Coal liquefaction using liquid clathrates

The method for the liquefaction of a solid carbonaceous substance whereby the liquefaction takes place as a result of the addition of a liquid clathrate to said substance. The liquid clathrate layer then contains the liquified petroleum oil products which are then separated from the liquid clathrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to low temperature coal liquefaction, and 
more particularly to liquefaction of coal using a liquid clathrate. 
2. Description of the Prior Art 
The oldest of the modern direct liquefaction processes, dating back to 
about 1962, is the Solvent Refined Coal (SRC) process, developed by 
Spencer Chemical. 
In the original process, now known as SRC-I, pulverized raw coal is mixed 
with a process-derived solvent and a small amount of hydrogen at high 
temperature and pressure. The coal dissolves; most of its ash and much of 
its sulfur settle out and can be removed by filtration. The resulting 
relatively clean liquid can be burned in that form, or it can be cooled to 
a tarlike solid for easier transportation and storage. 
A later, modified version, SRC-II, uses more hydrogen and operates under 
more severe conditions of temperature, pressure, and residence time. Most 
of the coal is converted to liquids mainly naphtha and boiler fuel. 
Recently, two 6000 ton-per-day demonstration plants--a modified SRC-I in 
Kentucky and an SRC-II in West Virginia--have been proposed. Conceivably, 
commercial-scale plants using either of these processes could be in 
operation by 1989 or 1990. 
Another approach to coal dissolution is the Exxon Donor Solvent (EDS) 
process. Crushed, dried feed coal is slurried with a hydrogenated recycle 
solvent (the donor solvent) and fed, along with gaseous hydrogen into an 
upward plug-flow reactor of fairly simple design. The effluent is 
separated by distillation into several fractions: the recycle solvent, 
depleted of its hydrogen; light hydrocarbon gases; heavier distillates, 
boiling at up to 1000.degree. F.; and a heavy vacuum bottoms stream 
containing still heavier liquids, unconverted coal, and ash. 
The recycle solvent is rehydrogenated catalytically in a conventional 
fixed-bed reactor. Bottoms are fed with steam and air to an Exxon 
Flexi-coking unit, which produces additional liquids and low Btu gas. In 
contrast to the other processes, hydrogen is obtained by steam-forming the 
light hydrocarbon gases. 
The third direct liquefaction process currently being seriously considered 
for commercialization is the H-Coal process, developed by Hydrocarbon 
Research Inc. The H-Coal process employs no solvent. Instead, dried, 
crushed coal is slurried with heavy distillate from the process, 
pressurized, mixed with compressed hydrogen, preheated and fed to an 
ebullated-bed catalytic reactor. 
Effluent gases are cooled to separate heavier components as liquids. Light 
hydrocarbons, ammonia and hydrogen sulfide are absorbed from the remaining 
hydrogen-rich gas, which is recompressed and recycled to the input slurry. 
The liquid-solid portion, containing unconverted coal, ash and oil goes to 
a flash separator. The lighter portions go to an atmospheric distillation 
unit, while the bottoms are separated with a hydrocyclone, a liquid solid 
separator, and a vacuum still. 
All of these direct liquefaction procedures require considerable energy 
input and are not truly cost effective techniques. 
A need therefore exists for the development of a low-energy input 
liquefaction process. 
SUMMARY OF THE INVENTION 
Accordingly, it is one object of the present invention to provide a process 
for the liquefaction of coal at low temperatures with minimum input of 
energy. 
It is another more particular object of the present invention to produce 
petroleum oil fractions from coal. 
These and other objects of the invention, as will hereinafter become more 
readily apparent by the following description, have been attained by 
providing a method for the liquefaction of coal which comprises admixing 
said coal with a liquid clathrate, maintaining said admixture for a period 
sufficient to form a liquid clathrate layer containing liquified petroleum 
oil products, decomposing said clathrate to separate said clathrate from 
said petroleum oil, whereby a petroleum oil phase is produced, and 
separating said petroleum oil phase from said decomposition products. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the present invention, coal is liquified by admixing the coal 
with a liquid clathrate. The chemical mechanism for this phenomena is not 
fully known, but seems to have similarities to a solvent or catalytic 
effect. The really surprising aspect of this process is that it occurs at 
or near room temperature, and in any event, at temperatures which are very 
far below those temperatures for any other known liquefaction technique. 
For instance, liquifaction can be effected at temperatures of from 
10.degree.-80.degree. C. and preferably 15.degree. C.-50.degree. C. In 
many cases, liquefaction will occur at or near room temperature with no 
application of heat. 
The coal or solid carbonaceous material used in this process can be any 
form of coal including bituminous and subbituminous coal, lignite, or such 
coal-like forms as oil shale or tar sands. The one which was used 
predominantly in our research was bituminous which is mined locally at the 
Chetopa mine, Mary Lee Seam. The analysis of the coal used can vary widely 
from 40 to 80% by weight carbon, 3 to 15 percent by weight hydrogen, 0 to 
10 percent by weight oxygen, 0 to 15 percent by weight nitrogen and 0 to 7 
percent by weight sulfur. Preferred carbon ranges from 80-100 parts per 
60-80 parts hydrogen, 0 to 8 parts oxygen, 0 to 8 parts nitrogen and 0 to 
4 parts sulfur. 
It is preferred to use very dry coal because many of the clathrates used 
herein are moisture sensitive. If necessary, the coal can be dried by 
conventional means to a dryness of 1 weight percent or less. 
The coal may be used in rock form, as mined, or may be crushed to a size of 
0.5 mm or smaller to increase the surface area to enable maximum contact 
with the liquid clathrate. It appears that the size of the coal used 
merely affects the period of time of liquid clathrate contact. Thus, the 
larger the coal formation, the longer will be the contact time necessary. 
The contact time can be reduced by applying mixing or gentle aggitation. 
Thus, the larger chunks of coal can be liquefied as fast as smaller 
particles if aggitation or stirring is applied during the contact time. It 
is believed to be possible in some instances to pump the liquid clathrate 
into a coal mine shaft whereby the coal liquefaction will occur in situ 
within the mine and thereafter to pump out a petroleum oil-like product 
directly from the shaft. If this will work, the commercial advantages are, 
of course readily apparent. 
"Liquid Clathrate" is a term of art which refers to certain enclosure 
compounds. A liquid clathrate is a loose structure of a complex salt and 
an aromatic whereby the aromatic is entrapped into the complex. The 
aromatics can be retrieved unchanged by lowering the temperature. The 
liquid clathrate will only accomodate a certain number of aromatic 
molecules and the excess aromatic will be immiscible with the clathrate. 
See J. L. Atwood et al, Journal Organometallic Chemistry 66, 15-21 (1974) 
42, C 77-79, (1972), 61. 43-48 (1973), 65, 145-154(1974). 
In general, the clathrates used herein are any of those disclosed and 
claimed in U.S. Pat. No. 4,024,170 which disclose crown ether containing 
clathrates. 
The liquid clathrates used herein may have the formula 
EQU M(Q.sub.n R.sub.3m X).vY 
wherein M is a mono, di- or trivalent cation, X is an anion of a mono-, di- 
or tri-negative salt, Q is Al or Ga, n is 2-4, v is 4 to 40 and Y is a 
hydrocarbon aromatic compound. 
The cation M may be a multidentate macromolecular complex salt cation, 
hereafter referred to as "crown ether complex salt cation", or a simple 
salt cation. For instance, suitable simple salt cations include alkali, 
alkaline, quaternary ammonium, phosphonium, arsonium, sulfonium, tellurium 
or mixtures thereof, including K+, Rb+, Cs+, Nr'.sub.4 +, Cr(C.sub.6 
H.sub.6).sub.2 +, Co(C.sub.5 H.sub.5).sub.2 +, TlR'.sub.2 +, PR'.sub.4 +, 
wherein R' is hydrogen, alkyl of C.sub.1-10, phenyl or naphthyl. 
Alternatively, crown ether complex salt cations may be used, such as those 
of the formula 
##STR1## 
wherein q is 4-8 and R.sup.2 is a lower alkyl, aryl or aryl which is fused 
to said ring, and r is an integer of 0-4. The complex can be formed 
wherein the metal cation M.sub.1.sup..sym. is complexed with one or more 
than one crown ether in the form 
##STR2## 
M.sub.1.sup..sym. in the above structure may be the same as M defined 
above, or may be a divalent cation such as Ba.sup.++ or Ca.sup.++ which 
when complexed with the crown ether is characterized by the said complex 
having the charge of the naked cation. 
Particularly suitable crown ethers which can be used in the complexes 
include 18-crown-6, 15-crown-5 or dibenzo-18-crown-6, and those described 
in the said copending crown ether patent application. 
It has been found that the larger the cation M.sub.1, the greater will be 
the number of molecules of hydrocarbon aromatic compounds which can be 
entrapped by the complex salt. 
Suitable hydrocarbon aromatic compounds which can be used in forming the 
clathrate include benzene, toluene, o-, m-, or p-xylene, mesitylene, 
tetramethylbenzene, dipropylbenzene, diisopropylbenzene, naphthalene, 
tetralin, anthracene or phenanthracene. Benzene and toluene have been 
demonstrated to give good results. 
The anion X may be any mono-, di or tri-negative salt anion such as halide, 
particularly Cl.sup.-, F.sup.-, Br.sup.-, or I.sup.-, azide, SCN.sup.-, 
SeCN.sup.-, nitrite, nitrate, lower alkyl acyl such as CH.sub.3 
--COO.sup.-, or HCOO.sup.-, hydroxide, carbonate, sulfate or phosphate. 
R in the formula may be a lower alkyl of 1-8 carbon atoms, particularly 
methyl, ethyl, propyl or butyl when Q is Al, the Al.sub.n R.sub.3n 
component is derived from an aluminum trialkyl compound such as trimethyl 
aluminum, triethyl aluminum, or the like. However, other aluminum alkyl 
compounds can be used such as tri-n-propyl aluminum, tri-isopropyl 
aluminum, or tri-n-butyl aluminum. When Q is Ga, the corresponding 
Ga.sub.n R.sub.3n component is derived from the corresponding gallium 
containing compound. 
In general, one must have a complex salt having the angular geometry 
##STR3## 
to form the aromatic clathrate; whereas a symetrical anionic structure 
EQU QR.sub.3 -- X --QR.sub.3 
will not normally clathrate. 
The nature of the clathrate interaction is thus related to the nature of 
the anion, the latice energy, size of the cation and the size of the 
aromatic molecule. It is expected that other clathrates based on systems 
other then the QR.sub.3 or GaR.sub.3 models as discussed above, will be 
useable. For instance, liquid clathrates of the form 
EQU K[CH.sub.3 Se{Al(CH.sub.3).sub.3 {.sub.3 ].6C.sub.6 H.sub.6 
can be used herein. 
Further information concerning liquid clathrates can be obtained by 
reference to Atwood "Liquid Clathrates," Recent Developments in Separation 
Sciences, CRC Press, Cleveland, 1977, pages 195-209 (1978). 
Mixtures of different clathrates can be used in the admixture to liquify 
the coal. 
The complex salt used however, can be prepared by reacting the aluminum 
alkyl compound with a simple salt such as the alkali nitrate carbonate, 
sulfate, azide or the like. Upon introduction of the hydrocarbon aromatic 
compound, the salt is converted into the liquid clathrate. The liquid 
clathrate can be produced in one step or in multiple steps. Thus, the 
simple salt, the aluminum alkyl and the aromatic, such as benzene or 
toluene, can be admixed together in one step, or the simple salt and the 
aromatic can be admixed and then combined with coal and aluminum alkyl to 
form the clathrate in situ with the coal. The former is the method of 
preference. Any such combination is suitable for forming the clathrate. 
The clathrate forming reaction can occur at room temperature or higher, up 
to about 190.degree. C., depending on the particular choice of materials. 
Beyond 190.degree. C., the aluminum alkyl will decompose. Good results are 
attainable in the range of 15.degree.-80.degree. C. Upon cooling, a 
temperature will be reached at which the clathrate will decompose back to 
the complex salt and the aromatic compound. The only limitation on the 
contact conditions between the clathrate and the coal seems to be that the 
temperature must be selected such that the liquid clathrate will be in 
existence during the period of contact. If one is concerned with materials 
which will clathrate at a temperature of say. 60.degree. C. but wherein 
the clathrate decomposes back to the complex salt below that temperature, 
of course, the temperature of coal-clathrate contact must be above 
60.degree. C. 
When the crown ether complex salt is used, the crown ether, simple salt, 
aluminum alkyl and aromatic must all be brought together. The order of 
addition does not seem to be critical. In fact, the complex salt can first 
be formed, converted into a clathrate, and then admixed with the crown 
ether, whereupon the new ether containing complex will clathrate with 
additional aromatic compound, as compared to the quantity clathrated by 
the non-crown ether complex salt. 
If the liquid clathrate is formed in an excess of the aromatic, which is 
usually the case, the existence of the clathrate can be visually detected 
by a phase separation between a top aromatic hydrocarbon compound solvent 
layer and the bottom liquid clathrate. 
When the clathrate is admixed with the coal, the clathrate is immediately 
discolored by a black petroleum-like product. Thereafter, the aromatic 
solvent layer gradually becomes discolored as the lighter liquified 
petroleum products are leached into that layer. The light oil dissolved in 
the solvent accounts for 5-10% of the oil recovery. From 250 parts to 2500 
parts or more of liquid clathrate per part by weight of coal is sufficient 
to obtain the desired effect. 
Petroleum oil separation is attainable almost immediately, with useable 
yields appearing after 30 minutes. Contact, however, can be maintained for 
an indefinite period of time and often 1-2 days is desirable. 
At the termination of the contact period, the aromatic solvent layer can be 
decanted off and the hydrocarbon oils contained therein recovered by 
ordinary separation means. If the contact time has been permitted for a 
sufficiently long period, most of the petroleum like oil will have been 
either solvent extracted into that solvent phase or, in particular, the 
heavier oils, will have been precipitated out at the bottom of the 
container. Alternatively, the temperature of the clathrate is reduced or 
the mixture is subjected to distillation in order to decompose the 
clathrate and to crystallize out the complex salt. A petroleum oil phase 
and a solvent (aromatic hydrocarbon) phase above it will appear upon the 
decomposition of the clathrate. As asphalt-like material is found to cover 
the complex salt crystals, which asphalt has been found to have an average 
molecular weight of 300-400 and a pour point above 200.degree. C. The fact 
that the residue is an asphalt-like material evidences that the coal is 
being chemically modified in some manner although the precise mechanism 
for this modification is as yet unknown. The ability of the clathrate to 
be discolored rapidly after contact with the coal is evidence of a solvent 
type activity. On the other hand, the fact that the structure of the 
residue seems to be altered and the fact that no discernable amounts of 
the complex salt used to make the clathrate is lost, leads to a catalysis 
explanation. 
Instead of decomposing the clathrate by reduction in temperature, where the 
clathrate selected is water or oxygen sensitive, it can alternatively be 
decomposed by introduction of water moisture or oxygen into the system 
which attacks the aluminum alkyl component of the salt. For this reason, 
it is desirable to carry out the coal-clathrate contact in a dry, inert 
atmosphere. A blanket of nitrogen, argon or other inert gas can be 
desirably maintained over the mixture during the period of contact. 
The petroleum oil layer formed after the decomposition of the clathrate, is 
then collected. Tests of the oil confirm that it is a petroleum oil having 
a weight average molecular weight of from 40 to 300 and a boiling point of 
from 30.degree. to 300.degree. C. Spectroscopic analysis has confirmed 
that the oil being produced is a hydrocarbon oil containing a multiplicity 
of different hydrocarbon products. In the 110.degree. C. boiling fraction, 
more than seventy different compounds result. The present inventor expects 
that the limitation in the yield of petroleum oil produced is a function 
of the amount of hydrogen present in the coal sample. Oil recovery amounts 
to about 10-15% by weight based on the weight of bituminous coal treated. 
Much higher yields have been obtained for lignite and for tar sands. Good 
results have been attained with clathrates of the form: 
EQU K[Al.sub.2 Me.sub.6 N.sub.3 ] 
EQU NMe.sub.4 [Al.sub.2 Me.sub.6 Cl] 
EQU NMe.sub.4 [Al.sub.2 Me.sub.6 I] 
EQU NEt.sub.4 [Al.sub.2 Me.sub.6 I] 
EQU NPr.sub.4 [Al.sub.2 Me.sub.6 I] 
EQU NEt.sub.4 [Al.sub.2 Me.sub.6 NO.sub.3 ] 
EQU NEt.sub.4 [Al.sub.2 Et.sub.6 NO.sub.3 ] 
EQU K.sub.2 [Al.sub.2 Me.sub.6 SO.sub.4 ] 
The economics of the present technique for coal liquefaction are extremely 
attractive. Unlike all other known liquefaction procedures, little or no 
heat input is required. Moreover, the cost of producing the clathrates is 
very modest and the loss factor of the complex salts used to produce the 
clathrates is small.