Changing the Pressure and Temperature

The optimum conditions for catalytic methanation involve moderately elevated temperatures and normal to moderately high pressures. 

Let us see whether we can gain insight into why these might be the optimum conditions for the reaction. 

A pressure change obtained by changing the volume can affect the yield of product in a gaseous reaction if the reaction involves a change in total moles of gas. The methanation reaction, CO + 3H2 CH4 + H2O, is an example of a change in moles of gas. When the reaction goes in the forward direction, four moles of reactant gas (CO + 3H2) become two moles of product gas (CH4 + H2O). 

In this way,the total number of molecules is reduced, which reduces the initial pressure 

You find the same result by looking at the reaction quotient Q. Let [CO], [H2], [CH4], and [H2O] be the molar concentrations at equilibrium for the methanation reaction. When the volume of an equiUbrinm mixture is halved, the partial pressures and therefore the concentrations are doubled. You obtain the reaction quotient at that moment by replacing each equilibrium concentration by double its value. 

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This definition cannot be applied directly to mixtures, as phase equilibria of mixtures can be very complex. Nevertheless, the term supercritical is widely accepted because of its practicable use in certain applications [6]. Some properties of SCFs can be simply tuned by changing the pressure and temperature. In particular, density and viscosity change drastically under conditions close to the critical point. It is well known that the density-dependent properties of an SCF (e.g., solubihty, diffusivity, viscosity, and heat capacity) can be manipulated by relatively small changes in temperature and pressure (Sect. 2.1). 

In reactions involving only liquid components without phase change, the pressure and temperature variation do not have any significant effect on the volume of the reaction mixture, and at the same time, the expansion factor is always zero. Thus, V= Vi in batch or Q = Qt in continuous-flow systems and eqs. (3.96) and (3.97) are applicable. 

A supercritical fluid (SCF) is any substance above its ailical temperature and critical pressure (P ). In general terms, SCFs have properties like density, viscosity and diffusivity between those of a gas and a liquid. One of the most important properties of those is that by changing the pressure and temperature of the fluid, its solvent power can be tuned. In Table 8.1, the critical properties of several supercritical fluids are presented [5]. 

The catalyst-product separation was also studied. By changing the pressure and temperature after the reaction they created a phase separation from which the aldehydes could be collected as a colorless liquid (containing less than 1 ppm rhodium). The catalysts of ligands 13-15 were used in 5 consecutive runs without significant changes in the catalytic performance. 

Supercritical fluids have been used as mobile phases in liquid chromatography for about 30 years, and this modality is known as supercritical fluid chromatography (SFC). When both the temperature and the pressure of the system exceed the critical values - that is, the critical temperature T ) and the critical pressure (Pc)-the fluid is considered to be critical in nature. These fluids have a mixture of the properties of liquids and gases. Supercritical fluids (SFs) are highly compressible like gases, and their density and viscosity can be maintained by changing the pressure and temperature conditions, as in the case of liquids. In chromatographic systems. 

The choice of conditions, including catalysts, can be very irr5)ortant to the success of a reaction. Removing a product from the reaction mixture, for example, shifts the equilibrium composition to give more product. Changing the pressure and temperature can also affect the product yield. Le Chatelier s principle is useful in predicting the effect of such changes. 

From this equation, it appears that the better way to significantly increase the overall energy efficiency is to increase se (the potential efficiency) and 6p (the faradic efficiency), since 8rev. (the reversible efficiency) is given by the thermodynamics (it can be slightly increased by changing the pressure and temperature operating conditions). 

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