Scrubbing with liquid methane

In some cases, a second intermediate liquid methane scrubbing unit is added, at 0.3. 10 Pa absolute, capable of yielding 99 per cent pure CO directly at the top of the 

A refrigeration cycle with carbon monoxide, operaong between 0.2 and 1.7.10 Pa absolute, contributes the refrigeration required for the installation. It includes the expansion of a large fraction of the cycle gases in a cryogenic turbine. 

For some uses of by-product hydrogen, such as ammonia manufacture in particular, it is essential to avoid the presence ofexcessive amounts of residual methane. Thb is replaced by one of the synthesis reactants, with scrubbing by liquid nitrogen (see Section 1.3.1.1). 

Among the various other industrial methods, particuiariy chemical for produdng re carbon monoxide, is the process recently developed by Mitsubishi Gas ChemicaL It is based on the two-step conversion of methanol to CO by the following reaction mechanism  

The methanol is dehydrogenated in the gas phase around I90. at atmospheric pressure, in the presence of a copper-based catalyst on a support, promoted by other metals such as Zr, Zn. M etc 

This type of unit features the main steps encountered in partial condensation. The available gas mixture, previously dried on molecular sieves, is cooled to around — 120 C by heat exchange in a series of piaicexchangers operating in countercurrent flow on the cold purified products, and is then introduced at the bottom of a tray column with a downflow of liquid methane. The operation takes place at a pressure of abont 1.6.10 Pa absolute. It yields hydrogen with a purity better than 98.5 per cent volume, and which contains less than 10 ppm of CO. After expansion and partial vaporization, the extract feeds a distillation colnmn operating at 0.2. 10 Pa absolute, which separates the carbon monoxide at the top with a yield of at least 80 per cent at a purity over 99 per cent volume, and liquid methane at the bottom. This methane provides reflux to the scrubbing column and partly to the feed cooling uuiL 

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This operation (Fig. 1.15), which is specific to the use of hydrogen produced for ammonia synthesis, is similar in principle to the one used to separate carbon monoxide by scrubbing with liquid methane (see Section 1.2.3.2). Its value is therefore twofold ... 

In the former case, which is discussed in this study, only partial condensation is performed industrially. In the latter, scrubbing can piso be carried out with liquid methane or nitrogen (see Section 1.2). However, this means that the residual hydrogen gas is polluted by one or the other of these two compounds, and this could restrict its field of application or, on the contrary, facilitate its use. Hence the use of Hquid nitrogen offers an interesting solution for ammonia synthesis from an effluent produced by partial oxidation of organic feedstocks (see Section 1.3.1.1) in so far as, introduced with hydrogen at the rate of 2 to 8 per cent volume, this represents the use of one of the reactants for the production of ammonia, whereas methane, on the contrary, is a diluent... 

The carbon dioxide is removed the gas is dried and scrubbed with liquid nitrogen to remove residual carbon monoxide, argon, and methane and then it is compressed and sent to ammonia synthesis. The removal of argon and methane improves the utilization of hydrogen in the synthesis loop. 

Among the adaptations made to the liquid methane scrubbing scheme is the replacement of the carbon monoxide refrigeration cycle by a similar system operating with nitrogen. 

One kmol/s of a gas consisting of 75 mol% methane and 25% n-pentane at 300 K and 1 atm is to be scrubbed with 2 kmol/s of a nonvolatile paraffin oil entering the absorber free of pentane at 308 K. Estimate the number of ideal trays for adiabatic absorption of 98.6% of the pentane. Neglect the solubility of methane in the oil, and assume operation to be at constant pressure. The pentane forms ideal solutions with the paraffin oil. The average molecular weight of the oil is 200 and the heat capacity is 1.884 kJ/kg-K. The heat capacity of methane over the range of temperatures to be encountered is 35.6 kJ/kmol-K for liquid pentane, is 177.5 kJ/kmol-K for pentane vapor, is 119.8 kJ/kmol-K. The latent heat of vaporization of n-pentane at 273 K is 27.82 MJ/kmol (Treybal, 1980). 

The reactor effiuent passes into a feed-product heat exchanger, where it is partially condensed. After washing with dilute caustic soda to neutralize traces of phosphoric acid, it passes into a second exchanger and on to a high-pressure separator tO give a liquid and a vapor stream. The condensate goes to purification and the vapor to recycle. The vapor is cooled by the recyde-gas cooler and scrubbed with water to remove alcohol. The build-up of impurities like methane and ethane is controlled at this point by venting a small stream of the recycle gas. 

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