Temperature Control In a Small Volume Pressure Chamber Edmund Y. Ting1 1 Avure Technologies Incorporated, 23500 64th Avenue South, Kent, WA, 98031 USA
ABSTRACT
INTRODUCTION
8 cc sample
RESULTS & DISCUSSION
80
70
500
80
Adiabatic change= 27oC 80
70
Pressure, KSI
100
Tc Center [C]
2) TC inserted into test cell
60
60
HPc Ext. [C] Bath [C] Pressure [ksi]
Pressurization is started at this predetermined temperature.
50
40 40
1) Sample inserted into test cell 20
3) Pressure on
30
•
3) Pressure on
20
0 0
200
400
600
800
1000
1200
1400
1600
Tim e, sec
60
Olive oil in olive oil 400
50 300 Water in olive oil
40
200 30 100
CONCLUSIONS High pressure experiments under near constant temperature conditions can be achieved in small test system is an “isothermal-endpoint”test method is used. While this procedure requires more work, it can produce more constant temperature behavior desired in certain process modeling.
20
0 0:00:00
90
Figure 3 Temperature history in an “isothermal endpoint” procedure shows near constant temperature for long duration test. By starting pressurization when the sample was at a predetermined lower temperature than the metal pressure chamber, a near “isothermal-endpoint”condition can be achieved after reaching pressure.
Thermal Decay, Oil and Water
600
120
4) Pressure off
Sample temperature increases due to heat transfer from hotter pressure chamber prior to pressurization. Temp, C
Figure 2 illustrates the effect of starting pressurization with the sample, pressure medium, and pressure vessel under isothermal condition (“Isothermal startpoint” ). It is apparent that the magnitude of the maximum temperature achieved depends on the sample and pressure medium composition and that all of the samples cool rapidly after reaching maximum temperature. The “Isothermal endpoint”test mode is illustrated in Figure 3. Given instant pressurization, by starting the sample pressurization at a temperature approximately equal to the target process temperature minus the anticipated adiabatic temperature, the temperature reached after reaching pressure should be near isothermal with that of the steel pressure chamber. This results in no significant driving force for heat transfer and thus the sample can maintain a constant temperature for a long period of time. The sample start temperature is determined by knowing the compression heating behavior of the sample. In reality, the start of pressurization temperature is typically lower to compensate for heat transfer during non-instantaneous pressurization.
700
Steel sleeve
100
Water in water
0:01:26
0:02:53
0:04:19
-100
0:05:46
0:07:12
REFERENCES
10 Time
Figure 1: Small volume high pressure test chamber and lab machine.
Furthermore, the “isothermal endpoint”procedure requires the sample insertion temperature to be typically 10 to 15oC lower than the start of pressurization temperature in order to allow time for the sample to be loaded into the high pressure equipment and the connection of thermocouple(s). The optimum values for these temperatures can be determined by a few iterations.
T90-8-s-N40c
Pressure, MPa
It is now recognized that the inactivation behavior of microorganisms is sensitive to pressure as well as temperature (1,2). In the use of small pressure chambers to determine inactivation kinetics, the effect of temperature has frequently not been considered due to the inability to monitor and/or control temperature. The ability to control temperature is made complex due to the unavoidable effect of sample compression heating. Two extreme examples of experimental procedures are “Isothermal-startpoint”and “isothermal-endpoint”. These are contrasted and the resulting different time-temperature histories under pressure are illustrated. The “isothermal-endpoint”test mode allows unlimited duration tests at a desired temperatures but requires greater experimental setup. The “isothermal-startpoint”mode allow simpler testing but is limited to short test durations where heat transferred will not result in significant temperature loss.
High pressure exposure was conducted in an approximate 50 ml (25mm ID x 76mm OD) stainless steel pressure chamber instrumented with a centerline 1mm stainless steel ungrounded jacketed type K thermocouple. Temperature was recorded using a calibrated PC data acquisition system (IO Tech) at 1Khz and averaged to 1 value per second (DasyLab software). The self contained high pressure test system (Avure PT-1) is shown in Figure 1. Samples were contained in a polypropylene 10 cc syringe.
Temperature, C
Temperature variations during high pressure experimentation can confuse the characterization of the process (i.e. inactivation kinetics) . Unlike thermal death time studies in which a small sample is plunged into a heated bath to produce a step-like temperature change, rapid pressurization of a sample results in simultaneous pressure and temperature change. The temperature change is unavoidable due to adiabatic compression. Following compression, due to major differences in compression temperature changes between the sample/medium and the pressure vessel, a temperature gradient develops, and leads to the cooling of the sample/medium by conventional heat transfer. In order to produce a more step-like pressure change while keeping a constant process temperature, an “isothermal-endpoint”experimental procedure can be used. This procedure attempts to produce an isothermal temperature condition immediately after reaching maximum pressure. This approach relies on starting the pressurization of a sample when the sample temperature is approximately at a value equal to the intended process temperature minus the anticipated compression heating.
MATERIALS & METHODS
Figure 2 Variations in temperature history depend on sample composition, pressure medium composition as well as heat transfer due to temperature differences due to compression heating. Water based sample treated in oil pressure medium would experience very different temperature history than in water medium.
(1) Ting, E. Y., Balasubramaniam, V.M., Raghubeer E., 2002. “Determining thermal effects in high-pressure processing”, Food Technology. Vol. 56, No.2 (2) V.M. Balasubramaniam, Ting, E.Y., Stewart, C.M., Robbins, J.A.; 2004. “Recommended laboratory practices for conducting high-pressure microbial inactivation experiments”, Innovative Food Science and Emerging Technologies, Elsevier, In-press
