The Pitfalls of Process TOC analysis and how to avoid them case study

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Courtesy of TOC Systems, Inc.

Since the introduction of Total Organic Carbon (TOC) Analyzers by Dow, Union Carbide and Automated Environmental Systems in thel960,s,l-5\ they have proven to be an invaluable tool in the environmental and process monitoring fields. While technologies have advanced since then, certain performance characteristics required for true process control have been notably absent. The purpose of this paper is to discuss those limitations as evidenced by field experience, maintenance requirements and other practical considerations. Failure mode and effect analyses are summarized for critical components, as well as suggested corrective design, operation, and maintenance approaches, with a view toward minimizing cost of ownership.

Generally, all TOC Analyzers employ the same basic technique as depicted in figure 1. A liquid sample is initially introduced to an Inorganic Carbon (IC) removal stage, where acid is added to the sample, dropping its pH to approximately 2.0. At this point, the IC is converted to carbon dioxide (CO2) gas, which is stripped out of the liquid by a sparge carrier gas. The remaining inorganic carbon-free sample is then delivered to the Oxidation Chamber. The Oxidation Chamber is normally either a chemical reagent (Persulfate) with an Ultraviolet (UV) Lamp Reactor*41 or a High Temperature Combustion Reactor'3' for catalytic or non-catalytic oxidation. The Reactors oxidize the remaining organic carbon to CO2 gas, which is directed to and measured by the CO2 gas detector.

The CO2 gas detector is a Non- Dispersive-Infrared Analyzer (NDIR), to meet EPA and ASTM Standards'3,4' and provide interference-free detection of CO2 gas. The CO2 generated from the oxidation process is directly related to the TOC in the sample.

While the principle objective of this paper is to provide a guide to avoid common TOC operational problems, a brief description of the chemical analysis should be given, since there has been some unnecessarily complicated treatment of sub-categories of organic carbon. Quite simply, do you need to measure volatile and purgeable organic carbon, as well as dissolved and suspended-solid organic carbon, for a 'true' TOC analysis or not? If only the dissolved and suspended solid TOC are of interest, categorized as 'NPOC' (Non-Purgeable-Organic-Carbon), then a 'TOCdirect' measurement will suffice. Figure 1 illustrates the 'TOCdirect' method. It indicates the physical effect of flowine a sparse carrier gas through the INORGANIC CARBON SPARGER to strip out the dissolved CO2. created by the acidification process to remove the inorganic carbon from the sample, as previously described. Note that 'POC' (Purgeable Organic Carbon) and 'VOC' (Volatile Organic Carbon) are also stripped out and lost to analysis. If the measurement of all the organic carbon is required, then a 'TOCtrue' analysis must be made.

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