U.S. patent number 3,929,615 [Application Number 05/409,914] was granted by the patent office on 1975-12-30 for production of hydrocarbon gases from oil shale.
This patent grant is currently assigned to American Gas Association, Inc.. Invention is credited to Harlan L. Feldkirchner, Henry R. Linden, Paul B. Tarman.
United States Patent |
3,929,615 |
Linden , et al. |
* December 30, 1975 |
Production of hydrocarbon gases from oil shale
Abstract
A process for the production of hydrocarbon gases from oil shale
comprising the steps of gradually preheating oil shale in a preheat
and prehydrogenation zone to a temperature of about 700.degree. to
about 950.degree. F. in the presence of hydrogen-rich gas without
substantial production of liquid or gas in the preheat and
prehydrogenation zone, then hydrogasifying the preheated and
prehydrogenated oil shale in a hydrogasification zone at a
temperature of about 1,200.degree. to about 1,500.degree. F. in the
presence of hydrogen-rich gas to form predominately low molecular
weight paraffinic hydrocarbon gases from the preheated and
prehydrogenated organic hydrocarbon portion of the oil shale. The
hydrogen-rich gas may be passed countercurrent in thermal exchange
relation to the spent shale to recover heat from the spent shale
heating the hydrogen-rich gas for passage countercurrent and in
thermal exchange relation to fresh oil shale in the preheat and
prehydrogenation zone. The improvement of this process lies in the
exceptionally high conversion of the organic component of oil shale
to products of high value having high content of low molecular
weight paraffinic hydrocarbon gases.
Inventors: |
Linden; Henry R. (La Grange
Park, IL), Tarman; Paul B. (Elmhurst, IL), Feldkirchner;
Harlan L. (Elk Grove Village, IL) |
Assignee: |
American Gas Association, Inc.
(Arlington, VA)
|
[*] Notice: |
The portion of the term of this patent
subsequent to December 25, 1992 has been disclaimed. |
Family
ID: |
27003183 |
Appl.
No.: |
05/409,914 |
Filed: |
October 26, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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365973 |
Jun 1, 1973 |
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Current U.S.
Class: |
208/408; 48/210;
208/412; 48/197R; 208/411 |
Current CPC
Class: |
C10B
53/06 (20130101) |
Current International
Class: |
C10B
53/06 (20060101); C10B 53/00 (20060101); C10B
053/06 () |
Field of
Search: |
;48/197R,211,202
;208/11 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Bashore; S. Leon
Assistant Examiner: Kratz; Peter F.
Attorney, Agent or Firm: Speckman; Thomas W.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of copending application Improved
Process for Production of Hydrocarbon Liquids and Gases from Oil
Shale, Ser. No. 365,973, filed June 1, 1973.
Claims
We claim:
1. In the process for the production of pipeline quality gas by
gasification of organic components of oil shale wherein above about
77 percent of the organic carbon in said oil shale is converted to
liquids and gases the improvement comprising:
introducing fresh oil shale into a preheat and prehydrogenation
zone;
gradually preheating oil shale in a preheat and prehydrogenation
zone to a temperature of about 700.degree. to about 950.degree.F.
in the presence of hydrogen-rich gas with less than about 20 weight
percent of the organic component of oil shale converted to liquid
and gas in said preheat and prehydrogenation zone, said preheating
at a rate of less than about 50.degree.F. per minute above about
500.degree. to 600.degree.F. and said hydrogen-rich gas and shale
passing countercurrently in said preheat and prehydrogenation
zone;
hydrogasifying the preheated and prehydrogenated organic components
of oil shale in a hydrogasification zone at a temperature of about
1200.degree. to about 1500.degree.F. in the presence of at least a
stoichiometric amount of hydrogen-rich gas to form low molecular
weight paraffinic gases and liquid hydrocarbons from the preheated
and prehydrogenated organic portion of said oil shale;
said hydrogen-rich gas being supplied to said preheat and
prehydrogenation zone in larger volumes than the hydrogen-rich gas
supplied to said hydrogasification zone;
removing spent shale from the hydrogasification zone separate from
said gases; and
purifying and upgrading said gases by removing liquids and
undesired steam, carbon monoxide, carbon dioxide, ammonia and
hydrogen sulfide followed by methanation to provide pipeline
quality gas.
2. The process of claim 1 wherein spent shale is passed from said
hydrogasification zone to a heat recovery zone wherein
hydrogen-rich gas recycled from said preheat and prehydrogenation
zone passes countercurrent and in thermal exchange relation to said
spent shale cooling the spent shale and heating the hydrogen-rich
gas for introduction directly to said preheat and prehydrogenation
zone.
3. The process of claim 2 wherein said heated hydrogen-rich gas is
further heated during passage from said heat recovery zone to said
preheat and prehydrogenation zone.
4. The process of claim 2 wherein the hydrogenrich gas stream
leaving said preheat and prehydrogenation zone is split, with one
portion, containing sufficient gas to provide at least the chemical
hydrogen requirements in the hydrogasification zone, being passed
to said hydrogasification zone and the other portion being recycled
to said heat recovery zone.
5. The process of claim 4 wherein said one portion of hydrogen-rich
gas stream is further heated during passage to said
hydrogasification zone.
6. The process of claim 1 wherein the oil shale is preheated to
about 750.degree. to about 850.degree.F. in said preheat and
prehydrogenation zone.
7. The process of claim 1 wherein the preheated and prehydrogenated
oil shale is heated to about 1300.degree. to about 1400.degree.F.
in said hydrogasification zone.
8. The process of claim 7 wherein said hydrogasification zone is
heated by methanation of carbon dioxide.
9. The process of claim 7 wherein the input hydrogen-rich gas to
the hydrogasification zone is heated by thermal exchange with the
output of the hydrogasification zone.
10. The process of claim 1 wherein the preheat and prehydrogenation
zone and the hydrogasification zone is at a total gas pressure of
about 100 to about 2000 psia.
11. The process of claim 10 wherein said total gas pressure is
about 500 to about 1500 psia.
12. The process of claim 1 wherein the preheat and prehydrogenation
zone and the hydrogasification zone is maintained at a hydrogen
partial pressure above about 20 psia.
Description
This invention relates to an improved process for the production of
predominately low molecular weight paraffinic hydrocarbon gases
from oil shales. Low molecular weight paraffinic hydrocarbon gases
include molecules of 4 and less carbon atoms, namely methane,
ethane, propane, butane and isobutane. The production of small
amounts of aliphatic and alicyclic hydrocarbon liquids including
straight or branched chain molecules which may be saturated or
unsaturated and alicyclic hydrocarbon molecules substantially free
from aromatic double bonds, is not detrimental since they are
especially suited for various further processing. Such liquids may
be used for preparing a high methane content pipeline-quality gas
suitable as a substitute for or as a supplement to natural gas by
direct recycling to the hydrogasifier. The process of this
invention provides high methane content pipeline-quality gas.
Oil shales are sedimentary rocks which are thought to have been
formed from finely divided mineral matter and organic debris from
aquatic organisms and some plant matter which were deposited on the
bottoms of shallow lakes and seas and later solidified. The
resulting oil shales are fine-grained impermeable rocks in which it
is almost impossible to separate the organic component and the
inorganic mineral matter without changing the structure of the
organic component. One of the major inorganic constituents of oil
shales is carbonates. Oil shales vary in the amount and in the
constitution of the organic component which is called kerogen.
Typical oil shales contain about 10 to 30 weight percent kerogen.
Kerogen is a high molecular weight hydrocarbon having a molecular
weight of over 3000 and a carbon to hydrogen weight ratio (C/H)
typically of about 7/1 to 8/1.
Due to the high demands upon natural gas supplies and their limited
reserves, synthetic pipeline gas will be needed to supplement such
natural gas in the United States and other countries of the world.
The interest in an economical process for producing synthetic
pipeline gas from oil shales is high. There are abundant reserves
of commercial grades of oil shales in the United States,
particularly in the northwestern areas of Colorado and adjoining
areas of Utah and Wyoming.
To be suitable as use for pipeline gas, the heating value must be
about 900 to 1100 BTU/SCF which results from high methane content,
normally 80% by volume or greater. Such specifications require that
for pipeline-quality gas the carbon to hydrogen weight ratio be
low, approaching as low as 3/1.
To produce a high yield of pipeline-quality gas from oil shales, it
is desirable to limit the coking and aromatization of the oil
shales' organic component to limit formation of aromatic oils and
carbon or coke during gasification. It is also desired to limit
decomposition of mineral carbonates present in the oil shale and
resultant carbon dioxide formation which increases process heat
requirements, consumption of hydrogen, and increases the cost of
purification.
Previous processes for the production of hydrocarbon fuels from oil
shale have the disadvantages of lower thermal efficiency and/or
lower conversion of the organic component of oil shale to suitable
gas.
It is an object of this invention to optimize the production of
paraffinic hydrocarbon gases from oil shale.
It is another object of this invention to provide a process for the
production of low molecular weight paraffinic hydrocarbon gases
from oil shale wherein the oil shale is preheated and
prehydrogenated by countercurrent flow of hydrogen-rich gas and may
be hydrogasified by countercurrent or cocurrent flow of
hydrogen-rich gas.
It is still another object of this invention to provide a process
for high yield production of pipeline-quality gas from oil shale by
a process characterized by its high thermal efficiency.
Further objects of this invention will appear to one skilled in the
art as this description proceeds and by reference to the
figures.
Preferred embodiments of this invention are illustrated in the
drawing wherein:
FIG. 1 is a block flow diagram illustrating the production of low
molecular weight paraffinic hydrocarbon gases from oil shale using
one embodiment of the process of this invention.
The process of this invention is applicable to a wide variety of
oil shales. For high efficiency it is desired that the Fischer
Assay, which indicates the oil yield obtained by conventional
retorting of the oil shale, be 25 gallons per ton or more. However,
the process of this invention is also applicable to oil shales
having lower Fischer Assays, down to about 10 gallons per ton.
The size of the shale particles used in the process of this
invention is not important, but particles generally of the size
one-sixteenth inch to 1 inch diameter are utilized. Use of very
fine particles may give difficulty in plugging during processing
and large particles have a lower surface area and may result in
longer processing times.
The fresh oil shale is fed into a preheat and prehydrogenation zone
and gradually preheated to a temperature of about 700.degree. to
about 950.degree. F. in the presence of hydrogen-rich gas. It is
preferred that the temperature to which the oil shale is heated in
the preheat and prehydrogenation zone is about 750.degree. to about
850.degree. F. At 700.degree. F. the rate of adequate
prehydrogenation is slow, but may be achieved by a residence time
of several hours. At the higher temperatures of about 950.degree.
F. prehydrogenation occurs in a few minutes. The criteria of
adequate prehydrogenation is the ultimate increased recovery of
organic matter from the shale which is expressed most conveniently
in terms of organic carbon recovery. By the process of this
invention, as much as about 95 percent of the organic carbon can be
removed from oil shale in the form of valuable liquid and gaseous
hydrocarbons. Longer residence times at the higher temperatures
lead to undesired production of oil and gas by hydroretorting in
the preheat and prehydrogenation zone. It is desired to limit oil
production in the preheat and prehydrogenation zone to avoid
plugging of the shale in this zone. It is desired not to produce
substantial quantities of hydrocarbons in the preheat and
prehydrogenation zone, preferably less than about 15 to 20 weight
percent of the organic component of the shale is converted to
normally liquid or gaseous hydrocarbons. It is especially preferred
to convert less than about 10 weight percent of the organic
component of the shale to liquid or gas in this zone.
The terminology "hydrogen-rich gas" throughout this description and
claims, means gases with sufficient hydrogen partial pressure to
effect high organic carbon recovery from the organic material in
the oil shales, which is essential to high yield hydrogasification.
Such hydrogen-rich gases may be obtained by a number of processes
well known in the chemical process industry. Table I shows the
effect of the hydrogen partial pressure and the heatup rate upon
organic carbon recovery from prehydrogenated oil shales which had
an original organic carbon content of 21.1 weight percent. The
maximum temperature to which the shale was heated was 1300.degree.
F. and the data was obtained at sustained hydrogen partial
pressures as shown in Table I.
TABLE I
__________________________________________________________________________
Percent Organic Carbon Recovery Total Pressure (psia) at Indicated
Hydrogen at Indicated Heating Rate Partial Pressure Hydrogen
Content (.degree.F/min)
__________________________________________________________________________
(psia) 25% 50% 90% 10 30 50
__________________________________________________________________________
0 Inert gas (500 psig Helium) 77 77 77 35 140 70 39 87 87 87 115
460 230 128 91 88 87 215 860 430 239 94 90 87 515 2060 1030 572 97
95 89
__________________________________________________________________________
The data in Table I show that increases in hydrogen partial
pressure are beneficial to organic carbon recovery. Hydrogen
partial pressure should be maintained above about 20 psia
throughout the system. In practical operation, the hydrogen partial
pressure is substantially constant throughout the preheat and
prehydrogenation zone. However, the hydrogen partial pressure
decreases from the inlet to the outlet of the hydrogasification
zone due to the formation of gaseous hydrocarbons by reaction of
hydrogen and organic carbon. To maintain sufficient hydrogen
partial pressure throughout the system, while avoiding hydrogen
dilution of the product gas from the hydrogasifier and resultant
lowering of the heating value of the final gas, the hydrogen
concentration at the outlet of the hydrogasification zone should
not exceed about 20 volume percent. Therefore, the lower limit of
total pressure in the hydrogasification zone should be maintained
above about 100 psia since the minimum hydrogen partial pressure
should be above about 20 psia. Inasmuch as zones 10, 11 and 12 are
at substantially the same total pressure, this lower total pressure
limit applies to the entire system. At higher total pressures, the
benefits of higher hydrogen partial pressures are obtained. The
upper operating pressure is limited only by equipment and economic
considerations. Normally the process of this invention is carried
out at total pressures of about 100 to about 2000 psia, preferably
about 500 to 1500 psia.
It is preferred that the oil shale not be thermally shocked by
abrupt temperature changes, but that it be gradually heated at a
rate in the order of less than about 100.degree. F. per minute in
the preheat and prehydrogenation zone. It is preferred that the
heating rate be less than about 50.degree.F. per minute above about
500.degree. to 600.degree.F.
The oil shale and hydrogen-rich gas may be heated by external
heating means or internal heating means in the preheat and
prehydrogenation zone by a wide variety of methods which are
readily apparent to one skilled in the art. Such variety of heating
means allow both cocurrent and countercurrent operation of the
preheat and prehydrogenation zone with respect to the shale and
hydrogen-rich gas. One method which is preferred is the
introduction of oil shale at ambient temperatures at one end of the
preheat and prehydrogenation zone and the introduction of the
hydrogen-rich gas at the other end of the preheat and
prehydrogenation zone at a temperature in quantities sufficient to
heat the oil shale to about 700.degree. to about 950.degree. F. by
countercurrent flow of the hydrogen-rich gas in thermal exchange
relation to the oil shales.
The preheated and prehydrogenated oil shale is hydrogasified in a
hydrogasification zone at a temperature of about 1200.degree. to
about 1500.degree. F., preferably about 1300.degree. to about
1400.degree. F., in the presence of hydrogen-rich gas to form
predominately low molecular weight paraffinic hydrocarbon gases
from the preheated and prehydrogenated organic portion of the oil
shales. It is necessary to reach a temperature of about
1200.degree. F. in order to obtain the desired hydrogasification in
a reasonable period of time. The residence time of the shale in
this zone is not considered critical since most of the organic
material or kerogen in the shale will be rapidly removed from the
shale due to prehydrogenation in the previous or oil shale preheat
zone 10. The light oils from the organic material in the shale are
vaporized immediately upon reaching the high temperatures.
Therefore, in the hydrogasification zone 11, the shale may be in a
free fall condition or a moving bed condition, whichever is
convenient. The length of time at which the shale is at the
hydrogasification temperature need not be more than about ten
seconds although longer times, as up to several minutes, is not
considered detrimental. The residence time of the gas in the
hydrogasification zone is more significant.
The hydrogen-rich gas stream is preferably introduced at the bottom
of the hydrogasification zone 11 and flows upwardly or
countercurrent to the shale passing downwardly. The hydrogen rich
gas stream may also flow cocurrent with both the hydrogen rich gas
and shale being introduced at the top of hydrogasification zone 11.
The hydrogen-rich gas flow rate and size of the hydrogasification
zone 11 is designed for a gas residence time of about 20 to 50
seconds within the hydrogasification zone, to allow conversion of
vaporized hydrocarbons to low molecular weight paraffinic
hydrocarbons, although somewhat longer periods of the time are not
considered detrimental to the process. This gaseous mixture is
carried upwardly and out of the hydrogasification zone 11 to a
series of processing steps for the separation of paraffinic
hydrocarbons from the other gases or volatilized liquids in the
gaseous mixture.
The hydrogasification zone may be heated by any suitable means as
will be obvious to one skilled in the art. One method is to supply
hydrogen-rich gas of sufficiently high temperature to raise the
temperature of the shale to the desired temperature by direct
thermal exchange. The hydrogasification zone may be optionally
internally heated by any suitable means such as a fuel oil/oxygen
burner. Carbon dioxide may be added to the hydrogasification zone
with the hydrogen-rich gas input and methanated to methane
providing heat by methanation. The actual hydrogasification
reaction is exothermic and when hydrogasification is established,
the heat requirements become lowered.
The hydrogen-rich gas supplied to the hydrogasification zone should
contain sufficient hydrogen to meet the chemical requirements
sufficient to convert the organic portion of the oil shale to
paraffinic hydrocarbon gases and hydrogasifiable aliphatic and
alicyclic hydrocarbon liquids. It may also be desirable to add a
controlled excess of hydrogen to the hydrogasification zone. For
example, sufficient excess hydrogen may be added to the
hydrogasification zone to ultimately convert all of the
hydrocarbons recovered, and the carbon monoxide remaining after
final purification, to methane. However, such excess hydrogen may
be added at a later stage. Less than such excess may lead to
undesired carbon deposition when the aliphatic and alicyclic
hydrocarbon liquids are hydrogasified. Lesser than the amounts of
hydrogen required chemically for conversion of the prehydrogenated
component of oil shale to low molecular weight paraffinic
hydrocarbon gases results in lower organic carbon recovery.
Referring to FIG. 1, the fresh oil shale is supplied to preheat and
prehydrogenation zone 10 wherein hot hydrogen-rich gas passes
countercurrent and in thermal exchange relation to the oil shale at
a temperature sufficient to gradually preheat the oil shale to a
temperature of about 700.degree. to about 950.degree.F. The
preheated and prehydrogenated oil shale is then passed to
hydrogasification zone 11 wherein it is passed countercurrently and
in thermal exchange relation with hydrogen-rich gas of sufficient
temperature to heat the oil shale to a temperature of about
1200.degree. to about 1500.degree.F. In hydrogasification zone 11,
the organic component of the oil shale is hydrogasified to form low
molecular weight paraffinic hydrocarbon gases. The mixture of
predominantly low molecular weight paraffinic hydrocarbon gases,
containing some aliphatic and alicyclic hydrocarbon liquids,
remaining hydrogen-rich gas and carbon dioxide are removed from the
hydrogasification zone. The mixed stream may be purified by further
treatment to form desired products such as pipeline-quality
gas.
The spent shale is removed from hydrogasification zone 11 and
passed through heat recovery zone 12 in countercurrent and thermal
exchange relation to hydrogen-rich gas which cools the spent shale
to less than about 300.degree.F., preferably to about 150.degree.F.
The hydrogen-rich gas is heated in heat recovery zone 12 for
recycle to preheat and prehydrogenation zone 10.
One advantage of the process of this invention is the high thermal
efficiency wherein the hydrogen-rich gas may remove a large portion
of the thermal energy of the spent shale for reutilization in
preheating of the fresh oil shale. FIG. 1 shows a preferred
embodiment of this invention wherein hydrogen-rich gas from preheat
and prehydrogenation zone 10 passes through separator 13 for
removal of liquids, namely water and hydrocarbon liquids which may
be formed in preheat and prehydrogenation zone 10. The organic
hydrocarbon liquids from separator 13 may be recycled to
hydrogasification zone 11.
The hydrogen-rich gas leaving separator 13 follows a split stream,
one portion recycling to heat recovery zone 12 and another portion
supplying hydrogasification zone 11. Valve 17 adjusts the split in
the hydrogen-rich gas flow dependent upon the chemical hydrogen
requirement in hydrogasification zone 11. The hydrogen-rich gas
passing from separator 13 to hydrogasification zone 11 may be
heated by passing through heat exchanger 20 in thermal exchange
relationship with the output stream of hydrogasification zone 11
followed by further heating by any suitable heating means shown as
15, prior to introduction to hydrogasification zone 11.
Alternatively, the hydrogen-rich gas may be supplied directly to
the hydrogasification zone 11 without preheating and
hydrogasification zone 11 may be heated by any suitable means shown
in FIG. 1 as heat input means 16. Heat input means 16 may be
combustion of fuel with oxygen.
The other portion of the stream of hydrogen-rich gas from separator
13 is recycled to heat recovery zone 12. The amount of
hydrogen-rich gas makeup is determined by the amount of hydrogen
consumed in the prehydrogenation and hydrogasification zones and
discharged from hydrogasification zone 11. The hydrogen-rich gas is
heated in heat recovery zone 12 and upon passing from heat recovery
zone 12 may be further heated by any suitable heating means 14
prior to introduction to preheat and prehydrogenation zone 10 to
obtain the desired temperature for entry to preheat and
prehydrogenation zone 10. A larger volume of hydrogen-rich gas
passes through preheat and prehydrogenation zone 10 than passes
through hydrogasification zone 11.
The advantages of the process of this invention appear to be
achieved by the controlled gradual preheating and prehydrogenation
in zone 10 followed by higher temperature hydrogasification of the
preheated and prehydrogenated oil shale in zone 11. While prior
processes of hydrogasifying oil shale without gradual preheating
and prehydrogenation of the organic component have resulted in less
than 80 percent recovery of the organic carbon from the shale, we
have found that with our two-zone hydrogenation process, it is
possible to recover as much as about 95 percent of the organic
carbon from the oil shale.
A preferred embodiment of this invention in the production of
pipeline-quality gas results in further increases in thermal
efficiency of the process being obtained by utilization of heat
from heat exchanger 20 cooling the gas output of the hydrogasifier
to heat hydrogen-rich gas input to hydrogasifier zone 11.
The effluent gas from hydrogasifier 11 generally including methane,
hydrogen, carbon dioxide and vaporized liquids is passed through
heat exchanger 20 wherein the hydrogen-rich gas passing to
hydrogasification zone 11 may be heated by thermal exchange with
the hydrogasified gas. The cooled hydrogasified gas is then passed
through liquidgas separator 21 removing liquids including water,
benzene, toluene, xylene and other organic liquids which are passed
through liquid separator 18 which separates water, high boiling
hydrocarbon liquids fraction which gives greatest coking
difficulties; and low boiling hydrocarbon liquids including
benzene, toluene and xylene from middle boiling hydrocarbon liquids
which are recycled to the hydrogasification zone. The low-boiling
hydrocarbon liquids may be used to supply heat for the process. The
gases from separator 21 pass through purifier 22 to remove any
remaining quantities of undesired steam, carbon monoxide, carbon
dioxide, ammonia and hydrogen sulfide. Following such purification,
the hydrogasified product is methanated in a conventional manner to
increase the amount of methane in the resulting pipeline-quality
gas.
Suitable apparatus for use in the process of this invention will be
readily apparent to one skilled in the art. It is apparent that the
process of this invention may be operated in a physically separated
preheat and prehydrogenation zone and hydrogasification zone or the
preheat and prehydrogenation zone and the hydrogasification zone
may be physically contained in one vessel appropriately separated.
When operated on a batch basis, the preheat and prehydrogenation
conditions may first be subjected to a single zone to which same
zone the hydrogasification conditions are later applied. It is
readily apparent the process of this invention may be carried out
on either a batch or continuous flow basis. A continuous flow
process is preferred.
While no specific means of distribution of the hydrogen-rich gas
throughout the zones containing oil shale is shown, it is readily
apparent that it is desirable to have a suitable gas distribution
means such as a gas manifold distribution system at the
introduction area of the gas to the particular zone. The desirable
factor is that the hydrogen-rich gas be effectively distributed to
the crosssectional area of the zone upon its introduction or
shortly thereafter.
Suitable materials for construction of an apparatus suitable for
the process of this invention are well known to persons skilled in
the art and need only be sufficient to contain the pressures
obtained in the process and to effect suitable heat retentions in
the different thermal zones of the process of this invention.
While in the foregoing specification this invention has been
described in relation to certain preferred embodiments thereof, and
many details have been set forth for purpose of illustration, it
will be apparent to those skilled in the art that the invention is
susceptible to additional embodiments and that certain of the
details described herein can be varied considerably without
departing from the basic principles of the invention.
* * * * *