Multiple-stage-flash

Dilute polymer solutions containing relatively large amounts of volatiles are devolatilized in ordinary, relatively low-cost, single or multiple stage flash tanks. The flash tank is fed via a preheater that superheats the solution. The vapors of the foamingboiling solution are removed at the top of the tank by a vapor takeoff system, and the concentrated solution is removed at the bottom via a gear pump. 

The thermal technologies consist of multiple-stage-flash (MSF) evaporation, multiple-effect distillation and vapour compression, while the membrane technologies are reverse osmosis (RO) and electro-dialysis (ED). MSF and RO are the most frequently used techniques, and together account for 87% of the world wide desalination activity (Meindersma et al., 2006). 

In this chapter, the fundamental principles and relationships involved in making multicomponent distillation calculations are developed from first principles. To enhance the visualization of the application of the fundamental principles to this separation process, a variety of special cases are considered which include the determination of bubble-point and dew-point temperatures, single-stage flash separations, multiple-stage separation of binary mixtures, and multiple-stage separation of multicomponent mixtures at the operating conditions of total reflux. 

The relationships given by Eq. (1-26) may be reduced to one equation in one unknown in a variety of ways, and a variety of forms of the flash function may be obtained. One form of the flash function is developed below and a different form is developed in Chap. 4 in the formulation of multiple-stage problems. Elimination of the yf, s from the last expression given by Eq. (1-26) by use of the first expression, followed by rearrangement, yields... 

The gap between the treatment of binary and multicomponent mixtures is closed in Chap. 1. This chapter is initiated by presenting the fundamental relationships and techniques needed for making bubble-point and dew-point calculations, and it is concluded by the presentation of techniques for solving a variety of special types of problems such as the separation of a multicomponent mixture by a single-stage flash process and the separation of a multicomponent mixture by use of multiple stages at the operating condition of total reflux. 

Figure 14. Energy requirement and cost of various desalination technologies. Data for multi-stage flash (MSF), multiple effect distillation (MED), reverse osmosis (RO), and two-stage RO (brine concentrator) taken from [57]. More recent data reportedfor a high flux membrane taken from [58].

Desalination of saline water (sea and brackish waters) is a well-established means of water supply in many countries. Basically, desalination processes in this area can be divided into two groups (1) phase-change/thermal, and (2) membrane-based separation processes. Phase-change processes include multi-stage flash,multiple effect boiling, vapour compression,freezing,humidi-fication/dehumidification and solar stills. RO, ED and membrane distillation (MD) are typical membrane separation processes (Charcosset, 2009). 

Beyond these general guidelines, beware of azeotropes and multiple phases in equilibrium, especially when water and organics are present. Special techniques are available to deal with these problems, some of which are discussed later. On the other hand, if a single-stage flash will do the separation, do not use a column with reflux. 

The overhead from the second stage is heated by an exchange with hot solvent. The fired heater further raises the temperature of the solvent/demetallized oil mixture to a point above the critical temperature of the solvent. This causes the demetallized oil to separate. It is then flashed and steam-stripped to remove all traces of solvent. The vapor streams from the demetallized oil and asphalt strippers are condensed, dewatered, and pumped up to process pressure for recycle. The bulk of the solvent goes overhead in the supercritical separator. This hot solvent stream is then effectively used for process heat exchange. The subcritical solvent recovery techniques, including multiple effect systems, allow much less heat recovery. Most of the low grade heat in the solvent vapors from the subcritical flash vaporization must be released to the atmosphere requiring additional heat input to the process. 

The values of vy and Xp may be obtained from an adiabatic flash for a single phase feed or from the constant relative volatility estimated with the converged compositions at the feed stage and feed quality. This procedure can be reformulated for multiple feeds and side products as well as different key components. A pinch point near the feed stage occurs for nearly all binary ideal mixtures. However, for nonideal multicomponent systems, the pinch point exists in rectifying and stripping sections. 

Disregarding economics, there is a maximum number of effects in a multiple-effect system which is fixed by the boiling point elevation (BPE). The number depends on the over-all temperature range, the initial salt concentration, and the per cent yield. For the special case of a 35,000-p.p.m. NaCl feed, temperature range of 100° to 25° C., and 50% recovery, the maximum is about 107. Economics fixes a much smaller number and present indications are that the optimum number of effects will be somewhere between 10 and 20. This is for the case of boiling on the heat-transfer surface and not for flash evaporation, where the optimum number of stages is probably much greater. 

With respect to the incrustation of heat transfer areas of multiple effect plants, multistage flash evaporators can be more favorable. In Fig. 7.6-2 such a unit is illustrated. As a rule 20 stages are installed. The seawater is preheated in the various stages and is fed into the first stage after the passage through a final heater. A certain part of the seawater is flash evaporated by depressurization and is con-... 

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