A Deeper Look at Wankai's Yearly Optimization of PET Production: Focusing on Quality, Efficiency, and Sustainability

Driving Efficiency in PET Production

Optimizing Solid-State Polymerization 

As society progresses, PET end products are evolving towards differentiation and functionalization, leading to an increasing demand for higher intrinsic viscosity (IV) polyester resins. These products require diverse IV levels and higher stability standards. However, the polyester resins produced typically have a characteristic IV of only about 0.64 dl/g, which is insufficient for applications like carbonated soft drinks and large-capacity barrels.

Wankai’s solid-state polymerization , sourced from Switzerland's Polymetrix, is equipped with high-precision electrical components and Honeywel’s DSC central control system. This setup enables the production of a variety of PET bottle-grade chips—including water bottle grade, carbonated bottle grade, hot-fill bottle grade, oil bottle grade, and fast reheat bottle grade—tailored to market demands.

Moreover, Wankai continuously improves the solid-state polymerization process, addressing challenges such as inefficient yield, IV adjustments, excessive transitional material production, and equipment complexity.

Automatic Regulation of Terminal Carboxylic Acid Content

The terminal carboxylic acid content in bottle-grade PET chips is critical for downstream product plasticity and processing performance. It must be adjusted based on end applications. The esterification rate indicates the extent of the esterification reaction of PTA (terephthalic acid), reflecting the terminal carboxylic acid content.

Terminal carboxylic acids primarily form during the esterification phase and remain stable during the polycondensation phase. The industry predominantly uses direct esterification methods. To enhance efficiency and maximize value, the amount of ethylene glycol (EG) added or the reaction temperature is increased to promote the reaction. Regardless of the esterification method employed, a target esterification rate of approximately 95% is maintained.

Wankai has developed an automatic method to adjust the esterification rate by mixing ethylene glycol (EG) and terephthalic acid (PTA) in a specified ratio, then feeding the mixture into a reaction kettle under controlled temperature and pressure. During the reaction, evaporated EG and generated water are separated, and the liquefied EG is returned to the kettle. The control system monitors water output and product yield in real-time, calculates the current esterification rate, and adjusts the reflux of EG or reaction temperature accordingly. This approach achieves flexible regulation of the esterification rate, enhancing polyester production efficiency.

Reducing EG Water Content to Improve Production Efficiency

The production of PET bottle grade involves two stages of esterification and three stages of polycondensation. Throughout the process, the reaction temperature increases while pressure decreases. Esterification is an endothermic process under positive pressure, while polycondensation is an exothermic process under negative pressure.

To improve the efficiency of esterification and polycondensation reactions, enhance vacuum conditions, and minimize side reactions and byproducts, Wankai has introduced a system designed to reduce the water content of ethylene glycol (EG) during polyester production.

This system comprises multiple reaction kettles and process towers for material processing. Fresh EG is introduced into the terminal polycondensation reaction to lower moisture content to as little as 0.3%. Excess water-laden EG is gradually replaced through a liquid sealing tank, reducing moisture to 2% and 5%. Ultimately, water-laden EG from the first sealing tank is purified in a process tower down to 1% before being returned to the EG mixing tank for recirculation.

Seamless Transition for Different Grade PET Chips

The PET bottle grade market is primarily categorized into four types: hot-fill bottles, water bottles, oil bottles, and carbonated beverage bottles. The key distinction among these categories lies in the varying levels of isopropyl alcohol (IPA) content. Some grade PET chips incorporate IPA in place of purified terephthalic acid (PTA) during the polymerization process. This adjustment aims to reduce the molecular regularity of PET, thereby lowering the crystallization rate and melting point while enhancing flexibility and transparency. These modifications improve processing performance during subsequent blow-molding operations and reduce processing temperatures.

When transitioning between products on a polyester production line, there are two primary methods. The first involves fully emptying all reaction kettles before starting a new product, eliminating transition material but proving time-consuming and economically inefficient. The second method, online transition, adjusts the amount of isopropyl alcohol (IPA) to facilitate direct production continuation. While this approach is quicker and more cost-effective, it generates a considerable amount of transition material. Typically, mixing to polymerization completion takes 12-15 hours, while online transition time is only 5-8 hours. Consequently, the industry is actively seeking methods to minimize transition material during online production.

Wankai's approach to transitioning between different grade PET chips presents significant advantages. The company directly mixes the required amount of IPA into the first polymerization reactor as an ester via a pipeline reactor before the polymerization process begins. This innovation eliminates the need for the 7-10 hours of esterification time typically required during the first and second esterification stages. By facilitating multiple transitions between products with varying IPA contents, the transition time is reduced to only 0.5-1 hour. This method significantly decreases the quantity of transition material generated during production changes between different IPA levels, resulting in substantial economic benefits for the company.

Strict Control of Color Value for PET resin 

PET bottle grade are primarily used as raw materials for various food-grade plastic bottles. In the production process, maintaining color stability is essential to meet downstream customer requirements. Key color indicators include L-value (whiteness and transparency), a-value (red/green index), and b-value (yellow/blue index), which are primarily influenced by PTA and ethylene glycol (EG). 

Consequently, color adjustment is a critical step in production. Currently, the industry commonly sends chip samples to a laboratory for color analysis, and production teams adjust the flow of colorants based on the lab results. This process consumes significant manpower and resources and results in delayed feedback, negatively impacting production efficiency.

To address this issue, Wankai offers a straightforward and efficient method for detecting and adjusting the color values of bottle-grade polyester. This color control method establishes target color values for PET chips, utilizes a color analysis instrument to measure the color value of the polyester melt, and feeds the results back to a control system. The system then adjusts the flow of red and blue colorants accordingly, effectively resolving the inefficiencies and resource wastage associated with current adjustment processes.

Promoting Sustainable Development in PET Production

Energy-Saving Vacuum Pumps in Precondensation

The production of PET bottle grade involves two esterification reactions followed by three stages of polycondensation, each utilizing different reaction kettles. Esterification occurs under positive pressure, while polycondensation is conducted under negative pressure, necessitating vacuum pumps to remove gases from the reaction kettles.

Vacuum pumps are essential for extracting small molecules (such as water, acetaldehyde, and ethylene glycol) generated during the polycondensation process, facilitating successful reactions. Typically, multiple vacuum pumps operate simultaneously—one for each precondensation reactor and another for the final polycondensation reactor. This arrangement leads to high power consumption and increased production costs.

Wankai has developed an energy-saving vacuum pump system for polyester production. This system connects the exhaust ports of both precondensation reactors to a single vacuum pump intake through a piping network, reducing the need for multiple pumps operating simultaneously. This innovative design decreases overall energy consumption and production costs.

Energy-Efficient Preheating in Solid-State Polymerization

Solid-state polycondensation refers to reactions that occur in the solid phase, suitable for synthesizing crystalline high polymers.

In existing solid-state polycondensation processes, base chips are introduced into a crystallizer at ambient temperature (around 30°C), where they are heated with nitrogen gas (99.95% purity). The chips exit the crystallizer at approximately 170°C, while the heated nitrogen reaches around 210°C. The nitrogen undergoes dust removal, purification, and filtration before recirculation.

However, in the crystallization process, nitrogen must be heated from ambient to the required high temperature, leading to excessive production loads. This scenario can increase nitrogen dew points and the temperature range needed for cooling, resulting in higher energy consumption.

Wankai provides an energy-efficient solid-state polycondensation preheating system that includes a nitrogen heat exchanger, a nitrogen heater, a nitrogen oxidation combustion reactor, a nitrogen cooler, and a fan. This system connects the nitrogen heat exchanger and cooler to a chip preheating chamber, which features multiple stainless steel tubes arranged vertically with adequate spacing. This design enhances preheating efficiency, reduces nitrogen dew points, and minimizes the cooling range, effectively addressing high energy consumption issues in current processes.

Conclusion

Through these innovative measures, Wankai demonstrates its commitment to producing high-quality, efficient, and sustainable PET resin, striving to be a leader in the PET manufacturing industry. The company's dedication to continuous improvement ensures that it meets the diverse needs of its customers while contributing to a more sustainable future.