Contaminated waste disposal during a manufacturing process is a significant cost to the pharmaceutical industry. HereUwe Gottschlich argues that vacuum pumps with a uniform temperature control throughoutoffering the ability to control heat – accelerated degradation/ polymerisationgives the best solution.
Dry-running designs of vacuum pump are increasingly becoming the dominant choice of vacuum generating machine within the pharmaceutical industry. The decision to adopt such technology generally being on the basis of reduced effluent/abatement costs. No service liquid or internal lubricant within the machine results in no contaminated waste disposal costs.
Machines that offer universal compatibility can only handle corrosive gasses and vapoursof a particularly wide-ranging nature. Reliance upon corrosion resistant metals and polymers provide a degree of protectionbut can be susceptible due to the thermal conditions that are particularly arduous within dry running vacuum pump technology. Additionallyhigh nickel coatings on ferritic base metals can accelerate galvanic corrosion as soon as any flaws appear. It is widely recognised that highly corrosive media can be handled without the need for such susceptible exotic materials.
On the basis that machines specifically designed for reliable temperature control ensure that corrosive media remains in the vapour phaseductile irons provide a good metallurgical solution. Machines with a uniform temperature profile throughoutoffer the ability to control heat-accelerated degradation/polymerisation.
Generallyvacuum is utilised within the chemical process industry in order to ensure that product degradation does not take place with heat sensitive media. Therefore applications such as distillationvaporisationand drying are undertaken at relatively low temperatures. As a rulethere is a pre-condenser located upstream of the vacuum pump in order to reduce the volumetric flow rate by condensing the vapour into a liquid. The purpose of the vacuum pump isthereforenot really to cope with the excessive free vapour and gas loadbut rather to handle an optimum flow of pour saturated gas. The pharmaceutical and fine chemical industry are particularly reliant on
multi-purpose plantsie plants designed for continuously changing media and process conditions.
The list of demands placed upon a vacuum pump that must be reliablein this fieldare extensive and can be prioritised as follows:
- Safe operation with flammable vapours (Low gas temperatureno mechanical ignition source.
- Problem-free handling of corrosive media (elevated gas temperatures avoid condensationas corrosion occurs only in the liquid phasepumping of thermally sensitive media/covering agents.
- Reliable temperature profile that ensures (1) gas temperatures that is suitably low enough to avoid crackingwhile (2) high enough gas temperatures to avoid crystallisation – easy rinsing/flushingeasy maintenance/clean-in-place (Fig.1).
From these demandsit is easy to deduce the ideal temperature condition inside of the pump is very important. It must neither be too cold nor too warm.
Optimum cooling
Dry-running vacuum pumps are characterised by almost adiabatic compression with very small mass flow rates. Since the mass flow rate is almost zero during operation at very low suction pressuresand there are no service liquids presentthe
compression-generated heat cannot be automatically dissipated. Alsothe heat created by (1000000:1) compression ratios is exacerbated because of the low density of the gas under such levels of vacuum.
The simplest way to remove the heat is through jacket cooling of the pump. Fig. 2ahowevershows that the low gas density adversely affects heat dissipation through convection to the cooled jacket. In other wordsthe temperature distribution throughout the pump is relatively inhomogeneous. As the pump becomes largerso does the heat transfer rate per displacement volume. Peak temperatures in excess of 200°C are common place with pump capacities in the region of 250m3/h.
Since elevated temperatures can cause ‘cracking’ products to suffer accelerated polymerisationdegradationor simple sublimationthere exists an increased danger of an active ignition source. In other wordsas the clearances within the machine are consumed by product depositionfriction can generate hotspots to the extent of localised ignition. Lowering the peak temperature is only possible if the coolant temperature is extremely low. Along with practical constraints of providing coolant at such low temperaturesproblems occur with condensation at the inner wall of the jacket. The cold inner surface of the casing chills any corrosive vapoursas they enter the pumpand causes condensation that leads to corrosion.
Consequentlyit can be deduced that dry-running vacuum pumps in excess of 250m3/h must employ a heat dissipation mechanism more advanced than simple jacket coolingif they are to be suitable for multipurpose plants. Internal rotor cooling provides a solution to this problem when utilised in conjunction with the typical jacket cooled system.
Fig. 2b illustrates the principle of dissipating heat from all gas-contact surfaces. Importantlythe jacket temperature can subsequently be increased in order to remove problematic quench zoneswhilst maintaining internal temperatures of less than 200°C.
Howeverthe heat transfer rate per displacement volume becomes detrimentally affected as the vacuum pump size is increased. Moreoverpeak temperatures below 200°C are seen to be achievable only with a suction capacities less than 400m3/h. The thirdand most effectiveway of cooling is through direct gas cooling and is shown in Fig.2c. This system feeds cold gas directly into the compression cycleand permits heat transportation through the unit.
Such effective cooling is not only achieved through the mixing of hot and cold gasbut also by two additional aspects: (1) The coefficient of heat transfer (Cp) is improved with an increase in gas densityand (2) The increased mass flow enhances heat dissipation. The result is a very homogeneous temperature profile that allows further elevated jacket temperatures without reaching detrimentally high internal gas temperatures.
With regard to the pumping of corrosive mediathis solution offers additional advantages. As stated earlier; corrosion will not occur in whilst it remains in the gas/vapour phase. It will only take place if the media is allowed to condense. Furthermorecondensation will only take place if the partial pressure of the condensable component reaches saturation during compression from vacuum to atmospheric pressure. The dilution effect of the Direct Gas Cooling system reduces the partial pressure of the condensable(s) within the pump and actually prevents condensation. Should the vacuum pump be equipped with a post (abatement) condenserrelatively cold gas can be taken from the exhaust flow. Importantlythis downstream condenser must be constructed from suitable corrosion-resistant materials. As Fig.3 shows; the cold gas is injected into the machine at a point where compression is greatest. Because this point is significantly downstream of the pump suctionneither suction pressure nor flow rate is compromised.
Practical mode of operation
The key to pumping corrosive media with dry-running vacuum pumps is to avoid condensation. Moreoverthis is basically achieved by utilising the heat associated with very high compression ratios within the pump.
A prerequisitehoweveris that the pump is always at the required operating temperature when corrosive media is introduced. Control of such temperatures is predominantly important during periods of start-up and shutdown. The process should always commence with an adequate warming-up cycle. Generallythis is achieved by running the pump against a closed suction-line valve for a pre-defined period of time.
Once the desired internal pump temperature is correctthen the suction line valve can be opened in order to introduce the corrosive media. When shutting down the pump and processit is important that the pump/system is completely clear of any residual corrosive media. Otherwisecondensation can occur within the pump and/or surrounding pipe-work and valves during the cooling phase.
And once the process is completethe suction-line valve should be closed while the pump continues to run for a given time. Howeverwith the suction-line valve closed there is insufficient inert gas flow needed to remove all remaining corrosive substances. For this reasonit is important to flush the machine with an inert media such as nitrogen. Only when the nitrogen inertisation is completeshould the pump be switched off. In many process plantsseveral vacuum pumps are connected in parallel and linked together by means of a common exhaust manifold. A cold vacuum pump that is open to the exhaustacts as a condenser.
In order to avoid warm vapour saturated corrosive gas from getting into a cold static machinesuitable isolation measures are advisable. This is possible by incorporating an isolation valve on the discharge side of the each vacuum pump.
Many fields of application do not permanently require a vacuum pump. Although continuous running is ideal for the pumpthis would be a blatant disregard of cost. Because of the necessary warm-up and shutdown cyclesit is only worth switching off the pump if it is to be idle for long periods. Pumps with variable speed operationand those with switched phase modes of operationprovide the optimal solution. In other wordswhen there is no requirement for vacuum the pump automatically switches to a stand-by speed in order to retain heat while ensuring that the pump is inertised. During such timesthe suction valve remains closed. Importantlythe power absorbed is significantly reduced in order to save energy and subsequent cost. As soon as vacuum is requiredthe suction valve can immediately be opened to corrosive flow without the need for additional warm-up cycles.
Uwe Gottschlich iis with Sterling SIHI GmbHItzehoeGermany. www.sterlingfluidsystems.de
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