Critical Analysis Points in the VCM Balanced Process

Critical Analysis Points in the VCM Balanced Process

Required analytes, measurement ranges, and suitable Applied Analytics detectors indicated.

The Need for Monitoring FeCl₃ and Cl₂ in Direct Chlorination Effluent (EDC)

Location: AT-1

The direct chlorination catalyst is known to contaminate the effluent EDC with FeCl₃ — a highly corrosive compound which fouls machinery. Reasons for controlling FeCl₃ and Cl₂ levels using analysis at this point include:

• Coke formation. FeCl₃ has been found to be a major contributor to coke formation in EDC crackers; a cracker can be shut down within 3 weeks of startup for cleaning due to FeCl₃ contamination as low as 30 ppm.
• Loss of VCM product quality. VCM containing appreciable FeCl₃ is often deemed unacceptable for commercial use.
• Degradation of Oxychlorination Catalyst. FeCl₃ levels as low as 0.1% can have deleterious effects on the oxychlorination reactor, including deactivating or shortening the life of the cupric chloride catalyst as well as coke formation. Controlling the level using AT-1 analysis serves to prevent FeCl₃ from entering the oxychlorination process as a contaminant in the HCl recycle gas.
• Corrosion of stainless steel piping. This corrosion not only requires maintenance but also serves to generate more FeCl₃ in the stream by freeing the iron content from the stainless steel, thus compounding related issues.
• Unwanted reactions. FeCl₃ can react with water to form HCl contamination in the EDC product.
• Reaction inefficiency. Monitoring free chlorine concentration at AT-1 provides a real-time measure of direct chlorination conversion rate: excess Cl₂ indicates incomplete reaction with ethylene and/or waste of Cl₂ feedstock. This allows for fast response to problems and upsets at the first stage of the VCM balanced process.
• Downstream FeCl₃ formation due to free chlorine. Even if FeCl₃ levels at AT-1 are below the threshold of concern, free chlorine reacts with ferrous material from the piping to generate FeCl₃ downstream from this analysis point. This is a major source of FeCl₃ in the EDC cracker feed stream and is extremely important to control. Cl₂ levels as low as 20 ppm at AT-1 are considered problematic for the EDC cracker. Excess ethylene is often used to minimize free chlorine output, but this safeguard can still allow 200-3000 ppm Cl₂ in the direct chlorination effluent. Analysis at AT-1 allows control of Cl₂ as well as optimized use of ethylene feedstock.

ANALYTICAL SOLUTION: OMA-300 PROCESS ANALYZER

The OMA-300 simultaneously monitors FeCl₃ and Cl₂ in the direct chlorination effluent stream. Using a high resolution dispersive UV-Vis spectrophotometer, this system provides unrivaled dynamic range in measuring chlorine compounds from trace to high concentration. Since the device uses an ultra-safe fiber optic cable design, the corrosive sample fluid is confined to a specialized flow cell and never enters the analyzer electronics enclosure.

The Need for Monitoring Moisture in EDC and VCM

Location: AT-4, AT-5

Moisture is known to form a highly corrosive mixture with HCl and Cl₂. Above 100 ppm, H₂O will aggravate the corrosive effects of the produced EDC and VCM streams. The moisture also carries other undesirable impurities which reduce the quality of these products. Since removal of moisture is more economically viable than removing other components of the corrosive mixture, this is the main target of corrosion control.

VCM process operators typically measure 0-200 ppm moisture at these analysis points for process control.

ANALYTICAL SOLUTION: MICROSPEC MCP-200 MODULAR IR ANALYZER

The MCP-200 is a rugged NDIR concentration monitor designed for simple reliability in continuous moisture measurement. This solid state device has a zero offset feature which removes the need to obtain totally dry (zero H₂O) zeroing fluid, thus making the Auto Zero functionality practical and inexpensive without sacrificing accuracy. This system provides the fast response 0-200 ppm moisture reading required for tight VCM process control.

The Need for Monitoring CO, CO₂, Ethylene, and Oxygen in Vent Gas

Location: AT-2, AT-3

This measurement set provides a range of benefits to the process operator:

• Safety. Oxygen in the vent gas should be controlled below the flammable limit, thus obligating a 0-15% O₂ measurement at AT-2 and a 0-5% O₂ measurement at AT-3.
• Ethylene waste prevention. Ethylene measurement in the vent gas provides feedback control for the flow rate of the ethylene feedstock into the direct chlorination and oxychlorination reactors. While the tendency to use excess ethylene is justified by the effect of reducing Cl₂ and HCl in the EDC and preventing corrosion, overshooting the optimal flow rate leads to waste of an expensive raw material. Measuring ethylene at AT-2 and AT-3 allows for real-time optimization of the ethylene feedstock flow rate into the initial stage of VCM production.
• Oxychlorination efficiency validation and optimization. CO/CO₂ ratio measurement in the oxychlorination reactor vent gas is an excellent indicator of reaction efficiency (an increasing ratio signifies decreasing efficiency). The concentration readings also provide a control parameter for the reactor temperature, as higher temperature promotes CO and CO₂ formation instead of the desired products.

SOLUTION: MICROSPEC SERIES MODULAR ANALYZERS

The MicroSpec series of rugged, ultra-modular concentration monitors offers a dedicated solution for each of these measurements. An integrated interface reads the concentration outputs from all of the connected MicroSpec units. This design leverages the modular design of the MicroSpec series by performing Auto Zero on all detectors synchronously (maximizing system uptime) and minimizing the system’s physical footprint.