Processes Combining Distillation with Desorption

Distillation can also be combined with desorption (or stripping). An example of such a process is the fractionation of nitric acid with the entrainer sulfuric acid (Geriche 1973). Nitric acid has a maximum azeotrope at a concentration of 37 mol% HNOj. Thus, a high boiling entrainer has to be used. 

Water is removed as overhead fraction in column C-1 (Fig. 11.4-4). The bottom fraction B with azeotropic concentration is fed into column C-2 to be mixed with the entrainer and further fractionation. Pure HNO3 is recovered as overhead product t 2. The bottom fraction lying close by the boundary distillation line is stripped with steam to remove residual HNO3. Thus, the boundary distillation line is crossed by the stripping step. The diluted sulfuric add B2 is fed into a single stage distillation unit for removal of most of the water. Nitric acid, when boiled, always forms some NO that colors the liquid. Therefore, NO is removed by further stripping with air to get a clear hquid. 

---

A suitable combination of two separation techniques like distillation and adsorption leads to a desired separation of a mixture which is either infeasible or expensive with a single technique. This combination of the different separation techniques is called hybrid separation. The techniques of hybridization are viewed as the techniques of process integration. Stankiewicz [60] presented the examples of extractive distillation, adsorptive distillation, membrane distillation, membrane absorption/stripping, and adsorptive membranes. A detailed discussion on hybrid separation processes involving distillation and one of the separation processes, namely, absorption, desorption, extraction, adsorption and membrane processes is available [61]. 

Many wastes can be effectively purified by activated carbon,1 but the cost is generally higher than with other methods. The use of carbon often can be justified, however, when the process involves the simultaneous recovery of useful ingredients. Depending on the nature of the adsorbed substances, they may be extracted from the carbon by steam distillation, by elution with a suitable solvent, or by a combination of both. The desorption restores some adsorptive power to the carbon, but the regeneration is seldom complete because many impurities adsorbed from waste liquors cannot be removed either by steam or by a solvent. 

A laser-induced acoustic desorption (LIAD) device combined with a chemical ionization source was employed for the analysis of crude oil distillates under atmospheric pressure. In general, LIAD, a matrix-free and laser-based approach, is usually performed under vacuum conditions. The desorption process in LIAD is induced by the action of a shockwave that is generated as a pulsed laser irradiated on the backside of a metal foil. As the energy is transferred from the metal foil to the sample, which is deposited on another side of the foil, it induces the desorption of analytes. By the interaction of the analyte with an ion cloud generated by a chemical ionization (Cl) process, analytes with a wide range of polarity are successfully ionized. Marshall et ah have combined an atmospheric pressure AP-LIAD/ Cl with a 9.4 T FT-ICR/MS to perform high resolution chemical analyses imder ambient conditions. It was demonstrated that not only polar but also non-polar compounds in the crude oil distillates could be successfully characterized by this AP-LIAD/Cl/FT-ICR/MS approach. 

---