Process for producing purified terephthalic acid

Abstract

Disclosed is a process for producing purified terephthalic acid, which employs new para-xylene (PX) oxidation technology to make the intermediate product, crude terephthalic acid (CTA) produced by oxidation does not contain two purities of 4-carboxybenzaldehyde (4-CBA) and para-toluic acid (PT acid), so that the factory processing operation of “purification” can be omitted. PTA product produced by the present process is more suitable to be used as the polymer raw material of food packaging material, beverage bottle material, medicament packaging material and medical appliance since the content of formaldehyde group thereof is extremely low and it has the advantages of being safer and having no poison.

1 . this invention refers to a new manufacturing process for Pure Terephthalic Acid (PTA), whose technical characteristics are as follows:
 Adopt an oxidation reactor with the structure of segregating the raw material of Paraxylene (PX) from target product Terephthalic Acid (TA) obtained from oxidation reaction, e.g., the oxidation reactor with the structure of internal and external chambers, the oxidation reactor with the structure of internal, medium, and external chambers; horizontal plug-flow oxidation reactor; tower type plunger-flow oxidation reactor; and the tandem oxidation reactor combination of multi-reactor, the difference between the present PX oxidation reactor and those reactors or the combination thereof is that mixed oxidation reaction is conducted inside the existing one, but step-by-step or gradual oxidation reaction inside the new ones, and the oxidation reaction basically follows the steps of tandem oxidation reaction;   Control the condition of oxidation reaction to meet the crystallization system demand, make sure that, the paraxylene (PX), P-methyl benzaldehyde (TALD), P-methyl benzoic acid (PT acid, p-TA) and 4-carboxyl benzaldehyde (4-CBA) of these reaction materials at this area have been all oxidized into terephthalic acid (TA) when target product terephthalic acid (TA) of oxidation reaction is being crystallized into solid, so the intermediate product crude terephthalic acid (CTA) obtained from crystallization is free from two impurities of 4-carboxyl benzaldehyde (4-CBA) and P-methyl benzoic acid (PT acid);   In order to fully dissolve the produced TA into the mixed solvent of water and acetic acid, add water as the secondary solvent of TA into the liquid phase materials in the 4-CBA-to-TA reaction region, rise the reaction temperature appropriately and increase the solubility parameter of TA in the this solvent;   To rise the reaction temperature can also prevent the reaction rate from deduction which is caused by reducing of concentration of the organic phase of the reactant when water is added into the liquid phase materials;   Also, the temperature in the reaction area from PT acid to 4-CBA can be properly increased, so as to increase the reaction velocity of control step of tandem oxidation reaction, the water as solvent can be added into oxidation reaction area from PT acid to 4-CBA step by step, also can be added into the reaction area before producing PT acid.   
     
     
         2 . according to  claim 1 , respectively control the supply of compressed air for different regions in the reactor and increase the supply of it per liquid phase volume in the PT Acid-to-4-CBA reaction region. 
     
     
         3 . by controlling the inlet amount of PX and the solvent-acetic acid and water, as well as the steam output and recycled amount of the solvent, the concentration of the produced TA in the liquid phase in the reactor can be controlled to be close to but less than the saturation point of PA under oxidation reaction condition, which can avoid producing TA crystallization in the reactor. 
     
     
         4 . according to  claim 1 , for the reactors with the structure of internal, and external chambers, the bottom of the external chamber has been separated effectively from the inlet on the internal chamber; the supply of compressed air per liquid phase volume in the external chamber is more than it in the internal one, and the excess air goes into the internal chamber through gill shaped bars. 
     
     
         5 . according to  claim 1 , for the reactors with the structure of internal, medium, and external chambers, the liquid phase at top of external chamber is effectively separated from the liquid phase at top of medium chamber, the bottom of medium chamber is effectively separated from the inlet of materials at internal chamber, and the outlet of liquid phase materials at top of external chamber is effectively separated from the inlet of materials at internal chamber. 
     
     
         6 . according to  claim 1 , for a horizontal oxidation reactor with the plug-flow reaction processes, the liquid phase at the end of reaction can not be mixed at will with liquid phase at the material inlet, from inlet of raw material to outlet of reaction material inside reactor, the oxidation reaction is basically carried out in the sequence of tandem oxidation reaction. 
     
     
         7 . according to  claim 1 , the tower type oxidation reactor with the approximative plunger-flow reaction processes is applied. 
     
     
         8 . according to  claim 1 , the oxidation reactor combination with two or three or more tandem reactors can keep the oxidation reaction materials in a approximative plug-flow state;
 It is difficult for two conventional reactors to keep the oxidation reaction materials in an approximative plug-flow state, but the combination of “oxidation reactor with the structure of internal and external chambers”, or “oxidation reactor with the structure of internal, medium, and external chambers”, or “horizontal oxidation reactor with the plug-flow reaction processes”, or “tower type oxidation reactor with the approximative plunger-flow reaction processes” can commendably do it well.   
     
     
         9 . Equip with a spare middle charging bucket at the inlet of oxidation reactor to collect the crystallized pulpous materials produced in the oxidation reactor; after the reaction returns to normal, add appropriate acetic acid and water as the solvent and push the solution turned by these pulpous materials into the reactor from the feeding system, so that a little of the 4-CBA and PT Acid remained in the crystallized pulpous materials will be continually oxidized into TA. 
     
     
         10 . according to  claim 1 , the solvent water added into liquid phase materials inside oxidation reactor can be heated by the method of tube heat exchange, add water into tubes, the tube outlet is the place to adding water to liquid phase at reactor as assistant solvent, and the end surface of water inlet of tubes is closed, the tubes are buried at the upstream of liquid phase material flow of reactants, the liquid phase materials of reactants are at outside, and fetch the heat from reaction heat of upstream materials.