Continuous hydroformylation of olefinically unsaturated compounds

Olefinically unsaturated compounds are continuously hydroformylated under from 2 to 30 bar and at from 80.degree. to 130.degree. C. using, as a catalyst, a rhodium complex which contains, as ligands, sparingly volatile compounds of the general formula I ##STR1## where A is phosphorus, arsenic, antimony or bismuth and R.sup.1, R.sup.2 and R.sup.3 are each organic radicals, by a method in which the hydroformylation mixture consisting of liquid and gaseous components is removed from the reactor and is subjected to relatively high temperatures and/or relatively low pressures for a short time in a devolatilization column, the mixture at the same time being separated into a gas phase and a liquid phase, the gas phase is separated into the product and the recycle gas in a separator, and the recycle gas and the liquid phase from the devolatilization column are recycled to the reactor.

The present invention relates to an improved process for the continuous 
hydroformylation of olefinically unsaturated compounds under from 2 to 30 
bar and at from 80.degree. to 130.degree. C. with the aid of, as a 
catalyst, a rhodium complex which contains, as ligands, sparingly volatile 
compounds of the general formula I 
##STR2## 
where A is phospnorus, arsenic, antimony or bismuth and R.sup.1, R.sup.2 
and R.sup.3 are each organic radicals. 
Apart from the improvement according to the invention, this process is well 
known, and is disclosed in, for example, DE-A-1 186 455 and DE-A-1 793 
069. 
Furthermore, it is well known that the products (mainly aldehydes but also 
the corresponding alcohols) can be obtained by either freeing the liquid 
reaction mixture from gaseous components and then working it up by 
distillation, or discharging the products in gaseous form, together with 
the gaseous reactants, from the hydroformylation reactor by the recycle 
gas method, isolating them from the gas stream and then recycling the 
major part of the gases to the reactor, as described in, for example, 
DE-A-2 715 685. 
The first-mentioned method has the basic disadvantage that the catalyst 
present in the liquid reaction mixture is deactivated, if not damaged, 
under the distillation conditions, mainly owing to the absence of the 
CO/H.sub.2 atmosphere in the presence of which the hydroformylation takes 
place. 
The recycle gas method, in which the catalyst remains in the 
hydroformylation reactor, does not have the disadvantage described above; 
however, this method is only feasible where the products have a 
sufficiently high partial pressure to permit them to be discharged in 
sufficient amounts together with the recycled gas. If, however, the 
partial pressure were very low, disproportionately large amounts of gas 
would be required to discharge the products, with the result that the 
hydroformylation process would become uneconomical. 
The possibility of making the recycle gas procedure more effective by 
increasing the hydroformylation temperature and hence the partial pressure 
of the products is not a reasonable one because it would lead to a shift 
from the optimum hydroformylation temperature and poorer results would be 
obtained: either undesirable hydrogenation of the olefin to a paraffin or 
of the aldehyde to an alcohol would take place, the isomerization of the 
olefins would increase at higher temperatures, or the proportion of linear 
aldehydes, which are generally preferred, would decrease. 
It is an object of the present invention to provide a remedy for the stated 
disadvantages, and to recover the products for the hydroformylation 
mixtures in a more effective manner. 
We have found that this object is achieved by an improved process for the 
continuous hydroformylation of olefinically unsaturated compounds under 
from 2 to 30 bar and at from 80.degree. to 130.degree. C. with the aid of, 
as a catalyst, a rhodium complex which contains, as ligands, sparingly 
volatile compounds of the general formula I 
##STR3## 
where A is phosphorus, arsenic, antimony or bismuth and R.sup.1, R.sup.2 
and R.sup.3 are each organic radicals, wherein the hydroformylation 
mixture consisting of liquid and gaseous components is removed from the 
reactor and is subjected to relatively high temperatures and/or relatively 
low pressures for a short time in a devolatilization column, the mixture 
at the same time being separated into a gas phase and a liquid phase, the 
gas phase is separated into the product and the recycle gas in a 
separator, and the recycle gas and the liquid phase from the 
devolatilization column are recycled to the reactor.

The process is illustrated with reference to the drawing in which the 
single FIGURE is a schematic flow sheet of one embodiment of the 
invention. This process may be described as follows: 
the partially liquid and partially gaseous reaction mixture which leaves 
the hydroformylation reactor R is advantageously brought, in a heat 
exchanger W1, to the higher temperature at which the devolatilization 
column E is operated in accordance with the invention. 
The devolatilization in E can be carried out under the same pressure as in 
R, but it is advisable to reduce the pressure via a flow-control valve D1. 
Separation of the reaction mixture into a gas phase and a liquid phase 
takes place in E. The residence time of about 2-10 minutes required for 
this is very much shorter than the residence time required in R for the 
hydroformylation, the latter time being about 4-8 hours; hence, in spite 
of the higher temperature, virtually no undesirable side reactions and 
secondary reactions are observed, although the reaction mixture is still 
under hydroformylation conditions here. 
The liquid phase which leaves the devolatilization column can, if required, 
be cooled in the heat exchanger W2. In this case, it is advantageous from 
the point of view of heat technology if W1 and W2, and possibly W3 as 
well, are combined to form a common unit (not shown in the drawing). 
The temperature in the devolatilization column E should as a rule be no 
less than 5.degree. C. higher than the hydroformylation temperature. In 
practice, however, this temperature difference is generally from 
10.degree. to 50.degree. C. 
With regard to the discharge of the products from the devolatilization 
column E in gaseous form, the pressure is also important and can be the 
same as, or even lower than, the hydroformylation pressure since, for a 
lower total pressure corresponding to the partial pressure determined by 
the temperature, the volume of the hydroformylation products in the gas 
stream increases. Hence, it is advisable in general also to reduce the 
pressure when the temperature is increased. This pressure difference is 
preferably from 2 to 20 bar, with, of course, the proviso that it cannot 
be greater than (p-1) where p is the hydroformylation pressure. 
Since the partial pressure equilibria in E are established very rapidly, 
residence times here need be only from 1 to 30, as a rule from 2 to 10, 
minutes. 
Since the liquid phase from R becomes enriched with high-boiling products 
in the course of time, it is necessary from time to time to separate off 
some of this liquid phase via the flow-control valve D2. The same applies 
with regard to the removal of the waste gas via D3. Both measures and 
apparatuses are, however, not features of the invention, and have 
therefore been mentioned only for the sake of completeness. 
The liquid phase together with fresh synthesis gas (CO/H.sub.2), fresh 
olefin and the recycle gas is passed once again into the reactor via the 
pump P2, which compensates both the unavoidable and the intentional 
pressure losses. 
The gas phase from E is cooled in a conventional manner in the separator A 
or, advantageously, in an upstream heat exchanger W3 to such an extent 
that the products, mainly the aldehydes but also the alcohols and 
unreacted olefin and any paraffin formed, separate out in liquid form; 
these products are then removed from the system via pressure-release valve 
D4, and are treated further in a conventional manner. After a waste gas 
bleed stream has been separated off via D3, the gas phase, ie. the recycle 
gas which consists mainly of CO, H.sub.2 and N.sub.2, with or without 
small amounts of olefin and small amounts of the corresponding paraffin, 
is likewise recycled in a conventional manner to the hydroformylation 
reactor via the compressor P1. 
The novel process hence embodies a reasonable decoupling of the conditions 
for optimum hydroformylation and an optimum discharge of gaseous product 
by the recycle gas method. Otherwise, within the general hydroformylation 
conditions according to the invention, the process is independent of the 
type of hydroformylation, so that a few basic explanations are sufficient 
here. 
Suitable olefinically unsaturated compounds (which are sometimes 
abbreviated to olefins here) are mainly .alpha.-olefins of not more than 
12 carbon atoms, but, for example, other .alpha.-olefinically unsaturated 
compounds, such as allyl alcohol, allyl acetate, acrylates, styrene and 
acrolein acetals, can also be used. 
Olefinically unsaturated compounds having nonterminal double bonds undergo 
hydroformylation under the stated reaction conditions as a rule only to a 
small extent, if at all. However, for exceptional cases, the novel process 
would of course be just as suitable. 
Although the lower olefins, eg. ethylene, propylene and but-1-ene, can also 
be more effectively hydroformylated using the novel process, the latter is 
more important in the case of .alpha.-olefins of 5 to 12 carbon atoms, 
since the partial pressure of the resulting aldehydes at the 
hydroformylation temperature is so low that the recycle gas will have to 
be circulated a disproportionately large number of times, ie. with 
substantial energy consumption. 
A characteristic feature of the rhodium-catalyzed hydroformylation is the 
presence of the complex-forming ligands I, which are employed as a rule in 
a 3-fold to 500-fold molar excess, based on the rhodium. In general, the 
processes are not carried out using a ready-prepared Rh complex of this 
type, since the latter forms in situ from an Rh salt, eg. the acetate, and 
the ligands I under the hydroformylation conditions. Among the large 
number of ligands disclosed (cf. for example the literature cited at the 
outset), virtually only the phosphorus compounds, such as trialkyl 
phosphines, triaryl phosphines, trialkyl phosphites and triaryl 
phosphites, are of commercial importance, and among these ligands in turn 
triphenyl phosphine is preferred. In a particular case, the choice of 
ligand depends on the specific hydroformylation tasks, but these are not 
critical with regard to the present invention. 
For practical reasons, care should be taken to ensure that the ligands I 
have sufficiently low volatility that they pass in no more than traces 
into the gas phase of the devolatilization column E, since otherwise they 
would contaminate the crude aldehyde and make it more difficult to work 
this up. 
The rhodium concentration is in general in a conventional range, ie. about 
50-500 ppm, based on the reaction mixture. 
The molar ratio of CO to H.sub.2 can be from about 10:90 to 90:10, 
depending on the object of the hydroformylation. In general, it is from 
45:55 to 55:45, particularly where an aldehyde is desired as the product. 
Compared with the prior art recycle gas method, the novel process permits 
the recovery of about 5-20 times the amount of products with the same 
amount of recycle gas. The amount of recycle gas, which in the 
conventional process is as a rule from 100 to 200 times the amount of 
fresh gas, can of course also be reduced correspondingly. In the novel 
process, the amount of recycle gas is advantageously about 10-30 times the 
amount of fresh gas. 
The process according to the invention is particularly important for the 
preparation of C.sub.6 -C.sub.13 -alkanals from the corresponding 
.alpha.-olefins. These aldehydes are mainly reduced to the corresponding 
alcohols, which are used as components of ester-type plasticizers for 
plastics. Furthermore, they are oxidized to the corresponding carboxylic 
acids, which are important industrially as components of lubricants. 
EXAMPLE 
Hydroformylation of oct-1-ene 
An experimental hydroformylation reactor R having a capacity of 40 liters 
was charged with 2.2 kg/hour of oct-1-ene, 0.94 m.sup.3 (S.T.P.)/hour of a 
mixture of CO and H.sub.2 in a volume ratio of 48:52, 24 m.sup.3 
(S.T.P.)/hour of a recycle gas, essentially consisting of 80 vol % of 
H.sub.2, 15 vol % of CO and 5 vol % of N.sub.2, and 4.0 kg/hour of 
recycled liquid. 
The volume ratio of recycle gas to fresh gas was hence about 26:1. 
The hydroformylation temperature was 105.degree. C. and the pressure was 14 
bar. 
The concentration of the catalyst components was 100 ppm of rhodium (used 
in the form of Rh acetate) and 4.7% by weight of triphenyl phosphine 
(molar ratio of Rh to phosphine=1:185). 
The reaction mixture leaving the reactor, and containing about 6.7 kg of 
liquid constituents in addition to the recycle gas, was heated to 
120.degree. C. in heat exchanger W1, let down to 3 bar, and introduced 
into the devolatilization column E, and the recycle gas became laden with 
the products. 
The catalyst-containing liquid phase E was cooled in the heat exchanger W2 
and then recycled to the reactor via the pump P2. 
The recycle gas was cooled to 20.degree. C. under constant pressure (3 bar) 
by means of the heat exchanger W3, and was then separated, in the 
separator A, into a product-containing liquid phase and the recycle gas 
phase, which was returned to the reactor via the compressor P1. 
2.71 kg/hour of crude product was obtained via the pressure-release valve 
D4, this product essentially having the following composition: 
______________________________________ 
n-nonanal 2.16 kg 
isononanal 0.24 kg 
nonanols 0.03 kg 
octenes 0.25 kg 
octane 0.03 kg 
2.71 kg 
______________________________________ 
The yield of the desired product n-nonanal was hence 77%, based on the 
octene employed, and the n-nonanal/isononanal ratio was 9:1. 
The olefin conversion, which serves as a measure of the catalyst 
reactivity, was 89% initially, 88% after an operating time of 1 week, 86% 
after 3 weeks and 85% after 6 weeks. 
COMATIVE EXAMPLE 1 
Conventional recycle gas method at the same hydroformylation temperature. 
The hydroformylation of the octene was carried out in the same manner as in 
the process example, except that only the gas phase, ie. the recycle gas, 
was taken off from R; from this gas, the crude product was separated off 
in liquid form in the separator A. 
The results essentially correspond to those of the process example, except 
that 210 m.sup.3 (S.T.P.)/hour of recycle gas were required to discharge 
the product in gaseous form, ie. the recycle gas/fresh gas ratio was 223:1 
in this case. 
This uneconomical situation resulted in a consumption of about 30% more 
energy compared with the process example. 
COMATIVE EXAMPLE 2 
Conventional recycle gas method at a higher hydroformylation temperature 
In contrast to the process example, the hydroformylation of the octene was 
carried out at a temperature at which the recycle gas/fresh gas ratio was 
about the same as in the process example. This temperature was about 
170.degree. C. The other conditions were the same as for the process 
example. 
The recycle gas was fed directly to the separator A, and was freed there 
from the liquid components. 
Although the recycle gas/fresh gas ratio and the total energy consumption 
were about the same as for the process example, the yield of n-nonanal 
deteriorated substantially, and was only 0.68 kg/hour (=25%). Moreover, 
0.29 kg/hour of isononanal, 0.97 kg/hour of nonanols, 0.43 kg/hour of 
octenes and 0.23 kg/hour of octane were obtained as undesirable products. 
Furthermore, as a result of the decreased catalyst activity, which in turn 
is caused by the higher thermal load, the conversion of the olefin to the 
hydroformylation products dropped from an initial value of 70% to 60% in 
the course of 7 days.