Front, contaminant

Process Concepts. Hybrid systems involving gas-phase adsorption coupled with catalytic processes and with other separations processes (especially distillation and membrane systems) will be developed to take advantage of the unique features of each. The roles of adsorption systems will be to efficiently achieve very high degrees of purification to lower fouUng contaminant concentrations to very low levels in front of membrane and other separations processes or to provide unique separations of azeotropes, close-boiling isomers, and temperature-sensitive or reactive compounds. 

Because of the railroad-car shortage that has persisted for many years, boxcars are often used for bulk materials. Lined with suitable materials to prevent contamination and with special bulkheads at each door, these cars are acceptable substitutes for covered hopper cars even though unloading is more difficult. Vacuum conveying wands are used to pick up the material, as are front-end-loader-type vehicles. 

For many operations, workers are assigned to assist in conducting decontamination of workers wearing Level A or B PPE during the decontamination process. Decontamination workers stationed at the front end of the decontamination line may require more protection from chemical and radiological contaminants than decontamination workers assigned to the latter stages of decontamination. 

FIGURE 10.3 Possible air and contaminant movements around person in front of opening (or with supply air coming from behind). Seen from above. 

Tools are normally close to the opening. One way to diminish tlie disturbances from a tool is to shape the opening around that part of the tool where contaminants are generated. Another is to size the opening specifically foi the source and place it very close to the source and away from disturbances front the tool. 

Rim exhausts are slot hoods located on or around the edge of a source such as an open surface tank. Flanges may be added to decrease the airflow from behind the slot (uncontaminated air) and therefore increase the airflow from in front of the slot (contaminated air). The plenum downstream of the slot, if located above the slot, may act as a flange. Flanges may also be added to the sides of the source (tank) away from the slot hood. These flanges also act to increase the flow of contaminated air into the tank. Tank flanges, however, may interfere with process activities by limiting access to the tank. 

Another design method uses capture efficiency. There are fewer models for capture efficiency available and none that have been validated over a wide range of conditions. Conroy and Ellenbecker - developed a semi-empirical capture efficiency for flanged slot hoods and point and area sources of contaminant. The point source model uses potential flow theory to describe the flow field in front of a flanged elliptical opening and an empirical factor to describe the turbulent diffusion of contaminant around streamlines. 

Disturbances can be caused by all processes that take place in or that pass through the remaining openings of the enclosure. If the location of the worker is in the opening, between the contaminant source and the surrounding room, recirculation areas in front of the worker can be created that could carry contaminants into the breathing area. 

The operation of flow dampers can cause pressure fluctuations in the ductwork system. Measurements by Melin indicate that pressure oscillations in an exhaust system can cause instabilities in the airflow through a fume cupboard sufficient to give rise to outward leakage of contamination, especially when a person stands in front of the cupboard. 

Class U The Class U (Types A, Bl, B2, and biological safety cabinets provide personnel, environmental, and product protection. Airflow is drawn around the operator, through the hood opening and into the front grill of the cabinet, which provides personnel protection, in addition, the downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination along the work surface of the cabinet. Because cabinet air has passed through the exhaust HEPA filter, it... 

Class HA in a Class IIA BSC, an internal blower (Fig. 10.9,St draws sui-ficient room air into the front grill to maintain a minimum calculated measured average velocity of at least 0.37 m s at the opening of the cabinet. The supply air flows through a HEPA filter and provides particulate-free air to the work surface. Laminar airflow reduces turbulence m the work zone and niim-mizes the potential for cross-contamination. 

One common combination is a jet and an exhaust hood. The jet can be circular or plane and situated around or in front of a (hot) contaminant source. The intention is to direct the contaminant into a basic opening or a receptor hood. Mostly these jets are directed upward into hoods, but may be directed sideways or downward. There is a difference to jets covering openings. When directed into a hood the jet is intended to help the natural flow into the hood and not to act as a shield, even though it sometimes also has this function. Figure 10.105 illustrates two principal ways that air jets could be used to direct contaminants into a hood (see also Section 10.4.5). 

Different protection factors have been defined. One method is to define it as the ratio of the concentration of a contaminant in the exhaust duct (CJ to the concentration in the breathing zone (C[,) of a person standing in front of the enclosure, for example, a laboratory fume hood ... 

Breakthrough A condition that exists when the backup section of a sorbent tube is found to contain 20-25 percent of the total amount of contaminant captured in the front section. 

The greatest risk of contamination of a pharmaceutical product comes from its immediate environment. Additional protection from particulate and microbial contamination is therefore essential in both the filling area of the clean room and in the aseptic unit. This can be provided by a protective work station supplied with a unidirectional flow of filtered sterile air. Such a facility is known as a laminar airflow unit in which the displacement of air is either horizontal (i.e. from back to front) or vertical (i.e. from top to bottom) with a minimum homogenous airflow rate of 0.45 ms" at the working position. Thus, airborne contamination is not added to the work space and any generated by manipulations within that area is swept away by the laminar air currents. 

To recondition the pots, the potliner is dug out and discarded Prior to discovery of the Spokane aquifer contamination, the procedure had been to remove the pot to an outdoor concrete slab where the pot was filled with water and allowed to soak for a few days to fracture and soften the cathode The contaminated water was presumably reused for soaking and not discharged to the industrial waste treatment system because of the cyanide content The pots were Jack-hammered and the potliner dumped on the slab The potliner was transfered by a front-end loader to an unprotected pile next to the slab ... 

U.S. EPA has shown that 90% of process water can be recycled to the front end of the system for slurry preparation, and the rest must be treated on site or transported to an off-site facility.80 During the aerobic process, some contaminated air may be formed and emitted from the reactor. Depending on the air characteristics, a compatible air pollution control device may be used, such as activated carbon. Slurry biodegradation has been shown to be successful in treating soils contaminated with soluble organics, PAHs, and petroleum waste. The process has been most effective with contaminant concentrations ranging from 2500 mg/kg to 250,000 mg/kg. 

Shoes became contaminated with a chlorate weed-killer solution which dried out in wear. A welding spark later fell into the shoe and the front was blown off. 

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