Page 15 - Gas Purification 5E

Introduction 5

cient. For dew-point depressions between about 60" and 100°F, either type may prove more
economical based on the specific design requirements and local operating costs. For dew-
point depressions consistently above lOOT, solid desiccant processes are generally specified,
although the use of very highly concentmted glycol solutions for attaining water dew points as
low as -130°F is gaining favor. Solid desiccant processes are also preferred for very small
installations where operating simplicity is a critical factor. Gas dehydration by absorption in
liquids is covered in Chapter 11, and by adsorption using solid desiccants in Chapter 12.
Fewer options have been commercialized for the removal of sulfur dioxide and nitrogen
oxides than for hydrogen sulfide and carbon dioxide. The predominant process for flue gas
desulfurizaton (FGD) for large utility boiler applications is wet scrubbing with a lime or
limestone slurry. This process can provide greater than 90% sulfur dioxide removal. Where
lower removal efficiency can be tolerated, spray drying and dry injection processes have
been found to be more economical. Commercially proven processes for the removal of nitro-
gen oxides from the flue gas of large boiler plants are currently limited to selective catalytic
reduction (SCR) and thd reduction processes. The SCR process is the only one capable
of 90%-plus NO, removal efficiency. Further details on the selection of FGD and nitrogen
oxide removal processes are provided in Chapters 7 and 10.
Volatile organic compounds (VOCs) can be removed from gases by at least five types of
processes: thermal incineration, catalytic incineration, carbon adsorption, absorption in a liq
uid, and condensation. Preliminary guidelines for selection of these are given in Figure 1-1,
which is based on the data of McInnes et al. (1990). For typical VOC concentrations in the
range of 100 to 1,OOO ppmv, only thermal incineration cau provide 99% removal efficiency.
Of course, feed concentration and removal efficiency are not the only hctm to be consid-
ered. For example, if energy consumption is a significant factor, Catalytic incineration may be
preferable to thermal incineration because it operates at a lower temperature and therefore
requires less heat input. If chemical recovery as well as removal is required, a process other
than incineration must be specsed. A detailed discussion of factors to be considered in select-
ing the best VOC control strategy is given by Ruddy and Carrol(1993). Specific VOC
removal processes are described in several chapters, Chapter 12-ad~tion processes, Chap
ter 13-atalytic incineration and thermal incineration processes, Chapter 15-membrane
permeation processes, and Chapter 1Londensation and absqtidoxidation processes.

Thermal
Incineration 95% -99%
catalytic
Incineration
Carbon
Adsorption 50% 95% - 99%
Absorption 90%- 95% - 98%

Condensatton - 50% 90% - 95%
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