Synthesis method of Dimethyl terephthalate

Background technology

Dimethyl terephthalate abbreviated as DMT, white needle crystal, melting point 140.7 ℃, boiling point 284 ℃, easy to sublimate. It is generally used in the synthesis of polyester fibres, resins, films, polyester paints and engineering plastics.

At present, there are usually two production methods of dimethyl terephthalate hereinafter referred to as DMT, one is to produce refined terephthalic acid with paraxylene first, and then esterify refined terephthalic acid and alcohols such as methanol or glycol to produce DMT, such as Patent Application No. CN99810145.1 ‘Preparation Method of Terephthalic Acid and/or Dimethyl Terephthalate’, which oxidises paraxylene and then makes terephthalic acid first. Terephthalic acid is first made, and then the terephthalic acid is further reacted to form dimethyl terephthalate; the other is to use concentrated sulfuric acid, which is now commonly used, as a catalyst, to produce crude dimethyl terephthalate by combining purified terephthalic acid and methanol at a low temperature, generally at about 100°C, and at a low pressure of 0.3-0.5 MPa, and then refining it to form a dimethyl terephthalate product. The most direct problem of the above two methods is that both of them use refined terephthalic acid as the direct raw material. Raw material requirements for high cost, and the production of terephthalic acid as a raw material of terephthalic acid there are high production costs, the process is too complex, because the first to face the problem of its production costs, to the current market price of fine terephthalic acid and dimethyl terephthalate tonne selling price of more than 10,000 yuan, the price is not much difference, so the direct use of fine terephthalic acid as a raw material, the production of DMT in the commercial value is not very big, and the use of sulfuric acid as a catalyst, waste acid treatment cost is high. The use of sulfuric acid as catalyst, high cost of waste acid treatment, large loss of methanol, environmental pollution, which in environmental protection, energy saving and emission reduction, there are shortcomings.

Using the existing two production process methods, the production of DMT equipment investment is also large, generally need to make two sets of relatively independent system devices to carry out, thus resulting in large equipment investment, high production costs.

Contents of the invention

The present invention is to provide a production method of dimethyl terephthalate (DMT), which produces crude terephthalic acid as raw material from alkali reduction wastewater, and then further produces dimethyl terephthalate (DMT), with low investment in equipment, greatly reduced production cost, energy saving and environmental protection.

Technical solution of the present invention: it comprises (1) applying alkali reduction wastewater as raw material to produce terephthalic acid; and (2) producing dimethyl terephthalate by subjecting terephthalic acid and methanol to high-temperature, high-pressure, and esterification reaction.

The method for producing dimethyl terephthalate of the present invention is that it is carried out in the following method steps:

(1) Preparing terephthalic acid containing 40-60% wt% water from alkali reduced wastewater;

(2) Refining, drying and crushing said terephthalic acid, and methanol with a content of 90-99.5%, reacting with the ester at a temperature of 220-300°C and a pressure of 2-2.8 MPa to produce an esterified liquid containing dimethyl terephthalate;

(3) Separating said esterified liquid of dimethyl terephthalate to make crude dimethyl terephthalate;

(4) filtering and distilling the crude dimethyl terephthalate to produce a dimethyl terephthalate product.

Said method of producing dimethyl terephthalate is that tin, zinc and antimony compounds may be added as catalysts during the esterification reaction.

The said method for producing dimethyl terephthalate is that the esterified solution containing dimethyl terephthalate after the completion of the esterification reaction is separated using a flash evaporator by decompression directly into the flash evaporator to complete the separation of methanol and water vapour from the solution of dimethyl terephthalate.

Said method of producing dimethyl terephthalate is that the esterified solution containing dimethyl terephthalate solution generated after completion of said esterification reaction is filtered with activated carbon and then distilled to produce dimethyl terephthalate product.