Continuous-contact equipment

For continuous contact equipment the material and energy balances are written around a section of column of differential height as shown in Figure 12.8. For the vapor phase the component material balance reads... 

Density. A difference in densities of the contacted phases is essential and should be as great as possible. Not only is the rate of disengaging of the immiscible layers thereby enhanced, but also the capacity of the contacting equipment is increased. It is insufficient to examine merely the relative densities of the solution to be extracted and the pure extracting solvent, since on admixture mutual solubility of the two will alter the densities for continuous contacting equipment, it is important to be certain that a satisfactory density difference for the contacted phases exists... 

In stagewise operations the time of contact is intimately connected with stage efficiency, whereas for continuous-contact equipment the time leads ultimately to the volume or length of the required device. The factors which help establish the time are several. Material balances permit calculation of the relative quantities required of the various phases. The equilibrium characteristics of the system establish the ultimate concentrations possible, and the rate of transfer of material between phases depends upon the departure from equilibrium which is maintained. The rate of transfer additionally depends upon the physical properties of the phases as well as the flow regime within the equipment. 

Density, A difference in densities of the saturated liquid phases is necessary, both for stagewise and continuous-contact equipment operation. The larger this difference the better. In systems of the type shown in Fig. 10.3, the density difference for equilibrium phases will become less as C concentrations increase and will be zero at the plait point. It may reverse in sign before reaching the plait point, in which case continuous-contact equipment cannot be specified to operate at the concentrations at which the density difference passes through zero. 

Equipment for continuous steady-state operations can be broadly classified into two major categories, according to whether it operates in stagewise or in continuous-contact fashion. Stagewise equipment is sometimes assembled in multiple units to produce multistage effects, whereas continuous-contact equipment may provide the equivalent of many stages in a single device. 

Liquid-hquid contacting equipment may be generally classified into two categories stagewise and continuous (oifferential) contact. 

Equipment in this category is usually arranged for multistage countercurrent contact of the insoluble hquids, without repeated complete separation of the hquids from each other between stages or their equivalent. Instead, the liquids remain in continuous contact throughout their passage through the equipment. 

Inventory Reduce inventory of chemicals Continuous operation may be preferable to batch Low residence time contacting equipment may be better than cheaper alternatives etc. Monitor temperature, pressure flow, composition, freedom from contamination and other appropriate properties of all streams where relevant. Consider automatic control... 

Health Hazards Information - Recommended Personal Protective Equipment Face shield to avoid splash Symptoms Following Exposure Very high vapor concentrations irritate eyes and upper respiratory tract. Continued contact with skin may cause irritation General Treatment for Exposure EYES immediately flush with plenty of water for at least 15 min. get medical attention. SKIN flush with water wash with soap and water Toxicity by Inhalation (ThresholdLimit Value) QO ppm Short-Term Inhalation Limits Data not available Toxicity by Ingestion Grade 2, LDjq 0.5-5 g/kg Late Toxicity None Vapor (Gas) Irritant Characteristics Vapor is moderately irritating such that personnel will not usually tolerate moderate or high vapor concentrations Uq or Solid Irritant Characteristics Liquid may irritate skin Odor Threshold Data not available. 

Continuous operation may be preferable to batch Low residence time contacting equipment may be better than cheaper alternatives etc. 

Because of the diversity of contacting equipment available, it is unlikely that all these contactors will be available in any one laboratory or pilot plant. Consequently, unless test work is carried out on similar contactors, the system may not be optimized. Since mixer-settlers are the easiest to construct, are simple to operate, and require little room and low-flow rates, these contactors are, in many cases, the only ones used to investigate a continuous solvent extraction process. This is by no means ideal and may result in abandonment of a process that, using another type of contactor, could be found to be entirely satisfactory. 

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