Weekly Topic for December 7: How to Control and Reduce Wax Formation in Methanol and Dimethyl Ether Production
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This post was last edited by Chemical Gas Purification on 2010-12-9 at 15:52: How to control and reduce wax formation in the production of methanol and dimethyl ether.
This post was last edited by 654262293 on 2010-12-8 08:27. To address the issue of wax formation, the key is to focus on prevention. First, measures should be taken during the catalyst loading process in the synthesis tower to prevent impurities such as rust from entering the tower. Second, during startup, shutdown, and in case of accidents, it is necessary to avoid operating within the temperature range where paraffin is likely to form; instead, the initial temperature of the synthesis reaction should be raised above 225°C (as reactions at 180–190°C tend to produce paraffin the most). Third, during normal operation, the operating pressure and space velocity should be adjusted appropriately to minimize the occurrence of wax formation. Fourth, a certain amount of CO2 in the feed gas must be ensured to effectively suppress the occurrence of methanol side reactions!
The key is to select a suitable catalyst: According to the mechanism of paraffin formation, carbon monoxide and hydrogen in methanol synthesis gas, under different copper-based catalysts and various process conditions, produce aliphatic hydrocarbons, that is, a mixture of various higher alkanes commonly known as paraffins. When elements such as Fe, Co, and Ni are present in the catalyst, reactions such as CO+H2 =CH4+H2O and 2CH4=C2H6+H2 occur; when elements such as Co, Mo, and ThO2 are present in the catalyst, this occurs at a concentration of 0.01! Under 1.0 MPa, the reaction is CO + 2H2 → H2O; the catalyst contains basic metals such as iron, nickel, and magnesium, and under conditions of 0.01–4.0 MPa and 180–300 degrees Celsius, the reaction 2CO + H2 → CO2 occurs. Therefore, the choice of catalyst is of great importance ； Then there is the skill level of the operators.
This post was last edited by Chemical Gas Purification on 2010-12-9 at 22:57. In medium and low-pressure methanol production, copper-based catalysts are often used; these catalysts tend to produce hydrocarbons with long carbon chains as well as paraffins, which can cause disruptions in the production process. For example, it leads to an increase in system pressure differences, preventing normal operation of the production process. 1. Principle of paraffin formation: It is primarily CO and H2 that, under suitable conditions, produce aliphatic hydrocarbons, namely paraffins. The representative molecular formula for these compounds is CnH2n+2, with a melting point of around 70°C. The conditions for their formation are as follows: (1) If the catalyst contains elements from group 8 such as iron, cobalt, or nickel, reactions such as CO + 3H2 → CH4 + H2O and 2CH4 → C2H6 + H2 occur. (2) If the catalyst contains elements such as cobalt or molybdenum, the reaction CO + 2H2 → … + H2O takes place under pressures of 0.01 to 1.0 MPa. (3) If the catalyst contains alkali metals such as iron, nickel, or magnesium, the reaction 2CO + H2 → … + CO2 occurs under conditions of 0.01 to 4.0 MPa and temperatures of 180 to 300°C. 2. Hazards: It reduces the yield of methanol ； It increases the operating costs of the system. 3. Wax removal measures: To ensure the proper operation of the methanol system, wax removal measures must be taken. (1) Parking degreasing: Using steam to cook at a temperature of over 70°C. (2) Online dewaxing: carried out with two production water coolers available. 4. Precautions in daily use: Keep the inlet temperature of the system above 200°C ； The system pressure must be above 4.5 MPa ； The pipeline must be thoroughly cleaned; no impurities such as iron shavings should remain ； Control the operating conditions properly ； Choose a good catalyst ； After parking, the system should be purged with nitrogen ； Enhance the purification of gases entering the system, and use oil-free lubrication equipment, etc.
We’ve only been driving for half a month, and we haven’t noticed any wax buildup. The main requirement is to keep the inlet temperature of the synthesis tower at least at 190°C; in fact, it is also necessary to control the modulus properly. The selectivity of the catalyst should also have an impact, and the composition of the feed gas certainly needs to be taken into account as well
The modulus refers to the hydrogen-to-carbon ratio, right! This concept doesn’t seem to be used very often!
Paraffin: It is a mixture of long-chain alkanes and belongs to the category of saturated fatty hydrocarbons; its molecular formula is CnH2n+2. Its specific gravity ranges from 0.786 to 0.800, and its melting point is above 36.8 degrees Celsius (as it is a mixture, it does not have a fixed melting point). 1. Catalysts with high activity and selectivity should be used ； 2. Avoid feeding material when the temperature at the inlet of the synthesis tower is below 195°C ； 3. The outlet temperature of the synthesis tower should preferably be maintained above 225°C ； 4. Reduce the number of start-up and shutdown cycles ； 5. When loading the catalyst, be sure to clean the interior of the synthesis tower of any rust (including transition metal ions such as iron, cobalt, and nickel) in order to reduce the likelihood of carbonyl iron formation and minimize the residue of alkali metal salts ； 6. When parking, it is best to keep the synthesis loop in circulation for 2 hours to consume as much of effective gases such as CO as possible, and only then stop the circulation and proceed with N2 purging ； 7. Reduce the proportion of CO in the inlet gas to the synthesis tower, maintain a high H/C ratio appropriately, and keep the levels of CO2 and inert gases at slightly elevated values ； 8. Maintain stable bed temperature to prevent localized overheating or undercooling ； 9. If the compressor uses an oil seal, it is also necessary to prevent oil from leaking into the process system ； 10. It is advisable to install a gas purification filter at the inlet of the synthesis tower to remove harmful impurities ； 11. Maintain a relatively high air velocity and a relatively low pressure ； 12. If the cooling efficiency of the methanol water cooler has deteriorated due to paraffin accumulation, the tropical method or shutting down the system for wax removal can be employed. (Weighing pros and cons)
Reply to 4# caizongxu: “The system pressure should be above 4.5 MPa.” I wonder where Brother Lou came across such a statement? A friend of mine has a device that uses coal as raw material at 3.7 MPA, but no wax formation was observed
Controlling the inlet temperature of the synthesis tower is crucial; an inert gas is used to raise the temperature to 195 degrees before feeding the gas in.