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The gear pump for climate protection

Although the title may seem a bit dramatic, the polyol that the Witte pump recently began delivering is an absolute novelty. Covestro has succeeded in putting carbon dioxide to use as a component for polyol production, and, in a first step, has thereby replaced around 20% of the oil generally used. Yet it is not just the actual product, but also the gear pump used - one of the installation's key components - that is a new development.

The newly developed process makes high demands on equipment: high system pressure, the gas content in the product, a minimal allowable temperature increase as well as flushing of the system with solvents. In addition to safety aspects and the requirement of a hermetic seal, the use of a magnetic coupling is absolutely necessary to ensure reliable pump operation. The pressure on the suction side would put massive stress in an axial direction on the drive shaft if a conventional shaft seal was used since one of the end faces is exposed to system pressure, the other to atmospheric pressure. The use of a metal separating can naturally generate eddy currents and thus transfers heat to the product. Since a loss-free ceramic separating can was not an option, the aim was to find an effective way to allow the heat produced to dissipate as close as possible to the point at which it was generated. A double-walled can is used for this purpose which is otherwise also more commonly used to heat magnetic couplings. The separating can was slightly modified to increase the heat exchange properties. The decisive innovation, however, is in the actual pump itself: while pumps are generally equipped with product lubricated slide bearings, this pump has a product lubricated roller bearing. In using this type of bearing, three of the relevant demands can be met.

 

The demands which the newly developed Covestro process makes on the equipment are high: high system pressure, the amount of gas in the product, a minimum allowable temperature increase as well as a flushing of the system with solvents.

 

 

The hybrid bearing used in the pump consists of metallic inner and out rings and ceramic rolling elements made of silicon nitride

 

Requirements fulfilled

Significantly less heat is generated in roller bearings than in slide bearings of a similar size. The generating of less heat is just as important in this context as it is for the magnetic coupling's containment can. An increase in temperature would result in outgassing of CO2 and thus to separation. The pump's geometry is conducive to low heat input as it has been decisively optimized in regard to its overall efficiency. This means only a small portion of the operating power is converted into heat energy due to friction losses. This is a major advantage of this pump design.

Despite careful temperature control gas bubbles may form in the product during individual phases. When using media lubricated slide bearings, this type of a gas cushion can lead to a breakdown of the hydrodynamic lubricating film and thus to dry friction and wear and in the worst case even to failure.

As is the case for gas cushions, the use of low-viscosity solvents also entails the risk of an unsustainable lubricating film, in particular because the pump dimensions are not suitable for this operating point. Using a single pump for such a large viscosity range cannot readily be achieved with slide bearings.

Reliable bearing technology

Pumps that use roller bearings are not necessarily a new invention. What is special in this case, however, is the use of hybrid bearings. This means that roller bearings with metallic inner and outer rings and ceramic rolling elements made of silicon nitride (Si3, N4) are used. This material combination guarantees reliable operation even if they are inadequately lubricated as can occur in the applications described above.

Unlike roller bearings lubricated with oil or grease, when using hybrid bearings with media lubrication, no sophisticated seals are needed to prevent the introduction of the lubricant into the medium or vice versa. The risk of contamination is thus eliminated, and consistent product quality can be guaranteed under all operating conditions. In this context, it is worth mentioning that naturally pumps with external bearings, i.e. product chamber and bearings are separated, are in order for various applications, for example, if the product would otherwise be damaged in the bearings.

The pump size used in this case is readily scalable. All required components can also be offered for much larger and as applicable also for much smaller pumps. There is nothing to keep Covestro from expanding production for this product on the side of the process pump.

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