Combination multi-effect distillation and multi-stage flash evaporation system

Abstract

The combination multi-effect distillation and multi-stage flash evaporation system integrates a multi-stage flash (MSF) evaporation system with a multi-effect distillation (MED) system such that the flashing temperature range of the MSF process is shifted upward on the temperature scale, while the MED distillation process operates in the lower temperature range. The multi-stage flash evaporation system includes a plurality of flash evaporation / condensation stages, such that the multi-stage flash evaporation system receives a volume of seawater or brine from an external source and produces distilled water. The multi-effect distillation system includes a plurality of condensation / evaporation effects, such that the multi-effect distillation system receives concentrated brine from the multi-stage flash desalination system and produces distilled water.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]The present invention relates to desalination, and particularly to a system for producing desalinated water from saltwater, such as seawater using both multi-effect distillation and multi-stage flash evaporation.[0003]2. Description of the Related Art[0004]A falling film evaporator is an industrial device to concentrate solutions, especially with heat sensitive components. The evaporator is a special type of heat exchanger. In general, the evaporation takes place on the outside surfaces of horizontal or vertical tubes, although it should be noted that there are also applications where the process fluid evaporates inside vertical tubes. In all cases, the process fluid to be evaporated flows downwards by gravity as a continuous film. The fluid creates a film along the tube walls, progressing downwards, hence the name “falling film”.[0005]In a falling film evaporator, the fluid distributor must be designed carefully in ord...

AI Technical Summary

Problems solved by technology

The result can be a high film velocity of a progressively thinner film, resulting in increasingly turbulent flow.

However, due to the intimate contact of the film liquid with the heating surface, such evaporators are susceptible to fouling from precipitating solids; liquid velocity, typically low at the top rows of a bank of horizontal tubes, is usually not sufficient to perform an effective self-cleaning of the tubes.

Conventional multi-effect distillation systems, such as the above, which generally rely on falling film evaporation, suffer from a number of drawbacks, each of which typically limits the design capacity of the units and the maximum permissible operating temperatures.

On a broad level, many MED designs involve complex and often circuitous paths for heated seawater and vapor to minimize usage of pumps, maintain wettability of the tubes to avoid scaling, and to maximize energy recovery from the flashing brine and distillate.

The farther the pumps, vessels, water routes and vapor routes are from minimal, optimized paths, the more the design suffers from excessive losses.

Feed brine reaches the first stage at an elevated temperature, however it is not high enough to start flashing, and therefore, additional heat must be supplied to the brine.

Such an evaporation mechanism makes heat transfer surfaces highly vulnerable to scale formation and precipitation, especially since only chemical treatment can be used to retard scale formation while on-line mechanical cleaning is not possible.

This situation imposes severe restrictions on the maximum practical operating temperatures in the MED process, which must be kept within a safe range (i.e., below 70° C.).

The conventional MSF process suffers from three primary sources of thermodynamic loss, namely boiling point elevation loss, pressure drop loss, and non-equilibrium loss.

However, these losses have far less significance as far as the thermal performance of the MED process is concerned, mainly because the evaporation temperature range is already limited to a narrow low temperature stretch, and also because the overall heat transfer coefficient at these low thermal levels is almost double that of the MSF process.

There are, however, certain technical and economic limitations to the upper values of the GOR or PR that can be achieved for any process.

Images