Nitrogen separation from methane

The separation of nitrogen from natural gas reHes on the differences between the boiling points of nitrogen (77.4 K) and methane (91.7 K) and involves the cryogenic distillation of a feed stream that has been preconditioned to very low levels of carbon dioxide, water vapor, and other constituents that would form soHds at the low processing temperatures. 

Table 4 summarizes commercial and precommercial gas separation appHcations (86,87). The first large-scale commercial appHcation of gas separation was the separation of hydrogen from nitrogen ia ammonia purge gas streams. This process, launched ia 1980 by Monsanto, was followed by a number of similar appHcations, such as hydrogen—methane separation ia refinery off-gases and hydrogen—carbon monoxide adjustment ia oxo-chemical synthetic plants. 

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

Recycle designs of this type are limited to applications in which the components of the gas mixture, if sufficiently concentrated, can be separated from the gas by some other technique. With organic vapors, condensation is often possible adsorption, chemical scrubbing or absorption can also be used. The process shown in Figure 8.20 is used to separate VOCs from nitrogen and air or to separate propane, butane, pentane and higher hydrocarbons from natural gas (methane). 

A demethanizer fractionation tower is frequently positioned first. This tower is often operated at 34 x 105 Pa, with temperatures low enough to obtain liquid methane. This tower is usually a tray-type column, although more recently packed towers have been introduced. The noncondensible gases (hydrogen, nitrogen, and carbon monoxide) and relatively pure methane can thus be separated from the C2 and higher hydrocarbons. 

Of course, some of the hydrogen comes also from the hydrocarbons themselves (methane has the highest content), and the carbon acts as a reducing agent for water and in some processes may also facilitate the separation of oxygen from nitrogen by formation of carbon dioxide, which can be removed by various operations. 

Nitrogen is to be separated from methane in a perfect-mixing membrane separator as specified below. Calculate the required membrane area, the methane permeance, and the product rates. 

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