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Process analysis in gas scrubbers - avoid overdosage, reduce operating costs

In several sections of the chemical industry, the connection of quality assurance and cost reduction gains in importance. There is not necessarily a conflict between both parameters. Determining the concentration of process liquids in gas scrubbers these both targets can be combined.

When cleaning gas streams, the focus is on avoiding an under- or overdosage of the scrubbing liquid. Hence, it needs to be prevented the insufficient gas cleaning, on the one hand, and excessive material usage with costs involved, on the other hand. In a gas scrubber, a liquid flow (scrubbing liquid) is brought into contact with the gas stream to be cleaned. The scrubbing liquid circulated in a loop needs to be refreshed and regenerated depending on the amount of absorbed matter. With concentration measurements directly in the process, an optimal dosage can be assured. Such an inline measuring system is the LiquiSonic® 40 by SensoTech that determines the concentration of scrubbing solution as well as the residual product separately without time delay in the running process and enables an automatic control. The precise concentration measurement allows a defined re-sharpening and thus a stabilization of the liquid concentration in the range of the maximum solubility or absorption.

The system consists of two sensors and one controller. For application in gas scrubbers, the LiquiSonic® 40 is usually equipped with a Halar or E-CTFE coated flange type sensor and a PFA or PEEK coated conductivity sensor. These sensors are resistant to corrosion as they are chemically resistant to a number of process liquids.

Besides, the flange type sensor is characterized by a highly efficient ultrasonic ceramic that ensures the precise measurement even with increased gas content. The rugged construction of sensors withstands mechanical vibrations and pressure peaks. There are no gaskets or moving parts, so that the measuring devices are completely working maintenance-free. The measuring values can be made available to the user or to the Process Control System via analog outputs or via Fieldbus. By using innovative signal processing technologies, an extremely precise and fail-safe measurement is ensured.

A procedure of the chemical industry is, for example, the carbon dioxide wash in a synthesis gas scrubber at Benfield. The gas stream is cleaned by adding potassium carbonate as scrubbing liquid in a plant with ethylene oxide. The gas component CO2 will thus be absorbed. The final product consists of potassium bicarbonate as well as cleaned gas. At temperatures from 95 to 105 °C and pressure up to 22 bar, the concentrations of potassium bicarbonate and potassium carbonate fluctuate in the range between 0 and 30 %wt. With the installation of LiquiSonic® 40 system, the concentration can be detected online without extensive and safety-critical sampling. With the connection to the Process Control System, the process is running in an optimal way and is controlled automatically.

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