Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2001-09-14
2002-10-08
Solola, T. A. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06462209
ABSTRACT:
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. 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 2000 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 U.S. Pat. No. 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.
Methods for improving the quality of propylene oxide via adsorption of poly(propylene oxide) are known in the art. Several adsorbents have been reported to be useful for this purpose. For instance, U.S. Pat. No. 4,692,535 discloses the use of activated carbon, charcoal or attapulgite as suitable adsorbents. In EP-A-0,601,273 non-calcined diatomaceous earth is mentioned as adsorbent for removing poly(propylene oxide). In JP-A-08/283253 zeolites and magnesia are mentioned as adsorbents. Suitable zeolites have a pore diameter between 3 and 10 Ångstrom, while the magnesia should suitably consist for at least 90 wt % of magnesium oxide.
Although the known adsorbents, and in particular activated carbon, perform satisfactorily in removing poly(propylene oxide) from propylene oxide, there is still room for improvement. The present invention aims to provide a process for improving the quality of propylene oxide by adsorption, wherein the adsorbent used has at least a similar performance in terms of poly(propylene oxide) removal as activated carbon.
According to U.S. Pat. No. 5,493,035 there are various difficulties associated with using activated carbon as the adsorbent for purifying propylene oxide, particularly during the initial or start-up phase of the activated carbon treatment. The adsorption of propylene oxide onto the activated carbon, namely, is highly exothermic and hence causes excessive temperature increases during said start-up. This has many undesired consequences, one of which is propylene oxide vaporisation and migration in the bed which in return causes secondary exotherms with very high temperatures. This is extremely hazardous and may even cause reactor damage according to U.S. Pat. No. 5,493,035. The solution proposed in U.S. Pat. No. 5,493,035 is a pretreatment of the activated carbon involving contacting this activated carbon with a glycol, such as propylene glycol.
It was envisaged that the adsorbent to be used in the process according to the present invention should not have the above risks associated with the use of activated carbon. On the other hand, the purification performance of the adsorbent to be used should be at least similar to that of activated carbon.
Accordingly, the present invention relates to a process for improving the quality of an propylene oxide contaminated with poly(propylene oxide), which process comprises the steps of:
(a) contacting the liquid propylene oxide with an adsorbent consisting of magnesium silicate and/or calcium silicate 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 has already been indicated above, the way in which the propylene oxide 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 adsorbent is magnesium silicate, calcium silicate or a mixture of both. In principle the known, commercially available magnesium silicates and calcium silicates may be used. Preferred magnesium silicates are the synthetic ones, e.g. prepared by reacting a magnesium salt like magnesium sulphate with sodium silicate. Similarly, synthetic calcium silicates may be used. Typically, the magnesium and calcium silicates are used in their hydrated form, although the dehydrated or water-free silicates may also be used. The use of magnesium silicate as the adsorbent is preferred.
The adsorbent may be used as a powder to form a slurry with the propylene oxide or may be used in extruded form in a bed through which the propylene oxide is passed.
Accordingly, step (a) may in a first embodiment comprise contacting the liquid propylene oxide with a fine powder of the adsorbent. The average particle size of such powder will typically be in the range from 1 to 100 &mgr;m, preferably from 2 to 40 &mgr;m. Suitably, the adsorbent is dispersed in the liquid propylene oxide yielding a slurry. In this embodiment of the present invention, step (b) advantageously is a filtration step yielding a permeate (or filtrate) containing the purified propylene oxide product. The retentate, consequently, contains the adsorbent with poly(propylene oxide) adsorbed thereon. Filtration may be carried out by microfiltration methods known in the art. The filter used should have such openings that the adsorbent with poly(propylene oxide) adsorbed thereon cannot pass these openings. The exact filter to be used, accordingly, depends on the size of the adsorbent powder particles used. Suitable filters for instance include glass filters, plate filters and multi-t
Beckers Johannes Gerhardus Joseph
Blom Johannes Jozias
Solola T. A.
Tsang Y. G.
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