in the Chemical laboratory Edited by NORMAN V. STEERE, School o f Public Health. University o f Minnesota, Minneooolis. Minn., 55455
XXVIII. Unattended Laboratory Operations-Part
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One
The Safe Operation o f Laboratory Distillations O v e r Night D. R. Conlon, Instruments for Research & Industry, Cheltenhom, Pa. I. Introduction Scientific laboratories differ greatly in the extent to which various laboratory apparatus are permitted to run mattended during the evening hours. Most scientists consider the familiar "dist,illed water" still to be free of trouble. Many labor* tories have had these stills operating day and night for years with the very minimum of attention. Other distilling units are, however, often regarded as hnzardoiis either hecause of their constn~etion or because the material being dist,illed is toxic or flammable. If these other stills are examined carefully, the various sources of potential brouble can be eliminated and these stills too can be safely operated into or t,hnmgh the evening holm. This article discusses the problems that may arise in running distillations unattended, and suggests various solutions to these problems.
II. Failure of the Cooling Water Mast distillation units operate with water-cooled condensers; therefore, i t is
essential for safe operation that the water supply be dependable. However, wat,er pressures change: 1. There are decremes in water presswe which may be caused by failure of a, pomp, by partial blocking of the mains, or by increased consumption by other lahorat,ories or by other usen on the same water main. 2. There are increases in water presswe which may aenrr when auxiliary pumps are being switched on, usually a t the beginning of the working day; or when other laboratories tnrn off bheir wat,er valves, usually a t theend of Lhe day. Either type of pressre variation can cause trouble. Too low a pressure results in an inadequate wat,er flow to the cooling condenser, t,hereby allowing dist,illat,e vapors to escape. Too high a. pressure can cause the tubing connections to the rondensers to expand and burst. This is doubly mdesirable: the laboratory is flooded, and, since t,here is no cooling in the condenser, again vapors may be relea5ed. I t is not s t all difficult to orotect aeainst, both hazards if the following four-point approach is used: A. Install a. simple pressure r e p l a t a r
HYDROSTATIC PRESSURE
Figure 1. Cooling woter-Row monitors (1A glars, i B metoll. When the Row ceorer, the column droinr through the capillary, and the mercury contqct or the pressure switch opens.
Donie1 R. Conlon wor trained aa a phyricol chemist a t Union College, receiving o Bachelor of Science degree in 1935, and worked ten year. in petroleum research a t Atlontic Refining Compony and eleven years ot Rohm ond Haor (three in physic01 chemistry. and eight in instrumentotion). In 1957, Mr. Conion storled "laR-o company that specializes in products which orrirt research, provide safety, and reduce the tedium of repetitive laboratory opemtionr. Mr. Conlon is president of "IIR"-lnrtrumen* for Rereorch ond Industry, ond o member of the American Chemical Sociely, the lnrtrvment Society of America, and the Americon Association for the Advancement of Science.
(Ref. 1) in the water main ahead of the valve that is used to adjust the flow to the condenser. The regulator can be adjusaed to maintain an intermediate water pressure; i t will then minimize the effect of both increases and decreases in the snpply pressure. B. Protect the pressure regulator with a suitable water filter, which will prevent particles of rust and other foreign materials from affecting its operation. Commercial filters with replaceable cartridges made of cotton or nylon twine are readily available (Ref. 2). C. Monitor the flow of cooling water flowing from the cooling condenser to the drain. Monitoring this emergent waler flow ensures that water has flowed through the condenser. Thus, it is inherently a. more reliable approach than monitoring the main pressure. D. Install a solenoid valve (Ref. 3) in the water mpply line and connect i t electrically to the water-flow monitor so that electrical power to this valve and t,o the still-pot heater will he turned offwhen the water-flow monitor responds ta a water failure. A number of water-flow monitor
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devices have heen desoribed in the titers, ture (Ref. 11-16). A modification of Houghton's device is shown in Figure 1A made of glass m d in 1B made of metal. This design has the advantages that i t imposes no back pressure an the apparatus being monitored; it possesses a short time delay so that adjustments in the flow oan he readily made; and its flow-restricting capillary can he readily inspected. Figure 2 shows a, commercially available water-flow monitor (Ref. 4) based on this
fluctuations: the constant-voltage transformer. A 500, 1000, or 2000 watt m i t would he suitable. A 2000 watt Sola constant-voltage trttnsformer (Ref. 5) costs approximately 5250 and can sccommodate several distillation columns. Such a unit is somewhat bulky (27 X 32 X 51 cm); however, i t may he mounted vertically under the other electrical controls. The insurance i t gives against fluctuations in line voltage greatly outweighs any inconvenience. These units are inherently reliable and trouble free. B. A constant-voltage transformer eannot cope with a line-voltage failure. Sueh failure in itself is not serious: the distillation merely stops. However, when the power is turned on again, "bumping" may well take place in the still pot. If "humping" is a serious problem, one should eonsider wiring a suitable relay into the stillpot heating circuit. Failure of the electrical supply should cause the relay to "drop out" and to stay out until i t is manually reset. An alternative approach to preventing humping by keeping the power off once it is turned off is to use the solenoid valve referred to in Section IID plugged into the water-flow monitor (Section IIC). Failure of the electric supply will cause the solenoid valve to close, shutting off the flow of cooling water, and hence causing the water-flow monitor to de-energize its outlets. Since the solenoid valve is connected to one of these outlets, both i t and
whether to leave an unattended distillation running during the night is how to stop the distillation if it is not to run until morning. There are severd methods; the choice depends upon the nature of the distillation.
A . Timer Terminations If the distillation is similar to the redistillation of a solvent, the still-pot and the still-head temperatures will not change greatly during the course of the distill* tion. Consequently if, as has been recommended, a. constant-voltage transformer is used to guard against variations
Figwe 2. Woter-Row monitor equipped with alorm ond convenience outlets.
same principle. The commercial unit has an a l a m built into it plus a number of electrical receptacles, several of which are automatically turned off when the cooling waler fails. The four-point approach recommended above is shown in Figure 3, items A, B, C, & D.
Ill. Failure of the Electrical Supply I t is important that thelinevoltage used for the distillation remain relatively constant since even moderate changes will aHect the rate of distillation. However, electric voltages, like water pressures, change: 1. There are decreases in line voltage due to increased consumption by other laboratories, usually e d y in the day. This will he most noticeable if the lines are inadequate. 2. There are inoreases in tine voltage when additional generating equipment is turned on, or when other labomtories turn off their electrically operated equip ment, a t the end of aday. The following two-point approach will guard against difficulties from these variations: A. As with water pressure, there is a convenient device for overcoming source
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Figure 3. Dirtillotion ~olvmnshowing IAl water presrure regulator. (81 woter filter, ICI emergent water-now monitor, (Dl solenoid vdve in woter IEE'I IFF') copocitonce-o~tuoted line, controllerr for monitoring still-pot and rtilihead thermameten.
the water-flow monitor are unable to reset themselves should the power failure be corrected. This approach has the advantage of having time delay inherent in it-it will not respond to momentary electrical cessations.
IV. Termination of Distillation A question that often arises in deciding
Figure 4. Distiliation column with still-head temperature being monitored for overtempemlure or undertemperature by AA', and still pot being monitored for overtemperature b y either Note: If AA' is not used for 08' or CC'. safety monitoring, it may be used to control the reflux ratio wtomotirolly b y turning the refiux timer off whenever the still-head temperature exceeds the l e t point.
in line voltage, the distillation rate will be reasonably constant. Withexperienceone can, therefore, estimate rather closely the amount of distillate that should eccumulate during the night, and the distillation can he safely terminated with an electric timer. Sueh timers are familiar tools in most laboratories.
B. Temperatu~eTerminatiom If the boiling range of the material being distilled is wide rather than narrow, the rate of distillation will not remain constant. I n this case, it will he better to
proaches may be osed: 1. If the system need not be airtight and the product receiver can be supported on a pan balance, movement of the balance can be used to open an electrical circuit and terminate the distillstion. When the balance moves, the heater is tunled off (Fig. 5). The weights used on the belance should allow for the force used to actuate the switch. This approach can be modified if the svstem must be airtieht: flexible tubine &n he run from the condenser to t h i product receiver mounted on the balance. I t may be necessary to run two flexible lines-ne for liquid flow, the other for vent. Care should be used in selecting the tubing used. I t must not be attacked by the materials being distilled. 2. If a volume termination is preferred, there are several choices. Some of these make use of merculy contack, of capacitance-actuated controllers, or of the optical detection of liquid level. Figure 6 shows schematically three examples.
terminate the distillation a t a predetermined temperature rather than a t a predetermined time. This is a common situstian, not only with distillations left running for days on end but also with one-day nms that are not quite finished hy "quie ting time." The scientist can readily terminate such a distillation through the use of a capacitance-actuated controller (Ref. 6) which senses the position of the mercury in a thermometer located in the still pot or in the still head. This is shown in the photograph Figure 3, items EE' and FF'. I t is shown diagrammatically in Figwe 4, AA' and BB'. When the still-pot heater
V. Possible Overheating of the Still Pot Figure 5. Termination of dirtill.tion of product.
Once the foregoing factors have been taken into considerat,ion, the more subtle hazards should receive attention. Labor* tory distillations involving many liters of raw material are not likely to ruu dry overnight. However, it is conceivable that sooner or later this unlikely situation might develop because of some overright. The still can be monitored either by an aver-temperature or an under-temperature approach: l a . If a heating mantle is osed with the still pot, one can make use of the thermocouple that is emhedded in most heating mantles (Ref. 7). This can be attached to a suitable millivoltmeter equipped with an adjustable overtemperature-sen-
b y weight
is plugged into the controller and its capacitanowensing head is attached to the thermometer, the heat to the st,illpot will be cut off automatically when the temperature rises to the desired value. (A limit switch in the controller which must be manually reset keeps the power from turning on again when the temperature drops.)
C. Weight and Volume Terminatiom If one prefers to terminate the distillation on the basis of the weight or the volume of the distillate, several other ap-
1
PHOTO CELL
MERCURY
F
A
B
C
MERCURY CONTACT
CAPACITANCE SENSING
OPTICAL S t N S I N G
; 6. ~Termination ~ ~ of distillotion ~ by volume of product.
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sing pointer (Fig. 4, CC'). These units, known as meter-relays, are commercially available (Ref. 8). They should be ordered complete with auxiliary relays capable of interrupting heater currents of several amperes. l b . The still pot itself can he monitored for overtemperature by the use of a thermoregulator plus an electronic relay or by a still-pot thermometer with a eclpaoitance-actuated controller which senses the position of the mercury-as in Section IVB above (but for safet,ymonitoring rather t,han for termination). See Figure 4, BB'. 2. In some cases it is convenient to monitor the still-head temperature against a decrease in t,emperat,ure. A decrease should occur if the liquid in the still pot becomes quite small in volume: the rate of distillation will then decrease greatly and the still-head temperature will drop. This monitoring of the still-head temper* ture can also he aeeomplished readily with either the thermocouple plus meter-relay (Ref. 8) or with the at,ill-head thermometer plus a capacitance-actr~ated controller (Ref. 6). SeeFigure 4, AA'.
VI. Breakage and Physical Separation I n any safety discussion it is important to consider the possible breaksge of the glass equipment. Such breakage is highly unpredictable since it may result either from residual internal stresses in the glms, from stresses generated by improper external supports, or from an accidental hlnw. .-Internd stresses in the glass apparatus may arise from inexperienced glass blowing or from careless commercial manufacture. I t is stmngly mcommmded that all parts of Wle glass distillation equipment be carefullq annealed and checked for residual atress by means of polarized light. Commercial polarized-light units are available for this purpose (Ref. 9) or a stress-analysis unit can be improvised from two sheets of polamid and a light source. Some lsrge research laboratories inspect with poI d z e d light all glass apparatus which they receive. The method of supporting a distillation column is of the greatest importance in minimizing external stresses. Thought must be given to the design of the s u p porting framework, the method of clamping, and the design and location of the individual clamps. A complete distillation column is usually rather tall and narrow. The column, head, and receivers can, therefore, be supported by a single support rod. To ensure that the sebup is not flimsy, the rod should he a t least 1.90 em ('/,") in diameter and should he securely fastened to a wdl. The use of a rigid support rod and heavy-dut,y clamps and clamp holders is import,ant in order to prevent one part of the apparatus from shifting with respect to the other parts. The following procedures are recommended: ( a ) The column itself is first firmly (Continued a page A694)
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clamped either a t its top, bottom, or a t some enlarged portion. The clamp must he strong enough to carry the full weight of thc column and head. ( b ) A second clamp is then placed on the column. This is tightened only slightly; it is intended as a guide against lateral movement. ( e ) When the still pot is attached to the bottom of the column, i t can be held in place by a. cradle supported by springs. The still pot will then be free to align i& self with the column. (d) The distillation head sits on the top of the column and is held loosely in position by a clamp which allows i t to move vertically but largely restricts lateral movement. (e) The receivers are also supported with care in order to prevent them from pulting strain on the distillation head. Careful supporting of the apparatus is essential in order to prevent physical parting of the ground joints as well as to prevent strain and breakage. If the apparatus is extensive, or if the receivers when loaded will be heavy, then i t may be desirable to introduce semi-flexible connecting links between the column and the receivers. One way of doing this is shown in Figure 7. The links illustrated are easilv fabricated from ground-glass bdl-and-socket joints.
the apparatus in a comer of the laboratory, out of the main line of traffic, and by (2) the use of adequate safety shields. Such shields may he flat, rectangular, or semicylindrical. Those shown in Figure 8 are hinged and particularly convenient to use (Ref. 10).
Figure 7. Ball-and-socket links in transfer liner can Rex and hence eliminate any stress present.
I n passing, mention should be made of an alternative method of supporting distillation equipment: namely, to mount the assenlhly on a large panel made of metal or other non-flammable material. In this case, a large number of supporting brackets may be used to clamp all parts of the assembly. A glrnsblower may well seal the parts together after they are supported, thus eliminsting any residual stresses due to the clamping. Finally, hreakage due to accidentel blows should be prevented by (1) locating
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In any safety program it is sometimes difficult to agree on the exact sources of potential trouble and their relative importance. Hence it is difficult to agree an what precautions are reamnable and what are unreason%ble. But there are few reasons why distillstions cannot be operated unrtttended-not only during the night, but during the day as well-if careful plans are worked out along the lines suggested here. Next month's issue will contain an article on the safe operation of unattended chemical reactions.
References Components Cited
~
SPRING CLAMP*** on each j o i n t *
IX. Conclusion
Figme 8. Modern acrylic rhieldr protect oppordus agoinat externol blows, yet ore easily opened for rnonipulation.
VII. Overflow Once a laboratory distillation is set up and allowed to run aver night, the opersr tion may prove so convenient and satisfactory that the scientist forgets his initial caution. He might, a t this time, forget to empty the receiver a t the proper time, or might underestimate the distill* tion rate. A simple safeguard is to use a product receiver that is large enough to contain the entire charge to the distillation. If this is undesirable, one can use a small receiver, but connect it to a large overnow receiver. A five- or ten-liter bottle equipped with s, two-hole rubber stopper and a drying tube in the vent line k a convenient safeguard. Even after these precautions have been taken, it is worthwhile to mount the entire distillation apparatns in or over a metal tray large enough to contain all the material involved. A tray 5 cm deep is usually adequate and is not tao cumbersome. VIII. Monitoring by the Woichman I n large, well-staffed laboratories, the safety committee may insist that the night watchman, on his periodic trips through the laboratory, glance a t any equipment that is running. Some scientists may feel that their "gxard" has no feeling for scientific apparatus. Nevertheless, whenever apparatus is running a t night i t is wise to have the apparatus so labeled that an inexperienced, scientifically untrained guard can turn it off-preferably by tuming no mare than two or three canspicuously labeled switches and valves.
NOTE: These are typical components that may he used in the manner described in the text. Model numbers of electrical components correspond to 110 volts, 60 cycles. 1. Water Pressure Regulator, Catalog #1/4-N263. Watts Regulator Co., P. 0. Box 810, Lawrence, Mass. 01842. 2. Water Filter, Model XLIOU-3/4. Filterite Corp., Timonium, Md. 3. Solenoid Valve, Catalog 8826225. Butomatic Switch Co., Florham Park, N.J. 07932. 4. Warer Flow Switch, Model HD-3P-2. Instn~mentsfor Research & Indnstry, Cheltenham, Pa. 19012. 5. Constant Voltage Transformer, type CVS; for 2000 watts. Catalog 823-26-220. Sola Electric Co., Elk Grove Village, Ill. 60007. 6. Therm-O-Watch Controller, Model L-6. Instruments far Research & Industry, Cheltenhsm, Pa. 19012. 7. Glas-Col Heating Mantles (various sizes and styles). Gla5-Col Apparatus Co., Terre Haute, Ind. 8. Meter-Relay, Model 503K Compack I . Specify temperature range and t,hermocouple wire. API Instruments Co., Chesterland, 0 . 44026. 9. Bethlehem Apparatus Co., 825 Front Street, Hellertawn, Pa. 10. Lsh Guard, Model H-16-30-1C. Instruments for Research & Indnstry, Cheltenhrtm, Pa. 19012. 11. BURFORD,H. C., "Improved FlowSensitive Switch," Journal of Scientific In&umats, 37, 490 (1960). 12. Cox, B. C . , "Protection of Diffusion Pumps Against Inadequate Cooling," Jounvll of Scienlifie Instruments, 37, 148 (1960). 13. HOUGHTON, G., "A Simple Water Failure Guard for Diffusion Pumps and Condensers," Journal of Sciatific Instruments, 33, 199 (1956). 14. MOTT, W. E., AND PETERS, C. J., "Inexpensive Flow Switch for Laboratory Use," Rariew of Scientific Inslrumenls., 32., 1150 11961). . , PIKE,E. R., AND PRICE,D. A,, "Water Flow Controlled Electrical Switch," Review of Scientific Instruments, 30, 1057 (1959). PRESTON, J., "Bellows Operated Water Switch." Journal of Scienli$c Instruments, 36, 98 (1959).
