Vacuum technology in the chemical industry
Pfeiffer Vacuum GmbH
Headquarters · Germany
T +49 6441 802-0

For many economic sectors, the chemical industry is an indispensable supplier of raw materials. The automotive industry, mechanical engineering, plastics, foodstuff, glass, or building materials industries, for example, are all reliant on substances that are produced by the chemical industry. Plastics or synthetic resins, which are used as the basis for coatings or foams, play by far the most important role. These are employed in a countless number of end products that we use daily. Vacuum technology is crucial for a large part of these applications in the chemical industry .

Polycondensation is a chain reaction of small molecule compounds, or monomers. The functional groups of the monomers involved generally react by losing water and becoming long-chain molecules, or polymers. Accordingly, only monomers with at least two functional groups can create chains or networks. The product that is formed at the end of the polycondensation depends on the number and types of functional groups of the reacting monomer . During this process, vacuum technology is used in order to prevent unwanted byproducts of polycondensation. In polymer chemistry, polycondensation is one of the most important processes. It is used to produce plastics such as polyethylene terephthalate (PET), polyethylene or polycarbonate in large quantities. Adhesives are also produced with polycondensation as well as brake pads for automobiles.

Vacuum conditions in the medium vacuum range between 1 and 10 mbar are an essential part of the polycondensation process, especially in the production of highquality plastics. Together with temperature, vacuum technology steers the complete polycondensation process and significantly affects the resulting end product. Even the smallest fluctuations in pressure during the reaction can lead to damage of the end product or even render it unusable. The thermal load is reduced by vacuum during the process, especially with temperaturesensitive materials. Without vacuum, the production of certain plastics would not be possible; they would burn.

High-quality plastics such as PET, which the food industry uses to manufacture, among other things, millions of beverage bottles, must fulfil strict quality requirements. Those include a long life span and low diffusion in order to avoid leakage of carbon dioxide, for example. Vacuum technology guarantees high quality in the production of these plastics. In order to generate the necessary vacuum conditions, a multi-stage combination of Roots and liquid ring pumps is usually used. These must be exactly matched to fit the special requirements of the specific application. In the production of plastics, products that tend to stick and bake on to things are the order of the day. When configuring a vacuum system, it must thus be ensured that no process components leave deposits in the vacuum pumps. With its extensive product portfolio, Pfeiffer Vacuum offers diverse solutions that can be individually adjusted to suit the requirements of your specific application. The aforementioned combination of Roots pumps - in normal and gas-cooled versions - with liquid ring pumps has established itself as a reliable solution. Especially this combination of liquid ring vacuum pump and gas-cooled Roots pump contributes significantly to the stability of the process. Besides numerous advantages for the chemical industry, liquid ring vacuum pumps also carry a crucial disadvantage in their use. This is based on their dependence on a liquid, which can lead to the pump not being able to reach the specified pressure or pumping speed in case of contamination or temperature fluctuations. These variations can be compensated by the gas-cooled Roots pump, as this pump principle is self -regulating. This is based largely on the formula:
p (pressure) x V (volume) = constant

The ratio between inlet and outlet pressure determines how much cold gas from the heat exchanger is fed into the hot gas inside the pump. In this way , any differential pressure could theoretically be realized. Limiting factors are the size of the heat exchanger and the power of the engine. Both are specifically coordinated for each application

Vacuum distillation
Distillation serves to separate liquid mixtures of substances into their pure starting substances.

This can be achieved by simple heating at atmospheric pressure, as is done with alcohol distillation.

But not all substances can withstand high temperatures. With some mixtures, it can lead to chemical reactions or to disintegration of one or more individual substances. For all these applications, distillation under vacuum is vital. Unlike distillation at atmospheric pressure, vacuum distillation occurs under lowered pressure. This allows for evaporation already at lower temperatures. The basis for this is the vapour pressure curve of the respective substance. Water, which begins to boil at 100OC under atmospheric pressure, can already boil at lower temperatures in a vacuum, for example at 90OC and approximately 800 mbar.

Figure 1: Functional principle of gas-cooled Roots pumps - illustration of the automatic regulation in relation to the differential pressure.

This makes it possible to carefully separate heat-sensitive materials from each other or those who would begin to break down at high temperatures. Vacuum distillation, for example, is used for separating oils, fatty acids, esters or monoglycerides. One of the most important areas of application is the processing of crude oil. During the refining process, the long-chain hydrocarbons of crude oil are separated with the aid of vacuum distillation. In this process, the mixture is heated in a container. A vacuum is generated at the condenser. With so-called short path distillation, medium vacuum conditions in the area of 1 mbar are created. In this way, the evaporation temperature is reduced significantly. If the pressure selected is even lower , the process is called molecular distillation. In this case, the mean free path length is greater than the distance between the vaporizer and condenser . This means that at a distance of approximately six centimeters between the vaporizer and the condenser, a pressure of 10-3 mbar is needed - and with an even larger distance, lower pressure is required, accordingly.

For short path and molecular distillation, Pfeiffer Vacuum offers a large selection of suitable vacuum equipment. In practice, Roots pumping stations with liquid ring pumps have established themselves as the ideal solution. Depending on the number of Roots piston stages, a pressure of 10-3 mbar can be reached without great effort. The liquid ring pump can also be operated by using the substance which is to be distilled. This way, none of the distillate can be contaminated with the pump's operating fluid.

Figure 2: Separating and insulating low-molecular components through distillation

One example is the processing of rolling oil. The oil contaminated by the rolling operation is reprocessed through distillation. For this, three-stage Roots pumping stations, consisting of two Roots pumps and one liquid ring vacuum pump, are used. As the operating fluid for the liquid ring pump, the rolling oil to be distilled is used. At a pressure of around 1 mbar, only the rolling oil evaporates and can be condensed out again in the liquid ring pump. In this way, contaminants are removed and the rolling oil can be used again. Depending on the application, dry backing pumps can be used instead of the liquid ring pump.

Polyisocyanates and vacuum
Polyisocyanates are highly reactive organic compounds. They are divided into aliphatic and aromatic polyisocyanates. Aliphatic polyisocyanates act as cross-linkers between two-component polyurethane, which is used to make coatings and foams.

Polyurethane is used to produce a multitude of different end products. The targeted hardening of the substance at room temperature and the use of a special coating device allows for adjustment of the hardening period for the individual application. In this way, coatings in a production system, for example, can simply be mixed right when they are really needed. Polyurethane created with the use of polyisocyanates plays a large role in numerous production processes. As a foam, for example, it is used for furniture upholstery, as mattress foam, carpet backing material, cleaning sponges, or filter material. Polyurethane as a foam is also used in vehicle construction , for example as a soft covering for handles, interior panelling, steering wheels or arm rests.

As a component in coatings and varnishes, polyurethane is characterized by good adhesive properties and high resistance against solvents, chemicals, and atmospheric influences. These are used in many applications, including floor coatings, textiles, leather, various materials in shoe manufacturing and in motor vehicle interiors. Especially in the last area of application, polyurethane has become indispensable.

Vacuum technology is also extremely important for the production of polyisocyanates. Following the production of the isocyanate, a multi-stage distillation process ensures the highest possible concentration. Medium vacuum conditions of up to 0.05 mbar are required for this.

The production of polyisocyanates takes place in potentially explosive areas . For this reason, system operators often use vacuum products that are certified according to ATEX directive 2014/34/EU. In order to cover all aspects of quality and safety in the process operation, exact configuration according to the specific requirements of the respective application is necessary.

Gas volume flows are calculated based on material data and flows in the process. These serve as parameters for selection of the appropriate vacuum pumps. Important requirements include, besides safety, the durability and reliability of the vacuum technology used. Pfeiffer Vacuum offers complete ATEX-certified vacuum systems for the production of polyisocyanates. Most processes require a multi-stage construction of the vacuum system. The experts at Pfeiffer Vacuum developed, for example, a sixstage system for a large German chemical corporation consisting of five Roots piston stages and one liquid ring stage. At different stages of the process, different gas inflows are to be taken into consideration. The system was created according to the customer's specifications and fulfilled all individual parameters.

Vacuum solutions for the chemical industry
In order to create the necessary vacuum conditions for different applications, Pfeiffer Vacuum offers a full, comprehensive range of customer -specific solutions. Especially with regard to applications in the chemical industry which require a pressure of less than 30 hPa, the Roots pumping stations of Pfeiffer Vacuum's OktaLine have established themselves as ideal solutions.

Depending on the required pumping speed and ultimate pressure, different pumping stages can be built in. They are available in gas-cooled or standard air-cooled versions as well as in different materials, for example, spheroidal graphite cast iron or stainless steel. Specific coatings and coupling types are also available and can be combined for individual requirements. Liquid ring, rotary vane, screw, and gas-cooled Roots pumps can be used as backing pumps. Furthermore, ATEXcertified Roots pumps are also available for use in potentially explosive environments. From conception to implementation, the experts at Pfeiffer Vacuum develop individual solutions together with customers from all different areas of the chemical industry. These solutions are exactly designed for the requirements of the individual application.