Separation by absorption

Another method to remove pollutants from air is to absorb the pollutants into a nonvolatile liquid such as oil. During absorption, pollutants move from the gas phase to the liquid phase. This movement is an example of mass transfer. To move between phases, the pollutants must cross the liquid-gas interface. Increasing the interfacial area increases the rate of mass transfer. An easy way to increase the interfacial area is to bubble the gas through the liquid. This concept is effective in delivering oxygen to water in fish tanks and it is effective in delivering benzene to oil. 

In other words, we will assume that the air/oil/benzene system reaches equilibrium in our absorber. But note - when mass transfer is complete there will still be benzene in the air. How much benzene That is what we will calculate in this section. 

we complete our absorber flowsheet. Of course, the oil should be recycled. Because the oil is nonvolatile, it is a simple matter to distill benzene vapor from the oil. 

And of course, one would want to consider adding a heat exchanger so the hot oil leaving the distiller could heat the oil/benzene mixture entering the distiller. 

We will assume that the bubbler is designed such that streams 2 and 4 are in thermodynamic equilibrium. Given a flow rate of polluted air, say 100 kg/min, what flow rate of oil is needed to reduce the benzene content of the air to an acceptable level As always, it is illustrative to begin with a qualitative analysis. We start with extremes - assume the air leaving the bubbler contains no benzene. Thus all the benzene that enters with the polluted air leaves with the oil. Assume that the flow rate of benzene is 1 kg/min in both streams 1 and 4 and that the equilibrium concentration of benzene in the oil is 1 kg benzene/100 kg oil. The flow rate of oil is therefore 100 kg/min. If the oil could hold twice as much benzene, the flow rate of oil would be half as large, and so on. We need to know the oil s capacity for absorbing benzene. 

The removal of one of more selected components from a mixture of gases by absorption into a suitable solvent (Mass Separating Agent, MSA) is the second major operation of chemical engineering after distillation. Absorption is based on interface mass transfer controlled largely by rates of diffusion. It is worth noting that absorption followed by a chemical reaction in the liquid phase is often used to get more removal of a solute from a gas mixture. 

The essential difference between distillation and absorption is that in the former, the vapour has to be introduced in each stage by partial vapourisation of the liquid, which is therefore at its boiling point, whereas in absorption the liquid is well below its boiling point. In distillation, there is diffusion of molecules in both directions, so that for an ideal system, equimolar counter-diffusion exists. In absorption, gas 

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In a typical process, the conversion of isobutene in the reactor stage is 97 per cent. The product is separated from the unreacted methanol and any C4 s by distillation. The essentially pure, liquid, MTBE leaves the base of the distillation column and is sent to storage. The methanol and C4 s leave the top of the column as vapour and pass to a column where the methanol is separated by absorption in water. The C4 s leave the top of the absorption column, saturated with water, and are used as a fuel gas. The methanol is separated from the water solvent by distillation and recycled to the reactor stage. The water, which leaves the base of the column, is... 

This reaction can be forced to effective complete conversion by first carrying out the reaction to approach equilibrium. The sulfur trioxide is then separated (by absorption). Removal of sulfur trioxide shifts the equilibrium, and further reaction of the remaining sulfur dioxide and oxygen allows effective complete conversion of the sulfur dioxide, Figure 6.6. 

Example 10.1 A hydrocarbon gas stream containing benzene vapor is to have the benzene separated by absorption in a... 

Figure 2. Flow diagram for methane separation by absorption/stripping method.

Number of transfer units Parameter that relates the change in concentration to the average driving force. It is a measure of the ease of separation by absorption. 

To be useful, this type of simulator must calculate the thermodynamic properties of multicomponent mixtures in both liquid and vapor phases while predicting bubble and dew points or partial vaporizations or condensations. Using this basic information, the simulator must then make calculations for other processes, such as gas cooling by expansion, gas compression, multiple flashes condensations, and separations by absorption... 

Ammonia may also be recovered from converter exit gases or from the gas separators by absorption into water when ammonia solutions are to be... 

Longer searches usually require a breakdown by special technical points one reference frequently contains information on two or three points. Each point can be designated with a number like 1, 2, or 3, which can be added on the cards as subjects indicate. For instance, in a search on separating low boiling olefins from hydrocarbon gases, point 1 may be separation by adsorption methods point 2, separation by absorption methods and point 3, separation by cold fractionation. The typist can copy all cards bearing number 1, then all bearing number 2, and so on. A breakdown by special interests results. 

Absorption is the process by which one or more components of a gas phase are removed by being solubilized (sorbed) into a liquid. The liquid phase is the mass-separating agent (MSA) it allows separation by absorption to occur. Stripping, or desorption, is the opposite of absorption. A reasonably volatile contaminant is moved from a liquid to a gas phase. In either case the contaminant to be removed is called the solute and the MSA which does... 

To obtain a more complete conversion of one fissionable isotope to another, it has been proposed to utilize a substantially pure fissionable isotope in solution or dispersion in a liquid moderator for the neutronic reaction, and then form the new fissionable isotope separately by absorption of leakage neutrons produced as a by-product of the reaction. 

The following example is a higly idealized example of absorption with solvent regeneration. Gas mixture of components C, and Cj is separated by absorption of C2 in a solvent where Cj is insoluble, and which can be regenerated by desorption of C2 under reduced pressure and heat supply. The solvent itself is of negligible equilibrium vapour pressure (nonvolatile) at the desorption temperature. The gas mixture (C, C2) is ideal. 

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