1. Project Background
In the process of Fischer-Tropsch synthesis of coal-to-oil, 3-5 tons of synthetic water are produced for every ton of oil produced, which contains 0.5%-3% of low-carbon alcohols such as methanol, ethanol, propanol, as well as aldehydes, ketones and organic acids. These alcohols are both recyclable chemical raw materials and pollutants due to direct discharge, which will cause COD to exceed the standard. The traditional distillation method is restricted by the azeotropic effect of alcohol and water, and the recovery rate of alcohol is only 70%-80%, and the consumption of steam per ton of water exceeds 1.5 tons, with high energy consumption and low efficiency. In order to achieve resource recovery and environmental protection standards, a certain enterprise built a synthetic water alcohol recovery device with a processing capacity of 22t/h, using extractive distillation + azeotropic distillation technology, using 304 stainless steel as the main material, and creating a DN1400×33900mm distillation tower to overcome the separation problem of multi-component azeotropic system.
2. Process Flow
(I) Pretreatment
Synthetic water is first filtered through a 50μm bag filter to remove solid impurities such as rust and catalyst powder, and then filtered through a magnetic rod filter to adsorb trace iron ions (≤5ppm). Then, a 0.1% mass fraction of NaOH solution is added to adjust the pH to 7-8 to inhibit organic acid corrosion of equipment and reduce the degradation of the extractant.
(II) Extraction Distillation
Ethylene glycol is selected as the extractant, which is miscible with water and does not azeotropize with alcohols. The relative volatility of methanol-water is increased from 1.3 to 2.1 through hydrogen bonding. In the DN1400×33900mm extraction distillation tower, synthetic water is fed from the 20th theoretical plate in the middle of the tower, and the extractant is injected from the 10th theoretical plate in the upper part (flow ratio 1:0.8). Under normal pressure, the bottom temperature is 105℃ (steam pressure 0.3MPa), the top temperature is 65℃, the methanol vapor at the top enters the azeotropic distillation tower, and the water phase at the bottom enters the wastewater treatment unit.
(III) Azeotropic distillation
For about 2% of ethanol in methanol after extractive distillation, benzene is added as an azeotropic agent, which forms an azeotropic mixture with ethanol at a boiling point of 68.2℃ and is separated from methanol. Benzene-ethanol azeotropes are evaporated from the top of the azeotropic distillation tower, and the benzene phase is recycled after condensation and stratification, and the ethanol phase is further processed; methanol with a purity of ≥99.5% is produced at the bottom of the tower, and the ethanol recovery tower processes the ethanol phase to obtain ethanol with a purity of ≥99%.
(IV) Post-treatment
The water-rich phase at the bottom of the tower is subjected to 0.05MPa vacuum distillation and 120℃ evaporation, with an ethylene glycol recovery rate of ≥99% and a recycling period of ≥1 year; the final wastewater is catalytically oxidized by ozone, with COD ≤80mg/L, and can be reused or discharged in compliance with standards.
3. Technical advantages
(I) Efficient separation and energy saving
Extractive distillation breaks the azeotropic effect of alcohol and water, increasing the methanol recovery rate to 95% and reducing steam consumption to 0.9t/h; azeotropic distillation accurately separates methanol and ethanol, shortens the process by 25%, and increases the ethanol purity to 99%.
(II) Durable equipment
The entire tower is made of 304 stainless steel, which can withstand the corrosion of trace organic acids in synthetic water and has a service life of 10 years; the optimized design of related components increases the anti-blocking ability by 30%, and the maintenance cycle is extended from 6 months to 12 months.
(III) Intelligent safety
The infrared spectrometer monitors the concentration of alcohols at the top of the tower in real time and automatically adjusts the flow rate of the extractant; safety devices such as the extractant low-level alarm and the tower kettle overtemperature cut-off valve are set, and an explosion-proof reboiler is used to ensure safe operation.
IV. Application areas
It is suitable for the treatment of alcohol-containing wastewater in coal chemical processes such as Fischer-Tropsch synthesis and methanol to olefins, and can also be extended to the recovery of alcohol-containing waste liquid in industries such as medicine and coatings; it provides a general solution for the separation of high-water content and multi-component azeotropic systems, and helps the green transformation of the coal chemical industry and the development of the circular economy in the chemical industry.
