Note: Descriptions are shown in the official language in which they were submitted.
- 2136~96
1 -
Method for the seParation of propylene/propane mixtures
The invention relates to a method for the separation of
propylene/propane mixtures by selective absorption. Water
serves as absorption medium to which phase transfer
reagents (solubilizers) can be added.
The separation of propylene/propane mixture i8 an object
which is frequently required in refinery technology as in
the chemical industry. Its aim is either the isolation of
the pure hydrocarbons or else the recovery of one of the
two components, in particular the propylene, eg. from
off-gases.
Depending on the required purity at which the constitu-
ents must be obtained from their mixtures, different
methods are used. Thus, by the method of DE-A-3 412 336,
the off-gas of propylene hydroformylation is partially
condensed, it conventionally contains 25 to 40% by volume
of propylene, 6 to 20% by volume of propane and, in
addition, hydrogen and carbon monoxide, the condensate is
rectified and the propylene fraction is returned to the
reaction stage.
The extractive distillation of propylene/propane mixtures
is described in Advan. Chem. Ser. 1972, pages 16 to 34 as
an alternative to simple distillation. Extraction media
are, inter alia, acetone, furfural and nitriles such as
acetonitrile, propionitrile, butyronitrile and benzo-
nitrile. A feature of the extractive distillation is the
influencing of the liquid/vapor equilibrium of the
propylene/propane mixture by a suitable solvent in a
similar manner to steam distillation. For industrial
realization, a rectification column with evaporation at
the bottom and reflux at the top is typical. In the
method described, propane is taken off overhead. Propyl-
ene is dissolved in the solvent and is isolated either in
a second rectification stage or, if a miscibility gap is
present, by phase separation. The authors conclude that
- 21~649~
-- 2
extractive distillation only has advantages over conven-
tional distillation in special cases.
A further method uses molecular sieves to separate off
propylene from propylene/propane mixtures of any composi-
tion (DD-A-150,885). It is based on the fact that propyl-
ene can penetrate into the zeolite pore structure and is
adsorbed there, whereas propane is excluded from the
diffusion, passes through the molecular sieve and arises
at high purity. The propylene is thermally desorbed and
is likewise obtained at high purity. The method iæ
suitable, in particular, for the separation of
propylene/propane mixtures having a low propylene propor-
tion. Yield-reducing attendant phenomena such as oligo-
merization or coke formation are substantially excluded.
The known processes are only of limited applicability or
they require complex measures which are not always
economically justifiable. The object was therefore to
develop a method which is technically simple and permits,
in particular, in the case of industrial-scale produc-
tion, the separation of propylene/propane mixtures withacceptable means. The method to be developed must be able
to be carried out independently of the composition of the
propylene/propane mixtures and also must be able to be
carried out in the presence of accompanying substances
due to the reaction. Moreover, it must be economical and,
if required, fit into the sequence of a plurality of
reaction stages without problem.
The invention achieves this object by a method for the
separation of propylene/propane mixtures by selective
absorption. It features the absorption medium used being
water.
Surprisingly, propylene/propane mixtures can be separated
into their components by the novel method in a simple
manner and with the use of industrially conventional
apparatuses. The absorption medium water is
213~96
-- 3
environmentally tolerated and inexpensive and is also
generally available in the required quality. Owing to the
solubility ratios, it is discharged only in very small
quantities together with the propane. The hydrocarbons
are isolated at a purity which satisfies the requirements
for most applications. hosses due to chemical conversions
of the hydrocarbons do not occur.
According to the invention, the separation of propylene/-
propane mixtures by the novel process is carried out by
extraction with the aid of water in the quality conven-
tionally available industrially. Drinking water, conden-
sate or deionized or softened water or else process water
of upstream or downstream process stages can be used.
Deionized water is preferably used.
According to a preferred embodiment of the method of the
invention, phase transfer reagents (also termed solubili-
zers or surfactants) are added to the aqueous phase. They
modify the solution power for the two hydrocarbon com-
ponents and as a result increase the separation effect.
The solubilizers which are known are compounds whose
hydrophilic groups are ionic (anionic or cati~nic) or
nonionic. The anionic compounds include sodium æalts,
potassium salts or ~mmo~;um salts of carboxylic acids
having 8 to 20 carbon atoms, in particular of saturated
fatty acids having 12 to 18 carbon atoms, in addition
alkylsulfates, alkylbenzenesulfonates and alkylbenzene-
phosphates. Examples of cationic solubilizers are tetra-
alkyl~mmo~;um salts and n-alkylpyridinium salts. The
nonionic phase transfer reagents cannot dissociate into
ions in aqueous solution. These include alkylpolyethylene
glycols, alkylphenyl polyethylene glycols, fatty alkanol-
amides and trialkylamine oxides. Finally, ampholytes such
as aminocarboxylic acids, betaines and sulfobetaines are
also used as solubilizers. In the context of the inven-
tion ionic surfactants have proved to be particularlyuseful, including as cationic surfactants
213~496
-- 4
trimethyltetradecyl~ ;um salts and anionic surfac-
tants, including dodecylsulfonate.
Generally, the propylene/propane mixtures to be separated
can be used in the form in which they arise in refineries
or in processes performed in the chemical industry. In
exceptional cases it can be necessary, prior to the
separation of the hydrocarbon mixture, to remove consti-
tuents interfering with or loading the absorption process
such as products and by-products of preceding process
stages which have not been separated off or only incom-
pletely separated off.
To carry out the absorption, the gas mixture is intro-
duced finely divided into the absorption medium and
conducted in counter-current or else in co-current to the
absorption medium spread out on a large surface area or
sprayed. In the course of this, propylene preferentially
enters through the phase boundary into the liquid and
forms a mixed phase, whereas the carrier gas, that is to
say the remaining components of the gas mixture, behave
neutrally and as a result of their considerably lower
solubility - compared with propylene - exit from the
absorption medium virtually completely. The absorption is
carried out in a single stage or multiple stages, discon-
tinuously or continuously in the known absorption appara-
tuses. Absorbers which are suitable are wash and spraytowers, plate columns, packed columns, wetted-wall
absorbers, rotary washers and rotary columns. The absorp-
tion is performed within broad temperature and pressure
ranges. Temperatures between -5 and 90C are preferred,
preferably 10 to 50C. The method can be carried out
without applied pressure; elevated pressures up to about
5.0 MPa, in particular 4.0 to 5.0 MPa increase the
absorption efficiency. In accordance with its solubility,
in particular propylene is absorbed by the aqueous phase,
whereas propane, together with any other inert gaseous
components, exits undissolved from the absorption medium.
- ` 2136~g6
-- 5
The number of the absorption stages used and thus the
completeness of propylene absorption is dependent on the
concentration of the propylene in the propylene/propane
mixture. The absorption proceeds according to Henry's law
and is dependent on the concentration of the hydrocarbons
in the mixture. Therefore, the separation process must be
terminated when more propane than propylene is absorbed
by the water because of insufficient propylene concen-
tration and high propane concentration.
The propylene dissolved in the absorption medium is
desorbed in a known manner, eg. by heating, by pressure
reduction or by expulsion with inert gas. The known
measures can be employed individually, together or as a
combination of two unit operations.
According to a particular embodiment of the method
according to the invention, the propylene/propane mixture
is conducted in counter-current to the water in a con-
tinuous absorption column. The solution of propylene in
water is then heated, it is depressurized in a desorption
apparatus, the unsaturated hydrocarbon escaping is taken
off and passed for further use. The low-propylene water
arising in the bottom of tr~e desorption apparatus is
likewise taken off, cooled, if required with utilization
of the heat content, and returned to the absorption
column. In accordance with the dependence of the amount
of propylene absorbed on the propylene concentration in
the gas mixture, the propane concentration in the propyl-
ene separated off and the propylene concentration in the
propane separated off can be adjusted to desired values
by appropriate specification of the amount of water.
The novel method can be applied in all cases in which
propylene and propane must be separated from each other.
This object is posed, e.g., in the hydroformylation of
propylene, in which, inter alia, a propylene/propane
mixture arises as off-gas. The specified procedure has
proved to be useful, in particular, when heterogene~us
- 2135496
- 6 -
catalysts, e.g. rhodium-phosphine complex compounds,
dispersed in water are used, since the absorption medium
water is also the reaction medium in the reaction of
olefin with synthesis gas.
The example below describes the specified procedure in
more detail, but does not restrict it to the embodiment
given.
Example
To the lower part of a counter-current absorption column
cont~;n;ng 10 theoretical plates are fed per hour 1853 kg
of a gas mixture which, in addition to other constitu-
ents, contains 60% by mass of propylene and 22% by mass
of propane and whose temperature is 51C. From the top of
the absorption column which is operated at 4.0 MPa flow
in counter-current to the gas mixture 130 m3 per hour of
water at 10C as absorption medium. At the top of the
absorption colnmn are taken off 586 kg per hour of a gas
mixture which contains 53% by mass of propane and 1.3% by
mass of propylene. At the bottom of the column the
propylene-laden absorption medium is taken off, heated to
70C, fed to the to~ of a desorption column which is
operated at 1.0 MPa and contains three theoretical stages
and the mixture is depressurized. 30 kg per hour of
synthesis gas at 15C are fed in at the bottom of the
desorption column and, at the top, 1312 kg per hour of a
gas mixture are taken off which contains 83% by mass of
propylene and 8% by mass of propane. The water freed of
propylene is cooled and returned to the absorption
column. The propylene-rich gas can be utilized directly.
