Chlorine and Hydrogen Chloride Monitoring Utilizing Ion Mobility Spectroscopy (IMS)

ABSTRACT

Increasing regulatory pressure has produced great interest in monitoring for hydrogen chloride (HCl) and chlorine (Cl2) in stacks and from other sources. Measurement of these gases has important process control implications as well¹. Traditional spectroscopic methods have been used in these applications, but have not always been found to be satisfactory. This paper describes the use of IMS in performing these measurements. This technique is shown to have
distinct advantages over competing techniques. Both compounds can be measured in a single instrument, limits of detection are in the area of 0.5 ppb, and no interference is found at high humidity levels. Theory of operation, hardware design, sensitivity and specificity will be discussed.

INTRODUCTION
HCl and Cl2 are used or produced in a wide variety of chemical processes. They may also be produced by incineration of chlorine containing materials. Very often they are present at the same time, depending on temperature, humidity and other parameters. When produced as a byproduct of a process or incineration, they are typically disposed of by a wet scrubbing process and released up a stack. While HCl is very water soluble even under relatively low pH conditions, CL2 typically requires a caustic scrubbing solution. Because of this, scrubber optimization has economic as well as environmental implications.²

The environmental concerns related to these compounds are an important factor. Both are toxic and corrosive. However, Cl2 is often considered a more dangerous compound. For this reason, specific monitoring for both compounds is becoming an important issue with local, state and federal agencies. These concerns have pushed desired detection limits to low ppm and in some cases ppb levels.

Traditional spectroscopic methods have been used to monitor for these compounds. HCl is usually monitored using infrared spectroscopy (IR), and Cl2 with ultraviolet spectroscopy (UV). However, these methods have some drawbacks. Two separate analyzers are required, one for each compound. In some cases coexisting compounds (in some cases even water vapor) present an interference problem. Limits of detection for these techniques are often insufficient to meet compliance or other requirements.

IMS has been used to monitor for both of these compounds in a wide variety of stacks, ambient air and process streams at levels down to 0.5 ppb. Because it is equally sensitive to both compounds, mixtures of the two provide a “total chloride” reading, without interference from organic chlorine containing compounds. Because of the differences in solubility, it is possible to scrub HCl from a stream without removing Cl2. This is easily accomplished by using a commonly used sample system component, the chiller. Chillers are very commonly used in extractive type sampling systems to remove water from the stream prior to introduction into analyzers. Chillers work by providing a chamber chilled to 4C using thermoelectric or other devices. Water in the stack sample is condensed out providing a sample with a 4C dewpoint. This process takes place very quickly, so that compounds of low water solubility pass through with little or no loss. However, compounds of high solubility are removed with the water.

This principle can be used to obtain independent measurements for Cl2 and HCl using a single IMS analyzer. The sample is presented directly to the analyzer to obtain a “total chloride” reading. The sample is then switched to the chiller to provide a chlorine only reading. HCl is then determined by difference. In practice, the non-chilled sample is run through a PermaPureÔ dryer which removes water but not HCl or Cl2. This is done so that both the total chloride and chlorine
measurements are obtained on a dry basis. Where a continuous output is required, the two streams (one through a chiller, the other through a PermaPure^TM drier) can be directed to a single analyzer with two sensors.

In streams that contain only one of these compounds (or if a total chloride measurement is satisfactory), the measurement can be made directly without removal of the water. This eliminate the need for the chiller or other driers. In some cases, a dilution type probe may be used.