Is it better to connect the synthesis towers in parallel or in series?
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Recently, a 300,000-ton methanol plant wants to install a methanol synthesis tower. Should the towers be connected in parallel or in series? Current status: 100,000 units of fresh gas, 660,000 units of gas entering the tower ； Carbon monoxide in fresh gas: 30%, carbon monoxide in gas entering the tower: 26%, carbon monoxide in gas exiting the tower: 25% ； 1,152 tons of crude methanol produced (with 1.5% water content) ； Inlet temperature: 225°C, outlet temperature: 254°C℃ ； 34.5 m3 of catalyst was loaded.
When examining strings and parallel arrangements, focus is mainly on the composition of fresh gas and the original tower design; furthermore, it is necessary to consider the definition of power. When the inertia is low and the performance is good, parallel connection is generally suitable; when the performance is average but the inertia is high, series connection is preferred. For towers of the original type, either water-cooled or air-cooled configurations with series or parallel connections can be used. If it is an air-cooled tower of the original type, series connection is the best option, with a larger water-cooled synthesis unit installed in front.
You should already have a synthesis tower in operation; if you want to add another one, it must be connected in series. Connecting them in parallel would result in waste of the catalysts used in the originally designed synthesis tower (as this would cause a significant deviation from the designed catalyst load), and the outlet temperatures of the two towers would affect each other greatly, especially when the load is increased or decreased. In series, it is further divided into one methanol synthesis system connected to the main circulation gas pipeline at the outlet of the methanol separator, and another system connected to the methanol off-gas stream. The overall methanol yield is higher in the parallel configuration; it is important that there be differences in the choice of catalysts, as this can help to control temperature and quality. If a system is installed for treating the off-gases, it is advisable to pair it with a compressor for carrying out high-pressure methanol synthesis decarburization processes. If the methanol off-gases are to be used for producing synthetic ammonia or extracting hydrogen, it is best to install another system on these off-gases, which would function as a high-pressure methanation decarburization unit.
From the perspective of catalyst utilization, parallel operation is of course better.
Series connection is equivalent to increasing the height of the catalyst bed, while parallel connection is equivalent to increasing its width; there are already successful examples of parallel connections in China.
Our facility’s large units use parallel and series connections: series for fresh gas, and parallel for recycled gas.