Pharmaceutical and Chemical Industry

Ethanol recovery

Distillation–Molecular Sieve Membrane Coupling Technology

　　Case 1: A 8,000-ton-per-year ethanol separation unit integrating distillation with a molecular-sieve membrane process.

　　In the synthesis of the drug enalapril, ethanol is used as a solvent for extraction or washing. The resulting ethanol mother liquor contains acetic acid, sodium chloride, by‑products from the synthesis process, and branched‑chain impurities; it is yellow in color, has a foul odor, exhibits an electrical conductivity of 600–1000 μS/cm, and contains approximately 10 wt.% water. To enable reuse, the ethanol must be dehydrated to a water content below 0.4 wt.% and rendered colorless and transparent. Previously, the client employed ethylene glycol and potassium acetate as extractants, performing separate extractive distillation on two distinct mother liquors, requiring four distillation columns each 20–25 m tall. Although this approach could produce ethanol meeting recycling specifications, it suffered from high energy consumption, complex operations, time‑consuming start‑up and shutdown procedures, and a tendency for column fouling during prolonged shutdowns. Moreover, ethylene glycol required periodic replacement. Accordingly, for this project, a coupled distillation–molecular sieve membrane process was adopted, integrating an acid–base neutralization unit, a distillation dehydration and desalination system, and molecular sieve membrane equipment into a single integrated operation. This enables the simultaneous removal of inorganic acids, organic acids, and water from the ethanol stream, ultimately yielding qualified ethanol with a water content ≤0.4 wt.%—colorless, transparent, and compliant with the company’s internal recycling standards.

Process flow

Process Comparison

Comparison of Ethanol Mother Liquor Recovery Processes for Enalapril

Ethanol Mother Liquor Recovery Distillation–Molecular Sieve Membrane Coupled Unit for Enalapril

Acetonitrile Recovery

　　Ceftriaxone sodium is a broad-spectrum, long-acting antibacterial agent launched by the Swiss company Roche in 1982. With its low dosage and minimal adverse effects, it commands a substantial market share and belongs to the third generation of cephalosporins, characterized by broad‑spectrum antimicrobial activity. In industrial production, when preparing crude ceftriaxone sodium from the starting material 7‑ACA, acetonitrile is used as the reaction solvent. After the reaction is complete, the resulting acetonitrile mother liquor is separated to obtain recovered acetonitrile that meets specified quality criteria, which is then recycled as the reaction solvent. During the manufacturing process, acetonitrile inevitably becomes contaminated with water and other impurities, generating acetonitrile waste liquid. Because acetonitrile forms an azeotrope with water—under atmospheric pressure, the acetonitrile–water azeotrope contains approximately 16 wt.% water—conventional distillation cannot directly purify this waste stream into a reusable reaction solvent, whose water content typically must be ≤0.05 wt.%. Consequently, the quality of the recovered acetonitrile exerts a decisive influence on the quality of the crude ceftriaxone sodium product.

　　In the conventional processes for recovering acetonitrile, four main separation and purification techniques are employed: pressure-swing distillation, azeotropic distillation, salting-out, and desiccant-based methods. All these approaches involve two stages—crude distillation followed by rectification—and rely on specialized distillation procedures for separation. However, they suffer from drawbacks such as complex process flows, high equipment costs, low separation efficiency, high energy consumption, and poor acetonitrile yields.

　　1 is the preheater, 2 is the concentration tower, 3 is the neutralization tank, 4 is the impurity removal tower, 5 is the superheater, 6 is the pervaporation separation unit, 7 is the permeate condenser, 8 is the vacuum pump, and 9 is the rectification tower.

　　The acetonitrile waste stream from the synthesis of ceftriaxone sodium is fed into a concentration column, where partial removal of water and high-boiling impurities is achieved via distillation. The concentrated acetonitrile distillate is then sent to a neutralization tank for pH adjustment. The neutralized acetonitrile distillate is further treated in a purification column to remove residual impurities; after impurity removal, it enters a pervaporation membrane separation unit. The bottom product is returned to the preceding concentration column to recover any remaining acetonitrile. Following pervaporation membrane separation, crude acetonitrile is obtained; the water and trace amounts of acetonitrile in the feed-side permeate pass through the membrane as vapor, forming the permeate, which is condensed and recycled back to the neutralization tank for additional acetonitrile recovery. The crude acetonitrile produced by the pervaporation unit is then sent to a rectification column for further purification, yielding the final acetonitrile product. The acetonitrile–water azeotrope distilled from the rectification column is returned to the purification column to recover the acetonitrile.

Project Cases

Isopropyl Alcohol Recovery

A distillation–pervaporation membrane coupling technology

　　The isopropanol feed stream, originating from the production process, is pumped by a feed pump and sequentially heat‑exchanged with the product vapor before entering the distillation column, which operates under pressure in a continuous mode. After stabilizing operation with total reflux, the overhead of the distillation column yields isopropanol vapor containing approximately 12 wt% water. This vapor is partially condensed in a condenser and then routed to a reflux drum; from there, a reflux pump returns it to the top of the distillation column. The remaining uncondensed isopropanol vapor is superheated in a superheater and subsequently fed into the membrane separation unit. The membrane separation unit consists of multiple membrane modules connected in series. Water and trace amounts of isopropanol in the feed permeate through the membranes from the upstream side to the downstream side, with the final stage on the upstream side producing the purified product. On the downstream side, a vacuum‑assisted condensation system is employed to establish a vapor‑pressure differential between the two sides of the membrane. The permeate vapor, drawn by a vacuum pumping system, enters a condenser, and the condensed permeate is sent for wastewater treatment. The bottoms liquid from the distillation column is cooled in a cooler and then forwarded to wastewater treatment.

Shenyang Sanjiu Pharmaceutical’s 3,000-ton-per-year distillation–molecular-sieve membrane‑coupled isopropanol separation unit

B Adsorption–Pervaporation Membrane Coupling Technology

　　Zhangjiagang Kailing Chemical’s 34,400‑ton‑per‑year ethanol–isopropanol dehydration project: The company uses isopropyl acetate and hydrogen as feedstocks, employing a catalyst to catalyze the hydrogenation of isopropyl acetate into ethanol and isopropanol. However, during the separation of these products, the presence of trace amounts of water complicates the process, necessitating prior removal of moisture from the reaction mixture before further purification can yield high‑purity ethanol and isopropanol. Under the original process, the reaction mixture was sent to a molecular sieve adsorption unit for dehydration, producing an anhydrous ethanol–isopropanol blend that was then further refined to obtain the final products. Yet in this molecular sieve system, once the sieve becomes saturated, it must undergo desorption; the resulting aqueous ethanol–isopropanol mixture, containing 2–3% water, cannot be effectively treated. With an annual production volume of approximately 34,400 tons, failure to recover this stream would severely compromise the overall plant’s yield, economic performance, and environmental compliance. To address this challenge, the company has commissioned Jiangsu Jiutian High‑Tech Co., Ltd. to supply a molecular sieve membrane dehydration unit capable of processing 34,400 tons per year of the ethanol–isopropanol mixture. This equipment will treat the desorption effluent from the molecular sieve adsorption system, reducing its water content to 0.3% before feeding it into the next stage for continued separation of ethanol and isopropanol.

Molecular sieve adsorption–molecular sieve membrane coupled apparatus for the hydrogenation of isopropyl acetate to produce ethanol and isopropanol.

　　After adopting this technology, calculations show that, compared with the conventional process, each ton of product saves 1.2 tons of steam and 90 m³ of circulating water. ³ At the same time, it significantly reduces labor and environmental protection costs, generating an additional annual benefit of RMB 9 million and substantially enhancing the company’s market competitiveness.