Principles of Expanded-Bed Operation

The expanded bed is, in principle, similar to a fluidized bed, a common unit-operation in the chemical industry.3 However, in the expanded-bed method, mixing of the adsorbent material is minimal during the operation, whereas a fluidized bed is turbulent. This means that an expanded bed is more effective for adsorption and elution processes than the fluidized bed. A complete review of all engineering aspects, comparing expanded beds and fluidized beds, can be found in Thommes.4 The expanded-bed adsorption operation is illustrated in Fig. 2. 

The overall operation is comprised of several phases. In the first phase, the adsorbent material is expanded and equilibrated by applying an upward liquid flow to the column. To allow for sufficient contact time and efficient binding of the target molecule, the expansion should be three times that of the sedimented bed ( expansion ratio ), to a height of approximately 50 cm. A stable bed is formed when the adsorbent particles achieve equilibrium between particle sedimentation velocity and upward liquid flow velocity. In the second phase, the sample is applied to the expanded adsorbent. The crude, unclarified protein solution of intact or disrupted biomass is pumped upward on the column. In a well-defined process, the expanded adsorbent will remain stable and will not change its expansion ratio. However, if the 

Sedimented gel volume (mL) % Average particle size (pm) Density (gm/mL) 

---

Sections I-V addressed the principles of expanded-bed operation, the instrumentation, and the special matrices. This section focuses on applications of the expanded bed, describing actual processes and providing operational guidelines. 

---