Edited by N O R M A N V. STEERE, School of Public Health, University of Minnesota, Minneapolis, Minn., 55455
XXVIII. Unattended Laboratory Operations-Part
Three
Running Laboratory Reactions Under Safe Control [concluded) D. R. Conlon, Instruments for Research & Industry, Chelfenhom, Po.
Ill. Exothermic Reactions Exothermic reactions, while usually not long-time reaotioni, a t least sa far 8s the exotherm is concerned, are particularly demanding of attention. While such reactions w e running, the scientist mcly well feel unable to leave the bench for even a few minutes. This need not be, for many families of exothermic reactions are not dangerous and, o ~ x ethey are understood, can he entrusted to control by suitable monitors. The monitors discussed under endothermic reactions may well be used with exot,hermio reaction ctppamt,ua, bot,h simple and complex, but with this difference: since the exothermic reaction evolves heat, the monitors may well have to came positive cooling to be applied to the apparatus. Thus, instead of wiring the monitor (or monitors) in such a way that i t merely interrupts power to the heater, it should be connected to turn on a
ing the thermometer, lowering the heating mantle and when neeersary substituting s n ice bath, and as soon as the temperature drop.; alqdying the heating mantle again!
cooling device. The nature of the cooling device will depend upon the nature of the exothermic reaction. Mildly exothermic reactions can be controlled by blowing air a t the reaction flask. T h k i~accomplished quite readily by having the temperature monitor turn OFF the hester and turn ON sn electric fan, or open B solenoid valve in an air tube connected to the laboratory air supply. More active but still "mild" exothermic reactions may require that the hester be removed from contact with the apparatus (so that any residual heat stored in the heater doe3 not reach the reaction), and that simultaneously the cooling air be turned ON. Still more active reactions require still more positive cooling. This can be sceamplished if the cooling is a p plied in the form of an ice bath that can he raised automatically mound the bottom of the reaction. (Heat to the resction is applied from the top of the flask-by radiant heating.) The steps described above can be applied automatically with a device known a5 Jsck-0-Matic (Fig 19) that is now commercially available (13). When used in connection with a temperature sensing system, Jack-0-Matic can either lower a heating bath from around a. reaction flask and apply positive air blast cooling, or turn off a radiant heater and raise and lower an ice bath. Thus it i5 able to control automatically reactions that chemists have ordinarily watched very closely by personal attention: read-
Figure 19. Jock-0-Matic used to control erothermic reactions by miring and lowering ice baths.
IV. Other Factors The foregoing nntr. diwl-.. ~ X I oiI 1he ~ poiv~t.~ t h l h a w !A> lw e m 4 v r m l i n xdding monitors and accessories to apparatus so that it can run unattended under constant conditions. The following are a few miscellsneoos items that are less frequently encountered: 1. Prormmmed Control: Sometimes instead of constant temperature (or constant pressure, flow, etc.) the reaction should be carried out a t increasing or decreasing levels. This procedure too can he developed-usually with the sssistance of the laboratory instrumentation group and a. research machinist. For example, a programmed temperature control t,het stepwise increases the lemperat,ure can be built around a grunp of constant-temperature regulators, each accurate to a few hundredt,hs of a degree. The regnlators would be selected in sequence by astepping switch controlled by a t,ime delay timer. Thus, one could maintain an apparatns a t each of a. series of fired temperatures for a predetermined time. This type of
1 ' I
feature
programming is particularly useful 11 connection with physical test apparatus. Auother type of programmed control is that in which the variable is allowed to increase (or decrease) gradually either linearly or nonlinearly. Programming controllers b a x d an rams for the programmed eoutrul of temperatiire have been used for many yean for plant-type process control (14). Although these are usually rather large physically, they can and are used with laboratory ovens and in pilot plant work. They can also be used with laboratory reactions. One can also program-control other variables such as prsstlre or vacuum with plant-type cam controllers. Another way is to make use of a commercially available presnre or vacmm regulator (15) with an sdjustahle control knob and couple t o the knob s slow-speed syehronous motor which will slowly dlive the control point upscale. If a limiling value is desired, i t o m be achieved by adding a suitable cam which activates a, microswitch and turns off the drive. 2. Sequential Contr.01: Occasionally it may be desirable t o program a sequence of operations. Thus, as an example, a. reaction can be programmed so that after a desired nomber of houn a charge of reagent is added to the reaction flask; and after still another time period, still another charge is added. A differentbasis for sequential control is to use a given stage of development of the reaction rather than time, to trigger a desired operation. 3. Foam Control: I n several rather different fields of laboratory work, excessive foaming in a laboratory operation can necessitate close personal a t t e n h n and be 8. vexing problem. Vacuum stripping although not a very long-time operation, i3 one example. As the lighter fractions are stripped from a mixture in a flask, foaming often occurs. If the stripping operation is not being watched quite carefully, part of the liquid in the still pot may be swept over as foam into the condensing appsrrttus. Boiling type foams in glass flasks can be sensed &her by probes and eaprtcitanee-rtci.uat,ed controllers or by optical means using a light source and a photocell light, beam. In either case the foam monitoring device can he connected to a solenoid valve (16) which open.; to admit a smsU amount of air or nitrogen to the apparatus (Fig. 20). The resulting pressure pulse will cause the foam to zuhside almost immediately. A* s, result one ran vscmm strip much mow rapidly and more effieierrtly than can be done manually. A second field for foam monitoring and control is in connection with biological preparations s i ~ as h fermentations. Such procescei are relatively slow compared
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with the foaming referred nlwvr. Xoreover, t,he fermentation foams sre not so clean as the hoiling fozms. Finally, the fermentation apparat,m may not he so accessible to optical sensing. Figure 31 shows the useof capacitance-~e~lsinp, using L: teflotl-insulated probe. The c a p a a i t a ~ w mntroller could eilher open a solenoid vnlve (17) and admit x small volllnle 01
":~ntilurm" or cc,\tld ton, ou :L sun:rll pump and inject "sntifoam." 4. Afaitoring the Monitor. Since this article is safety wieulrrl, it, should be pointed out that one can ronsider two different categoria of monitoring: "control monitoring" and "safety monitoring." I n designing apparatl~sthal. is to run unattended, one is conrerned with both. The followhg nolw illnstmt.e llnr
SOLENOID V A L V E OLE
ELECTRON lC
MICROSCOPE
Figure 20.
Photoelectric monitoring and control of stripping foams
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some mses, the "oalety numilu~a" will be extra monitors added to guard againsl failureof the "contml monitors". In other a s e s the "aafety monitors" will he extra monitors added to watch variables that were not being controlled. B. "Safety monitoring" systems o r ually need nut he as precise nor as fast ill response as "control monitoring" systems. For example, far "control monitoring" t,he temperature of a bath, one may use a sensitive mercury thermoregulator and hold the temperatwe constant to a few hundredths of a degree. Fur "safety monitoring" the same bath it would br quite satisfactory to add the relatively insensitive slow-responding bimetalli,: tlmrmoregulator. C. Whenever possible, "safety monilors" should be relatively unsaphisticst,ed devices whose mode of operation is visible, easily mderstood by the scientist, and readily checked. The mercury manometer is almost.ideal from these points of view. I t can be made into a n excellenl monitor for pressnre, rate of pressure rise, How of gas or liquid (or even flow of cooling water; see Figore 1 previous article). The electnmic viscosity-measuring device8 are an example of the opposite extreme. Admittedly, one is grateful for any method of memuring viscosity, hut one wish= f m devices that wodd be simple and readily cheeked. D. Although the "safely monitor" will usnally he R dm4c.e that is inherently
Safety
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simple and insensitive, there are some situations particularly involving exothermic reactions where the "safety monitor" must be s. highly sensitive d c vice with a fast response time. Under such conditions one may well be willing to use as the "safety monitor" a device of greater complexity. I n fact, i t can be s duplicate of the primary temperature controller. E. One may he tempted at times to consider using the "safety monitor" as a "oontrol monitor." This msv even. at times, he quite reasonable, particuiarly when used to terminate a reactmn. As an example, one can use a "safety monitor" attached to the stirrer motor to terminate a resotion when the react,ion mixture has reached the desired viscosity. However, one must guard aminst the opposite approach: considering that a "control monitor" eliminates the need for s. 'kafety monitor." The five possible monitors shown earlier in Figure 10 were really "safety monitors" added to guard against failure of the primsry controller. However, this approach brings up the following question: If we always have to assume that the primsry controller can fail and therefore needs monitorina. will the "safety monitor" then soo& or later fail? Should we monitor the "safety monitor?" Just how far should one go in monitoring the monitor, in guarding
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SOLENOID
I
Figure 21.
ritaring and control of fermentation foams with o probe and MOI
the guard, in policing the policemnn?? Sometimes the solution is not diffienll; lhe variables are not independent. For example, if the temperature increases,
0
copocitonce-actuated
other variables such as pressure, or reflux rate, etc., will probably increase and therefore several different monitors will respond.
1 Safety . . . In considering monitors and their de sign, one should remember that the degre' to which the monitors will he able t< cape wit,h the likely and the unlikely ma1 functions of the apparatus will depen~ upon t,he thought that has been given tc their design and installation. In the film analysis it is the good judgment of thl scientist that is involved.
Other Safety Recommendations Part I of these two article8 made ; number of specifio recommendatinns re1 ative to safe operation of glass dis tillation eqllipment. Some of these saml recommendations apply to the simple physical testing apparatus and some to th, mare complex reaction apparatus. T c review the recommendations briefly: 1. If eleotrieal power fluctuation! would affect the apparatns, these effect! may be minimized by use of constantvoltage transformea. 2. Breakage of glass apparatus can bc minimized by careful annealing, by inspecting the glass for rrtrain, by propel support of t,he equipment, and by rise of adeqrtste safety shields. 3. Whenever overAow of a liquid product is a possibility, oversize receiverr should be used. I t snmetimes is advisable to place large trays under the apparatns. 4. The aooaratus should be consoicuausly labeleh'so that the night watchman would have no trouble shutting it down.
Conclusion Although the suggestions made in this article are necessarily limited and will have to he modified to fit various local conditions, certainly there are great advantages in designing laboratory apparatus so that it can he run safely unattended. This is because: (a) Apparatus that is m n cantinuo~tslypradoces the desired resnlts much more repidly than wwonld otherwise be the case. ( b ) The qualit,y (color, etc.) of the experimental samples produced when running without interruption is often better. Likewise, the quality of physical tests that are run continuously rather than interruptedly is better. ( e ) Whenever the experiment can be run continuously the scientist will have greater assormce that the m n is valid and r e pradocible. I n fact, when interrupted rons RIP made. the scientist mav even have
lower the research costs per experiment. Certainly whenever a research project is under pressure to produce results in a qiven time, it is important that, safe w a y s be found to run the required apjaratus continooudy. However, even when a. project is not under great pressure, the benefits of continuous operation are so rea at that cresiive elhrts to design such ipperatus are most worthwhile. I n fact,, inch efforts by individual scientists and .heir supporting instrumentation perionnel have been extremely profitable and
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are one of the most pn,mi.;iug fields for the improvement of research.
References Note: The following list is but w pnrtial guide ti, e~,mpanentsand manofscti~reri. See buyem' guidw published by various j~,umals fur still other marn~faaturers. indicates that there are s. n ~ ~ m b eofr differen1 mudel.; nvailahle depending upon the applicatbu intended.
(1) THERMOSWITCH,' Fenwd, Inc., 11Y Pleasant St., Ashland, Mass. (2) OTER-TEMP GUARD. Instrlr mentr for Research & Industry, 108 Franklin Ave., Cheltenhnm. Pa. 19012. (3) YERSATRONIK Controller,' Model? H 7161 B & (: for sin& control point, Models R 7161 D-& E far two control points. 1Tone.vwell, Inc., 2701 Fourth Ave., S., Minneapolis, Minn. 55408. (4) GARDSMAN Controllers, Type J & J P for single control point,, Type JPT-3 for t u v control points. West Inst,rument Corp., Schiller Park, Ill. 60176. (5) CAPACITROL Controllers, Serieq 270 or 470. Barber-Cnlman Co., Indn~trialInstroment3 Div., Rockfr,rd,IIl. 61111. ( 6 ) COhIPACK I,*Meter Relay 503K. (Specify temperature range and thermocouple wire.) API Instruments Co., Chesterland, Ohio 44026. --. TERSA-TRAN Controller Model It 7079C. IIoneywell, Inc., 2701 Fourth Ave. S., Xinneapolir. hiinn. 55408. C Y C L F L E X Reriet Timer,* Model HP5 Series. (Specify maximum t,ime ranee desired-available from ~
.
(9) (10) (11)
(12)
Div., 726 Federal fit., l>avenpart, Iowa 52803. Time Delay Relay Type 412. (Specify maximum time range.) Cramer Div., Old Srtybrook, Conn. ULTRA-TrISCOSON.* Bendix Cincinnati Div.. 3130 Wasaan Road. Cincinnati, dhio 45241. Pressure and Vacuum Switches.. Bsrdsdsle Talveu, 5125 Alcoa. Ave., Loa Angeleq, Calif. 90058. T H E R M - 0 -WATCH Controller Model I.-6. Instruments far Re search & Indostry, Cheltenham, Pa. 19012. Stepping Switch (Spring Driven or Direct Drive).* C. P. Clare & Co., 3101 ~ & t tRlvd., Chicago, Ill. 60645. JACK-0-MATIC 3Iodel J 3 M - 3 or J-337-L6. Instruments for R e search & Industry, Cheltenham, Pa. 19012. ELECTRONIK 15 Circular Chart, Program Controller.' Honeywell, Ine. Fort Washineton. Pa. 19034. GARDSMAN Program Controller JGB. West Instrument Corp. Schiller Park, Ill. 60176.
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(15) Vacuum-Pressure Regulator, Series 44, and Pressure Regulator, Series 40. Moore Products Co., Springhouse, Pa. 19447. (16) Solenoid Valve Model #8262B1. Automatic Switch Co. Florham Park, N. J. 07932. (17) Solenoid Valve Model #8262B2. Automatic Switch Co. Florham Park. N. J. 07932.
Editor's N o t e From time to timeit is hoped t,hat these columns can report on accident case histories which may serve the very important function of reminding readers of potential hut often neglected hazards. Methyl Aride Explosion
A chemist was seriously injured when an explosion occurred while he was vacuum distilling methyl aaide. H e was either holding or shaking the trap containing the saide when i t detonated, se~iously injuring his right hand. He also sustained superficial burns and lacerations ahout the face and chest,. The shock wave broke the overhead fluorescent lights and hot,tles of organic reagents about the laboratory. Equipment in an area, five feet wide was completely destroyed. Burning liquid and glass werespattered on the chemist and the right lens of his safety glasses was broken. Ordinary glasses in a pocket crtse were completely demolished. The method of preparation used was the methylation of an aqueous sodium azide solution nit,h dimet,hyl sulfate. Four prwioudy succwsful preparations had been made in the same manner. The pH of this reaction is detected by useof methyl red indicator and controlled to a. pH of 5-7 b y the addition of sodium hydroxide if needed. If the pH should he close to 5 or drop for a. short time below 5, then hydrazoic acid, a powerful explosive, can be formed and be distilled out with the methyl a i d e . The investigators believe this may have occurred. Another case has been reported in which a slight shock caused the detonation of mercury szide formed by the reaction of the hydrmoic wid with the mercury in a manometer. This tragic accident is another example of the need for extreme care when dealing with potentially hazardous msle"s1s. ( M C A Case History 887) Effed of Low Temperatures on Mild Steel Cylinders
There have been several cases in which mild steel cylinders, used for compressed gases, have shattered after they had been immersed in dry ice/acetone baths during filling operations. I t has been discovered that mild steel cylinders are permanently weakened by this treatment. Special stainless steel cylinders should be used. ("Research Sentinel," Merck Sharp & Dohme Research Laborotorier Division, June 1964)
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