The Annulus

An electrochemical reactor containing two concentric cylindrical electrodes (Fig. 2.7), with or without a diaphragm, represents a practical and attractive geometry since it offers a uniform primary current distribution (see Chapter 5). Axial flow through the annular space between the two electrodes, or an electrode and a diaphragm, has characteristics between those of a pipe and a rectangular channel. A theoretical analysis for the condition of laminar or turbulent flow is approached in the same way as that for pipes and channels hence, we merely identify appropriate experimental correlations. 

FIGURE 2.7. Annulus between two concentric cylinders with axial flow. r,. = Radius of inner cylinder = radius of outer cylinder. 

The correlations provide good predictions and are suitable for electrode lengths of up to about 20d. Equation 2.21 can also be used for developing laminar flow. For design purposes, entry effect can be ignored. 

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In the event of a sudden loss of mud In an Interval containing overpressures the mud column in the annulus will drop, thereby reducing the hydrostatic head acting on the formation to the point where formation pressure exceeds mud pressure. Formation fluids (oil, gas or water) can now enter the borehole and travel upwards. In the process the gas will expand considerably but will maintain its initial pressure. The last line of defence leff is the blowout preventer. However, although the BOP will prevent fluid or gas escape to the surface, closing in the well may lead to two potentially disastrous situations ... 

The VC filter (Fig. 22) consists of two concentric hoUow cylinders mounted hori2ontally on a central shaft. The inner cylinder is perforated and carries the filter cloth, the outer cylinder is lined on the inside with an inflatable diaphragm. The slurry enters into the annulus between the cylinders and... 

BI SERT Blenders. The design of a BINSERT blender consists of a hopper-within-a-hopper, both of which ate usually conical ia shape (Fig. 15). Particles flow through the inner hopper as well as through the annulus between the inner and outer hoppers. By varyiag the relative position of these two hoppers as well as the configuration of the outlet geometry, it is possible to achieve between a 5 1 and 10 1 velocity differential between particles ia the inner hopper compared to particles ia the outer annular region (7,17). 

Fittings contribute strongly to the pressure drop on the annulus side. General methods for predicting this are not reliable, and manufacturer s data should be used when available. 

Historically the most common gas disperser for cross-flow plates has been the bubble cap. This device has a built-in seal which prevents liquid drainage at low gas-flow rates. Typical bubble caps are shown in Fig. 14-20. Gas flows up through a center riser, reverses flow under the cap, passes downward through the annulus between riser and cap, and finally passes into the liquid through a series of openings, or slots, in the lower side of the cap. 

When pumping down the draft tube, flow normally makes a more troublefree velocity change to a flow going up the annulus. Since the area of the draft tube is markedly less than the area of the annulus, pumping up the draft tube requires less flow to suspend sohds of a given settling velocity than does pumping down the draft tube. 

The suction pressure generated is equal to the liquid head difference across the annular baffle as magnified by G. The maximum suction that can be realized, assuming neghgible vapor pressure, is I atm. For example, with a differential liquid level of 2 cm (0.79 in) of water across the annulus under 500g, the suction generated by the siphon is 0.98 atm. 

The main bus and its enclosure is normally in a tubular form, in view of the advantages of a tubular section, as discussed already (concentration of current in the annulus. 

Locate and eliminate contact. If not possible for structural reasons— depending on the individual case—take other protection measures, e.g. insert hydraulic sealing medium or organic materials into the annulus. 

With electrical contact of pipe and casing, the absence of electrolyte within the annulus is not detectable from the outside by electrical measurements. 

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