Water as an Absorbent for Gas Impurities

As mentioned earlier, it is important to pay particular attention to the purity of the ILs and gases in the solubility measurements, as impurities may have a large effect on the measured solubility. Water, for example, has a profound effect on many of the physical properties of ILs [12]. This is important as many ILs are extremely hygroscopic [3,12], [BMIMJIPFe] can absorb up to 0.16 mole fraction water from the atmosphere [3], In general, water in the IL causes a decrease in viscosity, while increases in viscosity have been attributed to halide content. Similarly, vrater and halide content both cause a decrease in density [3,12]. Thus, impurities can have a profound affect on the transport properties of the IL. As an example, gas solubility in gravimetric solubility measurements is affected by uncertainty in the IL density as propagated by the buoyancy correction. Particular attention should be paid to IL purity and all resulting impurities should be reported for the solubility measurements. 

Referring to Figure 4-20, which represents a typical Phosam plant, the coke-oven gas (after cooling and cleaning to remove entrained solids, water, and tar) enters the bottom of the absorber where it is contacted countercurrently with an aqueous solution of ammonium phosphate. About 99% of the ammonia is removed from the gas stream in the absorber. The ammonia-free product gas is suitable as feed for a variety of processes to remove hydrogen sulfide and other impurities. The ammonia-rich solution is drawn off the bottom of the absorber for regeneration. 

As the absorbent circulates in the SO2 absorber, it accumulates impurities that need to be removed. These include fine ash particles, heat-stable salts, and other soluble compounds. Filters are used to remove the fly ash particles. Sulfates in the scrubbing solution, which result from SO3 and O2 in the gas stream, as well as other heat stable salts, are removed from a slipstream of lean absorbent using a proprietary process. The waste stream from the slipstream treating process is an absorbent-free, slightly alkaline, aqueous salt solution. For most applications, potable water should be suitable for make-up to the process, and general service water for make-up to the prescrubbo (Kirby. 1992). 

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