Ind. Eng. Chem. Process Des. Dev. 1980, 19,382-386
382
' = without chemical reaction
Greek Letters density of solid, g/cm3 I#J = enhancement factor
p =
Literature Cited Fujita, S . , Kagaku Kogaku, 27, 109 (1963). Kagaku Benran Kisohen", p 770, Japan Chemical Society, Maruzen, Tokyo, 1974a. "Kagaku Benran Kisohen", p 994, Japan Chemical Society, Maruzen, Tokyo, 1974b. Kustin, K., Taub, I. A., Weinstock, E., Inorg. Chem., 5, 1079 (1966). Sada, E.,Kumazawa, H., Tsuboi. N., Kudo, I., Kondo, T., Ind. Eng. Chem. Process Des. Dev., 17, 321 (1978a). Sada, E., Kumazawa, H., Kudo, I., Kondo, T., Chem. Eng. Sci., 33, 315 (1978b). Sada, E., Kumazawa, H.. Butt, M. A., J. Chem. Eng. Jpn., 12, 111 (1979). Teramoto, M., Hiramine, S.,Shimada, Y., Sugimoto, Y., Teranishi, H., J. C b m . Eng. Jpn., 11, 450 (1978).
Subscripts A = NO Ai = SO? A2 = NO B = FeII-EDTA E = MgS03 or SOZ-
F = HS03f = feed stream i = gas-liquid interface 0 = effluent stream s = surface of solid particle
Received for review May 10, 1979 Accepted January 22, 1980
Superscript
Coal Liquefaction Catalysts John F. Patzer, 11,' and Angelo A. Montagna Chemicals and Minerals Division, Gulf Research and Development Company, Pittsburgh, Pennsylvania 15230
A simple coal liquid hydrogenation catalyst evaluation technique is described for the screening of potential coal liquefaction catalysts. Eleven catalysts are evaluated by this technique. Three of the catalysts are also evaluated in actual coal liquefaction service. A correspondence between hydrogenation activity as measured by the coal liquid evaluation and hydrogenation activity in coal liquefaction service is noted. Also, a potential relation between denitrogenation activity in the coal liquid evaluation and hydrocracking in coal liquefaction is posited.
Most forecasts of future energy supply and demand show a projected shortfall in liquid hydrocarbons that becomes relatively severe by about 1990 (ERDA, 1975). Considerable research is currently being undertaken to develop processes which will produce ecologically acceptable liquid, gas, and solid fuels from coal. Various processes, such as SRC, Exxon Donor Solvent, Consol, H-Coal, Synthoil, and CCL have been or are currently being evaluated at the bench- or pilot-scale. Several of these rely on the use of a catalyst to aid in liquefaction of the coal. Development of significantly more active, longer life cycle catalysts is required to enhance the economic competitiveness of such processes. Coal hydroliquefaction processes involve a number of mechanical and chemical steps which collectively bring about the liquefaction of coal. Conceptually, the coal is solubilized (dissolved) in a recycle solvent which serves to depolymerize the coal particles into large molecular fragments. In a catalytic process, the catalyst provides the dual function of cracking the large molecular fragments to smaller fragments and hydrogenating such fragments to stabilize them. The hydrogenation step can result from either direct catalytic action or by hydrogen transfer from donor molecules in the solvent. In the latter case, the catalyst serves to rehydrogenate a donor molecule which has donated hydrogen so that it is capable of donating hydrogen again. Catalyst deactivation can occur by mineral deposition from the coal feed and coke buildup on the surface and in the pores (Stanulonis et al., 1976). Development of a suitable catalyst which exhibits acceptable activity in production of distillate range materials and has an acceptable lifetime is a primary requisite for
any catalytic coal liquefaction process. Kawa et al. (1974) present an extensive study of hydroliquefaction and hydrodesulfurization catalysts which provided the basis for selection of the Synthoil catalyst. Experiments with Mo, Sn, Ni, Co, and Fe impregnated as single components on high- and low-surface area supports indicated that Mo catalysts were best for sulfur removal and Sn catalysts were best for conversion of coal to oil. A commercial, highsurface area silica-promoted CoMo catalyst supported on alumina was chosen for the Synthoil process based upon combined activity for both liquefaction and desulfurization. Katzman (1974) provides an extensive review of the literature for catalysts used in coal conversion processes. However, improvements in catalyst performance are needed to improve the overall catalytic coal liquefaction process. This study reports the development of a relatively simple screening test for potential coal liquefaction catalysts which relies on the hydrogenation of a coal liquid. Several supported phase compositions are reported which serve to show the effects of utilizing tin and zinc, both known to be good homogeneous coal liquefaction catalysts, as promoters for heterogeneous coal liquefaction catalysts. A relationship between nitrogen removal in the coal liquid hydrogenation experiment and hydrocracking during coal liquefaction is observed. Experimental Section Catalysts. The catalysts used in this study are listed in Table I. The supported phase compositions shown are the weight percent of metal present based upon total catalyst weight. All of the catalysts are supported on the same y-alumina support which has a surface area of 208
0196-4305/80/1119-0382$01.00/00 1980 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 19,No. 3, 1980 383
Table I. Catalyst Cotnpositions Evaluated for Coal Liquid Hydrogenation Activity coal liquid hydrogenation linear regression" catalyst A
supported phase compn, wt % 3% Ni, 10% Mo
B
3% Ni, 1%Zn, 10% Mo
C
3% Ni, 3% Zn, 10%Mo
D
3% Ni, 5% Zn, 1 0 % Mo
E
4% Ni, 2% Sn, 10%Mo
F
4% Ni, 5% Sn, 1 0 % Mo
G
I
experimental-hy drocrac king experimental-hy drocrac king 3% Ni, 5% Ti, 8% Mo
J
4% Ni, 3% Mn, 10% Mo
K
0.5% Ni., 1%Co, 8% Mo
H
a
impregnation salts Ni( N 0 , ) , . 6 H 2 0 ammonium heptamolybdate Ni( N0,),.6H20 Zn(N03),*6H,0 ammonium heptamolybdate Ni(N0,),.6HZ0 Zn(NO,),~GH,O ammonium heptamolybdate Ni(N03),.6H,0 Zn(N03),.6H,0 ammonium heptamolybdate Ni(NO,),.GH,O SnCI, ammonium heptamolybdate Ni( NO ,),.6 H,O SnCI, ammonium heptamolybdate Ni( N03),.6H,0 TiCI, ammonium heptamolybdate Ni(N03),.6H,0 Mn(N0,),-50% solution ammonium heptamolybdate Ni(NO,),,6H2O Co(NO,),.6H2O ammonium heptamolybdate
intercept , sp. gr. 1.014 i 0.003
slope x lo4, (SP. gr.)/h 2.55 f 0.41
* 0.80
1.021 i. 0.005
1.66
1.021 f 0.004
1.99 i 0.64
1.039
f
0.003
0.89
?:
0.50
1.043
t
0.005
0.75
i.
0.84
1.061 t 0.006
1.34
?
1.03
1.018 i 0.003 1.012 i 0.007 1.024 t 0.002
2.23 f 0.60 3.48 * 0.97 1.93 ? 0.33
1.039 t 0.005
0.33
t
1.00
1.053 t 0.003
0.77
t
0.63
90% confidence interval.
m2/g, pore volume of 0.63 cm3/g, and average pore radius of 60.6 A. The salts used in the impregnation procedure, described below, are also provided in Table I. The catalysts were prepared by the following incipient wetness technique. (1)Molybdenum, dissolved in aqueous ammonium hydroxide, was first impregnated onto the support. The wet preparation was oven-dried for 16 h at 120 "C. The oven-dried catalyst was then calcined in air for 16 h a t 538 "C. (2) Nickel and the second metal were dissolved in distilled water and the resultant solution was used to impregnate the catalyst of step 1. The catalyst was then oven-dried and calcined in air as above to produce the final catalyst. Catalyst I differed in that aqueous ammonium hydroxide was used for the nickel-titanium impregnation. Coal Liquid Hydrogenation. The catalysts were evaluated for initial activity for the hydrogenation of a coal liquid in a continuous, single-pass trickle bed reactor. A catalyst bed consisting of 20 cm3 of 0.06 to 0.14 cm mean diameter particles was used for all experiments. The fresh catalyst was sulfded prior to an experiment by passing 0.09 m3/h of an 8% H2S-92% H2 gas blend through the reactor, which was maintained a t 316 "C and atmospheric pressure, for a period of 1 h. Evaluation conditions were 371 "C, 20.7 MPa, 1.0 liquid hourly space velocity (volume of coal liquid/volume of catalyst bed/h), and a hydrogen circulation rate of 1781 m3 of H2/m3 of coal liquid feed. An experimental evaluation was of 104 h duration, with an initial offstream period of 8 h followed by 12 8-h onstream periods. The liquid product specific gravity and wt % S were obtained for all on-stream periods. In addition, the 48-56-h and 80-88-h period liquid products were analyzed for w t 'TO C, H, N, and 0. The reactor off-gas was not analyzed. Liquid product recoveries averaged about 95 w t 70 of input. Coal Liquefaction. Three of the catalysts, B, I, and K, were evaluated for initial activity in catalytic coal li-
quefaction of Big Horn coal in a segmented bed reactor (Shah et al., 1978). It was attempted to run the catalysts at the same, constant level of solvation so that observed differences in hydrogenation and hydrocracking could be attributed to differences in the supported phase compositions without having to correct for solvation effects. The definition of solvation is given by the formula % solvation = 100 X
[(MAF coal feed) - (MAF dry cake)] /(MAF coal feed) (1)
and hydrocracking (conversion to distillate material) by the formula % hydrocracking = 100 X [(MAF coal feed)
-
(MAF dry cake) - (residue)]/(MAF coal feed) (2) where MAF is moisture and ash-free, dry cake refers to the recovered solids which have been filtered from the reactor effluent and oven-dried, and residue refers to the material remaining after distillation of the reactor effluent filtrate to a termination temperature of 400 "C a t 3 torr pressure. The catalysts were sulfided with a H2S-H2 gas blend prior to an experiment. The sulfided catalyst was then contacted with the feed and brought to evaluation conditions of 399 "C, 26.9 MPa, 1.2 WHSV (kg of coal/kg of catalyst/h, 40 w t 70 coal in anthracene oil slurry), and hydrogen circulation rate of 0.79 m3/ kg of coal. An initial 6-h off-stream period was followed by 66 to 70 h on stream. Feedstocks. A filtered catalytic coal liquefaction recycle liquid was used as the feedstock for the coal liquid hydrogenation evaluations. The relevant physical and chemical properties of the liquid are given in Table 11. A Big Horn coal containing approximately 21.1 w t % water and 4.98 w t '70 ash, slurried in anthracene oil, was used for the coal liquefaction runs.
384
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 3, 1980
Table 11. Rotory Drum Filtered CCL Recycle Product Inspections specific gravity carbon, wt % hydrogen, wt % sulfur, wt % nitrogen, wt % oxygen, wt % ash, wt % viscosity at 210 O F , cSt
1.128 90.15 6.68 0.36 1.01 2.50 0.13 8.28
>
I
I
20
40
80
60
100
TIME HOURS
Figure 2. Coal liquid initial hydrogenation activity of zinc- and tin-promoted catalysts: 0,catalyst A; 0,catalyst B; 0 , catalyst C; 0, catalyst D; V, catalyst E; A, catalyst F.
t 101
103 105 107 SPECIFIC GRAVITY OF PRODUCT
Figure 1. Relationship between coal liquid H/C atom ratio and specific gravity for coal liquid used in this study.
$ 1 @ 5 - '
2
0
A
-
,
P
z 1 0 4 u
G-
3 VI
Results Coal Liquid Hydrogenation. The major method of comparison between catalyst activity for coal liquid hydrogenation in this study was the liquid product specific gravity. Figure 1 presents the relation between the H/C atom ratio in the product and the product specific gravity. Linear regression of the data yielded the shown fit with a correlation coefficient of 0.86. Such a relation is particular to a given coal liquid feedstock and its hydrogenated products. Based upon the data in Figure 1,it was felt that product specific gravity provided an acceptable estimate of the product H/C atom ratio and, thus, was used to follow catalyst aging with respect to hydrogenation. In interpretation of the initial catalyst activity plots which follow, the higher the specific gravity, the lower the catalyst activity for hydrogenation. Quantification of catalyst initial coal liquid hydrogenation activity and aging rate is obtained through linear regression of the change in specific gravity with time. While this proved suitable for the relatively short aging periods used in this study, more realistic curvature will probably be required for longer term aging runs. Table I provides the linear regression coefficients and 90% confidence intervals for each catalyst. The intercept provides an estimate of initial activity and the slope of the aging rate. Catalyst A, 3% Ni and 10% Mo, is a convenient reference catalyst against which the effects of various promotors of the nickel-molybdenum system can be gauged. As can be seen from Figures 2 and 3 and Table I, catalyst A is the most active composition tested. It also, however, has one of the highest aging rates observed. Zinc-Promoted Catalysts. Zinc is known to be an excellent homogeneous catalyst for coal liquefaction (Consolidation Coal Co., 1969; Qader et al., 1973). The initial hydrogenation activity of three zinc-promoted catalysts, B, C, and D, containing 1%,3%) and 5% Zn, respectively, is shown in Figure 2. The response of the zinc-promoted catalyst system to an increase in zinc content is a decrease in hydrogenation activity. Only catalyst
1
5
103
-
-
0 3
P 102 0 .
lo'
-G
t--LuLJ 20
40
60
80
100
TIME HOURS
Figure 3. Coal liquid initial hydrogenation activity for hydrocracking, titanium- and manganese-promoted, and petroleum residue hydrodesulfurization catalysts: 0,catalyst A; 0,catalyst G; 0 , catalyst H; 0, catalyst I; V, catalyst J; A, catalyst K.
B, 1%Zn, has an activity comparable to the reference catalyst. The aging rates of the zinc-promoted catalysts are lower than that of the reference catalyst, most notably catalyst D, 5% Zn, which has an aging rate 35% that of the reference catalyst. Tin-Promoted Catalysts. The effect of tin as a promoter for heterogeneous catalytic coal liquefaction is of considerable interest because tin salts, such as SnC12,are known also to homogeneously catalyze coal solvation reactions (Hawk and Hiteshue, 1965; Qader and Hill, 1969). Two tin promoted catalysts, E and F, containing 2% and 5% Sn, respectively, were evaluated. The results, shown $ Figure 2, indicate that tin is not a particularly good promoter of the Ni-Mo catalyst system for this reaction. Both E and F are considerably less active than catalyst A. The aging rates for E and F are, however, 29% and 53%, respectively, that of the reference catalyst. Miscellaneous Promoted Catalysts. Catalysts G and H are experimental petroleum hydrocracking compositions, promoted Ni-W, which were evaluated to examine the effect of such compositions in coal liquefaction. As seen from Figure 3, catalysts G and H have initial hydrogenation activity comparable to the reference catalyst. The aging rates are also comparable to, or greater than, that of catalyst A. Catalyst I examines the effect of titanium promotion. This catalyst (Figure 3) is somewhat less active than catalyst A, with an aging rate about 76% that of A. The effect of manganese as a promoter, catalyst J, was examined briefly during a run which terminated early due
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 3, 1980
385
Table 111. Coal Liquid Product Inspection at Specified Times On Stream
~-
cata- time on lyst stream, h A
B C D
E
F
D
G
H 10
30
50
70
TIME, HOURS
Figure 4. Initial activity of zinc-, titanium-, and petroleum HDS catalysts for solvation, hydrocracking and hydrogenation of Big Horn coal in catalytic coal liquefaction: 0, catalyst B; 0, catalyst I; A, catalyst K.
to pressure drop problems. The results, shown in Figure 3, indicate that manganese promotion yields a catalyst which is less active than the reference, but also one with a very low aging rate. It is interesting to note (Table I) that only catalyst J and catalyst E (2% Sn) have aging rates not statistically different from zero. Catalyst K represents a typical petroleum residue hydrodesulfurization catalyst composition. Catalyst K is considerably less active th,an the reference catalyst (Figure 3). The activity and aging rate of K are comparable to that of the tin-promoted catalysts. Coal Liquefaction. Catalysts B, I, and K were evaluated for initial activity in catalytic coal liquefaction service. As was mentioned in the Experimental Section, an attempt to run these catalysts at constant coal solvation level was made so that differences in hydrocracking and hydrogenation activity could be attributed to catalytic activity and /or aging without correcting for solvation effects. As can be seen from Figure 4,the solvation levels for the three evaluations were fairly close, ranging between 74 and 78% for the most part. Linear regression was used to provide the linear fits to the data shown in Figure 4. The order of catalytic activity for hydrocracking (Figure 4) is I > B > K. Catalyst I is clearly the superior composition for hydrocracking activity. However, a reordering in catalytic activity for hydrogenation (Figure 41, again, as measured by liquid product specific gravity, is observed with an activity ranking B > I > K. Discussion The coal liquid hydrogenation initial activity test was developed as a quick, inexpensive screening tool to evaluate catalysts for potential use in catalytic coal liquefaction. The series of catalysts evaluated and reported in this study were believed to be representative of the types of catalysts which would be of general interest for catalytic coal liquefaction. The general conclusilons drawn from the results of the coal liquid hydrogenation experiments are as follows. (1) Addition of a promoter generally decreases the activity of the Ni-Mo catalyst sys tem. However, addition of a promotor also generally lowers the aging rate. (2) The response of both Zn- and Sn-promoted catalysts to increasing promoter levels is a decrease in catalyst activity. (3) Petroleum hydrocracking composition catalysts exhibit initial hydrogenation activity clomparable to unpromoted Ni-Mo,
I J K
48-56 80-88 48-56 80-88 48-56 80-88 4 8- 56 80-88 4 8- 56 80-88 4 8- 56 80-88 48-56 80-88 48-56 80- 8 8 48-56 80-88 48-56 72-80 48-56 80-88
H
%C
%
88.08 89.11 89.56 89.82 89.58 89.35 91.16 89.09 90.37 89.65 89.49 89.94 88.73 88.76 90.79 89.24 89.63 90.42 89.57 89.51 88.81 89.63
9.99 8.82 6.99 8.85 8.94 8.77 8.57 8.88 8.42 8.20 7.55 7.48 9.17 8.79 9.13 8.58 8.69 8.49 8.58 8.65 7.98 8.04
%S
%N
%0
0.28 0.34 0.24 0.23 0.26 0.25 0.27 0.32 0.22 0.30 0.33 0.32 0.30 0.26 0.25 0.21 0.19 0.17 0.30 0.33 0.19 0.09
0.66 0.71 0.56 0.55 0.65 0.59 0.65 0.69 0.62 0.67 0.75 0.77 0.59 0.64 0.54 0.65 0.50 0.54 0.66 0.70 0.69 0.67
1.49 1.52 1.29 1.10 1.61 1.33 1.34 1.35 1.44 1.24 1.57 1.54 1.46 1.42 1.00 0.93 0.93
-
1.40 1.78
-
-
but they age at a rate comparable to or greater than the reference (possibly due to coking from cracking a highly aromatic feedstock). (4)The manganese-promoted catalyst, J, was unusually stable. The relative activity of coal liquefaction catalysts for hydrogenation, as evaluated by the coal liquid hydrogenation experiments, was borne out in the catalytic coal liquefaction experiments. The zinc-promoted catalyst, B, was more active than the titanium-promoted catalyst, I, while the typical hydrodesulfurization catalyst, K, was less active than either in both sets of experiments. Thus, it appears that relative hydrogenation activity for coal liquefaction can be adequately gauged by the simpler coal liquid hydrogenation test. Hydrogenation activity, however, is only one parameter of interest in a coal liquefaction catalyst. Hydrocracking, which is a measure of catalyst ability to break large coal molecular fragments into stable smaller molecules, is also an important aspect of catalytic coal liquefaction. As can be seen from the coal liquefaction experiments reported here, catalyst activity for hydrogenation and hydrocracking need not coincide. Product inspections for the coal liquid hydrogenation experiments are given in Table 111. Examination of Table I11 for possible correlations of product properties from coal liquid hydrogenation with catalyst activity as measured in catalytic coal liquefaction shows that there are no apparent trends linking the product sulfur and oxygen content and catalyst activity for hydrogenation or hydrocracking. However, a tenuous link (because only three catalysts were tested in common between coal liquid hydrogenation and catalytic coal liquefaction) appears to exist between the coal liquid product nitrogen content and catalyst activity for hydrocracking in catalytic coal liquefaction. The catalyst's ability to remove nitrogen from a coal liquid appears to be related to catalytic coal liquefaction hydrocracking activity in that catalyst I, which removed the most nitrogen from the coal liquid, was also the most active for hydrocracking during coal liquefaction, while catalyst K, which was the least active in nitrogen removal, was also the least active for hydrocracking in coal liquefaction. The relationship is tenuous at the present time because the relative paucity of data precludes a quantitative correlation. However, such a relationship between hydrocracking activity and denitrogenation ac-
386
Xnd. Eng. Chem. Process Des. Dev. 1980, 19, 386-393
tivity is a common observation in petroleum hydrotreating operations. Thus, the simple coal liquid hydrogenation catalyst screening test developed for screening potential catalytic coal liquefaction catalysts appears to have predictive value in determining relative catalyst ranking for both hydrogenation and hydrocracking in catalytic coal liquefaction.
Conclusions A simple coal liquid hydrogenation screening test was developed for evaluation of potential catalytic coal liquefaction catalysts. Relative ranking of catalyst activity for hydrogenation of coal liquid in the screening test correranking Observed in 'POnded to the liquefaction. In addition, a tentative hypothesis is adthat the Of a to remove nitrogen from the coal liquid corresponds with the hydrocracking activity observed in catalytic coal liquefaction. Additional experimentation is required to quantify these relations.
Literature Cited Consolidation Coal Co., "Research on Zinc Chloride Catalyst for Converting Coal to Gasoline: Phase I-Hydrocracking of Coal and Extract With Zinc Chloride", R&D Report No. 39, OCR Contract No. 14-01-0001-310, Vol. 111, Books 1 and 2, 1969. ERDA-48, US. Energy Research and Development Administration, "A National Plan for Energy Research, Development, and Demonstration: Creating Cholces for the Future: Vol. I, the Plan", Government Printing Office, Washington, D.C., 1975. Hawk, C. O., Hiteshue. R. W., U . S . Bur. Mines Bull., No. 822(3) (1965). Katzman, H., "A Research and Development Program for Catalysis in Coal Conversion Processes", Report No. EPRI 207-0-0 for Electric Power Research Institute (1974). Kawa, W., Friedman, S., WU, W. R. K., Frank, L. U., 'favorsky, P. M., Presented at the 167th National Meetlng of the American Chemical Society, Division of Fuel Chemistry, Los Angeles, Callf., April 1974. Qader, S. A., Duraiswamy, K., Wood, R. E.,Hili, G. R., AICM Symp. Ser., 69(127), 102-104 (1973). Qader, S. A., HIII, G. R., Hydrocarbon Process, 48(3), 141-146 (1969). shah, Y. T., Cromuer, D. c., M C I KH.~G., , Paraskos, J. H., I&. ~ n g chem. . process D ~ S .DW., 17, 288-301 (1978). Stanulonis, J. J., Gates, 6. C., Olson, J. H., AIChE J., 22(3), 576 (1976).
Received for review May 16, 1979 Accepted J a n u a r y 9, 1980
Prediction of Vapor-Liquid Equilibria of Undefined Mixtures William J. Sim and Thomas E. Daubert Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802
The purpose of this work was to evaluate the procedures for predicting the vapor-liquid equilibria of undefined mixtures including light fractions, whole crudes, and heavy residua in order to determine which approach would be most satisfactory for all petroleum fractions. As a resutf of this investigation, it was concluded that the unmodified Soave (1972) procedure was the most accurate and reliable method for flash calculations, having an average error of 12.8% as compared to 15.9% for the Chao-Seader (1961) method and 24.6% for the Maxwell (1950) graphical procedure. While the unmodified Soave procedure was satisfactory for simulating flash data in the 25 to 90 vol YO range, it predicts low flash volume behavior less accurately. A modification of the Soave vapor pressure function and the a function helped to reduce the overall error in the low flash volume data. Improvement was limited by inaccuracies in the experimental data (TBP curve) and characterizing parameters (T,, P,, and a).
Since virtually all phases of petroleum refining involve the separation of petroleum fractions into liquid and vapor phases, a reliable estimate of the equilibrium flash vaporization curve is an essential prerequisite for the efficient design and operation of petroleum processing equipment. The methods currently available for flash calculations are empirical correlations which are not very reliable, especially at low flash volumes. Furthermore, the subatmospheric flash curves are calculated directly from the atmospheric flash curve which means that any error involved in computing the atmospheric data is compounded in going to other pressures. Computer methods, based on several equations of state have been proposed to replace the graphical procedures for flash calculations but none of these methods have been systematically evaluated for petroleum fractions. The purpose of the present work was twofold. First, the various computer and graphical procedures were tested to decide which approach showed the greatest promise. Secondly, various methods for improving existing techniques were investigated. Literature Survey and Analysis The methods presented in the literature for flash calculations are of two general types-graphical correlations and computer methods based on an equation of state. Graphical correlations were developed by Piroomov and Beiswenger (1929), Nelson and Souders (19311, Packie 0196-4305/80/1119-0386$01.00/0
(1941), Nelson and Harvey (1948), Edmister and Pollock (1948), Maxwell (1950), and Edmister and Okamoto (1959a). All of these methods convert an ASTM D86 or TBP distillation into an equilibrium flash vaporization (EFV) curve. House et al. (1966) evaluated the above mentioned flash methods using the same data base as that used to develop the original correlations. The evaluation indicated that for TBP data, the Maxwell correlation gave the best results, with an average error of 14.970,while the errors of the other procedures all exceeded 20%. For this reason, only the Maxwell correlation was considered in the present work for comparison with the computer methods proposed. Computer methods for making vapor-liquid equilibria calculations on petroleum fractions were developed by Chao and Seader (1961), Grayson and Streed (1963), Hoffman (1968), Soave (1972), Starling and Han (19721, Lee et al. (1973), and Peng and Robinson (1976). All of the methods, with the exception of Hoffman's, begin by assuming an initial set of K values and performing a flash calculation to find x i , yi, and V, based on the material balances
v = F(2i - X i ) / X i ( K I
- 1.0)
(1)
and c x i = 1.0
0 1980
American Chemical Society
(2)
