Composition processes, whether pH or "plon" (such as pCl and pAg), should be recognized as having two distinct aspects, one chemical and the other physical.
Several physical or process design considerations are associated with composition (pH or pIon) control applications. The most important is the primary device used to mix the reagent with the process stream. This can be as simple as reagent addition upstream of a pump or in-line mixer with a downstream measurement point or as complex as two mixed reaction vessels followed by an attenuation vessel.
Where reaction vessels are required, design decisions must be made to determine (1) size and number, (2) baffling, (3) agitation (how much and what type), (4) measurement probe location(s), and (5) reagent addition point location.
The other and equally important physical aspect of pH control is the design of the reagent delivery system. Both of these aspects will be discussed here.
The ease or difficulty of most industrial control applications is closely related to a property of the process referred to as dead time. Analogous terms, such as "transport time," "pure delay," and "distance velocity lag," describe the same effect. Dead time is defined as the interval between the introduction of an input disturbance to a process and when a measuring device first corrects for the effect of that disturbance. Qualitatively, the relationship between dead time and controllability is simple: The more dead time, the more difficult the problem of control. The presence of dead time in pH or pIon processes is extremely detrimental to controllability. The major reason is the severe sensitivity of the measurement of interest at the control point. One of the major goals of system design is to eliminate the dead time or to reduce it to an absolute minimum.
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