Process for producing ethyl tertiary butyl ether by catalytic distillation

A low pressure catalytic distillation process for producing high purity ethyl tertiary butyl ether that contains less than 0.6 weight percent ethanol, and preferably less than 0.07 weight percent ethanol, has been developed. The high purity ethyl tertiary butyl ether is withdrawn directly from a catalytic distillation column. No downstream processing is necessary to remove excess ethanol from the ether product. A stream containing a significant amount of one or more inert azeotropic agents such as normal butane, isopentane, and isobutane is introduced along with the isobutylene and ethanol reactants into an etherification zone containing a catalytic distillation column. The catalytic distillation column is operated under low pressure conditions which result in the reaction of the ethanol with the isobutylene to form ethyl tertiary butyl ether. The inert azeotropic agent must be present at the inlet to the catalytic distillation column in an amount sufficient to azeotrope excess ethanol and cause the excess ethanol to distill into an overhead stream under the conditions of operation. The preferred azeotropic agent is isopentane. Excess ethanol forms an azeotrope with the azeotropic agent and is distilled with other hydrocarbons into an overhead stream. The ethyl tertiary butyl ether and no more than 0.6 weight percent ethanol are distilled into a bottoms stream and withdrawn directly from the catalytic distillation column.

BACKGROUND OF THE INVENTION 
Ethers have become an important gasoline blending component in order to 
increase the octane rating of the gasoline without exceeding regulatory 
Reid vapor pressure limits and to reduce carbon monoxide emissions. 
Processes involving catalytic distillation have been used to produce 
ethers by the reaction of an alcohol with an isoalkene, see U.S. Pat. No. 
5,258,560; in particular, catalytic distillation has been used to produce 
ethyl tertiary butyl ether by the reaction of ethanol and isobutylene. For 
example, U.S. Pat. No. 5,248,836 discloses passing an 
isobutylene-containing stream and a stream containing ethanol and ethyl 
tertiary butyl ether through a straight pass reactor to selectively react 
ethanol and a portion of the isobutylene to form a first product stream 
which is sent to a catalytic distillation column. In the catalytic 
distillation zone the ethyl tertiary butyl ether is largely distilled from 
the ethanol and isobutylene that are then further reacted to form a second 
product stream. The distilled ethyl tertiary butyl ether is collected and 
the second product stream is recycled to the straight pass reactor. U.S. 
Pat. No. 5,368,691 discloses a configuration for catalytic distillation 
whereby reaction zones are alternated with, and clearly separated from, 
distillation zones without a continuous liquid mass between a reaction 
zone and an adjacent distillation zone. 
However, because excess ethanol is usually used to drive the etherification 
of isobutylene to high conversion and the unreacted ethanol distills with 
the product ethyl tertiary butyl ether, it is difficult to obtain a 
catalytic distillation product ethyl tertiary butyl ether having low 
levels of ethanol. Removing the ethanol from the ethyl tertiary butyl 
ether product results in a product having a lower Reid vapor pressure 
which, in turn, makes the ether product more valuable. U.S. Pat. No. 
5,158,652 discloses a process for separating ethyl tertiary butyl ether 
and ethanol using two distillation columns operating at different 
temperatures and pressures. U.S. Pat. No. 5,401,887 discloses a process 
where the reactants undergo etherification in a reactor and the reactor 
effluent is sent to a distillation column for separation. The distillation 
column bottoms containing both ethanol and ethyl tertiary butyl ether is 
further processed to separate the ethanol from the ethyl tertiary butyl 
ether by adsorbing the ethanol on a selective adsorbent. U.S. Pat. No. 
4,198,530 discloses a process for producing methanol-free methyl tertiary 
butyl ether. The methyl tertiary butyl ether is formed in an 
etherification reactor using a feed that contains methanol, C.sub.4 
hydrocarbons, and a significant amount of normal butene. The reactor 
effluent is passed to a distillation zone where substantially all of the 
methanol in the effluent is azeotropically removed together with the 
C.sub.4 hydrocarbons and whereby substantially all of the methanol is 
removed from the methyl tertiary butyl ether. U.S. Pat. No. 4,413,150 
discloses converting isobutylene and an alcohol to an ether in a reactor 
and then separating the product ether from the reaction mixture in a 
single distillation column where the product ether is substantially free 
of C.sub.4 hydrocarbons and alcohol. 
Applicants, however, are the first to realize that in a low pressure 
catalytic distillation process, by routing a stream containing at least 
one inert azeotropic agent to an etherification zone at a point prior to 
the low pressure catalytic distillation column, excess ethanol will be 
drawn into an azeotrope with the inert azeotropic agent and will not be 
able to contaminate the ethyl tertiary butyl ether product. The result is 
a high purity ethyl tertiary butyl ether product containing no more than 
0.6 weight percent ethanol available directly from the catalytic 
distillation column. No further downstream processing is needed for 
removal of ethanol and purification of the ethyl tertiary butyl ether. 
SUMMARY OF THE INVENTION 
The purpose of the invention is to provide a low pressure catalytic 
distillation process for producing high purity ethyl tertiary butyl ether 
that contains less than 0.6 weight percent ethanol, and preferably less 
than 0.07 weight percent ethanol. The high purity ethyl tertiary butyl 
ether is withdrawn directly from a catalytic distillation column. No 
downstream processing is necessary to remove excess ethanol from the ether 
product. A stream containing one or more inert azeotropic agents selected 
from the group consisting of normal butane, isobutane, and isopentane is 
introduced along with the isobutylene and ethanol reactants into an 
etherification zone containing a catalytic distillation column operated 
under low pressure conditions. The reaction of the ethanol with the 
isobutylene forms ethyl tertiary butyl ether. The inert azeotropic agent 
must be present at the inlet to the catalytic distillation column in an 
amount sufficient to azeotrope excess ethanol and cause the excess ethanol 
to distill into an overhead stream under the conditions of operation. The 
preferred azeotropic agents are isopentane and normal butane. Excess 
ethanol forms an azeotrope with the azeotropic agent and is distilled with 
other hydrocarbons into an overhead stream. The ethyl tertiary butyl ether 
and no more than 0.6 weight percent ethanol are distilled into a bottoms 
stream and withdrawn directly from the catalytic distillation column.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a process for producing an ethyl tertiary 
butyl ether stream that contains no more than 0.6 weight percent ethanol, 
and preferably 0.07 weight percent ethanol, by introducing a stream 
containing sufficient amounts of one or more inert azeotropic agents along 
with the ethanol and isobutylene reactants to an etherification zone 
containing a low pressure catalytic distillation column. The 
etherification zone also preferably contains at least one fixed bed 
reactor connected in series to the catalytic distillation column, and the 
most preferred etherification zone contains two serially-connected fixed 
bed reactors followed by one catalytic distillation column. The fixed bed 
reactors are used to perform a large portion of the etherification 
reaction and the catalytic distillation column allows for conversion 
beyond that which is achievable in a static system limited by equilibrium. 
Fixed bed reactors and catalytic distillation are known techniques for the 
production of ethers, and catalytic distillation involves one or more 
distillation zones and one or more reaction zones, all located within the 
same vessel. The distillation zones typically contain distillation 
structures such as inert packings or distillation trays. The reaction 
zones contain catalyst capable of catalyzing the etherification reaction 
and distillation structure for the distillation of components within the 
reaction zone. The exact fixed bed and catalytic distillation 
configurations used are not critical to this invention; any known 
catalytic distillation configuration for the formation of ethers may be 
used in the practice of this invention. Similarly, the catalyst used in 
the fixed bed and catalytic distillation columns may be any catalyst known 
to catalyze the etherification reaction including divinylbenzene 
cross-linked polystyrene ion exchange resins in which the active sites are 
sulfonic acid groups, and inorganic heterogeneous catalysts such as boric 
acid, bismuth molybdate, and metal salts of phosphomolybdic acids wherein 
the metal is lead, antimony, tin, iron, cerium, nickel, cobalt, or 
thorium. Boron phosphate, blue tungsten oxide, and crystalline 
aluminosilicates of the zeolitic molecular sieve type have also been 
proposed as heterogeneous catalysts for the reaction of ethanol and 
isobutylene. The conditions at which the fixed bed reactors are operated 
are well known and include pressures ranging from about 150 to about 250 
psig and temperatures ranging from about 35.degree. C. to about 75.degree. 
C. Conditions for the catalytic distillation unit are also known and 
include pressures ranging from about 80 to about 120 psig. Note that in 
the catalytic distillation column the conditions are limited to those 
compatible with the catalyst used and are therefore of a more narrow range 
as compared to a conventional distillation column. Specifically, the 
catalytic distillation column is restricted to operation at a lower 
pressure in order to control the temperature in the reaction portion. A 
conventional distillation column may be operated at higher pressure to 
increase the amount of azeotropic ethanol in the column overhead. 
Isobutylene and ethanol reactants are introduced to the etherification 
zone. The reactants may be obtained from any source. The isobutylene is 
typically in a stream that also contains other C.sub.4 hydrocarbons. A 
common source of isobutylene is a light hydrocarbon catalytic 
dehydrogenation process such as the preferred Oleflex process. The 
effluent of the light hydrocarbon catalytic dehydrogenation operation is a 
mixed C.sub.4 stream that includes isobutylene, normal butane, isobutane, 
butene-1, trans-butene-2 and cis-butene-2. The isobutylene is usually 
present in an amount ranging from about 10 to about 70 mole percent of the 
mixed C.sub.4 stream. It is contemplated that some propane and propylene 
may also be present. A common source of ethanol is the fermentation of 
grain. A greater than stoichiometric amount of ethanol is introduced to 
the etherification zone in an effort to increase the conversion of 
isobutylene. As the isobutylene and ethanol contact the catalyst in the 
fixed bed reactors, the etherification reaction is catalyzed and ethyl 
tertiary butyl ether is formed. Due to equilibrium limitations, not all of 
the isobutylene will be consumed and the effluent of the fixed bed 
reactors contains isobutylene, ethanol, ethyl tertiary butyl ether, trace 
byproducts and other unreacted hydrocarbons. To react the remaining 
isobutylene, the effluent is introduced to the catalytic distillation 
column. 
In the catalytic distillation column the isobutylene and ethanol contact 
the catalyst and further etherification is catalyzed. As ethyl tertiary 
butyl ether is formed, it is rapidly distilled and removed from the 
reactants, thereby facilitating the continuation of the etherification 
reaction. The product ether is distilled into a bottoms stream and removed 
from the catalytic distillation column. Most of the other components are 
distilled into an overhead stream and removed from the catalytic 
distillation column. A small amount of a few byproducts may distill into 
the bottoms stream. However, a problem arises due to fact that of the 
typical products present, ethanol has the highest pure component boiling 
point and by distillation alone would distill into the ether-containing 
bottoms stream. The ethanol present in the ether product stream decreases 
the purity of the ether product, and in order to take full advantage of 
the low blending Reid vapor pressure of the ethyl tertiary butyl ether 
product, the ethanol contaminant should be no greater than a 0.6 weight 
percent of the product. It is also contemplated that the blending octane 
number of the product ethyl tertiary butyl ether will be higher when the 
ethanol contaminant is kept to a minimum. Removal of the ethanol 
contaminant by techniques such as water extraction, stripping, or 
adsorption is difficult and costly, and the better approach is to minimize 
the amount of ethanol contamination occurring in the catalytic 
distillation column rather than attempt to further process the ether 
product stream to remove ethanol. Unfortunately, increasing the catalytic 
distillation column pressure to improve the distillation is of limited use 
since the distillation column also contains catalyst, and the limitations 
of the catalyst dictate the range of operating conditions. 
Therefore, the present invention requires that a stream containing at least 
one inert azeotropic agent also be introduced to the etherification zone. 
Specific agents are discussed at length below. The azeotropic agent is 
introduced to the etherification zone at a point prior to the catalytic 
distillation column and preferably after the fixed bed reactors. It is 
possible to introduce the azeotropic agent at the same location as the 
reactants, but that is less preferred since the volume is then increased 
at a portion of the etherification zone where the azeotropic agent is 
unnecessary. Ordinarily, introducing a significant volume of inert 
material to a reaction would not be expected to result in increased 
product purity. Flowing large volumes of inert material through a reaction 
zone is generally considered to be undesirable as the complete physical 
structure of the reaction zone must be enlarged to flow the combined 
volume of reaction mixture and inert material. Correspondingly, the costs 
of constructing and operating the physically enlarged reaction zone are 
increased in order to handle the increased total volume. However, in the 
low pressure catalytic distillation column described above, it is 
extremely beneficial to introduce a large amount of inert azeotropic 
agent. 
Specifically, by requiring the independent stream containing an inert 
azeotropic agent to be introduced to the catalytic distillation column, 
virtually all of the excess ethanol is incorporated into an azeotrope with 
the inert azeotropic agent and other inert hydrocarbons present and is 
carried from the catalytic distillation column in the overhead stream 
thereby leaving little ethanol to contaminate the bottoms ethyl tertiary 
butyl ether product stream. The bottoms ethyl tertiary butyl ether product 
stream is of increased purity and contains less than 0.6 weight percent 
ethanol contaminant. 
The azeotropic agents are inert alkanes containing 4 or 5 carbon atoms that 
have the capacity to azeotrope with ethanol under the catalytic 
distillation conditions. The term "inert" is meant to indicate that the 
azeotropic agents will not react to form appreciable amounts of byproduct 
in the fixed bed reactors or in the catalytic distillation column as 
described herein. Specific azeotropic agents include isopentane, normal 
butane, and isobutane. The preferred agents are those able to incorporate 
a higher amount of ethanol into an azeotrope at the operating conditions 
of the catalytic distillation column. The preferred agents are isopentane 
and normal butane with isopentane being the most preferred. The agents may 
be obtained from any source including, for example, by distillation of a 
crude C.sub.4 stream, fluidized catalytic cracking, steam cracking, and 
gas separation plants. 
One or more agents may be used, and the amount of agent necessary is 
dependent on several factors. First, the amount of ethanol that is able to 
azeotrope with each individual agent is different. For convenience, the 
term "ethanol capacity" will be used to refer to a characteristic of the 
azeotropic agent defined by that amount of ethanol that is drawn into an 
azeotrope with the individual azeotropic agent. Secondly, within a single 
agent the ethanol capacity varies with pressure. Thirdly, the amount of 
agent is dependent upon the desired purity of the ether product stream and 
the isobutylene conversion achieved. Therefore, the amount of overall 
agent necessary is determined by the weighted sum of the ethanol 
capacities for each agent used, where the ethanol capacity for each agent 
is determined by the pressure of operation, with the sum then being used 
in correlation with the amount of excess ethanol in the catalytic 
distillation column and the desired purity of the ether product. This 
determination is best described by examples. 
The first examples are those where the azeotropic agent is a single 
component. Isopentane and normal butane are particularly preferred as the 
azeotropic agents due to the large amount of ethanol incorporated into an 
ethanol-isopentane or ethanol-normal butane azeotrope. For example, at 85 
psig, an ethanol-isopentane azeotrope would contain about 14 mass percent 
ethanol, an ethanol-normal butane azeotrope would contain about 2.5 mass 
percent ethanol, while an isobutane-ethanol azeotrope would contain only 
0.5 mass percent ethanol. This comparatively large azeotropic capacity for 
ethanol in combination with an overall large concentration of isopentane 
or normal butane introduced to the catalytic distillation column causes 
the unreacted ethanol to form an isopentane-ethanol or normal 
butane-ethanol azeotrope and therefore would be unable to distill into the 
ethyl tertiary butyl ether bottoms stream. The isopentane-ethanol or 
normal butane-ethanol azeotrope is distilled along with other C.sub.4 
hydrocarbons into the overhead stream of the catalytic distillation column 
leaving the ethyl tertiary butyl ether product stream with no more than 
0.6 weight percent ethanol. As discussed above, the amount of azeotropic 
agent to be added depends on the ethanol capacity of the azeotropic agent, 
the operating pressure, the amount of excess ethanol present in the 
catalytic distillation column, and the desired purity of the ether 
product. In general, to achieve an ether product containing less than 0.6 
weight percent ethanol when isopentane is used as the sole azeotropic 
agent, the isopentane must be present in a weight ratio ranging from about 
5:1 to about 8:1 with the ethanol at the overhead from a catalytic 
distillation column designed to operate in the about 90 to about 120 psig 
range. When normal butane is used as the sole agent, the normal butane 
must be present in a weight ratio ranging from about 30:1 to about 50:1 
with the ethanol at the overhead from the catalytic distillation column 
designed to operate in the about 90 to about 120 psig range. When 
isobutane is used as the sole agent, the isobutane must be present in a 
weight ratio ranging from about 124:1 to about 450:1 with the ethanol at 
the overhead from the catalytic distillation column designed to operate in 
the about 90 to about 120 psig range. All the above ratios apply when the 
reactors, both fixed bed and catalytic distillation, are operating at 
design conversions for isobutylene. Should actual conversions fall outside 
of design expectations, the ratio of azeotropic agent to ethanol required 
may lie outside the above described ranges. 
When the azeotropic agent is a combination of two or more components, the 
determination of the quantity needed becomes more complex. A specific 
example of such a determination is as follows: the ether product is to 
contain no more than 0.07 weight percent ethanol, the catalytic 
distillation column is operating at 85 psig, and the reactants are 
introduced to the catalytic distillation column at a rate of 1000 kg/hr 
ethanol and 867 kg/hr isobutylene. These rates result in a 1.44:1 molar 
ratio of ethanol to isobutylene and the desired conversion of isobutylene 
is 99.0 weight percent. The azeotropic agent is a combination of 80 weight 
percent normal butane and 20 weight percent isobutane. At 85 psig, an 
ethanol-normal butane azeotrope would contain about 2.5 mass percent 
ethanol and an isobutane-ethanol azeotrope would contain only 0.5 mass 
percent ethanol. From these specifics an algebraic equation can be solved 
to determine that the azeotropic agent stream is introduced to the 
catalytic distillation column at 19,458 kg/hr. An example of a suitable 
equation is: the flowrate of excess ethanol=(the amount of compound 1 in 
the azeotropic agent stream) (R) (the ethanol capacity of compound 1 at 
the specified conditions)+(the amount of compound 2 in the azeotropic 
agent stream) (R) (the ethanol capacity of compound 2 at the specified 
conditions). Solving for R provides the flowrate of the stream containing 
the combination of components. The equation above is only one example of 
the many ways the flowrate of the azeotropic agent stream may be 
determined. 
In another example of a combination azeotropic agent stream, where the 
conditions, percent conversion, and product purity are the same as in the 
above combination azeotropic agent stream example, again reactants are 
introduced to the catalytic distillation column at a rate of 1000 kg/hr 
ethanol and 867 kg/hr isobutylene. However, in this example the azeotropic 
agent is combination of 50 weight percent normal butane and 50 weight 
percent isopentane. At 85 psig, an ethanol-normal butane azeotrope would 
contain about 2.5 mass percent ethanol and an isopentane-ethanol azeotrope 
would contain about 14 mass percent ethanol. Therefore, the azeotropic 
agent stream is introduced to the catalytic distillation column at 4,541 
kg/hr. 
For complex systems involving multiple azeotropic agents, determinations of 
the quantity of each azeotropic agent needed to remove the desired amount 
of ethanol from the ether product are best accomplished using commonly 
available process simulation software such as Hysys available from 
Hyprotech Ltd. and Aspen Plus available from Aspen Technologies, Inc. 
Manual calculations using established engineering procedures and known 
azeotropic characteristics, such as found in Holderbaum, T.; Utzig, A.; 
GMehling, J. Fluid Phase Equilibria, 1991, 63, 219-226; and Lecat, M. Ann. 
Soc. Sci. Bruxelles, 1929, 49B/I, 34 may be successfully performed, but 
would be labor intensive and therefore not recommended. 
Another factor to consider in determining the necessary quantity of 
azeotropic agent is that the compositions of isobutylene containing 
streams may vary, and some compositions may contain compounds that form an 
azeotrope with ethanol. In this case, the amount of additional azeotropic 
agent that must be added to the catalytic distillation column may be 
reduced to account for the azeotropic capacity of the compounds already 
present in the isobutylene containing stream. 
The ethyl tertiary butyl ether product stream having no more than 0.6 
weight percent ethanol can be collected and used in, for example, gasoline 
blending without further processing. The overhead stream may be treated to 
conserve and recycle components.