CALORIMETRIC DETERMINATION OF THESPECIFIC HEAT OF SOYBEANS

K. C. Watts

School of Agricultural EngineeringUniversity of Guelph

Guelph, Ontario

W. K. Bilanski

School of Agricultural EngineeringUniversity of Guelph

Guelph, Ontario

Soybeans have the potential of being an important livestock feed. Theygive high yields and have a highprotein content. However, inherent inraw soybeans is a chemical compoundcalled a trypsin inhibitor (1) whichcauses indigestion and has other adverse effects on simple-stomach animals. The usual commercial methodof breaking down this compound usessuper-heated steam in special processing plants, although a new processusing L.P. gas has been patented.However, a process that could be implemented right on the farm wouldsave livestock producers both timeand money.

One of these methods involves theuse of a microwave oven. It is necessary to heat the beans to some specificinternal temperature, dependent onthe animal being fed. However, temperature determination is very difficultin a microwave field; thermocouplesand mercury thermometers are subject to corona discharge, alcohol thermometers burst, and waxes (althoughthey are not affected by microwaves)first go through a plastic stage andthen liquify due to the heat from thebeans with which they are in contact.Other solutions included switchingoff the microwaves and then inserting a temperature-measuring deviceinto the beans, or, secondly, removing a number of beans and determining their bulk temperature by a calor-imetric method. The first alternativerequired an assumption concerningthe thermal resistance between thetemperature - measuring device andthe beans. Due to the ellipsoidalshape of the beans, it was decidedthat the error due to the virtuallypoint contact between the beans anda temperature-measuring device couldnot be determined to any degree ofprecision. Hence, this alternative wasset aside in preference to the second,which assumed a knowledge of thespecific heat of soybeans for the particular moisture content and temperature range.

The moisture content obviously af-

RtXUVLD FOR PUBLICATION

OCTOBER 15. 1969

CANADIAN AGRICULTURAL ENGINEERING,

fects the specific heat because thespecific heat of water is higher thanthat of the other materials involved.The temperature, too, has a sometimes unpredictable effect on specificheat values. For instance, the specificheat of water (2) drops from 1.00738calories/gram/°C at 0°C to 0.99795at 35°C and then increases to 1.00697at 100°C. Even though these changesare small, they indicate that one cannot always extrapolate specific heatvalues from one temperature range toanother with absolute certainty. Because the authors could not find anyliterature pertaining to the specificheat of soybeans in the moisture content and temperature range withwhich they were concerned, it wasdecided to determine the specific heatof soybeans at the approximate moisture content at which they are commercially stored, and for a temperature range which would enable further work to be carried out in thermally treating soybeans.

EQUIPMENT AND METHOD

A Parr Bomb Calorimeter (Figure1) was modified to take a polishedaluminum can with a slip-on cover.Aluminum stirrer blades were madeto fit around the aluminum can inorder to mix the water in the chrome-plated water jacket of the calorimeter.

Soybeans were weighed, placed inthe can, and covered with a knownweight of Stanolax, a paraffin-base

Figure 1. Disassembled Calorimeter

VOL. 12, No. 1, MAY 1970

oil with a known specific heat andgood thermal conductivity. The canand contents were heated in a FisherScientific isotemp forced-air oven fortwo hours or until an equilibriumtemperature was reached. Due to theoil impregnating the beans to someextent, it was not possible to determine their moisture content after atest. Therefore, it was assumed thatthe moisture content of the beans wasnot affected by the two-hour heatingperiod. Because the oil surroundedthe beans throughout the heating, itwas felt that this assumption wassomewhat valid.

Water of known volume (1750cubic centimeters) and temperaturewas placed in the calorimeter's waterjacket. The heated can and contentswere put into the water, the calorimeter was assembled, and the calorimeter stirrer motor started. Temperature measurements of the water inthe calorimeter jacket were takenevery minute until no change was observed for a period of three minutes.The temeparture rise of the watergave an indication of the heat in thebeans from which their specific heatcould be calculated.

In preliminary tests, attempts weremade to seal the can with silicon rubber so that no water could enter thecan when it was placed in the calorimeter. However, it was difficult tofind a compound that would dryquickly, not deteriorate due to theheating period in the oven, withstandthe pressure involved in the expansion of the air under heating and thesuction produced upon cooling, andstill be easy to remove. Also, no theoretical reason why the entrance ofwater should affect the specific heatof the soybeans could be found. Furthermore, the calibration of the calorimeter was run without sealant, soany errors induced would be largelycompensated for in the calibration.

The value of the specific heat wasdependent on three temperatures: 1)

45

always contained the same depth ofoil. In this way, the error due to anysteady state temperature gradientsthat might exist in the can would beabsorbed by the water equivalent,assuming that th^ gradients were thesame in all tests. The alligator clipwas left attached to the can throughout the heating period. The temperature was measured using a Honeywellpotentiometer and an ice-water reference bath; thus, it was possible toread the temperature to ±0.1° Centigrade at 135° Centigrade.

The temperature of the can andcontents decreased as they weremoved from the oven to the calorimeter. This loss of heat has beenaccounted for by calibration; however, to make the time as short aspossible from the oven to the calorimeter, a means had to be found ofquickly attaching the rotating shaft ofthe calorimeter to the can. Permanentmagnets gradually lost their strengthdue to the heat in the oven. Therewas insufficient space in the calorimeter shaft to put the coil for anelectromagnet. A press fit tended tobe too unreliable and slow. Therefore, a mechanical means (Fig. 2)

**s"*~«*,

the temperature to which the can,Stanolax and beans were raised, 2)the initial temperature of the waterin the calorimeter jacket, and 3) theambient temperature. An error of ±one degree Centigrade in the firsttemperature gave an error of ±1 percent in the specific heat. An error of±0.1°C in the second temperatureyielded an error of ±6 per cent inthe specific heat, and an error of±1°C in the third temperature resulted in an error of ±0.5 per cent inthe specific heat reading. To calibratethe calorimeter, the known amount ofheat was added using only Stanolaxin the aluminum can. The results ofthe specific heat determinations willbe discussed briefly along with theassumptions on which they are based.

Temperature of Can, Stanolaxand Beans

It was necessary to know the temperature to which the can, Stanolax,and beans were raised in order to obtain their temperature drop and theamount of heat added by the can andthe Stanolax. Temperature gradientsin the oven prevented the use of theoven temperature as the true temperature.

It was necessary, therefore, to insert a thermocouple inside the canand have leads from it which werereadily detachable. In short, the thermocouple leads were brought throughthe press-on cover. The following is adetailed description of the actualplacing of the wires.

Small sockets and plugs were usedat first, but the technical problems involved in securing the sockets to thecan and yet insulated from it provedtoo difficult. A terminal bar madefrom pieces of tin plate was fastenedonto the snap-on lid. The copper andconstantan wires from the thermocouple inside the can were solderedonto separate sides of the terminalbar, insulated both from each otherand from a third supporting piece oftin plate. An alligator clip was secured to the leads from the potentiometer, the copper and constantanwires being secured to and insulatedfrom the inside of the alligator clip,one on one side and one on the otherside of the clip. This was then clippedonto the terminal bar, the appropriatewires being matched.

The thermocouple was always setin the same point in the can which

Figure 2. Quick Catch Top

which proved to be satisfactory wasdevised. This consisted of six hingedhooks fastened to a piece of aluminum which was made to fit looselyover the cover of the aluminum can.This piece of aluminum was screwedonto the rotating shaft of the calorimeter. The hooks were forced in toward the center of the can by means

of an elastic band. The quick-catchtop was simply pressed down overthe aluminum can forcing the hooksoutward so that they engaged thecover of the can as it was lifted.

Initial Water TemperatureIn order to bring the water and the

calorimeter to an equilibrium temperature before a test, a Fisher Laboratory stirrer was used to rapidlymix the water for at least five minutes prior to each test. The water wascooled initially 1 to Wi degreesCentigrade below ambient. All watertemperatures were measured using aParr Calorimeter thermometer withgradations to 1/40 degree Centigrade.

Ambient Temperature

When the temperature of the waterin the calorimeter was different fromambient, there was a certain loss orgain of heat due to radiation. Thecalorimeter bath was sealed to cutdown any convection losses to theoutside air. To determine the amountof heat lost or gained, three radiationloss experiments were performed, andthe data was collected to yield asingle radiation loss curve. To do this,water was heated above ambient andput in the calorimeter water jacket.Measurements of the temperaturedrop per unit time were recorded andplotted against temperature aboveambient during the entire coolingtime.

Since the ambient temperature inthe room varied up to eight degreesCentigrade in the half-hour durationof a test, it was felt that the temperature which most affected theradiation loss of the calorimeter occurred at the point when the calorimeter was assembled. This was thelast point at which the ambient airwas in contact with the water bath.Thus, this was the value of ambienttemperature used when evaluatingthe radiation loss.

Investigation of Errors

Even though the Stanolax washeated for two hours in the oven before every test, the loss of Stanolaxdue to evaporation was found to benegligible when the top was on thepolished aluminum can. The heat input to the calorimeter by the continuous stirring in the duration of atest was found to be of no consequential magnitude. This was investigated

46 CANADIAN AGRICULTURAL ENGINEERING, VOL. 12, No. 1, MAY 1970

with the aid of a Yew wattmeter anda General Radio Strobotac. Measurements were taken of the power andspeed of the stirrer motor with andwithout water in the calorimeter.

Not all heat from the beans andStanolax was conducted out of thecan in the half-hour test period. Aplateau was reached in the calorimeter-water temperature rise; thenthere was a small increase of a fewhundredths to a few tenths of adegree. At this point, however, thetemperature of the calorimeter was4°C above ambient, and the radiationloss was about 0.004°C per minute.Had the time period been extended,there would have been greater dependence on the radiation loss curvesand less on the heat removed fromthe can. Hence, the tests were terminated at the first plateau on theassumption that most of the error induced in the specific heat tests wascompensated for in the water equivalent determinations.

Calibration

The calibration of the equipmentwas accomplished using the waterequivalent. The water equivalent isthe amount of heat required to raisethe water and calorimeter a unittemperature. In this study it is givenin calories per degree Centigrade.Transition time is the term used todefine the time taken to open ovendoors, lift the can out of the oven,put on the stirrer blades, and placethe can into the water. Several testswere made to determine the variationof water equivalent for various transition times (Fig. 3). Fifteen tests wererun at the ten and fifteen secondtransition times, and five tests at the

20 30 40

Transition Time In Seconds

Figure 3. Calibration Curveof the Calorimeter

thirty, forty-five, and sixty secondtransition times. The value at zerotransition time is the theoreticalwater equivalent, calculated from thespecific heats of the calorimeter scomponents. The curve apparentlyassumes a parabolic shape. This maybe due to the large scatter at theshort transition times. One possibleexplanation for the rise in the curveat low transition times is as follows.

With increasing transition time, thequantity of heat lost due to radiationis decreasing per unit time due todecreasing temperature of the can;but, as a total summed quantity, itincreases. Also, it would be expectedfor laminar flow that the film coefficient would increase as the squareroot of the velocity; but the transitorytime would decrease as the inverse ofthe velocity. Thus, more heat shouldbe lost at lower velocities (longertransition times). This, however, assumes that the motion has no effecton the internal conduction of heatout to the surface. It can be hypothesized that at the shorter transitiontimes the liquid contents of the canare agitated more vigorously, thusaiding in heat transfer to the outersurface of the can. Therefore, although the film coefficient does notincrease in equal proportion to thetransition time decrease, the vigorousmotion of the can at short transitiontimes maintains the can surface at ahigher temperature than if the canwere moved slowly and the contentsnot agitated. Hence, the heat lost atshorter transition times would begreater than at longer transitiontimes.

ard deviation of 24.6 calories per degree Centigrade (1.4 per cent deviation).RESULTS AND CALCULATIONS

All calculations were made on anI.B.M. digital computer. The potentiometer output was converted totemperatures using a straight line approximation for the temperaturerange of 110 to 120 degrees Centigrade. The specific heat of the Stanolax changed linearly with respect totemperature. Hence, the averagespecific heat for the temperaturerange used was calculated separatelyfor each test and was employed indetermining the heat input by theStanolax. The specific heat of thepolished aluminum can, terminalbars, and the Epoxy glue used tosecure the terminal bars to the canwere all assumed to have the specificheat of pure aluminum.

The total heat was obtained as theproduct of the water equivalent andthe calorimeter water temperaturerise which had been corrected forthe radiation heat loss.

The heat input by the can and theStanolax was obtained by the productof their specific heats, weights, andtemperature drops. The heat of thebeans were calculated by subtractingthe heat inputs by the can and Stanolax from the total heat input. Thespecific heat of the soybeans wasthen found knowing their weight andtemperature drop.

The results of the soybean determi-nations are shown in Table 1. The

continued on page 56

TABLE I. SPECIFIC HEAT OF SOYBEANS

Specific HeatOil

cals./gm.-C°

.511

.512

.512

.513

.513

.512

.511

.511

.510

.512

.514

.510

.511

Corrected Heat Oil

Temp. Rise

C° cals.

2.98

2.93

2.88

2.75

2.83

2.97

2.78

2193

2137

2360

2248

2252

2524

2360

2245

2146

2254

2328

2154

2366

Heat Can

cals.

741

767

731

724

723

813

803

766

796

762

769

761

788

Average Moisture Content is 7.4 Percent (w.b.)

Total Heat

cals.

5263

5111

5050

4898

5091

5387

5457

5365

5263

5030

5172

5426

5091

Heat Bean

cals.

2330

2207

1960

1926

2116

2050

2293

2354

2322

2013

2075

2511

1937

Specific HeatBean

cals./gm.-C0

.47

.46

.46

.48

.47

.42

.45

.43

.43

.45

.42

.50

.39

Temp. Range

30.7

31.2

31.8

34.4 •

34.4 •

27.0 •

27.4 •

29.8 •

27.5 •

31.0 .

32.2 •

27.6 •

27.1 -

126.5

127.6

126.2

125.5

127.6

131.7

128.2

127.7

127.7

127.0

131.3

125.9

128.9

The scatter in the data was less atfifteen seconds than at ten seconds;hence, the transition time used wasfifteen seconds. Consequently, thewater equivalent was 1829 caloriesper degree Centigrade with a stand-

TABLE II. MOISTURE CONTENTvrs. SPECIFIC HEAT

Specific Heatcals./gm.-C

0.45

0.47

0.49

Moisture Content

percent (w.b.)

7.4

17.72

21.72

Reference

Source

This paper(3)(3)

CANADIAN AGRICULTURAL ENGINEERING, VOL. 12, No. 1, MAY 1970 47