Separation of solids from coal liquids using an additive

Ash-containing solids are separated from coal liquid by mixing alcohol with said coal liquid, followed by a solids-liquid separation step.

This invention relates to a process for removing ash from coal liquids. 
Several solvation processes are now being developed for producing both 
liquid and solid hydrocarbons from coal. One such process is known as the 
Solvent Refined Coal (SRC) process. This process is described in a number 
of patents, including U.S. Pat. No. 3,892,654, which is hereby 
incorporated by reference. The SRC process is a solvation process for 
producing deashed solid and liquid hydrocarbonaceous fuel from coal. In 
this process, crushed raw coal is slurried with a solvent comprising 
hydroaromatic compounds in contact with hydrogen, or carbon monoxide and 
water, in a first zone at a high temperature and pressure to dissolve 
hydrocarbonaceous fuel from coal minerals by transfer of hydrogen from the 
hydroaromatic solvent compounds to the hydrocarbonaceous material in the 
coal. The solvent is then treated with hydrogen, or carbon monoxide and 
water, in a second zone to replenish the hydrogen lost by the solvent in 
the first zone. The hydrogenenriched solvent is then recycled. The 
dissolved liquids contain suspended particles of ash or of ash and 
undissolved hydrocarbons. The suspended particles are very small, some 
being of submicron size, and are therefore very difficult to remove from 
the dissolved coal liquids. Although certain approaches have been tried to 
agglomerate these particles in order to increase the rate of their 
separation, none of the present methods for removing solids from liquefied 
coal has proved to be entirely successful. 
It is the purpose of the present invention to treat the liquid product of a 
coal solvation process, such as the SRC process, containing suspended or 
dispersed ash-containing solids with an additive to agglomerate or 
otherwise affect these solids so that they can be subsequently removed 
from the coal liquid at a more rapid rate than would otherwise be 
possible. Any of the known methods for solids-liquid separation can be 
applied to the treated coal liquids, including filtration, settling, 
hydrocloning or centrifugation. If settling is employed, coal liquids 
treated in accordance with this invention will be relieved of their solids 
content without a subsequent manipulative step. However, because of the 
rapid rate of solids removal demonstrable by filtration, the present 
invention is illustrated in the following examples by the filtration 
method of solids separation. 
A composition containing alcohol and coal liquids having suspended or 
dispersed solid particles comprising ash or ash and undissolved 
hydrocarbons has been found to be considerably more amenable to solids 
removal than non-alcoholic coal liquid. Primary, secondary or tertiary 
alcohols are effective. Aliphatic alcohols containing 2 to 10 carbon atoms 
can be employed. Although longer aliphatic chains may be effective, they 
are more expensive and needlessly increase the cost of the operation. 
Particularly effective alcohols include isopropyl and normal, secondary 
and tertiary butanol. One or more alcohols can be employed. The alcohol 
can be present in the coal liquid in an amount between 0.05 and 15 weight 
percent. Alcohol concentration ranges between 0.1 and 10 weight percent or 
between 0.5 or 1.0 and 6 weight percent are effective. 
The alcohol employed in the present process does not perform any 
significant hydrogen donor or coal solvation function. For example, while 
butanol is a preferred alcohol of this invention, it is not an effective 
alcohol for purposes of coal solvation. In the present process, the 
alcohol is added to the coal liquefaction process after completion of the 
coal dissolving step, i.e. after at least about 85 or 90 weight percent of 
the coal has been dissolved. Furthermore, the use of alcohol in this 
process does not result in any significant increase in the hydrogen to 
carbon ratio of the coal liquid. There is no need to add alcohol to the 
process until after the coal dissolving and solvent hydrogenation steps 
are completed. Thereby, most of the alcohol is not consumed in the present 
process, nor is there significant conversion to another material, such as 
ketone, by hydrogen transfer. To prevent the alcohol from functioning as a 
hydrogen donor, the coal liquid to which the alcohol is added comprises a 
significant amount of a previously added and different hydrogen donor 
material, such as at least 2, 3 or 5 weight percent of hydroaromatic 
material, such as tetralin and homologues thereof. The hydroaromatic 
material present conserves the alcohol so that most of it can be recycled 
without hydrotreatment. Since the purpose of the alcohol is specific to 
solids removal, no prior removal of solids from the coal is required and 
the alcohol can be added to a coal liquid containing generally at least 3 
or 4 weight percent of ash. The alcohol does not require any co-additive, 
such as a base, in order to perform its function, such as would enhance 
its effect if it were to perform a hydrogen donor function. Also, the 
alcohol functions in the present invention in the liquid phase and 
therefore can be used for solids-liquid separation at a temperature below 
its critical temperature. 
It has now been discovered that the rate of solids romoval can be 
considerably improved by intermittent or spaced addition of increments of 
the alcohol to the coal liquid prior to solids removal, rather than 
employing a single injection. The temperature of the coal liquid should be 
at an elevated level prior to alcohol addition and should be between about 
100.degree. and 700.degree. F. (30.degree. and 371.degree. C.), generally, 
between about 150.degree. and 600.degree. F. (66.degree. and 316.degree. 
C.), preferably, and between about 400.degree. and 550.degree. F. 
(204.degree. and 288.degree. C.), most preferably. Following the addition 
of each alcohol increment, the coal mixture should be mixed to form a 
homogeneous composition within the liquid phase. Between additions of 
alcohol increments, the coal solution can be allowed to stand at the 
mixing temperature from 30 seconds to 3 hours, generally, from 1 minute to 
1 hour, preferably, or from 2 or 5 minutes to 30 minutes. These time 
intervals are also useful as a waiting period between the addition of the 
final alcohol increment and a filtration or other solids-removal step. 
Data are presented below which show that if an excessive quantity of 
alcohol is introduced in an individual increment, the effectiveness of the 
alcohol declines. However, if the same amount of alcohol is added 
incrementally with the stated time intervals between additions, a more 
beneficial effect can be realized. Since some of the alcohol can be 
recycled, there is very little incremental operating cost incident to the 
use of an enhanced quantity of alcohol. 
The incremental addition of an additive to a continuous process stream can 
be performed by addition of one increment upstream of a second addition. 
The process flow time delay accounts for the required time interval. 
In another mode of performing the present invention, alcohol is added 
incrementally to a hot, unfiltered slurry of dissolved coal and the 
mixture is stirred and allowed to age between increments and after the 
final increment. The mixture is then passed through a filter to which a 
diatomaceous earth precoat has previously been applied. The 
alcohol-containing filtrate is then distilled to recover the alcohol. The 
alcohol is then recycled and mixed with filter feed, together with any 
make-up alcohol that may be required. 
Filtration tests were performed to illustrate the present invention and the 
data obtained were interpreted according to the following well known 
filtration mathematical model: 
EQU T/W = kW + C 
where: 
T = filtration time, minutes 
W = weight of filtrate collected in time T, grams 
k = filter cake resistance parameter, minutes/grams.sup.2 
C = precoat resistance parameter, minutes/gram and, 
EQU T/W = (rate).sup.-1 
In the filtration tests reported below, the amount of filtrate recovered, 
W, was automatically recorded as a function of time, T. W and T represent 
the basic data obtained in the tests. Although the following variables 
were measured, they were held constant at desired levels in order to 
obtain comparative measurements: temperature, pressure drop across the 
filter, precoat nature and method of application, precoat thickness, and 
the cross-sectional area of the filter. 
The W versus T data obtained were manipulated according to the above 
mathematical model, as illustrated in the figure. The figure is based on 
Example 7 and shows four curves, each representing a separate filtration. 
The horizontal axis shows the value for W while the vertical axis shows 
the value for T/W, which is the reciprocal of the filtration rate. The 
slope or each curve is k, and the intercept of each curve with the 
vertical axis is C. 
In analyzing each curve, the parameter C is primarily a characteristic of 
the precoat because it is the reciprocal of the filtering rate at the 
beginning of the test before any significant amount of filter cake has 
deposited on top of the precoat. On the other hand, the slope k is a 
parameter of the filter cake which is being deposited upon the precoat 
during the filtration and is therefore representative of the filtration 
itself exclusive of the precoat. A relatively low slope (low value for k) 
represents an advantageously low cake resistance to filtration. Stated in 
another manner, any reduction in k represents an increase in the 
prevailing rate of filtration. By observing the figure, it is seen that 
the uppermost curve has the greatest slope (highest k) while the lowermost 
curve has the lowest slope (lowest k). The figure shows that after one 
minute of filtering time the upper curve has produced a smaller amount of 
filtrate than the lower curve. Viewed in another manner, although each 
curve indicates a lower filtration rate (i.e. a higher (rate).sup.-1) at 
the end as compared to the start of a test, a low curve slope indicates 
that the filtering rate has not diminished greatly during the test. 
It is noted that each filtering test is performed without solvent washing 
of the filter cake. Since a solvent wash is intended to alter the nature 
of the filter cake, it would also alter the k value. Many industrial 
filters are of the continuous rotary type wherein filtration cycles of no 
more than about one minute duration are continuously alternated with 
washing cycles wherein a wash solvent is sprayed through the filter cake 
to wash off the absorbed coal liquid. Therefore, all the tabulated 
filtering rates in the tests reported below represent the filtering 
operation during the first minute of filtration. 
In performing the filtration tests of the following examples, a 90 mesh 
screen located within the filter element was precoated to a depth of 0.5 
inch (1.27 cm) with diatomaceous earth. The filter element measured 1.9 cm 
I.D. by 3.5 cm in height and provided a surface area of 2.84 cm.sup.2. The 
screen was supported by a sturdy grid to prevent deformation. The precoat 
operation was performed by pressuring a 5 weight percent suspension of the 
dicalite precoat material in process light oil on to the screen using a 
nitrogen pressure of 40 psi (2.8 Kg/cm.sup.2). The precoat operation was 
performed at a temperature close to that of the subsequent filtering 
operation. The resulting porous bed of precoat material weighed about 1.2 
grams. After the precoat material had been deposited, nitrogen at a 
pressure of about 5 psi (0.35 Kg/cm.sup.2) was blown through the filter 
for about 1 - 2 seconds to remove traces of light oil. The light oil 
flowed to a container disposed on an automatic weighing balance. The light 
oil was weighed to insure deposition of the required quantity of precoat 
material. Following this operation, the light oil was discarded. The 
balance was linked to a recorder for later use which provided a continuous 
(at 5 second intervals) printed record of filtrate collected as a function 
of time. 
A 750 gram sample of unfiltered oil (UFO) without any additive was then 
introduced into a separate autoclave vessel which acted as a reservoir. 
The UFO was maintained at a temperature of 100.degree.-130.degree. F. 
(38.degree.-54.degree. C.) and was continuously stirred. Stirring was 
accomplished using two 5 cm turbines. The shaft speed was 2,000 rpm. The 
filtration was begun by applying a selected 40-80 psi (2.8 - 5.6 
Kg/cm.sup.2) nitrogen pressure to the autoclave. The UFO flowing from the 
autoclave passed through a preheater coil whose residence time was 
controlled by the manipulation of valves and which was provided with inlet 
and outlet thermocouples so that the UFO reaching the filter was 
maintained at a uniform temperature. The UFO passed from the preheater to 
the filter where solid cake was formed and filtrate obtained. The filter 
element and filter heater were also fitted with thermocouples. As 
indicated above, filtrate was recovered on a balance and its weight was 
automatically recorded every five seconds. The filtrate was collected in a 
clean container. 
Comparative tests to determine the effect of additives were performed using 
the same feed lot of UFO for which filtration data had been collected. 
First, the system tubing and the filter were purged of UFO with nitrogen 
at a pressure of about 100 psi (7 Kg/cm.sup.2). The additive substance was 
pumped into the autoclave reservoir containing UFO. A separate filter 
element was fitted and precoated in the same manner as described above and 
the tests employing an additive in the UFO were performed in the same 
manner as the tests performed on the UFO without an additive. Following 
each filtration, the residue on the precoat material in the filter was 
purged with nitrogen and washed with an appropriate liquid to eliminate 
the UFO and additive combination. 
Following is an analysis of a typical unfiltered SRC feed coal liquid 
employed in the tests of the following examples. Although light oil had 
been flashed from the oil feed to the filter in process pressure step-down 
stages, the filter feel oil had not experienced removal of any of its 
solids content prior to filtration. 
Specific gravity, 60.degree. F. (15.6.degree. C.), 1.15 
Kinematic viscosity at 210.degree. F. (98.9.degree. C.), 24.1 centistokes 
Density at 60.degree. F. (15.6.degree. C.), 1.092 
Ash, 4.49 weight percent 
Pyridine insolubles, 6.34 weight percent 
Distillation, ASTM D1160 
______________________________________ 
Percent Temp., .degree. F. (.degree. C.) at 1 
______________________________________ 
atm. 
5 518 (270) 
10 545 (285) 
20 566 (297) 
30 602 (317) 
40 645 (341) 
50 695 (368) 
60 768 (409) 
70 909 (487) 
71 
recovery of all 
distillables 
occurs at 925.degree. F. 
(496.degree. C.) 
______________________________________

EXAMPLE 1 
A series of filtration tests was performed to illustrate the effect upon 
filtration of the addition of various alcohols and of phenol to a coal 
liquid. These tests were performed at a temperature of 500.degree. F. 
(260.degree. C.) and with a pressure drop across the filter of 40 psi (2.8 
Kg/cm.sup.2). Following is a tabulation of the results of these tests. 
______________________________________ 
Additive k, (min/g.sup. 2) 
C, (min/g) Rate, (g/min) 
______________________________________ 
None .0256 .22 3.2 
n-propyl alcohol, 
.0245 .12 4.5 
2 wgt. % 
sec. butyl alcohol, 
2 wgt. % .0164 .13 5.0 
ter. butyl alcohol 
2 wgt. % .0236 .05 5.6 
iso amyl alcohol, 
2 wgt. % .0226 .28 3.1 
phenol, 2 wgt. % 
.0278 .27 2.8 
______________________________________ 
In considering the above data, it is reiterated that the filtering 
resistance parameter, k, is the best indicator of the effect of the 
additive upon the filtering operation because this parameter excludes all 
effects upon filtration inherent in the filtering system and the precoat. 
On the other hand, the value C is indicative of the effect of the 
filtering system and the precoat independently of the effect of the 
alcohol or phenol additives. 
The above data show that the filtering resistance parameter, k, was reduced 
to various extents by all the alcohols tested, with secondary butyl 
alcohol effecting the greatest reduction in the resistance parameter. In 
contrast, phenol increased the resistance parameter, showing that it is 
apparently a dispersion medium, rather than an agglomerant. Therefore, the 
presence of phenol has an adverse effect upon filtration of coal liquids. 
EXAMPLE 2 
Additional filtering tests were performed at 410.degree. F. (210.degree. 
C.) and with a filter pressure drop of 80 psi (5.6 Kg/cm.sup.2) to 
illustrate the effect of methyl alcohol and ethyl alcohol as additives to 
a coal liquid being filtered. The results of these tests are shown in the 
following table. 
______________________________________ 
Additive (2 wgt. %) 
k, (min/g.sup.2) 
C, (min/g) 
Rate, (g/min) 
______________________________________ 
None .0254 .07 5.0 
Methyl alcohol 
.0341 .07 4.5 
None .0376 .06 4.4 
Ethyl alcohol 
.0319 .10 4.6 
______________________________________ 
As shown in the above data, methyl alcohol has a detrimental effect upon 
the filtering resistance parameter, k, while ethyl alcohol has a slight 
beneficial effect. 
EXAMPLE 3 
Tests were performed to determine the effect of organic acids, aldehydes 
and ketones upon the filtration of coal liquids. The results of these 
tests are shown in the following table. 
______________________________________ 
Filtration at 500.degree. F. (260.degree. C.) 
and a pressure drop of 80 psi (5.6 Kg/cm.sup.2) 
______________________________________ 
Additive (2 wgt. %) 
k, (min/g.sup.2) 
C, (min/g) 
Rate, (g/min) 
______________________________________ 
None .0247 .20 3.5 
Butyl aldehyde 
.0258 .18 3.5 
None .0263 .32 2.5 
Acetic acid .0245 .35 2.5 
None .0239 .26 3.0 
Acetone .0372 .23 2.9 
Filtration at 410.degree. F. (210.degree. C.) 
and a pressure drop of 80 psi (5.6 Kg/cm.sup.2) 
______________________________________ 
None .0235 .15 4.1 
Methyl ethyl ketone 
.0256 .17 3.9 
______________________________________ 
As shown in the above data, butyl aldehyde, methyl ethyl ketone and acetic 
acid all exhibited an insignificant effect upon the resistance parameter, 
k. Acetone exhibited a slightly detrimental effect. The use of acids would 
not be desirable in an industrial process because of their corrosive 
nature. 
EXAMPLE 4 
Tests were performed to determine the effect of the amount of isopropanol 
additive upon the filtration of coal liquids. These tests were performed 
at 500.degree. F. (260.degree. C.) and at a pressure drop of 40 psi (2.8 
Kg/cm.sup.2). The results of these tests are shown in the following table. 
______________________________________ 
Additive and concentration, 
weight percent k, (min/g.sup.2) 
Rate, (g/min) 
______________________________________ 
None .0192 5.6 
Isopropanol, 1% .0119 7.3 
Isopropanol, 2% .0065 8.6 
Isopropanol, 2.7% 
.0086 9.2 
______________________________________ 
The above data show a progressive reduction in the resistance parameter, k, 
as the amount of isopropanol is incrementally increased from 0 to 1 to 2 
percent, respectively. However, the advantage at 2.7 percent is lower than 
that at 2 percent, indicating that an amount of alcohol beyond a critical 
level in a single injection decreases the beneficial effect obtainable. 
EXAMPLE 5 
In all the tests of the above examples a single additive injection was 
employed. However, the tests of the present example illustrate the effect 
of holding time and incremental addition of secondary and tertiary butyl 
alcohol. In these tests, the additive was added to a coal liquid feed 
maintained at a 120.degree. F. (49.degree. C.) holding temperature. The 
filtering tests were performed at 500.degree. F. (260.degree. C.) and 80 
psi (5.6 Kg/cm.sup.2) and included a holding time of two minutes at 
500.degree. F. (260.degree. C.). The results of these tests are shown in 
the following table. 
__________________________________________________________________________ 
Elapsed time at 
120.degree. F. (49.degree. C.) 
Additive and between addition 
Concentration, of additive and 
wgt. percent 
k,(min/g.sup.2) 
C, (min/g) 
Rate, (g/min) 
filtration, min. 
__________________________________________________________________________ 
None .0534 .06 3.8 -- 
sec. butyl 
alcohol-2% 
.0309 .29 2.8 1 
sec. butyl 
alcohol-2% 
.0301 .12 4.1 40 
sec. butyl 
alcohol-2% 
.0309 .29 2.8 80 
sec. butyl 
alcohol-4%* 
.0190 .16 4.2 85 
(5 min. after 
first addition) 
sec. butyl 
alcohol-4%* 
.0265 .17 3.7 135 
(55 min. after 
first addition) 
ter. butyl 
alcohol-2% 
.0236 .05 5.6 5 
ter. butyl 
alcohol-2% 
.0247 .15 4.1 45 
__________________________________________________________________________ 
*Includes original 2% plus an additional 2% added after 80 minutes. 
The above data show that the holding time between the introduction of the 
secondary butyl alcohol to the filter feed and the performance of the 
filtration operation has an effect upon the filtering resistance 
parameter, k. Within 80 minutes of the addition of the original 2 percent 
of secondary butyl alcohol, the effect of the alcohol increased to a peak 
and then declined, since the observed advantage of the additive is greater 
after 40 minutes than it is after either 1 or 80 minutes. Furthermore, 
after the addition of the second 2 percent of secondary butyl alcohol, the 
observed effect of the additive was greater after 5 minutes than after 55 
minutes. A similar observation on the effect of time is apparent in the 
case of tertiary butyl alcohol. Referring again to the secondary butyl 
alcohol data, it is seen that although the effect of the addition of the 
first two percent of secondary butyl alcohol peaked and declined with age, 
and the effect of the second addition of secondary butyl alcohol similarly 
peaks and declines with age, the second peak advantageously occurs at a 
lower filtration resistance than the first peak. This shows that 
intermittent addition of the secondary butyl alcohol permits achievement 
of an enhanced advantage due to the additive. This observation is 
surprising in view of the data of Example 4 which show that the advantage 
of isopropanol addition declines as the quantity increases in a single 
injection. Since, in practice, the alcohol employed can be recycled, it is 
a considerable advantage of the present invention that a method is 
provided for enhancing the effect of the alcohol additive via increase of 
the amount of the alcohol employed. By employing recycle, the increased 
amount of alcohol used in the process has very little effect upon 
operating costs. 
EXAMPLE 6 
A series of tests was performed using isopropanol to further illustrate the 
effect of holding time between the addition of the isopropanol to the coal 
liquid and the filtration of the liquid. The tests were performed at 
500.degree. F. (260.degree. C.) and with a pressure drop of 80 psi (5.6 
Kg/cm.sup.2). The results of these tests are shown in the following table. 
______________________________________ 
Elapsed time 
Additive and between addition 
Concentration, Rate, of additive and 
Wgt. Percent 
k, (g.sup.2 /min).sup.-1 
(g/min) filtration, min. 
______________________________________ 
None .0284 3.9 -- 
Isopropanol, 2% 
.0191 5.4 3 
Isopropanol, 2% 
.0144 7.0 6 
Isopropanol, 2% 
.0139 7.1 9 
None .0464 2.4 -- 
Isopropanol, 2% 
.0209 3.4 35 
______________________________________ 
The above data show an improved effect upon the filtration resistance 
parameter, k, resulting from an extended holding time between the addition 
of isopropanol and the filtration. These data tend to indicate the 
occurrence of a delayed reaction between the alcohol additive and material 
in the coal liquid. 
EXAMPLE 7 
Four filtering tests were performed to further illustrate the effect of the 
time interval between the introduction of isopropanol to the coal liquid 
and the filtering operation. In one test, isopropanol was not added. The 
coal liquid of the other three tests contained two weight percent 
isopropanol with holding times of two, four and six minutes, respectively. 
In all of the tests, the temperatures were about 500.degree. F. 
(260.degree. C.), and the pressure drop was 80 psi (5.6 Kg/cm.sup.2). The 
results of these tests are shown in the figure. The times noted at the 
data points along each parameter curve are the elapsed times between the 
start of the filtering tests and the times at which the data point was 
obtained. As shown in the figure, the use of isopropanol reduced the 
filtration resistance in all cases. However, progressively lengthened 
holding times between the addition of the isopropanol and start of the 
filtration test resulted in progressively lower filtering resistances. 
EXAMPLE 8 
A series of filtering tests was performed to further illustrate the 
advantage of intermittent addition of alcohol. In all of these tests, 
isopropanol was added to an unfiltered liquid coal mixture held at a 
temperature between 110.degree. and 130.degree. F. (43.degree. and 
54.degree. C.). The holding time between completion of alcohol addition 
and filtration was 5 minutes at a holding temperature of 500.degree. F. 
(260.degree. C.). The filtrations were performed at 500.degree. F. 
(260.degree. C.) with a pressure drop of 80 psi (5.6 Kg/cm.sup.2). 
Following are the results of the tests. 
______________________________________ 
Additive and 
Concentration, 
Wgt. Percent 
k, (min/g.sup.2) 
C, (min/g) 
Rate, (g/min) 
______________________________________ 
None .0510 .07 3.8 
Isopropanol-2% 
(added in a single 
increment) .0239 .09 4.9 
Isopropanol-4% 
(added in two in- 
crements with sec- 
ond 2% increment 
added 30 minutes 
after adding first 
2% increment) 
.0188 .03 6.5 
Isopropanol-4% 
(added in a single 
increment) .0218 .05 5.7 
______________________________________ 
The above data show that the addition of 4% of isopropanol in a single 
increment resulted in a slightly improved resistance parameter as compared 
to the addition of a single increment of 2% of isopropanol. However, the 
addition of 4% of isopropanol in two equal spaced increments resulted in a 
significant further improvement in the resistance parameter.