sle

Chemical Engineering Science62(2007)1948–
1957
/locate/ces
Structure and rate of growth of whey protein deposit from in situ electrical conductivity during fouling in a plate heat exchanger Romuald Guérin,Gilles Ronse,Laurent Bouvier,Pascal Debreyne,Guillaume Delaplace∗UR638,Génie des Procédés et Technologie Alimentaires,INRA,F-59651,Villenueve d’Ascq,France
Received7August2006;received in revised form13December2006;accepted15December2006
Available online30December2006
This paper is dedicated to the memory of Dr Jean-Claude Leuliet
Abstract
The influences of calcium concentrations(70.88mg/l),Reynolds number(2000–5000)and temperature(60.96◦C)upon the deposit structure and the rate of growth deposition have been investigated in a plate heat exchanger.This was done from in situ measurements of the deposit electrical conductivity via implementation of stainless steel electrodes in channels combined with assessments of deposit thickness.Calcium ions affect structures of deposits and increase the rate of deposit growth upon heated surfaces.This was attributed to the formation of weaker size aggregates at higher calcium concentrations and a higher number of calcium bindings,which reinforce adhesion forces between protein aggregates.Structures and appearances of deposits also were affected byflow rates whatever the calcium concentrations.Deposit growth rate was enhanced by increasingflow rate below a critical Reynolds number comprised between3200and5000.On the contrary,above the critical Reynolds number,a limitation of the deposit and/or an escape of the deposit from the fouled layer into the corefluid occurred,caused by the predominance of particle breakage on the deposit formation.Fouling tended to be reduced at higherflow rate.It was noteworthy that rates of growth decrease during fouling experiments which may be explained by an increase in local shear stresses leading to particle breakage.
᭧2007Elsevier Ltd.All rights reserved.
Keywords:Fouling;Whey protein;Calcium ions;Reynolds number;Shear stress;Deposit structure;Plate heat exchanger;Electrical conductivity
1.Introduction
Plate heat exchangers(PHEs)are widely used in food indus-tries.Several advantages in using PHEs have been discussed elsewhere(Corrieu,1980;Bond,1981).The main problems en-countered by users of heat exchangers are linked to fouling,cor-rosion or mechanical resistance.Bott(1992)shows that fouling of heat exchangers,classically observed with dairy products, is in the front row of the industrial preoccupations.Fouling of heated surfaces directly contributes toward increased costs in production and energy losses,cleaning,and hinders a constant product quality and overall process efficiency(Yoon and Lund, 1989;Delplace et al.,1994;Jeurnink and de Kruif,1995;Visser and Jeurnink,1997).
∗Corresponding author.
E-mail address:delapla@lille.inra.fr(G.Delaplace).
0009-2509/$-see front matter᭧2007Elsevier Ltd.All rights reserved. doi:10.1016/j.ces.2006.12.038
In dairy industries,deposits consist of a layer of protein aggregates and minerals(Tissier and Lalande,1986).Among all milk proteins, -lactoglobulin has been identified as one of the major contributors to fouling as it undergoes thermal denaturation(Lalande et al.,1985;Lalande and Rene,1988; Gotham et al.,1989).Consequently,whey protein concentrate (WPC)solutions often have applied as a modelfluid to mimic fouling reactions during pasteurisation of milk both in the bulk and in the deposit at heat surfaces.
There has been a considerable amount of work showing that fouling affects hydrodynamic and thermodynamic perfor-mances of heat exchangers.These studies,carried out with different dairy product compositions and process conditions, put forward the main parameters which interfere upon the foul-ing deposit mass(De Jong et al.,1992;Belmar-Beiny et al., 1993;Delplace et al.,1994;Delplace and Leuliet,1995;Fryer et al.,1996;Changani et al.,1997;Visser and Jeurnink,1997;
R.Guérin et al./Chemical Engineering Science62(2007)1948–19571949
Christian et al.,2002;Prakash et al.,2006).The main param-eters were for instance wall temperatures andflow rate as pro-cess parameters and ionic force,type of ions,pH and protein concentration as chemical composition parameters.
All these works represent an important step forward in the generation of predictive models both on -lactoglobulin denatu-ration or global thermal performance degradation for the whole heat exchangers(Fryer and Slater,1985;De Jong et al.,1992; Delplace et al.,1994;Fryer et al.,1996;Visser and Jeurnink, 1997).Unfortunately,these models are not powerful enough to explain the distinct cleaning behaviours experimentally ob-served.For instance,Christian et al.(2002)showed that overall cleaning times and cleaning rates,under standard conditions, were dependant on the deposit composition.There is a lack of knowledge concerning the influence of deposit structure and kinetic of deposit mass upon the cleaning efficiency to get rid off the total mass deposit.To overcome these difficulties,stud-ies that report the effect of process parameters and composi-tion upon the structure of the fouled layer are required.The aim of this work was partly to contribute to thisfield.In par-ticular,various controlled conditions offlow rate and calcium concentration of a WPC solution in a PHE were carried out to determine the influence of these parameters upon the structure and the kinetic of fouled layers.
The structure and the growth of the fouled layer were estimated in-line from in situ measurements of the electrical conductivity of the fouled deposit.This was done by the im-plementation of two opposite stainless steel electrodes in PHE channels.In the last section,the electrode system was imple-mented in various channels to determine the influence of the temperature upon the deposit.
2.Materials and methods
2.1.Modelfluid
The modelfluid used in this study was reconstituted from WPC75supplied by Armor Proteines(France).The compo-sition of the powder as given by the manufacturer is shown in Table1.Proteins are the main components of the WPC pow-der(76%w/w)in which -lactoglobulin and -lactalbumin represent63%(w/w)and11%(w/w),respectively.Minerals represented less than4%(w/w)of the total dry weight of the powder.To produce solutions with higher mineral concentra-tion,the powder was dispersed in controlled quality water. Water consisted of a mixture of tap water of Lille(France)and soft water using a water softener(HI-FLO1,Culligan, Purolite C100E resin,France).The calcium and sodium con-tents of tap water,determined by atomic absorption spectropho-tometry(Philips,Pye Unicam),varied in the range170–200 and44–64mg/l,respectively.The range of calcium and sodium contents of the soft water were1.0–3.0and304–341mg/l, respectively.The desired content of calcium of the fouling fluid was obtained by mixing raw water,soft water and afixed amount of powder(1%w/w).Water electrical conductivity var-ied from0.113to0.116S/m at20◦C for calcium concentration varying from35to55mg/l.The product electrical conductivity Table1
Composition of WPC powder(Armor Protéines,France)and1.0%WPC solution
Component WPC75powder
(%w/w)
1.0%WPC75
solution(%w/w) Water 5.599.05
Lactose100.1
Lipids 3.70.037
Protein760.76
Casein––
-lactoglobulin480.48
-lactalbumin8.40.084
Other19.80.198
Minerals40.04
Calcium0.450.007–0.00875 Sodium0.700.0277–0.0472 Potassium0.33N.D.
Chloride0.40N.D. Phosphorus0.30N.D. Magnesium0.045N.D.
Iron0.008N.D.
Other 1.77N.D.
Other0.8
N.D.:Not determined.
Fig.1.Schematic of the experimental setup.
varied from0.142to0.146S/m at20◦C for a range of calcium of70–90mg/l.The addition of protein powder to the mixing of water modified the electrical conductivity value of20%. The pH of the modelfluid remained between7.3and7.7.
2.2.Fouling experiment
The experimental set-up of pilot plant scale is shown in Fig.1.Although there are two heat exchangers(model V7 plates,Alfa-Laval Vicarb,France)in the setup,the fouling
1950R.Guérin et al./Chemical Engineering Science 62(2007)1948–1957
ELECTRICAL CONDUCTIVITY SENSOR TEMPERATURE PROBE Pi PLATE NUMBER
Ci RODUCT CHANNEL NUMBER
HOT WATER HOT WATER Fig.2.Heating plate heat exchanger flow arrangement and implementation of stainless steel electrodes inside channels.
observations were focused on the second.The first one was used only to pre-heat the model fluid up to 60◦C where fouling was negligible.Water was used as the heating medium.The model fluid was heated from 60to 96◦C in a countercurrent mode.The choice of temperatures was made taking into account the value of the denaturation temperature of the -lactoglobulin protein.Temperature value for the denaturation of -lactoglobulin is 74.76◦C (Matsudomi et al.,1991;Xiong,1992;Gotham et al.,1989;Liu et al.,1994).PHE setup consisted of 13plates form-ing six passes of one channel for the two sides (Fig.2).The equivalent space between two consecutive plates was 3.93mm.In order to keep the feed composition constant,the fluid was not re-circulated once it was heated through PHEs.During ex-periments,the inlet temperature of hot water was adjusted to ensure a constant outlet model fluid temperature close to 96◦C and a constant profile of product temperature along the PHE as a function of time (i.e.,constant heat flux).The fluid foul-ing layer interface temperature in each channel was assumed constant during fouling runs.In the beginning,the PHE was brought to thermal equilibrium and desired process temperature using reverse osmosis (RO)water.The feed was switched from RO water to model fluid and the experimental run was contin-ued for 330min.After the fouling experiment,model fluid was replaced by cold RO water to bring the temperature of PHE and deposits to ambient temperature.Experiments were performed for various calcium concentrations and Reynolds numbers as shown in Table 2.Reynolds numbers were computed based on physical properties of water,assuming that the presence of 1%WPC in water does not modify them significantly.Average Reynolds number for the clean heat exchanger was determined from the distribution of Re along the PHE (Re =2 Q/ w ).Inlet and outlet model fluids and hot water temperatures were measured with platinum resistance probes (type pt100)with a precision of 0.1◦C.Bulk and wall temperatures in chan-nels were measured from J-type thermocouples with a preci-sion of 0.5◦C.Flow rates were measured using electromag-netic flowmeters (Krohne IFM,Germany).All parameters were collected via a data acquisition system (Agilent Technologies 34970A,USA)with an acquisition period of 30s.
2.3.Measurements of fouled layer thickness
Deposit thickness on the different plates was obtained by two ways:
•Using a pneumatic lifting device of a uniaxial compres-sion machine (DY30Model,Adamel Lhomargy,TMI,USA)which allows to determine the distance between the support of the device and the upper of the fouled or cleaned plate as shown in Fig.3.The precision of the measurement was 0.01mm.The assessments were performed at nine positions on the plate surface.The average value of the deposit thick-ness was computed from the nine positions.
•By weighing plates before and after fouling runs using a Mettler apparatus (PM3000,Switzerland)with a preci-sion of 0.1g.From a wet deposit density value equal to 1000kg /m 3(Lalande et al.,1985),the average deposit thick-ness upon each plate was obtained.Of course,this method assumes that the deposit occurs uniformly upon the plate surface.2.4.Electrical conductivity of the deposit
Two AISI 304L stainless steel electrodes 0.015×0.01m were implemented in channels C3,C5and C6(Fig.2).Elec-trodes were connected to a commercial conditioning system (STRATOS 2402Cond,Knick,Germany).Electrodes were electrically insulated from metal plates using an insulating stick (Araldite A V138M-HV998,USA).The cell constant of the de-vice was determined with salt solutions whose electrical con-ductivity value was known with precision.
The stainless steel electrodes provide an indication of the equivalent electrical resistance R eq through the channel (Fig.4).For fixed operating conditions,the Kirchhoff’s rule allows decoupling the equivalent electrical resistance in terms of fouling fluid electrical resistance (R p )and deposit electrical resistance (R d )as follows:R eq =R p +2R d .
(1)
R.Guérin et al./Chemical Engineering Science 62(2007)1948–1957
1951
T a b l e 2S u m m a r y o f m e a s u r e d a n d c a l c u l a t e d p a r a m e t e r s d u r i n g h e a t t r a n s f e r t o s t u d y f o u l i n g b e h a v i o u r o f 1%W P C s o l u t i o n
R u n M e a n R e (–)
C a 2+(m g /l )
N a +(m g /l )
i ,p (◦C ) o ,p (◦C ) i ,h w (◦C ) o ,h w (◦C )
M a s s o f d e p o s i t
t =0t f
t =0
t f t =0t f t =0t f
i n c h a n n e l 5(g )
A 200072.9344.062.360.096.897.2102.5104.572.873.471.9
B 200378.9303.260.059.796.596.6102.6107.771.976.0118.6
C 204082.2280.061.561.397.196.3102.7107.973.479.1147.1
D 204085.6
277.4
60.461.595.8
96.9102.0109.271.980.4
180.6
E 339470.0323.663.863.995.595.7102.0107.475.080.8100.3
F 322076.3472.061.361.395.795.5101.7115.973.489.1170.0
G 321478.0364.962.662.295.095.0100.0112.374.083.9201.2
H 323286.5
331.2
62.763.6
94.6
94.6101.4121.773.2
93.2
240.6
I 493874.6329.060.861.495.495.2103.4110.974.383.490.2J 492077.4303.061.360.896.296.0103.1113.574.084.994.2K 494277.8340.063.263.995.495.1103.0111.875.286.8116.6L 492687.4
306.061.261.4
95.995.7103.5121.3
74.3
93.3
190.5
R u n
¯e d (M D -5)(m m )¯e d (U C M )(m m )
w *(◦C ) b (◦C ) e q *(S /m ) p a t b (S /m ) p a t 100◦C (S /m ) d *a t w (S /m )
d *a t 100◦C (S /m )
k ×104(S /m m i n )
A 0.480.4097.991.10.3030.3600.3880.208
0.2101.90B 0.800.61101.990.60.3030.3730.4030.2630.2612.33C 0.980.92102.491.60.3070.3710.3980.2850.2832.82D 1.201.16
105.4
90.10.2970.3720.404
0.275
0.270
3.57
E 0.670.69103.689.70.2480.3610.3940.1700.1673.96
F 1.201.28112.289.70.2840.4720.5040.2510.2408.19
G 1.341.42108.889.30.2320.3750.4090.2240.2166.50
H 1.601.55
118.7
89.80.3320.4770.509
0.337
0.320
6.88
I 0.600.59105.289.80.2460.3910.4230.1470.1425.67J 0.630.63108.590.10.2250.3600.3920.1410.1335.60K 0.780.75106.889.50.2130.3690.4020.1510.1454.75L 1.271.37
121.189.80.2730.379
0.411
0.228
0.209
6.70
p , d a n d e q :f o u l i n g p r o d u c t ,d e p o s i t a n d e q u i v a l e n t e l e c t r i c a l c o n d u c t i v i t y ,r e s p e c t i v e l y ; i ,p a n d o ,p :i n l e t a n d o u t l e t t e m p e r a t u r e o f t h e p r o d u c t ; i ,h w a n d o ,h w :i n l e t a n d o u t l e t t e m p e r a t u r e o f t h e h o t w a t e r ; w a n d b :w a l l a n d b u l k t e m p e r a t u r e i n c h a n n e l 5;¯e
d (U C M ):a v
e r a g e d e p o s i t t h i c k n e s s
f r o m t h e u n i a x i a l c o m p r e s s i o n m a c h i n e ;e d (M D -5):d e p o s i t t h i c k n e s s f r o m m a s s d e p o s i t i n c h a n n e l 5;k :r a t e o f c h a n
g e o f t
h e e q u
i v a l e n t e l e c t r i c a l c o n d u c t i v i t y .
∗A t
330m i n .
1952R.Guérin et al./Chemical Engineering Science 62(2007)1948–1957
Based on the general relationship linking the electrical resis-tance to the electrical conductivity for a pair of electrodes [ =e E /(AR)with e E the length between the electrodes,A the cross-section and R the electrical resistance]and assuming that (i)the cross-section A is a constant value and (ii)the space of the fluid flow (e fl)is defined as the difference between the
space
0.000 N
0.005 N e 1
0.000 N
0.005 N e 2
Fouling layer
Stainless steel plate
a
c
b
d
Fig.3.The thickness measurement technique using a pneumatic lifting device of a uniaxial compression machine (DY30Model,Adamel Lhomargy,TMI,USA).
P 8P 9
Isolating materia l Stainless steel electrodes Fluid flow Fouling layer R eq
R d
R d
R
p A
B
Flow direction
e
E
Fig.4.Schematic of the fouling layer and equivalent electric resistance diagram.
separating the two electrodes (e E )minus the total deposit thick-ness (2e d )(Eq.(2)),the deposit electrical conductivity (DEC, d )can be expressed as a function of model fluid ( p )and equivalent ( eq )electrical conductivities as shown in Eq.(3).e fl=e E −2e d ,
(2)
d (t =t f )=
e E
2e d (t =t f ) 1eq (t =t
f )
−1p
+1p
−1
.
(3)
At the beginning of the fouling experiment (i.e.,clean PHE),the value of the equivalent electrical conductivity,measured by the device,corresponded to the electrical conductivity of the model fluid ( p )at the product temperature.The electrical con-ductivity of the model fluid was invariant during fouling runs since the inlet temperature of hot water was adjusted to ensure a constant product temperature inside channels as a function of time.At the end of fouling runs (i.e.,fouled plates,t =t f )the deposit thickness was measured and the measurement of the equivalent electrical conductivity allowed to obtain the electri-cal conductivity of the deposit.In order to compare electrical conductivity values of each run,all conductivities were deter-mined at 100◦C as follows (Ayadi,2005): d(100◦C )= d( w )+0.0009×(100− w ), p(100◦C )= p( b )+0.0032×(100− b ),
(4)
where d(100◦C )represents the DEC value at 100◦C, d( w )is the DEC determined from Eq.(3)at the end wall tempera-ture w , p(100◦C )is the electrical conductivity of the fouling product at 100◦C, p( b )represents the value of the electrical conductivity of the product at the bulk temperature b .
Considering a deposit temperature nearly constant and an invariant viscosity value for the product,the only parameters
R.Guérin et al./Chemical Engineering Science 62(2007)1948–19571953
0.20.220.240.260.280.30.320.340.360.380.4
Time, (min )
E q u i v a l e n t e l e c t r i c a l c o n d u c t i v i t y , (S m -1)
Fig.5.Equivalent electrical conductivity during fouling run using 1.0%WPC solution with calcium concentration 78.0mg /l at Re =3200.
which affect DEC values are mobility and concentration of ions (Benoıˆt and Deransart,1976).However,considering a poor mobility and diffusion of ions from the bulk fluid through the fouled layers due to protein networks,the DEC values are affected in the majority by the concentration of ions embedded inside the protein structure.Thus,these values constitute a good indicator of the deposit structure.3.Results and discussion
3.1.Effect of calcium content on fouling
Typical equivalent electrical conductivity change as a func-tion of time,measured in-line from the electrodes in the fifth channel,is illustrated in Fig.5.After the switch from RO water to fouling fluid,the equivalent electrical conductivity reaches a maximum value at t =15min.This value corresponds to the electrical conductivity of the product at the bulk temperature.Data reported in Table 2show that product electrical conduc-tivity values at 100◦C are little affected by the modification of the ionic concentration (i.e.,calcium and sodium in the tested range of concentration)of the solution.
At the beginning of fouling stages,
very slow decreases in equivalent electrical conductivity are recorded with an initial rate k ∗(Fig.5).This region may be attributed to a homogeneous thin layer of irreversibly adsorbed individual protein molecules on clean metal surfaces (Arnebrant et al.,1985;Visser and Jeurnink,1997).Tissier and Lalande (1986)showed that this sublayer had a thickness of 0.02 m after only few minutes of contact;0.4and 1 m after 10and 30min of fouling run.This weak thickness may explain the slight decrease of the slope between t =15and 30min.The slightly decreasing slope (k ∗)indicates that the fouling mechanism starts immediately when fouling product is present in the heating zone,for a temperature higher than unfolding temperature of -lactoglobulin.After this period,the equivalent electrical conductivity decreases
2468106065
7075808590
Calcium content, (mg/l)
k x 104, (S .m -1.m i n -1)
parison of rates of deposit growth (k )as a function of calcium concentration in WPC solution for Re =2000,3200and 5000.(The trend lines represent the curve fit of data .)
linearly with time.The rate of electrical conductivity changes k is relatively high (Fig.5).The second decrease in eq may be attributed to the growth and structure changes of fouled layers.Indeed,whatever the Reynolds number,it is observed that the rate of electrical conductivity changes k rises with in-creasing calcium concentrations (Fig.6).This observation is in agreement with Li et al.(1994)observing that calcium induces conformational changes of the -lactoglobulin,facilitating the protein denaturation,but also increases the kinetic of the aggregate formation.A small change in the calcium concen-tration has an important impact upon the kinetic parameter k ,i.e.,the formation of the fouled layer.
Fig.7a illustrates the electrical conductivity values of the fouled layer (DEC)obtained at a wall temperature of 100◦C in the fifth channel for varying calcium concentrations at three Reynolds numbers.Whatever the Reynolds number,the DEC increases with the calcium concentration.Considering a low mobility of ions inside the deposit due to protein networks and a constant temperature in C5,this indicates that the DEC
1954R.Guérin et al./Chemical Engineering Science 62(2007)1948–1957
0.10.20.30.4
Calcium concentration,(mg.l -1)
E l e c t r i c a l c o n d u c t i v i t y o f t h e d e p o s i t , (S .m -1)
00.20.40.60.811.21.41.61.8Calcium concentration, (mg.l -1)
F o u l e d l a y e r t h i c k n e s s e d , (m m )
05001000150020002500300035004000Calcium content, (mg/l)
A m o u n t o f d e p o s i t i n c h a n n e l 5, (g /m 2)
parison of (a)deposit electrical conductivity at 100◦C,(b)fouled layer deposit and (c)amount of deposit in channel 5after 5.5h of heat transfer in PHE as a function of calcium concentration in WPC solution for Re =2000,3200and 5000.(The trend lines represent the curve fit of data.)
is affected by the deposit thickness and its structure which depends on the calcium concentration (Fig.7b).A small change in the calcium concentration has an important impact upon the fouling behaviour.
Figs.6and 7indicate that calcium ions (i)are essential in the growth of fouled layers as suggested by Xiong (1992)since amounts of deposit increase with calcium concentration (Fig.7c),(ii)modify the rate of protein aggregation and (iii)lead to a greater cohesion between protein aggregates modify-ing the deposit structure.Indeed,visual analysis of the deposit after fouling runs at Re 3200using 1.0%WPC solutions revealed that fouled layers formed with low calcium content
(78mg/l)have a spongy and soft texture whereas deposits formed at higher calcium content (86.5mg/l)are denser and elastic.This observation is in agreement with Pappas and Rothwell (1991)who showed that -lactoglobulin completely aggregated to form compact structures when heated with cal-cium.Simmons et al.(2007)also showed that increasing the levels of calcium had a dramatic effect on the size of the aggre-gates produced,which decreased with increasing mineral con-centration.An explanation for the difference in structure and kinetic is that calcium ions,essentially present in the deposit solid (Tissier and Lalande,1986),lead to lower size aggregates in the range of calcium concentration (70–88mg/l)and favour the growth of fouled layers by formation of bridges between adsorbed proteins and the protein aggregates formed in the bulk (Fig.8).Bridges may be formed via carboxyl groups of amino acids of -lactoglobulin as suggested by Xiong (1992).In-creasing the level of calcium would lead in a higher number of bridges resulting in a bigger stabilisation of protein aggregates as interpreted by Daufin et al.(1987)and Xiong (1992),forming a narrow network which embed other ions present in the solution (i.e.,sodium,magnesium,phosphate,calcium,…),and reinforce the adhesion forces between proteins.3.2.Effect of hydrodynamics conditions on fouling
Fig.5shows that rates of equivalent electrical conductivity changes are not constant as function of time since the slope of equivalent electrical conductivity decreased after t =180min.This slope modification in eq may be due to a decrease of the aggregate deposit and/or an escape of the deposit from the fouled layer into the core fluid caused by particle breakage.This can be a consequence of an additional local shear stress as deposit thickness evolved (Fig.9).Indeed,shear stress in a channel is a function of channel section which is reduced with the growing fouled layer [ = .¯u. /(2(e E −2e d ))].This confirms the assumption of Kern and Seaton (1959)who were the first to underline that the formation of a fouled layer is a consequence of the rate of aggregate entry and the rate at which they escape.
Fig.10illustrates the evolution of the kinetic parameter k during fouling with a 1.0%WPC solution at calcium concen-tration of 78mg /l as a function of Reynolds number.The in-crease of k between Re 2000and 3200can be explained by a weaker size of aggregates at higher shear rate for a fixed tem-perature (Simmons et al.,2007)favouring the growth of the deposit and resulting in a different deposit structures (Fig.8).Deposit masses in channel 5confirm this trend namely for a fixed calcium concentration,the amount of deposit in C5in-creases between Re 2000and 3200(Fig.7c).Nevertheless,the k parameter decreases between Re 3200and 5000at a fixed calcium concentration.Thus,the decrease of the rate of deposit is due to the increase of Reynolds number which may limit the deposit,compact the structure upon the heated surface and increase the rate of particle breakage.Visual analysis of the appearance of the deposit as a function of Reynolds number confirm the trends.Deposit formed after fouling with a calcium concentration close to 78mg /l at Re 2000has a granular aspect
R.Guérin et al./Chemical Engineering Science 62(2007)1948–1957
1955
Adsorbed protein Aggregates
Stainless steel electrodes Stainless steel plate With lower calcium concentraion
Embedded ions
Calcium bindings
With higher calcium concentraion
Fig.8.Schematic illustration of the proposed formation of the deposit with lower and higher calcium concentrations of the WPC solution.
0.20.40.60.811.21.470
76.378
86.5
Calcium concentration, (mg/l)
L o c a l s h e a r s t r e s s , (P a )
20406080100120140160180200Shear stress for fouled channel (C5)Shear stress for cleaned channel (C5)Increase of shear stress
I n c r e a s e o f s h e a r s t r e s s , (%)
Fig.9.Increase in shear stresses due to fouled layer growth as a function of calcium concentrations in 1.0%WPC solutions at Re =3200.
probably due to higher size aggregates whereas deposit formed at Re 3200appears more denser (i.e.,lower aggregates size).Finally,the deposit obtained after fouling at Re 5000appears more smooth and compact which may be the consequence of the increase of the local shear stress (Fig.9).
Another way to underline the influence of shear stress upon the structure of the deposit is the measurement of the electri-cal conductivity of deposits according to Reynolds numbers for a fixed calcium content and temperature (Fig.7a–c).Fig.7a shows that DEC values are similar at Re 2000and 3200while amounts of deposit in C5,and so the thickness of the deposit (Figs.7b and c),are completely different with a scatter close to 35%.In the same order,amounts of deposit in C5are similar for Re 2000and 5000while the corresponding
values of DEC
12345672000
3200
5000
Reynolds number, (-)
k x 104, (S m -1 m i n -1)
Fig.10.Rates of deposit growth (k )as a function of Reynolds number during fouling runs with a 1.0%WPC solution with calcium concentration comprised between 76.0and 78.0mg /l.
differ each other.Moreover,for a fixed calcium concentration,DEC values increase with a variation of Reynolds number from 2000to 3200while a decrease in DEC values can be observed above a critical Reynolds number,which could be comprised between 3200and 5000.Since calcium concentration and tem-perature are invariant and considering a poor mobility of ions due to protein networks,differences in the structure and/or the composition of the deposit can be explained by DEC variations.This indicates that shear stress has a dramatic effect upon the structure and the appearance of the deposit whatever the cal-cium concentrations.The differences in the structure of these。