- Email: [email protected]
cleaning detection by measuring electric resistance––equipment development and application to milk fouling detection and chemical cleaning monitoring
Journal of Food Engineering 61 (2004) 181–189 www.elsevier.com/locate/jfoodeng
On-line fouling/cleaning detection by measuring electric resistance––equipment development and application to milk fouling detection and chemical cleaning monitoring € zkan Xiao Dong Chen *, Dolly X.Y. Li, Sean X.Q. Lin, Necati O Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 29 March 2002; accepted 12 March 2003
Abstract An electrical resistance method, which has the potential to measure the extent of soft material fouling such as milk fouling on the surface of process equipment in situ and in real time, is described. An experimental fouling unit with the appropriate attachments has been devised and used to monitor the fouling build-up using the electrical resistance method. Reconstituted skim milks with solid contents of 10–30 wt.% were used to produce milk foulings, and these milk foulings were cleaned using a cleaning solution with 0.5 wt.% NaOH. Using the fouling unit, it was possible to measure the thermal resistance (essentially measuring heat flux) and electrical resistance simultaneously. As a result, the relationship between the electrical resistance and the thermal resistance during both fouling build-ups and cleaning processes was established. It has been shown that this technique is effective for measuring the extent of fouling, and it has the potential to be modified further so that it can be adopted in real process industries. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Fouling; Cleaning; Soft-deposit; Process monitoring
1. Introduction Monitoring fouling and cleaning can provide useful information for making operational decisions in food processing plants. Fouling is usually not visible from the outside of the industrial processing equipment, and can only be ascertained from significant effects, such as pressure drop measurement, which may not be sensitive enough to pick up small and local deposition. Fouling deposits in food and bioproduct processing plants contain significant proportions of liquid and they are often soft and fragile. As a result, they can be deformed easily by contact, so that their thickness may not be accurately measured with conventional mechanical instruments. Furthermore, the tendency of biological fouling deposits to shrink or slump outside their natural environment makes it difficult to gauge them satisfactorily in any other location (Tuladhar, Paterson, Macleod, & Wilson, 2000). A number of early research methods were discontinuous or invasive in nature (e.g., *
Corresponding author. Tel.: +64-9-373-7599x7004; fax: +64-9-3737463. E-mail address: [email protected] (X.D. Chen). 0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0260-8774(03)00085-2
gravimetric method and direct measurement of thickness). Continuous monitoring of the cleaning process may be done by measuring the contaminant in the effluent flow (Tuladhar et al., 2000). A number of techniques have been developed over the past 25 years. Some techniques are simple and require minimum instrumentation. Others are sophisticated and provide direct evaluation of a number of relevant parameters (Tuladhar et al., 2000). These devices are based on heat transfer measurement (Bott, 1995; Lalande & Rene, 1988), pressure drop (Corrieu, Lalande, & Ferret, 1979; Visser & Jeurnink, 1997), silicon sensor (Stenberg, Stemme, & Kittilsland, 1988), ultrasound method (Withers, 1996), microstrip monitoring technique (Root & Kaufman, 1992), photothermal deflection method (Fujimori, Asakura, & Suzuki, 1987), light transmission or reflectance (Tamachkiarowa & Flemming, 1999), ellipsometry (Karlsson, Wahlgren, & Tragardh, 1996), quartz crystal microbalance (Colberg et al., 1998), radiotracers (Scintillation detector) (Grant, Webb, & Jeon, 1996, 1997; Kabin, Saez, Grant, & Carbonell, 1996; Littlejohn, Saez, & Grant, 1998; Yan, Saez, & Eduardo, 1997), shear stress device (Bott, 1996; Fryer, 1989), pneumatic gauging technique (Gale, 1995;
182
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Tuladhar et al., 2000), and electrical conductivity method (Bott, 1996; Diehl & Van Gelder, 1990; Gale & Griffiths, 1995). The commercially available heat flux sensors have in fact been applied to the dairy industry in New Zealand, which have been shown to be a reasonably reliable tool for the operators to make rational decisions upon the termination of a production run and the degree of cleanness of their plant (Troung, 2001). Gale developed a pneumatic gauging technique to measure the thickness of leaf tissue (Gale & Griffiths, 1995). With this method, air at a constant pressure discharges through a nozzle in close proximity to the surface being examined; as the nozzle approaches the surface, the pressure profile in the nozzle apparatus is modified by the presence of the surface and this can be used to infer its distance from the nozzle. The pressure change is indicated by the formation of a stream of bubbles in a liquid. The thickness of a coating on the surface is then determined by difference, given the location of the ÔcleanÕ surface. Recently, the pneumatic gauging technique was improved by Tuladhar et al. (2000) for detecting a whey protein deposit layer. In the new system, the process liquid is sucked into a nozzle presented to the surface region to be gauged. The technique has been shown to be very successful for studying the fouling and cleaning kinetics on a laboratory scale (Fryer, 1997). For this technique, the deposition must be sufficiently rigid to resist significant deformation by impingement of the gauging fluid. The gauge may sometimes distort a soft deposit due to either the impinging jets or the suction streams. An automatic system was devised to measure dimensions of a stationary single soft sample (Diehl & Van Gelder, 1990). Size measurement was governed by the electrical conductivity of the material to be measured. The measurement system consisted of two subsystems: one to detect specimen contact and the other to determine the specimen dimension. Contact between the metal sensor and the specimen acted like a switch, which closed the circuit. This technique turns out to be quite relevant to the current study. Based on the same principle, Gale improved DiehlÕs system and developed a micrometer (Gale, 1995). When the electrical contact is established with the sample surface, further ÔsqueezingÕ is not necessary. For a single stationary sample, this approach is simple, reliable and easy-to-use. It may not be accurate when the soft layer contains air bubbles as the sensor has to contact the surface to be measured. The system is not suitable for on-line or in situ measurement. Milk fouling at low processing temperatures, is soft and fragile and thus can only be monitored conveniently if non-touching sensors are used. Heat flux sensors are appropriate from this viewpoint but they are not sensitive enough to pick up small thickness of a milk
deposition of say less than 0.1 or 0.2 mm (Troung, 2001). An optimal monitoring method would indicate the location and extent of the deposit accurately. In industry, this information must be acquired on-line, in situ, non-destructively, in real time reproducibly and automatically. The devices should be robust, cheap, and easy to use. As a development towards achieving such a goal, this paper describes a recently developed apparatus, which measures the electrical conductivity changes across a flow channel during both the fouling build-up and cleaning process. The principle of such an apparatus has been demonstrated successfully in this work. However, the practical aspects of the apparatus have to be improved in future work.
2. Experimental The current system has adopted two parallel stainless steel electrodes (50 110 mm, and 10 mm apart) to measure the electrical resistance of the fouling layer and the liquid between the electrodes. The thermal resistance was also measured at the same time on the fouling side of the test section in order to obtain a realistic benchmark of the electric resistance measurement for fouling (or cleaning) detection. The approach allowed the measurement process to be automated for real time sampling. A photograph and a schematic diagram of the fouling apparatus are shown in Fig. 1. The system was used to detect milk fouling under ÔacceleratedÕ conditions, which were achieved by recycling the reconstituted milk and setting a relatively high wall temperature. The measurement system consisted of two sub-systems: one to detect the fouling process through measuring the electrical resistance and the other to detect the fouling process through measuring the thermal resistance of the fouling as illustrated in Fig. 2. Essentially, the system comprised the flow system, the test section, the reference electrode, the electrical heating system, and the data acquisition system. The setup for ÔproducingÕ fouling and subsequent cleaning process mimicked a typical heat exchanger system. Before the fluid entered the test section, there was a 1.44-m-long flow channel before the test section to ensure that a well-developed laminar flow was established (Fig. 1). The flow system consisted of a milk heating tank (60 l), two 30-l cleaning solution storage tanks (one for the alkaline and the other for the acid solution), two pumps, two flow rotameters, a flow channel made of a stainless steel (fouling surface), and Perspex sheets. The sample fluid was preheated to 60 °C in the tank by an electric coil heater (PID + fuzzy logic control) before entering the test cell. A constant fouling surface temperature was maintained by adjusting the power
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Fig. 1. The photograph and schematic diagram of the electrical resistance monitoring system.
input (where the electrical resistance and thermal resistance were measured). The velocity of liquid flow was kept at 0.05 m s1 , corresponding to a Reynolds number of 352. In industry, Reynolds number is much higher. Again this low Reynolds number accelerates the buildup of fouling since the deposit removal rate was minimized. In the fouling experiments, reconstituted milk was pumped through the flow system to generate fouling and returned to the recycling tank. The recycling of the milk accelerated the fouling build-up. Furthermore, the procedure used in this study helped maintain the cost of running the trials at a manageable level. In the cleaning experiments, the rinsing (water or chemical solutions) fluid was pumped through the flow system to remove the deposit and returned to the tank
183
(recycling). The cleaning agent was also maintained at a temperature of 50 °C in the tank by the electrical coil heater. The fouling surface (or the test section; see Fig. 2) which was made of stainless a steel (304 l) with dimensions of 50 110 3 mm was embedded into a Tufnol plate. Meanwhile, this stainless steel surface was used as an electrode for the measurement of the electric resistance during the fouling build-up. Opposite the fouling surface, there is another stainless steel electrode (50 110 1 mm) with five viewing windows (15 10 mm) which permitted visual inspection of the fouling once the feed fluid was drained before the cleaning process started and during the entire cleaning process. The fouling surface was heated by an electric heater, which was made of ribbon wire. This heater ensured maximum surface contact between the heater and the heated surface and thus minimized the heat loss. The heater (at the outside of the fouling surface) was insulated by a 10 mm thick insulation. The inside and outside surface temperatures of the insulation were measured using type K thermocouples. The heat loss was calculated through the temperature difference between the inside and outside surface of the insulation. The input power of the heater was measured by a power meter (0.5%). The temperature of the back of the fouling surface (i.e. behind the steel plate for deposit formation) was measured by six sets of type K thermocouples, which were distributed and embedded in the fouling walls. The thermal resistance (RF ) can be deduced using these measurements. A reference electrode was used to measure the electrical resistance of the feed fluid. This cell had the same dimension as the above test cell. The difference in the electric resistance measured between the two cells was smaller than 1% when the surfaces were clean. Thus the electric resistance per unit distance of the feed fluid between the electrodes can be calculated for both the test and the reference electrode. The difference between the two resistances (RE ) was used to represent fouling buildup or removal. The inlet and outlet temperatures of the fouling fluid in the vertical rectangular channel were measured using type K thermocouples. The bulk temperature of the fouling fluid was taken to be the average of these two temperatures. For each fouling experiment, at the start there was no fouling on the test surface (this lasted for several minutes; the normal run time was >5 h). The thermal resistance of the convection heat transfer from the fouling surface was calculated from the following parameters: (a) the heat-flux that passed through the fouling surface; (b) backside surface temperature at the back of the fouling surface and (c) the bulk temperature of the fouling fluid. After the fouling became significant, the thermal resistance of the fouling was calculated by subtracting the convection resistance from the total
184
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Fig. 2. Details of the test section.
RF (m2.kW -1) Thermal resistance
2.0E-03
1.5E-03
1.0E-03
Run 1
5.0E-04
Run 2
0.0E+00 0
1
2
(a)
4
5
Electrical resistance
3
RE (Ω)
6
7
0.004
RF (m2.kW-1)
(i) Prepare the fouling liquid (reconstituting the milk powder to the desired solids content) or cleaning solution, and transfer them to the storage tank. (ii) Switch on the pump and the heater. (iii) Once the temperature of the fouling (or cleaning) liquid reaches the set-point, run the data acquisition system and switch on the heater of the fouling surface. (iv) As fouling increases (or decreases), the heater power input to the fouling surface is adjusted to keep the fouling surface temperature constant. (v) At the end of each fouling run, switch off the power of the heater. Let the fouling liquid run several minutes until the temperature difference between the fouling surface and the bulk liquid is less than 3 °C (otherwise, water will be removed from the fouling deposit) and then switch off the pump. (vi) Take the fouling sample from the surface if necessary for analysis.
2.5E-03
Thermal resistance
resistance (fouling resistance plus convection resistance). Here, the flow condition during the fouling build-up was assumed to be the same as that during the cleaning process. The general steps of the experiments are summarized below:
0.0035 0.003 0.0025 0.002 0.0015 0.001 Run 1 Run 2
0.0005 0 0
(b)
1
2
3
4
Electrical resistance
5
6
7
8
RE (Ω)
Fig. 3. Reproducibility tests.
3. Results and discussion 3.1. The electrical resistance and thermal resistance The reproducibility of fouling and cleaning measurements by detecting the electrical resistance changes during a fouling run is illustrated in Fig. 3. The electric resistances obtained in fouling runs at three different
temperatures are given in Fig. 4. At the beginning of the fouling runs, very low and nearly constant electrical resistance values were recorded, and this region is referred to as the Ôinduction periodÕ. The induction period was shortened when the temperature of the fouling surface was increased. After the induction period, the rate of change of the electrical resistance (dRE =dt) was
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189 4.0E-03 88°C 96°C
6
115°C
5
Thermal resistance
RE(Ω)
7
Electrical resistance
RF (m2.kW-1)
8
4 3 2 1
2
y = -0.001x + 0.0044x 2 R = 0.9886
3.0E-03
y = -9E-05x 2 + 0.0013x R2 = 0.9913
2.5E-03 2.0E-03 1.5E-03
2
y = -1E-06x + 0.0003x 2 R = 0.9947
1.0E-03 5.0E-04
0
0
20
40
60
80
100
120
140
160
180
200
220
Fouling time (min)
1
2
3
4
5
Electrical resistance R
E
6
7
8
(Ω)
Fig. 6. Comparison of the three fouling trials at different surface.
Fig. 4. Typical change of electrical resistance of the fouling with time using 10 wt.% skim milk, velocity ¼ 0.05 m s1 , Tbulk ¼ 62 °C: (a) general overview, (b) logarithmic view to highlight the initial period.
3.0E-03
R F (m2.kW-1)
88° 96°C 115°C
3.5E-03
0.0E+00
0
Thermal resistance
185
2.5E-03
times during fouling build-ups at various surface temperatures (88, 96, and 115 °C) are shown in Figs. 7–9. The photo in Fig. 7, taken 40 min after starting up the system shows the induction of the fouling process. At this stage, the surface was still reasonably clear and
2.0E-03
1.5E-03
1.0E-03 88°C 96°C 115°C
5.0E-04
0.0E+00 02
04
06
08
0
100
120
140
160
180
200
220
Fouling time (min)
Fig. 5. Typical change of thermal resistance of the fouling with time using 10 wt.% skim milk, velocity ¼ 0.05 m s1 , Tbulk ¼ 62 °C: (a) general overview, (b) logarithmic view to highlight the initial period.
relatively high and after a certain time it decreased slightly. These general trends are also reflected in terms of the thermal resistance due to fouling (see Fig. 5). It can be seen that the electrical resistance has a much more obvious dependence on temperature than that of the thermal resistance over the range tested. The relationship between the electrical resistance (RE ) and thermal resistance (RF ) during the fouling process for three different temperatures, which is illustrated in Fig. 6, can be represented by the following equation: RF ¼ aRE þ bR2E
Fig. 7. Photograph of fouling formed at the surface temperature 88 °C. Arrows denote the times at which photographs were taken.
ð1Þ
where a and b are constants. 3.2. Visualization of the fouling process The visualization of deposit growth during the fouling build-up has been studied using images of the fouling obtained during testing. Photos taken at different
Fig. 8. Photograph of fouling formed at the surface temperature 96 °C. Arrows denote the times at which photographs were taken.
186
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Fig. 9. Photographs of fouling formed at the surface temperature 115 °C. Arrows denote the times at which photographs were taken.
Fig. 10. Fouling distribution observed through the view windows of the opposite plate across the test section at the surface temperature 115 °C with a milk concentration of 10 wt.%, Tbulk ¼ 62 °C, velocity ¼ 0.05 m s1 .
3.3. The cleaning experiments A typical relationship between the electrical resistance and cleaning time (i.e. cleaning curve) for the foulant (about 1-mm thick) generated at the surface temperature of 96 °C is illustrated in Fig. 11. After a fast drop in the electrical resistance at the start of the cleaning process, a relatively slow decrease in the electrical resistance was observed until a persistent residual amount of milk fouling left at the end of the cleaning process. The initial dramatic drop in the electrical re-
4 3.5
Electrical resistance RE (Ω)
free of large deposits. The photo in Fig. 7, taken after 180 min shows a dramatic change in the deposit growth accompanying with an accompanying increase in the electrical resistance. At this stage, the fouling still occurred as isolated patches on the surface. This observation suggests that a Ôfouling layerÕ would have to be developed from these isolated ÔislandsÕ which merge into one another as they grow not only vertically but also horizontally along the steel surface. The photo in Fig. 7 taken at 320 min shows that the steel surface was almost completely covered by the foulant. After this stage, a significant increase in the thickness of the fouling layer can be seen. The ÔlayerÕ looked very porous with large voids due to the low velocity regime employed. The fouling trends observed at 98 and 115 °C (see Figs. 8 and 9) were quite similar to those observed at 88 °C (see Fig. 7). The fouling layers generated at 88 and 96 °C adhered only weakly to the metal surface and could be easily removed. However, at the surface temperature of 115 °C, the fouling layers became more compact and adhered strongly on the surface of stainless steel. The colour of the fouling deposits became darker as the surface temperature was increased. The fouling layers from the higher temperature case had even more brown under-layers, which were probably caused by extensive Maillard reactions. It was observed that the thickness of the fouling layer was not uniform along the stainless steel surface. The fouling in the vicinity of the ÔinletÕ of the steel surface plate (lower) test section was thinner than that of the ÔoutletÕ of the test cell (see Fig. 10). Typically, the fouling thickness at the inlet and outlet was about 0.8 and 1.2 mm, respectively after 100 min, and about 1.5 and 2.5 mm, respectively after 200 min. The formation of nonuniform fouling layers is due to the fact that the temperature of the milk close to the outlet is higher than that close to the inlet as a result of the heating by the test plate.
Electrical resistance decreases due to NaOH penetration
3 2.5
Actual cleaning starts
2 1.5
Penetration time 1 0.5 0 0
10
20
30
40
50
60
70
0 80
90
100
110
120
Time (second)
Fig. 11. The change in electrical resistance with time, skim milk fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189 4
0.0045
)
Cleaning effect
-1
0.004
Fouling
Thermal resistance RF (m2.kW
3.5
Electrical resistance RE (Ω)
187
Cleaning
3 Electrical resistance decreases due to NaOH penetration
2.5 2 1.5 1 0.5
0.0035 0.003
NaOH penetration
0.0025 0.002 0.0015 0.001 0.0005
0
0
05
0
100
150
200
250
300
350
400
450
500
0
0.5
1
1.5
2
2.5
Electrical resistance R E
Time (min)
Fig. 12. The change in electrical resistance with time, fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
sistance was observed once the system was filled with the cleaning solution, suggesting that this is due to the introduction of electrolytes. Fig. 12 shows the trend that combines both the fouling and cleaning process. The first six data points of the cleaning process in Fig. 12 correspond to the penetration of NaOH during the first six seconds. When the cell was initially filled with NaOH, there was an apparently ÔabruptÕ drop in the electric resistance (from 3.53 to 1.32 X). This drop was expected, because the NaOH solution is a strong conductive medium. If the cleaning solution penetrates into the foulant, it would substantially reduce the electrical resistance value. The thermal resistance as a function of time during the fouling build-up and the cleaning process is shown in Fig. 13. As can be seen from Fig. 13, a sharp drop in the thermal resistance was not observed during the cleaning stage. The relationship between the electrical resistance and the thermal resistance for the cleaning process is illustrated in Fig. 14. The ÔabruptÕ drop in the electrical
3
3.5
4
(Ω)
Fig. 14. Cleaning process, the relationship between electrical resistance and thermal resistance, fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
resistance was observed even though the removal of the deposit has not started. This is indicated by a large reduction in the electrical resistance (RE ) while almost no change in the thermal resistance was recorded (see Fig. 14). This shows that the NaOH penetration into the deposit was very fast and it can not be the limiting step for the cleaning process. The electrical resistance as a function of time for the foulant (about 2-mm thick) generated at a surface temperature of 115 °C during the cleaning process is shown in Fig. 15. The electrical resistance decreased from 7.43 to 5.86 X during the first 7 s of the cleaning process. Fig. 16 shows both the fouling build-up and the cleaning process in terms of thermal resistance. Similar trends were observed as those in Figs. 12 and 13. The drop in the electrical resistance for the 2-mm thick foulant (see Fig. 15) layer was not very abrupt compared to that for 1-mm thick foulant (see Fig. 12). This is reflected graphically in Fig. 17. The reduction in the electrical resistance at a constant thermal resistance for
0.004 Fouling
Cleaning Electrical reduction due to NaOH penetration
7
0.003
Electrical resistance RE (Ω)
Thermal resistance RF (m 2W-1)
8
0.0035
0.0025 0.002 0.0015 0.001 0.0005
6
Actual cleaning starts
5 4 3 2
Penetration time
1
0
0
0
50
100
150
200
250
300
350
400
450
500
Time (min)
Fig. 13. The change in thermal resistance with time, skim milk fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Time (second)
Fig. 15. The change in electrical resistance with time, fouling layer occurring at Ts ¼ 115 °C, d ¼ 2 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
188
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
References
Thermal resistance RF (m2.kW-1)
0.004 0.0035 Cleaning
Fouling
0.003 0.0025 0.002 0.0015 0.001 0.0005 0 0
50
100
150
200
250
300
350
400
450
500
Time (min)
Fig. 16. The change in electrical resistance with time, fouling layer occurring at Ts ¼ 115 °C, d ¼ 2 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
Thermal resistance RF(m 2.kW-1)
0.004
Effect of cleaning
0.0035 0.003 NaOH penetration
0.0025 0.002 0.0015 0.001 0.0005 0 0
1
2
3
4
Electrical resistance
5
6
7
8
RE (Ω)
Fig. 17. Cleaning process, the relationship between electrical resistance and thermal resistance, fouling layer occurring at Ts ¼ 115 °C, d ¼ 2 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
the thick foulant is less than that observed for the thinner more porous layer generated at 96 °C.
4. Conclusions A novel soft deposit gauging technique employing electrical resistance measurement has been developed and tested in laboratory. This technique is a simple and can be used in situ. The relationship between the electrical resistance and the thermal resistance during the fouling build-up and cleaning process has been established. The study provides good evidence that the effects of fouling can be detected via the measurement of the electrical resistance. This technique has the potential to measure the thickness of deposits on a surface in situ and in real time of a dairy plant operation.
Bott, T. R. (1995). Fouling of heat exchangers. New York: Elsevier. Bott, T. R. (1996). Fouling of the heat exchangers and its mitigation with special reference to biofouling. In: Second European thermalsciences and 14th UIT national heat transfer conference 1996. Colberg, M. T., Carnes, K., Saez, A. E., Grant, C. S., Hutchinson, K., & Hesterberg, D. (1998). Hydration and removal of supported phospholipid films in aqueous surfactant solutions. Thin Solid Film, 247–251. Corrieu, G., Lalande, M., & Ferret, R. (1979). New monitoring equipment for the control and automation of milk pasteurization plants. Food Process Engineering, 1, 165–171. Diehl, K. C., & Van Gelder, M. F. (1990). An automated dimension measurement system for soft biological materials. Journal of Agricultural Engineering Research, 47, 133–138. Fryer, P. J. (1989). A comparison of two possible fouling monitors for the food processing industry. In R. W. Field, & J. A. Howell (Eds.), Process engineering in the food industry (pp. 131–141). New York: Elsevier Applied Science. Fryer, P. J. (1997). Fouling and cleaning in food process plant. In P. J. Fryer, D. L. Pyle, & C. D. Rielly (Eds.), Chemical engineering for the food industry (pp. 365–380). Blackie Academic & Professional. Fujimori, H., Asakura, Y., & Suzuki, K. (1987). Non-contact measurement of film thickness by the photothermal deflection method. Japanese Journal of Applied Physics, 26, 1759–1764. Gale, G. E., & Griffiths, P. (1995). An automatic micrometer for measuring soft electrically conductive materials. Measurement Science and Technology, 6, 447–451. Gale, G. E. (1995). A thickness measuring device using pneumatic gauging to detect the sample. Measurement Science and Technology, 6, 1566–1571. Grant, C. S., Webb, G. E., & Jeon, Y. W. (1996). Calcium phosphate decontamination of stainless steel surfaces. AIChE Journal, 42, 861–875. Grant, C. S., Webb, G. E., & Jeon, Y. W. (1997). A noninvasive study of milk cleaning processes: calcium phosphate removal. Journal of Food Process Engineering, 4, 197–230. Kabin, J. A., Saez, A. E., Grant, C. S., & Carbonell, R. G. (1996). Removal of organic films from rotating disks using aqueous solutions of nonionic surfactants: film morphology and cleaning mechanisms. Industrial and Engineering Chemistry Research, 35, 4494–4506. Karlsson, C. A. C., Wahlgren, M. C., & Tragardh, A. C. (1996). Betalactoglobulin fouling and its removal upon rinsing and by SDS as influenced by surface characteristics, temperature and adsorption time. Journal of Food Engineering, 30, 43–60. Lalande, M., & Rene, F. (1988). Fouling by milk and dairy product and cleaning of heating exchange surfaces. In L. F. Melo, T. R. Bott, & C. A. Bernardo (Eds.), Fouling science and technology (pp. 557–573). Kluwer Academic Publishers. Littlejohn, F., Saez, A. E., & Grant, C. S. (1998). Use of sodium polyaspartate for the removal of hydroxyapatite/brushite deposits from stainless steel tubing. Industrial and Engineering Chemistry Research, 2691–2700. Root, L. F., & Kaufman, I. (1992). Non-contacting low-cost instrument for film thickness measurement. IEEE Transactions on Instrumentation and Measurement, 41, 1014–1019. Stenberg, M., Stemme, G., & Kittilsland, G. (1988). A silicon sensor for measurement of liquid flow and thickness of fouling biofilms. Sensors and Actuators, 13, 203–221. Tamachkiarowa, A., & Flemming, H. C. (1999). Optical fiber sensor for biofouling monitoring. In T. R. Bott (Ed.), Understanding heat exchanger fouling and its mitigation (pp. 343–347). New York: Begell House.
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189 Troung, T. (2001). Application of heat flux sensor in dairy industry. Ph.D. Thesis, Massey University. Tuladhar, T. R., Paterson, W. R., Macleod, N., & Wilson, D. I. (2000). Development of a novel non-contact proximity gauge for thickness measurement of soft deposits and its application in fouling studies. The Canadian Journal of Chemical Engineering, 78, 935– 947.
189
Visser, J., & Jeurnink, T. J. M. (1997). Fouling of heat exchangers in the dairy industry. Experimental Thermal and Fluid Science, 14, 407–424. Withers, P. M. (1996). Ultrasonic, acoustic and optical techniques for the non-invasive detection of fouling in food processing equipment. Trends in Food Science and Technology, 7, 293–298. Yan, J.-F., Saez, A., & Eduardo, S. G. C. (1997). Removal of oil films from stainless steel tubes. AIChE Journal, 43(1), 251–259.
On-line fouling/cleaning detection by measuring electric resistance––equipment development and application to milk fouling detection and chemical cleaning monitoring € zkan Xiao Dong Chen *, Dolly X.Y. Li, Sean X.Q. Lin, Necati O Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand Received 29 March 2002; accepted 12 March 2003
Abstract An electrical resistance method, which has the potential to measure the extent of soft material fouling such as milk fouling on the surface of process equipment in situ and in real time, is described. An experimental fouling unit with the appropriate attachments has been devised and used to monitor the fouling build-up using the electrical resistance method. Reconstituted skim milks with solid contents of 10–30 wt.% were used to produce milk foulings, and these milk foulings were cleaned using a cleaning solution with 0.5 wt.% NaOH. Using the fouling unit, it was possible to measure the thermal resistance (essentially measuring heat flux) and electrical resistance simultaneously. As a result, the relationship between the electrical resistance and the thermal resistance during both fouling build-ups and cleaning processes was established. It has been shown that this technique is effective for measuring the extent of fouling, and it has the potential to be modified further so that it can be adopted in real process industries. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Fouling; Cleaning; Soft-deposit; Process monitoring
1. Introduction Monitoring fouling and cleaning can provide useful information for making operational decisions in food processing plants. Fouling is usually not visible from the outside of the industrial processing equipment, and can only be ascertained from significant effects, such as pressure drop measurement, which may not be sensitive enough to pick up small and local deposition. Fouling deposits in food and bioproduct processing plants contain significant proportions of liquid and they are often soft and fragile. As a result, they can be deformed easily by contact, so that their thickness may not be accurately measured with conventional mechanical instruments. Furthermore, the tendency of biological fouling deposits to shrink or slump outside their natural environment makes it difficult to gauge them satisfactorily in any other location (Tuladhar, Paterson, Macleod, & Wilson, 2000). A number of early research methods were discontinuous or invasive in nature (e.g., *
Corresponding author. Tel.: +64-9-373-7599x7004; fax: +64-9-3737463. E-mail address: [email protected] (X.D. Chen). 0260-8774/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0260-8774(03)00085-2
gravimetric method and direct measurement of thickness). Continuous monitoring of the cleaning process may be done by measuring the contaminant in the effluent flow (Tuladhar et al., 2000). A number of techniques have been developed over the past 25 years. Some techniques are simple and require minimum instrumentation. Others are sophisticated and provide direct evaluation of a number of relevant parameters (Tuladhar et al., 2000). These devices are based on heat transfer measurement (Bott, 1995; Lalande & Rene, 1988), pressure drop (Corrieu, Lalande, & Ferret, 1979; Visser & Jeurnink, 1997), silicon sensor (Stenberg, Stemme, & Kittilsland, 1988), ultrasound method (Withers, 1996), microstrip monitoring technique (Root & Kaufman, 1992), photothermal deflection method (Fujimori, Asakura, & Suzuki, 1987), light transmission or reflectance (Tamachkiarowa & Flemming, 1999), ellipsometry (Karlsson, Wahlgren, & Tragardh, 1996), quartz crystal microbalance (Colberg et al., 1998), radiotracers (Scintillation detector) (Grant, Webb, & Jeon, 1996, 1997; Kabin, Saez, Grant, & Carbonell, 1996; Littlejohn, Saez, & Grant, 1998; Yan, Saez, & Eduardo, 1997), shear stress device (Bott, 1996; Fryer, 1989), pneumatic gauging technique (Gale, 1995;
182
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Tuladhar et al., 2000), and electrical conductivity method (Bott, 1996; Diehl & Van Gelder, 1990; Gale & Griffiths, 1995). The commercially available heat flux sensors have in fact been applied to the dairy industry in New Zealand, which have been shown to be a reasonably reliable tool for the operators to make rational decisions upon the termination of a production run and the degree of cleanness of their plant (Troung, 2001). Gale developed a pneumatic gauging technique to measure the thickness of leaf tissue (Gale & Griffiths, 1995). With this method, air at a constant pressure discharges through a nozzle in close proximity to the surface being examined; as the nozzle approaches the surface, the pressure profile in the nozzle apparatus is modified by the presence of the surface and this can be used to infer its distance from the nozzle. The pressure change is indicated by the formation of a stream of bubbles in a liquid. The thickness of a coating on the surface is then determined by difference, given the location of the ÔcleanÕ surface. Recently, the pneumatic gauging technique was improved by Tuladhar et al. (2000) for detecting a whey protein deposit layer. In the new system, the process liquid is sucked into a nozzle presented to the surface region to be gauged. The technique has been shown to be very successful for studying the fouling and cleaning kinetics on a laboratory scale (Fryer, 1997). For this technique, the deposition must be sufficiently rigid to resist significant deformation by impingement of the gauging fluid. The gauge may sometimes distort a soft deposit due to either the impinging jets or the suction streams. An automatic system was devised to measure dimensions of a stationary single soft sample (Diehl & Van Gelder, 1990). Size measurement was governed by the electrical conductivity of the material to be measured. The measurement system consisted of two subsystems: one to detect specimen contact and the other to determine the specimen dimension. Contact between the metal sensor and the specimen acted like a switch, which closed the circuit. This technique turns out to be quite relevant to the current study. Based on the same principle, Gale improved DiehlÕs system and developed a micrometer (Gale, 1995). When the electrical contact is established with the sample surface, further ÔsqueezingÕ is not necessary. For a single stationary sample, this approach is simple, reliable and easy-to-use. It may not be accurate when the soft layer contains air bubbles as the sensor has to contact the surface to be measured. The system is not suitable for on-line or in situ measurement. Milk fouling at low processing temperatures, is soft and fragile and thus can only be monitored conveniently if non-touching sensors are used. Heat flux sensors are appropriate from this viewpoint but they are not sensitive enough to pick up small thickness of a milk
deposition of say less than 0.1 or 0.2 mm (Troung, 2001). An optimal monitoring method would indicate the location and extent of the deposit accurately. In industry, this information must be acquired on-line, in situ, non-destructively, in real time reproducibly and automatically. The devices should be robust, cheap, and easy to use. As a development towards achieving such a goal, this paper describes a recently developed apparatus, which measures the electrical conductivity changes across a flow channel during both the fouling build-up and cleaning process. The principle of such an apparatus has been demonstrated successfully in this work. However, the practical aspects of the apparatus have to be improved in future work.
2. Experimental The current system has adopted two parallel stainless steel electrodes (50 110 mm, and 10 mm apart) to measure the electrical resistance of the fouling layer and the liquid between the electrodes. The thermal resistance was also measured at the same time on the fouling side of the test section in order to obtain a realistic benchmark of the electric resistance measurement for fouling (or cleaning) detection. The approach allowed the measurement process to be automated for real time sampling. A photograph and a schematic diagram of the fouling apparatus are shown in Fig. 1. The system was used to detect milk fouling under ÔacceleratedÕ conditions, which were achieved by recycling the reconstituted milk and setting a relatively high wall temperature. The measurement system consisted of two sub-systems: one to detect the fouling process through measuring the electrical resistance and the other to detect the fouling process through measuring the thermal resistance of the fouling as illustrated in Fig. 2. Essentially, the system comprised the flow system, the test section, the reference electrode, the electrical heating system, and the data acquisition system. The setup for ÔproducingÕ fouling and subsequent cleaning process mimicked a typical heat exchanger system. Before the fluid entered the test section, there was a 1.44-m-long flow channel before the test section to ensure that a well-developed laminar flow was established (Fig. 1). The flow system consisted of a milk heating tank (60 l), two 30-l cleaning solution storage tanks (one for the alkaline and the other for the acid solution), two pumps, two flow rotameters, a flow channel made of a stainless steel (fouling surface), and Perspex sheets. The sample fluid was preheated to 60 °C in the tank by an electric coil heater (PID + fuzzy logic control) before entering the test cell. A constant fouling surface temperature was maintained by adjusting the power
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Fig. 1. The photograph and schematic diagram of the electrical resistance monitoring system.
input (where the electrical resistance and thermal resistance were measured). The velocity of liquid flow was kept at 0.05 m s1 , corresponding to a Reynolds number of 352. In industry, Reynolds number is much higher. Again this low Reynolds number accelerates the buildup of fouling since the deposit removal rate was minimized. In the fouling experiments, reconstituted milk was pumped through the flow system to generate fouling and returned to the recycling tank. The recycling of the milk accelerated the fouling build-up. Furthermore, the procedure used in this study helped maintain the cost of running the trials at a manageable level. In the cleaning experiments, the rinsing (water or chemical solutions) fluid was pumped through the flow system to remove the deposit and returned to the tank
183
(recycling). The cleaning agent was also maintained at a temperature of 50 °C in the tank by the electrical coil heater. The fouling surface (or the test section; see Fig. 2) which was made of stainless a steel (304 l) with dimensions of 50 110 3 mm was embedded into a Tufnol plate. Meanwhile, this stainless steel surface was used as an electrode for the measurement of the electric resistance during the fouling build-up. Opposite the fouling surface, there is another stainless steel electrode (50 110 1 mm) with five viewing windows (15 10 mm) which permitted visual inspection of the fouling once the feed fluid was drained before the cleaning process started and during the entire cleaning process. The fouling surface was heated by an electric heater, which was made of ribbon wire. This heater ensured maximum surface contact between the heater and the heated surface and thus minimized the heat loss. The heater (at the outside of the fouling surface) was insulated by a 10 mm thick insulation. The inside and outside surface temperatures of the insulation were measured using type K thermocouples. The heat loss was calculated through the temperature difference between the inside and outside surface of the insulation. The input power of the heater was measured by a power meter (0.5%). The temperature of the back of the fouling surface (i.e. behind the steel plate for deposit formation) was measured by six sets of type K thermocouples, which were distributed and embedded in the fouling walls. The thermal resistance (RF ) can be deduced using these measurements. A reference electrode was used to measure the electrical resistance of the feed fluid. This cell had the same dimension as the above test cell. The difference in the electric resistance measured between the two cells was smaller than 1% when the surfaces were clean. Thus the electric resistance per unit distance of the feed fluid between the electrodes can be calculated for both the test and the reference electrode. The difference between the two resistances (RE ) was used to represent fouling buildup or removal. The inlet and outlet temperatures of the fouling fluid in the vertical rectangular channel were measured using type K thermocouples. The bulk temperature of the fouling fluid was taken to be the average of these two temperatures. For each fouling experiment, at the start there was no fouling on the test surface (this lasted for several minutes; the normal run time was >5 h). The thermal resistance of the convection heat transfer from the fouling surface was calculated from the following parameters: (a) the heat-flux that passed through the fouling surface; (b) backside surface temperature at the back of the fouling surface and (c) the bulk temperature of the fouling fluid. After the fouling became significant, the thermal resistance of the fouling was calculated by subtracting the convection resistance from the total
184
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Fig. 2. Details of the test section.
RF (m2.kW -1) Thermal resistance
2.0E-03
1.5E-03
1.0E-03
Run 1
5.0E-04
Run 2
0.0E+00 0
1
2
(a)
4
5
Electrical resistance
3
RE (Ω)
6
7
0.004
RF (m2.kW-1)
(i) Prepare the fouling liquid (reconstituting the milk powder to the desired solids content) or cleaning solution, and transfer them to the storage tank. (ii) Switch on the pump and the heater. (iii) Once the temperature of the fouling (or cleaning) liquid reaches the set-point, run the data acquisition system and switch on the heater of the fouling surface. (iv) As fouling increases (or decreases), the heater power input to the fouling surface is adjusted to keep the fouling surface temperature constant. (v) At the end of each fouling run, switch off the power of the heater. Let the fouling liquid run several minutes until the temperature difference between the fouling surface and the bulk liquid is less than 3 °C (otherwise, water will be removed from the fouling deposit) and then switch off the pump. (vi) Take the fouling sample from the surface if necessary for analysis.
2.5E-03
Thermal resistance
resistance (fouling resistance plus convection resistance). Here, the flow condition during the fouling build-up was assumed to be the same as that during the cleaning process. The general steps of the experiments are summarized below:
0.0035 0.003 0.0025 0.002 0.0015 0.001 Run 1 Run 2
0.0005 0 0
(b)
1
2
3
4
Electrical resistance
5
6
7
8
RE (Ω)
Fig. 3. Reproducibility tests.
3. Results and discussion 3.1. The electrical resistance and thermal resistance The reproducibility of fouling and cleaning measurements by detecting the electrical resistance changes during a fouling run is illustrated in Fig. 3. The electric resistances obtained in fouling runs at three different
temperatures are given in Fig. 4. At the beginning of the fouling runs, very low and nearly constant electrical resistance values were recorded, and this region is referred to as the Ôinduction periodÕ. The induction period was shortened when the temperature of the fouling surface was increased. After the induction period, the rate of change of the electrical resistance (dRE =dt) was
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189 4.0E-03 88°C 96°C
6
115°C
5
Thermal resistance
RE(Ω)
7
Electrical resistance
RF (m2.kW-1)
8
4 3 2 1
2
y = -0.001x + 0.0044x 2 R = 0.9886
3.0E-03
y = -9E-05x 2 + 0.0013x R2 = 0.9913
2.5E-03 2.0E-03 1.5E-03
2
y = -1E-06x + 0.0003x 2 R = 0.9947
1.0E-03 5.0E-04
0
0
20
40
60
80
100
120
140
160
180
200
220
Fouling time (min)
1
2
3
4
5
Electrical resistance R
E
6
7
8
(Ω)
Fig. 6. Comparison of the three fouling trials at different surface.
Fig. 4. Typical change of electrical resistance of the fouling with time using 10 wt.% skim milk, velocity ¼ 0.05 m s1 , Tbulk ¼ 62 °C: (a) general overview, (b) logarithmic view to highlight the initial period.
3.0E-03
R F (m2.kW-1)
88° 96°C 115°C
3.5E-03
0.0E+00
0
Thermal resistance
185
2.5E-03
times during fouling build-ups at various surface temperatures (88, 96, and 115 °C) are shown in Figs. 7–9. The photo in Fig. 7, taken 40 min after starting up the system shows the induction of the fouling process. At this stage, the surface was still reasonably clear and
2.0E-03
1.5E-03
1.0E-03 88°C 96°C 115°C
5.0E-04
0.0E+00 02
04
06
08
0
100
120
140
160
180
200
220
Fouling time (min)
Fig. 5. Typical change of thermal resistance of the fouling with time using 10 wt.% skim milk, velocity ¼ 0.05 m s1 , Tbulk ¼ 62 °C: (a) general overview, (b) logarithmic view to highlight the initial period.
relatively high and after a certain time it decreased slightly. These general trends are also reflected in terms of the thermal resistance due to fouling (see Fig. 5). It can be seen that the electrical resistance has a much more obvious dependence on temperature than that of the thermal resistance over the range tested. The relationship between the electrical resistance (RE ) and thermal resistance (RF ) during the fouling process for three different temperatures, which is illustrated in Fig. 6, can be represented by the following equation: RF ¼ aRE þ bR2E
Fig. 7. Photograph of fouling formed at the surface temperature 88 °C. Arrows denote the times at which photographs were taken.
ð1Þ
where a and b are constants. 3.2. Visualization of the fouling process The visualization of deposit growth during the fouling build-up has been studied using images of the fouling obtained during testing. Photos taken at different
Fig. 8. Photograph of fouling formed at the surface temperature 96 °C. Arrows denote the times at which photographs were taken.
186
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
Fig. 9. Photographs of fouling formed at the surface temperature 115 °C. Arrows denote the times at which photographs were taken.
Fig. 10. Fouling distribution observed through the view windows of the opposite plate across the test section at the surface temperature 115 °C with a milk concentration of 10 wt.%, Tbulk ¼ 62 °C, velocity ¼ 0.05 m s1 .
3.3. The cleaning experiments A typical relationship between the electrical resistance and cleaning time (i.e. cleaning curve) for the foulant (about 1-mm thick) generated at the surface temperature of 96 °C is illustrated in Fig. 11. After a fast drop in the electrical resistance at the start of the cleaning process, a relatively slow decrease in the electrical resistance was observed until a persistent residual amount of milk fouling left at the end of the cleaning process. The initial dramatic drop in the electrical re-
4 3.5
Electrical resistance RE (Ω)
free of large deposits. The photo in Fig. 7, taken after 180 min shows a dramatic change in the deposit growth accompanying with an accompanying increase in the electrical resistance. At this stage, the fouling still occurred as isolated patches on the surface. This observation suggests that a Ôfouling layerÕ would have to be developed from these isolated ÔislandsÕ which merge into one another as they grow not only vertically but also horizontally along the steel surface. The photo in Fig. 7 taken at 320 min shows that the steel surface was almost completely covered by the foulant. After this stage, a significant increase in the thickness of the fouling layer can be seen. The ÔlayerÕ looked very porous with large voids due to the low velocity regime employed. The fouling trends observed at 98 and 115 °C (see Figs. 8 and 9) were quite similar to those observed at 88 °C (see Fig. 7). The fouling layers generated at 88 and 96 °C adhered only weakly to the metal surface and could be easily removed. However, at the surface temperature of 115 °C, the fouling layers became more compact and adhered strongly on the surface of stainless steel. The colour of the fouling deposits became darker as the surface temperature was increased. The fouling layers from the higher temperature case had even more brown under-layers, which were probably caused by extensive Maillard reactions. It was observed that the thickness of the fouling layer was not uniform along the stainless steel surface. The fouling in the vicinity of the ÔinletÕ of the steel surface plate (lower) test section was thinner than that of the ÔoutletÕ of the test cell (see Fig. 10). Typically, the fouling thickness at the inlet and outlet was about 0.8 and 1.2 mm, respectively after 100 min, and about 1.5 and 2.5 mm, respectively after 200 min. The formation of nonuniform fouling layers is due to the fact that the temperature of the milk close to the outlet is higher than that close to the inlet as a result of the heating by the test plate.
Electrical resistance decreases due to NaOH penetration
3 2.5
Actual cleaning starts
2 1.5
Penetration time 1 0.5 0 0
10
20
30
40
50
60
70
0 80
90
100
110
120
Time (second)
Fig. 11. The change in electrical resistance with time, skim milk fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189 4
0.0045
)
Cleaning effect
-1
0.004
Fouling
Thermal resistance RF (m2.kW
3.5
Electrical resistance RE (Ω)
187
Cleaning
3 Electrical resistance decreases due to NaOH penetration
2.5 2 1.5 1 0.5
0.0035 0.003
NaOH penetration
0.0025 0.002 0.0015 0.001 0.0005
0
0
05
0
100
150
200
250
300
350
400
450
500
0
0.5
1
1.5
2
2.5
Electrical resistance R E
Time (min)
Fig. 12. The change in electrical resistance with time, fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
sistance was observed once the system was filled with the cleaning solution, suggesting that this is due to the introduction of electrolytes. Fig. 12 shows the trend that combines both the fouling and cleaning process. The first six data points of the cleaning process in Fig. 12 correspond to the penetration of NaOH during the first six seconds. When the cell was initially filled with NaOH, there was an apparently ÔabruptÕ drop in the electric resistance (from 3.53 to 1.32 X). This drop was expected, because the NaOH solution is a strong conductive medium. If the cleaning solution penetrates into the foulant, it would substantially reduce the electrical resistance value. The thermal resistance as a function of time during the fouling build-up and the cleaning process is shown in Fig. 13. As can be seen from Fig. 13, a sharp drop in the thermal resistance was not observed during the cleaning stage. The relationship between the electrical resistance and the thermal resistance for the cleaning process is illustrated in Fig. 14. The ÔabruptÕ drop in the electrical
3
3.5
4
(Ω)
Fig. 14. Cleaning process, the relationship between electrical resistance and thermal resistance, fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
resistance was observed even though the removal of the deposit has not started. This is indicated by a large reduction in the electrical resistance (RE ) while almost no change in the thermal resistance was recorded (see Fig. 14). This shows that the NaOH penetration into the deposit was very fast and it can not be the limiting step for the cleaning process. The electrical resistance as a function of time for the foulant (about 2-mm thick) generated at a surface temperature of 115 °C during the cleaning process is shown in Fig. 15. The electrical resistance decreased from 7.43 to 5.86 X during the first 7 s of the cleaning process. Fig. 16 shows both the fouling build-up and the cleaning process in terms of thermal resistance. Similar trends were observed as those in Figs. 12 and 13. The drop in the electrical resistance for the 2-mm thick foulant (see Fig. 15) layer was not very abrupt compared to that for 1-mm thick foulant (see Fig. 12). This is reflected graphically in Fig. 17. The reduction in the electrical resistance at a constant thermal resistance for
0.004 Fouling
Cleaning Electrical reduction due to NaOH penetration
7
0.003
Electrical resistance RE (Ω)
Thermal resistance RF (m 2W-1)
8
0.0035
0.0025 0.002 0.0015 0.001 0.0005
6
Actual cleaning starts
5 4 3 2
Penetration time
1
0
0
0
50
100
150
200
250
300
350
400
450
500
Time (min)
Fig. 13. The change in thermal resistance with time, skim milk fouling layer occurring at Ts ¼ 96 °C, d ¼ 1 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Time (second)
Fig. 15. The change in electrical resistance with time, fouling layer occurring at Ts ¼ 115 °C, d ¼ 2 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
188
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189
References
Thermal resistance RF (m2.kW-1)
0.004 0.0035 Cleaning
Fouling
0.003 0.0025 0.002 0.0015 0.001 0.0005 0 0
50
100
150
200
250
300
350
400
450
500
Time (min)
Fig. 16. The change in electrical resistance with time, fouling layer occurring at Ts ¼ 115 °C, d ¼ 2 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
Thermal resistance RF(m 2.kW-1)
0.004
Effect of cleaning
0.0035 0.003 NaOH penetration
0.0025 0.002 0.0015 0.001 0.0005 0 0
1
2
3
4
Electrical resistance
5
6
7
8
RE (Ω)
Fig. 17. Cleaning process, the relationship between electrical resistance and thermal resistance, fouling layer occurring at Ts ¼ 115 °C, d ¼ 2 mm, cleaning solution 0.5 wt.% NaOH, velocity ¼ 0.133 m s1 , Tsolution ¼ 45 °C, DT ¼ 10 °C.
the thick foulant is less than that observed for the thinner more porous layer generated at 96 °C.
4. Conclusions A novel soft deposit gauging technique employing electrical resistance measurement has been developed and tested in laboratory. This technique is a simple and can be used in situ. The relationship between the electrical resistance and the thermal resistance during the fouling build-up and cleaning process has been established. The study provides good evidence that the effects of fouling can be detected via the measurement of the electrical resistance. This technique has the potential to measure the thickness of deposits on a surface in situ and in real time of a dairy plant operation.
Bott, T. R. (1995). Fouling of heat exchangers. New York: Elsevier. Bott, T. R. (1996). Fouling of the heat exchangers and its mitigation with special reference to biofouling. In: Second European thermalsciences and 14th UIT national heat transfer conference 1996. Colberg, M. T., Carnes, K., Saez, A. E., Grant, C. S., Hutchinson, K., & Hesterberg, D. (1998). Hydration and removal of supported phospholipid films in aqueous surfactant solutions. Thin Solid Film, 247–251. Corrieu, G., Lalande, M., & Ferret, R. (1979). New monitoring equipment for the control and automation of milk pasteurization plants. Food Process Engineering, 1, 165–171. Diehl, K. C., & Van Gelder, M. F. (1990). An automated dimension measurement system for soft biological materials. Journal of Agricultural Engineering Research, 47, 133–138. Fryer, P. J. (1989). A comparison of two possible fouling monitors for the food processing industry. In R. W. Field, & J. A. Howell (Eds.), Process engineering in the food industry (pp. 131–141). New York: Elsevier Applied Science. Fryer, P. J. (1997). Fouling and cleaning in food process plant. In P. J. Fryer, D. L. Pyle, & C. D. Rielly (Eds.), Chemical engineering for the food industry (pp. 365–380). Blackie Academic & Professional. Fujimori, H., Asakura, Y., & Suzuki, K. (1987). Non-contact measurement of film thickness by the photothermal deflection method. Japanese Journal of Applied Physics, 26, 1759–1764. Gale, G. E., & Griffiths, P. (1995). An automatic micrometer for measuring soft electrically conductive materials. Measurement Science and Technology, 6, 447–451. Gale, G. E. (1995). A thickness measuring device using pneumatic gauging to detect the sample. Measurement Science and Technology, 6, 1566–1571. Grant, C. S., Webb, G. E., & Jeon, Y. W. (1996). Calcium phosphate decontamination of stainless steel surfaces. AIChE Journal, 42, 861–875. Grant, C. S., Webb, G. E., & Jeon, Y. W. (1997). A noninvasive study of milk cleaning processes: calcium phosphate removal. Journal of Food Process Engineering, 4, 197–230. Kabin, J. A., Saez, A. E., Grant, C. S., & Carbonell, R. G. (1996). Removal of organic films from rotating disks using aqueous solutions of nonionic surfactants: film morphology and cleaning mechanisms. Industrial and Engineering Chemistry Research, 35, 4494–4506. Karlsson, C. A. C., Wahlgren, M. C., & Tragardh, A. C. (1996). Betalactoglobulin fouling and its removal upon rinsing and by SDS as influenced by surface characteristics, temperature and adsorption time. Journal of Food Engineering, 30, 43–60. Lalande, M., & Rene, F. (1988). Fouling by milk and dairy product and cleaning of heating exchange surfaces. In L. F. Melo, T. R. Bott, & C. A. Bernardo (Eds.), Fouling science and technology (pp. 557–573). Kluwer Academic Publishers. Littlejohn, F., Saez, A. E., & Grant, C. S. (1998). Use of sodium polyaspartate for the removal of hydroxyapatite/brushite deposits from stainless steel tubing. Industrial and Engineering Chemistry Research, 2691–2700. Root, L. F., & Kaufman, I. (1992). Non-contacting low-cost instrument for film thickness measurement. IEEE Transactions on Instrumentation and Measurement, 41, 1014–1019. Stenberg, M., Stemme, G., & Kittilsland, G. (1988). A silicon sensor for measurement of liquid flow and thickness of fouling biofilms. Sensors and Actuators, 13, 203–221. Tamachkiarowa, A., & Flemming, H. C. (1999). Optical fiber sensor for biofouling monitoring. In T. R. Bott (Ed.), Understanding heat exchanger fouling and its mitigation (pp. 343–347). New York: Begell House.
X.D. Chen et al. / Journal of Food Engineering 61 (2004) 181–189 Troung, T. (2001). Application of heat flux sensor in dairy industry. Ph.D. Thesis, Massey University. Tuladhar, T. R., Paterson, W. R., Macleod, N., & Wilson, D. I. (2000). Development of a novel non-contact proximity gauge for thickness measurement of soft deposits and its application in fouling studies. The Canadian Journal of Chemical Engineering, 78, 935– 947.
189
Visser, J., & Jeurnink, T. J. M. (1997). Fouling of heat exchangers in the dairy industry. Experimental Thermal and Fluid Science, 14, 407–424. Withers, P. M. (1996). Ultrasonic, acoustic and optical techniques for the non-invasive detection of fouling in food processing equipment. Trends in Food Science and Technology, 7, 293–298. Yan, J.-F., Saez, A., & Eduardo, S. G. C. (1997). Removal of oil films from stainless steel tubes. AIChE Journal, 43(1), 251–259.