Technical paper: Vacuum technology in the chemical industry for producing Polyurethanes and Polyisocyanates

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The chemical industry is a supplier of indispensable raw materials for many industries. Not only the automotive and engineering industries, for example, but also the plastics, food, glass and building material industries rely on basic chemicals produced by the chemical industry. By far the most important role is played by polyurethane plastics or resins, which serve as the basis for foams or paints. These substances are employed in countless finished products that we use on a daily basis. Vacuum technology is indispensable for a large part of these applications.

For the production of high-quality foams, it requires medium vacuum conditions up to 0.05 hPa

Polyurethanes are compounds which are formed in a polyaddition reaction from multiple alcohols (di-, tri- or polyols) and polyisocyanates. Depending on the basic components used, thermosetting plastics, thermoplastics or elastomers are obtained which differ greatly in their properties and can accordingly be used in a large number of different end products. In the EU alone, more than 2 million tons of polyurethanes are produced annually; global demand rises on average by 5 % per year.

Polyisocyanates are highly reactive organic compounds and act as crosslinkers for two-component polyurethane, from which coatings and foams are formed. The targeted curing of the substance at room temperature and the use of a special coating device, allow to customize the curing time for the application. For example, in a production plant, coatings only need to be mixed if needed immediately.

Vacuum technology is enormously important for the polyisocyanate production. Following the actual production of the isocyanate, the highest possible concentration is ensured by means of a multi-stage distillation process. It requires fine vacuum conditions, i.e., absolute pressures in the range of 0.05 hPa.

 ATEX certified vacuum solutions for the production of polyisocyanates

TDI (2,4-toluene diisocyanate), which besides MDI (methylene diphenyl di-isocyanate) is the most widely-produced polyisocyanate in the world, is not only a very toxic liquid, but can also be used at higher temperatures to form flammable vapor- air mixtures. For this reason, the operators of the systems preferably use vacuum pumping stations which are certified in accordance with the ATEX Directive 2014/34/EU and which also meet particularly stringent requirements for tightness.

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. In the first step, the respective gas volume flows are calculated on the basis of substance data and flows in the process. The results then serve as parameters for selecting 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 Pfeiffer Vacuum experts developed a six-stage system for a large German chemical company consisting of a five-stage Roots and a liquid ring vacuum pump. At different stages of the process, different gas inflows are taken into consideration. The system was created according to the customer’s specifications and fulfilled all individual parameters.

CombiLine vacuum pumping stations

To enable companies to create the vacuum conditions required in the various applications effectively and in a cost-optimized manner, Pfeiffer Vacuum offers customized solutions based on its comprehensive range of products. Especially with regard to applications in the chemical industry that require a pressure of less than 30 hPa, the Roots pumping stations from Pfeiffer Vacuum’s CombiLine WS have established as solutions.

Depending on the required pumping speed and working pressure, different types and quantities of pumps can be installed in the individual pumping stages. Rotary vane, screw, liquid ring and gas-cooled Roots pumps are primarily available as backing pumps.

Roots pumps are usually used for additional stages, which are available as air-cooled (standard) or gas recirculated versions and in various materials (e.g., spheroidal graphite cast iron or stainless steel). Specific coatings and coupling types are also available and can be combined for individual requirements.

For applications in potentially explosive atmospheres, ATEX-certified 2G and 3G Roots pumps are available.

OktaLine ATEX Roots pump for use in potentially explosive environments

ATEX-certified Roots pumps

Thanks to their magnetic coupling, the OktaLine pumps are ATEX hermetically sealed. Their extremely low leakage rate of < 1E-6 Pa m³/s provides additional safety as it prevents zone entrainment. Zone entrainment means the risk that, for example, failure of a seal can cause explosive mixtures to escape from the pump to the outside, entraining the risk of explosion being carried over to the outside. Since the operator of the system usually does not expect explosive mixtures there, the risk potential is correspondingly increased. Due to this permanently low leak rate, the pumps are also TA-air compliant.

In addition to the advantages already mentioned, the magnetic coupling eliminates the shaft seals. Pumps with shaft seals can heat up due to lack of oil lubrication through friction and therefore represent a potential source of ignition. Experience from the field has shown that this condition – an empty oil tank for lubrication of the shaft seals – is very common. Furthermore, shaft seals are weak points in pressure surges and require regular maintenance.

Magnetic coupling of a Roots pump

The OktaLine ATEX has long maintenance intervals resulting in lower maintenance costs. The magnetic coupling also reduces power consumption due to the virtually lossless transfer of the engine torque. As a result, the power consumption at the operating point can be reduced by up to 20 %. This is realized in comparison with other magnetically coupled pumps by a non-metallic containment shell, which has a significantly lower turbulent flow induction than, for example, magnetically-coupled liquid ring pumps. Air cooling also requires considerably less energy than water cooling, which significantly reduces operating costs.

A non-blocked overflow valve makes pump replacement and operation very easy. In some cases, retro-fitting a frequency converter was not possible, especially in all applications where the ATEX-certified Roots pump should replace a previously used standard version. Moreover, if there are no additional pressure gauges, then starting the pump at a defined counter-pressure is not possible either.

Standard pumps can easily be replaced by new ATEX pumps with its unblocked overflow valve. Neither frequency converters nor pressure monitoring devices need to be installed. Once again, the Roots pump can be started at the same time as the backing pump, so that the new overflow valve ensures not only safe operation but also shorter evacuation times. If the ATEX overflow valve is used in addition to a frequency converter, it ensures faultless operation even in the event of inverter failure.

Frequency-controlled drives are suitable for increasing efficiency. This ensures that the system always works in optimum operating condition. Energy costs are saved – “vacuum on demand” becomes possible. Although it is now easier to replace older devices.

Experience shows that even small changes made in the course of time in the process flow may necessitate a new consideration. One way of doing this is to optimize the gradation of the Roots pump and the backing pump, allowing better distribution of loads and temperatures.

This results in an extension of the run-time. From conception to implementation, the experts at Pfeiffer Vacuum develop individual solutions together with customers from all different areas of the chemical industry.

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