Process for preparing alkyl tert-butyl ethers and di-n-butene from field butanes

A process for the coupled production of alkyl tert-butyl ethers and butene oligomers from a mixture containing isobutane and n-butane. The isobutane is converted to the alkyl tert-butyl ether and the n-butane is converted to the oligomers. The ratio of the two reaction products may be controlled by setting the ratio of n-butane to isobutane appropriately by isomerization.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for preparing alkyl tert-butyl ethers 
and butene oligomers from a mixture containing isobutane and n-butane, 
where the isobutane is converted to the alkyl tert-butyl ether and the 
n-butane is converted to the oligomers. The ratio of the two reaction 
products may be controlled by adjusting the ratio of n-butane to 
isobutane. 
2. Description of the Background 
Alkyl tert-butyl ethers (RTBE, where R represents alkyl) are used as 
additives to motor gasoline to increase the octane rating. They are 
generally prepared by addition of alkanols to isobutene, i.e., 
etherification. The isobutene may originate from four different sources: 
steam crackers, propylene oxide plants, petroleum refineries (i.e., FC 
crackers) and plants for the dehydrogenation of isobutane (cf. R. A. 
Pogliano et al., Dehydrogenation-Based Ether Production--Adding Value to 
LPG and Gas Condensate. 1996 Petrochemical Review, DeWift & Company, 
Houston, Tex.). In the first three sources, the isobutene arises as a 
constituent of the C.sub.4 fraction, that is as a direct byproduct. In the 
dehydrogenation of isobutane, isobutene is frequently a secondary 
byproduct of such plants, since the starting material isobutane is 
likewise obtained as a direct byproduct in steam crackers and petroleum 
refineries or by isomerization of n-butane, which itself is a byproduct in 
steam crackers and petroleum refineries. The current world production of 
RTBE is around 25 million metric ten/year, with an increasing trend. The 
production of butanes and butenes as byproducts in a particular cracker or 
a particular petroleum refinery is too small to be able to exploit 
completely the "economies of scale", which are latent in the RTBE process. 
Therefore, isobutene and/or isobutane (for dehydrogenation) would have to 
be collected from crackers and/or refineries, in order to be able to 
operate an RTBE plant at optimum capacity. Alternatively, sufficient 
C.sub.4 fraction could be collected from such plants and these could be 
worked up together on site to isobutene and isobutane. However, opposing 
both variants, and in particular the second, is the fact that the 
transport of liquid gases is expensive, in part due to the complex safety 
precautions necessary. 
The term dibutene refers to the isomeric mixture which, in addition to 
higher butene oligomers, is formed by dimerization and/or codimerization 
of butenes, i.e., of n-butene and/or isobutene, in the oligomerization of 
butenes. Generally, the term dibutene refers to the dimerization products 
obtained from a mixture of n-butene and isobutene. The term di-n-butene 
refers to the dimerization product of n-butene, i.e., 1-butene and/or 
2-butene. Significant components of the di-n-butene are 
3-methyl-2-heptene, 3,4-dimethyl-2-hexene, and, to a minor extent, 
n-octenes. Di-isobutene is the isomeric mixture which is formed by 
dimerization of isobutene. Di-isobutene is more highly branched than 
dibutene and this, in turn, is more highly branched than di-n-butene. 
Dibutene, di-n-butene and di-isobutene are starting materials for preparing 
isomeric nonanols by hydroformylation and hydrogenation of the C.sub.9 
aldehydes thus formed. Esters of these nonanols, in particular the 
phthalic esters, are plasticizers which are prepared in large quantities 
and are primarily used for poly(vinyl chloride). Nonanols from di-n-butene 
are linear to a greater extent than nonanols from dibutene, which in turn 
are less branched than nonanols from di-isobutene. Esters of nonanols from 
di-n-butene have application advantages over esters from other nonanols 
and are, therefore, particularly in demand. 
n-Butene is obtained for the dimerization, just as is isobutene, from 
C.sub.4 fractions, for example, that arise in steam crackers or FC 
crackers. The C.sub.4 fractions are generally worked up by first 
separating off 1,3-butadiene by a selective scrubbing, e.g., with 
N-methylpyrrolidone. Isobutene is a desirable and particularly valuable 
component of the C.sub.4 fraction because it may be chemically reacted, 
alone or in a mixture with other C.sub.4 hydrocarbons, to give 
sought-after products, e.g., with isobutene to give high-octane isooctane, 
or with an alkanol to give an RTBE, in particular with methanol to give 
methyl tert-butyl other (MTBE). After the reaction of the isobutene, the 
n-butenes and n-butane and isobutane remain behind. However, the 
proportion of n-butene in the cracked products of the steam crackers or 
the petroleum refineries is relatively small. In the case of steam 
crackers it is in the order of magnitude of barely 10 percent by weight, 
based on the principal target product ethylene. A steam cracker having the 
respectable capacity of 600,000 metric t/year of ethylene therefore only 
delivers aroung 60,000 metric t/year of n-butene. Although this amount 
(and that of the isobutenes) could be increased by dehydrogenating the 
approximately 15,000 metric t/year of n-butane and isobutane, which arise 
in addition to the n-butenes, this is not advisable, because 
dehydrogenation plants require high capital expenditure and are thus 
uneconomic for such a small capacity. 
Isobutene is, as discussed above, a sought-after cracking product and is 
therefore generally not available for the isomerization to n-butene. The 
amount of n-butenes which a steam cracker or petroleum refinery produces 
directly is not sufficient, however, to produce sufficient di-n-butene for 
a nonanol plant of a high enough capacity that it could compete 
economically with the existing large-scale plants for preparing important 
plasticizer alcohols, such as 2-ethylhexanol. Propylene oxide plants are, 
as already stated even less productive still. n-Butenes would therefore 
have to be collected from various steam crackers, refineries or propylene 
oxide plants (or C.sub.4 fraction from various sources worked up to 
n-butene) and the combined n-butene oligomerized in order to cover the 
dibutene requirement of a sufficiently large economical nonanol plant. 
However, the transport of liquid gases is expensive and dangerous, as 
discussed above. 
It would therefore be desirable if n-butene and isobutene could be provided 
at only one site without transport over relatively large distances in 
amounts as are required in a coupled production for the operation of a 
large economically advantageous plant for the preparation of di-n-butene, 
for example having a capacity of 200,000 to 800,000 metric t/year, and the 
same type of plant for preparing RTBE, e.g., having a capacity of 300,000 
to 800,000 metric t/year. It would further be desirable to arrange the 
link between these plants in such a manner that the ratio of n-butene to 
isobutene can be set in accordance with the desired amounts of RTBE and 
butene oligomers. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a process for the 
coupled production of alkyl tert-butyl ethers and butene oligomers. 
It is another object of the present invention to provide a a process for 
the coupled production of alkyl tert-butyl ethers and butene oligomers, 
where the ratio of the two reaction products may be controlled by 
selection of the starting materials. 
These objects and others may be accomplished with a process, which 
comprises: 
dehydrogenating a first mixture comprising n-butane and isobutane to 
produce a second mixture comprising n-butene and isobutene; 
etherifying the isobutene in the second mixture with an alcohol to produce 
an alkyl tert-butyl ether; and 
oligomerizing the n-butene in the second mixture. 
A more complete appreciation of the invention and many of the attendant 
advantages thereof will be readily obtained as the same becomes better 
understood by reference to the following detailed description when 
considered in connection with the accompanying drawing.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is a coupled production process in which alkyl 
tert-butyl ethers and oligomers of butene are produced. The preferred 
oligomeric product is di-n-butene, but other oligomeric products may also 
be produced as described below. 
In the first step of the present process a mixture containing n-butane and 
isobutane is dehydrogenated to produce a reaction product mixture 
containing n-butene and isobutene. A preferred starting mixture is a field 
butane. The term "field butane" refers to the C.sub.4 fraction of the 
"moist" portions of the natural gas and the gases associated with crude 
oil, which are separated off in liquid form from the gases by drying and 
cooling to about -30.degree. C. Low-temperature distillation produces 
therefrom the field butanes. The composition of the field butanes 
fluctuates depending on the field generally. Field butanes may contain 
about 30% isobutane and about 65% n-butane. Other components are generally 
about 2% C.sub.&lt;4 hydrocarbons and about 3% C.sub.&lt;4 hydrocarbons. Field 
butanes may be used without fractionation as feedstocks in steam crackers 
or as an additive to motor gasoline. They may be resolved into n-butane 
and isobutane by fractional distillation. Isobutane is used, for example, 
to a considerable extent for preparing propylene oxide by cooxidation of 
propylene and isobutane and as an alkylating agent, by means of which 
n-butene or isobutene is alkylated to isooctane, which, because of its 
high octane rating, is valued as an additive to motor gasoline. n-Butane, 
in contrast, has found fewer such important uses. It serves, for example, 
as butane gas for heating purposes or is used in comparatively small 
amounts, for example, for preparing polymers or copolymers or for 
producing maleic anhydride by atmospheric oxidation. Formerly, n-butane 
was also dehydrogenated via the n-butene stage to give 1,3-butadiene, but 
this process has become uneconomic in the interim. 
In a preferred embodiment of the process, the field butanes 1, prior to 
entry into the dehydrogenation stage 2, are subjected to hydrogenation 
conditions in the hydrogenation stage 9, passed into a separation stage 10 
to which an isomerization stage 11 where the ratio of n-butane to 
isobutane (n/iso ratio) can be adjusted in accordance with the desired 
ratio of di-n-butene to alkyl tert-butyl ether and the field butane 1a 
thus altered in its n/iso ratio is passed into the dehydrogenation stage 
2. 
This preferred embodiment of the invention is distinguished by high 
flexibility, since the amounts of di-n-butene and RTBE can be varied in 
accordance with the market requirements, within the limits which are set 
by the capacities of the di-n-butene plant and the RTBE plant. 
Because isobutane is the more sought-after component of field butane, 
n-butane is isomerized on a large scale to give isobutane (cf., for 
example, R. A. Pogliano et al., Dehydrogenation-based Ether Production, 
1996 Petrochemical Review, DeWitt & Company, Houston, Tex., BUTAMER 
Process, page 6; and S. T. Bakas, F. Nierlich et al., Production of Ethers 
from Field Butanes and Refinery Streams. AlChE Summer Meeting, 1990, San 
Diego, Calif., page 11). It was therefore not part of the technological 
trend to develop a process which utilizes n-butane as such or even 
converts isobutane into n-butane in order to prepare more di-n-butene 
therefrom. 
The process according to the invention is carried out in two sequential 
part-steps (A) preparation of RTBE and (B) preparation of di-n-butene. In 
principle, the sequence of these part-steps is optional, but it is 
advantageous to prepare RTBE initially and then di-n-butene, because 
isobutene is likewise active in oligomerization. The di-isobutene thus 
formed is, as previously mentioned, less highly branched and thus leads to 
isononanols having poorer application properties. 
(A) Preparation of RTBE 
The field butanes 1 or the field butanes which have been altered in 
composition by isomerization 1a (see section (C)) are passed into the 
dehydrogenation stage 2, which is an essential feature of the present 
invention. There, the field butanes are dehydrogenated to give a 
dehydrogenation mixture 3 containing n-butene and isobutene. The 
dehydrogenation is a codehydrogenation of n-butane and isobutane. It is 
remarkable that the dehydrogenation of the field butanes, which are 
mixtures of components having different dehydrogenation behavior, succeeds 
so readily. The process conditions substantially correspond to those which 
are known for n-butane and isobutane. Thus, ST. Bakas, F. Nierlich et al., 
loc. cit., pages 12 ff., incorporated herein by reference, describe the 
OLEFLEX process, which is generally suitable for preparing light olefins 
and by means of which, for example, isobutane can be dehydrogenated to 
isobutene with a selectivity of 91 to 93%. Further examples are provided 
by G. C. Sturtevant et al., Oleflex--Selective Production of Light 
Olefins, 1988 UOP Technology Conference, and EP 0 149 698, both 
incorporated herein by reference. The dehydrogenation is expediently 
carried out in the gas phase on fixed-bed or fluidized catalysts, e.g., on 
chromium(III) oxide, or adavantageously on platinum catalysts having 
aluminum oxide or zeolites as support. The dehydrogenation preferably 
takes place at temperatures of 400 to 800.degree. C., more preferably 550 
to 650.degree. C. Atmospheric pressure or a slightly elevated pressure up 
to 3 bar is generally employed. The residence time in the catalyst layer 
is generally between 1 and 60 minutes, depending on catalyst, temperature 
and the sought-after degree of conversion. The throughput is generally 
between 0.6 and 36 kg of n-butane and isobutane (as mixture) per ml of 
catalyst and hour. 
It is preferable to carry out the dehydrogenation only to the point that 
about 50% of the n-butane and the isobutane remain unchanged in the 
dehydrogenation mixture 3. Although higher degrees of conversion can be 
achieved at higher temperatures, cracking reactions which decrease the 
yield then proceed to an increasing extent, and, as a consequence of coke 
deposits, decrease the service life of the hydrogenation catalyst. The 
optimum combinations of reaction conditions which lead to the desired 
degrees of conversion, such as type of catalyst, temperature and residence 
time, may be determined without difficulty. 
The dehydrogenation mixture 3 generally contains 90 to 95% by weight of 
C.sub.4 hydrocarbons and, in addition, hydrogen, as well as lower- and 
higher-boiling portions. Preferably, it is subjected to preliminary 
purification prior to the oligomerization, namely in a first purification 
stage and in a selective hydrogenation stage 14. In the first purification 
stage, the C.sub.4 fraction and the higher-boiling portions are condensed 
out of the gas phase. The condensate is distilled under pressure, with 
cocondensed, dissolved C.sub.&lt;4 hydrocarbons passing through the head. 
From the higher-boiling portions, in a further distillation, the saturated 
and unsaturated C.sub.4 hydrocarbons are obtained as main product, which 
pass into the further process, and the relatively small amount of C.sub.&lt;4 
hydrocarbons are obtained as a residue. 
The C.sub.4 hydrocarbons generally contain small amounts, e.g., 0.01 to 5 
percent by volume, of dienes, such as propadiene and, in particular, 
1,3-butadiene. It is preferable to remove these dienes, since, even in 
markedly lower amounts, they can later damage the catalyst in the 
oligomerization stage 8. A suitable process is the selective hydrogenation 
14, which in addition increases the proportion of the desired n-butene. 
The selective hydrogenation has been described, for example, by F. 
Nierlich et al. in Erdol & Kohle, Erdgas, Petrochemie, 1986, pages 73 ff, 
incorporated herein by reference. It is carried out in liquid phase with 
completely dissolved hydrogen in stoichiometric amounts. Suitable 
selective hydrogenation catalysts are, for example, nickel and, in 
particular, palladium on a support, e.g., 0.3 percent by weight palladium 
on activated carbon or, preferably, on aluminum oxide. A small amount of 
carbon monoxide in the ppm range promotes the selectivity of the 
hydrogenation of the 1,3-butadiene to give the monoolefin and counteracts 
the formation of polymers, the so-called "green oil", which may inactivate 
the catalyst. The process generally proceeds at room temperature or 
elevated temperature up to 60.degree. C. and under elevated pressures 
which are preferably up to 20 bar. The content of 1,3-butadiene in the 
dehydrogenation mixture is decreased in this manner to values &lt;1 ppm. 
Advantageously, the selective hydrogenation is carried out under 
hydroisomerizing conditions. This simultaneously isomerizes 1-butene to 
2-butene, which, in contrast to 1-butene, can be separated from 
n-butane/isobutane by distillation in the separation stage 16 to be 
described below. For details of the selective hydrogenation under 
hydroisomerizing conditions see, e.g., F. Nierlich, Integrated Tert-Butyl 
Alcohol/Di-n-Butene Production from FCC C.sub.4 's, Erdol, Kohle 103 (11), 
pages 486 ff., 1989, incorporated herein by reference. 
Since the dienes may interfere with the later oligomerization, but less so 
the etherification, the selective hydrogenation stage 14 may also be 
arranged downstream of the etherification stage 4 in the stream of the 
residual dehydrogenation mixture 7, upstream or, preferably, downstream of 
the purification stage 15 to be described below. This arrangement permits, 
if appropriate, the reactor of the selective hydrogenation stage 14 to be 
designed to be smaller, because the volume of the residual dehydrogenation 
mixture 7 after the isobutene has been separated off in the etherification 
stage 4 is obviously smaller than that of the dehydrogenation mixture 3. 
The dehydrogenation mixture 3, if appropriate after preliminary 
purification and selective hydrogenation, is passed into the 
etherification stage 4 which is an essential feature of the process 
according to the invention. There, the isobutene present therein is 
reacted in a manner known per se with an alkanol 5 (see, for example, 
methyl tert-butyl ether, Ullmanns Encyclopedia of Industrial Chemistry, 
Volume A 16, pages 543 ff., VCH Verlagsgesellschaft, Weinheim). The 
alkanol preferably has 1 to 20 carbon atoms. The more preferred alkanols 
have 1 to 6 carbon atoms. The alkanol may have the formula R--OH, where R 
is a C.sub.1-20 hydrocarbon group. Preferably, the R group is unreactive 
under the conditions used in the present process. More preferably, R is an 
alkyl group, i.e., a saturated hydrocarbon group. The alkyl group may have 
any structure, such as linear, branched, cyclic or combinations thereof. 
Suitable examples of the alkanol include ethanol, isopropanol, isobutanol 
and, in particular, methanol. 
Since n-butene is considerably less reactive, a selective etherification 
takes place which consumes virtually only isobutene. The reaction proceeds 
in the liquid phase or gas-liquid phase, generally at a temperature of 50 
to 90.degree. C. and at a pressure which is established at the respective 
temperature. Preferably, a slight stoichiometric excess of alkanol is 
employed, which increases the selectivity of the reaction of the isobutene 
and suppresses its dimerization. The catalyst used is, for example, an 
acid bentonite or, advantageously, a large-pored acid ion exchanger. 
From the etherification stage 4 reaction mixture, the gaseous residual 
dehydrogenation mixture 7 and the excess alkanol may be separated off from 
the RTBE 6 formed by distillation. In the case of MTBE, the residual 
dehydrogenation mixture 7 and methanol form an azeotrope. The azeotrope is 
washed with water and separated into an aqueous phase and residual 
dehydrogenation mixture 7. The aqueous phase is worked up to methanol, 
which is recycled to the etherification, and to water, which is reused for 
the washing. The residual dehydrogenation mixture 7 passes onto the 
preparation of di-n-butene. 
(B) Preparation of di-n-butene 
The starting material for this reaction is the n-butene present in the 
residual dehydrogenation mixture 7. If no selective hydrogenation 14 has 
been provided upstream of the etherification stage 4, it may then take 
place upstream or downstream and, advantageously, downstream of the 
purification stage 15. The essential component of the latter is a 
molecular sieve on which other substances harmful for the oligomerization 
catalyst are removed, which her increases its service life. These harmful 
substances include oxygen compounds and sulfur compounds. The purification 
using molecular sieves has been described, for example by F. Nierlich et 
al. in EP-B1 U395 857, incorporated herein by reference. A molecular sieve 
having a pore diameter of 4 to 15 angstroms is expediently used, 
advantageously 7 to 13 angstroms. In many cases, it is expedient for 
economic reasons to pass the residual dehydrogenation 7 successively over 
molecular sieves having different pore sizes. The process can be carried 
out in the gas phase, in liquid phase or in gas-liquid phase. The pressure 
is correspondingly generally 1 to 200 bar. Room temperature or elevated 
temperatures up to 200.degree. C. are expediently employed. 
The chemical nature of the molecular sieves is less important than their 
physical properties, i.e., in particular the pore size, The most varied 
types of molecular sieves can therefore be used, both crystalline, natural 
aluminum silicates, e.g., sheet lattice silicates, and synthetic molecular 
sieves, e.g., those having a zeolite structure. Zeolites of the A, X and Y 
type are obtainable, inter alia, from Bayer AG, Dow Chemical Co., Union 
Carbide Corporation, Laporte Industries Ltd. and Mobil Oil Co. Also 
suitable are synthetic molecular sieves which, in addition to aluminum and 
silicon, further contain atoms introduced by cation exchange, such as 
gallium, indium or lanthanum, or nickel, cobalt, copper, zinc or silver. 
In addition, synthetic zeolites are suitable in which, in addition to 
aluminum and silicon, still other atoms, such as boron or phosphorus, have 
been incorporated into the lattice by mixed precipitation. 
n-Butene from the residual dehydrogenation mixture 7, if appropriate 
purified by selective hydrogenation 14 and/or treatment with a molecular 
sieve 15, is advantageously separated off in the separation stage 16 from 
the other gaseous components (residual gas 1 17), such as isobutane and 
isobutene which is unreacted in the etherification stage 4, and passed 
into the oligomerization stage 8 which is an essential part of the process 
according to the invention. This separation of the residual 
dehydrogenation mixture 7 upstream of the oligomerization is preferable, 
because otherwise the oligomerization stage 8 is loaded with unnecessarily 
high amounts of substance and, in addition, undesirable cooligomers may 
form from n-butene and isobutene. 
The oligomerization is carried out in a manner known per se, such as has 
been described, for example, by F. Nierlich in Oligomerization for Better 
Gasoline, Hydrocarbon Processing, 1992 (2), pages 45 ff., or by F. 
Nierlich et al. in the EP-B 0 395 857, both incorporated herein by 
reference. The procedure is generally carried out in liquid phase and, as 
homogeneous catalyst, a system is used, for example, which comprises 
nickel(II) octoate, ethylaluminum chloride and a free fatty acid (DE-C 28 
55 423, incorporated herein by reference), or, preferably, one of the 
numerous known fixed-bed catalysts or catalysts suspended in the 
oligomerization mixture based on nickel and silicon is used. The catalysts 
frequently additionally contain aluminum. Thus, DD-PS 160 037, 
incorporated herein by reference, describes the preparation of a nickel- 
and aluminum-containing precipitated catalyst on silicon dioxide as 
support material. Other catalysts which may be used are obtained by 
exchanging positively charged particles, such as protons or sodium ions, 
situated on the surface of the support materials, for nickel ions. This is 
successful with the most varied support materials, such as amorphous 
aluminum silicate (R. Espinoza et al., Appl. Kat., 31 (1987), pages 
259-266; crystalline aluminum silicate (DE-C 20 29 624); zeolites of the 
ZSM type (Netherlands Patent 8 500 459); an X zeolite (DE-C 23 47 235); X 
and Y zeolites (A. Barth et al., Z. Anorg. Allg. Chem. 521, (1985) pages 
207-214) and a mordenite (EP-A 0 233 302). 
The oligomerization is preferably carried out, depending on the catalyst, 
at 20 to 200.degree. C. and at pressures from 1 to 100 bar. The reaction 
time (or contact time) is generally 5 to 60 minutes. The process 
parameters, in particular the type of catalyst, the temperature and hence 
the contact time are matched to one another in such a manner that the 
desired degree of oligomerization is achieved, i.e., predominantly a 
dimerization. In addition, the reaction should preferably not proceed to 
full conversion, but conversion rates of 30 to 70% per pass are 
expediently sought after. The optimum combinations of process parameters 
may be determined without difficulty. 
The residual gas II 21 may be separated off from the oligomerization 
mixture 19 in the separation stage 20 by distillation. It can then be 
recycled to the dehydrogenation stage 2 or passed to the isomerization 
stage 11, if this is present and in operation. Finally, the residual gas 
II 21, can also be passed into the hydrogenation stage 18, whose function 
is described below. The alternatives for handling the residual gas II 21 
are indicated in the FIGURE by dashed lines. If a catalyst of the liquid 
catalyst type mentioned was used in the oligomerization stage 8, the 
residual gases II 21 is preferably purified to protect the dehydrogenation 
catalyst or the isomerization catalyst. The oligomerization mixture 19 is 
initially treated with water, in order to extract the catalyst components. 
The residual gas II 21 which has been separated off may be dried using a 
suitable molecular sieve, other minor components also being separated off. 
Polyunsaturated compounds, such as butynes, are then removed by 
hydrogenation, e.g., on palladium catalysts, and the residual gas II 21 
thus purified is finally conducted into the dehydrogenation stage 2 or 
into the isomerization stage 11. These purifying measures for the residual 
gas II 21 are not necessary if a fixed-bed oligomerization catalyst is 
used. 
Di-n-butene 22 and trimeric n-butene 23, i.e., isomeric dodecenes, may be 
further separated off from the remaining liquid phase of the 
oligomerization mixture 19 in the separation stage 20 by fractional 
distillation. The main product di-n-butene is directly suitable for 
preparing nonanols. The dodecenes 23 are a desirable by-product. They can 
be hydroformylated, the hydroformylation products can be hydrogenated and 
the tridecanols thus obtained can be ethoxylated, which produces valuable 
detergent bases. 
The residual gas I 17 arising in the separation stage 16 can be recycled to 
the dehydrogenation stage 2, provided that the field butanes 1 are 
dehydrogenated directly without changing the n-/iso ratio by 
isomerization. If an isomerization stage 11 is present and in operation, 
the residual gas I 17 can be passed directly, or via the hydrogenation 
stage 18, into the isomerization stage 11. The alternatives for treating 
the residual gas 1 17 are again depicted in FIG. 1 by dashed lines. 
(C) Variation of the amounts of di-n-butene and RTBE 
As mentioned above, it is expedient to incorporate an isomerization stage 
11 in the process, because by this means the ratio of the amounts of 
di-n-butene and RTBE (product ratio) can be varied. The possibilities for 
variation are limited only by the capacities of the di-n-butene and RTBE 
plants. Taking into account the capital expenditure, both plants are 
certainly rarely designed to be so large that all of the field butane 
stream available can be processed in only one of the plants, while the 
other plant is idle. Nevertheless, the isomerization stage 11 offers the 
opportunity of reacting flexibly to the requirements of the market within 
the given limits. 
If it is desired to change the present n-/iso ratio of the field butanes 1, 
they are expediently first passed into a hydrogenation stage 9, if they 
contain unsaturated compounds. The unsaturated compounds are hydrogenated 
there and can then no longer damage the catalyst of the isomerization 
stage 11. The hydrogenation is performed in a manner known per se (see, 
for example, K. H. Walter et al., in The Huls Process for Selective 
Hydrogenation of Butadiene in Crude C.sub.4 's, Development and Technical 
Application, DGKM Meeting, Kassel, November 1993, incorporated herein by 
reference). The procedure is preferably therefore carried out in liquid 
phase and, depending on the catalyst, at room temperature or elevated 
temperature up to 90.degree. C. and at a pressure of 4 to 20 bar, the 
partial pressure of the hydrogen being 1 to 15 bar. The catalysts which 
are customary for the hydrogenation of olefins, e.g., 0.3% palladium on 
aluminum oxide, are used. 
The hydrogenated field butanes 1 may be passed into the separation stage 
10, whose essential component is an effective column operated at low 
temperature and/or elevated pressure. If more alkyl tert-butyl ether is to 
be prepared than corresponds to the isobutane portion of the field butane 
1, an amount of n-butane 12 corresponding to the desired product ratio is 
taken off in the side stream (the C.sub.&lt;4 hydrocarbons arise as bottom 
product) and is conducted into the isomerization stage 11. The optional 
character of this measure is indicated in the FIGURE by a dashed line. In 
the isomerization stage 11, n-butane is converted into isobutane at the 
maximum up to equilibrium, which, depending on the temperature, is 40 to 
55% n-butane and 60 to 45% isobutane. The isomerization mixture 13 returns 
to the separation stage 10. As a result, therefore, the dehydrogenation 
stage 2 is fed with a field butane whose proportion of isobutane is 
increased with respect to the field butane 1. 
If more di-n-butene is to be prepared than corresponds to the n-butane 
proportion of the field butane 1, the isobutane-rich residual gas I 17 
from the separation stage 16 is expediently completely or in part, either 
directly or via the hydrogenation stage 18, passed into the isomerization 
stage 11. In this case, the residual gas II 21 is conducted directly into 
the dehydrogenation stage 2. As a result, the dehydrogenation stage 2 is 
then fed with a field butane whose proportion of n-butane is increased 
with respect to the field butane 1. 
The isomerization of n-butane and isobutane is a known reaction. The 
procedure is generally carried out in the gas phase at a temperature of 
150 to 230.degree. C., at a pressure of 14 to 30 bar and using a platinum 
catalyst on aluminum oxide as support, whose selectivity can be further 
increased by doping with a chlorine compound, such as carbon 
tetrachloride. A small amount of hydrogen is advantageously added, to 
counteract a dehydrogenation. The selectivity of the isomerization is 
high, cracking to form smaller fragments takes place only to a minor 
extent (approximately 2%) (see, for example, H. W. Grote, Oil and Gas 
Journal, 56 (13), pages 573 ff. (1958)). The yields of the desired isomer 
are correspondingly high. 
The isomerization mixture may be 13 is recycled to the separation stage 10, 
from which a field butane 1a having an appropriately altered n-/iso ratio, 
with respect to the original field butane 1, passes into the 
dehydrogenation stage 2. 
This application is based on German patent application serial No. 196 29 
905.5, filed Jul. 24, 1996 and incorporated herein by reference in its 
entirety. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.