Flooding Limitations in a Three-Phase Direct Contact Condenser for the Utilisation of Low-Grade Energy Sources for Desalination and Power Generation Application
Authors: Sharif Adel O. , Al-Muhammedawi Hameed B. Mahood , Campbell Alasdair N. , Thorpe Rex B. 
Published: July 2019

The continuing enhancement of energy recovery cycles that exploit low grade energy resources requires an efficient heat absorption and rejection system. Implementation of a conventional surface type heat exchanger, evaporator or condenser has many disadvantages. The low efficiency, fouling, corrosion problems, high cost and high heat transfer resistance are the most important shortcomings that emerge as a result of the metallic barriers involved. On the other hand, a direct contact heat transfer device offers a high heat transfer area and reduces or eliminates fouling and corrosion problems. They can also work with very low temperature differences and subsequently can enhance the cycle efficiency. Direct contact heat exchangers clearly have many advantages over surface type heat exchangers and can also be efficiently used in places where the surface heat exchanger cannot be.

Accordingly, direct contact heat exchanger can be found in different applications, such as water desalination and power generation from geothermal brine. Water desalination utilising direct contact heat exchangers can be achieved by two general methods: direct contact freezing-melting and direct contact evaporation- condensation. The former is based on the ability to remove water from a solution by freezing it out as crystalline ice. The ice or crystal should contain only pure water; therefore the partial freezing separates the fresh water from brine. The ice melts at another stage to produce distilled water. Direct contact freezing exploits the concept of direct contact evaporation of a low boiling point working fluid by absorbing heat from surrounding continuous fluid (water). The heat absorbed by the working fluid; which causes the water to freeze, is equivalent to the energy required to melt the ice. The working fluid used must have a low boiling point and a high freezing point along with other properties mentioned above. Refrigerants such as carbon dioxide and butane are widely used as a working fluid. For second application, the most common application of direct contact heat exchangers is in the generation of electricity from hot geothermal brine, utilising low boiling point fluids and the conventional Rankine cycle. The first plant utilising a spray column, which produces 500 k. We was designed by Barber-Nichols Engineering and installed at the East Mesa Geothermal Field in the U.S. in 1980. The spray column was 12 m in length and 1 m diameter. Hot brine is flashed to remove the non-condensable gases before entering the spray column from the top, while the isopentane liquid is injected in to the bottom of the column through a suitable distributer. Direct contact counter-current heat transfer between the two fluids takes place throughout the column height. According to the density difference, isopentane drops/bubbles rise upward with brine falling down and exiting the column at the bottom after being cooled down. On the other hand, isopentane heats up as a result of absorbing heat from the brine, and leaves the column as a superheated vapour from the top of the column. Isopentane vapour expands through a turbine to produce electricity, is liquefied in a condenser, and sent back to the direct contact heat exchanger again.

For a bubble type three-phase direct contact condenser, it is widely reported that its performance is characterised by a volumetric heat transfer coefficient, which is directly proportional to the holdup ratio in the column. It is both economical and practical for the bubble type direct contact condenser to operate at the maximum possible holdup ratio. This, of course, increases the possibility of flooding, which considerably impairs the performance of the column. Flooding can be either defined as the case when the continuous phase is completely held up by the dispersed phase or the dispersed phase is swept backward by the continuous phase.

Two mechanisms could lead to the inception of flooding in the three direct contact columns, depending on the critical velocity of each individual phase. The critical velocity can be defined as a maximum velocity that can be achieved by a given direct contact system and it is a function of the bubble size, the flow rate and the physical properties of the phases. The first mechanism depends on the reduction in the dispersed phase (bubbles) upward velocity due to the interactions within a swarm of two-phase bubbles. The two-phase bubbles move closer together when the dispersed phase flow rate increases, which results in a further dispersed phase slowing. Bubbles are swept down and drain out with the continuous phase from the bottom of the column. Therefore, the danger from this form of flooding form is the increased loss of the working fluid. The second scenario assumes that the continuous phase is swept upwards because of a high dispersed phase flow rate. This occurs when the dispersed phase velocity passes the critical velocity. The result of this flooding type lies in a change of geometry and makes the heat transfer relationships available invalid. Accordingly, flooding can be defined as the case when the continuous phase is completely held up by the dispersed phase or the dispersed phase is swept backward by the continuous phase.

As the first time, experiments to study the limitation of flooding inception of three-phase direct contact condenser have been carried out in a counter-current small diameter vertical condenser. The total column height was 70 cm and 4 cm diameter. Only 48 cm has been used as an active three-phase direct contact condenser height. Vapour pentane with three different initial temperatures (40°, 43.5° and 47.5°) and water with a constant temperature (19°) have been used as a dispersed phase and a continuous phase respectively. Five different continuous phase mass flow rate and four different dispersed phase mass flow rate have been tested throughout the experiments.

The experimental results showed that the effect of dispersed phase initial temperature on flooding inception to be insignificant at low continuous phase velocity (v_L^(*1/2).