Dosage control systems

Controlled release dosage forms typically have one of three different dissolution profiles square root of time or matrix diffusion, zero-order delivery as for erosional dosage forms or osmotic pumps, and zero-order delivery with depletion of the driving force as for a membrane-controlled system. For many controlled release dosage forms, zero-order release may be the holy grail. ... 

Kim, C.-J., Osmotically controlled systems, in Controlled Release Dosage Form Design, C.-J. Kim, ed. Lancaster, PA 2000, Technomic Publishing Company, 229-246. 

Figure 8.18 Consistent dosage control of antisealant based on the 3D TRASAR system. Note the relatively narrow range of antisealant dosages resulting in few underfed or overfed episodes.

Plants with system control and data acquisition (SCADA) systems may adjust chemical dosage automatically. The SCADA system communicates the feed setting to the feed equipment to achieve the needed chemical dosage. These systems also offer manual feed equipment control to achieve needed dosage changes. In either case, the dosage is determined first, and then the equipment is adjusted to dehver the correct amount of chemical to match the volume of water being treated (fiow). 

All chemical feed systems (gas, liquid, dry) have several components in common (Figure 2-1). All chemicals must be conveyed (delivered) to the process water flow, dispersed into the water, and their dosage controlled consistently and accurately. 

Gravity solution feed systems usually drop the chemical into the process stream from a single pipe or a perforated pipe distributor. The chemical solution is normally prepared from dry chemical or diluted hquid chemical (Figure 2-2). Gases are not often conveyed to the point of application by gravity. Where this is practiced, the gas is first dissolved in water to create a solution. This method is not common because dosage control of the solution is difficult. 

Chlorine dioxide dosage control is often based on the residual measurement. Analyzers measure chlorine dioxide in the treated water and feed a control signal back to the generator if adjustment is needed. Some systems instead have analyzers on the generator effluent. In this case, the chlorine dioxide concentration produced by the generator is used to adjust the feed parameters. 

The concentration of chlorine dioxide in the generator effluent is measured in some systems and is used as a dosage control. This value is calculated from the required chlorine dioxide production (calculators c4-12 and c4-13) and the water flow rate through the generator (for liquid sodium chlorite systems). 

The calculations for the ClOg production (consumption) can be substituted into calculators c4-12 and c4-13. The resultant calculators, c4-18 and c4-19, determine the concentration in the generator effluent that matches a specific water flow and dosage. Many systems use this value as a control, measuring the concentration of the generator effluent continuously. 

Dry chemical solution systems use dry chemical feeders to meter the chemical. The solution part of the system is used to predissolve the chemical or to help disperse the chemical more efficiently into the process water stream. The chemical is measured by the dry chemical feeder. The flow of water through the solution part of the system does not affect the amount fed because the entire chemical from the dry feeder is delivered to the application point. The flow of solution water and the size of the dissolving tank do, however, affect the time needed to deliver the chemical from the feeder to the application point. This delay can be problematic for control systems because changes in dosage are not instantaneous. It may take many minutes or hours to see the result of a chemical change. In addition, insoluble dry chemicals, such as activated carbon, may develop clogs in long pipelines from the solution tank to injection point. 

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