Method for processing etherified light hydrocarbon mixtures to remove methanol

Mixed hydrocarbons of predominantly four carbon atoms each are subjected to etherification with methanol, to make ether from the tertiary olefin in the mixture; the unreacted hydrocarbons, after separation from the ether by distillation, are contaminated with methanol which is removed by absorption or extraction with a glycol before the hydrocarbons are subjected to further processing in which the methanol is detrimental.

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, and 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 Ser. No. 863,499 filed Dec. 22, 1977 that tertiary branched 
chain olefins in light catalytically cracked gasoline (LCCG) and in 
partially hydrogenated pyrolysis gasoline (HPGB or dripolene) are 
advantageously converted to dialkyl 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 U.S. Pat. No. 3,482,952 
it is acknowledged that alkylation of the material, 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 
rich fraction from a less volatile ether rich fraction; thereafter the 
more volatile olefin rich fraction, having reduced alcohol content, is 
alkylated. 
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 U.S. Pat. No. 3,482,952 for etherification 
with methanol of the tertiary olefins in C.sub.4 -C.sub.6 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 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 hydrocarbon 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 (b) 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 miscible liquid which is ethylene glycol, diethylene 
glycol, triethylene glycol, propylene glycol, or a mixture of any of 
these, to remove methanol from the distillate, and 
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 isobutylene is used in the 
etherification reactor, in order to improve isobutylene conversion, but 
even without such an excess, there is bound to be 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 
polymerization to form polygas for blending into gasoline, but the 
methanol in the distillate must first be reduced to a much lower 
concentration to preclude interference with the catalysts used in either 
of the foregoing reactions. Both alkylation and polymerization reactions 
use strongly acidic catalyst which also, for example, promote 
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 
methanol by weight, it is not practicable from an economic viewpoint to 
separate methanol from the predominantly C.sub.4 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 pressure the azeotropic composition 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 
Wt. % Methanol 1.0 2.4 4.3 6.1 
Wt. % n-butane 99.0 97.6 95.7 93.9 
______________________________________ 
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 assessed 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 300 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 
C.sub.4 hydrocarbons is in the vapor phase; ethylene glycol is the most 
practicable scrubbing liquid to use as the absorbant, because it is an 
efficient absorber of the methanol while it minimizes absorption of 
hydrocarbons of the distillate, and it 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 C.sub.4 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 are ethers 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 34.degree. F. (1.degree. C.) to substantially 450.degree. F. 
(232.degree. C.). Preferably temperature in the range from 70.degree. F. 
to 200.degree. F. (21.degree. C. to 93.degree. C.) 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 from substantially 8.degree. F. (-13.degree. C.) 
to substantially 350.degree. F. (177.degree. C.); preferably the 
temperature is in the range from 50.degree. F. to 150.degree. F. 
(10.degree. C. to 65.degree. C.). Obviously pressure in the liquid-liquid 
extraction unit must be higher than in a gas absorber unit operating at 
the same 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 liquid-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 of 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 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 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 150.degree. F. to 250.degree. F. (65.degree. C. to 
121.degree. C.) 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 C.sub.4 hydrocarbon fraction 
containing 19% isobutylene includes a temperature of 180.degree. F., a 
pressure of 18 atmospheres, and a methanol: isoolefin feed ratio of 0.95; 
under such conditions a conversion of 82% of the isobutylene 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 (primarily 
C.sub.4 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. 
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 butane 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 give 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 19% by weight isobutylene and 
derived from the effluent of a fluid 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 temperature conditions at a liquid hourly space velocity of 
substantially 3.0. Reactor effluent containing 76% by weight of C.sub.4 
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 72.degree. F. 
(22.degree. C.) 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 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 was removed by the ethylene glycol which was withdrawn 
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) C.sub.4 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 
78.degree. F. (26.degree. C.) was fed to the top of the column at a mole 
rate relative to the distillate fed 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 180 mole ppm methanol was recycled to the top of the 
extractor. 
Numerous modifications of the specific expedients described herein can be 
made without departing from the scope of the invention which is defined in 
the following claims.