For many years, counter-flow scrubber methods have been used for the lion's share of the work in industries such as phosphate fertilizer and semiconductor chemicals manufacturing. Now these industries are exploring the use of cross-flow scrubber design, which offers consistently high efficiency and low operating costs. In addition, the unit's horizontal orientation makes maintenance easier than typical tower scrubbers. For certain classes of unit operations, cross-flow is now being recognized as a strong alternative to conventional counterflow technology.
When to use cross-flow
Compounds formed in different ways affect air emissions upstream of the scrubber. For example, aerosols formed by the reaction of basic (ammonia) and acidic gases, such as HF, HC1 and HN03, commonly yield very fine particulate compounds, such as NH4C1 and NH4N03. These are known as 'reaction aerosols' and are typically submicron . Acid vapor entering the scrubber may condense with water vapor to form a 'condensation aerosol.' Other examples are given in Table 1.
Contaminants must be removed using the correct combination of absorption and phase-separation techniques. Critical factors for absorption processes affect many traits of the scrubber, such as the choice of packing material and the flow rates of the gas and liquid. The bed structure and specific surface area are also important. Therefore, calculations of the number of transfer units (NTU) and the height of the transfer tower (HTU) are vital. Other factors that affect the proposedprocess are the humidity and cooling, the degree of saturation, the distribution of the liquid and its drainage, as well as the physical, chemical and thermodynamic properties of the effluents.
In any scrubber application, there are several aspects of design that must be addressed. Beyond those of the process being performed and the equipment to be used, are such overall concerns as energy consumption, efficiency and maintenance. Cross-flow technology often allows these items to be dealt with directly.
Several problems often arise due to unstable operating conditions, rate changes with control fluctuations and process upsets due to mechanical, electrical or instrument failures. Typical ones include dust, entrainment, and fog or aerosol formation.
Scrubber efficiency and NTU
Whether the goal is fulfillment of emission standards, or meeting a performance guaranty, the correct calculation of efficiency of a scrubber is vital. In general, Efficiency = 100 (1-PT)
In these operations, it is preferable to deal with penetration (PT), because gas rates can change considerably in a
scrubbing system. Such changes are particularly significant when hot, relatively dry gas is saturated with water.