ARTICLE pubs.acs.org/IECR
Ammonium Ion Removal with a Natural Zeolite in Monodispersed and Segregated Fluidized Beds Ays-e C-ec- en Erbil,* Elif Soyer, and Bilsen Beler Baykal Department of Environmental Engineering, Civil Engineering Faculty, Istanbul Technical University, Maslak-Istanbul 34469 Turkey ABSTRACT: This study focuses on the dynamic behavior of ammonium ion uptake during continuous operation with respect to liquid by means of ion exchange with a natural zeolite in fluidized and segregated fluidized beds. The major operating variables affecting the performance in both types of fluidized beds are particle size, bed expansion, and flow rate. Use of particles of about 300 μm size in monodispersed fluidized beds resulted in breakthrough capacities of about 0.5 meq/g or higher, with contact times in the order of 0.75.5 min. The column efficiencies in terms of breakthrough capacity/total capacity were in the order of 0.420.61. The segregated fluidized bed, as achieved by using two nonmixing particle size fractions, is a hybrid fluidized bed consisting of an expanded bed with reduced mixing which is topped by a regular fluidized bed. Segregation increased the column efficiency to 0.69 when a combination of 250300 and 500600 μm sized particles were fluidized with a contact time of 1 min. The breakthrough capacities for the segregated fluidized beds with the finer particle size combination were 0.600.89 meq/g. A new representation of the breakthrough curves in terms of C/C0 versus a dimensionless time t/tf suggests that these plots are a valuable tool for comparing performances for a wide variety of experimental conditions. The results of this study indicate that fluidization can well be applied to ion exchange operations to accommodate high influent flow rates.
1. INTRODUCTION The ammonium ion is an important constituent of domestic sewage and imparts a considerable nitrogen load to wastewater treatment systems. The ammonium ion is also an important source of nitrogen for plant growth which makes it a valuable substance worth recovering from domestic sewage. Urine-separating toilets facilitating the separate treatment of the nitrogen load are examples of recent efforts along this trend. Natural zeolites are aluminum silicate minerals with high cation exchange capacities and high ammonium selective properties.1 The possibility of reuse after regeneration adds to the value of zeolites in environmental engineering applications. Removal of the ammonium ion and also other cations by means of ion exchange using natural zeolites is a well studied subject. Studies include removal from both drinking water and wastewaters.112 Although studies on ion exchange have concentrated mainly on fixed beds, the use of fluidized and expanded beds is being investigated in the field of separation of biotechnology products by using specific adsorbents, and promising results are reported.1316 An expanded bed is defined as a fluidized bed operated with a wide particle size range or with a combination of both different particle sizes and densities resulting in segregation in the bed leading to decreased axial mixing of the particles in the column.17 An expanded bed can also result from lower flows. Segregation also decreases the axial dispersion coefficient of the liquid. An expanded bed can also be achieved by inducing a magnetic field onto a fluidized bed of magnetizable particles.18,19 Among the many advantages of using fluidized beds are the direct application of the effluent to the bed without any need for filtering as reported by Karau et al.13 and the lower pressure drop values with respect to a fixed bed at the same flow rate. Slater20 studied the ion exchange of various cations in fluidized columns and found that the performance was in many cases very encouraging. The advantage of the fluidized bed operation was summarized as lower pressure drop in the column, lower attrition r 2011 American Chemical Society
rates, and lower axial mixing of both liquid and solids in columns with a large height/diameter ratio. In addition, Slater20 mentioned that the treatment of high flow rate dilute waste streams was especially suited to fluidized bed systems. Some studies on ion exchange have been performed in fluidized and expanded beds by Koh et al.21 using a cation exchange resin for the removal of phenylalanine. They concluded that the breakthrough curves for the expanded bed and a packed bed were almost identical except for an earlier breakthrough in the case of the fluidized bed during the initial adsorption period. Hwang and Lu22 studied the removal of Cu2þ ions by means of a resin in a semifluidized bed which was formed when a mass of fluidized particles was compressed with a porous retaining grid. A packed bed was connected in series to this semifluidized bed. Hwang and Lu22 reported that the performance of a semifluidized bed was better than that of the fluidized bed or packed bed in terms of the volume of the liquid treated per unit pressure drop. Bruce and Chase15,16 have investigated the adsorption of pure lysozyme and bovine serum albumin on Streamline particles using an expanded bed. The authors reported that using particles of a wide size range reduces the liquid dispersion and the particle mixing in the fluidized bed due to segregation. The reduced dispersion by means of segregation in the fluidized bed led to an excellent breakthrough capacity as Bruce and Chase15,16 reported. Yun et al.23 reported that the axial dispersion decreased along the bed height when they used adsorbent particles with a density difference and a log-normal size distribution. Another way of reducing the liquid dispersion and particle mixing in fluidized beds is magnetic stabilization using magnetizable particles or composite particles with a magnetizable core. Received: July 7, 2010 Accepted: April 3, 2011 Revised: March 23, 2011 Published: April 03, 2011 6391
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Table 1. Monodispersed Fluidized Beds: Experimental Conditions and Results experiment 1
2
3
4
5
6
particle size, dp (μm)
250350
250350
250350
500600
250350
500600
mass, M (g)
100
100
100
100
200
200
fluidized bed height, Hf (cm)
13.8
14.8
45
20
85.8
40.8
fluidized bed voidage, ε
0.54
0.57
0.86
0.68
0.85
0.69
bed expansion, E (%)
17
25
246
60
264
63
flow rate, Q (L/h)
1.5
2.7
39.4
39.4
39.4
39.4
empty bed contact time, tc (min)
5.5
3.3
0.7
0.3
1.3
0.6
NH4þ feed concn, C0 (mg/L) loading, mg NH4þ /g 3 h
25.6 0.38
26.2 0.71
28.7 11.31
25.2 9.93
24.2 4.77
26.7 5.26
time needed for exhaustion (h)
112.6
74.6
4.6
11.7
7.1
12.8
breakthrough time (h)
29.50
8.30
1.25
0.62
2.33
1.13
breakthrough capacity, meq/g
0.60
0.42
0.54
0.23
0.61
0.39
breakthrough capacity/total cap.
0.48
0.25
0.45
0.19
0.58
0.33
linear velocity (cm/min)
2.5
4.5
66.2
66.2
66.3
66.3
interstitial velocity (cm/min)
4.7
7.9
77
96.7
77.8
96.1
B€ohm and Pittermann19 reported that by using magnetic particles a stabilized bed is obtained, which is more permeable than a packed bed adsorption column but simultaneously has less backmixing than a conventional fluidized bed. Powdered natural zeolite flocculated with a cationic-type polymer was used for ammonium removal in expanded/fluidized bed reactors.24 Ammonium ion concentration in the feed solution was 100 mg/L NH3N and the feed flow rate was kept constant for the expansion/fluidization of the flocs bed at an upflow velocity slightly higher than the minimum fluidization velocity. The authors concluded that the results had proved the high adsorption efficiency of powdered natural zeolites for the uptake of ammonia (11 mg NH3N/g) at a 38 m/h loading rate. Ammonium ion removal from municipal landfill leachate was studied in both upflow fixed-bed and fluidized bed clinoptilolite columns with an NH4þ concentration of 3000 mg/L in the feed solution.25 Fixed bed operation was found to have significantly higher performances than the fluidized bed operation. Increasing the expansion ratios over 10% resulted in decreased fluidized bed column treatment efficiencies. A review of the literature indicates that, in general, studies of fixed bed ion exchange outnumber the studies of ion exchange in fluidized beds. Further, ammonium ion exchange is a subject not fully investigated under fluidized bed conditions to this date. This work aims to fill the gap on the subject of ion exchange of ammonium in fluidized and segregated fluidized beds with special emphasis on laying out the fundamental frame of reference for the fluidization conditions such as flow rate, particle size, and segregation together with the ion exchange behavior and column efficiencies expressed as breakthrough capacity/total capacity. As segregation is expected to reduce both the mixing of solids in the fluidized bed and the liquid backmixing in the column leading to an improved breakthrough capacity, this idea is exploited in this work by inducing segregation using two nonmixing particle size fractions in the fluidized bed. Particles of the same density will not mix when their particle size ratio is greater than about 1.3 as stated by Wen and Yu.26
This study concentrates on the performance of fluidized beds in which complete segregation is induced by using two distinct nonmixing particle sizes. The segregated fluidized bed studied in this work is a hybrid bed composed of an expanded bed in the lower part and a regular fluidized bed in the upper part. The application of this idea to the field of ion exchange of the ammonium ion using the natural zeolite clinoptilolite is novel. The other extreme type of operation of fluidization using monodispersed particles is also investigated and comparisons are made between these two extreme types of operations and also with respect to the fixed bed operation on the subject of ammonium ion removal with clinoptilolite.
2. EXPERIMENTAL SECTION 2.1. Preparation of the Clinoptilolite Samples. Clinoptilolite is an ammonium selective natural zeolite which has a chemical formula of (Na4K4)(Al8Si40)O96 3 24H2O. Clinoptilolite in its raw form was obtained from the Bigadic-Balıkesir region in Turkey in the form of rocks. The rocks were broken into smaller pieces and then crushed and sieved into many fractions. Particles of very uniform size were obtained by collecting particles accumulating between two successive sieves. The skeletal density of the Bigadic clinoptilolite was determined by a Quantachrome Ultrapycnometer as 2.12 g/cm3. Its bulk density in water was 1.6 g/cm3. Each particle fraction was washed with distilled/reverse osmosis water to remove any dust and air-dried. After that, each fraction was conditioned with 1 M NaCl solution to convert the natural clinoptilolite into its homoionic form. After conditioning, the clinoptilolite samples were briefly rinsed with distilled or reverse osmosis water to remove any excess salt adhering to the particles and air-dried. Drying at a temperature of 105 °C followed to remove any moisture remaining in the pores without removing the crystal water from the clinoptilolite structure. Ion exchange experiments were performed at 20 °C, and the initial feed concentrations are listed in Tables 1 and 2. The initial feed concentration of 25 mg NH4þ/L is low enough not to alter the physical properties of the water. 6392
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Table 2. Segregated Fluidized Beds: Experimental Conditions and Results experiment
particle size, dp (μm)
a
7
8
9
10
11
12
250350
250350
250350
250350
500600
500600
500600
500600
500600
500600
8401000
8401000
mass, M (g)
200
200
200
100
200
200
fluidized bed height, Hf (cm)
46.8
54.3
64.5
41
37
42.3
fluidized bed voidage, ε
0.73
0.77
0.80
0.85
0.66
0.70
bed expansion, E (%)
84
113
153
173
35
54
flow rate, Q (L/h)
17b
25.2c
39.4
39.4
43.7
60
empty bed contact time, tc (min) NH4þ feed concn, C0 (mg/L)
1.6 26.1
1.3 25.9
1.0 28.5
0.6 26.7
0.5 26.5
0.4 25.1
loading, mg NH4þ /g 3 h
2.22
3.26
5.61
10.52
5.79
7.53
time needed for exhaustion (h)
30.6
14.1
10.8
8.3
13.6
14.3
breakthrough time (h)
6.67
3.33
2.75
0.92
0.58
0.50
breakthrough capacity, meq/g
0.87
0.66
0.89
0.60
0.26
0.16
breakthrough capacity/total cap.
0.63
0.60
0.69
0.45
0.21
0.13
linear velocity (cm/min)
37a
58.1a
66.3
66.3
73.6
101
interstitial velocity (cm/min) recycle
50.7 √
75.7 √
82.5
78.4
111.7
144
Linear velocity based on total flow in the column. b Fresh feed; recycle flow/fresh feed = 0.29. c Fresh feed; recycle flow/fresh feed = 0.37.
Tables 1, 2, and 3 list the particle sizes and the experimental conditions used in this study. 2.2. Capacity Measurement. The ion exchange capacity of the clinoptilolite at the feed concentration used in the experiments was obtained by the repeated batch equilibration method as explained in Helfferich.27 For this purpose, a precisely weighed amount of clinoptilolite was contacted with an NH4Cl solution of a chosen initial concentration and of measured volume for a day and the concentration of the ammonium ion remaining in the solution was determined. Then the solution was replaced with fresh solution of the same initial concentration. This procedure was repeated until the ammonium ion concentration in solution equaled that of the initial solution, meaning that ammonium ion uptake ceased. The ion exchange capacity determined with this method for a feed concentration of 25 mg NH4þ/L is 1.25 meq/g. Ammonium ion concentrations were measured by means of a Thermo Orion Advanced Ion Selective Electrode Meter 720Aþ using an ammonia selective electrode. 2.3. Column Studies. The ion exchange behavior of the Turkish clinoptilolite sample was studied in columns of 35.5 mm ID. This column diameter was chosen to avoid any wall effects with the particle sizes used. The columns were equipped with an inlet section of the same diameter which was filled with glass beads of 3 mm diameter to ensure an even distribution of the liquid. After this inlet section, the liquid entered the column passing through a fine mesh screen. The columns were loaded with the conditioned and dried clinoptilolite. A synthetic NH4Cl solution of a chosen initial concentration similar to that in domestic wastewater was prepared with reverse osmosis water and was continuously fed to the column at velocities high enough to fluidize the material. The synthetic NH4Cl solution is not representative of true wastewater and the present study focuses on the concept of the possible use of fluidized beds for ammonium ion removal rather than its application to wastewater. Centrifugal submerged pumps were used to avoid any pulsations during fluidization. The exiting liquid left the column at a
height of about 2.5 cm above the bed surface in most experiments. However, for the smallest particle size when the flow rate was very high, the liquid had to leave at an elevation of about 20 cm above the bed surface to avoid the entrainment of the particles. The breakthrough curves were constructed by determining the NH4þ ion concentration of the effluent from samples taken at suitable time intervals. The breakthrough capacity was determined from these curves as the capacity until about 1 mg NH4þ/L appeared in the effluent for all experimental conditions. The overall experimental system is shown in Figure 1. 2.3.1. Fluidized Beds. Fluidized bed ion exchange was studied with two different modes of operation regarding the particles in the bed: (i) Fluidized beds with a very uniform particle size distribution consisting of clinoptilolite particles obtained from between two successive sieves. (ii) Segregated fluidized beds containing two nonmixing particle sizes each obtained from between two successive sieves. In this type of fluidization, the bed segregated into two nonmixing layers when the ratio of the mean particle sizes of the larger to the smaller particles was at least 1.3 as stated by Wen and Yu.26 The flow rates were chosen such that the lower layer in the bed was also fluidized. Fluidization was achieved by using a recycle flow in addition to the fresh feed flow for two experiments to achieve the fluidized state at lower flow rates which would not fluidize the bed. The contact time is also increased by using a recycle flow without the need to use more clinoptilolite in the column. All fluidized beds and segregated fluidized beds in this study had a high bed aspect ratio which is the value of the expanded bed height to the bed diameter. The bed aspect ratio varied within the range of 3.9 to 24. Three experiments with fluidized beds were replicated and significant differences were not observed. 2.3.2. Packed Beds. Some packed bed experiments were also performed to be compared with the fluidized bed experiments. All packed beds were operated with an upward flow of the feed solution. The experimental conditions are listed in Table 3. 6393
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particle size, dp (μm) flow rate, Q (L/h) loading (mg NH4þ/g 3 h) empty bed contact time, tc (min) breakthrough time, (h) breakthrough capacity/total capacity 250350 39.4 4.77 1.3 2.33 0.58
expt 5
expt 1
expt 7 (250350) (500600) 17 2.22 1.6 6.67 0.63
250350 1.5 0.38 5.5 29.5 0.48
500600 4.4 1.14 1.7 9.5 0.50
expt 13
monodispersed mass: 100 g
500600 39.4 5.26 0.6 1.13 0.33
expt 6
monodispersed
particle size, dp (μm) flow rate, Q (L/h) loading (mg NH4þ/g 3 h) empty bed contact time, tc (min) breakthrough time, (h) breakthrough capacity/total capacity
particle size, dp (μm) flow rate, Q (L/h) loading (mg NH4þ/g 3 h) empty bed contact time, tc (min) breakthrough time (h) breakthrough capacity/total capacity
mass: 200 g
mass: 100 g
Table 3. Comparison of Main Results
expt 2
(250350) (500600) 25.2 3.26 1.3 3.33 0.60
expt 8
250350 2.7 0.71 3.3 8.3 0.25
expt 14
packed beds
(250350) (500600) 39.4 5.61 1.0 2.75 0.69
expt 9
segregated
250350 39.4 11.31 0.7 1.25 0.45
expt 3
fluidized beds
(250350) (500600) 1.3 0.17 11.7 93 0.78
monodispersed expt 4
segregated mass: 200 g
(500600) (8401000) 60 7.53 0.4 0.5 0.13
expt 12
500600 39.4 9.93 0.3 0.62 0.19
(500600) (8401000) 4.4 0.57 3.7 26 0.63
expt 15
(500600) (8401000) 43.7 5.79 0.5 0.58 0.21
expt 11
(250350)(500600) 39.4 10.52 0.6 0.92 0.45
expt 10
segregated
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Figure 2. Expanded bed heights for fluidized beds. Figure 1. Schematic diagram of the experimental system.
The total capacities for ion exchange were calculated by numerically integrating the area above the breakthrough curves plotted as C/C0 versus time for each experimental condition and values varied from 1.05 to 1.35 meq/g and these values are very close to the equilibrium value of 1.25 meq/g.
3. RESULTS AND DISCUSSION 3.1. Fluidized Bed Characteristics. The fluidized bed behavior of ammonium ion exchange on clinoptilolite was studied from the point of view of determining the effect of the feed flow rate and the mass of clinoptilolite in the column as the primary variables. The bed height itself is an intrinsic variable because it is determined both by the mass of material in the column and by the flow rate which determines the bed expansion. The empty bed contact time, an important variable in both packed and fluidized bed ion exchange, depends on the mass of material in the column and the flow rate which in turn causes the bed to expand. The empty bed contact time in a fluidized bed is based on the bed height in its expanded state. Operating conditions are limited by the fluidization characteristics of the particle sizes used in this study in the sense that the minimum fluidization velocity must be exceeded for both particle sizes in segregated beds. Both particle sizes are subjected to the same linear superficial velocity in the bed leading to two distinct regions with different voidage values. Very high velocities are needed to fluidize the larger sizes used in this study which in turn drastically decreases the contact time in the column. Packed bed operation was not attempted with the smallest particle sizes because a central channel was observed which acted like a spout through which a major portion of the flow was carried. The fluidized bed experiments covered a wide range of empty bed contact times ranging from 18 s to 5.5 min. The fluidized bed experiments also covered a wide range of bed expansion and flow rate values as seen in Tables 1 and 2. The effects of the particle size, flow rate (linear velocity), segregation, and initial bed height on expanded bed height are shown in Figure 2. As seen in Figure 2, these operating variables affect the bed expansion significantly which is a major difference to packed beds. The flow properties in fluidized beds in turn affect the breakthrough behavior. All results are listed in Tables 1 and 2. The discussion of column efficiency is based on the ratio of the breakthrough capacity to the total capacity and not the breakthrough capacity itself because there are slight variations in the total capacity values as mentioned in
Figure 3. (a) Breakthrough curves for experiments 1, 2, 3 and 4 with respect to BVs. (b) Breakthrough curves for experiments 3, 4, 5, and 6 with respect to BVs.
Section 2.3.2. Using the above-mentioned ratio overcomes this problem and enables a dimensionless comparison. 3.2. Evaluation of the Ion Exchange Performance by Means of the Breakthrough Curves with Respect to BVs. 3.2.1. Fluidized Beds with Monodispersed Particles. The breakthrough curves for the monodispersed fluidized beds are given in Figure 3a and b. The breakthrough behavior of a fluidized bed containing clinoptilolite particles in the size range of 250350 μm and with a relatively low bed expansion (expt 1) is compared with expt 2 to observe the effect of increased flow rate in Figure 3a, where C/C0 is plotted against effluent volume passed through the column/settled bed volume, referred to as BVs. The value of BVs is obtained by calculating the volume of effluent passed through the column until a specific time and then dividing that volume by the settled bed volume. The amount of 6395
dx.doi.org/10.1021/ie1014519 |Ind. Eng. Chem. Res. 2011, 50, 6391–6403
Industrial & Engineering Chemistry Research effluent passing through the column in terms of BVs therefore is related to the time passed and all columns working under different operating conditions are exhausted at different BVs values or times. The flow rates needed to calculate BVs are reported in Table 1. The breakthrough curves are quite similar for beds with lower bed expansion values and 100 g of clinoptilolite (expt 1 and expt 2) which have identical settled bed volumes, except for the initial period where breakthrough is observed at lower BVs for the experiment with the higher flow rate (expt 2) as seen in Figure 3a. The breakthrough curve shifts to the left when the flow rate is increased. Other conditions being the same, a higher flow rate means a higher interstitial velocity which is the superficial velocity divided by the bed voidage as seen in Table 1, because bed expansion is not increasing linearly with flow rate. The coefficient of mechanical dispersion in the liquid phase is proportional to the interstitial velocity times a characteristic medium length (particle size),28 and this justifies the earlier breakthrough at the higher flow rate of expt 2. A higher mechanical dispersion leads to an earlier breakthrough and thus affects the column performance. The value of the breakthrough capacity/total capacity is a good indicator of the efficient use of the bed material and as seen in Table 1, increasing the flow rate caused a major drop in the breakthrough capacity/total capacity value from 0.48 for expt 1 to 0.25 for expt 2. Karadag et al.26 have come to a similar conclusion in their work. Figure 3a also shows the breakthrough curve for a fluidized bed containing 100 g of clinoptilolite of 250350 μm size (expt 3) with a very high feed flow rate and a very high bed expansion. In this case, the interstitial velocity in this highly expanded bed is very high (Table 1). At the same time, the bed expansion has reached 246%, leading to a bed voidage value of 0.86 (Table 1). Even though the flow rate was very high, breakthrough occurred at a higher value of the effluent volume/settled bed volume as for expt 1 in Figure 3a. This means that a highly expanded fluidized bed can perform like a fluidized bed with a very low degree of bed expansion. Indeed, the highly expanded fluidized bed (expt 3) performs better than the beds with much lower expansion (expts 1 and 2) in that the exit concentration increases very slowly until about 600 bed volumes pass through the bed (Figure 3a). After that, a sharp increase in the exit concentration is observed resembling the breakthrough curve of a packed bed with a high contact time. The contact time of this fluidized bed actually is very short (42 s) as seen in Table 1 and the breakthrough capacity/total capacity value is increased to 0.45. Based on this, it can be concluded that expt 1 and expt 3 have similar column efficiencies regardless of the differences in the contact times, bed expansions, and the flow rates. A possible explanation can be based on flow characteristics which are due to a complicated relationship among flow velocity, expanded bed height of the fluidized bed, and the performance. An increased flow rate means an increased ion loading to the column as the feed concentrations remain similar. Values of the loadings expressed as mg NH4þ/h per gram of zeolite are listed in Tables 1, 2, and 3. To show the effect of the particle size at the same flow rate and the same amount of clinoptilolite as for expt 3, the breakthrough data for an experiment with the larger sized particles (expt 4) is included in Figure 3a. Expt 4, on a plot of C/C0 vs BVs, seems to perform quite similar to expt 3 initially, but breakthrough occurs at a lower value of BVs. After that the column capacity is filled up at an almost constant rate until the last quarter of the experiment duration. The breakthrough capacity/total capacity value has deteriorated to 0.19 from 0.45 for expt 4 as can be seen in Table 1.
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The loss of the immediate surface area upon increase of the particle size affects the number of active exchange sites available at short-term. The interstitial velocity for expt 4 (Table 1) is the highest among those compared because at the same feed flow rate the bed expands much less due to the larger particle size. A high interstitial velocity together with a lower bed height is expected to increase the mechanical dispersion; together with a decreased number of immediately available active exchange sites, these effects compound to lead to this poorer breakthrough performance. The effect of doubling the amount of clinoptilolite in the columns with a high bed expansion together with the effect of the increased particle size is shown in Figure 3b where the breakthrough curves for expts 5 and 6 are plotted. Expt 3 is also included in Figure 3b to aid the comparison to expt 5. Initially, the breakthrough curves for expts 5 and 3 look very similar on a plot of C/C0 vs BVs, however, after about 450 bed volumes of effluent have passed through the columns, the breakthrough curve for expt 5 remains above that for expt 3. This may seem incorrect at first sight because the bed with twice as much clinoptilolite cannot perform worse; the relation between the effluent volume passing through the column (BVs) and the time to exhaustion is obscured when the effluent quality (C/C0) is plotted against BVs. It is also very difficult to compare the breakthrough capacities on such a plot because BVs need to be converted to the time scale to determine the loading to the column. A comparison of the breakthrough capacity/total capacity values in Table 1, however, shows that keeping all other variables constant, doubling the amount of clinoptilolite has increased the breakthrough capacity/total capacity value from 0.45 for expt 3 to 0.58 for expt 5. This is due to the increased contact time and indirectly also due to the effect of the bed height on the mechanical dispersion. The breakthrough curve for expt 6 in Figure 3b shows the effect of the increased particle size, this time with the doubled amount of clinoptilolite. It is apparent from the breakthrough curves for expts 5 and 6 in Figure 3b that breakthrough occurs earlier when the particle size is increased with other variables being constant. The effect of increased interstitial velocity on the mechanical dispersion together with the decreased number of immediately available active exchange sites for a larger particle size can explain the drop in the breakthrough capacity/total capacity value for expt 6 with respect to that for expt 5. Doubling the amount of clinoptilolite in the column, all other variables being constant, also increased the breakthrough capacity/total capacity value for the larger particle size used in expts 4 and 6 as seen in Table 1. This is in accord with the generally accepted notion of improved performance upon increased contact time. The time needed for exhausting the column decreases with increasing feed flow rate at constant feed concentration when the same amount and size of particles are used as seen from Table 1 for expts 1, 2, and 3. Results in Table 1 show that the efficiency of the columns is influenced by the flow properties when a specific flow rate of 39.4 L/h (expts 36) is considered which corresponds to loading values between 4.77 and 11.31 mg NH4þ/g 3 h. The loading values alone do not explain the efficiencies. 3.2.2. Segregated Fluidized Beds. Ion exchange experiments in segregated fluidized beds were performed with two particle size combinations under the operating conditions listed in Table 2. The finer combination consisted of 250350 μm particles 6396
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Industrial & Engineering Chemistry Research fluidized together with 500600 μm particles. The coarser combination consisted of 500600 μm particles fluidized with 8401000 μm particles. Due to segregation of particles in the fluidized beds, the lower section is called an expanded bed (a special form of a fluidized bed with low mixing in both the liquid and solid phases) of the larger particles and is confined by the upper section which is a regular fluidized bed consisting of the smaller particles. However, both sections in the segregated fluidized bed expand freely in response to an increase in the flow rate. Visual observation of the lower layer in the fluidized bed by means of a video recording confirmed that particles did not mix between the layers and particle mixing was also significantly decreased within the lower layer itself. Particle mixing was observed in the upper fluidized layer. Two different types of operation were attempted with the use of segregated fluidized beds: (i) use of recycle flow to induce fluidization at fresh feed flow rates which would not fluidize the whole bed, and (ii) segregated fluidized bed operation where fluidization was achieved by the fresh feed flow only. A recycle flow was used in two experiments to observe its effect on the breakthrough curve. The presence of a recycle flow serves the purpose of fluidizing the bed material which will not fluidize at low fresh feed flow rates. The contact time is also increased without increasing the mass of the bed material because it is based on the fresh feed flow rate. The composition of the recycle flow varies with time as the column capacity fills up; therefore its effect on the breakthrough curve is expected to be time dependent. At early times into the ion exchange experiment, the flow exiting the column contains no ammonium ions, so that the important effect is the dilution of the feed stream by the recycle stream. Toward the end of the experiment, the ammonium ion composition of the recycle flow approaches that of the fresh feed. Therefore, at longer times into the experiment, the effect of the recycle flow is restricted to increasing the flow velocity in the column only. The concentration of ammonium chloride in the order of 25 mg/L is low enough not to alter the physical properties of the fresh feed. Figure 4a shows the breakthrough curves of the segregated fluidized bed experiments in terms of C/C0 plotted against BVs. Experiments 79 represent segregated fluidized beds each loaded with 100 g of clinoptilolite of size 500600 μm and 100 g of clinoptilolite of size 250350 μm. Experiments 7 and 8 had recycle flows in addition to the fresh feed flow shown in Table 2 because fluidization of both layers was not possible with the chosen fresh feed flow only. Experiment 7 had a fresh feed flow of 17 L/h with a recycle flow of 5 L/h, and the ratio of the recycle flow to fresh feed flow was 0.29. The breakthrough curve for expt 7 shows a sudden increase in C/C0 after the breakthrough for BVs values between 500 and 600; after that the rise in the breakthrough curve gradually slows down. The ratio of the breakthrough capacity to the total capacity was 0.63. Experiment 8 had a fresh feed flow of 25.2 L/h and a recycle flow of 9.3 L/h resulting in a ratio of recycle flow to fresh feed flow of 0.37. As shown in Figure 4a, breakthrough occurred at lower BVs values than in the previous case of expt 7. Still, the ratio of the breakthrough capacity to the total capacity was 0.60 as listed in Table 2. It is very hard to identify the effect of the increased recycle ratio in expt 8 on the plot of C/C0 against BVs in Figure 4a. Yet, the performance assessed in terms of the breakthrough capacity/total capacity seems to deteriorate as the recycle flow is increased (Table 2).
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Figure 4. (a) Breakthrough curves for expts 7, 8, 9, 11, and 12 with respect to BVs. (b) Breakthrough curves for expts 9 and 10 with respect to BVs.
As shown in Table 2, expt 9 had a very high fresh feed flow rate of 39.4 L/h and a very high average bed voidage (porosity) of 0.80. Experiment 9 had a remarkably high value of breakthrough capacity/total capacity equaling 0.69. Experiment 9 acted more like a packed bed than a fluidized bed in that the breakthrough curve showed a sharp rise after breakthrough was observed. However, this fact is almost impossible to conclude from Figure 4a on a plot of C/C0 against BVs. In addition, experiments with and without recycle flow (expts 79) look very similar on this plot. Overall, the performances of these segregated fluidized beds were similar to that of a packed bed with a sudden breakthrough in the first quarter of the time scale superimposed with a gradual approach to exhaustion in the second half, as will be discussed in more detail in the forthcoming sections. Experiments 1012 were performed to observe the effect of the bed height (mass of the bed material) with other variables being constant, and also the effect of particle size combination. The experimental conditions and results are listed in Table 2. To observe the column performance of the coarser particle combination as a segregated fluidized bed, 500600 μm particles were fluidized with 8401000 μm particles without any recycle flow in expts 11 and 12. This particle size combination required very high flow rates to achieve fluidization. The breakthrough curves for expts 11 and 12 are also plotted in Figure 4a as an insert. As seen from Figure 4a, the breakthrough curves for expts 11 and 12 are quite different from those for the segregated fluidized bed experiments with the smaller particles. Both curves are located to the left of the curves for expts 79, showing breakthrough at much lower BVs values. A sudden breakthrough is observed at a BVs value of about 50, followed by a gradual and continuous increase in C/C0. Experiment 11 had a flow rate of 43.7 L/h, slightly higher than that for expt 9, to fluidize the 8401000 μm particles in the lower part of the segregated fluidized bed. Even though the flow rate 6397
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Industrial & Engineering Chemistry Research was only about 10% higher than for expt 9, expt 11 had an unacceptably low breakthrough capacity/total capacity of 0.21 as given in Table 2. It also took a longer time for the coarser particles to be saturated with the ammonium ions as can be seen in Table 2 from the experiment duration. Similarly, expt 12 represents the same segregated fluidized bed operated at the even higher flow rate of 60 L/h. In this case, as expected, the breakthrough capacity/total capacity deteriorated to 0.13, the lowest value among all experiments. Because of the lower bed expansion with these larger particle sizes and the flow rate that was necessary to fluidize without recycle, the empty bed contact times in expts 11 and 12 were 30 and 24 s, respectively. Apparently, these contact times were too short to provide a reasonable breakthrough capacity. The shorter contact times and increased liquid dispersion due to both larger particle sizes and higher interstitial velocities, together with the decreased number of immediately available active exchange sites, may explain the much poorer column performance. Experiment 10 was another experiment with a segregated fluidized bed where 100 g of clinoptilolite was used under the same experimental conditions as for expt 9. As can be seen in Figure 4b, breakthrough occurs at lower BVs values than for expt 9 even though the effect of the initial settled bed height is included in the calculation of BVs values. As discussed earlier, the lower mass means a lower fluidized bed height and leads to a decrease in the breakthrough capacity/total capacity value of 0.45 as seen in Table 2. It is logical to conclude that to achieve a breakthrough performance similar to that of expt 9, a higher contact time is needed, which in turn would result from a greater mass of clinoptilolite in the column. The loading values for the segregated fluidized beds in Table 2 suggest that the flow properties in the columns are the key factors influencing the column efficiency rather than the loading value itself. 3.2.3. Comparison of Segregated Fluidized Beds to Monodispersed Fluidized Beds. To assess the effect of segregation on the breakthrough performance, a segregated fluidized bed is compared with two monodispersed fluidized beds operating under the same conditions except for the particle sizes. Experiments 9, 5, and 6 represent the first set where mass of clinoptilolite is 200 g. The breakthrough curves in terms of C/C0 against BVs are shown in Figure 5a. Experiments 10, 3, and 4 represent the second set where the mass of clinoptilolite is 100 g, and the corresponding breakthrough curves are shown in Figure 5b. Details of the experiments are in Tables 1 and 2. The breakthrough curves of the segregated bed (expt 9) shown in Figure 5a and the monodispersed fluidized bed with the fine particles (expt 5) are quite similar to each other; the monodispersed fluidized bed with coarser particles (expt 6), however, shows breakthrough at much lower BVs values and the curve rises more slowly. The breakthrough capacity/total capacity value was 0.69 for the segregated bed (expt 9), and for the monodispersed fluidized beds it was 0.58 for expt 5 and 0.33 for expt 6 (Tables 1 and 2). Clearly, segregation improved the breakthrough capacity/total capacity. When the mass of clinoptilolite in the columns is 100 g, Figure 5b shows that the breakthrough curves for both monodispersed fluidized beds (expt 3 and expt 4) look very similar when C/C0 is plotted against BVs. The breakthrough curve for the segregated fluidized bed (expt 10), however, seems to perform the worst since it lies to the left of the other two curves. At very low values of BVs, these three experiments are not
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Figure 5. (a) Breakthrough curves for expts 5, 6, and 9 with respect to BVs. (b) Breakthrough curves for expts 3, 4, and 10 with respect to BVs.
distinguishable. Yet, when the breakthrough capacity/total capacity values in Tables 1 and 2 are compared for these experiments, it is clear that the monodispersed fluidized bed with the coarser particles (expt 4) has the lowest one (0.19); the segregated bed (expt 10) and the monodispersed fluidized bed with the finer particle size (expt 3) have the same and much higher value (0.45). In this case, the effect of segregation is not very conclusive based on such plots and on the comparison of breakthrough capacity/total capacity values. The two sets of experiments discussed above agree with the general concept of improved performance upon increased contact time, whether there is segregation or not. 3.3. A New Approach to the Evaluation of Column Performance: Use of the Dimensionless Time Scale. The conventional displays of the breakthrough behavior as plots of C/C0 versus BVs are found to be of little help for comparing column performance because the time needed to exhaust the column contents is different for each experimental condition, complicating the direct comparison of experiments with different masses of material, feed concentrations, and feed flow rates, as expected. Especially, in the case of fluidized beds, the choice of the bed volume to calculate the contact time or the bed volumes of feed passed through the bed is quite elusive because of the bed expansion. In addition, slight variations in the feed concentrations are present in between some experiments which further complicates a direct comparison. Figures 35 indicate that valuable information about the fluidized bed performance and the efficiency of the operation is obscured on such plots, especially for low BVs values. Further, experiments with different initial settled bed heights cannot be directly compared on C/C0 versus BVs plots. This is evident in the discussions in Section 3.2. To overcome these difficulties, a new approach is suggested in this work by defining a dimensionless time scale in addition to the usual dimensionless concentration scale, C/C0. For this purpose, 6398
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Figure 6. (a) Breakthrough curves for expts 1, 2, and 3 with respect to t/tf. (b) Breakthrough curves for expts 3, 4, 5, and 6 with respect to t/tf.
the amount of ions fed to the column until time t (= Q C0 t) can be divided by the total amount of ions fed to the column until exhaustion (= Q C0 tf) where tf is the time needed to exhaust the column. The ratio (Q C0 t/Q C0 tf) becomes a dimensionless quantity representing a dimensionless loading and is simplified to t/tf, a dimensionless time, if the feed flow rate and the composition stay constant for each experiment. As both C/C0 and t/tf vary from zero to one, a dimensionless plot is obtained on which performances of ion exchange experiments can be compared within their own time scales regardless of the feed concentration or other operating variables such as flow rate and initial settled bed height. The proposed dimensionless plots of the breakthrough curves, therefore, provide a valuable visual tool for comparing the column performances on the same plot when any operating variable is changed. To demonstrate this idea, the discussion in the following parts is based on the dimensionless plots of C/C0 versus t/tf together with the calculated results listed in Tables 1, 2, and 3. 3.3.1. Fluidized Beds with Monodispersed Particles: C/C0 versus t/tf Plots. Figure 6a displays the breakthrough behavior of monodispersed fluidized beds with 100 g of the 250350 μm particles for expts 1, 2, and 3 in terms of C/C0 versus dimensionless time, t/tf. Figure 6a shows that expt 3, a fluidized bed with a very high voidage, containing 100 g clinoptilolite and with a feed flow rate of 39.4 L/h, performed better than beds containing 100 g clinoptilolite each fluidized with feed flow rates of 1.5 (expt 1) and 2.7 L/h (expt 2) even though the contact time was much shorter (Table 1) and the interstitial velocity was higher by an order of magnitude. The performance is judged by comparing the dimensionless exit concentration at the same t/tf value for each case. As seen from Figure 6a, the curve for expt 3 is below those for expts 1 and 2 for a significant portion of the dimensionless
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time scale. This means that during the first half of the experiment this column exchanges a higher fraction of the ions fed to the column. An efficient column use is indicated by a sharp rise in the curve after the breakthrough together with a higher t/tf value at breakthrough. A possible explanation for the more efficient use of the column may be the reduced axial dispersion in this bed with a higher expanded bed height. The flow characteristics, as mentioned in Section 3.2.1, are a suspected cause of the relation between expanded bed height, bed voidage, interstitial velocity, and performance. It is not possible to draw conclusions easily from the usual plots of C/C0 versus BVs as in Figure 3a because each experiment is completed at a different BVs value. Even though Figure 3a and Figure 6a are very similar, Figure 6a is easier to interpret. The C/C0 versus BVs plots are informative when the initial settled bed volumes are identical for the experiments to be compared as in Figures 3a, however, for cases with different initial settled bed volumes, this type of plot can be misleading and difficult to interpret as it is the case in Figure 3b. Figure 6b shows the effect of the initial settled bed height on the breakthrough behavior for two different particle sizes all other variables being constant as a plot of C/C0 versus t/tf. Experiment 3, having an identical flow rate, voidage, interstitial velocity, and particle size, but operating with 100 g of clinoptilolite instead of 200 g as in expt 5, looks very similar to expt 5 on this C/C0 versus t/tf plot (Figure 6b), indicating that this bed with half the initial settled bed height still operates with a similar efficiency. The breakthrough curves are nearly identical. The slightly lower effluent quality for expt 3 is hardly noticeable by careful inspection of the data in the first quarter of the t/tf scale, and reflects itself as a lower value of the breakthrough capacity/total capacity as discussed in detail the previous sections. When the C/C0 versus BVs plots for expts 3 and 5 in Figure 3b are compared with the C/C0 versus t/tf plots for the same experiments in Figure 6b, it is obvious that in Figure 3b, expts 3 and 5 look like the initial settled bed height made a pronounced difference on the breakthrough behavior. On Figure 6b, on the contrary, these experiments look almost identical when they are scaled with their own dimensionless time. To demonstrate the effect of the particle size on the breakthrough behavior, expts 4 and 6 are included in Figure 6b. All operating conditions except for the particle sizes are the same as for expts 3 and 5, respectively, as given in Table 1. As in the case of the breakthrough curves for expts 3 and 5, the breakthrough curves for expts 4 and 6 are almost identical. A deteriorated breakthrough capacity/total capacity value can be concluded from the dimensionless time scale, because breakthrough occurs at a much lower value of the t/tf scale. Therefore, due to the shape of the breakthrough curve, the breakthrough capacity is represented by a smaller fraction of the area above that curve. The effect of the increased particle size is very obvious in Figure 6b, in that the curves for expts 4 and 6 are located to the left of the curves for expts 3 and 5. This effect is obscured on the C/C0 versus BVs plots for the same experiments (Figure 3b). The breakthrough capacity/total capacity values for expts 4 and 6 listed in Table 1 confirm the visual information in Figure 6b. 3.3.2. Segregated Fluidized Beds: C/C0 versus t/tf Plots. The general features and results of experiments with segregated fluidized beds in terms of the breakthrough curves and the breakthrough capacity/total capacity values were discussed in detail in Section 3.2.2. An attempt is made in this section to 6399
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Figure 7. Breakthrough curves for expts 7, 8, 9, 11, and 12 with respect to t/tf.
analyze these aforementioned features in terms of the dimensionless C/C0 versus t/tf plots. Figure 7 is the dimensionless C/C0 versus t/tf plot for expts 7, 8, 9, 11, and 12 and can be compared to Figure 4a for the same experiments. On the BVs plot (Figure 4a), the only distinguishable feature is the effect of the particle size on the breakthrough curves. On the dimensionless time plot (Figure 7), the breakthrough curves for each experiment are very clear and a comparison regarding the column performances can be made directly. The effect of increasing the flow rate for the segregated fluidized bed with the larger particle size combination is seen clearly in Figure 7 (expts 11 and 12), whereas these two experiments looked the same in Figure 4a. Regarding the shape of the breakthrough curves together with the location of the breakthrough point on the dimensionless time scale, expts 7, 8, and 9 are the ones with the higher efficiencies because the breakthrough curve rises with a higher slope after breakthrough at about t/tf = 0.25. The effect of the recycle flow as explained in Section 3.2.2 causes the shapes of the breakthrough curves for expts 7 and 8. 3.3.3. Segregated Fluidized Bed - Monodispersed Fluidized Bed Comparison: C/C0 versus t/tf Plots. The dimensionless breakthrough curve for a segregated fluidized bed (expt 9) is compared with two monodispersed fluidized beds (expts 5 and 6) each operating with 200 g of clinoptilolite at the same flow rate in Figure 8a. This figure clearly shows the trend of the breakthrough performance with respect to particle size; the column with the finest particle size (expt 5) (250350 μm) has the lowest exit concentration at the same t/tf value. Even though the column exhaustion occurred faster, the column still showed a low exit concentration for a larger fraction of the total time needed. In contrast, the column with the larger particles (expt 6) (500 600 μm) had the highest exit concentration when compared at the same t/tf value as seen in Figure 8a. Both monodispersed fluidized beds showed similar slopes for the change in exit concentration with time indicating a similar value for the sorption effectiveness.8 The significance of expt 9, the experiment with the segregated fluidized bed, stands out in Figure 8a, not only because of the high breakthrough capacity, but also because of the sudden change in the exit concentration after the breakthrough. This behavior resembles that of a packed bed rather than a fluidized bed as will be discussed in the following section. The sudden change in the exit concentration indicates a high sorption effectiveness8 and reflects itself as a higher value of the breakthrough capacity/total capacity than for expts 5 and 6. Figure 8a
Figure 8. (a) Breakthrough curves for expts 5, 6, and 9 with respect to t/tf. (b) Breakthrough curves for expts 3, 4, and 10 with respect to t/tf.
carries very valuable visual information which is also confirmed by the results in Tables 1 and 2. The breakthrough curve for the segregated bed is located in between those for the two monodispersed fluidized beds. In Figure 5a, on the other hand, the steep rise in the breakthrough curve for the segregated bed is evident, but the location of the curves is not in that order. A similar analysis was performed by using a total of 100 g of clinoptilolite in the fluidized beds under the same operating conditions as for expts 9, 5, and 6. Figure 8b shows the breakthrough curves for expts 10, 3, and 4. The general trend of the breakthrough curves remained basically unchanged when the mass of clinoptilolite was halved. Again, the monodispersed fluidized bed with finer particles (expt 3) had the best effluent quality at all t/tf values; similarly, the monodispersed fluidized bed with the coarser particles (expt 4) had the worst. Furthermore, the breakthrough capacity/total capacity was the lowest for expt 4. The segregated fluidized bed (expt 10) and the monodispersed fluidized bed with the finer particles (expt 3) had the same value (0.45) for the breakthrough capacity/total capacity as listed in Table 1. These values are all lower than those for the corresponding experiments with 200 g of clinoptilolite (expts 9, 5, and 6), and this shows the importance of the mass since it implicitly affects the bed height, the contact time, and the liquid dispersion. When Figure 8a is compared to b, it is evident that the location of the breakthrough curve for the segregated fluidized bed is again between those for the two monodispersed fluidized beds. Such a conclusion is not possible from the BVs plots in Figure 5a and b. 3.3.4. Comparison of Fluidized Beds with Packed Beds: C/C0 versus t/tf Plots. Some packed bed experiments were performed to be compared with the fluidized bed experiments. Results and operating conditions of the packed beds are listed in Table 3 together with a summary representation of the fluidized bed experiments. The breakthrough curves of these experiments with respect to the dimensionless time (t/tf) are shown in Figure 9. A 6400
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Figure 9. Breakthrough curves for expts 5, 6, 9, 13, 14, and 15 with respect to t/tf.
segregated fluidized bed and two fluidized beds with uniform particle sizes are included in Figure 9 for comparison. Experiment 13 was a packed bed with 100 g of 500600 μm particles and an empty bed contact time of 1.7 min (102 s). The feed flow rate was 4.4 L/h. As shown in Table 3, this packed bed had a breakthrough capacity/total capacity of about 0.5. This means that when breakthrough was observed, the column had only filled up half of its operating capacity. As shown in Figure 9, the exit concentration rises steadily until about t/tf = 0.6, after that the remaining column capacity is filled up very slowly. Experiment 14 was a segregated packed bed with 100 g of 500600 μm particles in the lower part and 100 g of 250 350 μm particles in the upper part. The feed flow rate was 1.3 L/h and the empty bed contact time was 11.7 min, the longest among all experiments. Experiment 14 showed a very sharp breakthrough as seen in Figure 9 when t/tf was about 0.15. The feed flow rate was much lower than that for expt 13, and the column contained twice as much clinoptilolite, the ratio of the breakthrough capacity to the total capacity was about 0.78. This showed that the column capacity was used up very efficiently. Experiment 15 was also a segregated packed bed which contained 100 g of 8401000 μm particles in the lower part and 100 g of 500600 μm particles in the upper part. The feed flow rate was 4.4 L/h with an empty bed contact time of 3.7 min. A very sharp breakthrough was also observed with this packed bed. The ratio of the breakthrough capacity to the total capacity was 0.63. The packed bed with the shortest contact time (expt 13) performed the poorest in terms of breakthrough capacity/total capacity. This agrees with the generally accepted notion that increased contact time increases breakthrough capacity in packed beds.24 Experiments 13 and 15 had the same feed flow rate, however, expt 13 contained only 100 g of clinoptilolite where expt 15 contained twice as much. Therefore, the better performance of expt 15 can be attributed to the increased contact time. Since expt 14 also had a better breakthrough capacity than expt 13 as seen in Table 3, the increased contact time definitely improved the breakthrough capacity and also the ratio of the breakthrough capacity to the total capacity. This last fact is also apparent from the shape of the breakthrough curve for expt 13 in Figure 9. It is clearly seen in Figure 9 that the two packed beds with the longer contact times (expts 14 and 15) behave differently from fluidized beds because of the sudden rise in the exit concentration after breakthrough. The packed bed with the shortest contact
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time (expt 13) behaves similar to a fluidized bed both in terms of the capacity at breakthrough and also by showing a relatively slow rise in the exit concentration after the breakthrough. As can be seen in Figure 9, a segregated fluidized bed (expt 9) resembles a packed bed more because it shows a steeper rise in the exit concentration than the fluidized beds with uniform particle sizes (expts 5 and 6). Moreover, for the segregated fluidized bed (expt 9), the breakthrough capacity/total capacity was higher than that for the packed beds with the shorter contact times (expts 13 and 15) even though the feed flow rate was higher by 8.95 to 30 times. This is a major advantage implicating that feed streams with very high flow rates can be used for ion exchange in segregated fluidized beds consisting of the smaller particle size mixture without any loss in the column efficiency in terms of the breakthrough capacity/total capacity. The problem of frequent regeneration can be balanced by the advantage of the accommodation of waste streams with high flow rates. According to Table 3, segregated fluidized beds with the finer particle size composition (expts 7, 8, and 9) have column efficiencies comparable to the packed bed with a contact time of 1.7 min. Monodispersed fluidized beds also have comparable efficiencies depending on the particle size and flow rate used. The visual information in Figure 9 regarding the column efficiencies is confirmed by the related rows in Table 3. Table 3 also shows the drastic differences between the contact times for packed and fluidized beds which have similar column efficiencies. The advantage of using a segregated fluidized bed becomes apparent in Figure 9 because the shape of the breakthrough curve is similar to that of the packed beds with the longer contact times. Using a segregated fluidized bed for ion exchange clearly improves the breakthrough capacity/total capacity compared to monodispersed beds. A performance similar to that in fixed beds can be achieved in fluidized beds by means of segregation together with the proper choice of the remaining operating variables The effect of the particle size seems to be much more important in fluidized beds than in packed beds because of the significantly lower contact times in fluidized beds. Experiments 14 and 15 listed in Table 3 were packed beds with different particle size compositions and very different contact times; however, their breakthrough performances were quite similar on the C/C0 versus t/tf plot. Increasing the particle size drastically deteriorates the breakthrough capacity/total capacity of a fluidized bed, whether segregated or monodispersed, when the contact time is not increased by using more material in the column leading to a larger expanded bed height. The loading values for the packed bed experiments given in Table 3 are comparable with those for the monodispersed fluidized bed in expts 1 and 2. The loading values for all other fluidized bed experiments are much higher, however, as the values suggest, column efficiencies may not only depend on the flow rates or loadings, they depend on the flow rates and the flow properties as mentioned earlier. For the packed bed experiments (expt 1315), column efficiency decreases with increasing loading value. Overall, one can conclude that by the correct choice of the particle size, flow rate, and amount of bed material, ion exchange can be carried out in fluidized beds and satisfactory breakthrough capacities can be achieved. The dimensionless representation of the breakthrough curves as C/C0 versus t/tf allows the inclusion of any experiment with different masses, flow rates, contact times, configuration, and particle composition on the same plot. This representation is a useful tool which provides both visual and numerical information about the efficient use of an ion exchange column. 6401
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Industrial & Engineering Chemistry Research 3.3. Engineering Implications. The results of this study suggest that fluidized beds can be an alternative to packed beds based on their advantages and disadvantages. One major advantage is the possibility of handling high ion loadings due to high flow rates. A high flow rate in a fluidized bed means a short contact time, which as shown in this study is not a disadvantage when other parameters such as particle size are carefully chosen. However, frequent regeneration will be necessary in fluidized beds with high flow rates because the breakthrough time is much shorter when compared to packed beds as shown in Table 3. The use of particle sizes finer than about 0.3 mm is possible and favorable in fluidized beds; this may be a beneficial area of use for the fine sizes generated during the crushing and sieving operations for the regularly used fixed bed grain sizes. Fluidized beds will also avoid the problem of clogging when unfiltered feeds are used. The column efficiencies in terms of the breakthrough capacity/total capacity suggest that a scale-up to handle a higher wastewater flow rate may be possible by using a fluidized bed instead of a packed bed with a larger volume. Further, the use of a segregated fluidized bed may be suggested in place of a monodispersed fluidized bed to increase the breakthrough capacity/ total capacity instead of increasing the contact time by using more material in the column. An initial decrease in the breakthrough capacity with increasing flow rate is observed to be followed by an increase at much higher bed expansion values as observed with the preliminary data obtained in these experiments, which needs further investigation. In summary, the choice of flow rate, contact time, particle size, and segregation should be evaluated in a holistic manner rather than as individual parameters to optimize system performance in fluidized beds.
4. CONCLUSIONS This study leads to the conclusion that by the correct choice of particle size,flow rate, and amount of bed material, ion exchange can be carried out in fluidized beds with a satisfactory performance. Segregation in fluidized beds achieved by means of two nonmixing particle sizes is a valid operation for ion exchange as studied on the example of the ammonium ion exchange on clinoptilolite in this study. Segregation improves the breakthrough capacity/total capacity in fluidized beds. Performances comparable to those in packed beds can be achieved in fluidized and segregated fluidized beds with the additional advantage of accommodating very high flow rates despite very short contact times. Further research is needed to fully explain the relationship between flow rate and breakthrough capacity. The use of smaller particles is essential for fluidized bed ion exchange because of the very short contact times. Breakthrough capacity/total capacity is improved when smaller particle sizes are chosen which are not preferred in packed beds due to clogging and high pressure losses. Very high feed rates are possible with the use of finer particles in fluidized beds without loss of performance if the particle size, mass of solids, and other operating parameters are carefully chosen. Recycling fluid to fluidize the bed with lower fresh feed flow rates did not have an extremely adverse effect on the column performance, yet the effect of the recycle flow to fresh feed flow ratio needs to be carefully investigated.
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The agreement of the results obtained in this study with the representation of the breakthrough curves in terms of C/C0 versus a dimensionless time t/tf plot suggests that these plots are a valuable tool for comparing performances for a wide variety of experimental conditions which would not be possible with a C/ C0 versus time or BVs representation. To conclude, although more research would be desirable within this subject matter and despite more frequent need of regeneration, the results of this work have provided positive incentive on the use of fluidized beds as a viable alternative to fixed bed operation, especially at elevated flow rates.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ902122857063. Fax: þ902122856545. E-mail: erbila@ itu.edu.tr.
’ ACKNOWLEDGMENT :: : We gratefully acknowledge the financial support of TUB ITAK (The Scientific and Technological Research Council of Turkey) to the project 103I016. ’ NOMENCLATURE BVs settled bed volume, (Q 3 t)/(Π 3 Dc2/4) 3 Hsettled, C concentration, mg/L feed concentration of NH4þ ion, mg/L C0 particle size, μm dp column diameter, cm Dc E bed expansion, 100 3 (Hfluidized Hsettled)/Hsettled, % fluidized bed height, cm Hf Q feed flow rate, L/h t time, min empty bed contact time, Hfluidized 3 (Π 3 Dc2/4)/Q, min tc time passed until the column capacity is exhausted, min tf Q 3 C0 3 t amount of ion fed to the column until time t, mg Q 3 C0 3 tf total amount of ion fed to the column until column exhaustion, mg meq/g milliequivalents of ammonium per gram zeolite Greek Letters
ε
mean bed porosity (voidage) or void volume, -ε = volumevoids/volumecolumn = 1 {(mass/wet density)zeolite/ Hfluidized 3 (Π 3 Dc2/4)}
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