Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
2001-05-31
2003-02-25
Righter, Johann (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S868000
Reexamination Certificate
active
06525229
ABSTRACT:
The present invention relates to a process for separating ethylene glycol from its aqueous mixture with organic acids and salts. Such mixtures are generated in particular in the industrial production of ethylene oxide.
BACKGROUND OF THE INVENTION
Ethylene oxide is an important base chemical. It can be used as such and as an intermediate for various chemicals. Most ethylene oxide, however, is hydrolysed to produce ethylene glycol.
Herein unless otherwise specified the term “ethylene glycol” is used in a generic sense to denote all ethylene glycols produced from the reaction of ethylene oxide with water, and therefore encompasses for example monoethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, etc.
Ethylene oxide is produced by catalytic gas-phase oxidation of ethylene with oxygen. The hot reaction gas containing ethylene oxide is cooled rapidly and supplied to an absorption column wherein water is added to dissolve the ethylene oxide. The fat absorbent is then stripped with steam in a stripping column to release the ethylene oxide, the remaining lean absorbent being re-circulated to the absorption column. In many cases the absorption column proper is preceded by a separate quench section wherein the entering reaction gas is scrubbed by a cooled recirculated slightly alkaline aqueous quench stream to absorb and neutralise organic acids (such as formic acid, acetic acid and oxalic acid) which are typical by-products of the ethylene oxidation reaction. This neutralisation is maintained by the continuous addition of a caustic solution (typically sodium hydroxide) to the aqueous quench stream. Apart from the organic acids, carbon dioxide which is also present in the reaction gas also reacts with the added caustic and forms carbonate and bicarbonate. In aqueous solution these salts are completely dissociated and the organic acids are partly dissociated. When an organic acid is removed from the mixture, such as happens during flashing or distillation, it will be formed again from the respective salt in the bottom fraction, resulting in a rise of pH which will eventually reduce the formation of acid.
In order to maintain the water balance and prevent by-products from accumulating in the system, some fresh water is continuously added and the absorption column as well as the stripping column are purged. The purge stream from the (quench section of) the absorption column is herein termed the “quench bleed stream” and the purge stream from the stripping column is herein termed the “stripper bleed stream”. Both contain important amounts of valuable components.
The quench bleed stream originally contains, beside the above-mentioned by-products (salts and acids) and water, mainly ethylene oxide as its valuable component. In order to recover this ethylene oxide, the quench bleed stream can be stripped in a quench bleed stripper whereby the bottom solution which contains the by-products is discarded. Alternatively, as is disclosed in U.S. Pat. No. 4,822,926, the quench bleed stream is subjected to elevated temperature and pressure conditions in order to hydrolyse the ethylene oxide present therein to ethylene glycol. In that case the ethylene glycol is the valuable component of the hydrolysed quench bleed stream, which has to be separated from the by-products and dried.
The stripper bleed stream contains, beside the above-mentioned by-products (salts and acids) and water, mainly ethylene glycol which has been formed from ethylene oxide and water under the elevated temperature and pressure conditions of the stripping column. Again, the ethylene glycol is the valuable product of the stripper bleed stream, which has to be separated from the by-products and dried.
Usually, the ethylene glycol present in the quench bleed stream and in the stripper bleed stream is flashed and, if necessary, further dried in a dehydrator, which is essentially a distillation column wherein the water is evaporated. However, the dried product then still contains organic acids.
The organic acids are detrimental to the quality of the ethylene glycol, in that they are ultra-violet (UV) light absorbers. By reacting with the glycols, especially in the absence of water, the acids may form esters which are also UV light absorbers. Other, unidentified, UV light absorbers may also be present in the ethylene glycol containing streams.
It is an aim of the present invention to produce from the hydrolysed quench bleed stream and the stripper bleed stream an ethylene glycol stream from which monoethylene glycol of high (fibre grade) quality, which is essentially free of salts and organic acids and which has a high UV light transmittance (i.e. low UV light absorbance), can be obtained.
According to U.S. Pat. No. 4,822,926 the salts from the quench bleed stream are separated off by centrifuging the bottom slurry produced by the dehydrator, followed by passing the centrifuged liquid phase to a glycol flasher. However, there is no mention in this document of the acids, nor of their effect on UV light absorbance.
According to U.S. Pat. No. 3,904,656 the stripper bleed stream is upgraded by treatment with ion exchange materials to remove metal salts, by degassing to remove carbon dioxides and volatiles, and by treatment with activated carbon to remove UV light absorbers. A drawback of this method is that the ion exchange material when used to remove the salts will be exhausted very quickly. A second and similar drawback is that the capacity of an active carbon bed for removing UV light absorbers has been found to be disappointingly low.
GB-A 2 091 580 discloses a method for recovering ethylene glycol from washings containing ethylene glycol, organic acids and coloured materials, which comprises subjecting the washings to chromatography wherein a salt form of a cation exchange resin is used as the packing material, and water is used as the eluent. This document ignores the presence of salts in the washings.
SUMMARY OF THE INVENTION
A process for separating ethylene glycol from its aqueous mixture with organic acids, salts and UV light absorbers, the process comprising the steps of
a) flashing the aqueous mixture under conditions effective to produce a bottom slurry containing the salts and an overhead aqueous fraction containing the ethylene glycol, the acids and the UV light absorbers;
b) passing the overhead aqueous fraction from step a) through at least one bed of ion exchange resin effective to produce a product stream having a reduced UV light absorbance; and
c) drying the product stream of step b) in a dehydrator.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a process for separating ethylene glycol from its aqueous mixture with organic acids, salts and unidentified UV light absorbers, the process comprising the steps of
a) flashing the aqueous mixture under conditions effective to produce a bottom slurry containing the salts and an overhead aqueous fraction containing the ethylene glycol, the acids and the unidentified UV light absorbers;
b) passing the overhead aqueous fraction from step a) through at least one bed of ion exchange resin effective to produce a product stream having a reduced UV light absorbance; and
c) drying the product stream of step b) in a dehydrator. Typically, the aqueous mixture is the hydrolysed quench bleed stream and/or the stripper bleed stream of an ethylene oxide production plant.
In step b), preferably the ion exchange resin is an anion exchange resin. More preferably a combination of an anion exchange resin with a cation exchange resin is used. Such combination may be done in a mixed bed of anion and cation exchange resins or in two subsequent beds. Most preferably a bed of anion exchange resin is used, followed by a bed of cation exchange resin.
An important feature of the present invention is that in the sequence of steps a) and b) the salts are removed firstly and the organic acids and optionally other UV light absorbers are removed secondly, thereby avoiding rapid exhaustion of the ion exchange resin used in step b) with salts. The removal of th
Baars Hendrikus Jacob
Bos Alouisius Nicolaas Rene
Kars Jelte
Price Elvis O.
Righter Johann
Shell Oil Company
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