U.S. patent number 3,876,527 [Application Number 05/345,642] was granted by the patent office on 1975-04-08 for hydrocarbon cracking in a regenerable molten medium.
This patent grant is currently assigned to Exxon Research and Engineering Company. Invention is credited to John J. Dugan, Israel S. Pasternak, Keith C. Yao.
United States Patent |
3,876,527 |
Dugan , et al. |
April 8, 1975 |
Hydrocarbon cracking in a regenerable molten medium
Abstract
Hydrocarbon feedstocks, preferably heavy hydrocarbon feedstocks
containing sulfur, are cracked at elevated temperatures in a novel
regenerable alkali metal carbonate molten medium containing a
glass-forming oxide, such as an oxide of boron, to produce high
yields of light olefins such as ethylene, which olefins are useful
in the synthesis of polymers and other valuable chemicals. The
carbonaceous materials, i.e. coke, which are formed and dispersed
in the molten medium during the cracking operation are gasified by
contacting a portion of said carbonaceous materials with a gaseous
stream containing oxygen, steam or carbon dioxide at temperatures
of from about above the melting point of said medium to about
3000.degree.F. in order to regenerate the melt. Preferably, the
amount of glass-forming oxide expressed as the oxide thereof,
present in the alkali-metal carbonate melt is maintained in the
range of from about 0.1 to about 25 weight percent based on the
total weight of the molten medium in order to increase the overall
dispersion of the carbonaceous materials which are formed in the
molten medium during the cracking reaction.
Inventors: |
Dugan; John J. (Sarnia,
Ontario, CA), Pasternak; Israel S. (Sarnia, Ontario,
CA), Yao; Keith C. (Sarnia, Ontario, CA) |
Assignee: |
Exxon Research and Engineering
Company (Linden, NJ)
|
Family
ID: |
23355869 |
Appl.
No.: |
05/345,642 |
Filed: |
March 28, 1973 |
Current U.S.
Class: |
208/125; 48/211;
208/235; 252/373; 423/351; 423/572; 423/658.2; 585/635; 585/651;
585/652; 585/901; 585/912; 585/950 |
Current CPC
Class: |
C10G
9/34 (20130101); Y10S 585/912 (20130101); Y10S
585/901 (20130101); Y10S 585/95 (20130101) |
Current International
Class: |
C10G
9/00 (20060101); C10G 9/34 (20060101); C10g
009/34 () |
Field of
Search: |
;208/125,128,130,107 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Levine; Herbert
Attorney, Agent or Firm: Caulfield; Donald C.
Claims
What is claimed is:
1. A process for cracking a hydrocarbon feedstock which comprises
contacting said feedstock with a regenerable alkali metal carbonate
molten medium containing from about 0.1 to about 25 wt.%,
calculated as oxide and based on total molten medium, of a
glass-forming oxide selected from the group consisting of oxides of
boron, vanadium, silicon, phosphorus and mixtures thereof at a
temperature in the range of from 1200.degree.F. to about
2500.degree.F. to form cracked hydrocarbon products and to
uniformly suspend the carbonaceous materials formed during said
cracking operation throughout the molten medium and thereafter
gasifying a portion of said carbonaceous materials formed during
said cracking process by contacting said molten medium containing
the carbonaceous materials with a reagent selected from the group
consisting of oxygen, steam, carbon dioxide and mixtures thereof at
a temperature in the range of from about above the melting point of
said medium to about 3000.degree.F.
2. The process of claim 1 wherein the temperature of the molten
medium is maintained in the range of from about 1200.degree. to
about 2000.degree.F.
3. The process of claim 2 wherein the amount of glass-forming oxide
expressed as the oxide thereof in the molten medium is in the range
of from about 1 to about 20 wt. % of the total molten media.
4. The process of claim 3 wherein the alkali metal carbonate is
selected from the group consisting of lithium carbonate, sodium
carbonate and mixtures thereof.
5. The process of claim 4 wherein the alkali metal carbonate is a
mixture of a major amount of sodium carbonate and a minor amount of
lithium carbonate.
6. The process of claim 3 wherein the glass-forming oxide is an
oxide of boron.
7. The process of claim 6 wherein said gasifying reagent is
oxygen.
8. The process of claim 7 wherein the molten medium contains an
alkali metal borate.
9. The process of claim 8 wherein the hydrocarbon feedstock is a
heavy hydrocarbon feedstock containing sulfur.
10. The process of claim 9 wherein the amount of sulfur present in
the molten medium is reduced by:
a. contacting the sulfur compounds in the molten medium with a
reducing agent; and
b. thereafter contacting the reduced sulfur compounds formed in
step (a) with carbon dioxide and steam to form hydrogen sulfide as
a recoverable product.
11. The process of claim 10 wherein said reducing agent is
carbon.
12. The process of claim 11 wherein at least a portion of said
heavy hydrocarbon feedstock boils above about 650.degree.F. at
atmospheric pressure.
13. The process of claim 12 wherein said molten medium contains an
oxide of boron in an amount expressed as the oxide thereof in the
range of from about 1 to about 12 wt.% of the total molten
media.
14. The process of claim 13 wherein the heavy hydrocarbon feedstock
contains from about 2 to about 6 wt.% sulfur.
15. A process for cracking a heavy hydrocarbon feedstock comprising
a component selected from the group consisting of crude oils and
residua containing from about 2 to about 6 wt.% sulfur which
comprises contacting said heavy hydrocarbon feedstock with a
regenerable alkali metal carbonate molten medium comprising a
mixture of lithium and sodium carbonates containing from about 1 to
about 12 wt.%, calculated as oxide and based on the total molten
medium, of an oxide of boron at a temperature in the range of from
about 1300.degree.F. to about 1700.degree.F. to form cracked
hydrocarbon products and to uniformly suspend the carbonaceous
materials formed during said cracking operation throughout the
molten medium and thereafter gasifying a portion of said
carbonaceous materials formed during said cracking process by
contacting said molten medium containing the carbonaceous materials
with oxygen at a temperature in the range of from about
1400.degree. to about 1800.degree.F.; the amount of sulfur present
in the molten medium being reduced by: (a) contacting the sulfur
compounds in the molten medium with a reducing agent; and (b)
thereafter contacting the reduced sulfur compounds formed in step
(a) with carbon dioxide and water.
Description
FIELD OF THE INVENTION
This invention relates to the preparation of unsaturated organic
compounds such as ethylene from hydrocarbon feedstocks. More
particularly, this invention relates to cracking a hydrocarbon
feedstock at elevated temperatures in a regenerable alkali metal
carbonate molten medium. Still more particularly, this invention
relates to the cracking of a heavy hydrocarbon feedstock, e.g.,
hydrocarbons such as gas oils, crude oils, atmospheric or vacuum
residua, in a regenerable alkali metal carbonate molten medium
containing a glass-forming oxide such as an oxide of boron to
produce cracked hydrocarbon products such as ethylene and
carbonaceous materials. At least a portion of the carbonaceous
materials, i.e. coke which are formed during the cracking process
are gasified by contacting said carbonaceous materials in the
molten medium with a gaseous stream containing an oxygen, steam, or
carbon dioxide reagent at elevated temperatures in order to
regenerate the melt. The cracked hydrocarbon products find use in
the synthesis of polymers and other valuable chemicals.
DESCRIPTION OF THE PRIOR ART
The thermal cracking of hydrocarbons at elevated temperatures to
produce olefinic compounds such as ethylene by employing a molten
salt such as eutectic mixtures of lithium and potassium chloride as
the heat transfer medium is well known in the art. The cracking of
hydrocarbon feedstocks in molten heat transfer medium such as lead
to produce ethylene has likewise been disclosed in the art.
However, the molten medium which have heretofore been employed to
crack hydrocarbons have suffered from one or more disadvantages
which has resulted in limited industrial application of these
processes. The difficulty primarily encountered in the prior art
processes such as molten lead was the fact that the carbonaceous
particles produced during the cracking operation were not
sufficiently dispersed in the melt, but formed a separate phase
which contaminated the liquid and gaseous products. Further with
molten medium that partially suspended the coke, such as
lithium-potassium chloride eutectics, the buildup of such
carbonaceous material in or above the molten medium necessitated
additional steps such as to physically remove the carbonaceous
particles from the melt. In addition, numerous contacting media
have been proposed in the literature including metals, alloys,
slags, basalt and glass (see Czechoslovakian Pat. No. 109,952 and
U.S. Pat. No. 3,647,358) in order to effectuate the thermal
cleavage of hydrocarbon feedstocks.
It has been suggested that hydrocarbon feedstocks can be cracked in
a molten alkali metal carbonate, alkali metal hydroxide, or a
mixture thereof, to form hydrocarbon products containing ethylene
and thereafter regenerating the molten medium by intimate contact
with oxygen or steam (see U.S. Pat. Nos. 3,553,279; 3,252,773;
3,252,774; 3,505,018; 3,438,727; 3,438,728; 3,438,733; 3,438,734;
3,516,796; 3,551,108; Oil and Gas Journal Sept. 27, 1971; German
DT-OS No. 2,149,291.
While molten alkali metal carbonate melts tend to absorb or
disperse the coke formed during the cracking reaction; the extent
of coke dispersion is relatively low. This limited coke dispersion
in the molten medium may cause process difficulties in a commercial
environment.
Recently, it has been proposed to crack a hydrocarbon feedstock in
a regenerable molten medium containing an alkali oxide in
combination with a glass-forming oxide such as an oxide of boron.
(U.S. Ser. No. 280,185, filed Aug. 14, 1972). Such a molten medium,
while exhibiting sufficient coke dispersion, suffers from the
disadvantage of being corrosive in nature, thereby resulting in a
significant materials of construction problem.
SUMMARY OF THE INVENTION
It has now been discovered that hydrocarbon feedstocks are
converted to produce high yields of light olefins such as ethylene
by contacting the hydrocarbon feedstock with an alkali-metal
carbonate molten medium containing minor amounts of a glass-forming
oxide selected from the group consisting of an oxide of boron,
vanadium, silicon, phosphorous including mixtures thereof at a
temperature in the range of from about above the melting point of
the medium to about 2500.degree.F. for a time sufficient to form
cracked hydrocarbon products and carbonaceous materials.
Thereafter, the carbonaceous materials formed and dispersed in the
molten medium during the cracking operation are contacted with a
gaseous stream containing oxygen, steam, or carbon dioxide
including mixtures thereof, at a temperature in the range of from
about the melting point of said medium to about 3000.degree.F. for
a period of time in order to regenerate the molten medium. It has
been discovered that not all oxides of primary and secondary
glass-forming compounds known in the art are amenable to the
process of the instant invention. Specifically, oxides of such
primary glass-forming elements as germanium, arsenic and antimony
have been found unsuitable for the cracking or gasification
processes described herein in that the oxides of these elements,
i.e., glass-forming oxides of germanium, arsenic and antimony, are
excessively reduced to their metal state in the presence of carbon
at temperatures which would normally be employed to crack or gasify
a hydrocarbon feedstock. Carbon in the form of coke will normally
be present in the molten medium to a greater or lesser degree in
the practice of the process of the instant invention, as will be
hereinafter described, in view of the fact that it is preferred to
gasify only that amount of coke which is being formed in cracking
zone in order to achieve a heat balanced system as well as to
maintain a steady state coke concentration in the melt. Further,
two primary glass-forming oxides which are not significantly
reduced by carbon, namely the oxides of molybdenum and tungstem are
not effective in increasing the dispersion of carbonaceous
materials formed during the cracking operation when employed in
minor amounts in a molten alkali carbonate medium. In addition, a
secondary glass-forming oxide, namely bismuth oxide, is likewise
reduced in a molten medium at elevated temperatures to its metal
state in the presence of carbon.
The glass-forming oxides of the instant invention are employed in
combination with an alkali metal (Group IA) carbonate, that is a
carbonate of lithium, sodium, potassium, cesium or mixtures
thereof. The molten medium may additionally contain other Group IA
or IIA constituents such as the oxides, hydroxides, sulfides,
sulfites, or sulfates of sodium, lithium, potassium, cesium,
magnesium, calcium, strontium and barium. Alkali metal sulfites,
sulfates and sulfides are formed in situ during the course of the
cracking or subsequent gasification reactions by the reaction of
the sulfur contaminants in the feedstock with the alkali or
alkaline earth metal constituents of the melt. Alkali metal oxides
are also generated in situ by the carbon reduction of alkali metal
carbonates. Alkali metal hydroxides may be formed if water is
present in the cracking or gasification zones. The amount of
glass-forming oxide in the total molten media is maintained between
about 0.1 to 25 weight percent, preferably between 1 to about 20
weight percent and more preferably from about 1 to about 12 weight
percent calculated as the oxide thereof, e.g. B.sub.2 O.sub.3,
V.sub.2 O.sub.5 , SiO.sub.2, P.sub.2 O.sub.5 and based on the total
molten media. It should be recognized that the glass-forming
element e.g. boron may exist in various valence states at various
points in the process. Accordingly, the expression "oxide of boron"
is intended to encompass any oxide of the applicable element e.g.
boron, vanadium, silicon and phosphorous.
The advantages of cracking a hydrocarbon feedstock, particularly a
heavy hydrocarbon feedstock containing sulfur, in the
above-mentioned molten medium reside in the ability of the molten
medium of this invention to: (a) suspend the carbonaceous materials
formed in situ during the cracking operations uniformly throughout
the melt, and (b) thereafter, upon contact with a gaseous stream
containing oxygen or steam at elevated temperature, to promote the
rapid gasification of said carbonaceous materials. Accordingly, the
instant invention permits the thermal cracking of a heavy
hydrocarbon feedstock such as atmospheric or vacuum residuum, the
cracking of which feedstocks have heretofore not been feasible due
to excessive coking in tubular reactors. In addition, in view of
the fact that heavy hydrocarbon feedstocks, as hereinafter defined,
such as residua and crude oils normally contain sulfur, e.g.,
thiols, thiophenes and sulfides, the molten medium of the instant
invention offers the additional advantages of significantly
lowering the emission of pollutants into the atmosphere by
retaining the sulfur compounds produced during the burning of the
carbonaceous materials with a gasifying reagent containing oxygen.
Further, sulfur impurities initially present in the hydrocarbon
feedstock are retained by the molten medium of the instant
invention in view of the fact that a major portion of the hydrogen
sulfide formed during the cracking operation is retained by melt,
particularly when the cracking step is conducted in the essential
absence of steam. Also, a portion of the sulfur impurities that are
present in the carbonaceous materials are believed to be leached
out of the carbonaceous materials by the molten medium of the
instant invention, thereby effectuating a further removal of sulfur
from the carbonaceous materials.
Furthermore, the molten medium of the instant invention exhibits
the additional advantage of being relatively noncorrosive in
nature, particularly when compared with a molten medium containing
a predominant amount of a glass-forming oxide. Accordingly,
maintaining the concentration of the glass-forming oxide below
about 25 weight percent, preferably below about 20 weight percent,
and more preferably below 12 weight percent, alleviates the
containment problem associated with employing a glass-forming oxide
as a major constituent in the molten medium cracking process. Thus,
it can be seen that the addition of a controlled amount of a
glass-forming oxide to an alkali metal carbonate melt results in a
molten medium exhibiting excellent coke dispersion properties
without being extremely corrosive in nature, which properties are
required for a commercial operation.
A still further advantage of employing a molten alkali metal
carbonate melt containing a controlled amount of a glass-forming
oxide in order to convert a heavy hydrocarbon feedstock containing
sulfur to valuable chemical products resides in the fact that
increased desulfurization rates are obtained at a given temperature
with the molten medium of the instant invention. The fact that the
desulfurization rate decreases markedly with an increase in
temperature in a molten alkali carbonate medium often necessitates
cooling the molten medium during the desulfurization step in order
to obtain a satisfactory desulfurization rate. It has been
discovered, however, that during the desulfurization of the molten
medium with steam and carbon dioxide that the steam conversion and
thus the overall desulfurization rate increases with the addition
of a glass-forming oxide to a molten alkali carbonate medium. Thus,
a molten alkali carbonate medium containing a glass-forming oxide
can be desulfurized with steam and carbon dioxide at the same rate
at a higher temperature than a molten alkali carbonate melt which
does not contain a glass-forming oxide thereby reducing the amount
of cooling and reheating required to bring the molten medium back
to the desired reaction temperature.
Accordingly, the only requirement of the molten medium of this
invention is that a sufficient amount of the glass-forming oxides
are employed to disperse the carbonaceous materials formed during
the cracking reaction uniformly throughout the melt and thereafter
promote the rapid gasification of said materials upon contact with
a gaseous stream containing oxygen or steam or carbon dioxide at
elevated temperatures. The use of the term "glass-forming oxide" is
not meant to imply that all of the molten medium described above
can be rapidly cooled without crystallizing. It should be
recognized, as stated above, that the molten medium of this
invention may be employed in combination with other components such
as metallic and nonmetallic oxides, sulfides, sulfates and various
other salts in varying amounts so long as a sufficient amount of
glass-forming oxide is employed to disperse by-product carbonaceous
material, i.e. coke.
It is to be understood that although the molten medium of the
instant invention is described in terms of an alkali metal
carbonate and the glass-forming oxides, it is clearly within the
scope of this invention to employ and define the molten medium of
this invention with respect to the compounds which are believed to
be formed when a glass-forming oxide is heated to the molten state
in combination with the alkali metal carbonate or other alkali
metal compounds. For example, a molten medium containing an alkali
metal carbonate (M.sub.2 CO.sub.3) and boron oxide (B.sub.2
O.sub.3) as the glass-forming oxide can also be expressed in the
molten state as an alkali metal borate, on the basis of the
following reaction:
B.sub.2 O.sub.3 + XM.sub.2 CO.sub.3 .fwdarw. X M.sub.2 O.B.sub.2
O.sub.3 + XCO.sub.2
wherein X ranges from 1 to 3 depending on the alkali metal
employed. Accordingly, it is within the purview of the instant
invention to employ as the molten medium of this invention an
alkali metal carbonate and a glass-forming oxide, as defined above,
in combination with an alkali metal or an alkali metal salt of the
glass-forming oxide employed, e.g., alkali metal borate. It is to
be understood that any of the molten glass melts of this invention
may be prepared by fusing any combination of raw materials, which
upon heating will form a glass-forming oxide or an alkali metal
salt of the glass-forming oxide employed in combination with an
alkali metal carbonate.
Individual regenerable molten medium which are most preferred are
those obtained when an oxide of boron is employed as the
glass-forming oxide. The most preferred melt system of the instant
invention comprises boron oxide in combination with a carbonate of
lithium, sodium and mixtures thereof. The most preferred alkali
metal carbonate is a mixture of a major amount of sodium carbonate
and a minor amount of lithium carbonate.
In the process of this invention, a wide variety of feedstocks may
be converted to produce high yields of light olefins such as
ethylene. Generally, cracking can be conducted in the
above-described molten medium with any hydrocarbon feedstock such
as low boiling hydrocarbons, e.g., ethane, propane, butane, as well
as high boiling hydrocarbons such as naphthas, gas oils and the
like. Preferably the hydrocarbon feedstocks of this invention are
heavy hydrocarbon feedstocks that contain from about 2 to about 6
weight percent sulfur such as crude oils, heavy residua,
atmospheric and vacuum residua, crude bottoms, pitch, asphalt,
other heavy hydrocarbon pitch-forming residua, coal, coal tar or
distillate, natural tars including mixtures thereof. Preferably,
the hydrocarbon feedstock which is cracked in the stable molten
medium of the instant invention comprises a hydrocarbon feedstock
which contains material boiling above about 400.degree.F. at
atmospheric pressure. The preferred hydrocarbon feedstocks which
can be employed in the practice of the instant invention are crude
oils, aromatic tars, and atmospheric or vacuum residua containing
material boiling above about 650.degree.F. at atmospheric pressure.
Aromatic tar, atmospheric or vacuum residua are particularly
preferred.
While not essential to the reaction, an inert diluent can be
employed in order to regulate the hydrocarbon partial pressure in
the molten medium cracking zone. The inert diluent should normally
be employed in a molar ratio of from about 1 to about 50 moles of
diluent per mole of hydrocarbon feed, and more preferably 1 to 10.
Illustrative of the diluents that may be employed are helium,
carbon dioxide, nitrogen, steam, methane and the like.
This invention will be further understood by reference to the
accompanying drawing FIG. 1 of which is a schematic flow diagram
for thermally cracking a heavy hydrocarbon feedstock in the molten
medium of the instant invention.
A heavy hydrocarbon residua fraction containing from about 2 to
about 6 weight percent sulfur and having a boiling point at
atmospheric pressure of above 650.degree.F. and a Conradson carbon
content of 12 is passed by the way of line 1 into the cracking zone
2. Within the cracking zone 2 is maintained a molten bed containing
an oxide of boron and alkali metal carbonate comprising a major
amount of sodium carbonate and a minor amount of lithium carbonate.
The liquid hydrocarbon feedstock passing by way of line 1 is
introduced into the cracking zone 2 by bubbling the feedstock
through the molten medium 3. Alternatively, the molten medium may
be sprayed into the reactor or trickled down the reactor walls as
the hydrocarbon feedstock passes through the reactor. The molten
medium may flow either cocurrently or countercurrently to the flow
of the hydrocarbon feedstock. In either event, means should be
provided to secure intimate contacting of the feed with the molten
medium.
The temperature of the molten medium 3 is maintained in the range
of from about 1200.degree. to about 2000.degree.F., and more
preferably from about 1300.degree. to about 1700.degree.F. in order
to form cracked hydrocarbon products and carbonaceous materials.
The temperature of the molten medium is maintained within the
above-mentioned range due to the exothermic gasification reaction
of a portion of the carbonaceous materials formed during the
cracking reaction, as will be hereinafter described, such that the
molten medium provides the heat for the cracking operation.
Depending upon the temperature and the specific type of hydrocarbon
feedstock, the rate at which the feedstock is passed via line 1
into cracking zone 2 is in the range of from about 0.1 to about 100
w/w/hr. (weight of feed/weight of melt/hour), and more preferably
from about 0.1 to about 20 w/w/hr. Pressures are not a critical
feature of the instant invention such that the reaction may be
conducted at a pressure ranging from subatmospheric, e.g. 0.1
atmosphere to about 50 atmospheres, preferably from about 1 to
about 10 atmospheres. The reaction time, as expressed in the amount
of time the feedstock is in contact with the melt 3, i.e.,
residence time, is in the range of from about 0.01 to about 20
seconds, and more preferably from about 0.3 to about 5.0
seconds.
After the hydrocarbon feedstock has been cracked in the molten
medium at the desired temperature and pressure, the gaseous
effluent emanating from the molten medium 3 passes overhead from
the cracking zone 2 and is recovered by way of line 4. The cracked
products passed by way of line 4 are cooled by being subjected to a
quenching medium introduced by way of line 5. Thereafter, the
cracked products are further cooled to condense and separate liquid
products from the gaseous products containing light olefins by
passing the quenched products by the way of line 6 to a
fractionation zone, not shown. Most of the hydrogen sulfide formed
during the cracking operation is absorbed by the melt particularly
when the cracking operation is conducted in absence of significant
amounts of steam. The product distribution obtained by cracking a
hydrocarbon feedstock in the manner described above is
substantially identical to the product distribution obtained by
subjecting the same feedstock, under identical conditions, to the
well known steam cracking process.
The significant advantage of employing the molten medium of the
instant invention is that the carbonaceous materials which are
formed during the above-described cracking process become uniformly
suspended throughout the melt and can be gasified, i.e., burned to
gaseous products, when contacted with a gasifying reagent such as
an oxidizing gas, i.e., air steam or carbon dioxide at elevated
temperatures in order to rapidly regenerate the molten medium.
Accordingly, the molten medium containing suspended carbonaceous
material is withdrawn from the cracking zone 2 by way of line 7 and
is passed by way of line 7 into a gasification zone 8. The rate at
which the molten medium is withdrawn from the cracking zone depends
on the type of hydrocarbon feedstock being pyrolyzed and the rate
at which the feedstock is being introduced into the cracking zone
2. Preferably, a vapor lift is employed in order to circulate the
molten medium by way of line 7 from the cracking zone 2 to the
gasification zone 8.
The carbonaceous materials which are formed during the thermal
cracking reaction may be generally described as solid particle-like
materials having a high carbon content such as those materials
formed during high temperature pyrolysis of organic compounds and
normally referred to as coke. While the carbonaceous material
heretofore discussed has been produced in situ during the cracking
of a hydrocarbon feedstock, as described above, it should be
emphasized that it is clearly within the scope of the instant
invention to gasify carbonaceous materials which may be added, in
conjunction with or independently of a thermal cracking reaction,
to the molten medium of the instant invention in the form of coal
of various grades, polygnite, lignite coal, coke of various types
such as coal coke and petroleum coke, peat, graphite, charcoal and
the like. Accordingly, the term gasification as used herein
describes the contacting of such carbonaceous materials in the
molten medium of the instant invention with a gasifying reagent
comprising a gaseous stream containing oxygen, steam, carbon
dioxide and mixtures thereof. The gasification reaction is carried
out by contacting the carbonaceous material in the molten medium 9
with the gasifying reagent introduced into the gasification zone 8
by way of line 10. The gasification reaction is carried out at
temperatures in the range of from about the melting point of the
molten medium to 3000.degree.F. or higher and at a pressure in the
range of from subatmospheric to about 100 atmospheres. Preferably,
the temperature at which the gasification reaction is carried out
is in the range of from about 1200.degree. to about 2000.degree.F.,
and more preferably from about 1400.degree. to about 1800.degree.F.
It is preferred to maintain the pressure in the gasification zone
in the range of from 1 to about 10 atmospheres.
When a gaseous stream containing oxygen is employed as the
gasifying reagent in order to regenerate the molten medium, the
amount of oxygen which must be present in the gaseous stream is in
the range of from about 1 to about 100 weight % oxygen, and more
preferably in the range of from about 10 to about 25 weight %
oxygen. Normally, the gaseous stream containing oxygen is passed
through the molten medium 9 at a rate of less than about 0.01
w/w/hr. (weight of oxygen/weight of molten medium/hour) to about 50
w/w/hr., and more preferably from about 0.01 w/w/hr. to about 10
w/w/hr. Most preferably, air is introduced by way of line 10 at a
temperature in the range of from about 100.degree. to about
1000.degree.F. in order to effect a rapid regeneration of the
molten medium.
Alternatively, a gaseous stream containing steam or carbon dioxide
may also be introduced as the gasifying reagent by way of line 10
into the gasification zone 8 in order to regenerate the molten
medium. When steam is employed as the gasifying reagent, the amount
of steam which must be present in the gaseous stream is in the
range of from about 10 to 100 weight % and more preferably from
about 50 to about 100 weight %. The steam is normally introduced by
way of line 10 at a temperature in the range of from about
300.degree. to about 1000.degree.F., and at a pressure in the range
from about 100 to about 500 psig in order to regenerate the molten
medium. In the event a gaseous stream containing carbon dioxide is
employed as the gasifying reagent, the amount of carbon dioxide
that must be present in the gaseous stream in in the range of from
about 10 to about 100 weight %. Preferably, the temperature and
pressure at which carbon dioxide is introduced into the
gasification zone 8 is in the range of from about 100.degree. to
1000.degree.F. and 100 to 1000 psig, respectively.
The specific gasification rate of the carbonaceous materials in
individual, regenerable molten medium, as defined by the amount of
carbonaceous material which is gasified per hour per cubic foot of
melt, is dependent upon the temperature at which the gasification
process is carried out, as well as the residence time of the oxygen
containing gas or steam in the melt, the concentration of
carbonaceous material in the melt, and feed rate of oxygen
containing gas into the medium. As a general rule, the carbon
gasification rate increases as the temperature of the melt,
concentration of carbonaceous materials and feed rate of the oxygen
containing gas increase. Preferably, the concentration of
carbonaceous materials in the molten medium is maintained in the
range of from 0.1 to about 60 weight %, and more preferably from
about 1.0 to about 20 weight %, and most preferably from about 1 to
about 5 weight % in order to effect a rapid gasification
thereof.
The gaseous products produced by contacting the carbonaceous
materials in the molten medium with either an oxidizing gas, steam
or CO.sub.2 are recovered from the gasification zone by way of line
11. When steam is employed as the gasifying reagent, a
hydrogen-rich gaseous effluent is produced and recovered by way of
line 11. The contacting of the carbonaceous materials wtih steam
under the preferred conditions of temperature and pressure for
regenerating the molten medium of the instant invention, normally
1500.degree.F. and atmospheric pressure, respectively, result in a
gaseous effluent containing about 75 mole % hydrogen and about 24
mole % carbon oxides. Based on thermodynamic considerations,
however, the gasification of carbonaceous materials with steam in
the molten medium of the instant invention at lower temperatures,
preferably below 1000.degree.F., and at elevated pressures would
result in formation of a methane-rich gaseous effluent.
As opposed to the production of either a hydrogen or methane-rich
stream when steam is employed as the gasifying reagent, the use of
an oxygen containing gas such as air as the gasifying reagent
results in the formation of a nitrogen-rich gaseous effluent.
During the gasification of the carbonaceous materials with an
oxidizing gas such as air, it is believed that the sulfur
impurities present in the carbonaceous material are oxidized to
sulfur oxides and are absorbed by the molten medium of the instant
invention. In addition, the process of the instant invention
further serves to remove other contaminants present in a heavy
hydrocarbon feedstock such as ash forming impurities which include
trace metals such as vanadium, iron and nickel that are normally
present to a greater or lesser degree depending on the specific
type of hydrocarbon feedstock being cracked and/or gasified.
The melt which has been regenerated as described above in
gasification zone 8 is withdrawn by way of line 12 and reintroduced
back into the cracking zone 2. Normally, the amount of carbonaceous
material that is gasified in the gasification zone 8 is
substantially equivalent to the amount of carbonaceous material
being formed during the cracking operation in the cracking zone 2,
such that an overall balance of carbonaceous material is maintained
throughout the system. A further advantage of employing a gaseous
stream containing oxygen as the gasifying reagent is the fact that
the gasification. i.e., burning of carbonaceous materials with
oxygen is an exothermic reaction. Thus, when an oxidizing gas such
as air is employed to gasify the carbonaceous materials in
gasification zone 8, a sufficient amount of heat is liberated in
order to provide an overall heat balance for both the gasification
and cracking processes. Accordingly, in addition to regenerating
the melt, the gasification of the carbonaceous materials with an
oxidizing gas maintains the temperature of the melt such that the
melt being passed by way of line 12 into the cracking zone 2
provides the heat required for the thermal cracking of the
hydrocarbon feedstock.
As can be appreciated, while the molten medium of the instant
invention effectuates the removal of sulfur and ash-forming
impurities from the carbonaceous material by retaining these
impurities during the gasification of the carbonaceous materials
with an oxygen containing gasifying reagent, the continual buildup
of these impurities in the melt requires that a slip-stream be
withdrawn from the integrated cracking and gasification processes
described above in order to restore the level of these impurities
present in the melt to an acceptable level. While the slip-stream
may be withdrawn from either the cracking or gasification zone or
from any of the transfer lines wherein the molten medium is being
passed to either the cracking or gasification zone, i.e., lines 7
and 12, respectively, it is preferred to withdraw a stream of the
molten medium from transfer line 7. The basis for this preference
of removing a portion of the contaminated molten medium from the
cracking zone resides in the fact that the cracking zone contains a
greater amount of carbonaceous material, which carbonaceous
material effects the reduction of alkali or alkaline earth metal
sulfur oxides to metal sulfides, thereby facilitating the
subsequent removal of the sulfur from the molten medium, as will be
hereinafter described.
The sulfur impurities present in the carbonaceous material as
mentioned above are retained by the molten medium during
gasification with an oxidizing gas stream as well as being leached
from the coke by the melt at elevated temperatures. When steam is
employed as the gasifying reagent, however, the sulfur impurities
are not converted to sulfur oxides and are not absorbed by the melt
but rather the sulfur in carbonaceous material is primarily
converted to hydrogen sulfide and is recovered in the effluent from
the gasification zone. Thus, when an oxidizing gas stream is
employed as the gasifying reagent, the sulfur impurities are
absorbed by the melt in the form of metal sulfites or sulfates. The
presence of carbonaceous materials in the molten medium serves to
reduce the metal sulfites or sulfates, predominantly alkali or
alkaline earth metal sulfites, to their sulfide form. The metal
sulfides are thereafter contacted with carbon dioxide and water in
order to recover the sulfur impurities as hydrogen sulfide.
Accordingly, a slip-stream 13 is withdrawn from line 7 and is
passed to a sulfur recovery zone 14, wherein carbon dioxide and
steam is introduced by way of line 15 and passed through the melt
16 at a temperature in the range of from about 800.degree. to about
1800.degree.F. Alternatively, the molten medium containing the
sulfur impurity as a metal sulfide is passed into a sulfur recovery
zone and is contacted with water in order to dissolve the melt and
recover precipitated metals and ash and thereafter carbon dioxide
is bubbled through the solution in order to recover the sulfur
impurity as a hydrogen sulfide rich stream. In either embodiment,
it is essential that the sulfur impurities be present in the
sulfide form before being contacted with water or steam and carbon
dioxide. In the event that a sufficient amount of carbonaceous
material is not present in the system described above, and
specifically in the cracking zone 2 in order to reduce the metal
sulfate and sulfite to their sulfide form, it may be necessary to
employ a reducing zone prior to passing the molten medium into the
sulfur recovery zone 16. If a reducing zone is required, it is
evident that the slip-stream may be withdrawn from any point in the
system and thereafter passed to the reducing zone wherein such
reducing agents as carbon, hydrogen, carbon monoxide, methane,
ethane or the like may be employed in order to reduce the metal
sulfite or sulfates to their sulfide form. If such a reducing zone
is required, it is preferred to have a holding zone below the
cracking zone wherein the addition of further amounts of carbon may
or may not be necessary, depending on the specific type of
hydrocarbon feedstock, to effectuate the reduction of substantially
all of the metal sulfate or sulfites to their sulfide form. In
either event, it is preferred that the total sulfur concentration
in the molten medium be maintained below about 5.0 weight % and
preferably below about 0.25 and 2.0 weight % based on the total
molten media.
The hydrogen sulfide rich stream is recovered from the sulfur
recovery zone by the way of line 17 and may be ultimately passed to
a Claus plant for sulfur recovery. The molten medium with a reduced
sulfur content is withdrawn from the sulfur recovery zone by way of
line 18, wherein this molten medium containing a reduced sulfur
level is returned to the gasification zone by way of line 19.
It will, likewise, be necessary to treat the molten medium in order
to remove trace metals and ash which have accumulated in the melt.
Accordingly, a stream of the melt with a reduced sulfur content is
withdrawn by way of line 20 from line 18 and is passed to an ash
recovery zone, not shown, wherein the ash is separated from the
melt by dissolution in water.
This invention will be further understood by reference to the
following examples:
EXAMPLE 1
A heavy residua hydrocarbon feedstock containing materials boiling
above 650.degree.F. was introduced by means of a pump at a rate of
about 2.3 grams per minute through a 1/8 inch inlet tube into a
stirred reactor containing a molten alkali metal carbonaceous
medium consisting of 70 mole % lithium carbonate and 30 mole %
sodium carbonate to which has been added 6 wt. % of boron oxide
(Run A) and 10 wt. % boron oxide, i.e. B.sub.2 O.sub.3 as the
glass-forming oxide component. The cracking zone was 4 inches in
diameter and 12 inches in length, and was placed in a Lindberg
furnace. The melt temperature was measured by a thermocouple
inserted into a thermowell positioned in the molten medium
connected to a portable pyrometer. The effluent gases were passed
directly to a gas chromatograph for analysis. The quantity of
C.sub.5 + liquid products and carbonaceous material, namely coke,
produced was also measured.
TABLE I ______________________________________ Feed: 650.degree.F.+
Arabian Heavy Resid (2.3 grams/minute) Melt: 1900 g. Li/Na (70/30
mole %) carbonate containing B.sub.2 O.sub.3 in stirred reactor
Temp: 1350-1400.degree.F. : Helium Diluent: 1.2-1.5 l/min. Run A
Run B Wt. % B.sub.2 O.sub.3 6 10 Product Yield, wt.% on Feed
______________________________________ H.sub.2 0.2 0.3 Methane 6.7
8.6 CO 6.0 9.1 CO.sub.2 10.9 11.2 Ethylene 12.7 13.0 Ethane 2.9 3.1
Propane 0.4 0.5 Propylene 9.5 8.9 Butenes 6.2 4.6 C.sub.5 + Liquid
40.8 38.2 Coke 12 12 ______________________________________
As can be seen from the results as shown in Table I, the cracking
of a heavy hydrocarbon residua feestock in a molten alkali
carbonate medium containing 6 wt. % and 10 wt. % boron oxide
results in a high conversion to C.sub.3.sup.- products.
EXAMPLE 2
This example indicates the carbon gasification rates that are
obtainable in accordance with the instant invention when coke,
which has been formed and dispersed during the cracking operation
as outlined in Example 1, has been contacted with air in a molten
alkali carbonate medium containing a glass-forming oxide, namely
boron oxide at a temperature of 1500.degree.F.
TABLE II ______________________________________ Coke Gasification
Melt: 500 grams of melt containing 1.9 wt. % coke Temp:
1500.degree.F.; Air Flow Rate: 2.0 STP Liters/min.; Pressure:
Atmospheric Oxygen Conversion Carbon Gasification Molten Medium (%)
Rate (lb/ft..sup.3 /hr.) ______________________________________
Li-Na (70/30 mole 99 3.3 %) Carbonate containing 6 wt. % B.sub.2
O.sub.3 ______________________________________
As can be seen from the results as shown in Table II, the molten
alkali metal carbonate medium containing a minor amount of a
glass-forming oxide results in a sufficient gasification of coke
present in the melt so as to permit the facile regeneration of the
melt after the melt has been employed as the cracking medium for a
hydrocarbon feedstock.
EXAMPLE 3
A heavy residua hydrocarbon feedstock containing materials boiling
above 650.degree.F. was introduced into a bubble-type,
graphite-lined stirred reactor containing a molten medium in the
same manner as described in Example 1. The molten medium comprised
70 % lithium carbonate and 30 mole % sodium carbonate to which 6
wt. % boron oxide, i.e. B.sub.2 O.sub.3 had been added. The
hydrocarbon feedstock was introduced along with a helium diluent (1
STP liter/minute) and cracked in this molten medium at a
temperature of 1350.degree.-1400.degree.F. for approximately 21/2
hours during which time the carbonaceous material, i.e. coke,
formed during the cracking reaction was uniformly dispersed at all
times throughout the medium. At the conclusion of the 21/2 hour run
the molten medium contained about 3 wt. % coke.
EXAMPLE 4
A heavy residua hydrocarbon feedstock was introduced into a
bubble-type, stainless steel stirred reactor containing a molten
medium in the same manner as described in Example 3. The molten
medium comprised 70 mole % lithium carbonate and 30 mole % sodium
carbonate. The hydrocarbon feedstock was introduced along with a
helium diluent (1 STP liter/minute) into this molten medium and
cracked at a temperature of 1350.degree.-1400.degree.F. for
approximately 40 minutes. The carbonaceous materials, i.e. coke,
formed during this cracking reaction were not uniformly dispersed
throughout the molten medium such that the molten medium formed two
layers, one of which was coked and one of which was relatively
clear. At the conclusion of the 40-minute cracking run, the molten
medium contained about 0.5 wt. % coke.
Upon carrying out the above-described cracking reaction for an
extended period of time, namely for 2 hours, the carbonaceous
materials became dispersed into a single homogeneous phase. At the
conclusion of this 2 hour run, the molten medium contained about
2.0 wt. % coke.
EXAMPLE 5
This example indicates the increase in the desulfurization rate as
measured by the steam conversion when a molten alkalimetal
carbonate medium containing a minor amount of boron oxide is
contacted with steam and CO.sub.2 at a given temperature. The
condition under which these runs were conducted is shown in Table
III below.
TABLE III ______________________________________ Melt: 70 mole %
Li.sub.2 CO.sub.3 /30 mole % Na.sub.2 CO.sub.3 Sulfur Conc. in
melt: 4.8 wt. % as sulfides CO.sub.2 /H.sub.2 O feed ratio: 37/63
mole % ______________________________________ B.sub.2 O.sub.3 Conc.
wt. % 1200.degree.F. 1500.degree.F. X 22 24 .DELTA. 12 12 0 0
______________________________________
As can be seen from the results as shown in FIG. 2, at a given
temperature, the presence of boron oxide in the molten medium
increases the desulfurization rate when the molten medium
containing sulfides is contacted with steam and carbon dioxide.
EXAMPLE 6
This example indicates that a molten medium containing an alkali
metal carbonate in combination with an oxide of boron is
significantly less corrosive to materials of containment than is a
molten medium consisting of an oxide of boron in combination with
an alkali oxide. The data shown below was obtained by partially
submerging a sample of the refractory into the molten medium under
the conditions specified in Table IV.
TABLE IV
__________________________________________________________________________
Corrosion Test Results Corrosion, Inches per Melt Year Refractory
Composition Additive Atmosphere Temperature Hours
Melt/Interface/Vapor
__________________________________________________________________________
95% Cr.sub.2 O.sub.3 Na.sub.2 O--Li.sub.2 O--B.sub.2 O.sub.3 0.2
Cl.sup.- Air 1550.degree.F. 50 4.2/28.8/4.7 Al.sub.2 O.sub.3 89 wt.
% Li/Na.sub.2 Co.sub.3 -- Air 1550.degree.F. 48 0.1/0.2/0.1 (70/30
mole %) 11 wt. % B.sub.2 O.sub.3 Al.sub.2 O.sub.3 83 wt. %
Li/NaCO.sub.3 -- Air 1550.degree.F. 48 .8/1.2/0.7-13.5 (70/30 mole
%) 17 wt. % B.sub.2 O.sub.3 Al.sub.2 O.sub.3 80 wt. % Li/Na.sub.2
CO.sub.3 -- Air 1550.degree.F. 49 1.2/1.2/1.2-13 (70/30 mole %) 20
wt. % B.sub.2 O.sub.3
__________________________________________________________________________
As can be seen from the results as shown in Table IV, the degree of
corrosion of the refractory materials is directly related to the
amount of boron oxide which has been added to the molten
medium.
* * * * *