E-book materials ~ Natural gas chemical process knowledge lecture (Chapter 4, Sections 2, 3, 4, 5, and 6)

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Section 2 Formaldehyde 1. Properties and uses of formaldehyde Formaldehyde is a colorless gas with a special odor. Its boiling point is 252K. When it is cooled to 254K under normal pressure, liquid formaldehyde can be obtained and condenses into a solid at 155K. Formaldehyde is toxic, with an explosion range of 7 to 73% (volume fraction). Using formaldehyde as raw material, polyoxymethylene resin, melamine resin, phenolic resin and urea-formaldehyde resin can be produced, and * * , pentaerythritol, l,4-butanediol, neopentyl glycol, vinylon fiber, nylon, etc. Formaldehyde can also be used as an insecticide in pesticides, a disinfectant in medicine, and a reducing agent in the dye industry. Ø Temperature. Raising the temperature is beneficial to the reaction, but too high a temperature can easily cause excessive oxidation, product decomposition, and catalyst sintering. The reaction temperature is generally between 600 and 720°C. Ø The ratio of methanol to air in the raw gas composition should be outside the explosion range ； An appropriate amount of water vapor must exist to prevent the catalyst from overheating. Ø The purity of the raw gas must not contain sulfur to prevent the formation of silver sulfide ； Do not contain aldehydes or chemicals to avoid resinification reaction. ； There should be no carbonyl iron to avoid decomposition of formaldehyde. 3. Process flow of formaldehyde production by methanol oxidation method (1) After the silver catalytic method raw material methanol is pumped into the high-level tank, it flows through the filter and indirect steam heating evaporator in a certain amount, and purified air is sent from the bottom of the evaporator. The methanol content in the mixture of methanol vapor and air is about 0.5g/L. In order to control the oxidizer temperature, an appropriate amount of water vapor is added to the methanol vapor and air mixture. The raw gas mixture is heated to 110-120°C by a heater, and then enters the reactor through a flame arrester and filter, where a catalytic oxidation reaction occurs under the action of a silver catalyst at 380-650°C. The cooled conversion gas enters two series-connected absorption towers. The absorbent in the first absorption tower is the dilute formaldehyde liquid from the second absorption tower. Most of the formaldehyde in the first absorption tower is absorbed. After cooling, the absorption liquid drawn from the bottom becomes a formaldehyde aqueous solution containing 10% methanol. A small amount of methanol can prevent formaldehyde polymerization. The top of the second absorption tower is absorbed with an appropriate amount of cooling water, and the effluent at the bottom of the tower is sent to the first suction. The formaldehyde in the tail gas at the top of the tower has been completely absorbed. (2) In the iron-molybdenum method, methanol and air are heated into gas through a vaporizer and then enter the shell-and-tube reactor. The iron-molybdenum catalyst is placed in the tube. The reaction temperature is 320 to 350°C. The reactants are quickly cooled after leaving the reactor to avoid side reactions. The reaction gas cooler generates steam for use in the vaporizer. The reaction heat generated in the reactor is taken away by the shell-side heat transfer medium and reaches the waste heat boiler as a by-product of 2MPa steam. The heat transfer medium (such as diphenyl ether and other heat transfer oil) naturally circulates, which not only recovers energy, but also facilitates temperature control. Formaldehyde is absorbed in the stainless steel absorption tower. By adjusting the amount of water added, a formaldehyde solution with any concentration below 60% can be obtained. The heat of the absorption process is taken away by the cooling system in the absorption tower. The formaldehyde solution obtained by this method only contains less than 0.02 formic acid and can be used as a commercial product without further treatment. Effects of temperature, raw material concentration, and contact time on the reaction: Ø Reaction temperature: Iron-molybdenum catalysts are not resistant to high temperatures, and the reaction temperature must be strictly controlled. The formaldehyde yield is the highest at about 350°C, reaching about 90%. The reaction temperature is about 350°C. Ø Raw material concentration: The methanol feed concentration is very sensitive to the oxidation temperature. If the methanol concentration increases or decreases by 1%, the reaction hot spot temperature changes by about 5°C. Therefore, the methanol concentration in the feed must be kept constant. The methanol concentration in the raw gas is about 6%. Ø Methanol is oxidized with excess air on the iron-molybdenum catalyst during contact space. It is suitable for large space speed operation and the contact time is 0.2~0.5s. Section 3 Acetic Acid 1. Properties and uses of acetic acid The molecular formula is CH3COOH and the molecular weight is 60.06. Pure acetic acid is a colorless, watery liquid with a pungent and sour smell, and is highly corrosive. Its vapor is easy to ignite and can form explosive mixtures with air. Acetic acid is one of the important saturated fatty carboxylic acids and a typical monovalent weak organic acid. Acetic acid can carry out a series of typical reactions of fatty acids such as esterification reaction, metal salt formation reaction, hydrogen atom halogenation reaction, amination reaction, nitrification reaction, acylation reaction, reduction reaction, aldehyde condensation reaction and oxidative esterification reaction. 2. Properties and uses of acetic acid Acetic acid is an important organic chemical raw material and can produce acetic anhydride, acetate ester, vinyl acetate, fiber acetate, etc., as shown in the figure below. It is widely used in fiber, plasticizer, paint, adhesive, copolymer resin, pharmaceutical, dye and other industries. Picture below: The important chemical product liquid methanol raw material obtained from acetic acid passes through the tail gas scrubber and is continuously added to the reactor together with dimethyl ether and carbon monoxide. The crude acetic acid and unreacted gas drawn from the top of the reactor are cooled and enter the low-pressure separator. The crude acetic acid coming out of the bottom of the low-pressure separator is sent to the refining section. The tail gas coming out of the top is washed with feed methanol to recover methyl iodide in the converted gas. The washed tail gas is used as fuel. In the refining section, the crude acetic acid is advanced into the degassing tower to remove low-boiling components, and then cobalt iodide is removed in the catalyst separator. Cobalt iodide is removed as a bottom residue in the acetic acid aqueous solution. The crude acetic acid from which the catalyst has been removed is dehydrated and refined in an azeotropic distillation column. The entrainer used is a by-product mixture that evaporates with steam, which is generated during the reaction and is separated in the catalyst separation column. Acetic acid without water and formic acid is obtained at the bottom of the azeotropic distillation tower, and is then processed into pure acetic acid with a purity of more than 99.8% in two distillation towers. 4. Catalyst and reaction mechanism are as follows:: Performance comparison of catalytic systems for carbonylation of methanol to acetic acid Catalytic system Reaction phase system Catalyst Reaction conditions Acetic acid yield % Catalyst characteristics and by-products Temperature/°C Pressure/MPa Co system homogeneous CoI-CH3I 200～250 50.0～70.0 87 Acetaldehyde, ethanol, methane Rh system homogeneous heterogeneous RhCl3-CH3I Rh/C-CH3I 150～220 170～250 0.1～3.0 0.1～3.0 99 30～95 High activity, by-product CO2 Unstable activity, little by-product Ir system homogeneous IrCl3-CH3I 150～200 1.0～7.0 99 Activity similar to rhodium Ni system homogeneous heterogeneous Ni compound-CH3I Ni/C-CH3I 150～330 180～300 3.0～30.0 0.1～30.0 50～95 40～98 The amount of CH3I is high and the by-products are CO2 and CH4 5. Production process (1) Reaction process Methanol carbonylation is a gas-liquid phase reaction. The catalyst solution is in the reactor. Methanol is heated to 185°C and carbon monoxide from the compressor is sprayed into the reactor at a pressure of about 3.0MPa. At the bottom, the reacted material enters the flash tank from the side of the tower. The solution containing the catalyst comes out from the bottom of the flash tank and returns to the reactor. The vapor containing acetic acid, water, methyl iodide and hydrogen iodide comes out from the top of the flash tank and enters the refining process. The CO2, H2, CO and methyl iodide discharged from the top of the reactor enter the condenser as relief gas, the condensate returns to the reactor, and the remaining non-condensables are sent to the light component recovery process. The reaction temperature is controlled between 180 and 190°C, with 185°C being the best. As the temperature rises, the by-products methane and carbon dioxide increase. (2) Refining process Ø Light component tower: The mixture from the flash tank is separated here, the condensate methyl iodide at the top of the tower is returned to the reactor, and the non-condensable tail gas is sent to the low-pressure absorption tower. A high-boiling mixture of water and acetic acid, a small amount of rhodium catalyst, and dissolved hydrogen iodide are discharged from the bottom of the tower and returned to the flash tank. Aqueous acetic acid is discharged from the side line and enters the upper part of the dehydration tower. Ø Dehydration tower: The water evaporated from the top of the tower still contains methyl iodide, light hydrocarbons and a small amount of acetic acid, and returns to the absorption tower. The bottom of the tower contains heavy component acetic acid and is sent to the heavy component tower. Ø Recombinant tower: Light hydrocarbons are evaporated from the top of the tower, and materials containing propionic acid and heavy hydrocarbons are sent to the spent acid stripping tower from the bottom of the tower. The finished product acetic acid is obtained from the side line of the tower. Ø Spent acid stripping tower: The acetic acid is further evaporated from the heavy components and returned to the bottom of the heavy component tower. The waste material is discharged from the bottom of the stripping tower, which contains propionic acid and heavy hydrocarbons and needs further treatment. Section 4 Dimethyl Carbonate 1. Properties and uses of dimethyl carbonate. Boiling point is 90.1°C, melting point is 4°C, and has a slight aroma. Dimethyl carbonate can be miscible with organic solvents such as alcohols, homologs, and esters in any proportion, and is non-corrosive ； Slightly soluble in water. Dimethyl carbonate is flammable, with an explosion limit of 3.8 to 21.3% in air. Dimethyl carbonate is a non-toxic or slightly toxic chemical, but it is irritating to human skin, eyes and mucous membranes. Dimethyl carbonate is an excellent solvent. Its main characteristics as a solvent are narrow melting point-boiling point range, large surface tension, low viscosity, small dielectric constant of the medium, low heat of evaporation, and fast relative evaporation rate. The dimethyl carbonate molecule contains CH3-, CH3O-, CH3OCO-, -CO- and other functional groups, and has good reactivity. It can replace phosgene as the carbonylation agent, replace dimethyl sulfate as the methylating agent, and replace methyl chloroformate in the carbonyl methoxidation reaction to generate a variety of chemicals. Therefore, dimethyl carbonate is a widely used organic synthesis intermediate. Phosgene and methanol are introduced into a packed tower with a height-to-diameter ratio of 20 at a molar ratio of 1: (1.5 to 2.5). The raw material gas is introduced from the bottom, or phosgene is passed through the bottom, and methanol is sprayed at the top. React at 60°C for 12 to 20 hours to obtain a crude product containing 90% of dimethyl carbonate. At 70°C, the crude product is refluxed in the reflux equipment for 1 to 2 hours. The unreacted phosgene and methanol react further, while removing most of the HCl. The removed HCl is further processed and recovered. The reaction liquid is introduced into the neutralizer, and Na2CO3 is used to neutralize the HCl that has not been removed. The neutralized reaction liquid is moved to a distillation tower for separation, and dimethyl carbonate with a purity of more than 95% can be obtained. The yield is 90% based on methanol and 99% based on phosgene. The entire process includes reactive distillation, light component removal, azeotrope separation, solvent recovery, methanol recovery and propylene carbonate recovery processes. Reactive distillation combines the two processes of reaction and separation in one device, which not only satisfies the reaction rules but also adheres to the principles of distillation. The advantages of reactive distillation are: Once the reaction product is generated, it will evaporate from the reaction zone, destroying the reaction balance, increasing the conversion rate of propylene carbonate, and increasing the reaction rate. Reactive distillation is completed in the reactive distillation tower. Methanol and PC are added to the reactive distillation tower in a certain proportion. Under the action of the catalyst, dimethyl carbonate and PG are generated. The azeotrope formed by methanol and dimethyl carbonate is evaporated from the top of the tower. The azeotrope is composed of dimethyl carbonate.: Methanol = 30:70 (mass ratio), the azeotropic point is as follows:: Comparison project of three DMC production process conditions Gaseous phase oxidation method Liquid phase oxidation method Transesterification method Japanese Ube company, American DOW company, Italian ENI company, Japanese Daicel company, German Bayer company Development stage Industrialization research and development Industrialization research and development Research and development Reaction conditions Catalyst Pd CuCl2 CuCl2 Pd-Cu Temperature/℃ 50～150 100～120 120～150 50～200 50～250 Pressure/MPa Normal pressure～0.3 1.96～3.92 1.96～3.92 Normal pressure～9.8 DMC yield 200～500① 40～80① 40～135② About 10② Selectivity 38% Conversion rate/% 90 85 95 76 99 ① DMC, g/(L catalyst·h) ； ② DMC, g/(L solvent·h). (1) Liquid-phase synthesis process The methanol liquid-phase oxidative carbonylation reaction is carried out in three continuously stirred tank reactors. There are three phases of gas, liquid and solid in the reactor. Cuprous chloride is the catalyst. The reaction temperature is 90-120°C and the pressure is 2-3MPa. Carbon monoxide is compressed to reaction pressure and then bubbled into the first reactor, the recovered catalyst and methanol are sent to the first reactor, and oxygen enters the first, second, and third reactors respectively. During the reaction, the oxygen concentration is always kept below the explosion limit. After passing through three reactors, the single-pass conversion rate of carbon monoxide is about 67%. The gas leaving the third reactor is cooled and separated into liquid and gas in the separation tank. The liquid is returned to the third reactor. The gas is washed with an alkaline solution in the scrubber to remove CO2, then pressurized and returned to the first reactor. The liquid exiting the third reactor is flashed to remove dissolved gas, and the gas is recycled back to the first reactor. The liquid is separated from the catalyst through a filter, and the catalyst and methanol are returned to the first reactor. The liquid containing dimethyl carbonate, water and methanol is sent for extractive distillation. (3) Extractive distillation of dimethyl carbonate. Since methanol and dimethyl carbonate form an azeotropic mixture, a relatively pure product cannot be obtained by one distillation. For this reason, a two-step separation method is usually used. The first step is initial distillation, which utilizes the azeotrope properties of methanol and dimethyl carbonate to obtain the CH3OH-DMC azeotrope in a packed tower and remove the by-products. The composition of the CH3OH-DMC azeotrope is 70% CH3OH and 30% DMC (mass fraction), and the azeotropic temperature is 63°C. Step 2 is the refining stage to obtain pure dimethyl carbonate. There are four methods: low-temperature crystallization, extractive distillation, alkane azeotrope, and pressure distillation. The reaction solution of the water extractive distillation method removes by-products after initial distillation, and the CH3OH-DMC azeotrope is heated to 69°C to vaporize it, then enters the extractive distillation tower from the middle, and sprays water at the top of the tower. The water-azeotrope mass ratio should be 10 times greater than the reflux ratio. The extractive distillation tower is a sieve plate tower with an operating pressure of 0.1MPa, a reflux ratio of 1, a reboiler temperature of 99°C, a tower top temperature of 79.5°C, an azeotrope inflow temperature (gas phase) of 69°C, and an extraction water of 76°C. The distillate at the top of the extraction tower contains a large amount of dimethyl carbonate. After cooling, it enters the decanter. The distillate is divided into two layers, one is the organic layer and the other is the water layer. The organic layer is collected as the product, which contains 97% dimethyl carbonate, 2.7% water, and 0.3% methanol. The water layer contains 87% water, 11.8% dimethyl carbonate, and 1.2% methanol. The aqueous layer and the organic layer are separated and returned to the middle of the extractive distillation tower. The liquid at the bottom of the distillation tower contains 95% water, 5% methanol and less than 0.05% dimethyl carbonate. This liquid is introduced into the distillation column to evaporate the methanol, and the water is returned to the top of the extractive distillation column. Section 5 Methylamine 1. Properties and uses of methylamine Methylamine (methylamine), monomethylamine (CH3NH2, NMA), dimethylamine ((CH3)2NH, DMA) and trimethylamine ((CH3)3N, TMA). All are colorless, toxic and flammable gases. Monomethylamine is a gas with an ammonia smell. It is soluble in water, ethanol, and ether. It is flammable and forms an explosive mixture with air. It is weakly alkaline, with a boiling point of -6.8°C and a freezing point of -93.5°C. Dimethylamine is soluble in water, alcohol and ether, with a boiling point of 7.4°C and a freezing point of -96°C. Trimethylamine has a spicy fishy smell and a salty taste. It will liquefy under pressure or condensation at room temperature. Its boiling point is 2.9°C and solidifies at -117.1°C. It is weakly alkaline and easily soluble in ether, benzene and chloroform. (1) Lenoad method methanol, anhydrous liquid ammonia and circulating liquid pass through the vaporizer, heat exchanger and superheater in a certain proportion, and enter the reactor as the catalyst is alumina. Under the conditions of 5MPa and 420~450℃, methanol and ammonia generate methylamine. Part of the heat released by this reaction is used to preheat the feed gas. The crude product is fed into four series-connected distillation towers. The first tower separates excess ammonia, operating pressure is 1.3~1.5MPa, part of the azeotrope of trimethylamine and ammonia is circulated, the bottom product is removed from the trimethylamine distillation tower, water is added at the top of the tower, the pressure is 0.8~0.9MPa, and trimethylamine is obtained at the top of the tower. The pure trimethylamine product goes to the storage tank or circulates, and the bottom product is sent to a methylamine distillation tower. In the monomethylamine distillation tower, the pure monomethylamine at the top of the tower goes to the product storage tank or circulation, and the bottom product goes to the dimethylamine tower. In the dimethylamine distillation tower, the pure dimethylamine at the top of the tower is sent to the product storage tank or recycled back to the reactor. According to needs, monomethylamine, dimethylamine and trimethylamine can be used as products respectively. In the four-tower separation process, the conversion rates of methanol and ammonia are both greater than 95%, and the purity of the three products of methylamine is greater than 99%. AAT uses five tower distillation. Using the five-tower distillation methylamine production process, the conversion rate of methanol and ammonia is 98%, and the product purity can reach 99.6%. Section 6 Dimethyl Ether 1. Properties and uses of dimethyl ether Dimethyl ether is a colorless gas at normal temperature and pressure, with a pleasant smell. The flame when burning is slightly bright and non-toxic. It is miscible with most polar and non-polar organic solvents and partially miscible with water. It can be miscible with water in any proportion after adding a small amount of additives. Dimethyl ether is easy to condense and vaporize, and its vapor pressure at room temperature is similar to that of LPG. It also has excellent environmental performance, with low ozone depletion potential (ODP) and global warming potential (GWP). Dimethyl ether is the simplest fatty ether. Its emerging use is to replace LPG as clean civilian fuel and diesel as automobile fuel. In addition, it can be used in fuel cells and as a feedstock for the production of light olefins. 3. Methanol, the raw material of the methanol dehydration to dimethyl ether process, is pressurized to about 0.9MPa by the feed pump. It is heated to the boiling point by the preheater, enters the gasifier, is heated and vaporized, and then exchanges heat with the reactor discharge through the heat exchanger. It is heated to the reaction temperature and enters the reactor catalyst bed to perform a gas phase dehydration reaction. Then, each component is separated through four distillation towers to obtain a high-purity dimethyl ether product. The resulting non-condensable gas and the discharged crude dimethyl ether containing aromatic hydrocarbons can be used as fuel. The operating conditions of this process are: The reaction temperature is about 280°C at the inlet and raised to 330°C at the outlet. The methanol conversion rate is 60% to 70%, and the dimethyl ether selectivity can reach more than 99%. The dehydration of methanol to dimethyl ether is an exothermic reaction. In a tubular reactor, the tubes are filled with catalyst, and circulating heat transfer oil is used between the tubes to absorb the reaction heat. The reaction pressure is about 0.8MPa. Commonly used catalysts for direct production of dimethyl ether from synthesis gas are Cu-Zn-Al/γ-Al2O3 or Cu-Zn-Al/molecular sieve (HZSM-5, HSY) (2) Production process There are two synthesis processes for direct production of dimethyl ether from synthesis gas.: Fixed bed process and suspended bed process. In the fixed bed process, the synthesis gas reacts on the surface of the solid catalyst, which is also called the gas phase process. In the suspended bed process, the synthesis gas diffuses to the surface of a solid catalyst suspended in an inert solution for reaction, which is also called a liquid phase process. table below: Fixed bed synthesis of dimethyl ether process test results Process TIGAS ASMTG Test scale catalyst synthesis gas ratio H2/CO Reaction temperature/℃ Reaction pressure/MPa CO single-pass conversion rate/% Selectivity/% Laboratory 2 250～280 > 3 75 95 1 t/d Cu – Zn –Al/Al2O3 2 210～290 7～8 18 70～80 117 kg/d STD – 60 2 270 4.5 Slurry bed synthesis of dimethyl ether process test results Process Tsinghua University LPDMETM NKK Test scale/(t/d) Reactor type Reactor inner diameter/m Reactor height/m Catalyst synthesis gas ratio H2/CO Reaction temperature/℃ Reaction pressure/MPa CO single-pass conversion rate/% Selectivity/% 10 Circulating slurry bed 0.6 (riser) 21.56 LP201 + Al2O3 1 255 4.5 63 > 95 10 Bubbled slurry bed 0.475 15.24 Cu – Zn – Al + Al2O3 0.7 250～280 5～10 22 40～90 5 Bubbled slurry bed 0.55 15 1 260 5 51 90 Advantages of slurry bed compared to fixed bed: Good heat transfer performance enables constant temperature operation. Small catalyst particle size and large surface area can speed up the reaction. Coking of the catalyst can be alleviated. Catalyst loading and unloading is easy. ； In addition, the structure of the slurry bed reactor is relatively simple and can reduce investment costs. 4. Technical and economic analysis table below: Technological and economic comparison of the three synthesis processes, product dimethyl ether, methanol, methanol and dimethyl ether cycle ratio, CO single-pass conversion rate/%, total conversion rate/%, exhaust gas volume ratio/%, apparent thermal efficiency/%, relative investment, relative natural gas consumption, production capacity/(t/d), production cost/($/t) 2 50 95 5 70.7 85.9 81.1 1797 127.4 5 14 77 23 55.0 100 100 2500 109.2 5 18 85 15 63.2 94.5 90.6 659/1325 128.0 Compared with the synthesis of methanol unit, the one-step synthesis of dimethyl ether can save investment by 14.1% and natural gas consumption by nearly 19%. ； Compared with the two-step method using methanol, the total production cost of dimethyl ether is reduced by more than 20%.