0.0 8.5 50.2 0.17 H* a427 "C; heat-up time, 100 ?- 1 0 min; no hold.
uncatalyzed, heat-up and cool-down in either hydrogen or nitrogen. It thus appears that thermal reactions are essentially completed within the initial heat-up period and can
Table VII. Rate-Controlling Steps0 Cellulose A
H,INi -Bitumens i+ 2 (Oils and Gases)
B
C
Conversions % Major reaction 2-Step Reaction 2-Step Reaction
100 min
100 min
AiEZ H 2i z +
B
H /Ni
A
C
Carbon X
DAFB X D
X/(l -X D )
33.0
65.0
0.94
33.7
64.5
0.95
100 min 100 min A kH B/Ni H 2 / N 1 C 34.7 64.5 0.98 1-Step Reaction 100 min total a Each step heat up time to 427 "C = 1 0 0 + 10 min; no hold time; cat. loading 0.5 wt % Ni on cellulose.
z
F i r s t O r d e r In U n c o n v e r t e d C a r b o n
I
H e a t i n g O f d - C O l l u l O S e In N i t r o g e n F o l l o w e d B y R e t e n t i o n In F l o w i n g H y d r o g e n
8
,
I
i
I
Paramatera Of Holding Time W i t h Hi. A t T e m p e r a t u r e
(L
W
> z 0
4 6 0 m i
