Two-Cell Conductivity Analyzer Applied to Ion Exchange

Take the Guesswork out of Bed Regeneration

Background
Deionization is a process that removes dissolved ionic materials for water purification. It incorporates two basic types of resins, one for removing cations - positive ions such as calcium and sodium, and one for removing anions - negative ions such as sulfate and chloride. Specific resins are chosen to optimize performance in treating a particular water composition. An industrial ion exchange unit along with its resin is referred to as a bed. Typical industrial systems use a cation bed, an anion bed, and a mixed bed in series. The mixed bed (also known as a polisher) contains both cation and anion resins for highest ion removal efficiency, although its regeneration is more difficult due to the combination of resins.

Ion-exchange operations are monitored by conductivity. More precise control can be achieved in cation and anion beds by using conductivity ratio measurement. Comparing inlet and outlet conductivity, across the bed, identifies breakthrough of unwanted ions and the need for bed regeneration. Comparing conductivity at measurement points within the bed can give warning before the need for regeneration by identifying depletion in the resin before breakthrough occurs. However, measurement within the bed requires access for the conductivity cells, with screening to prevent resin beads from entering and shorting the cells.

Properties of water vary at different stages of deionization, requiring specialized conductivity temperature compensation to provide consistent monitoring of performance. Raw water generally contains neutral minerals with one set of conductivity temperature characteristics. Cation-exchanged water is acidic and has different temperature characteristics. Deionized water is again close to neutral but requires high-purity temperature compensation to accommodate its high and variable temperature coefficients. The 9782 analyzer provides all of these algorithms, field- selectable for each cell depending on its location in the system.

Applications

Cation Exchange with Across-the-Bed Monitoring
In a cation exchange bed, mineral cations are exchanged for hydrogen ions, which are much more conductive. Thus the conductivity at the outlet is considerably higher than at the inlet. A high conductivity ratio indicates good mineral ion removal, while a ratio approaching 1 indicates no removal. A low ratio alarm can be used to initiate regeneration.

The increase of hydrogen ions in cation exchange produces an acidic solution at the outlet. Because the temperature coefficient of acids is different from the coefficient for neutral solutions, the cation temperature compensation algorithm should be selected in the 9782 analyzer for the Cell 1 conductivity measurement. The conventional salt temperature compensation is appropriate for Cell 2.

Cation Exchange with In-the-Bed Monitoring
Within the bed during normal operation, both cells "see" full mineral exchange for hydrogen ions. Conductivities are high and the ratio is near 1. As the resin is depleted from the top down or by channeling, Cell 2 will sense the less conductive mineral cations first, and the ratio will increase above 1. A high ratio alarm can then trigger regeneration before mineral breakthrough reaches the outlet.

Since both points are measuring an acidic solution, cation temperature compensation is selected for both cells in the 9782 analyzer.

Anion Exchange with In-the-Bed Monitoring
When both cells "see" full anion removal in normal operation, the ratio is near 1. As resin is depleted from the top down or by channeling, Cell 2 will sense the conductive anions first and the ratio will decrease below 1. A low-ratio alarm can then trigger regeneration before anion breakthrough reaches the outlet. Cell 1 also provides outlet monitoring¾ the primary indicator of system performance. The 9782 analyzer’s flexibility allows the display to monitor the outlet conductivity while the alarm can be set on the ratio.

Since both points are normally measuring a neutral solution, high-purity salt temperature compensation is selected for all points of measurement.

Mixed Bed Deionizers (Polishers) can be monitored in the same was as anion exchangers. Precise salt high purity water temperature compensation is especially critical in monitoring this ultrapure water.

Ion Exchange Regeneration uses acids and bases to periodically remove accumulated mineral ions from the resin. Since these regents are normally supplied at too high a concentration for regeneration, they must be diluted. The dilution concentration can be monitored by high range conductivity measurements with special temperature compensation and conductivity-to- concentration conversions specifically for these reagents.

Honeywell’s Conductivity Solutions
Using industry-accepted algorithms, the 9782 analyzer accurately compensates for conductivity changes with temperature, making it ideal for a wide variety of water-treatment applications. The superior electronic design ensures reliable signals from the cells over the full display range, allowing separation of cell and analyzer by as much as 1,500 feet without reduction of accuracy. A wide variety of conductivity cells with cell constants specified for individual processes allows reliable, continuous measurements.

Features

• High-purity water temperature compensation for neutral salts, cations/acid
• Conductivity-to-concentration conversion for reagents, sulfuric acid, hydrochloric acid, and sodium hydroxide
• Large digital display showing conductivity, concentration by weight, or total dissolved solids (TDS) and temperature
• Computed variables (% passage, % rejection, ratio, difference)
• Up to two independent cell inputs
• Multiple outputs for transmitting up to three measured or calculated variables
• Up to four relays available
• Ruggedly constructed cells withstand pressures up to 250 psig and temperatures up to 285° F (140° C)

Benefits

• Temperature compensation specific to high-purity water and/or solution concentration applications for improved accuracy
• Automatic math computations for improved process analysis
• Rugged materials of construction, reducing cell replacement costs
• Flexible mounting assemblies, reducing installation costs

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