Note: Descriptions are shown in the official language in which they were submitted.
~lZ2620
A PROCESS FOR THE PREPARATION OF AN
AROMATIC HYDROCARBON MIXTURE
The invention relates to a process for the
preparation of an aromatic hydrocarbon mixture from
natural gas.
Aromatic hydrocarbon mixtures are used on a large
scale as gasoline. They are, as a rule, obtained by
catalytic reforming of aliphatic hydrocarbon mixtures
based on mineral oil, which are boiling in the gasoline
range.
In view of the increasing need of gasoline and
the decreasing reserves of mineral oil there is a great
interest in processes permitting the conversion, in an
economically Justified way, of aliphatic hydrocarbons
boiling below the gasoline range into aromatic hydro-
carbon mixtures boiling in the gasoline range.
It is known that aliphatic hydrocarbons boiling
below the gasoline range can be converted into a
mixture of carbon monoxide and hydrogen, the so~called
synthesis gas. It is further known that synthesis gas
can be converted into a mixture of hydrocarbons, a part
~,~
~,
of which boils in the gasoline range by contacting the
synthesis gas with a suitable catalyst. Finally, it is
known that isobutane can be converted into a hydro-
carbon mixture boiling in the gasoline range by contact-
ing the isobutane, together with lower olefins such aspropene and butene, with an alkylation catalyst.
The Applicant has carried out an investigation in
order to find out how far the above-mentioned three
processes can be used in the preparation of gasoline
from natural gas. It has been found in this investigat-
ion that an aromatic hydrocarbon mixture which is very
suitable as gasoline can be prepared from natural gas
by combining the three above-mentioned processes,
provided that the following conditions are satisfied.
First of all, the natural gas should be converted
into synthesis gas. Then, the synthesis gas should be
converted into an aromatic hydrocarbon mixture, using
a catalyst containing a crystalline silicate which
(a) is thermally stable to temperatures above 600C,
(b) after dehydration at 400C in vacuum, is capable
of adsorbing more than 3 %w water at ~5C and
saturated water vapour pressure, and
(c) in dehydrated form, has the following overall
composition, expressed in moles of the oxides:
O. _
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- 3)(R)2/n ~ a Fe203. b Al203. c Ga203 /.
y (d SiO2. e GeO2), where
R = one or more mono- or bivalent cations,
a ~ 0.1,
b ~, 0,
c ~ O,
a+b+c= 1,
y ~, 10,
d ~ 0.1,
e ~ -
d+e= ~, 1, and
n = the valency of R.
From the aromatic hydrocarbon mixture thus obtain-
ed a number of fractions should then be separated,
viz.a C2 fraction which is recycled to the synthesis
gas preparation, an isobutane-containing gaseous
fraction which is contacted with an alkylation catalyst
and an aromatic liquid fraction boiling in the gasoline
range. Finally, a fraction boiling in the gasoline
range is separated from the product obtained in the
alkylation and this fraction is mixed with the aromatic
gasoline fraction obtained earlier.
The present invention therefore relates to a
process for the preparation of an aromatic hydrocarbon
mixture from natural gas, in which
(a) natural gas is converted into synthesis gas,
~b) the synthesis gas is corverted into an aromatic
~ .
~l;Z'~62~
hydrocarbon mixture, using a catalyst containing
a crystalline silicate as defined hereinbefore,
(c) a C2 fraction, an isobutane-containing gaseous
fraction and an aromatic liquid fraction boiling
in the gasoline range are separated from the
aromatic hydrocarbon mixture,
(d) the C2 fraction is recyled to step (a) of the
process,
(e) the isobutane-containing gaseous fraction is
converted by alkylation into a product from which
a fraction boiling in the gasoline range is
separated, and
(f) the two fractions boiling in the gasoline range
obtained according to (c) and (e) are mixed.
Natural gas, from which the process according to
the invention is started, consists substantially of
saturated aliphatic hydrocarbons with four or less
carbon atoms per molecule (C4 hydrocarbons), in
particular methane, ethane, propane and butane. In
addition to these C4 hydrocarbons, natural gas may
- depending on its origin - also contain up to about
10% saturated aliphatic hydrocarbons with five or more
carbon atoms (C5+ hydrocarbons), in particular hexane,
heptane and octane. The latter saturated aliphatic
hydrocarbons are liquid at normal temperature and
pressure; in natural gas, however, they are present
in the vapour phase. The process according to the
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invention may start both from a so~called crude natural
gas containing in addition to C4 hydrocarbons also
C5+ hydrocarbons, and from a so called residual natural
gas consisting substantially of C4 hydrocarbons and
which is left, after a so-called natural gas condensate
consisting substantially of c5+ hydrocarbons has been
separated from a crude natural gas. In attractive
variant of the process according to the invention is
one in which a crude natural gas is separated into a
residual natural gas and a natural gas condensate, the
residual natural gas being used as the feed for step(a)
of the process and the natural gas condensate as the
feed component for step(b) of the process. Besides
crude natural gas and residual natural gas a so-called
dry gas which contains only the methane and ethane
present in the natural gas can also very suitably be
used as the feed for step(a). When only this C2
fraction of natural gas is used as the feed for step(a),
LPG is also obtained as the end product in the process
according to the invention.
In step(a) of the process natural gas or at least
the C2 fraction thereof is converted into synthesis
gas. This conversion can very conveniently be effected
by catalytic steam reforming or by partial oxidation.
The catalytic steam reforming is preferably carried
out by contacting the feed together with stea~ at a
temperature of from 750 to 1000C and a pressure of
a~
112Z62~)
from 1 to 35 bar with a nickel, iron or platinum catalyst. The
partial oxidation is preferably carried out by treating the feed
with oxygen at a temperature of from 900 to 1600C and a pressure
of from 1 to 35 bar.
The synthesis gas which has been prepared according to
step(a) of the process is converted in step(b) into an aromatic
hydrocarbon mixture, using a catalyst containing a crystalline sili-
cate, which belongs to a special class. These silicates effect a
high conversion of aliphatic hydrocarbons into aromatic hydrocar-
bons in commercially desirable yields and are generally very activein conversion reactions in which aromatic hydrocarbons are involved.
In the process according to the invention preference is given to
the use of silicates in which no gallium and germanium are present,
in other words, silicates of which, in the above-mentioned overall
composition, c and e are o. Such silicates are the subject of
Canadian Patent Application No. 291,509 (see also United Kingdom
Patent 1,555,928). Further,preference is given to the use of sili-
cates of which, in the above-mentioned overall composition, a is
greater than 0.5. Particular preference is given to silicates in
which no aluminium is present, in other words, silicates of which,
in the above-men-
B
.
ll;~Z620
tioned overall composition, b is o. It should be noted that in thesilicates used in the process according to the invention, y is pre-
ferably less than 600 and in particular less than 300. Finally,
in the process according to the invention preference is given to
silicates whose X-ray powder diffraction pattern has, inter alia,
the reflections given in Table A of Canadian patent application No.
291,509.
In step(b) of the process synthesis gas should be converted
into an aromatic hydrocarbon mixture. Step(b) may in itself be
carried out as a one-step or as a two-step process. In the two-
step process the cynthesis gas is contacted in the first step with
a catalyst containing one or more metal components having catalytic
activity for the conversion of an H2/CO mixture into hydrocarbons
and/or oxygen-containing hydrocarbons ~for the sake of convenience
further designated catalyst X). In the second step the product thus
obtained is converted into an aromatic hydrocarbon mixture by con-
tacting it under aromatization conditions with the crystalline
silicate (for the sake of convenience designated catalyst Y). If
step(b) is carried out as a two-step process using a natural gas
20 condensate as the feed component, this natural gas
-7-
~B
l~ZZ620
condensate may be added both to the feed for the first
step and to the feed for the second step, or be divided
between the two feeds. If step(b) is carried out as a
one-step process, the synthesis gas is contacted with
a bifunctional catalyst which contains, in addition
to the crystalline silicate, one or more metal
components having catalytic activity for the conversion
of an H2/C0 mixture into hydrocarbons and/or oxygen-
containing hydrocarbons. The bifunctional catalysts
which may be used in step(b) of the process are
preferably composed of two separate catalysts X and Y.
If an X-catalyst is used which is capable of
converting an H2/C0 mixture into substantially oxygen-
containing hydrocarbons, preference is given to a
catalyst that is capable of converting the H2/C0
mixture substantially into methanol and/or dimethyl
ether. For the conversion of an H2/C0 mixture
substantially into methanol, catalysts containing one
or more of the metals zinc, copper and chromium are
very suitable. Preference is given to catalysts
containing at least two of these metals such as the
combinations zinc-copper, zinc-chromium and zinc-
copper-chromium. Particular preference is given to
the combination zinc-chromium. If desired, the said
metal combinations may be placed on a carrier material.
By introducing an acid function into these catalysts,
for instance by placing the metal combination on an
,,
ll;~Z6ZO
acid carrier, it may be secured that, apart from the
conversion of the H2/C0 mixture into methanol, a
considerable part of the mixture will be converted
into dimethyl ether. X-catalysts which are capable of
converting an H2/C0 mixture substantially into hydro-
carbons are referred to in the literature as Fischer-
Tropsch catalysts. Such catalysts often contain one
or more metals of the iron group or ruthenium together
with one or more promoters to increase the activity
and/or selectivity and sometimes a carrier material.
They can be prepared by precipitation, melting or by
impregnation. If in step(b) of the process according
to the invention use is made of a Fischer-Tropsch
catalyst as X-catalyst, it is preferred to choose for
this purpose an iron or cobalt catalyst, in particular
such a catalyst which has been prepared by impregenation.
If desired, it is possible in step(b) of the process
according to the invention to use X-catalysts which are
capable of converting an H2/C0 mixture into a mixture
containing both hydrocarbons and oxygen-containing
hydrocarbons in comparable quantities. An example of
an X-catalyst of this type is an iron-chromium oxide
catalyst. If desired, it is also possible in step(b)
of the process according to the invention to use
mixtures of two or more X-catalysts, for instance a
ll;~Z620
mixture of a first X-catalyst which is capable of
converting an H2/C0 mixture substantially into hydro-
carbons and a second X-catalyst which is capable of
converting an H2/C0 mixture substantially into oxygen-
containing hydrocarbons.
The crystalline silicates which are used as the
catalyst in step(b) of the process according to the
invention, are usually prepared from an aqueous
mixture as the starting material which contains the
following compounds in a given ratio: one or more
compounds of an alkali metal, one or more compounds
containing an organic cation or from which such a
cation is formed during the preparation of the silicate,
one or more silicon compounds, one or more iron
compounds, and, optionally, one or more aluminium,
gallium and/or germanium compounds. The preparation
is effected by maintaining the mixture at elevated
temperature until the silicate has been formed and
then separating the crystals of the silicate from the
mother liquor. Before being used in the process
according to the invention the organic cations
introduced during the preparation should be converted
by calcining into hydrogen ions. In the process it is
preferred to use silicates whose alkali metal content
is less than 1 ~w and in particular less than 0.05 %w.
Such silicates can be prepared from the above-mentioned
calcined silicates by ion exchange, for instance with
11;~;~620
an aqueous solution of an ammonium salt followed by
calcining.
Step(b) of the process according to the invention
is preferably carried out at a temperature of from
200 to 500C, a pressure of from 1 to 250 bar and a
space velocity of from 50 to 50,000 Nl gas.l catalyst.h.
In the conversion of an H2/C0 mixture into hydro-
carbons and in the conversion of oxygen-containing
hydrocarbons into an aromatic hydrocarbon mixture
in step(b) of the process water is formed. If step(a)
of the process is carried out by catalytic steam
reforming, it is preferred to recycle this water, at
least partly, to step(a). If step(b) is carried out as
a two-step process using an X-catalyst in the first
step which contains one or more metal components having
catalytic activity for the conversion of an H2/C0
mixture substantially into hydrocarbons, the water
formed in the first step is preferably separated from
the reaction product of the first step before the latter
is contacted with catalyst Y. Non-converted synthesis
gas which is present in the reaction product of the
first step of the two-step process is preferably
recycled to the first step.
In the process according to the invention a C2
fraction, an isobutane-containing gaseous fraction and
an aromatic liquid fraction boiling in the gasoline
range should be separated from the aromatic hydrocarbon
,
l~ZZ6ZO
12
mixture obtained according to step(b). The reaction
product resulting from step(b) is preferably separated
in step(c) into a C2- fraction, a propane fraction, an
isobutane-containing fraction, an n-butane fraction,
an aromatic liquid fraction boiling in the gasoline
range and, optionally, water. In step(d) of the process
the C2 fraction is recycled to step(a). If desired,
the propane and/or n-butane fractions may also be
recycled entirely or partly to step(a). Naturally, the
propane and n-butane fractions may also be combined to
LPG, if desired together with LPG which has been
separated from the natural gas in the preparation of
a C2 dry gas as the feed for the process according to
the invention.
In step(e) of the process according to the
invention the isobutane-containing gaseous fraction
should be converted by alkylation into a product from
which a fraction boiling in the gasoline range can be
separated. This alkylation can very conveniently be
2~ performed by contacting the fraction with a strong
acid as the catalyst, such as sulphuric acid or
hydrofluoric acid. Since the gaseous part of the reaction
product of step(b), as a rule, contains only small amounts
of olefins, the isobutane-containing gaseous fraction
which is separated from it will often have too low an
olefin content to permit a sufficient conversion of the
isobutane present therein by alkylation. It is there-
Z6ZO
fore preferred to increase the olefin content of thefraction before subjecting it to alkylation. An increase
in the olefin content of the isobutane-containing
fraction can conveniently be effected by mixing the
fractlon with an olefin-rich stream. This stream may
come from an external source or be prepared from
paraffins obtained in the process, such as a propane
fraction, an n-butane fraction or an LPG fraction.
From these fractions an olefin-rich stream can very
conveniently be prepared by dehydrogenation of these
fractions or by subjecting them to thermal cracking,
whereby ethene is formed as the main product and the
desired olefin-rich stream is obtained as a by-product.
From the product obtained in the alkylation a
fraction boiling in the gasoline range is separated and
this fraction is then mixed in step(f) of the process
according to the invention with the aromatic liquid
fraction boiling in the gasoline range obtained in
step(c). The non-converted isobutane is preferably
separated from the product obtained in the alkylation
and recycled to step(e). In order to increase the vapour
pressure of the gasoline mixture thus obtained, light
hydrocarbons are preferably added to it. Light hydro-
carbons very suitable for use include n-butane and LPG,
which may be obtained as products or by-products of the
process.
620
Three process schemes for the conversion of
natural gas into aromatic gasoline according to the
invention will be explained in more detail hereinafter
with the aid of figures.
5 Process_scheme I (see Fig.1)
The process is carried out in an apparatus compris-
ing successively a synthesis gas preparation section(1),
a hydrocarbon preparation section(2), the first
separation section(3)S a dehydrogenation section(4), an
10 alkylation section(5) and the second separation
section(6). A mixture of natural gas(7), a C2 fraction
(8), propane(9) and n-butane(10) is converted by partial
oxidation with oxygen(11) and steam(12) into a synthesis
gas(13). The synthesis gas(13) is converted over a
15 bifunctional catalyst according to the invention into
an aromatic reaction product(14), which is separated
into a C2 fraction(8), propane(15), isobutane(16),
n-butane(17), an aromatic gasoline fraction(18) and
water(19). The C2 fraction(8) is recycled to the
20 synthesis gas preparation section. n-Butane(17) is
divided into two portions(10) and (20), of which
portion(10) is recycled to the synthesis gas preparation
section. Propane(15) is converted by dehydrogenation
into a mixture of propene and propane(21). Isobutane(16)
25 is alkylated together with isobutane(22) and the mixture
of propene and propane(21). The alkylated product(23)
is separated into propane(9), isobutane(22) and a
l~ZZ620
gasoline fraction(24). Propane(9) is recycled to the
synthesis gas preparation section and isobutane(22) is
recycled to the alkylation section. The gasoline
fraction(24) is mixed with the gasoline fraction(18)
and with n-butane portion(20) to form gasoline(25).
Process scheme II (see Fig.1)
The process is carried out in the same apparatus
as used in process scheme I and in substantially the
same way as described therein, however, with this
difference that in the present case propane(9) is
conducted to the dehydrogenation section(4) instead of
to the synthesis gas preparation section(1).
Process scheme III (see Fig.2)
The process is carried out in an apparatus
15 comprising successively the first separation section(1),
a synthesis gas preparation section(2), a methanol
preparation section(3), the second separation section(4),
a hydrocarbon preparation section(5), the third
separation section(6), an alkylation section(7) and the
fourth section(8). Natural gas(9) is separated into a
C2 fraction(10) and a C3+ fraction(11). A mixture of
the C2 fraction(10) and a C2 fraction(12) is convert-
ed by partial oxidation with oxygen(13) and steam(14)
into synthesis gas(15). A mixture of the synthesis
gas(15) and a synthesis gas(16) is converted over a
methanol synthesis catalyst into a reaction product~17),
which ia separated into non-converted synthesis gas(16)
.~2~
16
and methanol(18). Synthesis gas(16) is recycled to the
methanol preparation section. Methanol(18) is converted
over a crystalline silicate according to the invention
into an aromatic reaction product(19) which is
separated into a C2 fraction(12), propane(20),
isobutane(21), n-butane(22), an aromatic gasoline
fraction(23) and water(24). The C2 fraction(12) is
recycled to the synthesis gas preparation section.
n-Butane(22) is separated into two portions(25) and
(26). Isobutane(21) is alkylated together with iso-
butane(27) and a propene/butene stream(28) originating
from an external source. The alkylated product(29) is
separated into isobutene(27) and a gasoline fraction(30).
Isobutane(27) is recycled to the alkylation section.
Gasoline fraction(30) is mixed with gasoline fraction(23)
and with n-butane portion(26) to form gasoline(31).
C3* fraction(11) is mixed with propane(20) and n-butane
portion(25) to form LPG(32).
Process scheme IV (see Fig.2)
The process is carried out in the same apparatus
as used in process scheme III and in substantially the
same way as described therein, however with this differ-
ence that in the present case for the preparation of
an aromatic reaction product(19) a natural gas condens-
ate(18a) is used, in addition to methanol(18), as the
feed component for the hydrocarbon preparation section.
, ~ .
'- ' ' : '
.
l~LT ~Z6~ZO
The present patent application also comprises
apparatus for carrying out the process according to the
invention, as schema~ically shown in Figs. 1 and 2.
The invention will now be further explained with
the aid of the following examples.
A crystalline iron silicate (silicate A) was
prepared as follows. A mixture of Fe(N03)3, SiO29NaNO3
and / (C3H7)4N /OH in water with the molar composition
Na 0. 1.5 / (C3H7)4N_/20. 0.125 Fe203 2
1O 468 H20 was heated for 48 hours in an autoclave at
150C under autogenous pressure. After the reactlon
mixture had cooled down, the silicate formed was filter-
ed off, washed with water until the pH of the wash
water was about 8 and dried for two hours at 120C.
Silicate A thus prepared had the following chemical
composition: 0.8 / (C3H7)4_/20. 3 2 2 3
200 SiO2. 55 H20. The silicate had an X-ray powder
diffraction pattern substantially as given in Table B
of United ICingdom patent No. l,555,928. The
silicate was thermally stable to temperatures higher
than 900C and was capable, after dehydration at 400C,
of adsorbing 7 %w water in v~cuum at 25C and saturated
water vapour pressure. With silicate A as the starting
material, silicate B was prepared by successively
calcining silicate A at 500 C, boiling with 1.0 molar
NH4N03 solution, washing with water, boiling again
with 1.0 molar NH4N03 solution and washing, drying at
~ '
120C and calcining at 500C.
A catalyst C was prepared by thoroughly mixing a
Co/Th/Mg/SiO2 Fischer-Tropsch catalyst, prepared by
impregnation, and silicate B in a weight ratio of 1:2.
Example I
This example was carried out according to process
scheme I. The starting material was a natural gas of
the following composition in %v:
C1 95.9
C2 3-4
C3 0.5
C4 0.2
C5 0.2
A mixture of 90 parts by volume of this natural
gas, 9 parts by volume of a C2- recycle stream and 1.5
parts by volume of a combined propane and n-butane
recycle stream was used as the feed for the preparatlon
of synthesis gas by partial oxidation. 70 Parts by
volume of oxygen and 0.05 part by volume of water were
added to the hydrocarbon mixture and the partial
oxidation was carried out at 1500C and 5 bar. After
purification of the reaction product a synthesis gas
was obtained which consisted of 35 parts by volume of
carbon monoxide and 65 parts by volume of hydrogen.
This synthesis gas was contacted at a temperature of
280C, a pressure of 30 bar and a space velocity of
250 l.l 1.h 1 with catalyst C. The conversion of the
0
19
synthesis gas was 95~. The hydrocarbon mixture obtained
had the following composition in %w:
C1 25
C2 5
C3 7
n-c4
i-C4 2
C5+ gasoline 60
The olefin content of both the C3 and the C4
fractlons was less than 1 %w. The reaction product was
separated by cooling into a C2 fraction and a C3+
fraction. The C2 fraction was recycled to the reactor
in which the partial oxidation of the natural gas was
carried out. The C3+ fraction was separated into a
propane fraction, an isobutane fraction, an n-butane
fraction and a C5+ gasoline fraction. The propane
fraction was converted into a mixture of propane and
propene by dehydrogenation at 600C over a Cr203
catalyst. The conversion of propane into propene was
32%. The mixture of propane and propene thus obtained
was mixed with the isobutane fraction and the mixture
was converted by contacting it at 40C with an HF
alkylation catalyst. From the product obtained in the
alkylation a propane fraction, an isobutane fraction
and a gasoline fraction were separated. By isobutane
recycling a constant isobutane/olefin ratio of 14 was
maintained in the alkylation reactor. The yield of
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alkylation gasoline was 95%. The alkylation gasoline
was mixed with the gasoline obtained earlier in the
process. To bring the vapour pressure of the mixture
to the proper value, part of the n-butane fraction was
added to it. The gasoline thus prepared had an octane
number (CRON) of 90. The remaining part of the
n-butane fraction obtained from the C3~ fraction of the
hydrocarbon synthesis product and the propane fraction
obtained from the alkylation product were recycled to
the reactor in which the partial oxidation of the
natural gas was effected .
Example II
This example was carried out according to process
scheme III. 100 Parts by volume of natural gas were
separated into 96 parts by volume of C1/C2 fraction
(dry gas) and 4 parts by volume of C3/C4 fraction. A
mixture of the 96 parts by volume of the C1/C2 fraction
and 4 parts by volume of a C2- recycle stream was used
as the feed for the preparation of synthesis gas by
partial oxidation. 70 Parts by volume of oxygen and
0.05 part by volume of water were added to the hydro-
carbon mixture and the partial oxidation was carried
out at 1500C and 5 bar. After purification of the
reaction product a synthesis gas was obtained which
consisted of 35 parts by volume of carbon monoxide and
65 parts by volume of hydrogen. 100 Parts by volume of
this synthesis gas were mixed with 2 parts by volume
a~ ~
of water and the mixture was contacted at a temperature
of 375C, a pressure of 80 bar and a space velocity of
25,000 l.l 1.h 1 with a ZnO-Cr203 methanol synthesis
catalyst. From the reaction product methanol was
5 separated by cooling. After removal of C02 the non-
converted synthesis gas was recycled to the reactor
in which the methanol synthesis was effected. Methanol
was converted into an aromatic hydrocarbon mixture
by contacting it at a temperature of 350C, a pressure
of 5 bar and a space velocity of 0.5 l.l 1.h 1 with
silicate B. The conversion of methanol was almost 100%.
The hydrocarbon mixture obtained had the following
composition in %w:
C1 + C2 11.4
C3 + n-C4 7.2
i-C4 2.1
C5+ gasoline79.3
The olefin content of both the C3 and the C4
fractions was less than 1 %w. The reaction product was
separated by cooling into a C2 fraction and a C3+
fraction. The C2 fraction was recycled to the reactor
in which the partial oxidation of the C2- fraction of
the natural gas was carried out. The C3+ fraction was
separated into a propane fraction, an isobutane fraction,
an n-butane fraction and a C5+ gasoline fraction. The
isobutane fraction was mixed with 80 %v of a C3-C5
olefin mixture originating from an external source and
~. .
-" ll;~Z620
22
the mixture was converted by contacting it at 40C
with an HF alkylation catalyst. By isobutane recycling
a constant isobutane/olefin ratio of 14 was maintained
in the alkylation reactor. The alkylate which was
obtained in a yield of 95% was mixed with the gasoline
obtained earlier in the process. To bring the vapour
pressure of the mixture to the proper value, part of
the n-butane fraction was added to it. The gasoline
thus prepared had an octane number (CRON) of 92. The
remaining part of the n-butane fraction and the propane
fraction obtained from the C3+ fraction of the hydro-
carbon synthesis product were mixed with the C3/C4
fraction separated from the natural gas to form LPG.
