Method for the preparation of 1,2-propanediol
A method for preparing 1,2-propanediol involves reacting propene with hydrogen peroxide, in the presence of a phase transfer catalyst and a heteropolytungstate, in a liquid two phase reaction mixture with an organic phase containing an alkylaromatic hydrocarbon solvent. The method then involves separating the reaction mixture into an aqueous phase containing 1,2-propanediol and an organic phase, recycling the oxygen depleted organic phase to the reaction, and recovering 1,2 propanediol from the aqueous phase. The reaction and separation are carried out in liquid flooded vessels at a pressure high enough to suppress desorption of gas from the liquid reaction mixture. The separated organic phase is contacted with a stream of a non-flammable gas to desorb from 10 to 75% of the oxygen dissolved in the organic phase into the stream of non-flammable gas, before recycling the organic phase which purges oxygen safely with little loss of propene.
1 . A method for the preparation of 1,2-propanediol, comprising:
a) reacting propene with hydrogen peroxide at a temperature of from 50 to 110° C. in the presence of a catalyst mixture, comprising a phase transfer catalyst and a heteropolytungstate, in a liquid reaction mixture comprising an aqueous phase with a maximum apparent pH of 6 and an organic phase comprising an alkylaromatic hydrocarbon solvent;
b) separating the liquid reaction mixture obtained in a) into an aqueous phase (P a ) comprising 1,2-propanediol and an organic phase (P o ) comprising non-reacted propene, the alkylaromatic hydrocarbon solvent, and dissolved oxygen from hydrogen peroxide decomposition;
c) contacting the organic phase (P o ) separated in b) with a stream of a non-flammable gas at a flow rate of the non-flammable gas, a temperature, and a pressure effecting desorption of from 10 to 75% of the dissolved oxygen in the organic phase (P o ) into the stream of the non-flammable gas, to provide an oxygen depleted organic phase (P a ) and an off-gas comprising the non-flammable gas and desorbed propene and oxygen;
d) recycling at least a part of the oxygen depleted organic phase (P a ) to the reaction a); and
e) recovering the 1,2-propanediol from the aqueous phase (P a );
wherein a mass ratio of the alkylaromatic hydrocarbon solvent introduced to a) to the hydrogen peroxide introduced to a) is from 4:1 to 30:1; and
wherein a) and b) are each carried out in a vessel flooded by the liquid reaction mixture at a pressure high enough to suppress desorption of gas from the liquid reaction mixture.
2 . The method of claim 1 , wherein a) to d) are carried out continuously.
3 . The method of claim 2 , wherein in a), a concentration of the hydrogen peroxide in the aqueous phase is from 0.1 to 5% by weight.
4 . The method of claim 1 , wherein a molar ratio of the propene introduced to a) to the hydrogen peroxide introduced to a) is from 1.2:1 to 5:1.
5 . The method of claim 1 , wherein the alkylaromatic hydrocarbon solvent comprises more than 50% by weight of at least one alkylated aromatic hydrocarbon having from 7 to 12 carbon atoms.
6 . The method of claim 1 , wherein in c), the non-flammable gas is passed through the organic phase (P o ).
7 . The method of claim 1 , wherein in c), the desorption is carried out with counter current flow of the non-flammable gas and the organic phase (P o ).
8 . The method of claim 1 , wherein in c), the flow rate of the non-flammable gas, the temperature, and the pressure are adjusted to provide an oxygen concentration in the off-gas of from 1 to 11 mol-%.
9 . The method of claim 1 , wherein nitrogen is used as the non-flammable gas.
10 . The method of claim 1 , wherein a) is carried out with a propene feedstock containing from 1 to 8 mol-% of propane, and
wherein in c), the flow rate of the non-flammable gas, the temperature, and the pressure are adjusted to provide the off-gas with a mass ratio of propane to a combined amount of propane and propene of from 0.2 to 0.7.
11 . The method of claim 1 , wherein a) is conducted continuously in a loop reactor comprising fixed internals in a tubular section, and the liquid reaction mixture is passed through the loop reactor at a flow rate sufficient to provide turbulent flow at said fixed internals.
12 . The method of claim 1 , wherein the heteropolytungstate is a polytungstophosphate.
13 . The method of claim 1 , wherein the phase transfer catalyst is at least one selected from the group consisting of a tertiary amine, a tertiary ammonium salt, and a quaternary ammonium salt, and
wherein the tertiary amine, the tertiary ammonium salt, and the quaternary ammonium salt each comprises in total at least 12 carbon atoms.
14 . The method of claim 13 , wherein the phase transfer catalyst comprises a tertiary or quaternary ammonium ion having the structure R 1 R 2 R 3 NR 4+ ,
wherein
R 1 , R 2 , and R 3 are the same or different and are each an alkyl group having from 8 to 10 carbon atoms, and
R 4 is hydrogen or methyl.
15 . A facility for preparing 1,2-propanediol, comprising:
a cooled loop reactor comprising an inlet for hydrogen peroxide, an inlet for propene, an outlet for a reaction mixture, and at least one recycle stream inlet;
a phase separator comprising an inlet, a first outlet for a separated aqueous phase and a second outlet for a separated organic phase, the inlet being connected by a first conduit to the outlet of the cooled loop reactor for receiving the reaction mixture;
an oxygen desorption unit having a liquid inlet, an inlet for inert gas, an off-gas outlet, and a liquid outlet, the liquid inlet being connected by a second conduit, comprising a first control valve, to the second outlet of the phase separator for receiving the separated organic phase, and the liquid outlet being connected by a third conduit comprising a recycle pump to the at least one recycle stream inlet of the cooled loop reactor; and
a separation unit, having an inlet connected by a fourth conduit, comprising a second control valve, to the first outlet of the phase separator, an outlet for separated water, an outlet for separated 1,2-propanediol, and an outlet for by-products.
16 . The facility of claim 15 , wherein the phase separator comprises a phase boundary sensor controlling the first control valve, and the second control valve is a pressure retention valve.
17 . The facility of claim 15 , wherein the phase separator comprises a phase boundary sensor controlling the second control valve, and the first control valve is a pressure retention valve.
18 . The facility of claim 15 , wherein the separation unit comprises
a first distillation column, the first distillation column having the inlet of said separation unit and a bottoms outlet, and
a second distillation column, the second distillation column having an inlet connected to the bottoms outlet of the first distillation column, an overhead product outlet for the separated 1,2 propanediol and a bottoms outlet for the by-products.
19 . The facility of claim 18 , comprising a hydrogenation unit arranged between the first outlet of the phase separator and the inlet of the separation unit.
20 . The facility of claim 15 , further comprising a pressure retention valve on the off-gas outlet of the oxygen desorption unit.