PLANT ENGINEERING

Ecology squaredSolvent recovery without coolantDr. Juergen Dirscherl

The Peltronic condenser with integrated electronic chiller offersadvanced possibilities for efficient solvent recovery using labor-atory vacuum pumps, even in applications where cooling water is un-available or undesirable. The minimal space, installation and maintenancerequirements and the costs for purchase and operation are much lower thanwith a closed-circuit chiller.

In virtually all vacuum applications inchemical and pharmaceutical laboratories,solvent vapours are transferred throughthe pump and released from the pump out-let. Typical examples include vacuum fi1-

tration, fluid aspiration, gel drying, va-cuum drying ovens and sophisticated sys-tems like vacuum concentrators and rotaryor parallel evaporators.The water aspirator pumps which werequite common in the past have many dis-advantages in terms of economy (highwater costs) and ecology (solvents enteringthe waste water). Chemistry diaphragmpumps avoid these problems. However,this pump type also releases solvent va-pours at the exhaust. The introduction of

pumping units with an integratedexhaust vapour condenser in

. 1988 put an end to this dilem-: rr1ä. The solvent vapours are

condensed and recovered on

a cold surface at atmospheric pressuredownstream of the pump exhaust. Manyusers even recycle the re-collected solvents.Using this weil-known exhaust vapourcondenser, up to 98 or 99"/" of the usual sol-vents can be recovered at a reasonable cool-ing water temperature.If another nearby device (e. g. the main con-denser of a rotary evaporator) also needscoolant, it is a straightforward matter touse the same coolant supply for both con-densers. This coolant might be either a

fluid (e.g. water or glycol) or ice (e.g. dry orwater ice). In the case of a fluid coolant, asimple pipe connection is recommended inseries from the main condenser (vacuumside) to the exhaust vapour condenser (at-mospheric side). If the main condenser isice cooled, an equivalent exhaust vapourcondenser that also uses ice as coolant isappropriate. Ice cooled exhaust vapourcondensers that meet this reouirement

are meanwhile available. Hoviever, thecosts for a continuous supply of dry orwater ice are comparatively high and thecoolant has to be checked frequently.Not all users by far of chemistry diaphragmpumps work with an exhaust vapour con-denser. There are many reasons for this,and often no coolant is at hand:. Not all laboratories have a closedlooo

coolant system.. The use of fresh water for cooling pur-

poses is very expensive and wastes animportant resource. In some countries itis already prohibited for this reason.

o Laboratory-scale, closed-circuit chillersare likewise cost-intensive and requirefrequent maintenance (coolant levelchecks).

o OnlI rotary evaporators (and some par-allel evaporators) actually have an inte-grated condenser (for the vacuum side)and only these devices require a separatecooiant supply. Most other vacuum ap-plications work without any a supply ofcoolant. This means that such a supplywould "merely" be required for theexhaust vapour condenser - somethingwhich many users try to avoid.

In other cases, pipes for a fluid coolant aresimply not desirable. In modern chemicaland pharmaceutical laboratories, local areavacuum networks (like the Vacuu-LANsystem) are more and more common. Thevacuum pumping unit (with its exhaust va-pour condenser) is often integrated into thelaboratory furniture (e.g. below the bench).Laboratory planners and users normallytry to avoid coolant pipes at such inaccess-ible sites through fear of water damagefrom loose or leaking coolant hoses. Va-cuum pumping units mounted on a trolleyfor mobiie use are another example - theycan hardly be equipped with a connectionfor fluid coolant. Complete systems com-prising, for instance, a vacuum concen-trator and a vacuum pump tend not to havean exhaust vapour condenser. A connectionfor coolant is thought to make the systemunattractive to customers.

Exhaust vapour condensationwithout a coolant supply

Vacuubrand has developed an exhaust va-pour condenser with an integrated chiller.It is based on the thermoelectric (Peltier)effect and works without anv internal or

Chemistry pumping unitswith an ordinary exhaust va-oour condenser. Left: Va-

cuubrand's PC 5 from '1988.

right: modern PC 3004 Variopumping unit

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Solvent recovery rates for ordinary and Peltronic condensers

external coolant. The Peltronic exhaust va-pour condenser cools the exhaust vapoursdirectly in the absence of fluid for heattransfer. The system is virtually mainten-ance-free and leakage of coolant is imposs-ible. The vapours only come into contactwith chemically resistant surfaces like spe-cial plastics and glass.This new approach was made possible byrecent advancements in the

Chemistry pumping unit with a Peltronic condenser: PC

600 LAN NT, Peltronic and liquid level sensor

cooling power and efficiency of Peltier ele-ments and by the realisation that the cool-ing port'er required for efficient conden-sation of vapours at the exhaust of a typicallaboratory vacuum pump is amazinglylow. All typical solvents are liquid at roomtemperature, which means that a coldsurface temperature slightly below roomtemperature is generally sufficient. Muchlower temperatures are regarded as dis-advantageous because the solvents mightfreeze. Practical experience shows that+10'C is absolutely sufficient to achievefairly high solvent recovery rates.ln addition, the amount of vapour - in

other words, the required cooling power -is mostly quite small, as the solvents areusually sucked into the pump at very lowpressure. Only for low-boiling point sol-vents like acetone do larger vapour quan-tities have to be handled.The Peltronic condenser has a very com-pact and functional design, offering highlyefficient solvent recovery for the majority

of laboratory applications. It was de-

^ . sisned for condensation of solventsK" downstream of the vacuum pump,

7 \ that is io say on the atmospheric' I side. lhe cold surface temperature

' J anci cooling power are not aci-

equate for solvent condensation onthe vacuum side. If large amounts ofsolvent are evaporated - as in a rotary

. evaporator - a vacuum-side condenser(with coolant supply) is still necessary.The Peltronic condenser features an in-

r tegrated, electronic, cold surface tem-perature control (preset to +10'C), a

controlled fan and hot-side temperatureprotection, making it very easy to use. In-stallation could not be more straightfor-ward - the equipment simply has to beconnected to a mains supply and to thevacuum pump outlet. If the Peltronic con-

denser is built into laboratory furniture, forexample, the additional heat release of be-tween 20 and 200 W (including the heat ofcondensation) has to be taken into account.Suitable fresh air circulation must be pro-vided. Nevertheless, the waste heat releaseis much lower than that of tvpical closed-circuit chil1ers.

Evaluation results

Evaporation tests were performed using a

rotary evaporator and a chemistry pumP-ing unit (Vacuubrand PC 2001 Vario). Thetotal solvent recovery rate was measuredfirst with an ordinary exhaust vapour con-denser (coolant temperature +10'C) andthen with a Peltronic condenser. The results

Total solventrecovery rate

(%)

99.8

98.9

98.8

97.8

98.3

are shown in the table. The differences inthe solvent recovery rates are within themeasuring tolerance. Even small vari-ations in the ambient and starting con-ditions can cause such fluctuations. Thetemperature of the coolant in the main con-denser of the rotary evaporator is espe-cially important. The evaporation timewas about 20 minutes for ethyl alcohol and10 minutes for acetone. To maintain itscold surface temperature at +10 oC, the Pel-tronic condenser was only at full coolingpower for acetone.In another performance test, the operationof a rotary evaporator on a local area va-cuum network was simulated. It was as-

sumed that multiple users of the vacuumnetwork introduce additional gas loadsinto the system, causing the vacuum pump(PC 2004 Vario with 2mbar ultimate va-cuum) to reach only 15 mbar inside the net-work at fuil pumping speed. In addition,the main condenser of the rotary evap-orator was only operated with coolant at18'C. These conditions imply a very highgas and solvent vapour load on the exhaustcondenser. The results are given at the bot-tom of the table.In short, the solvent recovery rates of thePeltronic condenser are excellent evenunder challenging (high load) operatingconditions - comparable to those of a con-ventional (water cooled) condenser. Thevapour pumping capacity of the vacuumpump will be the limiting factor for mostapplications, not the condensation capacityof the Peltronic condenser. The conden-sation power of the Peltronic was calcu-lated for various solvents (at 25 "C ambienttemperature). The vaiues are as follows:a00 g/h for acetone, 260 g/h for ethyl alco-hol, 600 g/h for toluene, 600 g/h for chloro-form and i00 g/h for water.Hall 5.1, Booth F3-F4

Vacuubrand GmbH + Co. KG

info@vacuubrand.de

www.vacuubrand.com

Exhaust vapourcondenser type

Common

Peltronic

Common

Peltronic

Peltronicunder high loadconditions

Solvent

Ethyl alcohol

Ethyl alcohol

Acetone

Acetone

Ethyl alcohol

Evaporatedquantity (g)

398.4

427.0

387.1

419.6

477.3

Solvent recovery atrotary evaporator

main condenser (g)

376.8

400.7

295,3

337.9

417.2

Solvent recoveryat exhaust vapour

condenser {g)

20,7

zt,)

87.1

72.3

51.8

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