Polyester resin

Polyester resins are synthetic resins and condensation products of di- or polyhydric alcohols (e.g. glycols or glycerol) and dicarboxylic acids. Unsaturated polyester resins (UPR) are used, among other applications, for the production of fiber-reinforced plastics, putties or casting resins. The applications for saturated polyester resins (SPR) can be identified as powder coatings, coil and can coatings, automotive coatings, packaging, industrial coatings and others.

Production plant philosophy

The industrial production of polyester resin in general is a batch process. The resins will be manufactured by adding the raw materials into a reactor of sizes typically between 5 and 50 m³ working capacity. The reaction mixture is heated and, if necessary, pressurized. The resulting reaction water needs to be removed during the conversion and the heat generated by the exothermic process will be withdrawn.

Polyester resin plants should support the universal production of a wide variety of polyester resins and be as universally applicable as possible. Multipurpose synthetic resin plants built from standard components in prefabricated frames are clearly the trend in the design and installation of high-quality but yet cost-effective reaction units.

Multi-purpose lines can run the production of polyester resins with all kinds of required temperatures and especially pressure that is needed to manufacture high-performance polyester resins for the world market.

Design & Installation

Typically, the polyester resin plants are designed to use the gravity in the material flow (Fig. 1). At the highest level (usually level 4), weighing tanks for pre-dosing of acid anhydrides and – if necessary – tanks for melting polyhydric alcohols are installed.

The 3^rd floor is the reactor level. The reactor is filled with the pre-dosed raw materials as well as additionally required reaction partners. On this level, hand components might be fed from containers or sewer ports into the reactor.

The heating/cooling level for the reaction vessels is located on the 2^nd floor. Heating and cooling facilities will be pre-assembled in skid units to minimize installation time and costs.

Beside the heating/cooling skids, the thinning tanks are located in levels 2 and 1. The hot resin will be transferred to the thinning tanks and diluted with solvents.

Raw material deposition and transport

The fully-automated charging of the reactors with solid and liquid raw materials requires one solid weighing hopper and, if necessary, one melting vessel for each reactor.

The solid weighing hopper is mounted on weighing cells and is dosed with the anhydrides of dicarboxylic acids, precisely measured according to the formulation. Big-bags or bags are commonly used, but it is more economical and attractive to store the solid raw materials in bulk. Dosing of the bulk solids into the solid weighing vessel will be executed by using a pneumatic, nitrogen-based transfer system.

The polyhydric alcohols are pre-dosed for the batch in the melting vessel according to the formulation values. The vessels, which are also stored on load cells, must be transferred under heated conditions, since the alcohols used can often crystallize out at room temperature. In addition to the load cells, mass flow meters may also offer an attractive, alternative method.

Mass meters, as compared to scales, are the most precise measurement devices, but when using a mass meter for several dosing vessels, only one vessel at a time can be filled.

Core equipment

The core equipment of a synthetic resin plant for the production of polyester resin includes the following devices, in addition to the above-mentioned dosing vessels:

Reactor, columns/dephlegmator, ascending pipe, total condenser, separation receiver and collection vessel for water. Equipment into contact with hot reaction media should be made of stainless steel, material no. 1.4571 (316Ti).

For safety reasons, the equipment is designed for a temperature of 300°C, even if the reaction temperature is not that high. The hot oil for heating the vessel will be at appr 300°C anyhow. The design pressure varies with the pressure at which the reaction is to take place. For average and high-molecular polyester resins, the design pressure should at least at 10 barg overpressure. Exact design pressure might be double checked with the formulations. When engineering the equipment, the consistent design of all parts according to temperature and pressure is a safety-relevant issue.

With low molecular resins, the process can also be carried out at normal pressure. However, the design pressure should not be less than 3 barg for safety reasons.All equipment connected directly to each other need to be designed for the same temperature and pressure for safety reasons.

The resulting reaction water is carried away from the reaction material by means of a column or Dephlegmator (correctly tempered vertical heat exchanger with baffle plates) to force the balance reaction to take place on the products’ side. The reaction water is fed into a collection vessel.

Vacuum units

The most robust solution for a vacuum system is certainly the water ring pump. To avoid contamination of the cooling water with reaction material, the vacuum pump should be installed in a secondary circuit in which the impurities are collected. This secondary circuit will be changed after a few days or weeks, depending on the mode of operation.

With water ring pumps, a minimum vacuum of 60 – 70 mbara might be achieved in the reactor. Further reduction of the pressure requires additional technical equipment.

One possibility to further reduce the pressure in the reaction vessel is to install steam or gas ejectors. Also the installation of an additional booster pump can ensure additional compression before the vacuum station and leads to a lower pressure in the process. It must be taken care to protect and/or clean the booster pumps with additional devices at specified intervals.

Another possibility is the use of dry-running vacuum pumps. Also here, attention must be paid to the cleaning and/or protection of the pumps. It is essential to check their use beforehand by means of a series of tests.

Control systems

The greatest development of synthetic resin plants has certainly been made in the last decade in the field of process control. Inherent safety control systems are capable of running the processes as safely as possible. However, I still consider mechanical safety devices to be necessary as a ‘last’ safeguard.

The installation of flow and temperature sensors on the heating/cooling inlet and outlet provides the opportunity to understand the process very well on the energy side. The incoming data from sensors can easily be evaluated by the control system and show the energy consumption of the whole installation.

The connection of the control system with Scada systems, ERP systems and preventive maintenance tools gives the operators and management level the opportunity not only to follow the production but also to taking preventive measures to maintain production capability.

The intelligent and integrated control system is part of the companies IT infrastructure and communicates to optimize the product quality and disposability under most sustainable conditions.