Dry vacuum technologies are now more flexible than ever, allowing the chemical processing industry to 'fit-and-forget'. On demand vacuum improves efficiency whilst reducing overall cost of ownership making vacuum more of a utility. Dr Don Collins reports.
Chemical processing industries face special challenges when it comes to vacuum because equipment often has to operate in damp, dusty, corrosive or flammable environments.
Dry vacuum pumps are now well established in the industry and are used to pump some of the most aggressive and problematic gases and liquids in a broad range of processes, including distillation, reactor service, evaporation, crystallisation, drying, filtration, solvent recovery, deodorization and for central or general vacuum duties.[Page Break]
New demands
New demands are now being placed on the market by chemical processors, driving an emerging trend for vacuum to be seen as a 'utility'. Chemical processors are seeking highly reliable, low maintenance vacuum systems that provide the process with vacuum as and when it is required.
Dry pump manufacturers are responding by developing technologies that offer high reliability, effluent-free pumping, high levels of controllability and long service intervals even in the harshest processing applications.
One such response is seen in the CXS from Edwards, featuring new discrete, variable pitch tapered-screw technology for exceptional energy efficiency, reliability and performance.
This, the very latest in chemical pump technology 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.
Developments in control systems and improved solids handling are key focus areas in dry screw pump innovation. All chemical dry pump systems require a control system and the ability to handle process upsets for best performance, which in turn affects quality, consistency, reliability and yield. The CXS dry screw pump's onboard controller provides low-cost pump controls for a wide range of vacuum duties, from simple setups to very complex installations. The CXS rotors shrink away from the stator on cooling and with no end compression plate or exhaust valve and nowhere to trap particles, the pump is excellent at handling solids with a much reduced risk of cold seizures compared to other dry pumps. A CXS pump is not damaged mechanically by liquid slugs either.[Page Break]
Environmental improvements
The leading dry screw pumps on the market are now demonstrating strong environmental improvements, the greatest of which is offering operating power reductions of at least 50 per cent or more if the pump utilises a 'standby' mode. In some cases cooling water usage can be reduced by around 20 per cent and seal purge nitrogen by about 40 per cent compared to other pumps. 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.
Selecting the right vacuum pump system offers engineers an opportunity to reduce costs and at the same time, make productivity, quality, yield and environmental improvements. When calculating costs the key challenge is to consider all the factors affecting ownership, from installation, through to maintenance, utility consumption and effluent handling cost. It is this 'total cost of ownership' that will dictate the optimum system for any individual application.
New 'fit and forget' dry screw pumps perform well when compared with an equivalent wet pump. The main wet process vacuum technologies used in chemical and pharmaceutical processes are vapour pumps, steam ejectors, liquid ring pumps (LRPs) and oil-sealed pumps, including rotary-piston and rotary-vane types. Although the capital cost of a dry vacuum pump is often higher than that of an equivalent wet pump in standard materials of construction, there tends to be very little difference when the total installation cost is considered. When running costs are taken into account, the dry system often shows a considerably lower cost of ownership.[Page Break]
CXS dry screw pumps are performing well in the field with customers seeing substantial benefits from its installation. A global manufacturer of specialty chemicals used the CXS dry screw pumps to overcome cold seizures.The manufacturer was operating dry vacuum pumps on an evaporation process that incorporates a phenol condenser ahead of the vacuum equipment and experienced major downtime and maintenance costs due to phenol (melting point 41°C) solidifying in the pump when it stopped or cooled down. Significant pump damage was caused by attempted restarts. Despite the evolution of techniques to recover the pump after cold seizures, maintenance costs remained high as the liquid slugging and subsequent cold seizures could not be completely eliminated without expensive pipework changes.
Triphenylphosphate (melting point 48-50°C) was also causing problems and the dry vacuum pumps additionally suffered from irregular liquid slugging caused by inconsistent drainage sequencing of the equipment ahead of the vacuum pumps.
The manufacturer installed an Edwards CXS chemical dry screw pump to replace one of the existing pumps under guidance from Edwards.
CXS was recommended predominantly for its excellent liquids and solids handling capability and ease of recovery, even if seized. Unlike some other screw pumps, a CXS does 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.
Furthermore CXS has a built-in energy saving mode that minimises power consumption when vacuum is not required but maintains a pre-set minimum pump temperature so the pump is available for immediate use. This is better than pump switch-off between batches because it saves warm up time (about 30 minutes) and reduces the chances of cold seizures and associated damage when restarting.[Page Break]
CXS is capable of continuously pump up to one litre of liquid per minute and up to 25litre slugs without stopping.
Edwards also expected the manufacturer to benefit from developments designed to reducethe cost and disruption of planned downtime. CXS has been developed to ensure long service intervals of up to five years and no routine maintenance requirements over a 25-year+ life expectancy.
Due to the excellent liquids and solids handling capability of the CXS dry screw pump, combined with its unique ability to avoid cold seizures, to date no downtime due to the original problems has been reported and the manufacturer reports an increase in vacuum performance of the process plant.
Like other leading chemical processors around the world this Italian manufacturer of active pharmaceutical ingredients (APIs) was very interested in the new 'plug and pump' technology because of its ease of installation, integral control, very low noise level and low maintenance requirements. This company tested the CXS dry screw pump for almost a year, and the CXS is now a firm favourite as they phase out wet dry pumps altogether.[Page Break]
The latest evolution in dry pump technology offers manufacturers who are still using wet pumps the opportunity to 'leap-frog' to the benefits of a 'plug and pump' dry system with integral process control, clean vacuum and low maintenance and running costs.
This is exactly what a major international manufacturer of high quality flavours and fine food, pharmaceutical and cosmetic ingredients did when they eventually replaced all their LRPs, oil-sealed and steam ejectors with dry pumps. Several months on, the new dry pumps continue to operate well under harsh conditions, with no problems.
Ongoing innovation
Innovation in this sector continues, with dry screw pumps delivering ever more 'utilitarian' high performing, highly controllable vacuum equipment that allows processors to minimise operating costs and reduce environmental impact.
Dr Don Collins is with Edwards Ltd, Bolton, UK. www.edwardsvacuum.com