Note: Descriptions are shown in the official language in which they were submitted.
1~89881
This invention relates to improvements in.refining
of petroleum refinery streams, more particularly light olefinic
hydrocarbon streams containing predominantly hydrocarbons of
four carbon atoms each, an~ specifically to a method in which
such streams are.subject to etherification with methanol for
the production of dialkyl ethers from their tertiary olefin
content.
- It has been suggested in the prior art, particularly
in copending application Serial No. 293,245 filed 16 Dec 77
that tertiary branched chain olefins in li~ht catalytically
cracked gasoline (LCCG) and in partially hydrogenated.pyrolysis
gasoline tHPGB or dripolene) are advantageously converted to
. dial~yl ethers by etherifying them with primary alcohol, either
in admixture with one another or separately subsequent to
substantial separation by fractionation of the tertiary olefins
of differing number of carbon atoms into separate hydrocarbon
fractions. It has also been suggested in the art that,
subsequent to etherification of tertiary olefins in such gasolines
or hydrocarbon fractions, other hydrocarbons in the fractions
can be processed, for example by alkylation, to increase octane
value of the material and/or reduce its volatility as a gasoline
component. In USP 3,482,952 it is acknowledged that alkylation
. of the materialj while it still contains unreacted alcohol from
the etherification, results in high consumption of acidic
alkylation catalyst, and fractional distillation of the
. etherified material is recommended to separate a low boiling
olefin richfraction from a less volatile ether rich ~raction;
-thereafter the more volatile olefin rich fraction, having
reduced alcohol content, is alkylated.
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It has now been found that, particularly when a
mixed hydrocarbon fraction containing predominantly hydrocarbons
of only four carbon atoms is processed in this manner, it is
not possible to achieve the necessary separation of methanol
from the low boiling, olefin rich fraction by simple fractional
distillation. Petroleum refineries having an alkylation unit
using hydrofluoric acid catalyst, or a polymerization (polygas)
unit using phosphoric acid catalyst, prefer a feed stream for -
such unit to contain less than substantially 100 mole ppm of
methanol. It has been discovered that a minimum boiling
azeotrope of methanol and n-butane exists, although its
existence does not appear to have been reported in the chemical
literature. This azeotrope prevents the efficient separation
of methanol, by simple fractional distillation, from hydrocarbon
fractions containing n-butane which generally is present in
significant amounts in such fractions following etherification
of the tertiary olefins therein. Hence the recommendation in
USP 3,482,952 for etherification with methanol of the tertiary
olefins in C4-C6 mixed hydrocarbon fractions, followed by
distillation separation of a more volatile, unetherified
hydrocarbon portion and alkylation of the separated unetherified
portion, is not practicable, particularly when the original
mixed hydrocarbon fraction to be etherified contains predominantly
hydrocarbons of only four carbon atoms. The present invention
was developed particularly to provide a method of processing an
olefinic mixed hydrocarbon stream containing predominantly
hydrocarbons of only four carbon atoms whereby the tertiary
olefin therein is substantially etherified with methanol and
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a portion of the unetherified hydrocarbons, separated as a
distillate from the ether containing residue, is refined to a
quality satisfactory for further utilization in gasoline alkylate
and polygas production. Such olefinic mixed hydrocarbon streams
to be etherified are available, for example, from the effluent
of a fluid catalytic cracking unit, from the effluent of a
thermal or steam cracker used primarily for ethylene production,
and various other sources of mixed olefinic hydrocarbons of
predominantly four carbon atoms.
The invention thus consists in a method for
processing an olefinic mixed hydrocarbon stream containing
predominantly hydrocarbons of only four carbon atoms each
including n-butane and isobutylene, said method comprising:
1. passing the stream in admixture with methanol
into contact with an etherification catalyst, in a reactor
under etherifying conditions, to etherify tertiary olefins in
the stream,
2. passing the resulting ether and mixed hydro-
carbon containing effluent to a fractional distillation column
and distilling to provide (a) a substantially ether-free
distillate containing a proportion of methanol distilling
azeotropically with n-butane in the distillate and lb) a
distillation residue containing substantially all of the ether
from the effluent,
. 3. passing said distillate through a methanol
removal unit in contact with a stream of methanol misci~le
liquid which is ethylene glycol, diethylene glycol, tri-
ethylene glycol, propylene glycol, or a mixture of any o~ these,
to ~emove methanol from the distillate, and
1~R398~1
4. separating distillate of reduced methanol
content from said liquid.
The catalytic etherification of tertiary olefins,
particularly isobutylene with methanol, is a well-known art
and modern catalytic processes therefor can readily achieve
single pass conversions to ether of up to 82% or more of the
isobutylene content of olefinic mixed hydrocarbon streams
containing predominantly four carbon atoms. Sometimes a slight
excess of methanol for stoichiometric reaction with the iso-
butylene is used in the etherification reactor, in order toimprove isobutylene conversion, but even without such an excess,
there is bound to ~e some methanol in the etherification
reactor effluent as the reaction cannot proceed past the point
of equilibrium concentration of the methanol and ether product.
When the effluent is distilled, the residual methanol can be
largely retained in the distillation residue with the higher
boiling ether product, for blending into gasoline for example,
but some of it must distill overhead from the effluent as the
azeotrope with n-butane previously mentioned herein; unreacted
butenes also readily distill overhead from the effluent. The
foregoing overhead distillate contains a high proportion of
butenes which can advantageously be reacted by alkylation to
form alkylate or by polymeri~ation to form polygas for ~lending
into gasoline, but the methanol in the distillate must first
be reduced to a muoh lower concentration to preclude inter-
ference with the catalysts used in either of the foregoing
reactions. Both alkylation and polymerization reactions use
strongly acidic catalysts which also, for example, promote
,
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etherification of methanol to dimethyl ether under the reaction
conditions, forming water as a co-product, and this water
is detrimental to the strongly acidic catalysts. Also, the
dimethyl ether is a low boiling ether, undesirable as a gasoline
component. Furthermore, methanol may react with strong acids,
thus destroying them and precluding them exercising any further
desired catalytic activity.
Because the binary azeotrope of methanol and n-butane
is a minimum boiling azeotrope generally containing in the
range from only one to six percent methanoi by weight, it is
not practicable from an economic viewpoint to separate methanol
from the predominantly C4 hydrocarbon distillate by further
distillation prior to using the latter as feed in an alkylation or
polygas unit. The foregoing azeotropic proportions of methanol
and n-butane relate to the most relevant pressure range from
one to four atmospheres. With higher pressures the azeotropic
compositlon of methanol and n-butane has higher proportions of
methanol. Available measurements showing the effect of pressure
on the azeotropic composition are given in the following
Table 1.
TABLE 1
Effect of Pressure on Composition of
Normal Butane-Methanol AzeotroPe
.
Pressure ~Atm. Ab.) 1.70 2.72 - 4.08 5.44
25Wt. ~ Methanol 1.0 2.4 4.3 6.1
Wt. ~ n-butane 99.0 97.6 95.7 93.9
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108988~
The reduction of methanol concentration which
must be achieved in any particular application of the
invention depends on the particular type of downstream process
that utilizes the hydrocarbon stream. The exact value of the
maximum permissible level of methanol in the feed to such
downstream process can be asses~ed for example by balancing
the capital and operating cost of methanol removal equipment
against the detrimental effect a specified methanol concentration
has on the downstream process. Current experience using a
polymerization reactor downstream indicates that methanol
concentration should be reduced to no greater than 100 mole
ppm by the methanol removal unit in the method of this
invention before the mixed hydrocarbon distillate is fed to
the polymerization reactor. Similarly it is felt that methanol
concentrations should be reduced to no greater than 30Q mole
ppm methanol and 50 mole ppm methanol respectively before the
mixed hydrocarbon distillate is fed to alkylation process
reactors utilizing sulfuric acid and hydrofluoric acid
catalysts respectively.
Methanol removal units suitable for use in the
present invention can be of either the liquid-liquid extractor
type or the gas absorber type. A gas absorber type is used
when it is desired to operate at temperature and pressure
under which the distillate containing methanol and predominantly
C4 hydrocarbons is in the vapor phase; ethylene glycol is the
most practicable scrubbing liquid to use as the absorbant,
~ecause it is an efficient absorber of the methanol while it
minimizes absorption of hydrocarbons of the distillate, and it
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is readily subsequently separated from the methanol by simple
distillation from which both the methanol and ethylene glycol
can be recovered for reuse. Liquid-liquid extractors for
removing methanol from the predominantly C4 hydrocarbon
distillate can use any of several methanol miscible glycols
as the extracting liquid. Ethylene glycol is preferred, for
the reason noted above that it is readily separated from the
extracted methanol by simple distillation, for recovery and
reuse of both materials. Diethylene glycol, triethylene glycol
(which areethers of ethylene glycol), and propylene glycol are
other suitable glycols.
When using a gas absorber type of methanol removal
unit to remove methanol from a vapor phase stream of effluent
in accordance with the present invention, the unit may operate
at temperatures in the range from substantially 34F (1C) to
substantially 450F (232C). Preferably temperature in the
range from 70F to 200F (21C to 93C) is used. The mole flow
rate of glycol absorption liquid in the absorber, in proportion
to the mole flow rate of hydrocarbon vapors containing methanol,
may be in the range from substantially 0.08 to substantially
5.0; preferably it is in the range from 0.15 to 0.5.
When using liquid-liquid extraction with glycol in
the methanol removal unit in the process of this invention, the
unit may operate at temperatures in the range fr~m sub tantially
2~ 8F (-13C) to substantially 3S0F (177C); preferably the
temperature is in the range from 50F to 150F ~10C to 65C).
Obviously pressure in the liquid-liquid extraction unit must
be higher than in a gas absorber unit operating at the same
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1(189881
temperature, in order to maintain the hydrocarbons in the
liquid phase. The mole flow rate of glycol through a liquid-
liquid extraction, in proportion to the mole flow rate of
liquid hydrocarbons containing methanol, may be in the range
from substantially 0.15 to substantially 6.0; preferably it
is in the range from 0.24 to 0.6.
The equipment for the methanol removal unit of
either the liquid-liquid extraction type or the gas absorption
type in the method of this invention can be any of the suitable
conventional types available for such operations. For example,
both packed and plate type vapor-liquid contacting columns can
be used, plate type columns normally being more efficient per
unit height than packed columns for absorption but the latter
having lower capital cost. Similar considerations apply to
counter-current li~uid-liquid extraction columns, but for
extraction, packed columns are generally preferred. Either
counter-current or co-current flows can be used, but counter-
current is generally more efficient. Alternatively a series
af mixers and settlers may be used for contacting and
separating various stages during liquid-liquid extraction.
The methanol miscible liquid used in the methanol
removal unit preferably is monoethylene glycol because of its
effectiveness and relatively low cost. The higher molecular
weight glycols: diethylene glycol, triethylene glycol, and
2~ propylene glycol, are generally more expensive without being
significantly more effective~
In the initial step of the method of this invention,
which step is generally a conventional catalytic etherification
~C~89881
with methanol of the isoolefin components of a fraction of
mixed hydrocarbons of predominantly four carbon atoms, the
mole ratio of methanol to isoolefin in the feed is generally
in the range from 0.7:1 to 1.3:1 and preferably is in the
range from 0.9:1.0, most preferably 0.95:1. Preferred
catalysts for the conventional etherifications are the
polystyrene-divinyl benzene type cation exchange resins.
Temperatures for the etherification are generally in the range
from 150F to 250F (65C to 121C) and pressures are at least
sufficiently high to maintain the etherification reaction
mixtures in the liquid phase. An example of conditions for a
typical etherification of a C4 hydrocarbon fraction containing
19~ isobutylene includes a temperature of 180F, a pressure
of 18 atmospheres, and a methanol: isoolefin feed ratio of
0.95; under such conditions a conversion of 82% of the iso-
butylene is obtained using conventional ion exchange resin
catalyst.
In the second step of the method of this invention,
the effluent from the preceding etherification step is
fractionally distilled. The effluent from the preceding step
typically contains, for example, 23~ ether (primarily MTBE,
i.e. methyl tertiarybutyl ether), 76% hydrocarbons (primariiy
C4 hydrocarbons) and 1% methanol. The distillation is
conducted under conditions of temperature, pressure and reflux
such that substantially all of the ether fed to the column is
withdrawn in the higher boiling bottom fraction and none of it
passes overhead in the distillate fraction, while at the same
time most of the hydrocarbons are withdrawn in the distillate.
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1-~8~8~1
Under these conditions, most of the methanol in the effluent
remains in the higher boiling bottom fraction but, because of
the formation of the binary azeotrope of methanol and n-butane,
some of the methanol appears in the primarily hydrocarbon
distillate. In achieving the separation of the hydrocarbon
distillate from the ether containing bottom fraction, a
proportion of, for example, 84% of the butane in the hydrocarbon
fraction distills into the distillate, the balance remaining with
the ether fraction. Typically this 84~ portion of the butanè
constitutes a proportion of, for example, 8% by weight of the
hydrocarbon distillate and brings with it into the distillate an
azeotrope with methanol, the proportion of methanol in the
distillate partially depending on the pressure maintained during
distillation and also on the proportion of n-butane in the
distillate. Typically there is, for example, a proportion of
0.4~ by weight of methanol in the distillate containing 6% n-
butane, from a column operating at 3.7 atmospheres pressure.
This invention may be more readily understood from
the following examples of specific embodiments thereof which
are given for illustration only and not to limit the ensuing
claims. The proportions given therein and throughout the rest
of the specification and claims are proportions by weight
unless otherwise specifically indicated.
EXAMPLE 1
An olefinic mixed hydrocarbon fraction containing
predominantly hydrocarbons of four carbon atoms including l9~
by weight isobutylene and derived from the effluent of a fluid
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1C~8~881
catalytic cracking process was mixed with methanol in a
proportion of substantially 0.95 mole of methanol per mole of
isobutylene in the fraction and passed in liquid phase into
contact with an etherification catalyst of ion exchange resin
under etherifying tèmperature conditions at a li~uid hourly
space velocity o~ substantially 3Ø Reactor effluent
containing 76% by weight of C4 hydrocarbons, 23~ by weight of
methyl tertiarybutyl ether, and 1~ by weight of methanol was
fractionally distilled in a 40 plate distillation column
operating at a pressure of 3.7 atmospheres to separate a bottom
fraction, containing substantially all of the ether together
with most of the methanol fed to the column and some of the
hydrocarbons,.from a distillate substantially free of ether
and containing 0.7 mole percent (0.4% by weight) methanol,
balance hydrocarbons including 8% by weight n-butane. The
distillate was fed at a rate of 3.3 lbs. per hour (1.5 kg/hr)
to the bottom of a sieve tray gas absorber column one inch
(2.5 cm) in diameter, having 7 trays and being maintained at
atmospheric pressure; a counter-current stream of ethylene
glycol maintained at 72F (22C) was fed to the top of the
column at a mole rate of 0.25 compared to the feed of distillate.
The glycol flowing down through the column contacted the
distillate which, under the temperature and pressure conditions
in the column, was in the vapor phase. The vapor phase
~5 effluent withdrawn from the top of the column contained 30 mole
ppm methanol in a hydrocarbon mixture which was eminently
suitable as feed to a hydrofluoric acid catalyzed alkylation
process; thus 99.5~ of the methanol fed to the absorber column
9881
was removed by the ethylene glycol which was with~rawn from
the bottom of the column and fed to a packed stripping column.
In the stripping column methanol was stripped from the glycol
for recycle to the etherification unit and glycol, withdrawn
from the bottom of the stripping column and containing a
residual 240 mole ppm methanol, was recycled to the top of the
absorber column.
EXAMPLE 2
This example illustrates the use of liquid-liquid
extraction of methanol from a hydrocarbon stream using ethylene
glycol as the extractant. The hydrocarbon distillate stream of
3.2 lbs/hr (1.45 kg) C4 hydrocarbons containing 0.7 mole percent
methanol, fed in the preceding example to a gas absorber column,
was directed instead into the bottom of an extraction column
five feet (1.52 m) high, two inches (5 cm) in diameter, packed
with half inch (1.25 cm) Raschig rings, and maintained at a
pressure of 3.5 atmospheres. A counter-current stream of
ethylene glycol maintained at 78F (26C) was fed to the top
of the column at a mole rate relative to the distillate feed
of 0.36. At the temperature and pressure condition in the
extractor, the distillate remained in the liquid phase. The
liquid hydrocarbon stream withdrawn from the top of the
extractor contained 95 mole ppm methanol, and was suitable as
feed to a polygas unit. Ethylene glycol withdrawn from the
bottom of the extractor was fed to a stripping column to strip
- methanol therefrom and the stripped glycol containing ~80 mole
ppm methanol was recycled to the top of the extractor.
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1~89881
Numerous modifications of the specific expedients
described herein can be made without departing from the scope
3 of the invention which is defined in the following claims.
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