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Advanced Dry Vacuum Systems for the Process Industry
Dr Don Collins. Chemical processing industries present special challenges related to vacuum technology because of the difficult environments required for operating equipment. Dry vacuum pumps have now become an effi cient, reliable option for processing industry applications. The article talks about the various advantages of dry pump technology.

Chemical processing industries - petrochemical, oleochemical and pharmaceutical - present special challenges when it comes to vacuum technology because of the demanding and difficult environments - damp, dusty, corrosive or flammable - in which they require equipment to operate. Consequently, it tends to be the demands of this sector that drive technology developments by the vacuum equipment manufacturers. Advanced companies in the pharmaceutical and fine chemicals sector were early adopters of dry vacuum pumping technology since the first pumps were installed in the late eighties. The sector has been quick to recognise the clear advantages of dry pumps over traditional wet technologies in most applications, including higher reliability and flexibility, better cost of ownership and lower environmental impact.

Dry vacuum pumps are now well established around the world as an efficient, reliable option for demanding chemical processing industry applications. They are used to pump some of the most aggressive and problematic gases in a broad range of processes, including distillation, evaporation, crystallization, drying, solvent recovery, deodorization, filtration and for house or general vacuum duties.

More recently, chemical processors are driving a new trend for vacuum to be seen as a ¬utility ; highly reliable, flexible, available on demand in a ¬plug and pump  system that requires minimum set-up and maintenance. When processors are specifying a new vacuum system, cost-of-ownership and environmental impact are now key issues. In response, dry vacuum technology is being developed and refined by manufacturers to meet this challenge. The most advanced chemical dry pumps feature new discrete, variable pitch tapered-screw technology for exceptional energy efficiency and performance. This technology offers high reliability effluent-free pumping, high levels of controllability and long service intervals, even in the most difficult of harsh chemical processing applications. It is cost-effective to run and environmentally sound.

Advantages of Dry Pump Technology Although the vacuum system represents an apparently small part of any chemical or pharmaceutical processing plant, its role is highly significant to product quality, consistency and yield, and to the cost-efficiency of the process. Selecting the right vacuum pump system offers engineers an opportunity to reduce the energy usage, environmental impact and carbon footprint of the process, and at the same time, make productivity and / or quality improvements (Table 1).

The key advantage of all dry pumps, including roots, claw and screw technologies, is that they do not use water or oil for sealing or lubrication of the vacuum stages. This eliminates the risk of process contamination and the generation of effluent associated with the ¬wet  technologies such as steam ejectors and Liquid Ring Pumps (LRPs). Dry pumps usually also offer clear savings in maintenance and running costs. It is true that for many standard applications, the capital cost of a dry pump may be higher than, for example, an equivalent oil-sealed or cast iron liquid ring vacuum pump. However when the cost of the total installed package and the running costs are taken into account, dry vacuum systems can be considerably more cost-effective.

Performance of Dry Pump
Dry pumps have very similar performance characteristics to oil-sealed pumps, typically covering the pressure range 1000 to 1 mbar at near constant volumetric efficiency with ultimate pressures of 10-1 to 10-2 mbar. Their operating range can be extended with the addition of one or two Roots pump stages thereby increasing the pump capacity to many thousands of m3h- 1 and decreasing the ultimate vacuum to 10-3 or 10-4 mbar. With the addition of one or more mechanical boosters in series, pump speeds up to nearly 40,000 m3h-1 can be achieved in a single train. These trains can be combined in parallel to increase capacities even further. Inlet and inter-stage condensers and knockout pots are also very efficient, non-polluting ways of increasing dry pump capacities.

Well designed dry pumps do not suffer from problems of compatibility with process gases as wet pumps do. Even though dry pumps are made from ductile cast iron, there is no corrosion when operating in the vapour phase. This is because the pressure and temperature profile inside the pump is maintained above the dew point of the process media, ensuring reliable operation even when pumping highly corrosive media (Figure 1). Solvents can be recovered relatively easily by inlet and/or exhaust condensation and recycled without any need for further purification. For this reason, the dry vacuum pump system is a particularly good solution for processing organic solvents and very corrosive vapours. Keeping corrosive vapours in the vapour phase has proven successful for many years, making the need for coatings and special material unnecessary. Users of dry pumps report that the performance is more consistently reliable than that of wet pumps. This is because the presence of contaminants in an oil-sealed pump can degrade its performance. Similarly, both LRPs and steam ejector pumps require large amounts of cooling water, which can become contaminated with process vapour as it condenses, with the effect that performance may be degraded. In a dry pump, no such contamination problems exist.

Cost of Ownership and Energy Consumption The capital cost of a dry vacuum pump is often higher than that of an equivalent wet pump, but there tends to be very little difference when the total installation cost is considered. And when running costs are taken into account, the dry system often shows a considerably lower cost of ownership. Dry pumps offer the best thermal efficiency of any process vacuumproducing system. Not only does the dry system use significantly less energy when it is running but, unlike a steam ejector, it can be switched off between cycles so that it uses no energy at all when it is not required. Inverters can also be used to minimise the power usage when in standby mode. Dry pumps are energy efficient and the reduced power consumption results in lower carbon footprint and environmental impact. The dry vacuum pump costs up to 90 per cent less to run than its steam ejector equivalent (see Figure 2). Similarly, compared to a liquid ring pump system, the running cost of a dry pump can be significantly cheaper. Even when the higher capital cost of a dry pump system is taken into account, the lower cost of ownership of the dry system means that it often pays for itself very quickly (see Table 2).

Safety of Dry Pump
Dry pumps are designed to pump flammable materials safely because they are contact-free pumping mechanisms with no ignition sources in normal operation. Two ignition sources are generally considered with dry pumps: spark ignition and auto-ignition (ignition by temperature alone). Auto-ignition is simple enough to deal with by running the pump in a pretested configuration, which avoids autoignition. However, spark ignition can never be ruled out entirely in a fault condition, no matter how rare. One must therefore implement a suitable protection strategy when pumping explosive gases.

The European ATEX directive requires process operators to undertake a risk analysis of the whole process, and to identify protection strategies to mitigate the risks. Edwards regularly advises its customers all over the world on these issues and, in areas where an explosion threat is identified, can provide guidelines on avoiding, eliminating, containing and limiting the effects of an explosion, to ATEX and other standards.

Innovation in Dry Pumping Technology The very latest in chemical pump technology is the new advanced discrete variable pitch taperedscrew technology, developed by Edwards for the next generation CXS pumps to meet demands for high reliability, highly flexible ¬vacuum on demand  from the chemical sector. The new design refines earlier screw mechanisms to give smooth, gradual compression along the length of the rotor for improved thermal control and optimised pumping at all inlet pressures.

It uses flooded air-gap potted motors, which are more efficient than standard motors, and integral drive and control systems, to improve performance and lower the cost of ownership. Unlike other screw pumps, these do not have a cantilever rotor design or an end compression plate. The result is superior solids handling, as material is not compacted during the cool down phase, thus preventing cold seizures within the pump. The technology has excellent liquid handling capability, continuously pumping up to one litre of liquid per minute and up to 25 litre slugs without stopping - making it robust and reliable even in severe processing environments. Noise levels in the new pump mechanisms are as low as 64 dB (A). The pumps can be close-coupled and vertically aligned with mechanical boosters to achieve minimum footprint and height. And finally, to avoid the cost and disruption of unplanned downtime, the new pumps have been developed to ensure long service intervals of up to five years and minimal routine maintenance requirements over a 25-year plus life expectancy. This technology is already installed at customer sites in UK, Italy, Germany, Switzerland, France and Israel on various pharmaceutical and fine chemicals applications, and interest continues to grow from customers in India, China, USA and all over the world as the benefits of the technology are realised.

Dry Vacuum Pump Systems in Action Worldwide Users worldwide regularly report significant process and cost benefits as a result of the installation of new dry vacuum pump technology. The following examples from around the world and are typical:

Improving Reliability of Vacuum Filter Drying - USA: A manufacturer of pharmaceutical intermediates in the Mid-Atlantic States area (USA) was using oil-lubricated, liquid ring vacuum pumps in a filter drying process. However, the system suffered from corrosion issues, frequent failures and loss of performance.

The LRPs were replaced with a dry vacuum pump system, which has been in operation for more than five years. In that time they have required nothing more than routine annual maintenance.

Multipurpose, Safe Operation for Speciality Chemicals - USA: A major global manufacturer of speciality chemicals needed a vacuum pump system that could cope with a wide range of process media and duty conditions for its facility in the Southeastern United States. Their process required a vacuum pump to operate under 150 degrees Celsius internally and they chose a dry vacuum pump with temperature control to safely operate under their specific conditions. Four years on, the pump continues to run well.

Handling Aggressive Agrochemicals - India: A major global agrochemicals manufacturer based in Gujarat (India), needed to pump aggressive chemicals including phosphorus oxy chloride, thionyl chloride and hydrogen chloride. To use the existing liquid ring pumps would have meant installing an expensive up-stream scrubber with high operating costs and potential pollution issues. Instead, a dry vacuum pump was installed and configured to operate relatively hot so that the pumped vapours could not condense internally during compression. After two years of operation the pump was still in pristine condition.

Cutting the Cost of Ownership for a Chemicals Manufacturer - India: Another agrochemical manufacturer based in Andhra Pradesh (India), switched from multistage steam ejectors to dry vacuum pumps and in a 12-month trial project calculated significant savings as a result of reduced energy consumption. The customer also found that they were able to recover almost all of the valuable uncontaminated speciality chemicals at the exhaust for reuse in the process. The results were so impressive that the customer ordered a further 12 systems, and has now replaced all its steam ejector systems with dry vacuum pumps.

Increasing House Vacuum Capacity - India: Another pharmaceutical plant was using oil-sealed piston pumps in its house vacuum system. The pumps were reliable, but they produced an oily waste that required disposal. When the decision was made to expand the plant, the existing pumps were replaced with four dry vacuum pumps configured for maximum flexibility and driven through inverters to ensure better control of the vacuum levels. Because of the cocktail of pumped media, full protection with flame arrestors and differential pressure measurement were specified. Solvents are now being recovered by exhaust condensation, capacity is increased and there is better pressure control.

Conclusion
Dry pumps are clean, reliable and provide low to high vacuum and require minimal maintenance. Importantly, they can be used quite safely even when pumping flammable and corrosive vapours. Innovation in this sector continues, producing ever more ¬utilitarian  high performing, highly controllable vacuum equipment that allows processors to minimise energy costs and reduce environmental impact.