Control the operation of the oxide stripper, PX distillation column, and BME distillation column, and as far as possible, the oxidation reaction by-product BME (methyl benzoate) and unreacted PX are stripped through the oxide stripper, and then passed through The PX distillation column separates the PX and discharges it to the V2101. The BME is separated into the discharge device through the BME distillation column. If the oxide stripper, the PX distillation column, and the BME distillation column work poorly, it is possible to make a large amount of BME enter the PX and In the PTE system, one affects the throughput of the device, and then BME also poisons the catalyst, affecting the normal progress of the oxidation reaction. Similarly, increasing the PTE content in the PTE of the oxidation tower feed can also reduce the consumption. Because if the content of DMT, DMI (dimethyl-terephthalate), DMO (dimethyl phthalate terephthalate), BME, etc. in PTE is too high, this part of the material circulates in the device, not only consumes energy, Moreover, it occupies a limited space of the reactor and other equipment, and the BME, DMO and the like contained in the PTE poison the cobalt and manganese catalysts, so that the activity of the catalyst is lowered, thereby affecting the oxidation reaction. Therefore, it is necessary to control the operation of the BME distillation column and the PTE distillation column to increase the purity of the PTE.
The PX and PTE feed mass ratio of the oxidation tower is theoretically 1:1, and the mass fraction should be 1:1142. In actual production, the PTE feed can be adjusted according to the PTE tank position. However, the ratio of PX to PTE cannot be too different. In the production process, especially in the process of raising the amount of 240t / d to 280t / d, due to the higher oxidation reaction temperature, the reaction depth of PX is large [the TPS (terephthalic acid) content in the oxide is relatively large, and the PTS (p-methylbenzoic acid) content is relatively small].
The conversion rate of PTE is also higher. After the esterification reaction, the PTE content in the crude ester is lowered, which causes the liquid level of the PTE storage tank to decrease, which in turn leads to a decrease in the PTE feed of the oxidation tower, so that the feed ratio of PX and PTE is reached. 1:0187. Because the PX feed is larger, it is more conducive to the oxidation reaction, resulting in higher TPS content, easy clogging of pipelines and equipment, and also unfavorable normal operation of P2201. In addition, due to the intense reaction, the peroxidation reaction causes PX. Consumption is increasing. In order to ensure the normal reaction, two large air compressors were opened, and the operating parameters were adjusted accordingly. DDD increased the air volume and lowered the reaction temperature. In order to generate more PTE, adjust PX and PTE to the appropriate ratio.
Residues Since the expansion of the DMT in 1996, a new residue (DMT distillation column bottoms) has been added to the oxidation tower. The purpose of adding the residue is to stabilize the oxidation reaction, the second is to inhibit the oxidation reaction to reduce the formation of high boilers, and the third is because the residue contains cobalt, manganese, etc., adding the residue can increase the concentration of cobalt and manganese in the oxide to facilitate the oxidation reaction. get on. When the concentration of the residue in the oxide is 6% to 9%, the oxidation reaction is better.
However, in the production process in the second half of the year, due to the large load of the single tower, if the concentration of the residue in the oxide is kept at 6% to 9%, the residue may inhibit the oxidation reaction, and the cobalt and manganese ions in the residue may be active. Poor (not as high as cobalt and manganese ions in fresh catalysts), so the concentration of this residue does not guarantee the smooth progress of the oxidation reaction, and the conversion rate of PX and PTE decreases, and the consumption increases. Therefore, adjustments have been made in the actual operation to control the concentration of the oxide residue at 4% to 6% to increase the feed amount of the fresh catalyst. Under normal circumstances, the amount of residue can be converted according to the feed amount of PX and PTE. This amount is also fixed under the same oxidation load, and the concentration of cobalt and manganese ions in the oxide is controlled by fresh catalyst. The amount of feed is adjusted. The change in the amount of residue in the chromatographic data of the oxide is generally caused by two factors, one is the change in the amount of residue feed, and the other is the change in the amount of residue generated by the oxidation reaction. If the feed amount of the residue is too large, in the case of ensuring a constant concentration of cobalt and manganese ions in the oxide, the amount of fresh catalyst is inevitably reduced, thereby affecting the normal progress of the oxidation reaction; if the amount of residue generated by the oxidation reaction Too much indicates that there is a problem with the oxidation reaction itself, both of which will cause an increase in PX consumption. Please refer to the results of the specific changes.
The amount of fresh catalyst depends on the oxidation reaction mechanism. Cobalt ions have a dual role in the oxidation process of PX and PTE. It can react with hydrocarbons to produce active radicals, accelerate the reaction, and form a molecular compound with free radicals to make the oxidation reaction. slow down. According to the relevant data and the experience of the operation of the device in the past two years, the cobalt ion concentration in the oxide is 170×10-6～210×10-6, and the oxidation reaction is better. (Note: There are two sources of cobalt ion in the oxide. One comes from fresh catalyst and the other from the residue feed). In the second half of the "two towers and one line" production process, especially in the process of raising the amount from 240t / d to 280t / d, although the measures to increase the temperature, increase the amount of air, etc., the acid value of the oxide is still low ( 190 mg KOH / g), it has been found that if the concentration of cobalt ions in the oxide is low, especially in the case where the amount of residue added is large and the amount of fresh catalyst added is small, the oxidation reaction is insufficient. Therefore, the amount of residue feed is reduced, and the feed amount of the fresh catalyst is increased to increase the content of fresh cobalt and manganese ions with high activity, and the cobalt ion concentration in the oxide is maintained at an upper limit of about 210×10-6. After adjustment, the oxidation reaction depth is increased, the acid value is increased, and the consumption is decreased.
Insufficient air volume will result in insufficient oxidation reaction depth, and intermediate products such as aromatic alcohol, aromatic aldehyde and aromatic acid will be esterified and condensed to form aryl ester and aromatic aldehyde high-boiling substances, which will increase the PX unit consumption. The amount of air is too large. First, it is prone to peroxidation. Second, it will cause an increase in the oxygen content of the exhaust gas, which makes the operation unsafe. When the oxygen content in the exhaust gas exceeds 6%, there is a danger of explosion. According to the production practice, the amount of air entering the oxidation tower is adjusted according to the oxygen content in the exhaust gas. The oxygen content in the exhaust gas is generally controlled at 215% to 315%.
In the case of low oxidation load, in order to save power, the "size compressor" operation mode is adopted, and the air volume can be satisfied under the load of 240t/d. However, as the load increases, the amount of air shows insufficient. In the process of lifting up to 280t / d, due to the limited capacity of a large and small compressor, the oxygen content of the oxidation tower tail gas can only be maintained at about 1% or even lower. The oxide acid value at this time was only about 190 mgKOH/g. In order to ensure the acid value of the oxide, the oxidation reaction temperature must be increased, and the upper limit of 162 ° C required by the process card is exceeded (the details will be described in detail in the section "Oxidation reaction temperature" below). However, this operating condition is extremely detrimental to the oxidation reaction. First, under high temperature conditions, the oxidation reaction is relatively intense. TPS and MMT (monomethyl terephthalate) may be deoxidized by excessive oxidation to form CO, CO2, H2O, BME, benzoic acid, toluene, phenols, formic acid, acetic acid, and the like. During this time, the PX consumption increased, the BME's external discharge increased significantly, and the combustion ratio of the oxidation reaction also increased significantly (detailed), the actual acid value of the oxide and the theoretical acid value (according to the respective triacids in the oxide chromatographic data) The content, and the molar value of the acid value of each of the triacids reacted with KOH, the theoretical acid value of DDD = 6.75 × TPS + 3.11 × MMT + 4.12 × PTS) is large (detailed), indicating that in addition to the target product triacid (TPS, In addition to PTS, MMT), there are other acidic substances. Secondly, due to insufficient air volume, the oxidation reaction depth is insufficient, which is consistent with the low acid value of the oxide and the higher TAE content in the oxide. Under such conditions, the oxidation reaction has both a peroxidation reaction and an incomplete reaction. Therefore, during this time, PX consumption increased significantly.
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