- Email: [email protected]
Experimental study of wash columns used for separating ice from ice-slurry
Desalination 218 (2008) 223–228
Experimental study of wash columns used for separating ice from ice-slurry F.G.F. Qina*, X.D. Chena,b, S. Premathilakab, K. Freea a
Freezcon Ltd., 70 Symonds Street, Auckland, New Zealand email: [email protected] b Department of Chemical and Materials Engineering, University of Auckland, New Zealand
Received 7 February 2007; accepted 8 February 2007
Abstract The separation of ice from ice slurries is the final step in the process of freeze concentration, where wash column can be employed. Phenomena found in washing an ice bed, such as channeling, viscous fingering, clogging, liquid entrainment, and permeability were studied in an experimental wash column. The appropriate operating conditions that lead to a successful washing were proposed in this paper. Keywords: Freeze concentration; Wash column
1. Introduction Separation of concentrated solutions from ice slurries is the final step in freeze concentration (FC). In FC, water in aqueous solutions is partially frozen forming ice crystal grains, and the concentrated unfrozen solution can be recovered provided the ice can be removed. As a promising alternative of the conventional evaporative concentration, FC has many advantages [1–3], such
as no denaturing of nutritional components in liquid foods or active components in bio-solutions; no aroma loss; longer operation time for the equipment because there are no thermal and microbialinduced fouling; great energy saving potential because the fusion heat of ice is only 334 kJ/kg, but the evaporation heat of water is 2340 kJ/kg [4] , which is seven times greater than the fusion heat. Among many separation methods, the wash column has its unique advantages in segregating
*Corresponding author. Presented in the Separation Sessions at Chemeca 2006, the 34th Australasian Chemical and Process Engineering Conference, Auckland, New Zealand, 17– 20 September 2006. Organised by the University of Auckland and the Society of Chemical Engineers New Zealand (SCENZ). 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.017
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
3-15 kg weight observation window
cold washing water inlet
ruler water line water h1
cold water
ice from the ice slurry because the liquid does not need to overcome the surface tension when it is separated with the solid ice particles. In wash column, ice slurry is compressed to form a packed ice bed, which is saturated with the concentrated solution (the mother liquid). Washing water is introduced into the wash column on top and passes through the ice bed slowly to displace the solution as a downward plug flow. When water displaces the concentrated solution during washing, mixing happens. Over-mixing of water and solutions would blur the wash front, so that the transitional region becomes larger and may even lead to failure of the separation. This study will focus on some fundamental problem found in the operation of the wash column, such as viscous fingering, channeling, and clogging etc.
stop watch
ice bed line washed ice h2
h1=8 cm h2=18~20 cm
mesh piston thermal insulation
v
mesh
draining control
2. Experimental The wash column used in this study was made of a perspex cylinder with an inside diameter of 80 mm and height of 500 mm. The exterior was wrapped with 50 mm rubber foam for thermal insulation, on which a double-glazed 30 mm × 200 mm observation window was constructed at the upper section of the column. The schematic diagram is shown in Fig. 1. The ice slurry was produced with a laboratory scale scraped surface heat exchanger. After aging for a given time, the ice slurry was manually filled into the wash column, and the ice bed was compressed using a mesh piston until it reached the ‘ice bed line’ indicated in Fig. 1. The compressing force was provided by weight(s) at the top end of the piston rod giving 0.1–0.5 kg/cm2 pressures. In this study the pressure used was 0.1 kg/cm2 unless stated otherwise. An ice-free section of mother liquid above the mesh piston was produced after compression. This part of the mother liquid was then drained to allow the liquid level to be flush with ice bed surface. The mesh piston was kept in the wash column to maintain the pressure and prevent water rushing into the ice bed. A given
11 12 1 2 10 9 3 8 4 7 6 5
v∞
compr essed i ce bed
224
Fig. 1. Experimental setup pf the wash column.
amount of cold washing water (~0°C) was added into the column from the top until it reached the level marked ‘water line’ before washing started. A ruler was placed beside the observation window for measuring. 3. Results and discussion Washing was achieved by using water to displace the mother liquid in the ice bed. During washing a downward plug flow was presumably maintained and the mother liquid was drained into a measuring cylinder. The washing (or draining) rate was measured using a stopwatch. However, wash operation may fail due to a number of problems, such as channeling, viscous fingering, and clogging etc. An often seen problem (an undesired phenomenon) was that a sharp, clear, horizontal wash front was not visible. Instead, the transitional region
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
between the washed and the unwashed ice-layer was blurred or too large, or even worse — no washed ice-layer was produced. The reason seemed to be over-mixing of the washing water with the mother liquid. But careful observations (with the help of dye-tracer) showed that this might be attributed to two phenomena: channeling and viscous fingering. When the local ice bed progressively became loose during washing because of melting or ice being washed away, channeling occurred. Channels often appeared as a number of downward caves in root-like shape as shown in Fig. 2a. Viscous fingering occurred when the wash front moved faster at some locations than others, as shown in Fig. 2b and 2c. The local porosity did not appear to change. The horizontal transitional zone was broken when viscous fingering happened, and the separation of ice from the mother liquid would be inefficient or even fail. Both channeling and viscous fingering occurred often in the vicinity of the column sidewall. An uncompressed piled ice bed could not be washed properly as particle cohesion and agglomeration caused the ice in the slurry to become lumpy. The interstices between ice lumps allowed
225
liquid to run down faster than in other parts; this caused channeling directly. When washing was done properly, it would show a horizontal, sharp wash front between the water and solution interface. The ice bed could also slide out of the wash column from the bottom after the supporting mesh at the bottom was taken off. The ice bed was rigid enough and could stand by itself on the table as shown in Fig. 2d. However, the unwashed ice beds remained soft even though they were compressed. Another potential problem is clogging in the ice bed resulted from over crystallization of ice in the porous ice bed. High concentration of the mother liquid with strong freezing point depression (FPD) will enhance this effect. A similar phenomenon was mentioned by others [5–7]. Experiments in this study also showed that it usually happened when the ripening time given to the ice slurry was insufficient. The ‘premature’ ice slurry perhaps contained a large number of very fine ice particles so that the permeability was poor. The situation became worse when the ice bed was over compressed, which also resulted in dead-end pores and liquid occlusion (liquid sack) inside ice lumps due to pressure-induced fusion of ice. Clogging
h2
(a)
(b)
(c)
(d)
Fig. 2. Phenomena found in washing. (a) Channeling. (b) Beginning of the viscous fingering. (c) Development of viscous fingering. (d) A successful washing.
226
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
of the ice bed prolongs the washing time and the ice close to the wall gradually melts and eventually leads to channeling near the wall. Ripening also influences the permeability of the ice bed. Fig. 3 shows the time needed for draining (or collecting) 630 ml mother liquid from the ice bed, which was compressed from different ice slurries with different ripening times. 630 ml is just the volume of the washing water used. The draining rate was also a measurement of separation rate of the mother liquid, which was dominated by the interstitial velocity of liquid in the ice bed (v). According to Darcy’s law, it can be written in the following one-dimensional form for natural draining: K ∆P ρgK v= = η h η
(1)
where v is an average value. K is the permeability of the ice bed, which is only a function of the pore structure of the medium. η and ρ the are viscosity (Pa·s) and density (kgm–3) of the liquid, respectively, at the wash front. They are considered to be the average of the washing water and mother liquid in this paper. P is the hydraulic (pressure) head, h the liquid height and g the acceleration due to gravity, P = ρgh. The flow velocity above the ice bed is slower than the interstitial velocity: v∞ = φ v
(2)
where φ is the porosity of the ice bed. v∞ is the flow velocity above the ice bed (it is also known as the velocity of approach, as shown in Fig. 1). v∞ can be obtained via the calculation of water height divided by the draining time.
v∞ =
h1 td
(3)
where h1 is the preset height of washing water in the column (h1 = 8 cm in this study). td is the time needed to allow the water level to be flush with
top of the ice bed, when 630 ml mother liquid was collected. The porosity of the ice bed can be calculated in this way: πd 2 h1 φπd 2 h2 = . 4 4
The left-hand side is the used water volume. The right-hand side is the water volume in the washed ice bed. So it equals the ratio of h1 and h2 :
φ=
h1 h2
(4)
h2 is the thickness of washed ice layer, which was 18~20 cm in this study (see Figs. 2 and 3c). Although the ripening time varied from no more than 1–18 h, the thickness of the washed ice layer did not show much difference. In other words, the porosity of the ice bed changed only a little, ranging from 0.4~0.44 under a constant pressure of 0.1 kg/cm2. However, the draining time (td) for collecting 630 ml mother liquid varied from 170 s to more than 1000 s (Fig. 3a), indicating that both the flow velocity of draining liquid and the permeability of the ice bed increases with ripening time as shown in Figs. 3b and 3c. K was obtained using Eq. (1) based on experimental measurement of draining rate. 4. Conclusion The movement of the wash front tends to induce viscous fingerings during washing of the ice bed. However, gravity is a stabilizing force that keeps the wash front horizontal. Freezing and thawing may both occur in washing, which change the porosity of the ice bed. Local thawing at the wash front may lead to the breakthrough of the washing water and develop into channeling. The opposite trend is freezing at the wash front when cold water comes into contact with the subzero
Draining time, td (s)
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
1200 1000 800 600 400 200 0 0
2
4
6
8
10 12
14
16 18
20
Ripening time, tr (h)
Flow velocity (mm/s)
(a) Draining time varying with the ripening time. 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
Interstitial velocity (v) Velocity of approach (v∞ )
0
2
4
6
10
8
12
14
16
18
20
Ripening time, tr (h)
Permeability, K (m2 )
(b) Flow velocity varying with the ripening time. 8×10-10 6×10-10 4×10-10 2×10-10 0 0
2
4
6
8
10
12
14 16
18
20
Ripening time, tr (h)
(c) Permeability of the ice bed varying with the ripening time. Fig. 3. Ripening of ice slurry affects the permeability of the ice bed.
ice. Over growth of ice can result in dead-end pores and liquid entrainment in the washed ice bed, and can even clog the ice bed. Ice grain size plays an important role in washing. Firstly, it determines the permeability of the (compressed) ice bed. Secondly, it impacts the
227
wash front height, which is the mixing region of water and the concentrated solution. For the ice slurries produced by scraped surface heat exchangers, larger grain size can only be obtained by given sufficient ripening time. An ice bed compressed from premature ice slurry would have poor permeability due to small grain size. If the permeability in the bulk ice pack is too small, the wall effect will become significant because (1) the porosity at the outside of the ice bed is greater than the bulk and, (2) the heat import from the environment tends to melt the ice in this region. The wall effect results in an easier breakthrough of the wash front, which in turn leads to channeling. Exterior thermal insulation of the wash column helps to reduce the influence from ambience. It is difficult to have the wash column well wrapped but still observable. A double glazed window with an air jacket is an option for direct observation. Other instrumental methods may be more important for industrial applications. Interstitial velocity of the washing water in the ice bed is a result of the pressure drop along the wash column and the flow resistance of the ice bed. A slow washing speed is preferable for stabilizing the wash front from being broken. But it was noticed that if the interstitial velocity is too slow because of poor permeability, breakthrough of washing water may occur at some weak points, such as in region close to the wall. Poor permeability may be due to over-compression of the ice pack, or small ice grains, or over growth of ice at the wash front. To obtain a uniform packed ice bed, compression is necessary. The pressure applied in this study was 0.1–0.5 kg/cm2. Porosity of the ice bed after compression is 0.25~0.5. A naturally piledup ice pack without being compressed shows a loose and lumpy structure. Channeling will develop at those looser positions. However if the ice bed is over compressed, dead-end pores may appear, which results in liquid entrainment in the wash ice bed.
228
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
5. Symbols
References
d g h1
[1] W.J. Swinkels, Recent developments in freeze concentration. in IDF seminar, Atlanta, Georgia, USA, IDF, 8–9 October 1985. [2] I.A. Langdon and G.C. Cox, Financial evaluation of freeze concentration for reduction in milk transport cost. Austral. J. Dairy Technol., 41 (1986) 54. [3] H. Thijssen, The economics and potentials of freeze concentration for fruit juices. Proc. XIX International Symposium of the Int. Fed. of Fruit Juice Producers, The Hague, 1986. [4] D.R.. Lide, ed., CRC Handbook of Chemistry and Physics. 76th ed., CRC Press, Cleveland, Ohio, 1995–1996, pp. 6–159. [5] H.A.C. Thijssen, Apparatus for the Separation and Treatment of Solid Particles from a Liquid Suspension. USA Pat. 3872009, 1975. [6] H.A.C. Thijssen, Continuous Packed Bed Wash Column. USA Pat. 4475355, 1984. [7] J.d. Dass and Grenco, Current large-scale commercial application of freeze concentration in the food industry. European Food and Drink Review, Spring 1991, pp. 19–24.
h2 K P td v v∞ φ η ρ
— Inside diameter of the wash column, m — The acceleration due to gravity, kgms–2 — Thickness of the wash water in the wash column, m — Thickness of the ice bed, m — Permeability of the ice bed, m2 — Hydraulic pressure head — Time needed to drain the water above the ice bed surface, s — The interstitial velocity of liquid in between ice particles, ms–1 — Water flow velocity (above the ice bed) in the wash column, ms–1 — Porosity of the ice bed — Viscosity, Pa·s — Density, kg.m–3
Experimental study of wash columns used for separating ice from ice-slurry F.G.F. Qina*, X.D. Chena,b, S. Premathilakab, K. Freea a
Freezcon Ltd., 70 Symonds Street, Auckland, New Zealand email: [email protected] b Department of Chemical and Materials Engineering, University of Auckland, New Zealand
Received 7 February 2007; accepted 8 February 2007
Abstract The separation of ice from ice slurries is the final step in the process of freeze concentration, where wash column can be employed. Phenomena found in washing an ice bed, such as channeling, viscous fingering, clogging, liquid entrainment, and permeability were studied in an experimental wash column. The appropriate operating conditions that lead to a successful washing were proposed in this paper. Keywords: Freeze concentration; Wash column
1. Introduction Separation of concentrated solutions from ice slurries is the final step in freeze concentration (FC). In FC, water in aqueous solutions is partially frozen forming ice crystal grains, and the concentrated unfrozen solution can be recovered provided the ice can be removed. As a promising alternative of the conventional evaporative concentration, FC has many advantages [1–3], such
as no denaturing of nutritional components in liquid foods or active components in bio-solutions; no aroma loss; longer operation time for the equipment because there are no thermal and microbialinduced fouling; great energy saving potential because the fusion heat of ice is only 334 kJ/kg, but the evaporation heat of water is 2340 kJ/kg [4] , which is seven times greater than the fusion heat. Among many separation methods, the wash column has its unique advantages in segregating
*Corresponding author. Presented in the Separation Sessions at Chemeca 2006, the 34th Australasian Chemical and Process Engineering Conference, Auckland, New Zealand, 17– 20 September 2006. Organised by the University of Auckland and the Society of Chemical Engineers New Zealand (SCENZ). 0011-9164/08/$– See front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.02.017
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
3-15 kg weight observation window
cold washing water inlet
ruler water line water h1
cold water
ice from the ice slurry because the liquid does not need to overcome the surface tension when it is separated with the solid ice particles. In wash column, ice slurry is compressed to form a packed ice bed, which is saturated with the concentrated solution (the mother liquid). Washing water is introduced into the wash column on top and passes through the ice bed slowly to displace the solution as a downward plug flow. When water displaces the concentrated solution during washing, mixing happens. Over-mixing of water and solutions would blur the wash front, so that the transitional region becomes larger and may even lead to failure of the separation. This study will focus on some fundamental problem found in the operation of the wash column, such as viscous fingering, channeling, and clogging etc.
stop watch
ice bed line washed ice h2
h1=8 cm h2=18~20 cm
mesh piston thermal insulation
v
mesh
draining control
2. Experimental The wash column used in this study was made of a perspex cylinder with an inside diameter of 80 mm and height of 500 mm. The exterior was wrapped with 50 mm rubber foam for thermal insulation, on which a double-glazed 30 mm × 200 mm observation window was constructed at the upper section of the column. The schematic diagram is shown in Fig. 1. The ice slurry was produced with a laboratory scale scraped surface heat exchanger. After aging for a given time, the ice slurry was manually filled into the wash column, and the ice bed was compressed using a mesh piston until it reached the ‘ice bed line’ indicated in Fig. 1. The compressing force was provided by weight(s) at the top end of the piston rod giving 0.1–0.5 kg/cm2 pressures. In this study the pressure used was 0.1 kg/cm2 unless stated otherwise. An ice-free section of mother liquid above the mesh piston was produced after compression. This part of the mother liquid was then drained to allow the liquid level to be flush with ice bed surface. The mesh piston was kept in the wash column to maintain the pressure and prevent water rushing into the ice bed. A given
11 12 1 2 10 9 3 8 4 7 6 5
v∞
compr essed i ce bed
224
Fig. 1. Experimental setup pf the wash column.
amount of cold washing water (~0°C) was added into the column from the top until it reached the level marked ‘water line’ before washing started. A ruler was placed beside the observation window for measuring. 3. Results and discussion Washing was achieved by using water to displace the mother liquid in the ice bed. During washing a downward plug flow was presumably maintained and the mother liquid was drained into a measuring cylinder. The washing (or draining) rate was measured using a stopwatch. However, wash operation may fail due to a number of problems, such as channeling, viscous fingering, and clogging etc. An often seen problem (an undesired phenomenon) was that a sharp, clear, horizontal wash front was not visible. Instead, the transitional region
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
between the washed and the unwashed ice-layer was blurred or too large, or even worse — no washed ice-layer was produced. The reason seemed to be over-mixing of the washing water with the mother liquid. But careful observations (with the help of dye-tracer) showed that this might be attributed to two phenomena: channeling and viscous fingering. When the local ice bed progressively became loose during washing because of melting or ice being washed away, channeling occurred. Channels often appeared as a number of downward caves in root-like shape as shown in Fig. 2a. Viscous fingering occurred when the wash front moved faster at some locations than others, as shown in Fig. 2b and 2c. The local porosity did not appear to change. The horizontal transitional zone was broken when viscous fingering happened, and the separation of ice from the mother liquid would be inefficient or even fail. Both channeling and viscous fingering occurred often in the vicinity of the column sidewall. An uncompressed piled ice bed could not be washed properly as particle cohesion and agglomeration caused the ice in the slurry to become lumpy. The interstices between ice lumps allowed
225
liquid to run down faster than in other parts; this caused channeling directly. When washing was done properly, it would show a horizontal, sharp wash front between the water and solution interface. The ice bed could also slide out of the wash column from the bottom after the supporting mesh at the bottom was taken off. The ice bed was rigid enough and could stand by itself on the table as shown in Fig. 2d. However, the unwashed ice beds remained soft even though they were compressed. Another potential problem is clogging in the ice bed resulted from over crystallization of ice in the porous ice bed. High concentration of the mother liquid with strong freezing point depression (FPD) will enhance this effect. A similar phenomenon was mentioned by others [5–7]. Experiments in this study also showed that it usually happened when the ripening time given to the ice slurry was insufficient. The ‘premature’ ice slurry perhaps contained a large number of very fine ice particles so that the permeability was poor. The situation became worse when the ice bed was over compressed, which also resulted in dead-end pores and liquid occlusion (liquid sack) inside ice lumps due to pressure-induced fusion of ice. Clogging
h2
(a)
(b)
(c)
(d)
Fig. 2. Phenomena found in washing. (a) Channeling. (b) Beginning of the viscous fingering. (c) Development of viscous fingering. (d) A successful washing.
226
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
of the ice bed prolongs the washing time and the ice close to the wall gradually melts and eventually leads to channeling near the wall. Ripening also influences the permeability of the ice bed. Fig. 3 shows the time needed for draining (or collecting) 630 ml mother liquid from the ice bed, which was compressed from different ice slurries with different ripening times. 630 ml is just the volume of the washing water used. The draining rate was also a measurement of separation rate of the mother liquid, which was dominated by the interstitial velocity of liquid in the ice bed (v). According to Darcy’s law, it can be written in the following one-dimensional form for natural draining: K ∆P ρgK v= = η h η
(1)
where v is an average value. K is the permeability of the ice bed, which is only a function of the pore structure of the medium. η and ρ the are viscosity (Pa·s) and density (kgm–3) of the liquid, respectively, at the wash front. They are considered to be the average of the washing water and mother liquid in this paper. P is the hydraulic (pressure) head, h the liquid height and g the acceleration due to gravity, P = ρgh. The flow velocity above the ice bed is slower than the interstitial velocity: v∞ = φ v
(2)
where φ is the porosity of the ice bed. v∞ is the flow velocity above the ice bed (it is also known as the velocity of approach, as shown in Fig. 1). v∞ can be obtained via the calculation of water height divided by the draining time.
v∞ =
h1 td
(3)
where h1 is the preset height of washing water in the column (h1 = 8 cm in this study). td is the time needed to allow the water level to be flush with
top of the ice bed, when 630 ml mother liquid was collected. The porosity of the ice bed can be calculated in this way: πd 2 h1 φπd 2 h2 = . 4 4
The left-hand side is the used water volume. The right-hand side is the water volume in the washed ice bed. So it equals the ratio of h1 and h2 :
φ=
h1 h2
(4)
h2 is the thickness of washed ice layer, which was 18~20 cm in this study (see Figs. 2 and 3c). Although the ripening time varied from no more than 1–18 h, the thickness of the washed ice layer did not show much difference. In other words, the porosity of the ice bed changed only a little, ranging from 0.4~0.44 under a constant pressure of 0.1 kg/cm2. However, the draining time (td) for collecting 630 ml mother liquid varied from 170 s to more than 1000 s (Fig. 3a), indicating that both the flow velocity of draining liquid and the permeability of the ice bed increases with ripening time as shown in Figs. 3b and 3c. K was obtained using Eq. (1) based on experimental measurement of draining rate. 4. Conclusion The movement of the wash front tends to induce viscous fingerings during washing of the ice bed. However, gravity is a stabilizing force that keeps the wash front horizontal. Freezing and thawing may both occur in washing, which change the porosity of the ice bed. Local thawing at the wash front may lead to the breakthrough of the washing water and develop into channeling. The opposite trend is freezing at the wash front when cold water comes into contact with the subzero
Draining time, td (s)
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
1200 1000 800 600 400 200 0 0
2
4
6
8
10 12
14
16 18
20
Ripening time, tr (h)
Flow velocity (mm/s)
(a) Draining time varying with the ripening time. 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
Interstitial velocity (v) Velocity of approach (v∞ )
0
2
4
6
10
8
12
14
16
18
20
Ripening time, tr (h)
Permeability, K (m2 )
(b) Flow velocity varying with the ripening time. 8×10-10 6×10-10 4×10-10 2×10-10 0 0
2
4
6
8
10
12
14 16
18
20
Ripening time, tr (h)
(c) Permeability of the ice bed varying with the ripening time. Fig. 3. Ripening of ice slurry affects the permeability of the ice bed.
ice. Over growth of ice can result in dead-end pores and liquid entrainment in the washed ice bed, and can even clog the ice bed. Ice grain size plays an important role in washing. Firstly, it determines the permeability of the (compressed) ice bed. Secondly, it impacts the
227
wash front height, which is the mixing region of water and the concentrated solution. For the ice slurries produced by scraped surface heat exchangers, larger grain size can only be obtained by given sufficient ripening time. An ice bed compressed from premature ice slurry would have poor permeability due to small grain size. If the permeability in the bulk ice pack is too small, the wall effect will become significant because (1) the porosity at the outside of the ice bed is greater than the bulk and, (2) the heat import from the environment tends to melt the ice in this region. The wall effect results in an easier breakthrough of the wash front, which in turn leads to channeling. Exterior thermal insulation of the wash column helps to reduce the influence from ambience. It is difficult to have the wash column well wrapped but still observable. A double glazed window with an air jacket is an option for direct observation. Other instrumental methods may be more important for industrial applications. Interstitial velocity of the washing water in the ice bed is a result of the pressure drop along the wash column and the flow resistance of the ice bed. A slow washing speed is preferable for stabilizing the wash front from being broken. But it was noticed that if the interstitial velocity is too slow because of poor permeability, breakthrough of washing water may occur at some weak points, such as in region close to the wall. Poor permeability may be due to over-compression of the ice pack, or small ice grains, or over growth of ice at the wash front. To obtain a uniform packed ice bed, compression is necessary. The pressure applied in this study was 0.1–0.5 kg/cm2. Porosity of the ice bed after compression is 0.25~0.5. A naturally piledup ice pack without being compressed shows a loose and lumpy structure. Channeling will develop at those looser positions. However if the ice bed is over compressed, dead-end pores may appear, which results in liquid entrainment in the wash ice bed.
228
F.G.F. Qin et al. / Desalination 218 (2008) 223–228
5. Symbols
References
d g h1
[1] W.J. Swinkels, Recent developments in freeze concentration. in IDF seminar, Atlanta, Georgia, USA, IDF, 8–9 October 1985. [2] I.A. Langdon and G.C. Cox, Financial evaluation of freeze concentration for reduction in milk transport cost. Austral. J. Dairy Technol., 41 (1986) 54. [3] H. Thijssen, The economics and potentials of freeze concentration for fruit juices. Proc. XIX International Symposium of the Int. Fed. of Fruit Juice Producers, The Hague, 1986. [4] D.R.. Lide, ed., CRC Handbook of Chemistry and Physics. 76th ed., CRC Press, Cleveland, Ohio, 1995–1996, pp. 6–159. [5] H.A.C. Thijssen, Apparatus for the Separation and Treatment of Solid Particles from a Liquid Suspension. USA Pat. 3872009, 1975. [6] H.A.C. Thijssen, Continuous Packed Bed Wash Column. USA Pat. 4475355, 1984. [7] J.d. Dass and Grenco, Current large-scale commercial application of freeze concentration in the food industry. European Food and Drink Review, Spring 1991, pp. 19–24.
h2 K P td v v∞ φ η ρ
— Inside diameter of the wash column, m — The acceleration due to gravity, kgms–2 — Thickness of the wash water in the wash column, m — Thickness of the ice bed, m — Permeability of the ice bed, m2 — Hydraulic pressure head — Time needed to drain the water above the ice bed surface, s — The interstitial velocity of liquid in between ice particles, ms–1 — Water flow velocity (above the ice bed) in the wash column, ms–1 — Porosity of the ice bed — Viscosity, Pa·s — Density, kg.m–3