1 (5.28)
The unused length, HUNB, represents the mass transfer zone. It depends on the fluid velocity, and it is essentially independent of the column’s total length. HUNB may be experimentally obtained in laboratory columns, of small diameter, packed with the adsorbent being studied. This means that the usable part of the bed may be calculated for large-scale equipment by means of simply calculating the length of the column, HB, used until the break point is reached. HB is directly proportional to tb. Finally, the total length of the column, HT, can be obtained simply by adding HB and HUNB.
This procedure is widely used to scale packed beds, and its validation depends on the working conditions of the laboratory column being similar to those of a large-scale unit. The small diameter of the first column is necessary to ensure that simi-larity with the large adsorption towers occurs, once in these cases the process is essentially adiabatic. The mass flow rate in both cases must be equal, and the bed should be sufficiently long to contain a mass transfer region at steady state. Axial dispersion or axial mixing are not the same in both conditions, but if care is taken, this is a very useful method.
5.7.3 adsorptionin expanded Beds
Adsorption in expanded beds is a one-step operation where unwanted compounds, such as protein, are adsorbed from a particularly dirty feed with no need for its clarification, centrifugation, and initial purification, which are, in general, demanded operations for fixed-bed processes (Figure 5.11). The expansion of the bed creates a distance between the adsorbent particles—that is, it increases the bed porosity, which in turn opens the way to cells, cell fragments, and other particles during the application of the solution.
The principle of adsorption in expanded bed considers that the adsorbent is expanded and equilibrated by the application of an ascending flow in the column. A stable fluidized bed is formed when the adsorbent particles are suspended in equilib-rium due to the balance between the settling velocity of the particles and the velocity of the liquid flow.
The unprocessed feed is subsequently fed to the expanded bed, maintaining the same liquid flow rate. Proteins adsorb (by ionic exchange, hydrophobic interactions, affinity) to the adsorbent and fragments of cells, whole cells, particles, and contami-nants pass through the bed. Once the adsorption step is finished, the materials that are loosely bound are washed with the initial buffer solution; when this operation has ended, the flow is stopped, therefore promoting the sedimentation of the adsorbent
Chromatographic Techniques Applied to Dairy Product Manufacturing 105
particles. This procedure is followed by the elution of the adsorbed proteins; their adsorption is promoted by the reverse flow of a buffer solution containing the appro-priate salt concentration.
Following the elution step, the sedimented bed is regenerated by a descend-ing flow of buffer solutions, specifically chosen for the chromatographic principle applied in the separation. This regeneration removes those proteins that are more strongly bound and therefore were not removed during the elution phase. Finally, a cleaning-in-place procedure is applied to remove nonspecifically bound, precipi-tated, and denaturated substances, so that the original performance of the adsorbent is restored.
5.7.3.1 stable Fluidization
Adsorption in expanded beds is based on the control of a stable fluidization, which combines the hydrodynamic properties of a fluidized bed with the chromatographic properties of a fixed bed. The achievement of stable fluidization with minimal back-mixing, vortexing, and turbulence in the bed leads to the formation of several mass transfer units, or theoretical plates, in the expanded bed, thus increasing the perfor-mance of a traditional packed column.
Fixed-Bed Process
Fermentation Fermentation
Clarification Concentration
Initial Purification Intermediate
Purification
Intermediate Purification
Final Purification Final
Purification
PRODUCT PRODUCT
Expanded-Bed Process
Expanded Bed
economy in process steps
FIGuRE 5.11 Comparison between the purification processes using adsorption in fixed bed and in expanded bed.
106 Engineering Aspects of Milk and Dairy Products
5.7.3.2 Critical Parameters
In addition to the adsorbent and the column, there are other critical parameters in adsorption which may be divided in physical and chemical parameters. Chemical parameters are those related with selectivity and volumetric capacity of the separa-tion processes and include pH, ionic strength, and types of ions and buffers used.
The influence of these parameters in the separation performance of expanded-bed adsorption and traditional fixed-bed chromatography is virtually the same. Physical parameters are those related with hydrodynamics and stability of a homogeneous fluidization in the expanded bed. Some physical parameters are related with the con-centration of the bulk, such as viscosity or density; others are related with operation conditions, such as temperature, flow velocity, and bed height.
In general, chemical parameters are optimized in fixed-bed operation, and physi-cal parameters are optimized in expanded-bed operation, once these are related with the hydrodynamic properties of the bed.
Example 5.1
b-Lactoglobulin, the main allergenic component of cheese whey, will be partially removed from 1000 L of whey by a batch adsorption process. Whey has 3.5 g/L of that protein, and the process will use 15 kg of an adsorbent that behaves according to the Langmuir isotherm model. Knowing that the whey and the adsorbent will be kept in contact until equilibrium is reached, determine the remaining concentration of b-lactoglobulin in the whey and the efficiency of the process.
Given:
Solution: assuming that the amount of adsorbed protein will not significantly change the total volume of whey and the total weight of adsorbent, the mass balance for this process will be
The diagram of q versus c, shown in Figure 5.12, is a graphical representation of the mass balance and the adsorption isotherm for the case evaluated in Example 5.1.
From this graphical representation, it is possible to conclude that for the given oper-ational conditions, equilibrium is achieved for a concentration of b-lactoglobulin in the whey (Ceq) equal to 1.2 g/L and a concentration in the adsorbent (qeq) of 155 g/kg. This means that the efficiency of the process equals
E C S C S
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