Stirred cell

Interfacial Contact Area and Approach to Equilibrium. Experimental extraction cells such as the original Lewis stirred cell (52) are often operated with a flat Hquid—Hquid interface the area of which can easily be measured. In the single-drop apparatus, a regular sequence of drops of known diameter is released through the continuous phase (42). These units are useful for the direct calculation of the mass flux N and hence the mass-transfer coefficient for a given system. 

Danckwerts and Gillham did not investigate the influence of the gas-phase resistance in their study (for some processes gas-phase resistance may be neglected). However, in 1975 Danckwerts and Alper [Trans. Tn.st. Chem. Eng., 53, 34 (1975)] showed that by placing a stirrer in the gas space of the stirred-cell laboratoiy absorber, the gas-phase mass-transfer coefficient fcc in the laboratoiy unit could be made identical to that in a packed-tower absorber. When this was done, laboratoiy data obtained for chemically reacting systems having a significant gas-side resistance coiild successfully be sc ed up to predict the performance of a commercial packed-tower absorber. 

The simplest ultrafiltration is the stirred cell, a batch operation. The most compex is a continuous stages-in-series operation incorporating diafiltration. Industrial practice incorporates the full gamut of complexity. 

These concepts were implemented according to the following scheme the liquid element surrounding the bubble and the bulk are considered as two separate dynamic reactors that operate independent of each other and interact at discrete time intervals. In the beginning of the contact time, the interface is being detached from the bulk. When overcome by the bubble, it returns to the bulk and is mixed with it. Hostomsky and Jones (1995) first used such a framework for crystal precipitation in a flat interface stirred cell. To formulate it for a... 

The intrinsic kinetics of the reactions taking place in the scrubber, i.e. the reaction of NO with the iron chelate forming an iron nitrosyl complex (eq. 1) and the undesired oxidation reaction of the iron chelate (xanpla (eq. 2) wae deteimined in dedicated stirred cell contactors. Typical process conditions were T = 25-55 °C [Fe"(EDTA) "] = 1-100 mol/m [NO] = 1-1000 ppm pH = 5-8 and an oxygen level ranging between 1 and 20 vol%. 

Laboratory reactors for studying gas-liquid processes can be classified as (1) reactors for which the hydrodynamics is well known or can easily be determined, i.e. reactors for which the interfacial area, a, and mass-transfer coefficients, ki and kc, are known (e.g. the laminar jet reactor, wetted wall-column, and rotating drum, see Fig. 5.4-21), and (2) those with a well-defined interfacial area and ill-determined hydrodynamics (e.g. the stirred-cell reactor, see Fig. 5.4-22). Reactors of these two types can be successfully used for studying intrinsic kinetics of gas-liquid processes. They can also be used for studying liquid-liquid and liquid-solid processes. 

Another device that finds frequent use is the stirred cell shown in Fig. 20-54. This device uses a membrane coupon at the bottom of the reservoir with a magnetic stir bar. Stirred cells use low fluid volumes and can be used in screening and R D studies to evaluate membrane types and membrane properties. The velocity profiles have been well defined (Schlichting, Boundary Layer Theory, 6th ed., McGraw-Hill, New York, 1968, pp. 93-99). 

FIG, 20-54 Stirred cell TFF module. (Courtesy MilUpore Corporation.)... 

Some of the earliest attempts to address the difficulties associated with making kinetic measurements at immiscible liquid-liquid interfaces were made by Lewis [16,17] using the stirred cell design illustrated in Fig. 2. The Lewis cell employs direct contact between the two immiscible liquids, and reaction rates are evaluated by measuring concentration changes in the bulk of one of the two phases, usually by a batch extraction technique. The rate of change of concentration, dc/dt, is related to the interfacial reaction flux, 7, by... 

The study of transport between immiscible phases requires a device which (1) yields contacting surfaces with known contact area, (2) has a stationary interface, and (3) has known hydrodynamics. Stowe and Shaewitz [6] developed and evaluated a cell which meets these criteria. Each liquid is stirred with a rotating disk. As noted by these investigators, if the fluids are rotated in opposite directions by the rotating disks, then the torque on the interface cancels. The criterion for operation of the stirred cell with a stationary fluid interface is... 

Liquid/liquid stirred cell (free boundary method) Mass transport between immiscible phases 6-8... 

L Stowe, J Shaewitz. Hydrodynamics and mass transfer characteristics of a liquid/ liquid stirred cell. Chem Eng Commun 11 17, 1981. 

Figure 4.4. High-Output Stirred Cell from Millipore. [12]...

Ultrafiltration was carried out in an Amicon stirred cell, 65 ml capacity, at 4 C with continuous stirring under C02 pressure until 25 ml filtrate had been collected. MWC0 = nominal molecular weight cut-off of the membrane. 

The absorption of carbon dioxide into aqueous amine solutions has been investigated by Danckwerts and McNeil(42) using a stirred cell. It was found that the reaction proceeded... 

Each point-by-point experiment requires a complete set of mixing, separation, sampling, and analysis. This usually leads to scattered results, though it may not be critical, if the D values cover a limited range from 0.1-10. However, the more the D values deviate from 1, the more accurate must be the measurements also the number of points required for a reliable extraction curve usually increases. To reduce the uncertainty and labor involved with the batch technique, the stirred cell technique has become popular. 

Eigure 4.32 shows a typical thermostated stirred cell. The liquid volumes are commonly 50 -i- 50 mL. The stirrer may consist of a single paddle at the interface, or a double paddle, one in the center of each phase. The cell may contain baffles, etc., to improve the mixing. Stirring may be violent, completely destroying the interface and producing very small ( 1 mm) droplets, or slow... 

Fig. 5.6 Typical extraction rate vs. stirring rate for constant interfacial-area-stirred cell.

In the case of constant interfacial-area-stirred cells, although zone A is certainly an indication that the process is controlled by diffusional processes, the opposite is not true for zone B. In fact, in spite of the increased stirring rate, it may happen that the thickness of the diffusion films never decreases below a sufficiently low value to make diffusion so fast that it can be completely neglected relative to the rate of the chemical reactions. This effect, sometimes called slip effect, depends on the specific hydrodynamic conditions of the apparatus in which the extraction takes place and simulates a kinetic regime. 

Constant interfacial-area-stirred cells Those techniques in which the two phases can be stirred to a variable extent, while a known and constant interfacial area is maintained. 

Fig. 5.11 Constant interfacial-area-stirred cell ( Lewis-type cell ) for hquid-hquid extraction kinetic measurements.

Prq[iaration of clarified digests sludge. Samples were removed under anaerobic conditions and moved immediately into the anaerobic chamber. Before analysis, the samples were clarified by centrifugation and filtration with 0.22-jLtm Acrodisc filters. Concentrated digester supernatant was also examined, and it was prepared by ultrafiltration in the anaerobic chamber with an Amicon stirred cell ultraconcentrator and Amicon PMIO membranes. 

Fig. 5. (a) Unstirred cell, (b) Stirred cell. Full arrows show eddies produced directly by stirrer, broken arrows show eddies induced by momentum transfer across the turbulent liquid interface. 

For plane, unstirred cells, the theoretical equations are (21) and (25) and for plane stirred cells there are the theories of Lewis and Whitman, Kishinevskii, Danckwerts, and Levich, and Eqs. (14) and (16) of the present writer. In particular, if Eqs. (7) and (11) represent processes... 

Vigorously stirred cells have liquid-phase resistances much lower than for unstirred cells, and are therefore useful in investigating interfacial resistances. In practice, Ri and R2 can be made quite small typical values are 10,000 sec. cm. to 100 sec. cm. , depending on the rate of stirring. Against this advantage must be set the complications of the turbulent flow the eddies near the interface are likely to be affected by the proximity of a monolayer. 

A different empirical correlation of the transfer across a clean interface in a stirred cell comes from the author s laboratory (Mayers (90)) ... 

Study in a stirred cell of the transfer of uranyl ions from water to organic solvents confirms the result for unstirred cells that transfer is faster than theoretical when interfacial turbulence is visible (53). After long times, in systems showing no visible turbulence, the transfer coefficients decrease, becoming less than those calculated from Eqs. (25) and... 

---