|
Authors |
S. Jacques, J. Peultier, J-C.
Gagnepain and P. Soulignac, INDUSTEEL, ArcelorMittal group |
|
Preview |
ABSTRACT
The number of countries, which suffer from a shortage of fresh water
resources, is increasing continuously. For this reason, the number of
desalination projects (production of fresh water from sea water) is booming.
In the past, carbon steel, clad steels, copper-nickel alloys, or austenitic
stainless steel grades such as 316L were the primary materials used for the
walls of thermal process evaporation cells. More recently the duplex grades
UNS S32304 and UNS S32205 have been selected for this application. This
paper provides some of the technical reasons, which brought about this
change in the choice of materials selected. Initially, the results of an
extensive corrosion study, performed in a representative environment for
evaporation cells and including both duplex and austenitic grades, are
presented and discussed. Then a joint corrosion testing program conducted in
cooperation with a well established desalination plant designer and run
under actual plant conditions is outlined. The results of these field tests
led to the development of a new concept for evaporation cells based on
utilizing duplex stainless steels.
INTRODUCTION
The number of countries, which suffer from a shortage of fresh water
resources, is increasing continuously due to population increases. The total
global water stock is 97.5% salt or brackish and only 2.5% fresh water.
Approximately 70% of this global fresh water stock is locked up in polar
icecaps and a major part of the remaining 30% lies in remote underground
aquifers. Indeed, only a miniscule fraction of fresh water (less than 1% of
total fresh water or 0.007% of the total global water stock) that is
available in rivers, lakes and reservoirs is readily accessible for direct
human use. Fresh water is of vital importance and is needed for different
purposes such as potable water or agriculture. Desalination is thus one of
mankind's earliest forms of separating fresh water from a salt-water
solution. Desalination [1] may be defined as: "the art of producing fresh
potable water from a saline supply at a reasonable cost with reasonable
reliability". There are different techniques to desalt water and for
production of industrial or high quantity potable water, distillation and
reverse osmosis are the most widely used [2-7]. The thermal processes have
been developed primarily in countries where the desalination plant can be
linked to a power plant, which can act as a low temperature heat source. The
major thermal processes are: Multi Stage Flash distillation (MSF) and Multi
Effect Distillation (MED) [4,8-12]. In this paper, the focus is on the MED
process, which is described below. This process can generate aggressive
conditions in some parts of the system, which may cause corrosion problems.
As a consequence, highly corrosion resistant stainless steels must be
selected in order to insure the continuous operation of the system.
Therefore the selection of materials must be done with a complete
understanding of the process parameters.
MULTI EFFECT DISTILLATION PROCESS (MED)
Description
The Multi Effect Distillation (MED) installation consists of several
consecutive cells (or effects) operating at decreasing levels of pressure
and temperature from the first cell (hot, 145.4°F / 63°C) to the last one
(cold, about 104°F / 40°C) [2-4] (FIGURE 1). |