Process of generating C.sub.3 - and C.sub.4 -olefins from a feed mixture containing C.sub.4 to C.sub.7 olefins

A feed mixture containing C.sub.4 -olefins to C.sub.7 -olefins is evaporated and mixed with steam in a weight ratio of H.sub.2 O:hydrocarbons in the range from 0.5:1 to 3:1. The steam containing feed mixture with an inlet temperature in the range from 380.degree. to 500.degree. C. is introduced into a reactor, which contains a bed of granular, form-selective zeolite catalyst. The zeolite is of the pentasil type and has an atomic ratio of Si:Al of 10:1 to 200:1. From the bed a product mixture is withdrawn whose temperature is 20.degree. to 80.degree. lower than the inlet temperature, and whose total content of propylene and butene isomers is at least 60 wt-% of the olefinic constituents of the feed mixture.

FIELD OF THE INVENTION 
Our present invention relates to a process for producing C.sub.3 -olefins 
and C.sub.4 -olefins from a feed mixture containing C.sub.4 -olefins to 
C.sub.7 -olefins by conversion of the feed mixture on a granular zeolite 
catalyst at a temperature from 380.degree. to 700.degree. C. 
BACKGROUND OF THE INVENTION 
A process of this type is known from U.S. Pat. No. 5,059,735. In this 
process, a large amount of propane (a C.sub.3 -alkane) is added to the 
feed mixture containing C.sub.4 -olefins to C.sub.7 -olefins before it is 
passed over the catalyst, and in addition to C.sub.2 -olefins to C.sub.4 
-olefins a considerable amount of C.sub.6 and higher aromatics is 
generated. 
OBJECTS OF THE INVENTION 
The principal object of the present invention is to provide an improved 
process for converting C.sub.4 -olefins, C.sub.5 -olefins, C.sub.6 
-olefins and C.sub.7 -olefins to C.sub.3 -olefins and C.sub.4 -olefins in 
an inexpensive manner and such that the product is rich in propylene. 
Another object of the invention is to provide an improved process of the 
type described which overcomes drawbacks of the earlier system. 
SUMMARY OF THE INVENTION 
In accordance with the invention, this is accomplished in that the feed 
mixture is evaporated and mixed with steam. With a weight ratio of H.sub.2 
O:hydrocarbons in the range from 0.5:1 to 3:1. The steam-containing feed 
mixture with an inlet temperature in the range from 380.degree. to 
700.degree. C. is introduced into a reactor which contains a bed of 
granular, form-selective zeolite catalyst, where the zeolite is of the 
pentasil type and has an atomic ratio of Si:Al in the range from 10:1 to 
200:1. From the bed and from the reactor a product mixture is withdrawn at 
a temperature of 20.degree. to 80.degree. C. lower than the inlet 
temperature, and whose total content of propylene and butene isomers is at 
least 60 wt-% and preferably at least 70 wt-% of the olefinic constituents 
of the feed mixture. 
Usually, the product mixture also contains ethylene in an amount of 5 to 20 
wt-%, based on the olefinic constituents in the carburizing or feed 
mixture. 
Preferably, the temperature of the product mixture is lower by 30.degree. 
to 50.degree. C. than the inlet temperature. 
The catalyst can be one which is known per se and described for instance in 
EP-B-0 369 364. 
Advantageously, the reactor is operated at relatively low pressures in the 
range from 0.2 to 3 bar. Usually, the pressures in the reactor are in the 
range from 0.6 to 1.5 bar. 
The composition of the carburization of feed mixture may vary within wide 
limits, and it is preferred to adjust the content of aromatics, calculated 
anhydrous, to not more than 5 wt-% and preferably not more than 2 wt-%. 
This is desirable because a higher content of aromatics leads to a 
premature degradation of the catalyst through carbon deposits. It is 
furthermore advantageous for the carburizing mixture to be free from 
components having carbon-carbon triple bonds or conjugated double bonds, 
as they likewise deactivate the catalyst. Suitable carburizing mixtures 
include, for instance, gas mixtures produced in a refinery, which may 
possibly be subjected to a hydrogenating pretreatment.

SPECIFIC DESCRIPTION 
Evaporated feed or carburizing mixture is supplied via line 1 and mixed 
with steam from line 2. Via line 3, the mixture is then passed through a 
heater 4, which is preferably designed as a fired heater. In line 5, the 
steam-containing carburizing mixture has a temperature in the range from 
380.degree. to 700.degree. C., and preferably from 400.degree. to 
600.degree. C. With this operating temperature it is introduced into the 
reactor 6. In the mixture in line 5, the weight ratio of H.sub.2 
O:hydrocarbons is 0.5:1 to 3:1, preferably at least 1:1. 
In the reactor 6, the granular zeolite catalyst is provided in the form of 
a bed 7. The grain sizes of the catalyst usually lie in the range from 1 
to 8 mm. The zeolite is of the pentasil type, and it has form-selective 
properties. It is important for the zeolite catalyst that the primary 
crystallites of the alumosilicate have a narrow grain-size distribution 
with diameters in the range from 0.1 to 0.9 .mu.m. The BET surface usually 
is 300 to 600 m.sup.2 /g, and the pore volume (according to mercury 
porosimetry) is about 0.3 to 0.8 cm.sup.3 /g. As a binder for holding the 
primary crystallites together, alumina hydrate is preferably used. 
The conversion in the reactor 6 is preferably effected adiabatically, so 
that the temperature of the mixture to be converted decreases when the 
same flows through the bed 7. As a result, the product mixture withdrawn 
via line 9 has a temperature which is 20.degree. to 80.degree. C. and 
usually 30.degree. to 50.degree. C. lower than the inlet temperature in 
line 5. The main product of the product mixture in line 9 is propylene. 80 
to 100 wt-% of the olefins in the carburizing mixture of line 5 are 
converted in the reactor 6. 
In the cooler 10 the product mixture of line 9 is cooled to temperatures of 
about 30.degree. to 60.degree. C., so that water and gasoline will 
condense out. Via line 11, the condensate-containing mixture is supplied 
to a separator 12. From the separator 12 water is withdrawn via line 13, 
in line 14 an organic liquid phase is obtained, and via line 15 a product 
gas is withdrawn. The product gas contains C.sub.2 -olefins to C.sub.4 
-olefins and in addition a small amount of paraffins. To separate the 
valuable substances, in particular ethylene and propylene, the gas of line 
15 is supplied to a non represented separating means, which is known per 
se. 
In the distillation column 17, the organic liquid phase of line 14 is 
separated into C.sub.3 -olefin and C.sub.4 -olefin fraction, withdrawn via 
line 18, and gasoline, withdrawn via line 19. The overhead product, which 
is withdrawn via line 18, usually still contains small amounts of 
saturated hydrocarbons. The gasoline in line 19 only contains small 
amounts (not more than 5 wt-%) of aromatics. The propylene-containing 
product of line 18 is usually likewise supplied to a separating means not 
represented here, so as to recover desired useful substances for instance 
through distillation or through adsorption. In addition to propylene, 
these useful substances include above all ethylene, n-butene-1 and 
isobutylene. n-butene-2 produced, for which there is no use, may be 
recirculated to the reactor 6. 
EXAMPLES 
In the laboratory, there is employed a plant corresponding to the drawing. 
The catalyst always is the same, and it is employed in the form of 
extrudates of the dimensions 1.5.times.3 mm. The catalyst has been 
described in detail in Examples 1 to 5 of the patent EP-B-0 369 364 
(Suedchemie). 
Example 1 
The following carburizing mixture is prepared: 
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n-pentene-1 (C.sub.5 H.sub.10) 
40 wt % 
n-hexene-1 (C.sub.6 H.sub.12) 20 wt % 
n-pentane (C.sub.5 H.sub.12) 20 wt % 
n-hexane (C.sub.6 H.sub.14) 20 wt % 
______________________________________ 
For each kilogram of the olefins, 1.5 kg water is added to the carburizing 
mixture, the mixture is heated to 460.degree. C., and at a pressure of 0.5 
bar is passed through the reactor 6, which contains the catalyst bed 7. 
The loading per hour is 1 kg hydrocarbons per kg catalyst. The product 
mixture, which is withdrawn from the reactor at a temperature of 
435.degree. C., contains 
______________________________________ 
ethylene 4.8 wt % 
propylene 23.5 wt % 
isobutylene 7.7 wt % 
n-butene-1 2.0 wt % 
c-butene-2 2.9 wt % 
t-butene-2 3.6 wt % 
Total: C.sub.2 -C.sub.4 -olefins 44.5 wt % 
______________________________________ 
Example 2 
The carburizing mixture of Example 1 together with 1.5 kg water per kg 
hydrocarbons is preheated to 500.degree. C., and at a pressure of 2 bar 
and the loading of 3 kg/kg/h (calculated anhydrous) is passed through the 
catalyst bed. The product mixture in line 9 has been cooled to 470.degree. 
C. and contains 41.6 wt-% olefins of the following composition: 
______________________________________ 
ethylene 9.9 wt % 
propylene 48.6 wt % 
butene 41.5 wt % 
______________________________________ 
Example 3 
Example 2 is modified such that there is now employed a pressure in the 
reactor 6 of 1.3 bar and a loading of the catalyst of 2 kg/kg/h 
(calculated anhydrous). The temperature in line 9 is 462.degree. C., and 
behind the cooler 10, the temperature in line 11 has decreased to 
35.degree. C. After the separation of water, which contains a small amount 
of organic compounds, the following remains as liquid and gaseous product: 
______________________________________ 
Olefins 
Paraffins 
Naphthenes 
Aromatics 
Total 
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1. C2-C4 Olefins: 
C2 (wt %) 7.22 
C3 (wt %) 26.07 
C4 (wt %) 18.33 
Total (wt %) 51.62 
51.62 
2. Combustion gas 
C1 (wt%) 0.13 
C2 (wt %) 0.31 
C3 (wt %) 2.1 
C4 (wt %) 2.05 
Total (wt %) 4.59 
4.59 
3. Gasoline 
C5 (wt %) 5.17 16.19 0.51 
C6 (wt %) 1.13 15.94 0.8 0.4 
C7 
+ (wt %) 0.62 
0.85 0.45 
1.73 
Total (wt %) 6.92 32.98 1.76 2.13 
43.79 
In all: (wt %) 
100 
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Example 4 
The following carburizing mixture is prepared, and the procedure is as 
described in Example 3: 
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t-butene-2 9.2 wt % 
n-pentene-1 35.9 wt % 
n-hexene-1 18.3 wt % 
n-pentene 18.3 wt % 
n-hexene 18.3 wt % 
______________________________________ 
Upon removal of the water through line 13 a C.sub.2 - to C.sub.4 -olefin 
fraction of the following composition is obtained in combined lines 14 and 
15: 
______________________________________ 
ethylene 12.1 wt % 
propylene 49.7 wt % 
isobutylene 17.5 wt % 
n-butene-1 5.3 wt % 
c-butene-2 7.1 wt % 
t-butene-2 8.3 wt % 
______________________________________ 
This olefin fraction constitutes 57.2 wt-% of the organic substances of the 
combined lines 14 and 15.