Dimethyl ether synthesis

Thread Content

The existing viable dimethyl ether production processes include the two-stage liquid-phase method, the two-stage gas-phase method, and the one-stage method. The original two-step liquid-phase dimethyl ether production process features low reaction temperatures (130–160°C); both the selectivity for methanol and its conversion rate are above 90%. The product separation is easy, and the process can be operated either intermittently or continuously, with relatively low investment requirements and simple operation. However, the equipment is severely corroded; the liquid in the tower and reactors as well as the wastewater are highly acidic and toxic. The operating conditions are harsh, and this process causes serious environmental pollution. It is difficult to scale up the volume of waste processed, which is why this technology has been largely phased out, with only a few companies in China still using it. However, thanks to technological improvements in recent years, the aforementioned drawbacks have been largely eliminated, and independent intellectual property rights have been obtained. The two-step gas-phase process for producing dimethyl ether features a high methanol conversion rate (75%–85%) and selectivity (greater than 90%). This process is well-established, simple to operate, does not require special materials for the equipment, generates virtually no waste or corrosion issues, and is amenable to scale-up (the largest plant in China currently has an annual capacity of 200,000 tons). However, throughout the entire process, the two-step method for synthesizing dimethyl ether has the drawbacks of a low one-pass conversion rate of CO in methanol synthesis, the need for extensive recycling of feed gas, and high energy consumption. Moreover, the process involves multiple steps such as methanol synthesis, methanol distillation, methanol dehydration, and dimethyl ether distillation, resulting in a relatively long process flow. In particular, the technology for synthesizing dimethyl ether using a two-step gas-phase process involves fixed-bed reactors, which results in high equipment costs and catalyst expenses, thereby increasing the cost of the product. The one-step process for dimethyl ether synthesis carries out both the methanol synthesis reaction and the dehydration reaction simultaneously, thereby overcoming the thermodynamic equilibrium limitations of the methanol synthesis reaction. As a result, the one-way conversion rate of CO is high (around 60% in existing technologies). This process has a shorter flow sequence, requires relatively less investment in equipment compared to the two-step gas-phase process, and results in lower product costs. However, this technology is still in the industrial pilot stage at present; the equipment is complex, the operating conditions are stringent, the investment required is high, and the preparation of composite catalysts as well as the maintenance of their activity have not yet been fully resolved.