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Cleaning-in-Place of Whey Protein Fouling Deposits
0960±3085/99/$10.00+0.00 Institution of Chemical Engineers Trans IChemE, Vol 77, Part C, June 1999
CLEANING-IN-PLACE OF WHEY PROTEIN FOULING DEPOSITS: Mechanisms Controlling Cleaning C. R. GILLHAM, P. J. FRYER1 (FELLOW), A. P. M. HASTING2 (FELLOW) and D. I. WILSON (MEMBER) Department of Chemical Engineering, University of Cambridge, Cambridge, UK 1
School of Chemical Engineering, University of Birmingham, Birmingham, UK 2
Unilever Research, Sharnbrook, Bedford, UK
T
he processes involved in alkali-based cleaning-in-place of whey protein deposits were investigated using cleaning solutions of 0.5 wt% NaOH over a range of surface temperatures (20±80°C) and ¯ ow rates (Re 500±5000). Cleaning was quanti® ed by measuring both mass removal and the change in the thermal resistance of the deposits. The results con® rmed that cleaning involved three stages, namely deposit swelling, uniform erosion and a ® nal decay phase. The evolution of structure in the protein network on contacting NaOH was elucidated with SEM and surface ® xation techniques on samples generated by a short contact time apparatus. The effects of temperature and ¯ ow rate on the cleaning rate changed between the uniform and breakdown stage. The protein removal rate in the uniform stage was strongly dependent on conditions at the deposit/solution interface, while that in the decay phase was more sensitive to ¯ ow rate (i.e. surface shear stress). Reaction and diffusion of protein within the swollen deposit matrix appear to control the uniform cleaning rate. Simultaneous measurements of thermal resistance and protein removal did not show a simple correlation and suggest that existing models for cleaning require further development. Keywords: cleaning-in-place; mechanisms; whey protein; fouling; microstructure
generates uncertainty in process parameters6 . Understanding the key steps in the cleaning mechanism is essential, nonetheless, to interpret factory data, formulate CIP chemicals and design CIP systems. The cleaning of milk and milk protein deposits has received considerable attention owing to their importance in the dairy industry7 . Proteins constitute the major fraction of many other food process deposits (e.g. chocolate dessert8 and tomato pastes9 ) and are notably dif® cult to remove; most CIP systems employ a combination of chemical and hydraulic treatment in order to remove these soils. The importance of chemical and hydraulic (or physical) factors on the cleaning rate is poorly understood. Table 1 summarizes the contributions attributed to these factors. Alkali-based solutions, frequently based on sodium hydroxide, are usually employed to clean proteinaceous deposits. Proteinaceous deposits are formed from denatured proteins, which agglomerate together and/or stick to the surface to form a compact, porous layer. This porous deposit may contain other components, entrained in the pores, or inverse solubility salts such as calcium phosphate which are co-deposited with the proteins. The organic non-protein components are often removed along with the protein during cleaning with NaOH-based solutions. Mineral deposits are not affected by NaOH treatment and are removed either by the use of appropriate sequestrants in alkali formulations, or by circulation of acidic solutions before or after the NaOH stage. Figure 1 shows the series of steps involved in
INTRODUCTION The rapid fouling of heat exchangers in the food industry (e.g. in continuous pasteurization and sterilization plant) leads to frequent interruptions for cleaning-in-place (CIP), which is usually performed by the circulation of formulated detergents. Plant downtime for UHT plant can be in excess of 40% of the available production time1 . Optimization of CIP processes will yield reduced downtime and costs for cleaning, decreased environmental impact (in disposing of the spent chemicals) and increased plant ¯ exibility. Optimization requires a reliable model for the cleaning process based on the physical and chemical principles involved in the removal of a particular deposit. Previous attempts at optimization of the fouling, cleaning and disinfection cycle have been largely unsuccessful as the mechanisms which govern cleaning, and thus the length of the CIP cycle, are still not well understood 2 ,3 . The incentive for improving the ef® ciency of cleaning cycles is large owing to the cost and environmental impact of cleaning chemicals4 and the need to ensure hygiene standards in production5 . Hygiene involves both the prevention of microbial contamination, and removal of product during changeover from one product to another. The cleaning mechanism will also depend on the fouling process as many food deposits are thermally sensitive; their structure and chemical behaviour being determined by their processing history. The variability of food materials also 127
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Table 1. Effects of chemical and hydraulic (or physical) processes on soil removal (after Graû hoff2). Factor
Effect
Chemical reaction /modi® cation
Swelling of deposit matrixÐ change of voidage DissolutionÐ erosion AgeingÐ change in deposit composition and structure over time Mass transfer of reagent and reaction products from deposit interface to bulk solution LiftÐ removal of particulate soils from surface ScouringÐ entrained particulates Surface shear stressÐ mechanical erosion
Hydraulic action of reagent ¯ ow
cleaning such materials with NaOH-based solutions1 0 . In the swelling stage, the deposit swells on contact with alkali to form an open protein matrix of high void fraction; this `uniform’ swollen layer is removed by a combination of surface shear and diffusion in the erosion phase. The ® nal `decay’ phase occurs when the swollen layer is thin and no longer uniform, and involves removal of isolated `islands’ by shear/mass transport. The process is complex and the interaction of NaOH with the protein matrix has been shown to be concentration dependent2 ,1 0 . The rate of cleaning in the decay phase has been shown to be more sensitive to surface shear stress than the other phases. Existing models for these processes1 0 ,1 1 have considered the cleaning process to be controlled by transport of hydroxide through the deposit and surface removal steps, but quantitative modelling has yet to provide an acceptable description of the process. The three stages of the cleaning process have been shown to be sensitive to different combinations of operating parameters and solution chemistry1 0 . This paper describes the use of different techniques to investigate the stages of removal depicted in Figure 1. Both removal rates and deposit structure have been studied in order to elucidate the mechanisms involved in cleaning. Experiments were performed on soils generated using simulated dairy ¯ uids (whey protein solutions) to reduce the variation in deposits encountered with real ¯ uids. These whey protein deposits featured minimal amounts of mineral scale and represent a
Figure 1. Schematic of the stages involved in removal of whey protein deposits (a) swelling phase; (b) uniform erosion phase; (c) decay phase.
model system for studying proteinaceous foulants. The results will be of direct relevance to whey protein applications and it is anticipated that the mechanistic information obtained will be transferable to other systems, particularly milk soils generated under pasteurization conditions (i.e. Type A deposits as described by Burton1 2 ). Milk soils generated under more severe temperature conditions (e.g. UHT processes) contain signi® cantly higher fractions of mineral salts, particularly calcium phosphates, which alter the cleaning process and require two-stage cleaning with alkali, acid or sequestrant formulations. The less dense type A deposits cause greater pressure drop problems in process plant. A novel technique employed here uses a heat ¯ ux sensor to monitor the thermal resistance of a deposit during cleaning. The use of heat ¯ ux sensors to study fouling and cleaning phenomena has increased noticeably as small, affordable and accurate devices have become available. The device used here was similar to that used by Davies et al.1 3 to estimate the thermal conductivity of whey protein soils; their results were subsequently con® rmed by Rose et al.1 4 using a specially designed fouling test unit. Similar devices have been used on-line in process plant to monitor the extent of fouling and ef® ciency of cleaning1 5 ; their use as effective fouling and cleaning monitors is presently limited by the simulation criterionÐ how well do such devices reproduce the behaviour occurring inside process plant? Such considerations do not affect their application to smallscale experimental studies. EXPERIMENTAL Generation of Fouling Deposits Cleaning experiments were performed on 10 cm sections of fouled tubes which were generated in a countercurrent double pipe heat exchanger using a proven protocol1 6 . The concentric tube heat exchanger has been described in detail by Gotham1 7 and by Davies et al.1 3 . The inner AISI 316 tube (i.d. 6 mm, 0.15 mm wall thickness, 2.2 m heated length) could be easily removed after an experiment and cut into sections or opened up for surface analysis. Tubes were cleaned in situ before fouling experiments using 2 wt% aqueous solutions of Decon 90 detergent (Decon Laboratories, Sussex) at 50°C, bulk velocity 0.5 ms ± 1 for 30 minutes followed by a 20 minute warm RO water rinse. This procedure also served to bring the apparatus to operating temperature. Whey protein concentrate (WPC, 35% protein, Carberry Milk Products, Eire) was reconstituted to a protein concentration of 3.5 wt% protein in 60 litres of RO water (cf. typical value of 3.1 wt%1 8 ) and the pH adjusted from 6.6 to 6.0 using 0.1 M HCl in order to yield larger deposit quantities per run on a consistent basis1 9 . Whey protein solutions were used on a once-through basis and were pumped from a holding reservoir at ambient temperature through a preheater coil (wall temperature ~ 80°C) to 72±74°C before passing through the heat exchanger at 0.8 l min ± 1 (Re 4600). Oil (Transcal N, BP) at 97°C ¯ owed on the shellside; the unit had a signi® cant shellside heat transfer resistance, so that the wall temperature varied between 80±87°C1 3 . Fouling runs were stopped after 60 minutes, accompanied by a decrease in outlet temperature of 2±3 K. The oil temperature was then reduced to 50°C and the system Trans IChemE, Vol 77, Part C, June 1999
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0
129
60 0
0.5
1
1.5
2
x /m Figure 2. Deposit distribution pro® les in foulant generation experiments. Open symbolsÐ wet deposit coverage; solid circlesÐ dry deposit coverage; solid trianglesÐ protein assay; solid lineÐ bulk temperature pro® le from simulation by Davies et al.13 .
¯ ushed through with RO water until no visible traces of protein in solution were evident. Temperature data were logged during the fouling run; typical values of overall fouling resistance (0.365 m2 K kW ± 1 ) corresponded to a fouling Biot number of 0.24. Figure 2 shows typical deposition pro® les exhibiting good experimental reproducibility. The variation in wet coverage along the exchanger is caused by the change in deposit water content (from 58±66% wb) as the bulk temperature increased along the exchanger, corresponding to the transition from surface reaction to bulk reaction control around 75°C. The protein coverage data, obtained using a modi® ed Bradford assay2 0 , were in very good agreement with the dry coverage data obtained gravimetrically. Coverage data are hereafter reported as g(protein) m ± 2 . Trials showed similar cleaning behaviour in deposits with different water contents as the process of swelling on contact with NaOH eliminated differences in initial deposit structure. Rose et al.1 4 reported similar water contents using 1 wt% WPC solutions. Microscope examinations con® rmed that the deposits were of the Type A variety. Small, almost negligible amounts of mineral scale were formed from the calcium phosphate present in the WPC powder. Cleaning Experiments Cleaning experiments were performed by mounting 10 cm sections of fouled tube in the once-through cleaning loop shown schematically in Figure 3. Sodium hydroxide solution (0.5 wt% NaOH (BDH, UK) in RO water) was pumped from a heated reservoir through the test section and then collected for analysis or sent to disposal. This NaOH concentration had been identi® ed previously by Bird and Fryer9 as the optimal concentration for cleaning the type of whey protein deposits involved in these experiments. Timperley and Smeulders3 also reported a similar optimum concentration of NaOH in cleaning dairy HTST plate heat exchanger deposits. The apparatus was brought to thermal equilibrium with a dummy piece in place. The ¯ ow was then Trans IChemE, Vol 77, Part C, June 1999
stopped, the test section quickly installed and the ¯ ow restarted at the desired setting. Switching test sections took approximately ten seconds. The concentration of protein in solution and thus the cleaning rate (in g m ± 2 s ± 1 ) was measured using the modi® ed Bradford assay with Coomassie Brilliant Blue G-250 (Pierce and Warriner, Chester) as the protein dye reagent. Flow rates of cleaning solution corresponding to Re 500±6500 were used for 30 minutes, after which time the test sections were usually free of protein. The error in cleaning rate measurements was estimated to be around 5%. Mass balances on test sections gave good agreement between the gravimetric result and that obtained from the removal rate data (<6% difference). Figure 4 shows the degree of reproducibility achieved in these experiments. The three sets of removal rate data show the three different stages in cleaning reported by other workers1 0 ,1 1 , namely the swelling stage, uniform or plateau stage, and decay stage. Cleaning rates were compared by quantifying the total time to remove all protein from the tube, t; the plateau cleaning rate P (in the uniform stage) and the length of the decay period, d. Cleaning runs used tubes with protein coverage of ~ 100 g m ± 2 . Experiments investigating the thermal resistance of the deposit during cleaning and the effect of deposit temperature both used the temperature controlled block shown
sampling
7
1 3
2
6
5
V
4
V
V V
drain
Figure 3. Schematic diagram of cleaning apparatus. Dotted zoneÐ temperature control: V denotes solenoid valve locations 1Ð NaOH reservoir; 2Ð rotameter; 3Ð heating coil; 4Ð ¯ uid ¯ ow forming section 5Ð test section mount; 6Ð back pressure valve; 7Ð connection to water rinse system for short contact time experiments.
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GILLHAM et al. 0.5 Experiment reference number
2 CLEANING RATE (g/m sec)
swelling 0.4
uniform removal
decay
#25 #26 #27
0.3
P
d
0.2
0.1
0.0 0
5
10
15
20
25
30
TIME (mins)
t Figure 4. Reproducibility of cleaning experiments. Protein coverage 145 6 2 g m ±2; Re 585, isothermal cleaning at 50°C. The Figure also shows the three cleaning parameters discussed in the text: total cleaning time t, plateau removal rate P, and length of decay phase d.
schematically in Figure 5. The block (dimensions 60 ´ 60 ´ 60 mm) was constructed from brass and acted as a heat sink or source. The central channel (diameter 6.35 mm) ® tted snugly around a test section and was held in place by retaining screws. Good thermal contact was ensured using high thermal conductivity paste (3 W m ± 1 K ± 1 ; RS Components, Corby). The block temperature was maintained within 6 1 K by a water bath recirculator system. The micro-foil heat ¯ ux sensor (dimensions 8 ´ 13 ´ 0.076 mm, Rhopoint Ltd, Oxford) was mounted in a machined recess in the middle of the block so that it rested ¯ ush on the tube surface. The block assembly featured ¯ exible couplings so that the test section could be inserted rapidly into the cleaning apparatus. The sensor voltage (typically 150±350 mV) was measured and ampli® ed (Keithley Instruments Type 148 nanovoltmeter, Ohio, USA) then recorded by a dedicated PC. The heat ¯ ux q was calculated from the manufacturer’ s calibration and the fouling resistance Rf , based on the overall heat transfer coef® cient U, calculated thus: Rf <
xf lf
1 U
1 Uclean
Tblock
Tbulk
1
q
Uclean
1
where Tb u l k was the temperature of the bulk liquid and Uc le a n was obtained at the end of a cleaning experiment. Consistent heat transfer results were obtained when temperature driving forces greater than 5 K were used. The reliability of the technique, and agreement with correlation predictions, are discussed in references 13 and 16. T Heating/cooling fluid
Fouled tube
Experiments investigating short contact times were performed in a purpose built fouling and cleaning rig, similar to that in Figure 3 but featuring computer-controlled sequencing of cleaning and rinsing solutions2 1 . Test sections for these experiments were generated as before using AISI 321 tubing instead of AISI 316 as the latter material contains molybdenum, which gives an X-ray spectrum peak coincident with that of sulphur, the element present in b-lactoglobulin used to indicate the presence of the protein. RO water was ® rst circulated around the system to establish thermal and hydraulic equilibrium; the test place was then quickly replaced and 0.5 wt% NaOH ¯ ow established at 250 mL min ± 1 and 50°C (Re 1520) for the required period (5±800 s) followed by a RO water rinse for 20 s at 50°C. The test section was then isolated, drained and freeze-dried in liquid nitrogen to quench any further reaction. Protein removal was measured as before. The surface analysis techniques summarized in Table 2 were used to study the microstructure of 10 ´ 10 mm deposit samples during the cleaning process. SEM visualization was performed on gold sputtered samples in an SEM model JEOL JSM-820 (Tokyo, Japan). X-ray elemental analysis was performed at 20 kV, 30 nA using a Camscan C4 X-ray microanalyser ® tted with a Link 860II analyser and software (Cambridge Scanning Co, Cambridge, UK). Cryogenic SEM (CSEM) studies were performed using a JEOL JSM630IF device featuring a cryo-prep chamber with a CT100 cold stage controller (Oxford Instruments, UK) located at Unilever Research, Colworth House. The cleaning runs for the CSEM studies were carried out immediately before analysis. Short test pieces containing wet deposit were immersed in 0.5 wt% NaOH, rinsed in deionized water, drained rapidly then mounted on the CSEM sample stub before immersion in slush nitrogen. The sample was gold plated in the cryo-prep chamber at 0.4 mbar before imaging at ±150°C and 10 ± 6 torr. The sample preparation protocol required considerable development in order to avoid the generation of ice crystals on the deposit surface. Both conventional SEM and CSEM preparation methods involved water removal via sublimation of ice crystals. The formation of these crystals can damage the specimen structure, particularly where high water contents are involved so the results were compared with SEM images generated using a chemical ® xation technique (subsequently referred to as FSEM). Glutaraldehyde was chosen as it has been used extensively for cross-linking and preserving tissue proteins2 2 . The ® xative did not penetrate the deposit completely but this method did effectively `freeze’ the surface structure at the deposit/solution interface. Chemically ® xed samples were generated by contacting the fresh sample with 2.5 wt% glutaraldehyde in 0.1 M cacodylic buffer at pH 7.2 for 1 hour. The glutaraldehyde was removed by successive rinses of buffer then distilled water. Water was removed by dehydration in ethanol followed by critical point drying using CO2 before being gold sputtered for conventional SEM imaging.
Cleaning solution
T
sensor Heating/cooling fluid
Figure 5. Layout of heat ¯ ux sensor monitor.
RESULTS AND DISCUSSION Operating Parameters The initial swelling stage of cleaning is controlled by the reaction of protein with NaOH and is dif® cult to study Trans IChemE, Vol 77, Part C, June 1999
CLEANING-IN-PLACE OF WHEY PROTEIN FOULING DEPOSITS
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Table 2. Summary of surface visualization techniques. Visualization technique
Cleaning conditions
Conventional SEM (SEM)
Re 1520 T 50°C Contact time² 5±300 s Water rinse time 20 s
Freeze drying in liquid nitrogen (±197°C)
X-ray elemental mapping
as SEM
as SEM
Cryogenic SEM (CSEM)
Deposits dipped in NaOH² bath T 20°C Contact time 5±300 s Water rinse time 20 s
Freeze in `slush’ nitrogen
SEM with chemically ® xed sample (FSEM)
as CSEM
Contact with glutaraldehyde in cacodylic buffer
² NaOH concentration
0.5 wt%
experimentally due to the speed of the process. The effect of operating parameters on the uniform and decay stages was studied; most protein removal occurs in this period. Experiments were performed to establish whether the mean plateau cleaning rate P was controlled by reaction, and thus strongly dependent on temperature, or hydraulic (shear/mass transfer) effects and therefore ¯ ow rate dependent. The effect of ¯ ow rate and temperature on P were studied over the range 500 < Re < 5000 and 20°C < Tb u l k ,
0.5
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(a)
Block
CLEANING RATE (g/m 2 sec)
0.4
70C 50C 30C 50C 50C
0.3
Experiment reference number
Bulk liquid 50C 50C 50C 30C 70C
0.4
#14 #15 #16 #18 #19
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0.1
0.1
0.0
0.0 0
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(b)
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0.5 Experiment reference Block number
CLEANING RATE (g/m 2 sec)
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80C 70C 50C 30C
0.3
0.4
#13 #14 #15 #16
0.3
0.2
0.2
0.1
0.1
0.0
0.0 0
Sample preparation method
5
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TIME (mins)
Figure 6(a). Effect of cleaning solution temperature on cleaning rate. Mean protein coverage 100 g m 2 ; NaOH ¯ owrate 800 mL min 1 . (b). Effect of block (wall) temperature on cleaning rate. Mean protein coverage 100 g m ±2; Tbulk 50°C; NaOH ¯ owrate 800 mL min ±1.
Trans IChemE, Vol 77, Part C, June 1999
Tw a ll < 80°C using the heat ¯ ux sensor monitor. Comparison of isothermal tests (at 50°C), with and without the sensor, con® rmed that the sensor arrangement did not affect the cleaning pro® les. Temperature differences (DT) >20 K were required to generate thermal resistance data; DT values were varied to investigate whether cleaning was more sensitive to (a) the deposit/liquid interface temperature, and thus reaction/diffusion in the swollen deposit or its surface, or (b) deposit/wall temperatureÐ and thus reactions in the unswollen deposit, or associated with swelling as proposed by Gallot-Lavalle e and Lalande1 1 . Calculations showed that the temperature in the deposit-liquid interface during the swelling and uniform removal stages is effectively controlled by the temperature of the cleaning solution. Cleaning pro® les for different bulk and different wall temperatures are shown in Figure 6 and the results are summarized in Table 3. The cleaning pro® les in Figure 6 show a very strong dependence on the cleaning solution temperature; the variation due to the block temperature is effectively due to its contribution to the mean temperature in the swollen deposit layer. Note that the initial swelling period does not seem to be a strong function of temperature by comparison with the other two stages. The temperature dependence of P is plotted as a pseudo-Arrhenius plot in Figure 7. The cleaning rate was corrected for the fraction of deposit not in contact with the heated block using results from isothermal runs1 6 ; TD L is the temperature of the deposit/liquid interface calculated from the heat transfer data. The activation energy calculated at Re 2340 was skewed by the datum used in calculating the cleaning rates. The activation energies do not show any signi® cant variation with Re and are smaller than that reported by Bird and Fryer1 0 for a similar system (~ 80 kJ mol ± 1 ). The values approach the range of values quoted for reaction with internal diffusion control2 3 . These results suggest that processes within the swollen deposit layer control the cleaning process in the uniform stage: however, the mean plateau removal rate is not a true reaction parameter. Figure 6(a) and 6(b) both show a noticeable sensitivity of the cleaning rate in the decay phase on temperature. The length of the decay phase decreased signi® cantly when the deposit-liquid interface temperature exceeded 50°C. Above this temperature there was very little effect of temperature on the d parameter. Figure 8 shows that this parameter was however signi® cantly sensitive to ¯ ow rate under ® xed
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GILLHAM et al. Table 3. Summary of cleaning experiments. Tbulk / °C
Flowrate / ml min ±1 (Re)
P /g m ±2s ±1
Protein coverage /g m ±2
t /min
80
50
70
50
50
50
30
50
20 50 50
50 30 70
100 (585) 260 (1500) 400 (2340) 800 (4680) 100 (585) 260 (1500) 400 (2340) 800 (4680) 100 (585) 260 (1500) 400 (2340) 800 (4680) 100 (585) 260 (1500) 400 (2340) 800 (4680) 800 (4680) 800 (3300) 800 (6520)
0.18 0.22 0.21 0.29 0.16 0.18 0.20 0.24 0.13 0.14 0.19 0.17 0.08 0.10 0.11 0.16 0.16 0.07 0.50
139 104 101 109 102 108 91 96 107 119 110 106 102 111 110 106 89 102 98
18 10.5 12.5 9 14 13 10.5 8.5 22 17 12 8 32 29 24 21 >25 5
Tblock / °C
temperature conditions. This Figure also shows that the total cleaning time t was less sensitive than d to Re owing to the contribution from the plateau cleaning rate; P increased with Re under ® xed temperature conditions, varying as Ren where n was in the range 0.20±0.35. The low value of the index con® rms that diffusive transport and reaction within the foulant layer affect P more than mass transfer from, and shearing at, the deposit surface. The effect of Re on d is consistent with a change in removal mechanism in the decay stage to break-up of the deposit layer near the surface by shearing and reaction. The relatively weak dependence of P on Re and the importance of reaction processes on P also indicates that these results should be applicable to CIP systems, which are normally operated under turbulent ¯ ow conditions in order to maximize hydraulic effects. Under such conditions the decay stage is likely to be short and t will be dictated by the length of the swelling and uniform cleaning stages, which are controlled by internal processes.
Thermal Resistance Direct measurement of the swollen layer thickness required for quantitative modelling is dif® cult, but thermal resistances and thus deposit thicknesses can be inferred from measurements of the local heat transfer coef® cient. The heat transfer data are presented here as fouling resistances for ease of comparison with cleaning rate data. Figure 9 shows typical results from simultaneous measurement of fouling resistance (Rf < thickness/thermal conductivity) and protein removal rate during a cleaning experiment. Replicate trials showed very good reproducibility. The fouling resistance data feature three stages; (i) an initial, rapid increase from Rf ,0 (unswollen deposit) to a maximum value (Rf ,m a x ); (ii) a `uniform’ stage where the fouling resistance decreases slightly (# 20%) over time, followed by (iii) a ® nal `recovery’ stage in which Rf decays rapidly to zero. The length of the initial stage corresponded closely to the deposit swelling stage in the cleaning rate data and was
-1 25
-1.5 20
ln P
Removal time /min
-2
-2.5
-3
15
10
5
-3.5 0.0029
0.003
0.0031
0.0032
0.0033 0
1/TDL
Figure 7. Pseudo-Arrhenius plot of corrected P versus 1/(TDL) at different Re. Mean protein coverage 100 g m ±2, Tbulk 50°C. SquaresÐ Re 585, Eact 44 kJ mol ±1; trianglesÐ Re 1500; Eact 51 kJ mol ±1; open circlesÐ Re 2340, Eact 63 kJ mol±1; ® lled circlesÐ Re 4680, Eact 50 kJ mol±1.
0
1000
2000
3000
4000
5000
Re Figure 8. Effect of ¯ ow rate on removal time. SquaresÐ length of decay stage, d; CirclesÐ total cleaning time, t. Isothermal cleaning at 50°C; mean protein coverage 100 g m ±2.
Trans IChemE, Vol 77, Part C, June 1999
CLEANING-IN-PLACE OF WHEY PROTEIN FOULING DEPOSITS 0.5
133
250
1.6
CLEANING RATE (g/m2s)
I
II
III
1.2 1
0.3
0.8 0.2
0.6 0.4
FOULING RESITANCE (m2K/kW)
0.4
0.1 0.2 0.0
0 0
5
10
15
20
25
30
TIME (mins)
Figure 9. Simultaneous measurement of protein removal and fouling resistance; (I)Ð swelling stage; (II)Ð uniform stage; (III)Ð recovery stage; Flow rate 800 mL min ±1 (Re 4680); Tbulk = 50°C; Tblock 30°C.
found to decrease with increasing Re. The ratio of Rf,m a x to Rf ,0 was also used to estimate the degree of swelling of the deposit by assuming that (a) negligible removal during the swelling stage; (b) the thermal conductivity of the unswollen deposit was that reported by Davies et al., and that of the swollen deposit was close to that of water. These calculations gave a swelling factor of 2.4±3.4, in reasonable agreement with the value of 2.5 reported previously2 ,2 0 . Further calculations indicated that the voidage of the deposit increased from ~ 0.62 to ~ 0.90 on swelling, consistent with microstructure images. Figure 9 shows signi® cant differences in the length of the uniform and decay stages between the thermal and mass removal pro® les. These differences were apparent in all the experiments performed in this study. The uniform/decay stage transition was always earlier in the thermal data, and the length of the thermal decay stage was always shorter than the corresponding mass removal stage. A noticeable increase in the thermal cleaning rate (i.e. reduction in Rf ) coincides with the onset of the decay stage in the mass removal data. The time taken to achieve complete heat transfer recovery was on average 34% shorter than the mass removal parameter t, even though the local cleaning rate at the heat ¯ ux monitor would be expected to be slightly lower for high Tw a l l cases owing to the extra local heat transfer resistance of the sensor (around 3 K1 6 ). Comparison of the data in Figure 9 also shows that the change in Rf during the `uniform’ stage was relatively small (~ 10%) compared to the amount of protein removed from the surface during the same period (~ 60%). The protein removal data were con® rmed by direct measurement of the unswollen deposit thickness using conventional SEM shown in Figure 10. Note that the swollen protein layer collapses during sample preparation so its thickness cannot be measured using this technique. SEM thus measures the unreacted form below the swollen layer. This thickness (see Figure 10) decreases linearly over ca. 10 minutes, suggesting zeroth order conversion of protein to the swollen form. This conversion time is noticeably longer than the deposit swelling stage (2±3 minutes in this case) and con® rms that the uniform stage cleaning rate is independent of the amount of native deposit present. Trans IChemE, Vol 77, Part C, June 1999
Deposit thickness (mm)
1.4
200 150 100 50 0 0
2.5
5
7.5
10
12.5
Contact time (minutes)
Figure 10. SEM measurement of unswollen deposit thickness v. NaOH contact time. Isothermal cleaning at 50°C; Re 1500; mean protein coverage 260 g m ±2.
The fouling resistance is a lumped measurement, combining the thermal resistance of the native deposit, swollen deposit (of varying voidage and thus thermal conductivity) and any enhancement of the convective heat transfer coef® cient due to reduction in duct dimension. The latter factor was estimated to be small. The small change observed in Rf during the uniform mass removal stage indicated that the thermal resistance of the swollen layer was signi® cant and compensated for the change in the thickness of the unswollen deposit layer. Attempts to simulate the fouling resistance pro® les using a model based on native and swollen protein layers, of ® xed voidages and thermal conductivities, with a uniform removal rate were not successful. Simulation parameters were obtained from experimental data, but predictions did not match experimental results very well. The assumption of constant voidage in the swollen layer was considered invalid; a more realistic model of reaction and diffusion within the swollen protein layer is required. The data in Figure 10 are not consistent with a model where growth of swollen deposit is controlled by diffusion of NaOH to the native deposit surface. X-ray elemental mapping studies of samples generated under similar conditions in the short contact time apparatus indicated that sodium ions (from the NaOH) were present at the deposit/metal interface within 30 seconds of initial contact. The growth of the swollen layer is thus controlled either by protein swelling kinetics or by the diffusion of larger molecular species with longer diffusion times. The decay stage in the heat transfer data starts just before the end of the uniform stage in the mass removal pro® le, and complete heat recovery is achieved before all the protein is removed. Studies using longer fouled sections con® rmed that this behaviour was consistent and was not caused by a cleaning `front’ moving downstream through the deposit1 6 . The length of the thermal decay stage was found to decrease noticeably with increasing ¯ ow rate. These observations are consistent with the previously hypothesized mechanism where the uniform foulant layer breaks down into a disordered structure, possibly lacking native protein to contribute mechanical strength. Such a weak, non-uniform deposit is then readily broken down and will be sensitive to shear forces at the surface. The thermal resistance of isolated lumps of deposit is furthermore reduced by their
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contribution to surface roughness. The thermal measurement technique becomes insensitive as the fraction of the surface covered, and deposit size decreases (i.e. « thermal boundary layer thickness). This suggests that such a device could not be used to monitor the absolute `cleanliness’ of the surface. These simultaneous thermal and protein removal studies con® rm certain aspects of previous mechanistic models of cleaning but also contradict the hypothesis that diffusion and reaction of NaOH with protein controls the removal process. These results indicate that the deposit layer thickness decreases slowly after initial swelling and suggest that cleaning is dominated by the reaction and diffusion of dissolved protein through the swollen protein matrix. Surface Studies The evolution of deposit microstructure during cleaning has been studied previously using SEM on samples isolated during the cleaning process1 0 . Three variants of SEM were used here to avoid reporting artefacts; especially in visualizing the deposit/solution interface for the swollen deposit, which has a high water content and would not survive conventional SEM preparation. All three techniques (SEM, CSEM and FSEM) gave identical results for imaging the native deposit, an amorphous structure of protein aggregates of size 1± 10 mm and up to ~ 200 mm thick for a coverage of 260 g m ± 2 . Figure 11 shows a SEM cross section of the deposit obtained after contact with NaOH at 50°C for 30 s in the short contact time apparatus; for all contact times, such images showed a thin (1±5 mm) smooth adherent layer on the cleaning solution/deposit interface, which was taken to be the swollen deposit which collapsed on drying. Figure 12(a) is a CSEM image of the cross section of a deposit structure near the surface at a magni® cation of 2500, after (i) contact with 0.5 wt% NaOH for 3 seconds and (ii) rinsing in deionised water for 10 seconds. There is a dramatic change in the deposit structure after just 3 seconds; similar results were obtained using conventional SEM. Figure 12(b) is a CSEM image of the cross section of the deposit structure near the surface after contact with NaOH for 4 minutes followed by rinsing and shows that the swollen protein has changed to a regular network of thin strands (0.2±0.5 mm), enclosing narrow pores (0.5 to 1 mm). How these structures evolve is not clear and poses problems in modelling. Figure 13 is a series of FSEM images of deposit undergoing cleaning by 0.5 wt% NaOH at 20°C and
Figure 11. SEM cross section of whey protein deposit showing native deposit and collapsed swollen layer.
Figure 12. CSEM cross sections of whey protein deposit after contact with 0.5 wt% NaOH.(a) after contact for 3 seconds; (b) after contact for 4 minutes.
shows the gradual evolution of a ® ne stranded network similar to that in Figure 12(b). The chemical ® xation technique used here can only be used to study (near) surface topography rather than matrix thickness but these images con® rm that Figure 12(a) shows an artefact generated during sample preparation. The porosity and thus the effective diffusion rate of hydroxide and dissolved protein increases monotonically over the ® rst few minutes, explaining the increase in cleaning rate during the swelling stage. CONCLUSIONS The mechanisms involved in the alkali-based cleaning of whey protein deposits on stainless steel surfaces during steady pipe ¯ ow have been investigated using mass removal rate, thermal resistance and surface imaging techniques. Protocols for generating reproducible soils and removal pro® les have been developed. A novel heat ¯ ux sensor unit was used to collect thermal resistance data during cleaning experiments and a novel cleaning apparatus was used to generate samples of deposit exposed to NaOH cleaning solution for short contact times. The results con® rmed that alkali-based cleaning of protein deposits is a three-stage process involving deposit swelling, uniform cleaning and a decay phase. Analysis of swelling kinetics was not performed but thermal resistance results and SEM visualization of samples prepared using chemical ® xation techniques showed that the swelling stage involves the gradual build up a of a swollen protein matrix. The cleaning rate during the uniform stage, corresponding to the removal of most of the protein deposit, was most sensitive to the deposit/solution interface temperature (activation energy ~ 50 kJ mol ± 1 ) and less sensitive to hydraulic/external mass transport conditions (3Re0 .2 0 ± 0 .3 5 ). The cleaning rate in the decay stage exhibited a strong dependence on hydraulic effects and a threshold Trans IChemE, Vol 77, Part C, June 1999
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monitors which could detect the change between cleaning stages were available. Devices based on heat ¯ ux sensors, as used here, could be used if the results could reliably correlated to plant performance. This work also indicated that such the sensitivity of these devices limits their scope for monitoring cleanliness. Industrial heat exchangers frequently operate under higher Reynolds numbers and surface shear stress conditions than those used in this study (Re 500±6500 and shear stress 0.009±0.95 Pa). Although the Reynolds numbers used here crossed the `transition regime’ for turbulent ¯ ow, pressure drop and heat transfer measurements indicated that the ¯ ow regime was not changing signi® cantly. It was not feasible to perform once-through experiments at larger ¯ ow rates. Application of the results of this investigation to industrial units, particularly those featuring plate heat exchangers, is complicated by the variation of the parameters affecting both fouling and cleaning in such units. The spatial distribution of temperatures, surface shear stresses and residence times in a plate heat exchanger means that deposition is not uniform. The variation in foulant thickness, allied with the variation in ¯ ow patterns, gives different cleaning rates at different points so that it is likely that some parts of the exchanger will be in the decay stage while others are still in the uniform cleaning stage. The challenge facing the process engineer in such circumstances is to identify regions of high fouling/low cleaning potential and apply the knowledge of deposit behaviour gained here to minimize the time taken to remove this `worst case’ deposit. Figure 13. FSEM images deposit surfaces after contact with 0.5 wt% NaOH (a) after 3 s contact; (b) after 30 s contact; (c) after contact for 3 minutes.
temperature of ~ 50°C for these experiments, above which there was little temperature effect. The simultaneous thermal measurements also exhibited the uniform and decay stages but with different characteristics. The fouling resistance did not decrease signi® cantly during the uniform protein removal stage, and the transition to the decay stage occurred earlier in the thermal pro® les. The length of the thermal decay stage was shorter than the analogous mass removal stage. The latter results are consistent with the uniform/decay stage transition corresponding to the breakup of a uniform deposit layer to a random deposit after all the native deposit had undergone swelling. Several of the observations are inconsistent with previous quantitative models for the protein removal rate during the uniform cleaning stage. The X-ray elemental spectroscopy results indicate that diffusion of NaOH through the deposit is not a limiting factor; reaction and diffusion of dissolved protein out of the swollen matrix is more likely to be the controlling stage. The fouling resistance results are not consistent with the models of Bird and Fryer, or GallotLavalle e and Lalande, suggesting that further development of quantitative models is required before rigorous optimization of the cleaning process can be carried out. These experiments con® rmed that the operating parameters controlling the cleaning rate under conditions of steady pipe ¯ ow found in many CIP systems change between each cleaning stage. Optimization of cleaning in such systems could therefore be improved greatly if Trans IChemE, Vol 77, Part C, June 1999
NOMENCLATURE P q Re Rf T DT U xf d lf t
mean plateau cleaning rate, g m ±2 s ±1 heat ¯ ux, W m ±2 Reynolds number, ± fouling resistance, m2 K W ±1 temperature, K temperature difference in heat ¯ ux device, K overall heat transfer coef® cient, W m2K ±1 deposit thickness, m length of decay stage in mass removal pro® les, s deposit thermal conductivity, W m ±1 K ±1 time to clean deposit, s
REFERENCES 1. Pritchard, N.J., de Groerden, G. and Hasting, A.P.M., 1988, The removal of milk deposits from heated surfaces by improved cleaning processes, Proc Fouling in Process Plant, St. Catherine’s College, Oxford, pp. 465±479. 2. Graû hoff, A., 1997, Cleaning of heat treatment equipment, in Fouling and Cleaning of Heat Treatment Equipment (ed) Visser, H., IDF Monograph #328, publ. IDF, Brussels, pp. 32±44. 3. Timperley, D.A. and Smeulders, C.N.M., 1988, Cleaning of dairy HTST plate heat exchangers: optimisation of the single stage procedure, J Dairy Tech, 41: 4±7. 4. Sandu, C. and Singh, R.K., 1991, Energy increase in operation and cleaning due to heat exchanger cleaning in food pasteurisation, Food Tech, 45: 84±91. 5. Hasting, A.P.M., 1995, Good enough to eat, The Chemical Engineer, 599: 19±20. 6. Kessler, H.G. and Schraml, J.E., 1996, Effects of concentration and product composition on fouling structure, Proc Fouling and Cleaning in Food Processing, Jesus College, Cambridge, March 1994. publ EU, Brussels, EUR 16894 EN: 26±33. 7. Visser, H. (ed.), 1997, Fouling and Cleaning of Heat Treatment Equipment, IDF Monograph #328, publ IDF, Brussels.
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8. Rene , F., Leuliet, J.C., Goldberg, M. and Lalande, M., 1988, Encrassement et nettoyage d’ un e changer de chaleur aÁ plaques lors du traitment UHT d’ un produit lacteÂchocolate , Comparaison avec lait, Le Lait, 68: 85±102. 9. Cheow, C.S. and Jackson, A.T., 1982, Circulation cleaning of a plate heat exchanger fouled by tomato juice: I. Cleaning with water & II. Cleaning with caustic soda solution, J Food Tech, 17: 417±440. 10. Bird, M.R. and Fryer, P.J., 1991, An experimental study of the cleaning of surfaces fouled by whey proteins, Trans IChemE, Food Bioprod Proc, 69C: 13±21. 11. Gallot-Lavalle e, T. and Lalande, M., 1985, A mechanistic approach of pasteurised milk deposit cleaning, in Fouling and Cleaning in Food Processing, Madison, USA, Lund, D., Plett, E.A. and Sandu, C. (eds) pp. 374±394. 12. Burton, H., 1968, Deposits from whole milk in heat treatment plant - a review and discussion, J Dairy Res, 35: 317±330. 13. Davies, T.J., Henstridge, S.C., Gillham, C.R. and Wilson, D.I., 1997, Investigation of whey protein deposit properties using heat ¯ ux sensors, Trans IChemE, Food Bioprod Proc, 75C: 106±110. 14. Rose, I., Epstein, N. and Watkinson, A.P., 1997, The effect of velocity on the initial fouling rate of whey protein solutions at elevated wall and low bulk temperatures, Proc Engineering Foundation Conference on Understanding Heat Exchanger Fouling and its Mitigation, Lucca, Italy. 15. Truong, T., Anema, S., Kirkpatrick, K. and Tuoc, K.T., 1998, In-line measurements of fouling and CIP in milk powder plants, Proc Fouling and Cleaning in Food Processing `98, Jesus College, Cambridge. 16. Gillham, C.R., 1998, Enhanced cleaning of surfaces fouled by whey proteins, PhD thesis (University of Cambridge). 17. Gotham, S.M., 1990, Mechanisms of protein fouling of heat exchangers, PhD dissertation (University of Cambridge). 18. Walstra, P. and Jenness, R., 1984, Dairy Chemistry and Physics, (J. Wiley and Sons Inc, NY). 19. Skudder, P.J., Brooker, B.E., Bonsey, A.D. and Alvarez-Guerrero, N.R., 1986, Effect of pH on the formation of deposit from milk on
20. 21.
22. 23.
heated surfaces during ultra high temperature processing, J Dairy Res, 53: 75±87 Bird, M.R., 1993, Cleaning of food process plant, PhD dissertation (University of Cambridge). Belmar-Beiny, M.T. Toyoda, I. and Fryer, P.J., 1997, The initial stages of fouling from milk proteins and minerals onto surfaces above 100°C, Fouling Mitigation of Industrial Heat-Exchange Equipment, Panchal C.N. (ed), Begell House, NY, pp. 601±612. Hyatt, M.A., 1981, Fixation for electron microscopy (Academic Press, London). Levenspiel, O., 1972, Chemical Reaction Engineering (John Wiley, NY).
ACKNOWLEDGEMENTS Support for CRG from the BBSRC, DiverseyLever and Unilever Research, assistance from Dr M.T. Belmar-Beiny with the short contact time experiments and Mr Mark Kirkland (both Unilever Research Colworth Laboratory) with the SEM work are gratefully acknowledged.
ADDRESS Correspondence concerning this paper should be addressed to Dr Ian Wilson, Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, UK. Email: [email protected]. The manuscript was received 17 November 1998 and accepted for publication after revision 19 March 1999. This paper is an extended and updated version of a paper presented by the authors at Fouling and Cleaning in Food Processing, 6±8 April 1998, Cambridge, UK.
Trans IChemE, Vol 77, Part C, June 1999
CLEANING-IN-PLACE OF WHEY PROTEIN FOULING DEPOSITS: Mechanisms Controlling Cleaning C. R. GILLHAM, P. J. FRYER1 (FELLOW), A. P. M. HASTING2 (FELLOW) and D. I. WILSON (MEMBER) Department of Chemical Engineering, University of Cambridge, Cambridge, UK 1
School of Chemical Engineering, University of Birmingham, Birmingham, UK 2
Unilever Research, Sharnbrook, Bedford, UK
T
he processes involved in alkali-based cleaning-in-place of whey protein deposits were investigated using cleaning solutions of 0.5 wt% NaOH over a range of surface temperatures (20±80°C) and ¯ ow rates (Re 500±5000). Cleaning was quanti® ed by measuring both mass removal and the change in the thermal resistance of the deposits. The results con® rmed that cleaning involved three stages, namely deposit swelling, uniform erosion and a ® nal decay phase. The evolution of structure in the protein network on contacting NaOH was elucidated with SEM and surface ® xation techniques on samples generated by a short contact time apparatus. The effects of temperature and ¯ ow rate on the cleaning rate changed between the uniform and breakdown stage. The protein removal rate in the uniform stage was strongly dependent on conditions at the deposit/solution interface, while that in the decay phase was more sensitive to ¯ ow rate (i.e. surface shear stress). Reaction and diffusion of protein within the swollen deposit matrix appear to control the uniform cleaning rate. Simultaneous measurements of thermal resistance and protein removal did not show a simple correlation and suggest that existing models for cleaning require further development. Keywords: cleaning-in-place; mechanisms; whey protein; fouling; microstructure
generates uncertainty in process parameters6 . Understanding the key steps in the cleaning mechanism is essential, nonetheless, to interpret factory data, formulate CIP chemicals and design CIP systems. The cleaning of milk and milk protein deposits has received considerable attention owing to their importance in the dairy industry7 . Proteins constitute the major fraction of many other food process deposits (e.g. chocolate dessert8 and tomato pastes9 ) and are notably dif® cult to remove; most CIP systems employ a combination of chemical and hydraulic treatment in order to remove these soils. The importance of chemical and hydraulic (or physical) factors on the cleaning rate is poorly understood. Table 1 summarizes the contributions attributed to these factors. Alkali-based solutions, frequently based on sodium hydroxide, are usually employed to clean proteinaceous deposits. Proteinaceous deposits are formed from denatured proteins, which agglomerate together and/or stick to the surface to form a compact, porous layer. This porous deposit may contain other components, entrained in the pores, or inverse solubility salts such as calcium phosphate which are co-deposited with the proteins. The organic non-protein components are often removed along with the protein during cleaning with NaOH-based solutions. Mineral deposits are not affected by NaOH treatment and are removed either by the use of appropriate sequestrants in alkali formulations, or by circulation of acidic solutions before or after the NaOH stage. Figure 1 shows the series of steps involved in
INTRODUCTION The rapid fouling of heat exchangers in the food industry (e.g. in continuous pasteurization and sterilization plant) leads to frequent interruptions for cleaning-in-place (CIP), which is usually performed by the circulation of formulated detergents. Plant downtime for UHT plant can be in excess of 40% of the available production time1 . Optimization of CIP processes will yield reduced downtime and costs for cleaning, decreased environmental impact (in disposing of the spent chemicals) and increased plant ¯ exibility. Optimization requires a reliable model for the cleaning process based on the physical and chemical principles involved in the removal of a particular deposit. Previous attempts at optimization of the fouling, cleaning and disinfection cycle have been largely unsuccessful as the mechanisms which govern cleaning, and thus the length of the CIP cycle, are still not well understood 2 ,3 . The incentive for improving the ef® ciency of cleaning cycles is large owing to the cost and environmental impact of cleaning chemicals4 and the need to ensure hygiene standards in production5 . Hygiene involves both the prevention of microbial contamination, and removal of product during changeover from one product to another. The cleaning mechanism will also depend on the fouling process as many food deposits are thermally sensitive; their structure and chemical behaviour being determined by their processing history. The variability of food materials also 127
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Table 1. Effects of chemical and hydraulic (or physical) processes on soil removal (after Graû hoff2). Factor
Effect
Chemical reaction /modi® cation
Swelling of deposit matrixÐ change of voidage DissolutionÐ erosion AgeingÐ change in deposit composition and structure over time Mass transfer of reagent and reaction products from deposit interface to bulk solution LiftÐ removal of particulate soils from surface ScouringÐ entrained particulates Surface shear stressÐ mechanical erosion
Hydraulic action of reagent ¯ ow
cleaning such materials with NaOH-based solutions1 0 . In the swelling stage, the deposit swells on contact with alkali to form an open protein matrix of high void fraction; this `uniform’ swollen layer is removed by a combination of surface shear and diffusion in the erosion phase. The ® nal `decay’ phase occurs when the swollen layer is thin and no longer uniform, and involves removal of isolated `islands’ by shear/mass transport. The process is complex and the interaction of NaOH with the protein matrix has been shown to be concentration dependent2 ,1 0 . The rate of cleaning in the decay phase has been shown to be more sensitive to surface shear stress than the other phases. Existing models for these processes1 0 ,1 1 have considered the cleaning process to be controlled by transport of hydroxide through the deposit and surface removal steps, but quantitative modelling has yet to provide an acceptable description of the process. The three stages of the cleaning process have been shown to be sensitive to different combinations of operating parameters and solution chemistry1 0 . This paper describes the use of different techniques to investigate the stages of removal depicted in Figure 1. Both removal rates and deposit structure have been studied in order to elucidate the mechanisms involved in cleaning. Experiments were performed on soils generated using simulated dairy ¯ uids (whey protein solutions) to reduce the variation in deposits encountered with real ¯ uids. These whey protein deposits featured minimal amounts of mineral scale and represent a
Figure 1. Schematic of the stages involved in removal of whey protein deposits (a) swelling phase; (b) uniform erosion phase; (c) decay phase.
model system for studying proteinaceous foulants. The results will be of direct relevance to whey protein applications and it is anticipated that the mechanistic information obtained will be transferable to other systems, particularly milk soils generated under pasteurization conditions (i.e. Type A deposits as described by Burton1 2 ). Milk soils generated under more severe temperature conditions (e.g. UHT processes) contain signi® cantly higher fractions of mineral salts, particularly calcium phosphates, which alter the cleaning process and require two-stage cleaning with alkali, acid or sequestrant formulations. The less dense type A deposits cause greater pressure drop problems in process plant. A novel technique employed here uses a heat ¯ ux sensor to monitor the thermal resistance of a deposit during cleaning. The use of heat ¯ ux sensors to study fouling and cleaning phenomena has increased noticeably as small, affordable and accurate devices have become available. The device used here was similar to that used by Davies et al.1 3 to estimate the thermal conductivity of whey protein soils; their results were subsequently con® rmed by Rose et al.1 4 using a specially designed fouling test unit. Similar devices have been used on-line in process plant to monitor the extent of fouling and ef® ciency of cleaning1 5 ; their use as effective fouling and cleaning monitors is presently limited by the simulation criterionÐ how well do such devices reproduce the behaviour occurring inside process plant? Such considerations do not affect their application to smallscale experimental studies. EXPERIMENTAL Generation of Fouling Deposits Cleaning experiments were performed on 10 cm sections of fouled tubes which were generated in a countercurrent double pipe heat exchanger using a proven protocol1 6 . The concentric tube heat exchanger has been described in detail by Gotham1 7 and by Davies et al.1 3 . The inner AISI 316 tube (i.d. 6 mm, 0.15 mm wall thickness, 2.2 m heated length) could be easily removed after an experiment and cut into sections or opened up for surface analysis. Tubes were cleaned in situ before fouling experiments using 2 wt% aqueous solutions of Decon 90 detergent (Decon Laboratories, Sussex) at 50°C, bulk velocity 0.5 ms ± 1 for 30 minutes followed by a 20 minute warm RO water rinse. This procedure also served to bring the apparatus to operating temperature. Whey protein concentrate (WPC, 35% protein, Carberry Milk Products, Eire) was reconstituted to a protein concentration of 3.5 wt% protein in 60 litres of RO water (cf. typical value of 3.1 wt%1 8 ) and the pH adjusted from 6.6 to 6.0 using 0.1 M HCl in order to yield larger deposit quantities per run on a consistent basis1 9 . Whey protein solutions were used on a once-through basis and were pumped from a holding reservoir at ambient temperature through a preheater coil (wall temperature ~ 80°C) to 72±74°C before passing through the heat exchanger at 0.8 l min ± 1 (Re 4600). Oil (Transcal N, BP) at 97°C ¯ owed on the shellside; the unit had a signi® cant shellside heat transfer resistance, so that the wall temperature varied between 80±87°C1 3 . Fouling runs were stopped after 60 minutes, accompanied by a decrease in outlet temperature of 2±3 K. The oil temperature was then reduced to 50°C and the system Trans IChemE, Vol 77, Part C, June 1999
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85
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0
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60 0
0.5
1
1.5
2
x /m Figure 2. Deposit distribution pro® les in foulant generation experiments. Open symbolsÐ wet deposit coverage; solid circlesÐ dry deposit coverage; solid trianglesÐ protein assay; solid lineÐ bulk temperature pro® le from simulation by Davies et al.13 .
¯ ushed through with RO water until no visible traces of protein in solution were evident. Temperature data were logged during the fouling run; typical values of overall fouling resistance (0.365 m2 K kW ± 1 ) corresponded to a fouling Biot number of 0.24. Figure 2 shows typical deposition pro® les exhibiting good experimental reproducibility. The variation in wet coverage along the exchanger is caused by the change in deposit water content (from 58±66% wb) as the bulk temperature increased along the exchanger, corresponding to the transition from surface reaction to bulk reaction control around 75°C. The protein coverage data, obtained using a modi® ed Bradford assay2 0 , were in very good agreement with the dry coverage data obtained gravimetrically. Coverage data are hereafter reported as g(protein) m ± 2 . Trials showed similar cleaning behaviour in deposits with different water contents as the process of swelling on contact with NaOH eliminated differences in initial deposit structure. Rose et al.1 4 reported similar water contents using 1 wt% WPC solutions. Microscope examinations con® rmed that the deposits were of the Type A variety. Small, almost negligible amounts of mineral scale were formed from the calcium phosphate present in the WPC powder. Cleaning Experiments Cleaning experiments were performed by mounting 10 cm sections of fouled tube in the once-through cleaning loop shown schematically in Figure 3. Sodium hydroxide solution (0.5 wt% NaOH (BDH, UK) in RO water) was pumped from a heated reservoir through the test section and then collected for analysis or sent to disposal. This NaOH concentration had been identi® ed previously by Bird and Fryer9 as the optimal concentration for cleaning the type of whey protein deposits involved in these experiments. Timperley and Smeulders3 also reported a similar optimum concentration of NaOH in cleaning dairy HTST plate heat exchanger deposits. The apparatus was brought to thermal equilibrium with a dummy piece in place. The ¯ ow was then Trans IChemE, Vol 77, Part C, June 1999
stopped, the test section quickly installed and the ¯ ow restarted at the desired setting. Switching test sections took approximately ten seconds. The concentration of protein in solution and thus the cleaning rate (in g m ± 2 s ± 1 ) was measured using the modi® ed Bradford assay with Coomassie Brilliant Blue G-250 (Pierce and Warriner, Chester) as the protein dye reagent. Flow rates of cleaning solution corresponding to Re 500±6500 were used for 30 minutes, after which time the test sections were usually free of protein. The error in cleaning rate measurements was estimated to be around 5%. Mass balances on test sections gave good agreement between the gravimetric result and that obtained from the removal rate data (<6% difference). Figure 4 shows the degree of reproducibility achieved in these experiments. The three sets of removal rate data show the three different stages in cleaning reported by other workers1 0 ,1 1 , namely the swelling stage, uniform or plateau stage, and decay stage. Cleaning rates were compared by quantifying the total time to remove all protein from the tube, t; the plateau cleaning rate P (in the uniform stage) and the length of the decay period, d. Cleaning runs used tubes with protein coverage of ~ 100 g m ± 2 . Experiments investigating the thermal resistance of the deposit during cleaning and the effect of deposit temperature both used the temperature controlled block shown
sampling
7
1 3
2
6
5
V
4
V
V V
drain
Figure 3. Schematic diagram of cleaning apparatus. Dotted zoneÐ temperature control: V denotes solenoid valve locations 1Ð NaOH reservoir; 2Ð rotameter; 3Ð heating coil; 4Ð ¯ uid ¯ ow forming section 5Ð test section mount; 6Ð back pressure valve; 7Ð connection to water rinse system for short contact time experiments.
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2 CLEANING RATE (g/m sec)
swelling 0.4
uniform removal
decay
#25 #26 #27
0.3
P
d
0.2
0.1
0.0 0
5
10
15
20
25
30
TIME (mins)
t Figure 4. Reproducibility of cleaning experiments. Protein coverage 145 6 2 g m ±2; Re 585, isothermal cleaning at 50°C. The Figure also shows the three cleaning parameters discussed in the text: total cleaning time t, plateau removal rate P, and length of decay phase d.
schematically in Figure 5. The block (dimensions 60 ´ 60 ´ 60 mm) was constructed from brass and acted as a heat sink or source. The central channel (diameter 6.35 mm) ® tted snugly around a test section and was held in place by retaining screws. Good thermal contact was ensured using high thermal conductivity paste (3 W m ± 1 K ± 1 ; RS Components, Corby). The block temperature was maintained within 6 1 K by a water bath recirculator system. The micro-foil heat ¯ ux sensor (dimensions 8 ´ 13 ´ 0.076 mm, Rhopoint Ltd, Oxford) was mounted in a machined recess in the middle of the block so that it rested ¯ ush on the tube surface. The block assembly featured ¯ exible couplings so that the test section could be inserted rapidly into the cleaning apparatus. The sensor voltage (typically 150±350 mV) was measured and ampli® ed (Keithley Instruments Type 148 nanovoltmeter, Ohio, USA) then recorded by a dedicated PC. The heat ¯ ux q was calculated from the manufacturer’ s calibration and the fouling resistance Rf , based on the overall heat transfer coef® cient U, calculated thus: Rf <
xf lf
1 U
1 Uclean
Tblock
Tbulk
1
q
Uclean
1
where Tb u l k was the temperature of the bulk liquid and Uc le a n was obtained at the end of a cleaning experiment. Consistent heat transfer results were obtained when temperature driving forces greater than 5 K were used. The reliability of the technique, and agreement with correlation predictions, are discussed in references 13 and 16. T Heating/cooling fluid
Fouled tube
Experiments investigating short contact times were performed in a purpose built fouling and cleaning rig, similar to that in Figure 3 but featuring computer-controlled sequencing of cleaning and rinsing solutions2 1 . Test sections for these experiments were generated as before using AISI 321 tubing instead of AISI 316 as the latter material contains molybdenum, which gives an X-ray spectrum peak coincident with that of sulphur, the element present in b-lactoglobulin used to indicate the presence of the protein. RO water was ® rst circulated around the system to establish thermal and hydraulic equilibrium; the test place was then quickly replaced and 0.5 wt% NaOH ¯ ow established at 250 mL min ± 1 and 50°C (Re 1520) for the required period (5±800 s) followed by a RO water rinse for 20 s at 50°C. The test section was then isolated, drained and freeze-dried in liquid nitrogen to quench any further reaction. Protein removal was measured as before. The surface analysis techniques summarized in Table 2 were used to study the microstructure of 10 ´ 10 mm deposit samples during the cleaning process. SEM visualization was performed on gold sputtered samples in an SEM model JEOL JSM-820 (Tokyo, Japan). X-ray elemental analysis was performed at 20 kV, 30 nA using a Camscan C4 X-ray microanalyser ® tted with a Link 860II analyser and software (Cambridge Scanning Co, Cambridge, UK). Cryogenic SEM (CSEM) studies were performed using a JEOL JSM630IF device featuring a cryo-prep chamber with a CT100 cold stage controller (Oxford Instruments, UK) located at Unilever Research, Colworth House. The cleaning runs for the CSEM studies were carried out immediately before analysis. Short test pieces containing wet deposit were immersed in 0.5 wt% NaOH, rinsed in deionized water, drained rapidly then mounted on the CSEM sample stub before immersion in slush nitrogen. The sample was gold plated in the cryo-prep chamber at 0.4 mbar before imaging at ±150°C and 10 ± 6 torr. The sample preparation protocol required considerable development in order to avoid the generation of ice crystals on the deposit surface. Both conventional SEM and CSEM preparation methods involved water removal via sublimation of ice crystals. The formation of these crystals can damage the specimen structure, particularly where high water contents are involved so the results were compared with SEM images generated using a chemical ® xation technique (subsequently referred to as FSEM). Glutaraldehyde was chosen as it has been used extensively for cross-linking and preserving tissue proteins2 2 . The ® xative did not penetrate the deposit completely but this method did effectively `freeze’ the surface structure at the deposit/solution interface. Chemically ® xed samples were generated by contacting the fresh sample with 2.5 wt% glutaraldehyde in 0.1 M cacodylic buffer at pH 7.2 for 1 hour. The glutaraldehyde was removed by successive rinses of buffer then distilled water. Water was removed by dehydration in ethanol followed by critical point drying using CO2 before being gold sputtered for conventional SEM imaging.
Cleaning solution
T
sensor Heating/cooling fluid
Figure 5. Layout of heat ¯ ux sensor monitor.
RESULTS AND DISCUSSION Operating Parameters The initial swelling stage of cleaning is controlled by the reaction of protein with NaOH and is dif® cult to study Trans IChemE, Vol 77, Part C, June 1999
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Table 2. Summary of surface visualization techniques. Visualization technique
Cleaning conditions
Conventional SEM (SEM)
Re 1520 T 50°C Contact time² 5±300 s Water rinse time 20 s
Freeze drying in liquid nitrogen (±197°C)
X-ray elemental mapping
as SEM
as SEM
Cryogenic SEM (CSEM)
Deposits dipped in NaOH² bath T 20°C Contact time 5±300 s Water rinse time 20 s
Freeze in `slush’ nitrogen
SEM with chemically ® xed sample (FSEM)
as CSEM
Contact with glutaraldehyde in cacodylic buffer
² NaOH concentration
0.5 wt%
experimentally due to the speed of the process. The effect of operating parameters on the uniform and decay stages was studied; most protein removal occurs in this period. Experiments were performed to establish whether the mean plateau cleaning rate P was controlled by reaction, and thus strongly dependent on temperature, or hydraulic (shear/mass transfer) effects and therefore ¯ ow rate dependent. The effect of ¯ ow rate and temperature on P were studied over the range 500 < Re < 5000 and 20°C < Tb u l k ,
0.5
0.5
(a)
Block
CLEANING RATE (g/m 2 sec)
0.4
70C 50C 30C 50C 50C
0.3
Experiment reference number
Bulk liquid 50C 50C 50C 30C 70C
0.4
#14 #15 #16 #18 #19
0.3
0.2
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0.1
0.1
0.0
0.0 0
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(b)
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0.5 Experiment reference Block number
CLEANING RATE (g/m 2 sec)
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80C 70C 50C 30C
0.3
0.4
#13 #14 #15 #16
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0.1
0.1
0.0
0.0 0
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25
TIME (mins)
Figure 6(a). Effect of cleaning solution temperature on cleaning rate. Mean protein coverage 100 g m 2 ; NaOH ¯ owrate 800 mL min 1 . (b). Effect of block (wall) temperature on cleaning rate. Mean protein coverage 100 g m ±2; Tbulk 50°C; NaOH ¯ owrate 800 mL min ±1.
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Tw a ll < 80°C using the heat ¯ ux sensor monitor. Comparison of isothermal tests (at 50°C), with and without the sensor, con® rmed that the sensor arrangement did not affect the cleaning pro® les. Temperature differences (DT) >20 K were required to generate thermal resistance data; DT values were varied to investigate whether cleaning was more sensitive to (a) the deposit/liquid interface temperature, and thus reaction/diffusion in the swollen deposit or its surface, or (b) deposit/wall temperatureÐ and thus reactions in the unswollen deposit, or associated with swelling as proposed by Gallot-Lavalle e and Lalande1 1 . Calculations showed that the temperature in the deposit-liquid interface during the swelling and uniform removal stages is effectively controlled by the temperature of the cleaning solution. Cleaning pro® les for different bulk and different wall temperatures are shown in Figure 6 and the results are summarized in Table 3. The cleaning pro® les in Figure 6 show a very strong dependence on the cleaning solution temperature; the variation due to the block temperature is effectively due to its contribution to the mean temperature in the swollen deposit layer. Note that the initial swelling period does not seem to be a strong function of temperature by comparison with the other two stages. The temperature dependence of P is plotted as a pseudo-Arrhenius plot in Figure 7. The cleaning rate was corrected for the fraction of deposit not in contact with the heated block using results from isothermal runs1 6 ; TD L is the temperature of the deposit/liquid interface calculated from the heat transfer data. The activation energy calculated at Re 2340 was skewed by the datum used in calculating the cleaning rates. The activation energies do not show any signi® cant variation with Re and are smaller than that reported by Bird and Fryer1 0 for a similar system (~ 80 kJ mol ± 1 ). The values approach the range of values quoted for reaction with internal diffusion control2 3 . These results suggest that processes within the swollen deposit layer control the cleaning process in the uniform stage: however, the mean plateau removal rate is not a true reaction parameter. Figure 6(a) and 6(b) both show a noticeable sensitivity of the cleaning rate in the decay phase on temperature. The length of the decay phase decreased signi® cantly when the deposit-liquid interface temperature exceeded 50°C. Above this temperature there was very little effect of temperature on the d parameter. Figure 8 shows that this parameter was however signi® cantly sensitive to ¯ ow rate under ® xed
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GILLHAM et al. Table 3. Summary of cleaning experiments. Tbulk / °C
Flowrate / ml min ±1 (Re)
P /g m ±2s ±1
Protein coverage /g m ±2
t /min
80
50
70
50
50
50
30
50
20 50 50
50 30 70
100 (585) 260 (1500) 400 (2340) 800 (4680) 100 (585) 260 (1500) 400 (2340) 800 (4680) 100 (585) 260 (1500) 400 (2340) 800 (4680) 100 (585) 260 (1500) 400 (2340) 800 (4680) 800 (4680) 800 (3300) 800 (6520)
0.18 0.22 0.21 0.29 0.16 0.18 0.20 0.24 0.13 0.14 0.19 0.17 0.08 0.10 0.11 0.16 0.16 0.07 0.50
139 104 101 109 102 108 91 96 107 119 110 106 102 111 110 106 89 102 98
18 10.5 12.5 9 14 13 10.5 8.5 22 17 12 8 32 29 24 21 >25 5
Tblock / °C
temperature conditions. This Figure also shows that the total cleaning time t was less sensitive than d to Re owing to the contribution from the plateau cleaning rate; P increased with Re under ® xed temperature conditions, varying as Ren where n was in the range 0.20±0.35. The low value of the index con® rms that diffusive transport and reaction within the foulant layer affect P more than mass transfer from, and shearing at, the deposit surface. The effect of Re on d is consistent with a change in removal mechanism in the decay stage to break-up of the deposit layer near the surface by shearing and reaction. The relatively weak dependence of P on Re and the importance of reaction processes on P also indicates that these results should be applicable to CIP systems, which are normally operated under turbulent ¯ ow conditions in order to maximize hydraulic effects. Under such conditions the decay stage is likely to be short and t will be dictated by the length of the swelling and uniform cleaning stages, which are controlled by internal processes.
Thermal Resistance Direct measurement of the swollen layer thickness required for quantitative modelling is dif® cult, but thermal resistances and thus deposit thicknesses can be inferred from measurements of the local heat transfer coef® cient. The heat transfer data are presented here as fouling resistances for ease of comparison with cleaning rate data. Figure 9 shows typical results from simultaneous measurement of fouling resistance (Rf < thickness/thermal conductivity) and protein removal rate during a cleaning experiment. Replicate trials showed very good reproducibility. The fouling resistance data feature three stages; (i) an initial, rapid increase from Rf ,0 (unswollen deposit) to a maximum value (Rf ,m a x ); (ii) a `uniform’ stage where the fouling resistance decreases slightly (# 20%) over time, followed by (iii) a ® nal `recovery’ stage in which Rf decays rapidly to zero. The length of the initial stage corresponded closely to the deposit swelling stage in the cleaning rate data and was
-1 25
-1.5 20
ln P
Removal time /min
-2
-2.5
-3
15
10
5
-3.5 0.0029
0.003
0.0031
0.0032
0.0033 0
1/TDL
Figure 7. Pseudo-Arrhenius plot of corrected P versus 1/(TDL) at different Re. Mean protein coverage 100 g m ±2, Tbulk 50°C. SquaresÐ Re 585, Eact 44 kJ mol ±1; trianglesÐ Re 1500; Eact 51 kJ mol ±1; open circlesÐ Re 2340, Eact 63 kJ mol±1; ® lled circlesÐ Re 4680, Eact 50 kJ mol±1.
0
1000
2000
3000
4000
5000
Re Figure 8. Effect of ¯ ow rate on removal time. SquaresÐ length of decay stage, d; CirclesÐ total cleaning time, t. Isothermal cleaning at 50°C; mean protein coverage 100 g m ±2.
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CLEANING-IN-PLACE OF WHEY PROTEIN FOULING DEPOSITS 0.5
133
250
1.6
CLEANING RATE (g/m2s)
I
II
III
1.2 1
0.3
0.8 0.2
0.6 0.4
FOULING RESITANCE (m2K/kW)
0.4
0.1 0.2 0.0
0 0
5
10
15
20
25
30
TIME (mins)
Figure 9. Simultaneous measurement of protein removal and fouling resistance; (I)Ð swelling stage; (II)Ð uniform stage; (III)Ð recovery stage; Flow rate 800 mL min ±1 (Re 4680); Tbulk = 50°C; Tblock 30°C.
found to decrease with increasing Re. The ratio of Rf,m a x to Rf ,0 was also used to estimate the degree of swelling of the deposit by assuming that (a) negligible removal during the swelling stage; (b) the thermal conductivity of the unswollen deposit was that reported by Davies et al., and that of the swollen deposit was close to that of water. These calculations gave a swelling factor of 2.4±3.4, in reasonable agreement with the value of 2.5 reported previously2 ,2 0 . Further calculations indicated that the voidage of the deposit increased from ~ 0.62 to ~ 0.90 on swelling, consistent with microstructure images. Figure 9 shows signi® cant differences in the length of the uniform and decay stages between the thermal and mass removal pro® les. These differences were apparent in all the experiments performed in this study. The uniform/decay stage transition was always earlier in the thermal data, and the length of the thermal decay stage was always shorter than the corresponding mass removal stage. A noticeable increase in the thermal cleaning rate (i.e. reduction in Rf ) coincides with the onset of the decay stage in the mass removal data. The time taken to achieve complete heat transfer recovery was on average 34% shorter than the mass removal parameter t, even though the local cleaning rate at the heat ¯ ux monitor would be expected to be slightly lower for high Tw a l l cases owing to the extra local heat transfer resistance of the sensor (around 3 K1 6 ). Comparison of the data in Figure 9 also shows that the change in Rf during the `uniform’ stage was relatively small (~ 10%) compared to the amount of protein removed from the surface during the same period (~ 60%). The protein removal data were con® rmed by direct measurement of the unswollen deposit thickness using conventional SEM shown in Figure 10. Note that the swollen protein layer collapses during sample preparation so its thickness cannot be measured using this technique. SEM thus measures the unreacted form below the swollen layer. This thickness (see Figure 10) decreases linearly over ca. 10 minutes, suggesting zeroth order conversion of protein to the swollen form. This conversion time is noticeably longer than the deposit swelling stage (2±3 minutes in this case) and con® rms that the uniform stage cleaning rate is independent of the amount of native deposit present. Trans IChemE, Vol 77, Part C, June 1999
Deposit thickness (mm)
1.4
200 150 100 50 0 0
2.5
5
7.5
10
12.5
Contact time (minutes)
Figure 10. SEM measurement of unswollen deposit thickness v. NaOH contact time. Isothermal cleaning at 50°C; Re 1500; mean protein coverage 260 g m ±2.
The fouling resistance is a lumped measurement, combining the thermal resistance of the native deposit, swollen deposit (of varying voidage and thus thermal conductivity) and any enhancement of the convective heat transfer coef® cient due to reduction in duct dimension. The latter factor was estimated to be small. The small change observed in Rf during the uniform mass removal stage indicated that the thermal resistance of the swollen layer was signi® cant and compensated for the change in the thickness of the unswollen deposit layer. Attempts to simulate the fouling resistance pro® les using a model based on native and swollen protein layers, of ® xed voidages and thermal conductivities, with a uniform removal rate were not successful. Simulation parameters were obtained from experimental data, but predictions did not match experimental results very well. The assumption of constant voidage in the swollen layer was considered invalid; a more realistic model of reaction and diffusion within the swollen protein layer is required. The data in Figure 10 are not consistent with a model where growth of swollen deposit is controlled by diffusion of NaOH to the native deposit surface. X-ray elemental mapping studies of samples generated under similar conditions in the short contact time apparatus indicated that sodium ions (from the NaOH) were present at the deposit/metal interface within 30 seconds of initial contact. The growth of the swollen layer is thus controlled either by protein swelling kinetics or by the diffusion of larger molecular species with longer diffusion times. The decay stage in the heat transfer data starts just before the end of the uniform stage in the mass removal pro® le, and complete heat recovery is achieved before all the protein is removed. Studies using longer fouled sections con® rmed that this behaviour was consistent and was not caused by a cleaning `front’ moving downstream through the deposit1 6 . The length of the thermal decay stage was found to decrease noticeably with increasing ¯ ow rate. These observations are consistent with the previously hypothesized mechanism where the uniform foulant layer breaks down into a disordered structure, possibly lacking native protein to contribute mechanical strength. Such a weak, non-uniform deposit is then readily broken down and will be sensitive to shear forces at the surface. The thermal resistance of isolated lumps of deposit is furthermore reduced by their
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contribution to surface roughness. The thermal measurement technique becomes insensitive as the fraction of the surface covered, and deposit size decreases (i.e. « thermal boundary layer thickness). This suggests that such a device could not be used to monitor the absolute `cleanliness’ of the surface. These simultaneous thermal and protein removal studies con® rm certain aspects of previous mechanistic models of cleaning but also contradict the hypothesis that diffusion and reaction of NaOH with protein controls the removal process. These results indicate that the deposit layer thickness decreases slowly after initial swelling and suggest that cleaning is dominated by the reaction and diffusion of dissolved protein through the swollen protein matrix. Surface Studies The evolution of deposit microstructure during cleaning has been studied previously using SEM on samples isolated during the cleaning process1 0 . Three variants of SEM were used here to avoid reporting artefacts; especially in visualizing the deposit/solution interface for the swollen deposit, which has a high water content and would not survive conventional SEM preparation. All three techniques (SEM, CSEM and FSEM) gave identical results for imaging the native deposit, an amorphous structure of protein aggregates of size 1± 10 mm and up to ~ 200 mm thick for a coverage of 260 g m ± 2 . Figure 11 shows a SEM cross section of the deposit obtained after contact with NaOH at 50°C for 30 s in the short contact time apparatus; for all contact times, such images showed a thin (1±5 mm) smooth adherent layer on the cleaning solution/deposit interface, which was taken to be the swollen deposit which collapsed on drying. Figure 12(a) is a CSEM image of the cross section of a deposit structure near the surface at a magni® cation of 2500, after (i) contact with 0.5 wt% NaOH for 3 seconds and (ii) rinsing in deionised water for 10 seconds. There is a dramatic change in the deposit structure after just 3 seconds; similar results were obtained using conventional SEM. Figure 12(b) is a CSEM image of the cross section of the deposit structure near the surface after contact with NaOH for 4 minutes followed by rinsing and shows that the swollen protein has changed to a regular network of thin strands (0.2±0.5 mm), enclosing narrow pores (0.5 to 1 mm). How these structures evolve is not clear and poses problems in modelling. Figure 13 is a series of FSEM images of deposit undergoing cleaning by 0.5 wt% NaOH at 20°C and
Figure 11. SEM cross section of whey protein deposit showing native deposit and collapsed swollen layer.
Figure 12. CSEM cross sections of whey protein deposit after contact with 0.5 wt% NaOH.(a) after contact for 3 seconds; (b) after contact for 4 minutes.
shows the gradual evolution of a ® ne stranded network similar to that in Figure 12(b). The chemical ® xation technique used here can only be used to study (near) surface topography rather than matrix thickness but these images con® rm that Figure 12(a) shows an artefact generated during sample preparation. The porosity and thus the effective diffusion rate of hydroxide and dissolved protein increases monotonically over the ® rst few minutes, explaining the increase in cleaning rate during the swelling stage. CONCLUSIONS The mechanisms involved in the alkali-based cleaning of whey protein deposits on stainless steel surfaces during steady pipe ¯ ow have been investigated using mass removal rate, thermal resistance and surface imaging techniques. Protocols for generating reproducible soils and removal pro® les have been developed. A novel heat ¯ ux sensor unit was used to collect thermal resistance data during cleaning experiments and a novel cleaning apparatus was used to generate samples of deposit exposed to NaOH cleaning solution for short contact times. The results con® rmed that alkali-based cleaning of protein deposits is a three-stage process involving deposit swelling, uniform cleaning and a decay phase. Analysis of swelling kinetics was not performed but thermal resistance results and SEM visualization of samples prepared using chemical ® xation techniques showed that the swelling stage involves the gradual build up a of a swollen protein matrix. The cleaning rate during the uniform stage, corresponding to the removal of most of the protein deposit, was most sensitive to the deposit/solution interface temperature (activation energy ~ 50 kJ mol ± 1 ) and less sensitive to hydraulic/external mass transport conditions (3Re0 .2 0 ± 0 .3 5 ). The cleaning rate in the decay stage exhibited a strong dependence on hydraulic effects and a threshold Trans IChemE, Vol 77, Part C, June 1999
CLEANING-IN-PLACE OF WHEY PROTEIN FOULING DEPOSITS
135
monitors which could detect the change between cleaning stages were available. Devices based on heat ¯ ux sensors, as used here, could be used if the results could reliably correlated to plant performance. This work also indicated that such the sensitivity of these devices limits their scope for monitoring cleanliness. Industrial heat exchangers frequently operate under higher Reynolds numbers and surface shear stress conditions than those used in this study (Re 500±6500 and shear stress 0.009±0.95 Pa). Although the Reynolds numbers used here crossed the `transition regime’ for turbulent ¯ ow, pressure drop and heat transfer measurements indicated that the ¯ ow regime was not changing signi® cantly. It was not feasible to perform once-through experiments at larger ¯ ow rates. Application of the results of this investigation to industrial units, particularly those featuring plate heat exchangers, is complicated by the variation of the parameters affecting both fouling and cleaning in such units. The spatial distribution of temperatures, surface shear stresses and residence times in a plate heat exchanger means that deposition is not uniform. The variation in foulant thickness, allied with the variation in ¯ ow patterns, gives different cleaning rates at different points so that it is likely that some parts of the exchanger will be in the decay stage while others are still in the uniform cleaning stage. The challenge facing the process engineer in such circumstances is to identify regions of high fouling/low cleaning potential and apply the knowledge of deposit behaviour gained here to minimize the time taken to remove this `worst case’ deposit. Figure 13. FSEM images deposit surfaces after contact with 0.5 wt% NaOH (a) after 3 s contact; (b) after 30 s contact; (c) after contact for 3 minutes.
temperature of ~ 50°C for these experiments, above which there was little temperature effect. The simultaneous thermal measurements also exhibited the uniform and decay stages but with different characteristics. The fouling resistance did not decrease signi® cantly during the uniform protein removal stage, and the transition to the decay stage occurred earlier in the thermal pro® les. The length of the thermal decay stage was shorter than the analogous mass removal stage. The latter results are consistent with the uniform/decay stage transition corresponding to the breakup of a uniform deposit layer to a random deposit after all the native deposit had undergone swelling. Several of the observations are inconsistent with previous quantitative models for the protein removal rate during the uniform cleaning stage. The X-ray elemental spectroscopy results indicate that diffusion of NaOH through the deposit is not a limiting factor; reaction and diffusion of dissolved protein out of the swollen matrix is more likely to be the controlling stage. The fouling resistance results are not consistent with the models of Bird and Fryer, or GallotLavalle e and Lalande, suggesting that further development of quantitative models is required before rigorous optimization of the cleaning process can be carried out. These experiments con® rmed that the operating parameters controlling the cleaning rate under conditions of steady pipe ¯ ow found in many CIP systems change between each cleaning stage. Optimization of cleaning in such systems could therefore be improved greatly if Trans IChemE, Vol 77, Part C, June 1999
NOMENCLATURE P q Re Rf T DT U xf d lf t
mean plateau cleaning rate, g m ±2 s ±1 heat ¯ ux, W m ±2 Reynolds number, ± fouling resistance, m2 K W ±1 temperature, K temperature difference in heat ¯ ux device, K overall heat transfer coef® cient, W m2K ±1 deposit thickness, m length of decay stage in mass removal pro® les, s deposit thermal conductivity, W m ±1 K ±1 time to clean deposit, s
REFERENCES 1. Pritchard, N.J., de Groerden, G. and Hasting, A.P.M., 1988, The removal of milk deposits from heated surfaces by improved cleaning processes, Proc Fouling in Process Plant, St. Catherine’s College, Oxford, pp. 465±479. 2. Graû hoff, A., 1997, Cleaning of heat treatment equipment, in Fouling and Cleaning of Heat Treatment Equipment (ed) Visser, H., IDF Monograph #328, publ. IDF, Brussels, pp. 32±44. 3. Timperley, D.A. and Smeulders, C.N.M., 1988, Cleaning of dairy HTST plate heat exchangers: optimisation of the single stage procedure, J Dairy Tech, 41: 4±7. 4. Sandu, C. and Singh, R.K., 1991, Energy increase in operation and cleaning due to heat exchanger cleaning in food pasteurisation, Food Tech, 45: 84±91. 5. Hasting, A.P.M., 1995, Good enough to eat, The Chemical Engineer, 599: 19±20. 6. Kessler, H.G. and Schraml, J.E., 1996, Effects of concentration and product composition on fouling structure, Proc Fouling and Cleaning in Food Processing, Jesus College, Cambridge, March 1994. publ EU, Brussels, EUR 16894 EN: 26±33. 7. Visser, H. (ed.), 1997, Fouling and Cleaning of Heat Treatment Equipment, IDF Monograph #328, publ IDF, Brussels.
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8. Rene , F., Leuliet, J.C., Goldberg, M. and Lalande, M., 1988, Encrassement et nettoyage d’ un e changer de chaleur aÁ plaques lors du traitment UHT d’ un produit lacteÂchocolate , Comparaison avec lait, Le Lait, 68: 85±102. 9. Cheow, C.S. and Jackson, A.T., 1982, Circulation cleaning of a plate heat exchanger fouled by tomato juice: I. Cleaning with water & II. Cleaning with caustic soda solution, J Food Tech, 17: 417±440. 10. Bird, M.R. and Fryer, P.J., 1991, An experimental study of the cleaning of surfaces fouled by whey proteins, Trans IChemE, Food Bioprod Proc, 69C: 13±21. 11. Gallot-Lavalle e, T. and Lalande, M., 1985, A mechanistic approach of pasteurised milk deposit cleaning, in Fouling and Cleaning in Food Processing, Madison, USA, Lund, D., Plett, E.A. and Sandu, C. (eds) pp. 374±394. 12. Burton, H., 1968, Deposits from whole milk in heat treatment plant - a review and discussion, J Dairy Res, 35: 317±330. 13. Davies, T.J., Henstridge, S.C., Gillham, C.R. and Wilson, D.I., 1997, Investigation of whey protein deposit properties using heat ¯ ux sensors, Trans IChemE, Food Bioprod Proc, 75C: 106±110. 14. Rose, I., Epstein, N. and Watkinson, A.P., 1997, The effect of velocity on the initial fouling rate of whey protein solutions at elevated wall and low bulk temperatures, Proc Engineering Foundation Conference on Understanding Heat Exchanger Fouling and its Mitigation, Lucca, Italy. 15. Truong, T., Anema, S., Kirkpatrick, K. and Tuoc, K.T., 1998, In-line measurements of fouling and CIP in milk powder plants, Proc Fouling and Cleaning in Food Processing `98, Jesus College, Cambridge. 16. Gillham, C.R., 1998, Enhanced cleaning of surfaces fouled by whey proteins, PhD thesis (University of Cambridge). 17. Gotham, S.M., 1990, Mechanisms of protein fouling of heat exchangers, PhD dissertation (University of Cambridge). 18. Walstra, P. and Jenness, R., 1984, Dairy Chemistry and Physics, (J. Wiley and Sons Inc, NY). 19. Skudder, P.J., Brooker, B.E., Bonsey, A.D. and Alvarez-Guerrero, N.R., 1986, Effect of pH on the formation of deposit from milk on
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heated surfaces during ultra high temperature processing, J Dairy Res, 53: 75±87 Bird, M.R., 1993, Cleaning of food process plant, PhD dissertation (University of Cambridge). Belmar-Beiny, M.T. Toyoda, I. and Fryer, P.J., 1997, The initial stages of fouling from milk proteins and minerals onto surfaces above 100°C, Fouling Mitigation of Industrial Heat-Exchange Equipment, Panchal C.N. (ed), Begell House, NY, pp. 601±612. Hyatt, M.A., 1981, Fixation for electron microscopy (Academic Press, London). Levenspiel, O., 1972, Chemical Reaction Engineering (John Wiley, NY).
ACKNOWLEDGEMENTS Support for CRG from the BBSRC, DiverseyLever and Unilever Research, assistance from Dr M.T. Belmar-Beiny with the short contact time experiments and Mr Mark Kirkland (both Unilever Research Colworth Laboratory) with the SEM work are gratefully acknowledged.
ADDRESS Correspondence concerning this paper should be addressed to Dr Ian Wilson, Department of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, UK. Email: [email protected]. The manuscript was received 17 November 1998 and accepted for publication after revision 19 March 1999. This paper is an extended and updated version of a paper presented by the authors at Fouling and Cleaning in Food Processing, 6±8 April 1998, Cambridge, UK.
Trans IChemE, Vol 77, Part C, June 1999