Liquid purification or separation – Processes – Treatment by living organism
Reexamination Certificate
2000-02-11
2001-07-03
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Treatment by living organism
C210S631000, C210S639000, C210S652000, C210S758000, C210S763000, C210S195200, C210S195300, C210S205000, C210S908000, C562S405000, C562S407000, C562S494000, C562S513000
Reexamination Certificate
active
06254779
ABSTRACT:
This invention relates to the treatment of effluent streams containing an organic acid species, e.g. the treatment of effluent arising from the production of aromatic carboxylic acids.
According to one aspect of the present invention there is provided a process for the treatment of an effluent stream containing water and an organic acid species which is a stronger acid than carbonic acid, comprising:
adding an alkaline medium to increase the pH of the effluent stream;
subjecting the effluent stream to oxidation treatment to convert the organic acid species to carbon dioxide, water and carbonate and/or bicarbonate ions;
separating the oxidised effluent stream into a concentrated aqueous stream rich in (bi)carbonate ions and cationic species derived from said alkaline medium and a depleted aqueous stream which is substantially free of said (bi)carbonate ions and said cationic species; and
recycling the concentrated stream for direct or indirect use as said alkaline medium.
Preferably the separation of the oxidised stream into said concentrated and depleted streams is effected by means of at least one reverse osmosis membrane. However, we do not exclude the possibility of carrying out this separation by other suitable means using for instance an evaporative technique such as multi-effect evaporation.
Typically the alkaline medium contains alkali metal or ammonium cations. Recycling of the concentrated stream allows the alkali metal/ammonium to be repeatedly recycled within the process and substantial savings may be secured since the consumption of alkaline medium, e.g. caustic soda, can be reduced significantly.
The oxidation stage may take various forms such as biological digestion, wet oxidation (e.g. Loprox or Zimpro wet oxidation processes), chemical oxidation (e.g. treatment with oxidising agents such as ozone and/or peroxide with or without the use of radiation such as ultra-violet light) and any combination of two or more of these treatment techniques.
The alkaline medium may be added prior to and/or during said oxidation step.
Prior to contacting the oxidised effluent stream with the reverse osmosis membrane or the like, the stream is preferably subjected to one or more stages of solids-liquid separation in order to reduce or eliminate any particulate matter that may be suspended in the effluent stream. Usually such solids-liquid separation is effected prior to separation of the the oxidised effluent stream into said concentrated and depleted streams.
In a more specific application thereof the invention relates to the treatment of an effluent stream or streams arising from the production of aromatic carboxylic acids, such as terephthalic acid and isophthalic acid.
Terephthalic acid for example is produced commercially by oxidising p-xylene with oxygen in a liquid phase which comprises a lower aliphatic carboxylic acid solvent, such as acetic acid, and a dissolved heavy metal catalyst system (usually cobalt and manganese and a bromine promoter). A slurry of terephthalic acid in the solvent is withdrawn from the reactor and is subjected to a solids-liquid separation process resulting in crude terephthalic acid crystals which may be subsequently processed further and a mother liquor filtrate which, in addition to catalyst and promoter used in the oxidation reaction, contains dissolved terephthalic acid and various by-products and impurities. These by-products and impurities arise from various sources such as minor impurities in the p-xylene feed stock to the reaction, incomplete oxidation of p-xylene resulting in partially oxidised products and by-products arising from the competing side reactions in the oxidation of p-xylene to terephthalic acid.
It is common practice to recycle a large proportion of the recovered mother liquor to the oxidation reaction in order to return catalyst and promoter to the oxidation reaction while purging a smaller proportion to a solvent recovery system so as to maintain the level of impurities and by-products in the reaction within tolerable limits. In the solvent recovery system, the mother liquor purge is subjected to evaporation to remove a substantial proportion of the aliphatic acid solvent present (which can be returned to the oxidation reaction) and water leaving a concentrate containing terephthalic acid and impurities/by-products together with some of the heavy metal catalyst present in the original mother liquor filtrate. The concentrate (the residues) may then be disposed of or, if economically justifiable, treated in order to recover valuable components for recycling, e.g. catalyst metals. Typical downstream treatments of the residues include catalyst recovery, incineration and anaerobic/aerobic biological treatment to reduce chemical oxygen demand (COD).
The terephthalic acid obtained from the liquid phase oxidation reaction usually contains impurities at levels which are unacceptable for subsequent uses of the acid, e.g. polyester fibre production. For instance, crude terephthalic acid produced in this way typically contains impurities such as 4-carboxybenzaldehyde (4-CBA), which tends to co-precipitate with terephthalic acid, and so-called colour bodies. To reduce the impurities to acceptable levels, it is well known to purify crude terephthalic acid by dissolving the same in an aqueous medium and then contacting the solution with hydrogen in the presence of a suitable hydrogenation catalyst. Such a purification process is described in our prior European Patent Applications Nos. 498591 and 502628, the entire disclosures of which is incorporated herein by this reference. In the process of EP-A-498591 and EP-A-502628, following reaction with hydrogen (which serves to convert 4-CBA to paratoluic acid and the so-called colour bodies to other non-coloured species), the solution undergoes crystallisation resulting in a slurry comprising crystals of purified terephthalic acid in aqueous mother liquor. The purified terephthalic acid is then recovered by solids-liquid separation as disclosed for instance in our prior International Patent Application No. WO 93/24440, the entire disclosure of which is incorporated herein by this disclosure. At least part of the recovered aqueous mother liquor (pure plant mother liquor) may be recycled within the process as described in EP-A-498591 so that use can be made of its water content and also its organic content, including terephthalic acid precursors such as paratoluic acid. Such recycle is effected by passing the mother liquor (with or without intervening treatment) to a distillation column associated with the oxidation reaction and used for separating water from aliphatic carboxylic acid solvent. At least part or substantially all of the pure plant mother liquor may be passed to effluent treatment plant. Conventional practice has been to discard all of the pure plant mother liquor.
As applied to the production of an aromatic carboxylic acid such as terephthalic acid and isophthalic acid, one aspect of the process according to the invention comprises feeding at least one organic acid-containing waste water stream generated in the course of operating the process to a waste water treatment plant in which the pH of the stream is adjusted by the addition of an alkaline medium and is subjected to oxidation treatment to decompose its organic components, separating the oxidised stream into a depleted stream comprising water having low organics content and a concentrated stream comprising water from said stream, cationic species derived from the alkaline medium and (bi)carbonate ions generated in the course of oxidation of the organics, at least part of the concentrated stream being recycled for use in said adjustment of the pH of the waste water stream and, optionally, at least part of the depleted stream being recovered and re-used in one or more water-consuming applications within and/or associated with the production process.
A feature of the process of the present invention is that water-containing effluent streams from an aromatic carboxylic production process can be managed in such a way as to all
Jackson Christopher Howard
Jeffery Ian Charles
E. I. Du Pont de Nemours and Company
Krukiel Charles E.
Prince Fred
Simmons David A.
LandOfFree
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