Interphase Mass Transfer

As diffusion of any substance within a single phase can be studied without considering the equilibrium characteristics of a particular operation but when two insoluble phases are brought into contact which leads to the transfer of constituent substances between the phases.

For example let us consider gas-liquid system of ammonia-air mixture, as a note ammonia is soluble in water so if a fixed amount of liquid water is placed in a closed container together with a fixed amount of ammonia and air mixture the complete setup is maintained at constant temperature and pressure. Now some of the ammonia molecules immediately start to transfer from the gas phase into the water (liquid phase) passing through the interfacial surface area which separates the two phases. A fraction of the ammonia molecules escapes back into the gas phase again at a rate proportional to their concentration in the liquid. As more ammonia dissolves in the liquids, which increase the concentration with in the liquid, the rate at which it enters the liquid accurately equals that at which it leaves. This mode of operation deals with the concept of inter phase mass transfer.

Consider diffusion of A through stagnant B. At 1 atm and 298 K, NA = 2.3x10--5 kmol / m2 s. If the pressure is increased to 10 atm (all other parameters including the driving force for mass transfer remain unchanged), NA will be 2.3x10-5 kmol/m2s. Penetration models will be most appropriate in describing the mass transfer process. Consider a case where mass transfer of a solute gas A takes place through stagnant B and the mass transfer coefficient based on mole-fraction when driving force is ky and the mass transfer coefficient ky’ applicable for equimolar counter diffusion is ky yBM = ky'.

In a gas absorption process; for this case L/mG = 1 this means driving force for mass transfer at the top of the column is equal to that at the bottom of the column and typical superficial gas velocity at the bottom in a countercurrent packed absorber is around 1 m/s. A careful examination of the solvent properties may give us a clue to the controlling resistance in a gas absorption process. During natural gas dehydration in a plate column triethylene glycol is used in order to reduce the water content of the gas so as to prevent formation of solid hydrates the liquid flow rate is much lower as compared to the gas flow rate. When plate column is fitted with bubble-cap trays and the liquid viscosity is high liquid-side resistance is likely to control the dehydration.

Many industries generate liquid streams containing 0.01 to 2 wt % NH3 nitrogen which have an adverse effect when discharged into the environment. Therefore, NH3 should be removed from aqueous waste streams by, say, stripping so solution pH should be maintained 11 to 12. Liquid flow rate has a significant effect on overall (KGa) value in gas absorption in a packed tower. Under otherwise uniform conditions, a doubling of the liquid irrigation rate typically will increase the overall (KGa) value by 23 %.

A gas absorption operation taking place in a packed column the operation is gas-film controlled for a fixed packing height, if the gas flow rate is increased, the solute removal efficiency will decrease slightly and total liquid hold-up in a packed column is the sum of static hold-up and operating liquid hold-up. Static hold-up depend based on the contact angle between the packing surface and the liquid. With increasing liquid viscosity, liquid hold-up in a packed column increases. At a constant liquid hold-up, if the liquid density is lowered (all other parameters remaining unchanged), gas pressure drop increases.

When gas and liquid flow downward in the same direction (cocurrent operation) the same liquid rate, if the gas flow rate is increased, total liquid hold-up will decrease the capacity of a packed column of a given diameter will be higher for cocurrent operation. As compared to a liquid-film controlled operation, the value of (KGa) for a gas-film controlled operation will be much higher. In the layout plan for a vacuum distillation unit operating at 60 mm Hg supported by a barometric condenser the appropriate place for location of the vacuum drum for collecting the distillate will be 10m above ground.

The presence of foam in a packed bed causes a marked increase in pressure drop. Let air-water contacting packed column operating at atmospheric pressure at uniform conditions, if water is replaced by an organic liquid for which the surface tension is much lower, the operating hold-up will remain practically unchanged. A absorption operation where CO2 is being absorbed in a 4% aqueous NaOH solution in a packed tower. The process is liquid-phase mass transfer controlled for such a process, the overall gas-phase mass transfer coefficient (KGa) will be mainly dependent on liquid flow rate dependent on both gas and liquid flow rates to almost the same extent. In a certain mass transfer experiment, pure acetone evaporates into air the process is gas-film controlled. The liquid hold-up below the loading region in a packed column is primarily a function of liquid flow rate independent of liquid flow rate a strong function of gas flow rate a function of both gas and liquid flow rates at the theoretical plate of a plate column the gas leaving the plate is in equilibrium with the liquid leaving the plate.

Plastic packing are extensively used in scrubbers as they have light weight and cheap, they are resistant to mechanical damage most of organic compounds, acids and alkalis. Absorber equipments are usually designed for a gas pressure drop between 0.1 to 0.4 inch H2O per ft of packed depth because there is practically no restriction on this parameter for rational design of a packed absorber. For systems that tend to foam moderately, the design pressure drop at the point of greatest loading should be a maximum of 0.25 inch H2O / ft of packed depth.

Distribution coefficient (m) for extraction 

m = solute concentration in extract / solute concentration in raffinate. For m > 1, major resistance to mass transfer will lie in raffinate phase. At the plait point, the distribution coefficient, m, is one. Consider an extraction system of H2S present in LPG by aqueous ethanolamine solution the major resistance to mass transfer in this system will be in organic phase.