Objective
To design, construct and operate a special type of evaporator to concentrate the feed brine of a salt plant situated between the last stage and the barometric condenser of a multiple stage evaporation plant for the production of sodium chloride, thus saving a substantial part of the energy required for brine evaporation.
The brine concentrator was brought on stream as scheduled (22.03.84) and despite no appreciable problems, commissioning proved so time consuming that test runs only began one year after start-up.
It soon became evident that, due to a too high demister load, salt concentration in the down-flowing cooling water of the barometric condenser was too high. This was overcome by the introduction of a demister washing programme in combination with a reduction of evaporator liquid level, which maintained salt concentration of down-flowing water within set standards.
During the first months of operation, salt scaling occurred in the heat exchanger tubes which was eventually overcome by optimising periods between washing and reversing direction in circulation pump rotation.
Other measurements obtained from 24 hour plant running, with the brine concentrator operational, and 24 hours without demonstrated that as a result of the brine concentrator +/- 9% less steam is used at 0.3 Bar lower pressure. The effect of this, both on fuel consumption and electricity production, suggests that at a salt production level of 5,454 T/d., net gas savings are 53.1 GJ/h., leading to a 4,921 TOE/y - an increase of 10% on orginal predictions. Annual savings at a load of 1,660,000 tons, amount to DFl 1,650,000 (in 1986).
The various technical risks involved in construction of the this (new) evaporator type, including a lower than expected flash efficiency; that measures to reduce pressure loss and improve fluid distribution on the heater tubes might fail, and that corrosion at the vapour side of the steel heat exchangers would reduce heat transfer coefficient were all in practice, seen to be unfounded. However, incrustation at the tube's inlet did, as expected, occur but was overcome by adapting the circulation with a water injection of brief duration.
It was also found that when running the brine concentrator under such conditions, an appreciable amount of crystals were formed, though byputting brine through to the E plant only, no problems were encountered.
It also became necessary to install more accurate temperature sensors.
There are no obstacles to future industrial development since, in principle, the equipment can be applied to any liquid-evaporating process, for example, caustic soda evaporation, evaporation of diaphragm cell liquor and of clarified juice in the production of sugar.
As a result of the project, know-how has been gained in the operation of horizontal vapour separators, on flash efficiency and crystallisation. The horizontal evaporator made it possible to install a very large heating surface with two heat exchangers in series, with a circulation pump between then. Neither patents or licenses have been granted although planning is underway for the erection of three brine concentrators.
In the special evaporator +/- 40 % of the heat normally transferred at low temperature to cooling water in the barometric condenser, can be used to concentrate the salt plant feed brine evaporation. The special evaporator has been designed in such a way that it can be operated at extreme low temperature difference, keeping investment costs low.
Innovative features of this project lie in the choice of a horizontal evaporator body, which means that a very large heating surface can be economically applied in a forced circulation circuit. Pressure loss in the heat exchanger has been kept low, and heat tranfer in the heat exchanger after the pump elbow has been enhanced.
During the process, vacuum salt is produced by evaporating brine at between 50 deg. C to +/- 130 deg. C in a number of evaporators (also called 'stages'). In the first of these low pressure steam (LPS) is condensed. The steam is generated at high pressure in a heat power plant then supplied after turbine expansion, thereby co-generating electricity.
The vapour developed in the first evaporator is supplied to the next, which operates at a lower temperature and pressure. The vapour from the last stage is condensed in direct contact with cooling water : A multiple-effect brine evaporation plant is generally comprised of four to six evaporator by way of a battery of preheaters. Evaporated brine, carrying the crystallised salt is purged from effect to effect.
The special evaporator is installed after the last evaporator. The brine concentrator consists of a horizontal evaporator, two vertical heaters and circulation pump. The evaporator body is fitted with a demister. Circulation brine leaves the evaporator at about 40 deg. C.
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Programme(s)
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Coordinator
7554 Hengelo
Netherlands
The total costs incurred by this organisation to participate in the project, including direct and indirect costs. This amount is a subset of the overall project budget.