Percent Concentration Control

Application

There are numerous industrial applications where measurements and/or control of a specific chemical strength of the process is critical for optimizing the production of the end product. These specific concentrations are obtained by mixing a full-strength solution with water to achieve the desired percent concentration.

Process

The desired chemical concentration is achieved using a two-stage mixing procedure. During the first stage, the flow ratio control unit on the missing tank is set to provide (x) gallon per minute of the full-strength solution and (y) gallons per minute of water. These values are adjusted to produce a concentration value that is slightly weaker than the desired value. This ratio control must include alarm capabilities to indicate "low flow" conditions for both the full strength solution and the water in order to prevent wasted chemicals or hazardous situations.

At the second stage, a conductivity sensor and analyzer function as a "trim control". This combination adds small amounts of full-strength solution to the mixing tank to produce the exact concentration desired.

For example, to produce a 4% caustic solution from a large bulk caustic supply at 50%, the flow ratio controller is adjusted to make a 3% solution and the conductivity information is used to add additional caustic to achieve the 4% concentration.

Conductivity is a very reliable index of the concentration for most acid and base (caustic) solutions. Figure 1 shows the correlation between conductivity and concentration for four common solutions.

Challenges

For most solutions, there is a peak conductivity value. Before this peak value is reached, conductivity correlates positively with concentration; after the peak, it correlates negatively. So, if the concentration range passes through the peak for that chemical the conductivity value (except the peak value) represents two different concentration values. Therefore, it is mandatory that any application near the peak of a particular solution be carefully controlled.

Solution

Conductivity measurement is a reliable indicator of the concentration of most acid or base solutions. In determining the proper loop components for a particular application, the material of construction will be of primary concern. A chemical resistance chart should be consulted (see Table 1), or an application data sheet completed and sent to the factory in order to insure an installation that will be suited for the intended application.

Key Advantage & Recommendations

One sensor that will measure the entire conductivity range. Since the sensor is non-contacting it is not affected by chemical attack and requires less overall maintenance.

Conductivity Measurement Systems

Process Liquid Analyzer:
•    2-wire FLEXA Inductive Conductivity Analyzer
•    4-wire FLXA402 Inductive Conductivity Analyzer
Sensor Selection: 
ISC40G General Purpose Sensor
ISC40S Intrinsically Safe Sensor
•    Available in PEEK and PFA material for more aggressive chemicals
 

Note: For additional information or assistance on this application, please contact the Yokogawa Analytical Product Marketing.