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BACKGROUND

Numerous industrial applications require solutions of aspecific chemical strength. Such specific concentra-tions are achieved by mixing a full strength solutionwith water in the desired proportions. Flow-paced con-trol is used as a crude control in this operation, and aconductivity measurement is used for trim control.

CONTROLLING CONCENTRATION

The full-strength solution and water are mixed in twostages illustrated in Figure 1:

1. A flow ratio controller on a mixing tank is set for agiven flow rate of the full strength solution and aproportional flow rate of the water. The flow-ratiocontroller is set to produce a concentration slightlyweaker than that which is desired. The flow-ratiocontroller should also be capable of warning oper-ators when no liquid is actually flowing in the fullstrength solution and the water lines.

2. A control valve receiving input from a conductivityanalyzer functions as a trim control. It adds asmall amount of full strength solution to the mixingtank to produce the exact concentration desired.

For example, in making a 35% caustic solution froma large bulk caustic supply at 50%, the flow ratio con-troller is adjusted to make a 30% solution and theconductivity signal is used to add additional caustic tomake the 35% solution.

Conductivity is an economical and widely used indexof concentration for most acid, base, and salt solu-tions. It is important to determine the range of possi-ble concentrations at a given location in order tointerpret conductivity measurements. A good conduc-tivity application will have a significant change inreading over the possible concentration range andonly one concentration value for any given conductiv-ity reading. Conductivity tends to increase with con-centration for dilute solutions, however concentrated

solutions can have very different properties. Caustic(NaOH) is an example in which conductivity actuallydrops with increased concentration over 15% (seeFigure 2).

Conductivity graphs, such as Figure 2 and the manyothers included in Emerson Process ManagementsConductance Data for Commonly Used Chemicals,usually apply to pure mixtures of water and the indi-cated chemical. Although the conductivity of a mix-ture is usually close to the sum of the conductivity ofits components, the presence of significant amountsof certain substances (iron, copper, and some sug-ars, for example) can actually lower the measuredconductivity.

Conductivity data is temperature dependent and isfrequently stated at a reference temperature of 25C(77F). Temperature correction is specific to the

application and can be very important when extremeand/or variable temperature is likely.

INSTRUMENTATION

Emerson Process Management's Liquid Division hasa complete line of electrodeless (toroidal) conductivi-ty sensors that are especially suitable for monitoringchemical concentration. The most popular and versa-tile sensor is the Model 228 insertion/submersionsensor, which can also be used in a high pressureretraction assembly. Sensors having other installationoptions include the large bore Model 226 sensors,sanitary Model 225, and the flow-through Model 242.Consult the appropriate Product Data Sheet for com-plete details.

Compatible Toroidal Conductivity analyzers includethe Models 5081-T and Xmt-T DC-powered HARTand FOUNDATION Fieldbus Analyzers, the Model 1055Conductivity Analyzer, and the Model 54eC HARTConductivity Analyzer/Controller. Each of these mod-els has been specifically designed for measuring per-cent concentration and contain conductivity and tem-perature curves for 0-12% Sodium Hydroxide, 0-25%Sulfuric Acid, 96-100% Sulfuric Acid, and 0-15%Hydrochloric Acid, in addition to a curve fitting routinefor other chemicals. Selecting the data points for the

curve fitting requires some consideration of therequired measurement range. In general, the narrow-er the required range, the more accurate the resultwill be, since conductivity curves can be complex.

CHEMICAL CONCENTRATION CONTROL

Appl ication Data SheetADS 2800-06/rev.B

August 2004Chemical Industry

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Emerson Process Management

Liquid Division2400 Barranca Parkway

Irvine, CA 92606 USA

Tel: (949) 757-8500

Fax: (949) 474-7250

http://www.raihome.com

Rosemount Analytical Inc. 2004

CONDUCTANCE

S/CM @ 25C

Model 54e C Conductivity Analyzer

Measures conductivity, resistivity, or percent (%) con-centration.

Percent concentration curves for 0-12% NaOH, 0-15%HCl, and 0-25% or 96-100% H2SO4.

Optional TPC and PID control capability.

Fully Descriptive Diagnostic Messages and easy-to-use interface spells

out each operation in English, French, German, Italian, or Spanish. Two Independent Outputs for conductivity and temperature.

Three alarms with programmable logic, plus one dedicated fault alarm.

INSTRUMENTATION

FIGURE 1. Chemical Concent ration Contro l

FIGURE 2. The Conductivity of Some Common Solutions

CONCENTRATION

Model 228 Toroidal Conductiv ity

Sensor

Toroidal measurement principle greatlyreduces sensor fouling problems.

High Temperature PEEK sensor operates at tempera-tures up to 200C (392F).

Installation options include insertion in a tee, submer-

sion on a standpipe, and a high pressure mechanicalretraction assembly.