Vapor solution evaporation

The fonnation of clusters in the gas phase involves condensation of the vapour of the constituents, with the exception of the electrospray source [6], where ion-solvent clusters are produced directly from a liquid solution. For rare gas or molecular clusters, supersonic beams are used to initiate cluster fonnation. For nonvolatile materials, the vapours can be produced in one of several ways including laser vaporization, thennal evaporation and sputtering. 

You may have noticed that concentrated aqueous solutions evaporate more slowly than does pure water. This reflects the fact that the vapor pressure of water over the solution is less than that of pure water (Figure 10.6). 

The GPC instrumentation and GPC procedures used here are the same as described earlier (I) but with slight alterations. The Kroeger HAK-1 pads furnished with the pressure filter were replaced by Millipore 5-/x Mitex membranes to reduce changes in solution concentrations arising from adsorption of the solute on the fibrous material of the pad. When retention of eluted fractions was desired, two shallow aluminum pans filled with tetrahydrofuran (THF) were placed in the syphon box, and then the door was closed and latched. The saturation of the enclosed air space by THF vapor minimized evaporation of the solution during drop formation, drop fall, and retention of fraction in the syphon. The possibility of diluting the sample solution when charging the sample loop was... 

Evaporation as a process operation involves the concentration of one more or solutes by transfer of the solvent from the liquid into the vapor phase. Evaporation also may be simply formation of vapor from a liquid. The heating medium, usually steam, is introduced in the steam chest connected to a set of tubes inside the evaporator body. The steam condenses causing some of the liquid outside of the tubes to vaporize. As a matter of economy, often a multiple series of evaporators are connected so that the vapor from one evaporator is introduced (at a lower pressure) into the steam chest of the next evaporator where it condenses, and so on. Dissolved solids can be deposited on the exterior of the heating tubes (scaling) so that different interior evaporator designs are used to reduce scaling. 

Rather than produce an atomic vapor by evaporation from a solid surface, an aerosol may be generated from an aqueous salt solution by an atomization procedure. The aerosol can then be evaporated so that the salt condenses into a particle. This is known as the spray-pyrolysis technique. The flame decomposition method is a modification of this technique, in which the aerosol is introduced into a high-temperature flame (1200-3000 K). The precursor is vaporized and oxidized to form metal-oxide particles. 

Platinum vapor, (generated by electron beam evaporation of the molten metal at ca. 2x10 torr), or palladium vapor (by evaporation from a resistively heated hearth at ca.l0 torr)) was allowed to dissolve in a solution of poly(isobutyl-aluminoxane), [ C4H9A10]n ... 

Glasses can also be prepared by methods other than cooling from a liquid state, including solution evaporation, reactive sputtering, vapor deposition, neutron bombardment, and shock wave vitrification . These techniques suggest that the purely kinetic explanation of the glassy state is subject to question, and that the previous definitions need to be modified. One proposal would define glasses based on isotropy and relaxation time measurements . 

A vapor-recompression evaporator is to concentrate a very dilute aqueous solution. The feed rate is to be 30,000 Ib/h the evaporation rate will be 20,000 Ib/h. The evaporator will operate at atmospheric pressure, with the vapor mechanically compressed as shown in Fig. 16.12 except that a natural-circulation calandria will be used. If steam costs 8 per 1000 lb, electricity costs 3 cents per kilowatthour, and heat-transfer surface in the heater costs 70 per square foot, calculate the optimum pressure to which the vapor should be compressed. The overall compressor efficiency is 72 percent. Assume all other costs are independent of the pressure of the compressed vapor. To how many effects will this evaporator be equivalent ... 

If single-solvent solutions do not yield the desired phase, mixtures of solvents can be tried. Multicomponent solvent evaporation methods depend on the difference in the solubility of the solute in various solvents. In this approach, a second solvent in which the solute is sparingly soluble is added to a saturated solution of the compound in a good solvent. Often a solvent system is selected in which the solute is more soluble in the component with the higher vapor pressure. As the solution evaporates, the volume of the solution is reduced and, because the solvents evaporate at different rates, the composition of the solvent mixture changes. 

When a multicomponent mixture forms nearly ideal liquid and vapor solutions, and the ideal gas law holds, the K values and relative volatility can be readily estimated from vapor pressure data. Such K values are referred to as ideal or Raoult s law K values. Then, the SF for vapor-liquid separation operations employing an ESA (partial evaporation, partial condensation, or distillation) is given by... 

In the second stage of the process, the salt solution is boiled at atmospheric pressure to remove much of the water from the initial solution and water generated by oligomerization reactions. The evaporation process continues until the combined monomer and oligomer concentration is approximately 65-75 wt%. HMD vapor, which evaporates along with the steam, may be recovered and recycled. The evaporation stage can be carried out using either a batch or a continuous reactor. 

Fig. 7-10. Balance scheme to determine the solvent vapor flow and steam consumption (a) and heat flow diagram for single stage solution evaporation (b).

Reduction of heat losses by appropiate insulation of equipment and unit parts Solution heating by vapor and vapor condensate of the evaporation unit Multistage solution evaporation including vapor heat use (the specific steam consumption dH decreases with increasing number of stages n proportional 1/n)... 

Single or favored multistage solution evaporation including vapor compression by mechanical or thermal means (additional steam savings by energy supply to vapor, which are usually condensed and fed back to the evaporator)... 

Solution Evaporation with Mechanicai and Thermal Vapor Compression... 

For solution evaporation, circulation evaporators are often used in the form of a tube bundle apparatus with vertical, horizontal or slanting tubes, operated with natural or forced solution circulation. TVibes may be mounted inside or, with respect to better cleaning, outside (Fig. 7-20) of the evaporator. The solution evaporates inside the tubes and is lifted by the vapor bubbles generated according to the principle of an air lift pump. Natural circulation of the solution is mainly intensified by adding circulation pumps (forced circulation). 

At the top distributed solutions flow down the inside of evaporator tubes as a falling film by gravitational force, vapor is evaporated from the liquid film, Z = 5-7 m, d approximately 50 mm, liquid load ca. 0.7-3 m /(h m), mean residence time 1-3 min, to calculate the heat transfer see [7.21]. 

B. Vapor-Recompression Evaporation. The existence of the BPR in a solution means that the condensing temperature of the vapor raised in an evsqiorator will be lower than the boiling point of the solution from which it came. In other words, the vapor as it forms is superheated. When the vapor is used in another effect, the superheat provides very little thermal energy, and the vapor temperature quickly drops to the saturation temperature of pure water at the operating pressure. 

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