in the Chemical Laboratory Edited by NORMAN V. STEERE, School of Public Health, University of Minnesota, Minneapolis, Minn., 55455
XXVIII. Unattended Laboratory Operations-Part
Two
Running Laboratory Reactions Under Safe Control
D. R. Conlon, Instruments for Research & Industry, Cheltedam, Pa. Introduction
quite complicated setups used for chemical operations and chemical reactions. In many cases thescientiit wodd find it to his advantage to have such equipment nm unattended either for minutes, hours, or days. The previaw article discussed the various faelon involved specifically in rnnning laboratory distillalions safely. This second article dkcnssei running other eqnipment unattended. Lahorntory scientists and lahoratory safety committew differ greatly in t,heir attitudes toward apparatus and in their willingness to have apparatus running unattended a t any time. Therefore, pmrtie= vary in different laboratories. I n some, prsctiexlly every pieceof equipment, is shut down a t the end of the working day. In abhen, numerous piece? of eqnipment and even chemical reactiann run under snlomatic control by day and hy night. In general, the determining facton in deciding whether it is worthwhile to work on1 a safe set of conditions so that a given piece of apparatus can nln ,mattended are: 1. I s the operalion repetitive and while not lengthy so demanding of close personal attention thnt it is quite tedious? (Exothermic reactions and vacuum s t r i p ping operations are examples.) 2. I s it a lengthy experiment requiring many hours or days to run?
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hy mi electric heater of either au external immerEioll type. Electrir%l power is s~lppliedto the heater cyelirslly through a temperature controller connerted to a sensing prohe in the appsrat~w.T I Idetect thermal malfunction of such an apparatus, one may w e a Monitor to watch:
and "safety
1, The ,,f the apl,nrat,,a, 2. The temoerature controller.
I. Simple Endothermic Experiment Figure 9 show3 three apparatus which
3. The temperatnre 4. The poaer supplied eyelicall?- to the heater. .j The temperature of the heater.
hoth "control monitoring" monitoring."
from a safety point of view may he eonsidered together: the laboratory oven, the constant temperature bath, and the glass reaction flask. The assumptions made in granping lhese t o ~ e t h e rare: (1) only the variableof temperatureneed beUwatched"; ( 2 ) should the temperature exceed s. desired value, this can he detected by a monitor which automatically turns off the electric power; (3) since the operation is not exothermic, once t,he power is turned off, the apparatus will cool safely. Apparatus such as ovens and constant temperature baths are not usoslly considered to be hazardous. Therein lies the danger: they are overlooked in safety inspections. T h k is particularly dangerou3 in the cme of oil baths. If the oil bath thermostat fails, the oil will overheat and a. fire may start. (Even if no fire result?, valuable apparatus may be damaged and irreplaceable sample5 may be lost.) Figwe 10 reprewnts schematically an experimental apparatus which i? heated
One might reason bhat since the experiment is primarily concerned with the temperature of the apparatus (Item d l ) this is what should be monitored. A good way of so monitoring k to use a secondary controller and probe that are independent of the primary temperature controller. If room ii available, a relatively rugged bimetallic thermoregulator ( I ) h ideal for this purpose. I t should he set to operate 5 to 10' ahove the normal temperature of t,hebath. It can be electrically connected to take over control of the temperature if the primary controller fails. In that event it would hold the hxth temperature a t the 5 to 10" higher level. I t may, on the other hand, be wired to a relay circuit (Fig. 11) designed so bhat once t,he thermoswitch opens the relay will keep the power off until a push-button switch is reset. Figure 1 2 shows a eammercially available "over-temperature cutoff" based on this design ($).
a. If il, is, are there real di.advantages in interrupting theoperation? h. If it were to he left running, codd it he terminated xr~tomniically a t ~omede.viredpoint? r. Could it be terminsted xt any point should some mslfunction develop? If the answer to these questions is ye?, then one should examine the apparatus, determining which parts should be "watched" or monitored automatically. In s m h an examination one usually finds there are very few setups so compliexted that t,hey cannot safely be left running under nntomatic control; but also one finds there are few units so inherently safe that they donot need:ornesafety monitoring. This paper considers both the simple apparatus and the complicated apparatus, both the endothermic reaction and the exothermic reaction, both t,he short-term operat,ion and ihe long-term operation,
...a
C CHEMICAL REACTION Figure 9.
Examples of thermostded endothermic apparatus.
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M O N I T O R
TEMP.
i Figure 10.
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M O N I T O R OPEN C I R C U I T
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ll0VOLT A.C.
Five possible ways to monitor an opporatvr for
With smaller apparatus it may not be convenient to use a bulky bimetdlic thermoregulator (although some are fairly small). If such is the case, one may uie s. thermoeonple connected either to x pyrometer controller (3-6)or to a meter relay ( 6 ) . A thermistor plus a bridge type eontmller may also be used (7). If one prefee to insert no secondary probe, then one should either w e n contact,
mdfunction of
the temperature controller.
probe or monilor item5 2, 3, 4, or 5. In some cases (Item #2) the primary eontroller may be partially able to monitor itself. Pyrometer controllem are often so designed that if certain internal eomponenk fail, the controller will cut off the heater power. (Thii h known as FAIL SAFE design.) A somewhat diierent a p proach i- to make use of a pyrameter that ha- trrm independent control points. The OUTLET FOR PRIMARY CONTROLLER and HEATER
*
THERMOSWITCH OPENS UPON TEMPERATURE INCREASE
110 v A.C.
SWITCH
BATH
Y
Figure 1 1 . Wiring diogrom re-energize circuit.
of
an over temperature reloy circuit.
second c a r ~ h mint. l can be set a t a hieher level to sonnd a n alarm, alarm. open onen B. a circuit, circuit. sound an ~~~~ or take remedial action. Admktedly, the pymmetric approaches arenot 1007, FAIL SAFE, hut they do help t,o guard against troohle. Thii is also t.he case when one monilori Item #3-the sensing probe. This, again, is common practice with pyrometer-co~~trolle~;; they are usually designed so bhat ihe controller will t w n off the heat if there is a thermocouple break. Item 84 corresponds to the monitoring of the cyclic turning ON and OFF of power to the heater. This can be ae~~
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Figure 12. Cornpoct unit bated on circuit shown in Figure 3.
Reret switch must be pulhed to
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(Conlinued on page A@&)
complkhed with a so-called "reset," "intervd," or "time delay" timer (8). Such timers aredesigned to open or to closes. circuit after a period of seconds, minutes, or hours. When connected parallel with the heater, one of these timem can sound art alarm or turn off the heat if for any reman the electric power remains continuously ON. This approach has the advantage that i t monitors the apparatus without physical contact with the apparatus. Item 85 is sometims the most convenient to monitor-particularly when one k using heating mantles into which the manufacturer hss built a thermocouple. Again one can attach the thermocouple either to a conventional pyyrometer or to a meter relay. Once the choice is made ss to which approach (#I-5) one prefers, one then proceeds to the next decision: what action should the monitor take if an excessive temperature occurs? (1) Should the monitor permanently shot dawn the apparatus until an operator pushes s. reset button? (2) Should the monitor allow the apparatus to continue to operate but a t some slightly higher value? (3) Or if the disturbance is only momentary, should the monitor take no action, but a t least make s. record that a. disturbance occurred? Note: These choices apply not only to the monitors described above but also to many of the monitors described in the fallowing seotiana. For these situations, the monitor should be designed to shut down the apparatus rabher than to mert corrective action. I n some other eases, however, the monitor
Figure
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PULLEY-TOP V I E W
r'7 MOTOR
Figure 14. An impmvised stirrer monitor which user Almico Magnets in an aiuminum pulley to generate o rototing mognetic fleld. Thir changing mognetic fleld is sensed b y a meter relay connected to the pickup coil.
can be designed to take corrective action because normal operating conditions u n be readily rstored.
13. Schematic illustrotion of a complex reastion apparatus.
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II. Complex Apparatus While the scienlist is almost always concerned wit,h temperature, the apparatus is often more complicated then the examples in Fieures 9 and 10. Fieure 13 illustrates schematically a more complicated setup. Ten circles symboliee the components and/ or variables that the scientist would naturally keep an eye on if he were personally watching the setup. They r e p resent, in some cases, malfunction of the apparatus and, in other ceses, the progress of the reaction. They are as folloas: 1. The rate of the stirring. 2. The viscosity of the reaction mixture. 3. The pressure in the apparatn.;: either hydrostatic pressure, gaseous pressure, or vacnum, depending upon the nature of the apparatus. 4. The rate a t which reflux i? rising into the reflux condenser. 5. The rate of distillation of product or by-product out of the reaction flask. 6. The flow of cooling water Lo any water-cooled part. i. The volume of product or byprodnet in the distillate receivers. 8. The rate of flow of gas (or liquid) to or from the reaction. 9. Depending upon the specific r e action, there will he still other indicators of the progress of the reaction. 10. Time. I t should he remembered that with each variable one eusnally has the choice of several different monitoring approach&% To illustrate: the rotation of the stirrer ( # l ) may be monitored by (a) a device .mch as a meter relay which measures the electric current to the stirrer motor; or ( h ) a commercidly available tachometer
PRESSURE MONITORS
side stream. ( b ) If the rate of s t i r r i r ~is~ sensitive to the viscosity (and i t may he if one select.? an A.C. motor having "poor regulat.ionH) then one can monitor v i s cosit,yby monitoring jyl as diicussed above. (el , If the reactor is a continuous rather than batch-type reactor, the viscosit,y may be monitored by watchmg the bark pressure as the reaction mixture is pumped. The pressure in the apparatus (#3) can be monitored by use of either (a) commercially wd.ilsble pressure swit,ches of the bourdan or bellows type (10); or ( b ) mercury manometers with contact probes; or (c) mercury manometers t o which have been dipped s. capacitanceactuated controller (11) which senses ~, movement of the mercury level (Fig. 15A). The rate of reflux (#4) can he monitored quite readily by sensing the vapor Lemperature in the reflux condenser eit.her in the region of the condensing zone or slightly above this zone. The temperature in these regions gives an indication of the reflux rate because an increase in the rate cilnses the condensing zone to move farther n p into the condenser. This results in a temperrtbme increase a t any given point in the oondensat,ion zone. Such an increase i?easy to monitor using either a thermoregulator, a thermoconple plu.. pyrometer, or a thermometer with a capacitanceactuated eont,raller attached (Fig. 16). The monitor select,ed should be set 5 to 10" below the normal condensing temperature of the refl~rxvapors. The exact locat,ion of the sensing unil nit,h respect to the condensing zone (and the
.
contact
contactless
FLOW MONITORS Figure 15A & 158. Prerrvre and Row monitors bored on manometers equipped either with contoch or capasitmce-actuated cantrollerr
coupled to either the motor or to the stirrer shaft; or ( c ) an improvised tachometer such as is shown in Fieure 14. The viscosity of thk mixture (62) is often more difficult to monitor. Three
possibilities can h e suggested: (a)S o m e times a commercially available viscometer such as an Ultraviscoson ( 9 ) can be adapted to the a p p a r a t u eithkr b y immersion into the reaction area or into a.
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Figure 16. ReRux rate monitoring with a thermometer located in condensing mne of reRvx condenser.
electrical connections) will depend on whether one wishes the monitor to turn off the heat cyclically when the rate of reflux gets too high, or to terminate the
I
Figure 17.
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distillation if the rate drops too low or increases too much. The rat,e of distillation (#5) is more difficult to monitor. The approach is discontinuous: to monitor the time required for successive samples of a given volume to accumulate. One could employ either a relatively simple sample collector such as a syphon or one of the many fixedvolume sample collecting and distributing devices that have now been developed for chrornrttogrbphie purpose?. To monitor the time required for successive samples to accumulat,e (and the syphon to discharge) or for the sample collector to shift, one would again use the time-delay timer. The timer, in this case, could monitor the time elapsed per sample as being too long or too short, depending on how the timer was connected. (To monitor both high and low rate would require two t,imera.) The flow of cooling water ( # 6 ) to the apparatus and the volume of product in a dist,illate receiver (f7) are of such critical importance t o distillation that they were discussed in considerable detail in Part I of these two articles. The rate of gas or liquid flow (#S) either to the reaction or from the reaction can he a very important variable. The monitoring of a clean flow can usually be accomplished with a flaw meter which develops a pressure differential across an orifice or capillary. Monitoring can he based on contacts in a mercury U tuhe connected across the capillary. It also can be monitored with a capacitance act.uat.ed controller attached to t,he U tuhe (Fig. l5B). Category #9 can be quite broad. I t includes variables such as pH, color, presence of precipitate, etc. I n general, it represents variables thal can only he
A L A R M
I llOYOLT A
C
A number of normally closed circuit monitors may b e r e o d i i ~connected in series
Figure 18.
Normally open circuit monitors m a y be connected in parallel.
monitored with rather sophisticated electronic devices. These may he either quite specialized units or may be general purpose laboratory tooh that the scientist uses quite frequently. Time, the final variable ($10) on our list, can be monitored either (a) on the basis of the time of day or ( b ) the elapsed time after a given stage of the reaction is attained. Scientists are quite accustomed to using electric clocks and timers to shut down apparatus a t a predetermined time. They areless likely to heaccustomed to the many possible ways of using the time-delay timer. Such a. device if inter-connected to one of the other monitors listed above can terminate the reaction "n" h o u e after the other monitor indicated progress of the reaction. I t is possible, for example, to connect the time-delay timer to the primary temperature controller and have the timer triggered by the first O F F cycle. This will enable it to cont,rol the time that the reaction was actually at temperature.
Coupling of Monitors The monitoring units referred to above fall into several categories: mercury contact systems, mechanical-electrical devices (such a- pressure switches), and various electronic devices. Thew different units generally have different electrical outputs: 1. The mercury contact systems are either "normally open" or "normally closed," and may well be able to carry only a few milliamperes of current. 2. The electromechanicd devices often have a. microswitch wlth a, singlepole doublethrow output: it can be wired either "normally open" or "normally closed," and is able to carry a t least 5 amperes of current. 3. The
various electronic devices terminate in either (a) a low voltage (or low current) control circuit or (b) an indicating meter that should he changed t,o a meter relay, or ( c ) a 110-volt circuit either normally ON or normally OFF, or (d) a single-pole double-throw relay equivalent to the microswitch. The output of any single monitor might be wired so that it interrupts the heat to the reaction (or causes some other type of corrective action to take place). On the other hand, if a number of monil.oring devices are used, it. may well be that all of the units can be wired together so that if any one monitor is actnated t,he heater can he turned OFF. A seriw circuit e m be used if the output of every monitor is a "normrtlly closed" relay or switch which opens npon mslfrmction (Fig. 17). A parallel circuit would he wed when every monitor ha? a. "normally open" output which clo~eriupon malfnnction (Fig. 18). If one hes a hybrid system of monitorssome "normally open," some "normally closed," some supplying s. law-voltage output, and some suppl,ying 110-volt outpu-then in order t,o combine them one may need to add relays to the various monitors or groups of monitors so that. resulting syst,ems can he connected in series or in parallel. Mention should he made of a more elaborate method of coupling monitors: the "sampling" t,echnique in which the monitors are periodically rather than continuously checked. This may be accomplished with a st,epping type switch (12) wired to the monitors. As it steps it connects each monitor in t,um to t,he slsrm and control circuits. ( T o be cmclzcded in Seplember issue) Volume 43, Number 8, August
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