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1 Project Introduction: The expansion and renovation of the dimethyl methane project by Shaanxi Weihe Coal Chemical Group Company includes a 200kt/a methanol plant and a 10kt/a dimethyl ether plant. The project was initiated in June 2003 and came online in March 2006. The dimethyl ether plant uses pure methanol as raw material, and under certain temperature and pressure conditions, along with the action of an active alumina catalyst, it dehydrates to produce dimethyl ether. The main components of the reaction products are dimethyl ether, methanol, and water. Pure dimethyl ether is obtained through distillation separation. 2 Dimethyl ether plant process: The raw material, methanol, is metered and then fed into the methanol storage tank. The methanol from this tank is pressurized by a pump and preheated in a preheater before entering the methanol vaporization tower where it is vaporized. The vaporized methanol is then superheated to the reaction temperature in a heat exchanger before being sent to the reactor. The crude methyl ether exiting the reactor is cooled through heat exchange before being fed into a crude methyl ether storage tank for gas-liquid separation. The liquid phase after separation is crude dimethyl ether, while the gas phase consists of non-condensable gases along with saturated dimethyl ether and methanol vapors. The gaseous material enters the alcohol scrubber, where dimethyl ether and methanol are absorbed using methanol; the absorbent solution is returned to the crude dimethyl ether storage tank, while the exhaust gas after absorption is sent to the flare line for combustion. The crude dimethyl ether in the crude dimethyl ether storage tank is pressurized by a pump and then fed into the distillation tower; after distillation and separation, the dimethyl ether product is obtained from the upper part of the tower and flows naturally into the product metering tank. The bottom liquid from the distillation tower is discharged into a bottom liquid storage tank, and after measurement, it is sent back to the methanol vaporization unit for methanol recovery and reuse. The process wastewater is discharged from the bottom of the vaporization tower, cooled in a wastewater cooler, and then sent to the coal grinding section for use. The process flowchart is shown in Figure 1 ； http://www.nmtech.com.cn/jishuwang/upload1/0804211444283284.jpg 3 Main equipment of the dimethyl ether plant 3.1 Reactor The reactor is a cold-jet fixed-bed reactor; approximately 1.8 tons of catalyst are placed in two sections within this reactor. Its process parameters are as follows: volume of 3.84 m3, inner diameter of 1000 mm, total height of the reactor around 5355 mm, operating pressure of 0.8 MPa, and operating temperature ranging from 260 to 400°C. 0Cr18Ni10Ti steel is used as the material for the main pressure-bearing components. 3.2 Distillation Tower The distillation tower is of vertical skirt-supported design, equipped with corrugated packing made of pressed perforated plates; a total of 17.5 m of packing is installed across five sections. Its process parameters are as follows: the inner diameter of the tower bottom is 1000 mm, the inner diameter of the tower body is 700 mm, the total height of the equipment is ~23735 mm, the operating pressure is 0.9–1.0 MPa, and the operating temperature is 150°C. 0Cr18Ni10Ti is used for the distillation section and the internal components of the tower, while 20R is used as the steel material for the main pressure-bearing elements in the stripping section. 3.3 Dimethyl ether storage tank: The dimethyl ether storage tank in this project is a 1000 m3 spherical tank, and it is one of the key equipment in this project. Its process parameters are a volume of 1000 m3, an inner diameter of 12300 mm, a design pressure of 1.16 MPa, and a design temperature of 50°C; the spherical shell is made of 16MnR. 4 Operation of the dimethyl ether plant 4.1 During the startup phase, ensuring an adequate amount of methanol vaporization and gradually increasing the reactor inlet temperature in a steady manner are key to a smooth startup. During the driving phase, fluctuations in steam volume or instability in the feed rate can cause changes in the load on the methanol vaporization tower, which in turn leads to variations in the amount of methanol that is vaporized. If the fluctuations are too large, the temperature rise of the catalyst bed will be uneven, making it difficult to maintain stability. Especially after the reaction begins when the catalyst bed temperature exceeds 240°C, large fluctuations in the amount of methanol vaporized can lead to significant temperature differences within the bed, thereby even increasing the likelihood of the reactor overheating. The designed full-load methanol vaporization rate is 2642 kg/h; based on past operating experiences, it is considered appropriate to keep the methanol vaporization rate at around 1500 kg/h to ensure a stable temperature rise in the catalyst bed. During the startup phase, vaporized methanol must be superheated by a startup heater in order to gradually raise the catalyst bed to the reaction temperature; if the increase in inlet temperature is too slow, this will delay the startup process and affect production progress. If the reactor inlet temperature is increased too rapidly, it will result in a large temperature difference between the upper and lower parts of the catalyst bed. The upper part of the catalyst may already be at or above the reaction temperature, while the lower part is still quite far from the catalyst’s active temperature. As a result, the reaction begins in the upper part of the catalyst; as the reaction proceeds, the temperature continues to rise. The higher the temperature, the more intense the reaction becomes, thus creating a vicious cycle in which the temperature difference grows larger and larger, which very easily leads to overheating. Based on operational experience, it is considered appropriate to maintain a temperature difference of 20–50°C between the reactor inlet and the catalyst bed during startup. Moreover, the closer this temperature difference is to the catalyst’s active temperature (240°C), the smaller it should be. Since the reaction of methanol dehydration to dimethyl ether is an adiabatic reaction, it is very difficult to handle if the catalyst bed overheats. Blindly reducing or cutting off the vaporized methanol does indeed decrease the amount of methanol participating in the reaction, which theoretically reduces the reaction heat. However, it also eliminates the circulation flow; the methanol adsorbed on the catalyst bed continues to react, and the heat accumulated in that catalyst bed cannot be removed in a timely manner. As a result, the higher the temperature, the more intense the reaction becomes ； The vicious cycle of more intense reactions and higher temperatures is difficult to eliminate even after introducing nitrogen into the system, posing a serious threat to driving safety. Therefore, during the startup phase, it is essential to strictly control the amount of methanol vaporized and the temperature at the reactor inlet, as well as to effectively manage the temperature of the catalyst bed, in order to ensure a safe and smooth startup process. 4.2 Analysis of problems in the initial stage of production and corresponding technical improvements: The venting that occurs when the entire system exceeds its operating pressure comes from the crude dimethyl ether storage tank, the distillation tower, and the metering tank. According to the design concept, these vent gases are all collected in the alcohol scrubber tower, where they are washed with methanol; the methanol and dimethyl ether absorbed are then released into the flare line for combustion. The diameters of these three vent pipelines are φ25, φ25, and φ32 respectively, and they converge onto a φ32 main pipe before entering the alcohol washing tower. As a result of the mutual influence among the gases released from these three sources, if the distillation system experiences overpressure, the gases released from the crude dimethyl ether storage tank cannot be discharged in a timely manner; this will lead to overpressure in the synthesis system. Consequently, it will directly affect the stability and normal operation of the catalyst bed temperature, and may even cause overheating. The cooling water used for the reflux cooler in the distillation column is the recycled water from the crude methanol cooler, which inevitably results in a relatively high temperature of the cooling water supplied to the reflux cooler ； The distillation tower cooler is located at a relatively high position, on the top layer of the dimethyl ether framework; as a result, the pressure of the circulating water is also relatively low (around 0.12 MPa). For these two reasons, during seasons with higher ambient temperatures, the increase in the temperature of the circulating water prevents the vapor at the top of the distillation tower from condensing effectively under full load. This leads to an increase in pressure within the distillation system, more venting, and a significant rise in production losses. To address the issues in these two areas, after thorough consideration and with the approval of the design team, it was decided to change the current practice of collecting the three types of exhaust gases in a single pipeline: the cooling water used for the return cooler will be changed from secondary cooling water to raw cooling water. To facilitate the technical renovation work, the off-gases from the distillation tower are first introduced into the crude methyl ether cooler; the methanol and dimethyl ether components in these gases are cooled there before they proceed to the alcohol washing tower via the crude methyl ether tank. The cooling water supply for the reflux cooler has been rerouted from the previous cooling return pipeline of the crude methyl ether cooler to the cooling supply pipeline. The renovation plan was implemented in May 2007; the specific renovations are shown in Figures 2, 3, and 4. http://www.nmtech.com.cn/jishuwang/upload1/0804211445303732.jpg http://www.nmtech.com.cn/jishuwang/upload1/0804211446374295.jpg 4.3 The entire system is now operating at full capacity smoothly, is easy to control, produces high-quality products, and its wastewater and exhaust gas emissions meet the required standards. The technical upgrades carried out in May 2007 completely eliminated the problems that existed during previous operations. Currently, the temperature at the upper part of the catalyst is between 350–360°C, while the temperature at the lower part is between 370–375°C. The constant load temperature remains stable, making it easy to control. The purity of the product can reach 99.99% of the designed level, with CH3OH levels in the product being less than 10×10—6 and H2O levels being less than 100×10—6. The wastewater generated by this device comes from the methanol vaporization tower; under normal conditions, the methanol concentration in this wastewater is below 100×10—6. The exhaust gas comes from the alcohol washing tower, with CH3OH at 0.03%, CH3OCH3 at 22.37%, CO at 25.99%, CH4 at 33.14%, CO2 at 20.71%, and H2 at 17.4%. The entire installation is currently operating at an overload level; last month, the average daily production of dimethyl ether was 44.5 tons, representing a high load of 131%. Even when the ambient temperature reaches around 38°C in summer, the system can operate stably at a load of 115%–120%, completely changing the previous situation where it was difficult to maintain stable operation at 100% load during hot seasons. 5 Conclusions 5.1 The technology for producing dimethyl ether through the dehydration of methanol vaporization, developed by Sichuan Tianyi, is mature; the product quality is stable, and the amount of waste generated is low, meeting the environmental requirements of chemical production. 5.2 The entire system is easy to operate and control, capable of running under overload conditions smoothly while maximizing production efficiency.