By Jens Bekker Nielsen
Thermal vapour compression desalination units produce low-cost distillate for use as boiler makeup and process water from seawater at a fertiliser plant in Indonesia.
A fertiliser plant on the coast of Indonesia is desalinating seawater for use as boiler makeup and process water. The first two of three Alfa Laval Type Thermal Vapour Compression (TVC) desalination units turn seawater into 1,680 m3 per day of high-purity distillate. Two units were commissioned in December 2001, and a third plant, also rated as 1,680 m3 per day, is now being installed.
The evaporative TVC desalination process uses Alfa Laval's high-efficiency plate-type heat exchanger technology, which the company describes as "pressed plate using a falling film" or PPFF. Fertiliser plant boilers provide steam for the evaporative process with energy saved by operating in a vacuum (150 mbar abs) environment created by ejector systems from Croll-Reynolds of Westfield, New Jersey.
The multi-effect evaporator increases efficiency. Energy balance for the process is further improved by the use of Croll-Reynolds thermocompressors, which take spent steam from the last (cold) effect and recombine it with higher pressure steam from an external source for reuse at the first (hot) effect.
In the PPFF process, seawater falls down one side of each titanium plate as a thin film and evaporates because of the heat being transferred from condensing water vapour on the other side of the plate. The vapour is collected, condensed and extracted as pure distilled water, while the brine is returned to the sea.
Alfa-Laval claims that the PPFF configuration achieves greater efficiency than conventional tube-and-shell desalination systems, which translates into smaller installations with lower site requirements and capital costs.
Placing the evaporative process under vacuum allows condensing water to boil at a much lower temperature, resulting in considerable energy savings. The Croll-Reynolds ejector vacuum system for each desalination unit consists of two ejector stages, each 100 mm x 100 mm, and a vertical vapour-in-shell aftercondenser. The ejectors and condenser shell are made of stainless steel and the condenser tubes are made of titanium with titanium-clad tubesheet.
In each desalination unit, a thermocompressor recompresses spent steam from the multi-effect evaporator, decreasing steam consumption and raising the efficiency of the system. Since the Indonesian plant employs primary energy - boiler bleed steam - the importance of efficient energy use is increased.
The thermocompressor uses high-pressure steam as a motive fluid to entrain the low-pressure spent steam from the process. The two fluids are combined in the mixing chamber and recompressed to an intermediate pressure through the diffuser, which reconverts velocity energy to pressure energy.
Each 900-mm x 900-mm thermocompressor in the Indonesian installation features seven nozzles and a spray ring at discharge for desuperheating.
The plate-based desalination system is constructed from titanium and can be dismantled for manual cleaning should hard scaling occur. The ejectors and thermocompressors feature no moving parts and are practically maintenance-free. The only recommended spare parts are nozzles - one for each ejector stage and seven for each thermocompressor - with gaskets.
Similar applications for seawater desalination plants are found in crude oil refineries, oil and gas off-shore installations, power generation plants, food and beverage industries and processing plants in general. Typical key success factors in these markets are based on process efficiency linked to a reliable and safe operation plus an effective minimum lifetime of 25 years, which is a typical lifetime for industrial processing plants.
Thermocompressors help reuse heat energy
Rising energy costs and projected shortages of fuel oil feed stocks,have re-emphasised the need to develop alternate fuels and resource conservation. Reclamation and reuse of valuable heat energy from processing operations represents one of the best opportunities for fuel conservation.
The use of thermocompressors continues to increase as process engineers in the power and process industries find new areas of plant operations where energy from steam can be recovered and reused. Since thermocompressors operate on a wide variety of motive gases and pressures, they offer an unusual degree of flexibility to answer the varied needs of today's process requirements.
Jens Bekker Nielsen is the project manager for Alfa Laval Water Technology, based in Sob rg, Denmark.