Unlimited flow conditions

A Maxwell element shows an instantaneous elastic deformation and thereafter unlimited flow. For polymers in the solid condition the latter is not realistic. For a fluid polymer it is more relevant moreover, the instantaneous elastic deformation is in accordance with the real behaviour when the stress is released a polymer fluid shows an instantaneous recoil. 

By interchanging the flow conditions of the adsorption and desorption sections, one obtains an enriching unit, as sketched in Figure 12.4. In an enriching unit the extent of enrichment is unlimited but stripping is limited. 

The great advantage of this arrangement is that once the flow rate and temperature are stabilized and invariant operation is achieved, the composition at the outlet of the reactor does not change with clock time. The PFR is therefore a steady state reactor. This allows us to sample repeatedly at a given condition and verify our results. In terms of BR operation, it is as if we had frozen the progress of the reaction at some level of conversion and can now allow ourselves unlimited quantities of sample for analysis as the product issues out of the PFR. Arguably, this is the principal reason for the dominance of the PFR over the BR in kinetic studies. This ideal situation is achieved only under certain conditions. Care must therefore be taken that the PFR is built and operated so as to approach the required conditions as closely as possible. 

With the advent of reliable, commercially available CFD software has come an increasing interest in the simulation of the flows in large scale, geometrically complex, industrial vessels (Bakker and Fasano [1], Colenbrander [2] and Sharratt [3] for some examples). CFD simulations provide access to almost unlimited data on the flow field, infinite variations on the geometry, infinite scale-up possibilities, and visually appealing results which are easily digested by everyone involved with the process. In addition, the numerical experiments performed using CFD seem to require much less investment than wet experimental work, which may be difficult or impossible under hostile plant conditions. Like traditional experimental work, however, CFD has its limitations. 

When compared to the solid state filters, liquid-phase retention systems prove to have several advantages. The heat introduced by the venting flow as well as by the decay of the absorbed radionuclides is passively removed by boiling of the water phase, the load capacity for aerosols is virtually unlimited, and the retention of volatile iodine compounds can be guaranteed by appropriate chemical conditioning of the water phase. An important prerequisite for high retention efficiency, besides the thermal and radiation stability of the chemicals applied, is a very intimate contact between the gas-steam flow and the liquid phase of the retention system, in order to obtain a fast and effective exchange of matter. 

For a large liner surface with an unlimited laterally extended gap, the water in the gap spreads as far as the pressure subsides to zero at some radius Rw and then the lateral flow succumbs. The boundary conditions, which describe this case, must therefore be formulated as follows ... 

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