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Dimethyl ether, as a new type of clean energy product, has very broad market prospects. Dimethyl ether can be used as a domestic fuel, replacing liquefied petroleum gas or being mixed with it in any proportion, and it can also be used as a fuel for diesel engines, making it an ideal alternative to diesel. Therefore, vigorously developing the dimethyl ether industry is of great significance for both achieving substitution for petroleum and coal-based fuels and reducing air pollution. Building on its research and development of the slurry-bed one-step dimethyl ether production technology, Tsinghua University has in recent years also studied the gas-phase methanol dehydration method for dimethyl ether production. In May 2007, an industrial demonstration plant with a capacity of 10 kt/a for producing dimethyl ether via methanol dehydration was built and put into operation at Shandong Kaifu Chemical Co., Ltd. More than a year of continuous production testing has shown that the synthesis and separation processes of this demonstration unit are advanced, and all economic and technical indicators, product quality, and production capacity have met the design requirements. This article provides a brief overview of the process and technical features of Tsinghua’s vapor-phase dimethyl ether production technology, in the hope of sparking further discussion and contributing to the development of the dimethyl ether industry as well as to advancements in this technology. 1 Process flow: The synthesis of dimethyl ether using methanol as a raw material is essentially a catalytic dehydration reaction of methanol. The reaction equations are shown in http://www.nmtech.com.cn/jishuwang/upload1/080519940094531.jpg and http://www.nmtech.com.cn/jishuwang/upload1/080519941008865.jpg. Figure 1 shows a simplified diagram of the process for synthesizing dimethyl ether through methanol dehydration in the gas phase. This synthesis reaction is a moderately exothermic reaction, with an adiabatic temperature rise of approximately 170 °C during the reaction process. In engineering, the differences among various proprietary dimethyl ether synthesis technologies mainly lie in aspects such as the catalyst system, catalyst performance, and the methods for controlling the thermal balance of the reactor. Based on the characteristics of the methanol dehydration reaction process, and through extensive experimental research and engineering simulation optimization, Tsinghua University has developed a process flow for producing dimethyl ether via gas-phase methanol dehydration, as shown in Figure 1. During the production process, the raw material methanol is pressurized and metered by pump P101. It is then preheated in heat exchanger E101 before entering the methanol vaporizer H101 where it is vaporized. The resulting methanol vapor proceeds to heat exchanger E102, where it exchanges heat with the high-temperature reaction products, thereby raising its temperature further. Once it reaches the temperature required by the reactor, it enters the synthesis reactor R101. The reaction products that leave the reactor after the catalytic reaction first enter heat exchanger E102, where they exchange heat with methanol vapor and cool down. Then they go to the reboiler E203 of the methanol distillation tower, where they serve as a heating source for the bottom of the tower and thus cool down themselves. Finally, they pass through heat exchanger E101 again to exchange heat with the raw methanol, thereby causing the majority of the water vapor and gaseous methanol to condense. The cooled gas-liquid mixture enters the separation section and goes directly into the dimethyl ether distillation tower T201. The separation section consists of the dimethyl ether distillation tower T201 and the methanol recovery distillation tower T202. In the dimethyl ether distillation tower, the dimethyl ether product and trace non-condensable gases rise to the top of the tower and then enter the condensation heat exchanger E201. A small portion of the condensed and liquefied dimethyl ether flows back to the tower by gravity, while the majority is collected as a product and directly fed into the product metering tank. The methanol solution from the bottom of the tower enters the methanol distillation column T202 via a pressure reducing valve. Within the column, methanol rises as a gas to the top, where it enters the condensation heat exchanger E202 where it is condensed and liquefied. Some of the condensed liquid returns to the column by gravity, while the rest is collected and sent to the methanol raw material storage tank. In the tower bottom, the liquid at the bottom of the tower that meets the discharge requirements is removed through a drain valve. In water-scarce areas, the by-product water can be used as recycled water, while in areas with an ample supply of water, the extracted water can be discharged directly. 2 Technical Features: The Tsinghua vapor-phase dimethyl ether production technology features unique designs and proprietary technologies in both the synthesis and separation processes. The synthesis process utilizes a specialized catalyst for efficient solid acids, developed in collaboration with partner organizations, along with an isothermal fixed-bed reactor that is suitable for the performance of this catalyst. The separation process employs high-pressure closed distillation to separate and purify dimethyl ether, while low-pressure distillation is used to recover the methanol feedstock. The overall technology features a short process, safety and reliability, as well as energy efficiency and environmental protection. 2.1 High-efficiency specialized catalysts A stable and efficient catalyst is the key technical factor in developing dimethyl ether synthesis processes. The vapor-phase dimethyl ether technology developed at Tsinghua University uses TH-16 series solid acid dehydration catalysts, whose main active component is γ-Al2O3, supplemented by various modified catalytic components. Through nearly 10 years of improvement and optimization, these catalysts have achieved high catalytic activity and selectivity. Figure 2 shows the results of the sample activity evaluation for large quantities of industrial TH-16 catalysts. As can be seen from the graph, the TH-16 catalyst exhibits very good low-temperature activity, with a fairly high initial activity already at 230°C. At a relatively high liquid phase space velocity of 12 ml/gcat·h, the conversion rate of the methanol dehydration reaction at 280°C is over 80%, whereas when the temperature exceeds 290°C, the dehydration reaction approaches the thermodynamic equilibrium conversion rate. Therefore, the TH-16 catalyst can be used at higher space velocities, which allows for a significant reduction in the amount of dehydration catalyst required in industrial reactors; its optimal operating temperature range is 280–300°C. http://www.nmtech.com.cn/jishuwang/upload1/080519941528915.jpg Figure 2: Catalytic activity of the TH-16 series of methanol dehydration catalysts. http://www.nmtech.com.cn/jishuwang/upload1/080519942377327.jpg Figure 3: Selectivity in the gas-phase methanol dehydration reaction process. In addition to catalytic activity, selectivity for the desired product is another important indicator for evaluating catalyst performance. For catalytic reactions, generally as the reaction temperature increases, the selectivity for the desired product decreases. The selectivity of methanol dehydration catalysts follows a similar pattern; Figure 3 shows how the selectivity for the target product and methane varies with reaction temperature for the TH-16 and Topsoe DMK-10 catalysts, respectively. As can be seen from the graph, for the TH-16 catalyst, as long as the reaction temperature does not exceed 360°C, the selectivity for the target product in the synthesis reaction remains above 99.8%; however, when the temperature exceeds 370°C, the selectivity for DME begins to decline, and this decline is significant. For the DMK-10 catalyst, when the reaction temperature exceeds 340°C, the selectivity for by-products such as methane increases significantly. If ZSM-5 molecular sieve is selected as the methanol dehydration catalyst, the temperature at which DME selectivity begins to deteriorate is around 280°C. In comparison, the TH-16 catalyst exhibits higher selectivity for the target product, and operates within a wider temperature range with high selectivity, which lays the foundation for simplifying the distillation separation process. 2.2 Isothermal fixed-bed reactor: Due to the good low-temperature activity of the TH-16 catalyst, it presents both advantages and challenges in the selection and design of synthesis reactors. The advantage is that the catalyst’s good low-temperature activity allows the synthesis reaction to take place at lower temperatures, thereby reducing the requirements regarding reactor design, manufacturing, and material selection, and making it easier to choose a heat exchange medium for temperature control. The challenge lies in the high activity of the catalyst, as its heat release intensity per unit is approximately 3 times that of similar catalysts. Even with an adiabatic fixed-bed design and a multi-stage cooling strategy, it is difficult to meet the requirements for heat dissipation and temperature control. Through extensive engineering simulation optimization and experimental verification, an isothermal fixed-bed reactor fully compatible with the TH-16 catalyst was developed. This reactor uses heat transfer oil as the medium for heat dissipation and temperature control; it not only enables effective regulation of the reaction temperature in the catalyst bed but also facilitates the efficient recovery of the heat generated during the catalytic reaction. The heated heat transfer oil is then used as a heat source for reheating the bottom liquid in the methanol vaporization and dimethyl ether distillation columns. The synthesis reactor features an isothermal fixed-bed design, which offers the following advantages. ● The axial temperature distribution of the catalyst bed in the reactor can be artificially controlled as needed, allowing the methanol dehydration reaction to take place at a precisely controlled temperature. The high efficiency of the catalyst bed enables a significant reduction in the amount of catalyst required. ● There are no hot spots in the catalyst bed along the axial direction; the selectivity for the desired reaction product is high, with almost no by-products formed during the reaction. The dimethyl ether product has no unpleasant odor, and it also helps to extend the service life of the catalyst. ● Start-up and shutdown are convenient; no auxiliary start-up furnace or start-up heat exchanger is required. During start-up, once the boiler is ignited to raise the temperature, the reactor will automatically ignite and begin operating. ● The production process does not require complex control measures; proper control of the boiler oil temperature is sufficient to meet the temperature control requirements of the reactor. 2.3 High-pressure closed distillation for separation: The product separation and purification unit employs a simple two-column process. The operating pressure of the dimethyl ether distillation column is determined based on the temperature of the circulating water in different seasons, and can be adjusted within the range of 0.8–1.2 MPa. The methanol recovery distillation column operates under slightly positive pressure. The methanol distillation column is designed with two reboilers, one of which uses the sensible heat and latent heat of condensation from the material exiting the synthesis reactor as a heat source. Due to the extremely high selectivity of the TH-16 catalyst, combined with the isothermal reaction catalysis process, the methanol dehydration reaction produces almost no by-products, thereby providing the prerequisite for closed-loop distillation in the separation and purification of dimethyl ether as well as in the separation and recovery of methanol. By optimizing the operating conditions of the synthesis reaction and the distillation process on an industrial demonstration unit, full closed-system operation for dimethyl ether distillation and methanol recovery distillation was successfully achieved. Under conditions with no release of non-condensable gases, the product obtained from the dimethyl ether tower meets all specifications; the contents of non-condensable gases such as methane, CO, and H2 in this product do not exceed the **standards** for fuel-grade dimethyl ether. Through the optimized design of the dimethyl ether distillation tower, it is also possible to produce both fuel-grade dimethyl ether and high-purity dimethyl ether simultaneously. The methanol recovery distillation process also does not emit non-condensable gases, and the methanol content in the distillation wastewater is below 50×10‑6, allowing it to be used directly as cooling circulating water. By adopting a high-pressure double-column distillation process, full enclosed operation of the distillation separation section is achieved; the mixture from the synthesis reaction is sequentially distilled into dimethyl ether product, water generated by the reaction, and unconverted methanol, which then go respectively into the product metering tank, the circulation tank, and the raw material tank. Another important advantage of the distillation separation process in Tsinghua’s vapor-phase dimethyl ether production technology is that the main impurity in the distilled product is methanol, with very little water contamination. Figure 4 shows the typical analysis results of the products sold by Shandong Kaifu. As can be seen from Figure 4, the water impurity content is only about one-tenth of that of the alcohol impurities, and this distribution of impurities facilitates the use of the product as fuel. When the water content in dimethyl ether is high, detonation or extinguishment can occur during combustion; therefore, when it is blended with liquefied petroleum gas, this affects the blending ratio. Products manufactured using Tsinghua’s vapor-phase dimethyl ether technology can be blended with liquefied petroleum gas in any proportion (0–100%), and the resulting mixture does not experience deflagration or extinguishment during combustion. http://www.nmtech.com.cn/jishuwang/upload1/080519943302912.jpg Figure 4: Typical chromatogram of dimethyl ether product. 2.4 Pumpless distillation process: In the separation unit, the transfer of process materials is achieved solely through pressure differences and gravitational gradients; as shown in Figure 1, no pumps are required for transferring material from the feed to the distillation tower, nor for extracting the product at the top of the tower or the liquid at the bottom of it. Apart from the cooling water and heat transfer oil circulation pumps, there are no process fluid pumps in the entire distillation and separation process, which fundamentally solves the difficulty in selecting pumps for high-pressure dimethyl ether, and also avoids shutdowns caused by pump leaks and failures. Thanks to the rational design of the operating pressures for the synthesis reaction, dimethyl ether distillation, and methanol recovery distillation processes, the process flow has been effectively simplified, and transfer pumps between different units have been eliminated; as a result, the entire production facility only requires one P101 methanol pump to meet the requirements of the process flow. The single-pump process **improves the safety of the production facility, enhances the stability of the production process, and reduces equipment investment as well as operating costs during production.** 3 Industrial demonstration plant: Starting in 2006, Tsinghua University and Shandong Kaifu Chemical Co., Ltd. collaborated on the development of industrial technologies for dimethyl ether production via the gas-phase method. Building on the technology developed in Tsinghua University’s laboratory for gas-phase dimethyl ether production, an industrial demonstration plant capable of producing dimethyl ether by dehydrating methanol at a rate of 10 kt/a was constructed and put into operation at Shandong Kaifu Chemical Co., Ltd. in May 2007, as shown in Figure 5. More than a year of continuous production testing has shown that all the economic and technical indicators related to the synthesis and separation processes, as well as the product quality and production capacity of this demonstration unit, meet the design requirements. http://www.nmtech.com.cn/jishuwang/upload1/080519944153848.jpg Figure 5: 10,000-ton/year pilot plant for dimethyl ether production using Tsinghua University’s vapor-phase method 3.1 Basic information: The first industrial pilot plant developed using Tsinghua University’s vapor-phase method for dimethyl ether production was built with the aim of refining the laboratory results obtained by Tsinghua University regarding the use of solid-gas catalysis for the dehydration of methanol to produce dimethyl ether, and of developing a complete set of dimethyl ether production technologies with excellent economic and technical parameters. The basic information of the industrial demonstration plant of Shandong Kaifu Chemical Co., Ltd. is shown in Table 1. Table 1 Basic information on Tsinghua’s gas-phase dimethyl ether demonstration plant http://www.nmtech.com.cn/jishuwang/upload1/080519945407801.jpg 3.2 Control parameters The Tsinghua gas-phase dimethyl ether technology offers a wide range of operational flexibility; the process parameters that need to be controlled during production include those related to utility systems, synthesis reactions, and distillation processes, as shown in Table 2. Table 2 Process parameter control indicators http://www.nmtech.com.cn/jishuwang/upload1/080519946394883.jpg 3.3 Evaluation indicators: After more than a year of industrial demonstration operations at Shandong Kaifu Chemical Co., Ltd., Tsinghua’s vapor-phase dimethyl ether production technology has proven to feature simple operation, stable performance, and low energy and material consumption. Table 3 lists the economic and technical indicators compiled based on 10 months of operation of the industrial demonstration unit. As can be seen from the data in the table, aside from slightly higher power and coal consumption due to the relatively small scale of the facility, all other indicators are good, which fully demonstrates the advancement of this technology. Table 3 Operational evaluation indicators for industrial demonstration units http://www.nmtech.com.cn/jishuwang/upload1/080519948461543.jpg 4 Conclusions The gas-phase dimethyl ether production technology developed by Tsinghua University features low investment costs, stable operation, low raw material consumption, low energy usage, and environmental friendliness with no pollution. An industrial demonstration unit capable of producing dimethyl ether via methanol dehydration at a rate of 10 kt/a has been built and put into operation at Shandong Kaifu Chemical Co., Ltd. More than a year of continuous operation testing has shown that this demonstration unit is simple to operate and stable in performance, with all economic and technical indicators related to the synthesis and separation processes meeting the design requirements. Dimethyl ether has an extremely low water content, allowing it to be blended with liquefied petroleum gas in high proportions without any unpleasant odor. The maturity of this technology meets the requirements for industrial deployment, and it can provide a complete set of technologies for an annual production capacity of 10–300 kt of dimethyl ether.