I
H. R. BATCHELDER, R. 8. FILBERT, Jr., and W. H. MINK Battelle Memorial Institute, Columbus, Ohio
Processing l o w Temperature lignite Tar Dual solvent extraction of low-temperature lignite tar provides a simple processing means and a new class of materials for commercial use
The following articles from the Symposium on Tars, Pitches, and Asphalts appear in the January 1960 issue of the Journal o f Chemi c c l and Engineering Data: “A Laboratory Evaluation of Pitch Binders Using Compressive Strength of Test Electrodes,” H. L. Jones, Jr., A. W. Simon, and M. H. Wilt; “Surface Properties of the Quinoline-Insoluble Fraction of Coal-Tar Pitch,” M. S. Morgan, W. H. Schlag, and M. H. Wilt; “Composition of Low-Temperature Thermal Extracts from Colorado Oil Shale,” W. E. Robinson and J. J. Cummins; “X-Ray Analysis of Electrode Binder Pitches and Their Cokes,” S. S. Pollack and L. E. Alexander; and “Characterization of Components in Low-Temperature Lignite Tar,” E. J. Kahler, D. C. Rowlands, J. Brewer, W. H. Powell, and W. C. Ellis.
Table I. Lignite Tar from the Parry Process Contains About One-Third Tar Acids Ash, % 0.4 c-I, % 1.3-1.6 Water, ?& 3-4 Specific gravity, 25/25’ C. Viscosity, cp., 28’ C. Elemental compn., % (water-free basis)
H
N S
0“
for this cvork was produced in a prototype Parry (2, 3 ) carbonizer a t the Rockdale works of the Aluminum Co. of America. The carbonizer has a capacity of about 50 tons of lignite per hour, only a fraction of the total boiler fuel required. If all of the fuel were carbonized, tar production would be about 50 million gallons per year. I n the Parry process? carbonization temperatures are low and residence time short so that a minimum of secondary cracking occurs. Under these conditions, the tar is a very complex mixture of hydrocarbons and their oxygen, nitrogen, and sulfur derivatives. T a r acids comprise almost one third of the tar, and the hydrocarbons, together with tar TAR
USED
81.0 8.7 0.7 0.7 8.5
Distillation, ASTM D20-52, weight yo (dry basis)
To 170’ C. 170’-235’ C. 235’-270’ C. 27O0-3OO0 C. 300D-decompn.temp. Residue at decompn. temp.
Loss Residue at 300’ C. Decompn. temp., O C . a
THE
0.9867 820
2.5 16.4 11.6 12.6 31.0 21.0 4.9 52.0 337
B y difference.
bases and some minor components, comprise the balance (Table I). T o develop methods for using these tar products commercially, the work reported here describes a method of processing. A rough separation between tar acids and hydrocarbons seemed desirable, and this was accomplished by dual-solvent (aqueous methanol and hexane) extraction of the tar (Figure 1). Most of the laboratory processing was done in a small, glass rotating disk contactor set u p for continuous countercurrent extraction.
Rotating Disk Contactor Unit T h e rotating disk contactor column is constructed of two 4-inch flanged glasspipe sections. The top section is 3 feet
and the bottom section 4 feet long. There are 44 stages 1 3 / 4 inches high. Each stage comprises a stator ring 2“/8 inches in inside diameter and a rotor 21/8 inches in outside diameter centered in the stage. Gaskets between the stator and the column wall compensate for irregularities in the glass. T h e joint between column sections provides a tar feed entry and a bearing for the rotor shaft. A stilling zone is provided at the top and bottom of the column. T h e methanol extract is removed through a second settling chamber to allow for removal of insoluble material. A variable-height syphon breaker on the extract stream is used to control the column interface level. T a r is fed by a variable-speed gear pump; solvents are fed from constanthead tanks through rotameters. Both are preheated in water baths, and heated air is passed u p through the column enclosure to maintain temperature. The extract and raffinate are stripped of solvent in a 100-gallon batch still, substantially without fractionation.
Operating Variables After a series of batch shake-outs made to establish initial operating conditions for the column, continuous runs were made to explore the effect of operating variables. The information derived from these two types of experiment can be summarized as follows. Operating Temperature. This was determined by shake-outs. As the temperature was increased from 6 6 ” to 120” F., the dispersion of insoluble material decreased and the degree of extraction increased (Figure 2). Results a t 120’ F. seemed satisfactory for column operation. Since higher temperatures would have raised problems with hexane boiling, no further exploration was made. VOL. 52, NO. 2
FEBRUARY 1960
131
HEXANE SOLUBLE R A FFINATE Figure 1 . Tar acids and hydrocarbons were separated satisfactorily by extracting the tar with a dual solvent
METHANOL SOLUBLE EXTRACT INSOLUBLE
Feed Rate and Rotor Speed, Unstable column operation was observed when the feed rate was equivalent to a total loading of 350 gallons per hour per square foot of cross-sectional area. All subsequent runs were made at a column loading of 270 gallons per hour per square foot. As the rotor speed was increased from 180 to 215 r.p.m., the amount of caustic solubles in the methanol solubles increased; but, when the speed was increased further to 270 r.p.m., a dispersion formed in the column which made phase separation impossible. All subsequent runs were made a t 215 r.p.m. Linear rim velocity was then about 120 feet per minute.
Phase Density Difference. Enough difference must be maintained between the density of the raffinate and extract so that the phases may be separated. AS each solvent dissolves components of the tar, the density of each solution increases; therefore, the density and density difference will depend not only on the solvents used, but also on the solvent ratios as well. As more water is added to the methanol solution, its density increases so that, for each methanol-water concentration, there is a solvent ratio below which the column is not operable (Figure 3). For any given methanol concentration, operations to the left of the proper curve will not be feasible because of insufficient
difference in density between the two phases. Thus, with 7070 methanol, a 1 : 3: 3 (tar-methanol-hexane) ratio is satisfactory, but a 1 : 5 : 3, or a 1 : 1 : 2 ratio is not. These curves are not sharp and precise boundaries, but rather the areas of approach to inoperable conditions. Solvent Ratio. In this work, the desired goals were low yield of material insoluble in both solvents, high yield of caustic-soluble material in the methanol extract, and minimum contamination of this extract with hexane-soluble material. Substantially no attempt was made to follow the minor components of the tar. Since the major emphasis was on finding a t least satisfactory conditions for the production of material for characterization studies, exploration of the full range of conditions was not made. However, some conclusions can be drawn for operation with 7070 methanol, and some inferences made as to the effect of changes in methanol concentration. Material insoluble in both phases decreases steadily with increasing ratio of total solvent (methanol plus hexane) to tar (Figure 4 ) . The lowest value of insoluble material is approaching the solids content of the feed tar. The insoluble fraction is quite fluid and easily handled as it leaves the column. After removal of solvent, it is an easily crumbled solid. Methanol extract and the yield of caustic-soluble material in the extract increase regularly with increase in ratio of methanol to hexane, independent of the ratio of either solvent to tar. The runs plotted in Figure 5 are with total solvent ratios of 6.6 to 17.6 randomly distributed through the range of methanol-tohexane ratios used. Presumably at ratios higher than used here, the yield of caustic-soluble material would have to reach or approach a limit. The area between the two curves represents the caustic-insoluble material in the methanol extract. As the methanol-tohexane ratio is increased from 0.5 to 2.0, the caustic insoluble increases from about
Hexane
Solution
Insoluble
Insoluble (did) 66 F Figure 2.
1 32
(liquid) 87 F
104 F
115 F
120 F
Increasing temperature increases the degree of extraction and the ease of phase separation
INDUSTRIAL AND ENGINEERING CHEMISTRY
L O W TEMPERATURE L I G N I T E T A R
6
I
I
I
I
12
i
Methanol Concentration, per cent 10 8
6
4 2
0 4
8
6
IO
14
12
16
18
Ratio-Total Solvent to Tar Figure 4. As the ratio of total solvent to tar increases, the amount of material insoluble in both solvents increases
Ratio - Hexane to Tar Figure 3. Operation of the solvent-extraction unit i s limited by density difference. For each methanol-water concentration, there i s a solvent ratio below which the column i s not operable
3l, 2 to about 9% of the methanol extract (Figure 6). The yield of the desired material increases as the purity decreases so that the choice of solvent ratio is a compromise between the two factors, plus the economic burden of a very high ratio of either solvent to feed tar. Based on the work to date, ratios of about four parts methanol and four parts hexane to one of tar seem optimum. Because there is no simple method for determining caustic-soluble material in the hexane-soluble raffinate, this must be calculated on the basis of certain assumptions. Many distillations had been made of the primary tar to yield distillates of various boiling ranges. Substantially all of these distillates showed about 27Yc of caustic-soluble material. If it is assumed that the whole tar contains 27% caustic-soluble material, it is possible to calculate the amount of caustic-soluble material in each hexanesoluble raffinate. and then to examine the relationship of this to the extraction conditions. Errors in the assumed figure of 2770 would not greatly affect the derived relationship. O n the basis of such an analysis, the following conclusions are probably valid. T h e yield of caustic-insoluble material in the hexane raffinate increases regularly with increases in the ratio of total solvent (methanol plus hexane) to tar, regardless of the methanol-to-hexane ratio. The yield of total hexane-soluble raffinate similarly increases with total solvent ra-
tio, but along a different parallel line for each methanol-to-hexane ratio. The spread between the two yields increases with decreasing methanol-to-hexane ra-
tio (Figure 7 ) . In other words, at a given total solvent ratio, the higher the methanol, the less caustic-soluble material in the hexane-soluble raffinate. The concentration of caustic soluble in the hexane-soluble raffinate decreases steadily with increasing methanol-t:ohexane ratio. This is calculated to be about 12% at a methanol-to-hexane ratio of 0.5, and about 2% at a ratio of
28
/
Total Methanol
26 L
t=
24
h
L
n c O b o
22
18 I
0.5
I
Methanol Soluble
20
I
I
0.6
0.8
I
1.0
I
I
1.4
2 .o
Ratio - Methanol t o Hexane Figure 5. Yields of extract and tar acid material depends on the ratio of methanol to hexane VOL. 52, NO. 2
FEBRUARY 1960
133
Table II.
Run No.
Ratios Total Hexane tar tar
For Most of the Experiments
e.
Meth. tar
Methanol hexane
7070 Methanol in Water Was Used Yields Hexane Soluble Caustic Total insoluble"
Methanol Soluble Caustic Total soluble
Purity Methanol Hexane soluble soluble" 70 % caustic caustic Insoluble insoluble soluble
70% Methanol Concentration 5 10 15 16 2 19 4 11
2.2 5.1 4.3 4.0 4.4 4.9 9.1 9.6
4.4 9.8 6.8 4.3 4.6 5.0 8.5 4.7
6.6 14.9 11.1 8.3 9.0 9.9 17.6 14.3
0.50 0.52 0.63 0.93 0.96 0.98 1.06 2.04
19.3 19.7 20.1 22.9 22.7 23.1 25.4 28.5
12 8 13
4.9 4.3 10.1
10.3 4.4 8.6
15.2 8.7 18.7
0.48 0.98 1.18
24.9 28.7(7) 28.5
18 14
4.9 5.1
5.4 4.8
10.3 9.9
0.91 1.06
14.2 17.7
18.6 18.8 19.3 21.4 21.2 21.6 23.8 25.9
69.9 76.8 74.0 71.9(?) 69.8 70.1 72.6 67.9
61.6 68.5 66.3 66.0(?) 64.0 64.6 69.5 66.6
10.8 3.5 5.9 5.2(?) 6.8 2.0 3.6
...
3.3 4.5 4.2 6.3 6.9 6.6 6.1 9.1
12.0 10.7 10.4 7.8(?) 8.3 7.7 4.4 1.6
67.1 67.5 65.9
4.9 3.4 4.7
4.1 7.4 8.4
4.4 0.6 1.4
66.6 65.6
5.6 6.5
5.7 4.9
17.0 13.5
75% Methanol Concentration 70.2 67.9 66.8
23.9 26.6 26.1
60% Methanol Concentration 13.4 16.8
80.2 75.8
Calculated.
2 (Figure 6). T h e data on which the curves were based are given in Table 11. Methanol Concentration. Only a few experiments were made with methanol concentrations of 75y0 and 6OY0 by weight. Changes in concentration affect the operability ranges of the extraction column by changing the
Table 111.
density of the methanol phase as explained previously. There are not sufficient data to establish quantitatively many of the effects of changes in methanol concentration on yield and quality of products. It is probable, however, that the following qualitative conclusions are valid.
Properties of Hexane-Soluble Fractions Foreruns, to 2000 c.
Specific gravity, 25/25' C. Refractive index, n: Viscosity, kinematic cs., 100' F. Flash point, F. Fire point, ' F. Copper strip corrosion Pour point, O F . Tar acids, vol. % Tar bases, vol. yo Neutral oil, vol. Yo Paraffins, vol. % ' Olefins, vol. yo Aromatics, vol. % ' Elemental compn., yo (water-free basis) C H
N S
0.
Loss
INDUSTRIAL AND ENGINEERING CHEMISTRY
c.
0.9325 1.5242 8.13 208 320
Pass
Pass
