FR2496485A1 - PROCESS FOR THE PURIFICATION OF EXHAUST GASES CONTAINING ALKYLENE OXIDES - Google Patents

Abstract

Waste gases containing alkylene oxides are formed, for example, in the production of polyether-alcohols, an intermediate in polyurethane production. It is the object of the invention to develop a process, by means of which the alkylene oxide content in the waste gas can be reduced to less than 0.5% by volume. Moreover, it should be possible to carry out the process in an energetically advantageous manner and to ensure the re-usability of the alkylene oxides removed from the waste gas. To achieve the object, a process has been developed which first removes about 90% of the alkylene oxides contained in the waste gas by condensation at about 243 K. After processing, this condensate can be re-used. The waste gas depleted of alkylene oxide is then subjected to chemisorption with a dilute acid, for example oxalic acid, the alkylene oxides being converted into glycol by reaction and being removed almost completely from the waste gas. This process is mainly used for the purification of waste gases arising in the production of polyether-alcohols.

Description

 The invention relates to a method for the purification of exhaust gases containing alkylene oxides.

 The invention finds everywhere its application where exhaust gases containing alkylene oxides are produced, in particular in installations for the production of polyether alcohols, which constitute an intermediate product for the manufacture of polyurethanes.

 The invention makes it possible to decontaminate exhaust gases and constitutes a condition for the recovery of alkylene oxides as a reusable material.

 The alkylene oxides cause, due to their high toxicity, a considerable danger of pollution by exhaust gases. Mixing with air can also lead to the formation of explosive mixtures. Therefore, it is necessary to strive to minimize the content of alkylene oxides in the exhaust gas, to an order of magnitude which excludes any risk to humans, nature and industry.

 In practice, a method is used for the purification of exhaust gases containing alkylene oxides, in which the exhaust gases are subjected to absorption by H2O and then released into the atmosphere. No more than 95 of the alkylene oxides can be removed in this way. The alkylene oxides absorbed from the exhaust gases are then removed from the absorption agent by desorption using steam, and then condensed. As the alkylene oxide / water mixture is not yet suitable, even in this form, for being discharged into the drain, the liquid alkylene oxides are transformed into glycols, in the presence of sodium hydroxide solution at 1500 ° C. and at 0.5 MPa. This aqueous glycol solution is treated, during the subsequent purification of the wastewater, without negative influence on the environment.

 This process has the disadvantage of reaching an insufficient degree of purification, since approximately 5% of the alkylene oxides present at the outset remain in the exhaust gases. The alkylene oxides removed from the exhaust gases must undergo a treatment in accordance with the legislation of watercourses before being discharged to the sewer. For this purpose, an additional treatment is carried out in the presence of sodium hydroxide solution. The energy expenditure required to carry out this process is relatively high, because the hydrolysis takes place at 150D C and it is necessary to use an even hotter vapor for the desorption of the alkylene oxides. In addition, with this process we are led to destroy alkylene oxides, which are precious materials.

 The object of the invention is to reduce the content of alkylene oxide in the exhaust gases so that neither the toxicity nor the danger of explosion of the alkylene oxides no longer constitutes a risk of pollution to the environment. .

 In addition, the energy expenditure during the still necessary exhaust gas cleaning should be reduced. It is also proposed to be able to recover the alkylene oxides as usable materials.

 The object of the invention is to propose a process for the purification of exhaust gases containing alkylene oxides, according to which a low use of energy makes it possible to virtually eliminate the alkylene oxides from the exhaust gas. exhaust and recover most of it as usable material.

 To this end, the invention proposes a method according to which the exhaust gases containing alkylet oxides are first cooled to around 243 K. About 90% of the alkylene oxides contained in the exhaust gases then condense and are removed from the gas stream. After further processing, these alkylene oxides are available as a reusable material.

 The exhaust gases depleted in alkylene oxides are then subjected, according to the invention, to a chemical sorpsic tion using dilute acids, for example oxalic acid. The alkylene oxides absorbed by the washing agent react, according to the invention, at atmospheric pressure and at a temperature of 293 to 298 K, with water to form glycols, the reaction being carried out so that the transformation of the alkylene oxides into glycols is practically total.

 The content of alkylene oxides in the exhaust gases amounts to a maximum of 0.5 ffi after this chemical sorption.

 The advantage of the process according to the invention lies in the practically complete purification of the exhaust gases and the recovery of a large part of the alkylene oxides contained in these exhaust gases, which allows to perform the purification process under very favorable cost conditions.

 Another advantage lies in the greater ease of purification of the waters, since the quantities of glycolic solution formed are much lower than the process according to the current state of the art.

 The invention will be better understood from the example of embodiment described below.

 Example.

In the manufacture of polyether alcohols, exhaust gases are produced which contain alkylene oxides. The content of alkylene oxides is very variable #
200 m3 of exhaust gas at 50% by volume of propylene oxide was cooled to 243 K; 240 kg of propylene oxide were then condensed. The propylene oxide content of the exhaust gas was reduced to about 7% by volume. This depleted exhaust gas is seen, at a temperature of 243 K, in a column of a chemical type.

 On contact with 65 kg of oxalic acid at ls, at atmospheric pressure and at 295 K, propylene oxide hydrolyses into approximately 25 kg of propylene glycol.

This hydrolysis is carried out in such a way that the pylene oxide is almost completely transformed into propylene glycol, which has made it possible to discharge into the atmosphere an exhaust gas practically free of propylene oxide. The proportion of propylene oxide in the exhaust gas was # 0.5% 0.5% by volume. Propylene oxide contained in the condensate of the first stage could be, after treatment by distillation, recycled in the synthesis of polyetheralcoo
Energy consumption, for the purified.

tion according to the invention, 200 m3 of gas amounted to env ron 175 x 103 kJ

Claims (1)

 CLAIM  Process for the purification of exhaust gases containing alkylene oxides, by absorption and hydrolysis of alkylene oxides into glycols, process characterized in that the exhaust gases containing dalkylene oxides are first subjected at low temperature condensation, the alkylene oxides being first condensed in the form of a mixture of usable materials, and the exhaust gases depleted in alkylene oxides are then subjected to chemical sorption at atmospheric pressure and at a temperature of at least 293 K, with a dilute acid, for example oxalic acid, the alkylene oxides then hydrolyzing into glycols.