Contact Operations

Continuous-contact operations are diametrically different from staged operations in almost every aspect. The two phases are in continuous flow and in continuous contact with each other, rather than repeatedly separated and re-contacted in an array of stages. Second, the attainment of equilibrium is shunned. An active driving force is maintained at all times and its constituent concentrations vary continuously from the point of entry to the exit. The result is that the concentrations are now distributed in space and, assuming a normal steady-state operation, are invariant in time. Thus, while staged operations vary at most with time, but not at all with distance, the exact opposite holds in continuous-contact operations. 

In the following, we divide continuous-contact operations into two distinct categories. The first deals with classical packed-column operations in countercurrent flow. We revisit the packed-gas scrubber we first saw in Chapter 2 and provide a general survey of packed-gas absorption operations. Packed-column distillation is addressed next and, in a somewhat unusual departure from the norm, we reexamine coffee decaffeination by supercritical extraction. 

The process involves a moving-bed configuration, which althougji superficially different from the conventions of packed column processes has identical operational parameters and can be analyzed by identical procedures. 

The second category involves membrane processes, a contemporary topic of considerable importance with the promise of a bright future. Among the subcategories considered here are reverse osmosis, hemodialysis, and membrane gas separation, and the text provides some useful relations to address problems in these areas. 

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A gas-sohds contacting operation in which the sohds phase exists in a dilute condition is termed a dispersion system. It is often called a pneumatic system because, in most cases, the quantity and velocity of the gas are sufficient to lift and convey the sohds against the force of gravity. Pneumatic systems may be distinguished by two characteristics ... 

Introduction Insoluble hquids may be brought into direct contact to cause transfer of dissolved substances, to allow transfer of heat, and to promote chemical reaction. This subsection concerns the design and selection of equipment used for conduc ting this type of liquid-hquid contact operation. 

Objectives There are four principal purposes of operations involving the direct contact of immiscible liquids. The purpose of a particular contact operation may involve any one or any combination of the following objectives ... 

Cooling or heating a liquid hy direct contact with another Although liquid-hquid-contact operations have not been used widely for heat transfer alone, this technique is one of increasing interest. Applications also include cases in which chemical reaction or hquid extraction occurs simultaneously... 

Contact operations to start the pump and allow the stuffing box to leak freely. Tighten the gland bolts one flat at a time until the desired leakage is obtained and the pump runs cool. 

In Vermeulen s work, a paddle impeller stirred fixed amounts of gas and liquid in a closed vessel. When the impeller was brought to the proper speed (240-360 rpm), the liquid and the gas that had been above it were dispersed together and completely filled the vessel. It is impossible to extrapolate from this experimental set-up to the usual type of gas-liquid contacting operation. 

Hooke, C. J., Surface Roughness Modification in Elastohydrodynamic Line Contacts Operating in the Elastic Piezovis-cous Regime," Proc. Inst. Mech. Eng., PartJ.J. Eng. Tri- [69] fooZ.,Vol.212,1998,pp.l45-162. 

Condensation Scrubbing The collection efficiency of scrubbing can be increased by the simultaneous condensation of water vapor from the gas stream. Water-vapor condensation assists in particle removal by two entirely different mechanisms. One is the deposition of particles on cold-water droplets or other surfaces as the result of Stefan flow. The other is the condensation of water vapor on particles as nuclei, which enlarges the particles and makes them more readily collected by inertial deposition on droplets. Both mechanisms can operate simultaneously. However, for the buildup of particles by condensation to be effective, there must be adequate time for the particles to grow substantially before the principal gas-liquid-contacting operation takes... 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

The labyrinth compressor is suitable for gases of higher density (molecular mass > 15 kg / kmol). The labyrinth-seal of the piston (see Section 4.1.5 ) is non-contacting, operates without... 

Many operators, like the Fermi-contact operator, changes both the spatial-and spin functions. 

Barone and coworkers250 also determined EPR hyperfine splittings nN of the radical 40 at the UMP2/DZ + P level of theory using the Fermi contact operator and a finite field method with an increment size of 0.001 a.u. Expectation values of aN, < aN >, at higher temperatures T were calculated by assuming a Boltzmann population of vibrational levels according to equation 23 ... 

Curve B of Figure 2 is typical of gas-liquid contacting operations. Here the rate pf mass transfer between phases increases to a maximum at small impeller diameter and then decreases as impeller diameter is increased. The significance is that more turbulence is available with the small impeller and that turbulence is more important than flow in this operation. 

A gas-liquid contact operation is illustrated in Figure 3.8. Gas is contacted with a liquid from a spray, resulting in both diffusion and heat transfer between the gas and liquid. The gas exits the system at conditions of humidity and temperature quite different from the entrance conditions. Assume the operation to be adiabatic. Perform a material and energy balance for the system. 

We note from Figure 1 that scalar relativistic calculations (7) are entirely unable to reproduce the experimental trend for X = Br and I. Indeed, scalar relativistic and non-relativistic calculations gave almost identical results in this case (7). However, with the inclusion of spin-orbit/Fermi contact operators, the experimental trend is reproduced nicely (9). 

Wear is not a direct property of polyurethane but is the result of a complex system of materials being in contact, geometry of contact, operating conditions, and environment (Mardel et al., 1995). Polyurethanes find successful... 

Systems that exhibit behavior of the type illustrated in Fig. 4 cannot be purified in a single crystallization stage. They represent situations in which multiple stages or continuous-contacting devices may be useful. The principles of such operations are analogous to those of other countercurrent contacting operations—for example, distillation, absorption, and extraction. 

Figure 1.9 Arc energy A, nitrous oxide production S and yield y of nitrous oxides in a contactor with contacts operated in air at normal ambient pressure, switching off a three-phase system with 1000 V, 260A rms at 50cps, versus the ratio resistance R to impedance Z (or versus the phase angle

Flameproof components (Fig. 6.100) show a body of insulating material, acting both as a mechanical support for terminals, magnetic coils, switching contacts, operating rods or spindles and as a flameproof enclosure for the... 

T he solubility data of gases in aqueous mixed-salt solutions are funda-mentally important in research on liquid-gas mass transfer and in designing gas-liquid contacting operations, especially gas absorption accompanied by chemical reaction. 

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