Process principle of the methanol project

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Process principles of the methanol project 1. Compression process principle: Due to the large distances between gas molecules and weak intermolecular forces, work is applied to the gas using devices such as compressors, which reduces the distance between molecules and increases the intermolecular forces. Macroscopically, this results in a decrease in the gas volume and an increase in pressure. This project has a total of 5 compressors, with two sets operating in one active and one standby configuration, while the other compressor is used separately. To bring the dry gas to the conversion pressure, the dry gas compressor pressurizes the dry gas coming from outside the system from 0.45 MPa (G) to 2.7 MPa (G) and sends it to the conversion process. Additionally, to bring the stripped gas to the conversion pressure, the stripped gas compressor pressurizes the stripped gas coming from outside the system from 0.01 MPa (G) to 2.4 MPa (G) and sends it to the conversion process. The fresh gas/cycle gas compressor pressurizes the fresh gas from the gas production process from 1.5 MPa (G) to 5.35 MPa (G) and sends it to the mixture buffer tank. Additionally, it pressurizes the cycle gas from the synthesis process from 4.7 MPa (G) to 5.35 MPa (G) and sends it to the mixture buffer tank, where it is mixed with the pressurized fresh gas before being sent to the synthesis process. 2. Principle of the gas generation process: In this step, a mixture of dry gas and desorbed gas is combined with steam to undergo a steam reforming reaction, thereby producing fresh gas containing CO, CO2, and H2. (1) Hydrodesulfurization: Since even small amounts of sulfur can poison the conversion catalyst and render it inactive, it is necessary to remove sulfides completely before gas production in order to meet the requirements of the conversion catalyst during steam conversion. Additionally, copper-based catalysts used in methanol synthesis are also highly sensitive to sulfur; poisoning of these catalysts by sulfides, nitrides, heavy metals, etc., can affect their activity and service life. Therefore, sulfides in the feed gas must be removed prior to gas production. Sulfides in dry gas exist in various forms, generally classified into inorganic sulfides and organic sulfides. Organic sulfides cannot be removed directly through reaction with zinc oxide desulfurization agents; they must first be converted into inorganic sulfides via hydrogenation before they can be removed through oxidation and adsorption by zinc oxide. In raw materials, organic sulfides typically include thiolles, thioethers, disulfides, and cyclic sulfides. The majority of sulfides in the feed gas are organic sulfides. The hydrogenation process also involves the conversion of organic chlorines into inorganic chlorines; high-activity metal oxides are used as active components. The dechlorinating agent reacts with hydrogen chloride and is fixed on a carrier, thereby achieving the removal of chlorides. Thiol hydrogenation: R-SH + H2 = RH + H2S; Thioether hydrogenation: R-S-R’ + H2 = RH + R’H + H2S; Thiophene hydrogenation: C4H4S + 4H2 = C4H10 + H2S; Carbon disulfide hydrogenation: CS2 + H2 = CH4 + H2S; Zinc oxide desulfurization: H2S + ZnO = ZnS + H2O. (2) Hydrocarbon steam reforming: Hydrocarbon steam reforming involves the use of water vapor as an oxidant, along with a Ni catalyst, to convert the hydrocarbons present in dry gas and desorbed gases into feedstock for methanol synthesis; this is an endothermic process. Production is carried out by introducing water vapor into the mixture; the heat required for the reaction is provided by burning fuel gas in the radiant section of the converter. The following reactions occur in the Ni catalyst layer: CH4 + H2O(g) ==== CO + 3H2 – 206.29 kJ/mol; CnH2n+2 + nH2O(g) ==== nCO + (2n+1)H2 – Q; CO + H2O(g) ==== CO2 + H2 + 41.19 kJ/mol; CH4 + 2H2O ==== CO2 + 4H2 – 165.1 kJ/mol; C2H6 + 2H2O ==== CO + 5H2 – Q. The main side reactions are: 2CO ==== C + CO2 + 172.50 kJ/mol; CO + H2 ==== C + H2O – 131.47 kJ/mol; CH4 ==== C + 2H2 – 74.30 kJ/mol. Since the conversion of hydrocarbon vapors is an endothermic, volume-increasing reversible process, increasing the conversion temperature is beneficial for the reaction. From a thermodynamic perspective, increasing the pressure is unfavorable for the reaction; however, an appropriate increase in conversion pressure can accelerate the reaction rate, reduce the amount of catalyst needed and the investment in equipment, resulting in better overall economic outcomes ； Furthermore, under certain temperatures and pressures, increasing the water-to-carbon ratio not only enhances the conversion rate of hydrocarbons but also prevents carbon deposition on the catalyst ； Air velocity also has an impact on the conversion reaction. Increasing the air velocity can enhance the production capacity of the equipment, but if it exceeds the allowable range, it will increase system resistance, reduce the conversion rate, and raise the residual methane content. The last three reactions are the causes of carbon deposition in the converter, and should be avoided as much as possible. 3. Principle of the synthesis process: Methanol is produced by reacting CO, CO2, and H2 in the presence of a copper-based methanol catalyst. The basic reaction equations are as follows: C0 + 2H2 ==== CH3OH + 90.64 kJ/mol; CO2 + 3H2 ==== CH3OH + H2O + 48.02 kJ/mol. In reality, many other reactions occur between these reactants as well as with the products formed, such as: CO2 + H2 ==== CO + H2O; CO + H2 ==== HCHO (formaldehyde); 2CO + 4H2 ==== (CH3)2O + H2O (dimethyl ether); 2CO + 4H2 ==== C2H5OH + H2O (ethanol); 4CO + 8H2 ==== C4H9OH + 3H2O (butanol). Additionally, methyl formate, methyl acetate, as well as other higher alcohols and higher alkanes are also produced. By selecting appropriate catalysts and operating conditions, the reaction can be directed essentially toward the production of methanol as the product, thereby minimizing the formation of impurities. Copper-based catalysts, with copper as the main component, exhibit very high selectivity for methanol synthesis and also high activity at relatively low pressures and temperatures. However, such catalysts are highly sensitive to poisons such as sulfur and chlorine, requiring thorough purification of the syngas; otherwise, their activity is lost rapidly. Additionally, they have poor heat resistance, necessitating operation of the catalyst at its optimal temperature. Copper catalysts can generally be operated at 270–290°C; depending on the type of catalyst and the design of the reactor, the optimal operating temperature range varies slightly. The optimal operating temperature for shell-and-tube reactors is 230–260°C. For the synthesis of methanol using copper catalysts, an appropriate pressure range is 4.0–10.0 MPa(G). In cases where the CO2 content in the syngas is high, increasing the pressure has a significant effect on boosting the reaction rate. Due to the small scale of this facility, a synthesis pressure of 4.5–5.3 MPa(G) was chosen; raising the operating pressure further yields no significant benefits and instead results in increased energy consumption. The composition of syngas has a significant impact on the methanol synthesis reaction. As can be seen from the aforementioned reaction equation, to reduce energy consumption, it is appropriate to use syngas with a higher CO2 concentration; however, if the CO2 content in the syngas is too high, it will increase the burden on the distillation process and raise energy consumption. The appropriate composition of the fresh gas is: (H2 – CO2)/(CO + CO2) ≈ 2.05. Methanol synthesis is an exothermic reaction; heat must be continuously removed during the reaction for it to proceed properly. Shell-and-tube reactors utilize the medium-pressure steam generated between the tubes and the shell to remove heat. The control of the synthesis reaction temperature is achieved by regulating the steam pressure, as a certain steam pressure corresponds to a certain saturated steam temperature. 4. Principles of distillation process: Distillation is one of the most commonly used unit operations for separating homogeneous liquid mixtures. The distillation tower is the main equipment used for distillation, with a reboiler at the bottom and a condenser at the top. The bottom liquid is heated by steam in the reboiler, partially vaporizing to produce steam. Steam rises layer by layer along the tray columns, and at each tray it encounters the liquid flowing down from the upper part of the column. The light components in this liquid are vaporized, while the heavier components are liquefied, producing condensate that flows downward together with the other liquid materials. The steam that reaches the top of the column enters a condenser where it is completely condensed into liquid. Part of this liquid can be used as distillate, while another part is used as reflux and sent back into the column from the top. The reflux liquid descends plate by plate, where it meets the rising steam on each tray and is partially vaporized; the liquid that is ultimately extracted from the bottom of the tower is known as the bottom stream. Thus, the rising vapor condenses partially on multiple occasions, causing the temperature to decrease gradually; the concentration of volatile components increases over time, while the concentration of non-volatile components decreases. The temperature distribution within the tower decreases from the bottom to the top, whereas the concentration of volatile components increases from the bottom to the top. Distillation is a unit operation that utilizes multiple partial vaporizations of the liquid phase and multiple partial condensations of the gas phase to facilitate mass and heat transfer, thereby enabling the thorough separation of light and heavy components within a liquid mixture. Although copper-based catalysts for methanol synthesis exhibit very high selectivity, the synthesis of methanol by these catalysts inevitably results in the formation of numerous impurities; aside from water, the total amount of these impurities is very small. To produce a product that meets the quality requirements for pure methanol, it is necessary to remove impurities from crude methanol. The distillation process for pure methanol takes advantage of the different volatilities of the various components in crude methanol, as well as the fact that no azeotropes are formed. The method of multiple partial vaporizations and partial condensations is employed to achieve complete separation of the various components. This process uses three-column distillation and one-column recovery. Soluble gases and low-boiling-point impurities are removed in the pre-distillation tower, while water and high-boiling-point impurities are removed in the pressurized tower and the atmospheric pressure tower. Methanol contained in the recovered water is recovered, thereby producing high-quality purified methanol. 5. Working principle of the drum: After the boiler water enters the heat exchanger to absorb heat, it forms a vapor-liquid mixture. Due to the difference in specific gravity, this mixture circulates separately into the drum, where vapor and liquid are separated by gravity and adsorption forces; the steam is then stored and delivered.