Measuring water impurities in EDC and other key hydrocarbon processing applications

The ethylene dichloride production process

To produce EDC, ethylene and chlorine are combined in a chlorination reactor. This creates a crude EDC stream which is sent to a clean-up fractionator, resulting in almost pure EDC.

Gas analysis is required throughout the process, most notably at the chlorination reactor where oxygen measurements are critical to ensure maximum process safety. However, minimising trace amounts of water in EDC is also critical to the operation of the plant. As the thermal cracking process produces large amounts of hydrogen chloride (HCl) and EDC, any residual water will greatly increase the corrosive nature of the process stream, causing damage to plant equipment. Therefore, the measurement of water in EDC is of critical importance.

The first point at which water measurements are required is where the pure EDC leaves the clean-up fractionator.

The EDC stream then undergoes pyrolysis to produce a combined vinyl chloride monomer (VCM) and HCl stream. Once again, it is crucial to monitor the water concentration to minimise corrosion before the stream is quenched and sent to the splitter.

The VCM is then sent for storage or immediate use in PVC manufacture, while the HCl is recycled back into the EDC process.

Other key applications for water measurements in HP applications

While measuring water in EDC is a key process requirement, water monitoring is of great importance in other applications within the hydrocarbon processing sector. For example, VCM is created from the reaction of hydrogen and chlorine (Cl2) to form HCl, which in turn is combined with acetylene to produce VCM. In this instance, it is important to monitor moisture in the Cl2 stream to avoid compressor corrosion.

Challenging process conditions, such as condensation and corrosion, can affect the monitoring equipment used in this process. Analytical systems must not only deliver reliable measurements for process control and safety but must be able to do so without being impaired by the conditions themselves.

Infrared sensing for gas and liquid analysis

Infrared sensing is a flexible measurement technology based on the unique light-absorbing properties of some gases. It delivers a non-contact, real-time detection of a selected gas’ concentration in a mixture and is widely used in a range of industrial applications.

Infrared (IR) sensors focus an IR light source through a sample cell holding a continuously flowing sample of the gas mixture, and onto a detector after wavelength selection. The property of some gases to absorb unique light wavelengths can then be used to detect the concentration of a selected gas in a mixture.

For water measurements, a single beam dual wave (SBDW) IR sensor technology is commonly used. This often uses a pair of optical filters mounted on a rotating disc, which pass through a beam of IR light alternately. One filter (the measure filter) is chosen to pass light only at a wavelength that the gas to be measured absorbs, while the other filter (the reference filter) has a light passed through it at a wavelength unaffected by the gas to be measured, and the background gases that are also in the sample. The difference in absorbance is measured by the detector and provides a direct output of the gas concentration.

The technology works in the same way for liquid samples, so there is no need for adaptations to be made to achieve an accurate measurement.

Compared to a gas sample, the key difference is that in a liquid sample the molecules are significantly closer together, so the required measurement path length is much smaller. This creates a challenge for accuracy as the mechanical tolerances and variations, which have a minimal impact over longer path lengths, are more significant over a shorter measurement. Analysers that are used for liquid measurements must, therefore, be designed in such a way as to negate these effects and deliver a stable measurement, even with variation in sample or ambient temperature.

A solution for liquid analysis of water in HP applications

Servomex’s SERVOTOUGH SpectraExact 2500F analyser is designed to achieve trace water measurements in hazardous conditions. For water, it uses a Single Beam, Dual Wavelength non-dispersive infrared (NDIR) measurement technique which offers high performance, increased measurement sensitivity, and a high degree of stability.

This technique is virtually unaffected by contamination on the sample cell windows, since it influences both measure and reference wavelengths equally. A 50% loss of signal due to obscuration of the sample windows produces no more than a 3% FSD error in reading. Maintenance requirements are low, and the cells can be easily changed or cleaned.

The SpectraExact 2500F has a rugged construction and is certified for use in hazardous areas and for the measurement of flammable samples. It has UKCA, CE, ATEX, UKEx, IECEx, and North American hazardous area approvals.

It uses special ultranarrow band pass measurement and reference IR filters, and a measurement cell designed to handle both gaseous and liquid samples. Typically, measurement ranges down to 0 - 50ppm H2O in EDC are achieved using quartz windows. The instrument is calibrated using the Karl-Fischer titration method, a standard laboratory technique for trace water determination.

Servomex also offers surrogate endboss calibration using carbon dioxide (CO2) which negates the requirement for laboratory analysis.

A special requirement in trace water measurement is the use of sample temperature compensation. Without compensation, the cell temperature must be maintained within ±0.5°C, as sample temperature fluctuations will produce errors in the measurements. Digital communications enable the SpectraExact 2500 to be operated remotely and safely, with Modbus implemented through MODBUS TCP.

Conclusion

By monitoring liquid water in process gas streams, plant operators can ensure greater product quality while minimising the effects of corrosive damage caused by the water reacting with the other components present.

In the widely used EDC production process, this is essential to prevent damage to plant equipment, particularly where HCl is also present, creating a corrosive environment. NDIR sensing is an effective and stable method of measuring liquid water concentrations within process streams, delivering the accuracy and reliability needed to achieve optimum process performance.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/06092024/servomex-he-feature-2500f-liquid/