Optimizing Vinyl Chloride Production with the TDLS8000 and TDLS8200

Prevent minor issues in Vinyl Chloride Production from escalating into serious problems with the In-Situ Gas Analyzer TDLS8000 and Probe Type Analyzer TDLS8200. These advanced analyzers help you detect and address potential issues early, ensuring smooth and efficient operations.
 

Introduction to PVC

Polyvinyl Chloride (PVC), commonly known as vinyl, is a material you'll encounter frequently in everyday life. PVC comes in two basic forms: rigid and flexible.

Rigid PVC is widely used in construction for applications like piping, flooring, doors, siding, windows, and roofing. On the other hand, flexible PVC is found in products such as electrical cables, insulation, faux leather, inflatables, sporting goods, and various automotive components.

The widespread use of PVC increased significantly during World War II, thanks to its excellent properties, including chemical resistance, strength, abrasion resistance, durability, weather resistance, and its role as an alternative to rubber. Today, PVC is one of the top three synthetic polymers produced globally, with over 40 million tons manufactured each year.

PVC is a type of polymer, which consists of large molecules made up of repeating smaller units. These large molecular chains give PVC its desirable physical properties, such as toughness and elasticity.

 

Introduction to Vinyl Chloride

Vinyl Chloride (also known as Vinyl Chloride Monomer or VCM) is the primary precursor used in the production of PVC. All commercial vinyl chloride production plants use a C₂ hydrocarbon feedstock, such as acetylene, ethylene, or ethane.

Acetylene can be highly effective for converting to vinyl chloride, but the mercuric chloride catalyst required is highly volatile, which limits its commercial viability. Ethane has been explored as an alternative feedstock due to its abundance and cost-effectiveness. However, the high reaction temperatures — up to 500°C — pose significant metallurgy challenges because chlorine becomes more aggressive at these elevated temperatures.

The development of a commercially viable process using ethylene became feasible in the mid-20th century. This method, which involves reacting chlorine with ethylene, now accounts for over 95% of global vinyl chloride production.

 

Vinyl Chloride Production

The Vinyl Chloride Plant can be summarized into 5 main processes for production highlighted in the figure below:

1. Direct Chlorination of Ethylene and Chlorine to produce 1,2-Dichlorethane, commonly referred to as Ethylene Dichloride or EDC
2. Oxychlorination that combines ethylene, air, and recycled chlorine to form EDC
3. Purification of EDC
4. EDC pyrolysis or thermal cracking of EDC to form VCM and Hydrogen Chloride (HCl)
5. VCM purification

 

Process Challenges & Solutions

Direct Chlorination

In the direct chlorination process, ethylene (CH₂CH₂) and chlorine (Cl₂) react in the liquid phase using a ferric chloride (FeCl₃) catalyst. To maximize the conversion of chlorine to EDC, a slight excess of ethylene is typically fed into the reactor.

A well-optimized direct chlorination process can produce an effluent stream with nearly 95%EDC purity, leaving only trace amounts of HCl, ethylene, chlorine, and trichloromethane. The waste gas stream from this step is saturated with HCl, which is repurposed for use in the oxychlorination steps.

Measurement Points on Direct Chlorination and Oxychlorination Reactors

Monitoring critical measurement points in both the direct chlorination and oxychlorination reactors is essential. For the direct chlorination reactor, it is crucial to monitor the outlet for oxygen to ensure proper reactor control and prevent oxygen levels from surpassing the Limiting Oxygen Concentration (LOC), which could lead to explosive conditions. The outlet stream typically has a temperature of around 80°C and can exceed 70 psig. The combination of high pressure, temperature, and aggressive components makes in-situ measurements challenging.

Traditional oxygen measurement technologies often require sample extraction, which can delay response times, increase costs and complexity, and introduce inaccuracies due to fluctuating sample return pressures.
Yokogawa’s TDLS8000 is designed to function in harsh environments with high pressures and temperatures, eliminating the need for sample extraction. Its steady-state electronics reduce the need for routine calibration, thereby lowering overall ownership costs.

Similarly, for the oxychlorination reactor, monitoring outlet oxygen levels is crucial for both efficiency and safety. The oxychlorination outlet stream typically has pressures over 60 psig and temperatures above 400°F, complicating in-situ measurements. The robust design of the TDLS8000 allows for reliable, direct installation on the oxychlorination reactor outlet, providing continuous and accurate oxygen measurements. Its onboard diagnostics and internal reference cell ensure stable absorption peak positions even under low oxygen conditions, maximizing measurement reliability and availability.

EDC Purification and Pyrolysis

The EDC produced from direct chlorination, oxychlorination, and recycling streams is combined and purified before undergoing pyrolysis, or thermal decomposition. Initially, the combined stream passes through a wash tower to remove water from the oxychlorination reaction and to separate the highly soluble FeCl₃ for later reuse.

After the wash tower, the EDC is sent to a series of distillation columns where it is purified to achieve over 99 wt% EDC. This purified EDC is then fed into the cracking furnace, which operates at approximately 500°C to thermally decompose EDC into the desired component, VCM.

The process also produces HCl and various chlorinated hydrocarbon byproducts, with an EDC conversion rate of around 50-60%.

The furnace effluent is typically quenched to minimize the formation of unwanted deposits such as coke or tar on the furnace walls. Finally, the stream undergoes treatment in flash drums before proceeding to the VCM purification stage.

EDC Pyrolysis Combustion Measurement Points

EDC pyrolysis is performed in a thermal cracking furnace to decompose EDC into VCM and HCl. Maintaining continuous and reliable measurements of O₂ and CO is critical for this process, similar to other combustion systems such as incinerators, boilers, and process heaters.

Legacy technologies that use extractive measurement systems can suffer from issues like plugging, slow response times, and the need for frequent calibration. In contrast, the in-situ TDLS8000, when installed in the radiant section of the process heater, provides near real-time responses (approximately 2.3 seconds). This allows operators to quickly react to changing conditions. The TDLS8000 offers a path-averaging measurement, which gives a more accurate representation of actual operating conditions compared to point-style measurements from older technologies.

If installing a cross-duct TDLS8000 is not feasible due to physical obstructions or lack of maintenance access, the TDLS8200 is an ideal alternative. The TDLS8200 features a single flanged design and can replace legacy technologies, offering the capability to measure O₂, CO, and CH₄ in a single analyzer. Both the TDLS8000 and TDLS8200 are reliable enough to support fuel overrides and can be integrated into SIL2 or SIL3 rated safety systems.

In addition to controlling combustion, enhancing efficiency, and ensuring safety, Yokogawa provides solutions for detecting HCl during EDC pyrolysis. Over time, the heater tubes can be damaged or fail, especially if the furnace operates at higher temperatures in the convective section due to inefficiencies such as excessive air and coke build-up. Elevated temperatures and reduced heat transfer efficiency can lead to tube deterioration and potential ruptures, posing significant risks and potentially causing unplanned plant shutdowns. The TDLS8000 and TDLS8200 can both measure HCl in the EDC pyrolysis furnace to detect tube leaks early and prevent small issues from becoming catastrophic.

Other Measurement Point Considerations

In the various stages of the VCM production process, Cl₂ and HCl are frequently present. Both compounds are toxic, so proper precautions must be taken when handling or working with them. At room temperature, HCl is a colorless gas. When it comes into contact with water, it reacts to form hydrochloric acid (denoted as HCl(aq), where “aq” stands for aqueous or dissolved in water).

In most industrial applications, such as vinyl chloride production, hydrogen chloride (HCl) is used rather than hydrochloric acid. While HCl(aq) is toxic to humans, it is also highly corrosive to many materials, including carbon steel, 304/316 stainless steel, and aluminum. The presence of even trace amounts of moisture can lead to the formation of hydrochloric acid, which significantly shortens the lifespan of process equipment and increases the risk of unexpected outages.

Therefore, in the vinyl chloride process and other processes involving HCl, it is crucial to continuously and reliably monitor moisture levels to prevent the formation of HCl(aq) and minimize equipment damage.

Traditional Technologies vs. Yokogawa’s Solution for ppm Moisture Detection

Traditional ppm moisture detection technologies typically use capacitance, impedance, or piezoelectric (quartz crystal) analyzers. Capacitance-based analyzers require the sample and ambient temperatures to be equal to provide accurate readings. They can also be affected by the presence of polar components or sulfur/iron oxides, which can damage the sensor. Similarly, inductive-based analyzers face issues when polar components, salts, acids, or bases are present, potentially leading to sensor damage.

Piezoelectric sensors, or quartz crystals, are commonly used but also suffer from long recovery times after large moisture concentration spikes. This saturation can take hours to resolve, resulting in lost production and unplanned outages. Additionally, sensors in processes with particulates or aggressive chemicals may degrade quickly, leading to premature failure.

Yokogawa’s Pre-engineered Moisture in Chlorine Solution addresses these issues by integrating a TDLS8000 with a Monel or Hastelloy C flow cell, rack, and sample system. This pre-engineered solution reduces the need for custom engineering and design work, streamlining project execution and offering a standardized setup across multiple sites or process units.

The TDLS8000 provides rapid and accurate trace moisture detection, with response and recovery times in seconds for ppm changes. It eliminates the need for routine calibration due to its steady-state components, which exhibit negligible drift over the analyzer's lifetime. By isolating the sample stream from sensitive components, the TDLS8000 reduces maintenance costs and minimizes preventive maintenance activities.

 

Product Recommendations

TDLS8000 (O₂ Analyzer)
TDLS8000 (CO/CH₄ analyzer)
TDLS8000 (ppmH₂O Analyzer)
TDLS8200 (O₂ and CO/CH₄ Analyzer)
TDLS8200 (HCl Analyzer)

Key Benefits

• SIL 2 Certified/ SIL 3 Capable
• Non-contacting sensor isolated from harsh processes
• Onboard diagnostic with 50 days of historical and spectral data
• Interference Free Measurement
• Adaptable platform for in-situ or extractive measurement analysis

Summary

Yokogawa’s Tunable Diode Laser Spectrometers is the industry’s most trusted laser analyzer designed specifically to meet all of your requirements in one robust device that is easy to operate and maintain. Process owners achieve an unprecedented opportunity to optimize combustion with In-Situ Gas Analyzer TDLS8000 near real-time measurement of O₂/CO/CH₄ in the radiant section of a large-scale combustion furnaces and process heaters. Yokogawa’s second generation of TDLS analyzers, Probe type Tunable Diode Laser Spectrometer TDLS8200 platform retains its improved reliability, ease of installation, and reduced maintenance requirements, but now does so with lower total-installed cost for O₂ and CO/CH₄, NH₃, HCl measurements.

Yokogawa’s TDLS technology houses all the industry’s leading features in one robust device. The platform is designed for a variety of in-situ measurements such as process oxygen or combustion measurements using the TDLS8200 or TDLS8000. The adaptability of the platform allows for extractive measurements as needed such as Yokogawa’s Pre-engineered Moisture in Chlorine Solution. The use of a non-contacting sensor allows for installation in a variety of process types that contain corrosive and/or abrasive components. The platform maintains 50 days of data and spectral storage to ensure that minor issues can be identified before becoming serious problems. Yokogawa TDLS analyzers are designed for reliability and quality.