Multi-stage evaporation

In most of the cases, the vapour Is used within an evaporation plant, which is designed according to the requested capacity. There are generally two possibilities the multi-stage evaporation and the vapour recompression. 

The vapour can be used by being supplied as heating steam to a second evaporator stage, which works at a lower pressure. The vapour from this can be supplied to a third stage, and so on. In this way, in case of n stages in the ideal case n kg of vapour can be evaporated with 1 kg of heating steam. 

Therequired pressure graduation is generally carried out in the vacuum range, as far as the vapour of the last stage still can be condensed with cooling water. 

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Description Purified EO or a water/EO mixture is combined with recycle water and heated to reaction conditions. In the tubular reactor (1), essentially all EO is thermally converted into monoethylene glycol (MEG) with diethylene glycol (DEG) and triethylene glycol (TEG) as co-products in minor amounts. Excess water, required to achieve a high MEG selectivity is evaporated in a multi-stage evaporator (2,3). The last evaporator (4) produces low-pressure steam, which is used as a heat medium for other units in the plant. Crude glycol is purified in a series of vacuum columns (5,6,7,8). Selectivity toward MEG can be controlled with the feed composition. 

The purified juice (thin juice) with a dry mass content of some 15% is concentrated in a multi-stage evaporator station to 60-70% (thick juice). 

The evaporation with mechanical vapour recompression corresponds to the ideal evaporation the vapour energy is completely used since no pressure graduation is required as it is the case for the multi-stage evaporation. Apart from the starting steam only electric energy is required to drive the compressor. 

The steepwater, containing 6-7% d.s., is concentrated in multi-stage evaporators. The average composition becomes then ... 

Two-phase flows in micro-channels with an evaporating meniscus, which separates the liquid and vapor regions, have been considered by Khrustalev and Faghri (1996) and Peles et al. (1998, 2000). In the latter a quasi-one-dimensional model was used to analyze the thermohydrodynamic characteristics of the flow in a heated capillary, with a distinct interface. This model takes into account the multi-stage character of the process, as well as the effect of capillary, friction and gravity forces on the flow development. The theoretical and experimental studies of the steady forced flow in a micro-channel with evaporating meniscus were carried out by Peles et al. (2001). These studies revealed the effect of a number of dimensionless parameters such as the Peclet and Jacob numbers, dimensionless heat transfer flux, etc., on the velocity, temperature and pressure distributions in the liquid and vapor regions. The structure of flow in heated micro-channels is determined by a number of factors the physical properties of fluid, its velocity, heat flux on... 

New spatial forms of carbon - fullerenes, nanotubes, nanowires and nanofibers attract significant interest since the time of their discovery due to their unique physicochemical and mechanical properties [1-3]. There are three basic methods of manufacturing of the carbon nanomaterials (CNM) - laser evaporation, electric arc process, and catalytic pyrolysis of hydrocarbons. However, the multi-stage manufacturing process is a serious disadvantage for all of them. For example, the use of organic solvents (benzol, toluene, etc.) for separation of fullerenes from graphite soot results in delay of the synthesis process and decrease in the final product quantity. Moreover, some environmental problems can arise at this. 

There are two obvious ways of removing the remaining hydrocarbon contamination. Multi-stage stripping could be used but the most economical and simplest method is to add a carbon bed adsorption step to the process as indicated in Figure 1. Carbon bed removal can be justified only in conjunction with the evaporator system where the hydrocarbon contamination is very low. The concentration of soluble organic components in a system where the evaporator is not used is simply too high and the bed is exhausted too quickly. 

Recently another system of evaporation has been developed in Europe, in which vapors coming from the evaporator are compressed in a multi-stage turbo-compressor, and then returned into the steam chest at higher temperature. It is claimed that under certain conditions, the fuel consumption of a single effect is less than that of a quadruple effect. For detailed information, see an article by Carlsson in Chemical and Metallurgical Engineering, Apr. 13, 1921, p. 645. 

In the laboratory, a multi-stage liquid-liquid extraction can be performed by a simultaneous distillation-extraction process according to Likens-Nickerson [29] (Fig. 2.10). Here, the liquid matrix with the solute in one flask is evaporated together with an immiscible solvent in a second flask. Extraction takes place in the vapour phase where an intensive distribution of both phases is ensured. The condensed vapours from the two phases are separated via a siphon using their different densities and their reintroduction into the original flasks. As the distillation process is continued, extraction is repeated until the solute is exhausted in the original matrix. This method is very useful when traces of non-volatile solutes are present, which are only partly miscible... 

ATF Agitated Thin Film Evaporator VTE Vertical Tube Evaporator MSF Multi Stage Flash... 

In this method, inert gas (nitrogen with less than 7% oxygen) is circulated between the evaporation zone and multi-stage condensation unit. Because the gas is inert, the restriction which calls for solvent concentration never to exceed a fraction of the... 

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