GB2348200A

GB2348200A - Purification of propylene oxide

Info

Publication number: GB2348200A
Application number: GB0007010A
Authority: GB
Inventors: Johannes Gerhardus Jos Beckers; Johannes Jozias Blom; Jan Hermen Hendrik Meurs
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1999-03-23
Filing date: 2000-03-22
Publication date: 2000-09-27
Also published as: GB0007010D0

Abstract

A process for purifying propylene oxide contaminated with poly(propylene oxide), comprises the steps of: <SL> <LI>(a) subjecting the contaminated liquid propylene oxide to an ultrafiltration treatment using a ceramic membrane having pores with a pore size up to 20 nm under such conditions that the amount of poly(propylene oxide) is reduced to the desired level, and <LI>(b) recovering the purified propylene oxide product as the permeate. </SL>

Description

PROCESS FOR IMPROVING THE QUALITY OF PROPYLENE OXIDE The present invention relates to a process for improving the quality of propylene oxide.

Propylene oxide is widely used as precursor for preparing polyether polyols, which upon reaction with polyisocyanate compounds yield polyurethanes. Typically, methods for preparing polyether polyols involve reacting a starting compound having a plurality of active hydrogen atoms with propylene oxide, optionally together with one or more other alkylene oxides like ethylene oxide or butylene oxide. Suitable starting compounds include polyfunctional alcohols, generally containing 2 to 6 hydroxyl groups. Examples of such alcohols are glycols, glycerol, pentaerythritol, trimethylolpropane, triethanolamine, sorbitol, mannitol, etc. Usually a strong base like potassium hydroxide is used as a catalyst in this type of reaction.

The quality of the propylene oxide used to prepare the polyether polyol has significant impact on the quality of the polyurethane foams eventually obtained, especially when these foams are high resilience flexible polyurethane foams. Particularly the presence of poly (propylene oxide) is known to cause undesired effects in the polyurethane foam formation. Examples of such undesired effects are the occurrence of blow holes, low foam rise and even collapse of the foam formed.

Particularly, in moulding applications the presence of poly (propylene oxide) in the propylene oxide used for preparing the starting polyether polyol may cause problems in terms of quality of the polyurethane foam.

The term"poly (propylene oxide)"as used throughout the present specification refers to poly (propylene oxide) having a molecular weight of 4500 Dalton or higher as determined by polypropylene glycol-calibrated gel permeation chromatography.

Methods for manufacturing propylene oxide are well known in the art. Commercial production normally takes place via the chlorohydrin process or via the hydroperoxide process. In the latter process propene is reacted with an organic hydroperoxide. This hydroperoxide is either tert-butyl hydroperoxide or ethylbenzene hydroperoxide. In the first case tert-butyl alcohol is formed as a co-product (to be further converted into methyl tert-butyl ether), in the second case styrene is formed as the co-product. In the chlorohydrin process chlorine, propene and water are reacted to form propylene chlorohydrin, which is subsequently dehydrochlorinated with calcium hydroxide to form propylene oxide. For the purpose of the present invention it is immaterial which preparation route is used. Namely, in all processes poly (propylene oxide) is formed in undesirably high quantities. Moreover, it is known (e. g. from US-4,692,535) that high molecular weight poly (propylene oxide) may be formed during storage or transport, for example upon contact with a metal, such as carbon steel.

One method for improving the quality of propylene oxide via filtration is known from US-5,248,794.

According to the method disclosed in this U. S. patent purification of liquid propylene oxide takes place via membrane filtration. Suitable membranes mentioned are poly (vinylidene fluoride) membranes and acrylonitrile membranes. The membrane pore size is defined in terms of molecular weight cut off. The membrane filtration is suitably effected by filtering the propylene oxide to be purified through one or more flat membranes in plate and frame mountings. Preferably several membranes are arranged in parallel and additional membranes could be used in series to attain the desired throughput as well as a better separation.

The present invention aims to provide a process for purifying propylene oxide which is relatively easy to operate, which can be continuously operated and which has a very good performance in terms of propylene oxide purification.

Accordingly, the present invention relates to a process for improving the quality of propylene oxide contaminated with poly (propylene oxide), which process comprises the steps of: (a) subjecting the liquid propylene oxide to an ultrafiltration treatment using a ceramic membrane having pores with a pore size up to 20 nm under such conditions that the amount of poly (propylene oxide) is reduced to the desired level, and (b) recovering the purified propylene oxide product as the permeate.

In general, ultrafiltration is a pressure driven membrane filtration technique, wherein porous membranes are used which typically have membrane pore sizes of 1 to 20 nm. The use of ceramic membranes, as is the case in accordance with the present invention, offers certain advantages. First of all, the pores of the membrane are rigid. Secondly, the ceramic membrane is chemically inert, which is required when filtering an aggressive liquid like propylene oxide. One type of suitable ceramic filters are those comprising a filtration tube made of a porous and sufficiently rigid material like a-alumina, onto which a thin layer of the actual filter material having nanometer-size pores (for instance y-alumina) is applied. This can suitably be achieved by using the sol gel technology.

For the purpose of the present invention it has been found particularly advantageous when the pores of the ceramic membrane have a size of 1 to 10 nm, more preferably 2 to 7 nm.

In order to effect the filtration a pressure gradient is applied across the ceramic membrane. Typically, the pressure difference across the membrane is from 0.1 to 15 bar. Preferably, however, this pressure difference is in the range of from 0.2 to 10 bar, more preferably from 0.4 to 5 bar. The pressure at the low pressure side of the ceramic membrane will typically range from 0.5 to 2 bar.

As has already been indicated above, the way in which the propylene oxide to be purified in accordance with the present invention is prepared is immaterial to the present invention. Any known preparation process may be applied. The propylene oxide to be treated in the process according to the present invention may be the product directly obtained from the known preparation processes.

Alternatively, said directly obtained propylene oxide also may have been subjected to conventional purification and recovery techniques before it is treated in accordance with the present invention. Assuming that the propylene oxide is produced in a hydroperoxide process, such purification and recovery techniques typically involve the removal of unreacted propene and organic hydroperoxide, by-products (like propane, aldehydes and alcohol) and other treating agents. in general, the propylene oxide stream to be treated in the process of the present invention consists for at least 95 wt% of propylene oxide.

The conditions applied in step (a) should preferably be such that the concentration of poly (propylene oxide) is reduced to 1 mg/1 or less, more preferably to 0.6 mg/1 or less. Furthermore, the conditions should be such that the propylene oxide remains in the liquid state. Thus, at atmospheric pressure temperatures from 0 C up to 34 C may be applied. Suitably, step (a) is carried out at a temperature in the range of from 5 to 30 C. More than one ceramic filter may be used depending on the size of the filter and the amount of propylene oxide to be treated. One very feasible way of operating the process according to the present invention is using one or more parallel arranged ceramic filters and one swing filter, which can be put into operation as soon as one of the other filters is plugged.

The invention is further illustrated by the following example without limiting the scope of the invention to these particular embodiments.

Example A 6 nm ceramic membrane tube having a length of approximately 33 cm, an outer diameter do of 25 mm and an inner diameter di of 17 mm was installed in a laboratory scale ultra filtration unit. The pump was filled with impure propylene oxide. A small sample of this impure propylene oxide (PO blank) was analysed with GPC-ELSD.

The impure propylene oxide was pumped at a flow of 200 cc/h. The temperature applied was 15 C. A pressure difference of 0.5 bar was applied across the ceramic membrane with a 1% retentate purge. After filtering 1 litre of impure propylene oxide a permeate sample was analysed on for the content poly (propylene oxide) having a molecular weight above 4500 Da using the GPC-ELSD technique as described herein after.

The concentration of poly (propylene oxide) was determined by means of combined gel permeation chromatography and evaporative light scattering detection (GPC-ELSD). The ELS detector used was the ALTECH 500 (ALTECH is a trademark), used at 55 C with a nitrogen flow of 1.9 ml/min. In the GPC-ELSD technique the poly (propylene oxide) having a molecular weight of 4500 Da and higher is separated from lower molecular weight material by means of GPC and is subsequently passed into the ELS detector, where it is nebulized into a fine mist of droplets using nitrogen as the nebulizing gas. The droplets thus obtained flow through an evaporation tube, where they are partially evaporated leaving clouds of small, non-volatile particles. These particles pass through a light beam and are detected by light scattering on a photo multiplier. The concentration of poly (propylene oxide) can then be calculated from the ELSD peak area found via the relation Y = a * Cb wherein Y represents the ELSD peak area, C the poly (propylene oxide) concentration and a and b are constants. The constants a and b were determined from a series of standard solutions of poly (methyl methacrylate) (molecular weight 24,400 Da) with known concentrations.

The results of the ultrafiltration are indicated in the table below. It was found that the 6 nm ceramic membrane removed about 50% poly (propylene oxide).

TABLE I Ultrafiltration results

UF 6 nm PPO (mg/1) Blank 1. 03 Permeate 0. 54

Claims

C L A I M S 1. Process for improving the quality of propylene oxide contaminated with poly (propylene oxide), which process comprises the steps of: (a) subjecting the liquid propylene oxide to an ultrafiltration treatment using a ceramic membrane having pores with a pore size up to 20 nm under such conditions that the amount of poly (propylene oxide) is reduced to the desired level, and (b) recovering the purified propylene oxide product as the permeate.
2. Process according to claim 1, wherein the pores of the ceramic membrane used in step (a) have a pore size in the range of from 1 to 10 nm.
3. Process according to claim 1 or 2, wherein the pressure drop across the ceramic membrane is in the range of from 0.1 to 15 bar, preferably from 0.2 to 10 bar.
4. Process according to any one of claims 1 to 3, wherein step (a) is carried out at a temperature in the range of from 5 to 30 C.