Contaminants removal

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

Process condensate from reforming operations is commonly treated by steam stripping. The stripper is operated at a sufficiently high pressure to allow the overhead stripping steam to be used as part of the reformer steam requirement (71). Contaminants removed from the process condensate are reformed to extinction, so disposal to the environment is thereby avoided. This system not only reduces atmospheric emissions, but contributes to the overall efficiency of the process by recovering condensate suitable for boiler feedwater make-up because the process is a net water consumer. 

A U.S. EPA study (41) showed that soil vapor extraction (SVE) is an effective treatment for removing volatile contaminants from the vadose zone. Sandy soils are more effectively treated than clay or soils with higher organic content because higher air flows are possible in sand and clays—organic soils tend to adsorb or retain more contaminants. Removal of volatiles is rapid in the initial phase of treatment and thereafter decreases rapidly thereafter-an important consideration in the design of air emissions control over the life of the project. 

Tank settling as a means of contaminant removal is not very efficient with fuels having the viscosity of kerosene. It is common practice to design tanks with cone-down drains and floating suctions to facilitate water and solids removal. 

For organic contaminant removal from surface water packed-tower aeration, granular activated carbon (GAC), powdered activated carbon (PAC), diffused aeration, advanced oxidation processes, and reverse osmosis (RO). 

For inorganic contaminants removal membranes, ion exchange, activated alumina, and GAC. 

Chemical pretreatment is often used to improve the performance of contaminant removal. The use of chemical flocculants is based on system efficiency, the specific DAF application and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeClj or FeSO or aluminum, such as AISO. Organic and inorganic polymers (cationic or anionic) are generally used to enhance the DAF process. 

Swimming pools Ozone injection for removal of organic contaminants Removal of residual ozone and control of chloramine levels... 

Ventilation efficiency has traditionally been defined as the ratio between contaminant concentration in the occupied spaces and the concentration in the exhaust air. Sandberg and Skaret differentiate between the terms air change efficiency and contaminant removal effectiveness. Air change efficiency is a measure of how effectively the air present in a room is replaced by fresh air from the ventilation system, whereas contaminant removal effectiveness is a measure of how quickly an air-borne contaminant is removed from the room. A third similar criterion that is used is contaminant removal efficiency. ... 

The contaminant removal effectiveness can be used when emission data for contaminant sources are available. 

As mentioned above, the traditional definition of ventilation efficiency or, in approved terms, contaminant removal effectiveness, is the ratio between contaminant concentration in the exhaust air and the concentration at a point in the occupied space, i.e.,... 

In other words, this contaminant removal effectiveness is a measure of how much cleaner the air is in the occupied spaces than in the exhaust. See Fig. 8.8. 

When detailed information on heat and contaminant sources is available, assessment of design is improved by evaluating the effectiveness of contaminant removal achieved by space ventilation. The set of contaminant removal effectiveness indices in Table 8.5 is given in accordance with contemporary use of indices. 

Application of contaminant removal effectiveness indices is relatively simple for scenarios with one or a few dominant contaminants being released. That is often the case in industrial mails. Where there are many polluting substances to consider the contaminant removal efficiency should ideally be evaluated for each one. Consequently, applications for regular indoor climate— for example, in a restaurant—are limited, except when addressing specific pollutants like smoking and ctxrking hunes. 

The contaminant removal efficiency can be derived from the coniaminaiu removal effectiveness as follows ... 

Advantages include the whttle flow pattern can be controlled, areas upstream of sources can be kept clean, and high contaminant removal and tern... 

A similar temperature and contaminant distribution throughout the room is reached with stratification as with a piston. The driving forces of the two strategies are, however, completely different and the distribution of parameters is in practice different. Typical schemes for the vertical distribution of temperature and contaminants are presented in Fig. 8.11. While in the piston strateg) the uniform flow pattern is created by the supply air, in stratification it is caused only by the density differences inside the room, i.e., the room airflows are controlled by the buoyancy forces. As a result, the contaminant removal and temperature effectiveness are more modest than with the piston air conditioning strategy. 

Advantages include low concentration in the ventilated zone can be achieved and relatively high contaminant removal and temperature effectiveness. However, the srratificacion strategy is sensitive to disturbances and stagnant areas with high... 

The zoning method offers better contaminant removal and thermal effectiveness than with mixing, limited control of the flow patterns in the ventilated zone, and the ability to avoid stagnant areas with high local concentrations in the ventilated zone. However, partial mixing of contaminants in the ventilated zone decreases its effectiveness. 

The contaminant removal and temperature effectiveness in the mixing strategy are equal to 1. In practical installations incomplete mixing in the room and unfavorable temperature gradient and location of the exhaust openings in relation to air supply may, however, cause short-circuiting of the supply air into the exhaust openings and the efficiency may remain below 1. 

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