Coal chemical industry

Low-boiling fusel oils in coal-to-methanol production

Background Art

　　Methanol is one of the most important basic organic raw materials, used to produce a variety of organic products such as chloromethane, methylamine, and dimethyl sulfate. It also serves as a feedstock for agrochemicals (insecticides, acaricides) and pharmaceuticals (sulfonamides, polymyxins, etc.), and is a key precursor in the synthesis of dimethyl terephthalate, methyl methacrylate, and methyl acrylate. At present, methanol is primarily produced via the synthesis process, with the following chemical reaction: 2H₂ + CO → CH₃OH; the main feedstocks are petroleum and natural gas. In recent years, as the prices of oil and natural gas have continued to rise, coal‑based chemical processes have attracted increasing attention. Synthetic gas is obtained through coal gasification and purification, and after removing acidic gases by low‑temperature methanol scrubbing, it is used to synthesize crude methanol.

　　In the typical process for separating and purifying crude methanol, a four‑column separation and purification sequence is commonly employed, comprising a pre‑distillation column, a pressurized distillation column, a atmospheric distillation column, and a methanol recovery column. Crude methanol enters the pre‑distillation column, where process water is sprayed at the top to carry out extractive distillation, removing light components—primarily noncondensable gases and dimethyl ether—while the methanol and high‑boiling constituents at the bottom are pressurized and sent to the pressurized distillation column. The vapor phase condensed at the top of the pressurized column is routed to a reflux drum; part of it serves as reflux for the pressurized column, while the remainder is withdrawn as the purified methanol product. The methanol, high‑boiling components, and water from the bottom of the pressurized column are fed into the atmospheric column, from which the purified methanol product is taken off at the top. A side draw is installed below the feed tray, yielding a stream primarily composed of methanol, water, and high‑boiling components; this stream is directed to the methanol recovery column for further methanol recovery, with the bottom wastewater sent to the biochemical treatment system. The recovery column also features a side draw for low‑boiling fusel oils, ensuring that the overhead purified methanol meets quality specifications and that the total alcohol content in the bottom wastewater complies with regulatory limits; the bottom wastewater is then conveyed to biochemical treatment. The low‑boiling fusel oil drawn from the recovery column consists mainly of methanol, isopropyl ether, n‑hexane, ethanol, methyl ethyl ketone, propanol, n‑heptane, and water, with the methanol content dependent on the synthesis conditions and the water content ranging from 30 to 50 wt.%. In this fraction, water molecules form numerous binary and multicomponent azeotropes with the other components, making it exceedingly difficult to remove water by conventional distillation and significantly complicating efforts to recover additional methanol from the fusel oil.

Process flow

　　1. Preheater; 2. Concentration tower; 3. Condenser; 4. Superheater; 5. Pervaporation membrane separator; 6. Permeate condenser; 7. Vacuum unit; 8. Rectification column; 9. Heat pump.

　　In the refined methanol unit, the low-boiling fusel oil drawn from the side stream of the methanol recovery column is fed to a concentration tower for further concentration. The vaporized fusel oil, after preliminary dehydration in the concentration tower, is distilled off from the top of the tower; part of it is condensed and returned as reflux to the concentration tower, while the remaining vapor, either directly or after passing through a heat pump, is sent to a pervaporation membrane separator. There, it is separated into a dehydrated product vapor containing 0.01–1 wt.% fusel oil. Meanwhile, water and trace organic impurities in the feed solution permeate the pervaporation membrane in vapor form, yielding a permeate that, upon condensation, is returned to the concentration tower; the overhead product from the concentration tower is then routed to the pervaporation membrane separator. The dehydrated product vapor from the pervaporation membrane separator, bypassing condensation, enters the refining tower in the vapor phase. At the top of the refining tower, the methanol product is withdrawn, while the bottoms discharge residual liquid for further separation and recovery of organic components.

Ethanol production from syngas

Background Art

　　Ethanol finds extensive applications in defense‑related chemical industries, healthcare, the food sector, and both industrial and agricultural production. Ethanol can be produced via biological fermentation or chemical synthesis. Among the chemical synthesis routes, the technology for producing ethanol from syngas has reached maturity; its production costs are significantly lower than those of the grain‑based route, giving it strong competitiveness and promising prospects for future development. There are three primary pathways for synthesizing ethanol from syngas: direct conversion of syngas to ethanol; bioconversion of syngas to ethanol; and hydrogenation of acetic acid derived from syngas to ethanol. Regardless of the production method, the resulting product is an aqueous ethanol solution. Because ethanol and water form an azeotrope, conventional methods cannot directly yield anhydrous ethanol. Currently, industrial production of anhydrous ethanol relies mainly on azeotropic distillation, extractive distillation, and adsorption‑based separation. However, these traditional dehydration techniques suffer from drawbacks such as complex process flows, high energy consumption, and significant environmental pollution.

Process flow

　　1. First heat exchanger; 2. First heat exchanger; 3. Distillation column; 4. Reboiler; 5. Top condenser; 6. Superheater; 7. Vapor-permeable water‑removal membrane unit; 8. Heat pump.

　　Syngas undergoes a multi-step reaction to produce a reaction mixture containing ethanol and water. Before entering the vapor‑permeation membrane, this mixture is separated in a distillation column, with the overhead or side‑draw stream from the final column yielding an ethanol vapor containing 0.1–30% water. The aqueous ethanol vapor is superheated or passed through a heat pump before being fed into the membrane unit; after separation by the membrane module, the permeate (on the pressurized side) is a finished ethanol product with a water content of 0.01–1%.

Process Advantages

　　By employing a coupled process of distillation and vapor‑permeation membrane separation, this technology integrates conventional methods with advanced separation techniques. It is free from azeotropic limitations and requires no addition of third‑party components, making it an efficient, energy‑saving, and environmentally friendly separation approach.

Ethanol co-produced in the synthesis of ethylene glycol from coal

　　Ethylene glycol, an important petrochemical feedstock, is primarily used to produce polyester fibers, polyester plastics, antifreeze, lubricants, and plasticizers. It is also employed in industries such as explosives, coatings, and inks, and can be further processed into more than 100 different chemical products, making it highly versatile. Its applications in the production of polyesters, antifreeze, and fine chemicals account for approximately 94%, 2.5%, and 3.5% of total consumption, respectively.

　　Indirect ethylene glycol synthesis technologies mainly include methanol dehydrogenation–dimerization, dimethyl ether oxidative coupling, glycolic acid-based processes, formaldehyde condensation, formaldehyde hydroformylation, and oxalate ester hydrogenation. Among these, the oxalate ester hydrogenation process is currently the most widely applied and extensively studied coal-to-ethylene glycol technology.

　　The coal-based oxalate ester process produces syngas via coal gasification and simultaneously generates nitrite esters by reacting alcohols with N2O3; these nitrite esters undergo oxidative coupling over a Pd catalyst to afford diester of oxalic acid, which is then converted into ethylene glycol via catalytic hydrogenation. Among the alcohols, methanol and ethanol have been the most extensively studied.

　　The process for producing ethylene glycol via hydrogenation of dimethyl oxalate comprises four main units: the esterification unit, the carbonylation unit, the hydrogenation unit, and the ethylene glycol purification unit. The esterification unit converts nitric oxide (NO) produced in the carbonylation unit and methanol (ME) obtained in the hydrogenation unit into methyl nitrite (MN) through an oxidation reaction. The carbonylation unit catalyzes the coupling of carbon monoxide (CO) with methyl nitrite to form the intermediate product dimethyl oxalate (DMO), while simultaneously generating NO, which is recovered by a separation system and returned to the esterification unit. A small amount of dimethyl carbonate (DMC) co‑produced in this unit is separated from DMO by the same separation system. The hydrogenation unit hydrogenates DMO to yield crude ethylene glycol (EG), concurrently producing ME, which is routed back to the esterification unit via an ME separation system. This unit also generates minor amounts of methyl glycolate (MG), ethanol (Et), water, and 1,2‑butanediol as by‑products of the hydrogenation process. The purification unit further refines the crude EG to produce polyester‑grade EG and refrigerant‑grade EG.

　　In addition to the polyester-grade and freeze‑grade EG products, this process also generates a by‑product stream composed of ethanol (Et), methyl glycolate (MG), water, and 1,2‑butanediol. For an industrial plant with an annual ethylene glycol production capacity of 200,000 tons, this ethanol by‑product stream amounts to approximately 10,000 tons; recovering this ethanol could significantly enhance the process’s profitability.

　　Process flow

　　1. First heat exchanger; 2. First heat exchanger; 3. Distillation column; 4. Condenser; 5. Superheater; 6. Vapor-permeable water‑removal membrane unit.

　　The ethanol byproduct generated in this process is first subjected to distillation to remove high-boiling impurities such as methyl glycolate and 1,2-butanediol. The overhead stream from the distillation column consists of ethanol vapor containing a small amount of water; this aqueous ethanol vapor is either superheated or passed through a heat pump before entering the membrane unit. Following separation by the membrane module, the permeate on the pressurized side yields the final ethanol product.