Trace Moisture in Chlorine Production

Rapid Detection of Moisture in Chlorine Processing

Plant Name:    North American Electrolysis Plant

Industry:          Chemical

Product(s):      Chlorine

Application

Chlorine is used as an intermediate chemical in the production of high-demand products such as bleach, acids, and PVC. To achieve the quality required for each product, chlorine manufacturers know the importance of measuring moisture levels in the chlorine gas at key points as it moves through the process toward liquefaction. 

Electrolysis plants create hydrogen and chlorine from a brine solution. Chlorine gas generated from the anolyte of the electrolysis tank generally contains between 0.5 to 2.0 vol% H₂O (figure 1). The sample is then cooled and filtered to remove brine, subsequently emerging as wet chlorine gas. The wet gas flows to a drying tower where it is treated with sulfuric acid to reduce the moisture content down to ppm levels. Acid mist remaining in the dry chlorine gas flows to a dry chlorine tower where it is removed. The dry chlorine gas is then sent to a reciprocating compressor for liquefaction. 
 

Challenges

Learning immediately if there is ineffective drying or ingress of moisture into the process is critical to safe and smooth operation. For example, the presence of even trace levels of moisture after the dry chlorine tower causes corrosion of the chlorine compressor and contaminates the product with chlorine hydrate and hydrous iron chloride. When a specified level of corrosion is detected, the process must be shut down to correct these issues.

Since manufacturers must respond rapidly to the presence of moisture, they rely on sensors for alerts. A conventional moisture sensor with a range of 1-5 ppm could have a T90 response time of an hour (note: T90 is the amount of time it takes for the sensor to measure 90% of the maximum analyte level). After measuring moisture in the process, the conventional sensor must recover quickly to allow it to recognize another event. Due to sensor saturation, recovery time from large concentration spikes of moisture can take more than 24 hours. A slow-recovery sensor could miss the presence of additional moisture and lead to further contamination and corrosion. 

If a sensor is not operating, the manufacturer cannot detect moisture in the chlorine and must stop the process. Unfortunately, sensors can deteriorate and deliver inaccurate measurements due to contact with the process. This deterioration leads to maintenance costs that are associated with recalibrating or replacing the sensor. 

Solution

To maximize chlorine production, manufacturers need to prevent corrosion of equipment and increase the time between shutdowns. To support safe and continued chlorine production, Yokogawa’s TDLS8000 Laser Analyzer responds accurately to low quantities (in ppm) of moisture, recovers quickly after a moisture detection event, and simplifies maintenance. Since it uses solid-state electronics, the sensor will not drift and will not require re-calibration.

The TDLS8000 can respond to single-digit ppm step changes and recover from the presence of moisture in as fast as one second. Since the sensor does not contact the process, it does not degrade and will provide accurate readings over a longer period of time than conventional sensors; therefore, manufacturers are alerted more reliably to immediate, potential issues.

Since device design is modular and provides isolation from the process, maintenance is greatly simplified. In an extreme event such as a lightning strike or contaminated gas purge, any part of the TDLS8000 can be serviced or repaired in the field. For added ease, all modular components are automatically detected by the analyzer and no additional configuration is required.

In addition to measurement, the TDLS8000 Laser Analyzer assists in troubleshooting by storing 50 days of spectral data and diagnostics. This enables manufacturers to review spectral images and attain positive verification that an event was caused by moisture and not an analyzer fault. This information can be conveniently accessed via a USB flash drive, remotely via a network, or through the HMI.

Key Advantages

•    High sensitivity in the sensor enables users to address process issues more quickly. Minimum range is 0-30 ppm H₂O. 
•    The sensor’s rapid recovery times enable faster corrective action to address moisture issues.
•    Since the sensor does not drift, maintenance and shutdown costs are substantially reduced.
•    TDLS is a non-contacting measurement that never requires calibration or consumables.