Paweł Madejski
Tomasz Kuś
Tomasz Kuś
Piotr Michalak
Piotr Michalak
Michał Karch
Michał Karch
Navaneethan Subramanian
Navaneethan Subramanian
Department of Power Systems and Environmental Protection Facilities, Faculty of Mechanical Engineering and Robotics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krakow, Poland
Author to whom correspondence should be addressed. Energies 2022, 15(24), 9312; https://doi.org/10.3390/en15249312Submission received: 10 October 2022 / Revised: 29 November 2022 / Accepted: 4 December 2022 / Published: 8 December 2022
Direct contact heat exchangers can be smaller, cheaper, and have simpler construction than the surface, shell, or tube heat exchangers of the same capacity and can operate in evaporation or condensation modes. For these reasons, they have many practical applications, such as water desalination, heat exchangers in power plants, or chemical engineering devices. This paper presents a comprehensive review of experimental and numerical activities focused on the research about direct condensation processes and testing direct contact condensers on the laboratory scale. Computational Fluid Dynamics (CFD) methods and CFD solvers are the most popular tools in the numerical analysis of direct contact condensers because of the phenomenon’s complexity as multiphase turbulent flow with heat transfer and phase change. The presented and developed numerical models must be carefully calibrated and physically validated by experimental results. Results of the experimental campaign in the laboratory scale with the test rig and properly designed measuring apparatus can give detailed qualitative and quantitative results about direct contact condensation processes. In this case, the combination of these two approaches, numerical and experimental investigation, is the comprehensive method to deeply understand the direct contact condensation process.
Direct Contact Condensers (DCCs) have been used in industry since the beginning of the 20th century [1], covering a wide range of various applications in chemical engineering, water desalination, air conditioning, and energy conversion processes.
In this device, the cooling liquid is directly mixed with gas or vapour, which results in condensation and a significant decrease in device volume [2]. Involving a surface condenser of the same capacity direct condenser has several advantages. Due to direct contact with process fluids, its construction is simpler and more corrosion resistant [3], less expensive [4], easier to maintain, and simpler in operation [5].
Direct contact condensers are generally divided into spray-type, film-type, and bubbling type [6]. In the first solution, the sprayed liquid phase flows downwards and is in contact with flowing upwards gas. In the second case, both phases flow counter currently. In the latter solution, the bubbling gas phase passes through the liquid layer. Furthermore, spray condensers can exist with constant pressure or constant area jet ejectors [7]. Despite these apparatuses’ wide range of applications, plenty of studies summarize theoretical and practical aspects of their development. Aidoun et al. [8,9] presented results of experimental and numerical studies focusing on ejectors and their applications in refrigeration systems. Mil’man and Anan’ev [10] focused on the application of air-cooled condensing units in thermal power plants. Xu et al. [11] discussed recent advances in humidification-dehumidification desalination processes, including direct and indirect contact condensers. They are also commonly used as dehumidifiers in solar-driven humidification-dehumidification desalination [12,13] and seawater greenhouse [14] systems. The application of direct and indirect condensers in the pyrolysis of waste plastics was discussed by Kartik et al. [15], and in pyrolysis of biomass to bio-oil was presented in [16].
The wide range of possible and actual applications of these devices indicates the need to investigate more deeply the direct contact condensation process by using simulation and experimental studies devoted to developing direct contact condensers. This paper aims to present various analyses and their main outcomes to give the full view of the most important factors that must be considered during theoretical and experimental research.
Direct Contact Heat Exchangers (DCHEs) play an important role in various technological processes, including humidifying air, cooling water, and removing excess heat from flue gases. The exchange processes in such apparatuses occur under contact with the liquid phase (e.g., water) and gas phase (e.g., air) at the interphase surface. In this case, heat and mass transfer are mainly determined by the geometric dimensions of the surface area for contact between the two phases. A specific value of this surface area (attributed, for instance, to the volume of the active zone of DCHE) depends on the method of interaction of the contacting phases, i.e., on the DCHE design. The most commonly used designs of heat exchangers are the following [6]:
spray-type (gas phase flows upwards and comes into contact with the liquid phase, which is sprayed from the nozzles and flows downwards),
film-type (liquid phase flows downwards as a thin liquid film on the inside wall of the vertical tube while the air flows counter currently) [17,18,19,20],
bubbling-type (bubbling of the gas phase through a layer of the liquid located on a hole tray [21,22,23] or in a vertical channel [24,25].
Direct Contact Condensers (DCCs) have a variety of purposes. They can be used to heat the liquid for heat recovery. The hot liquid can be used to heat rooms, preheat raw materials, or melt solids such as ice. DCCs can be used to cool the gas to generate condensate. Condensate can be used to purge a reaction product, such as acids coabsorbed in the DCC, condense a particulate by converting it from a vapor to a liquid or solid, and grow particulate by condensing directly on the particulate surface to improve its capture or reclaim water in arid regions. Direct Contact Condensers also can reduce gas volume, suppress stack plumes, and lower energy requirements.
In direct-contact condensers, the gas and liquid come in direct contact. The cooling liquid is sprayed into the gas region to start a rapid condensation, which maximizes the thermal efficiency of condensers. The heat is transferred from a gas to a liquid, and the condensate temperature is the same as that of the cooling liquid leaving the condenser. The condensate cannot be reused as feed water if the cooling water is not pure and free from harmful impurities. The occurrence of the other gases strongly impacts the heat transfer rate and condensation efficiency. This process is one of the important issues investigated experimentally or numerically to determine overall efficiency and properly design Direct Contact Condensers. The general classification of condensers is presented in Figure 1.
In a parallel flow jet type condenser, the exhaust steam and cooling water find their entry at the top of the condenser and then flow downwards, and condensate and water are finally collected at the bottom (Figure 2).
The steam and cooling water enter the condenser from opposite directions in a counter jet type condenser. Generally, the exhaust steam travels upward and meets the cooling water, which flows downwards. In this low-level jet-type condenser (counter jet type condenser), presented in Figure 3, the exhaust steam enters slightly lower than in a parallel flow jet-type condenser, and the cooling water is supplied from the top of the condenser chamber (Figure 3). The direction of the steam is upward, and the cooling water is downward. An air pump creates a vacuum and is placed on top of the condenser. The vacuum sucks the cooling water, and a hollow cone plate collects the falling water, which joins the second series of streams and meets the exhaust steam entering from below. The resulting condensate is delivered to the tank through a vertical pipe by the condensate pump. Another solution of counter jet type condensers is called barometric condenser and is presented in Figure 4. In this type, the shell is placed at the height of about 10.363 m above the hot well; thus, there is no need to provide an extraction pump. Provision of providing injection pump is observed, where water under pressure is unavailable.
In Figure 4, the discharge pipe is connected to the bottom of the condenser shell. The exhaust steam enters the system in the lower part of the condenser, with the flow direction pointing upwards. Cooling water enters at the top and is collected by a punched cone plate. An air pump creates the vacuum on top of the shell. Steam and cooling water mix together and are carried through a discharge pipe to the tank. The difference between low and high-level jet condensers is that there is no pump between the tank and the discharge pipe in the high-level type.
The last type of jet condenser is an ejector flow jet type condenser (Figure 5). Here the exhaust steam and cooling water mix in hollow truncated cones. Due to this decreased pressure, exhaust steam and associated air are drawn through the truncated cones, finally leading to the diverging cone. In the diverging cone, a portion of kinetic energy is converted into pressure energy which is more than the atmospheric, so that condensate consisting of condensed steam, cooling water, and the air is discharged into the hot well. The exhaust steam inlet is provided with a non-return valve which does not allow the water from the hot well to rush back to the engine in case of cooling water supply to the condenser.
The cooling cycle makes use of a steam ejector condenser. The steam ejector condenser is classified into two types based on the mixing method in the primary nozzle exit [7,26]. The first one is the constant pressure jet ejector (CPJE), and the other one is the constant area jet ejector (CAJE). The CPJE performs better than the CAJE due to better turbulent mixing [7,27]. In addition to having no moving parts, the steam ejector condenser benefits from lower maintenance and capital cost than the compressor.
After the exhaust passes through the Direct Contact Condenser (DCC) for condensation, the condensate water from the DCC consists of a proportion of non-condensable gases such as CO2, air, or other gases. The stream is passed through the separator or non-condensable gas removal system, which separates the water and the non-condensable gases. By this method, the separated CO2 can be sent to the CCU unit for further utilization, or the separated air gases can be removed from the system. Gas separation from the DCC outlet stream can be carried out in various methods. The axial flow cyclone separator is the most commonly used gas-liquid separation method widely used in industries. Kou et al. [28] simulated and experimentally proved gas-liquid separation using an axial flow cyclone separator. The experiment is conducted by passing the gas-liquid mixture stream into the cylindrical axial flow separator. A guide vane at the bottom of the cyclone separator produces centrifugal force in the fluid passing through it. Once the fluid passes into the separator, the centrifugal force created in the fluid separates the gas and liquid due to the density difference. While the liquid is collected at the bottom, the gas escapes through the top of the separator [28,29]. Ji et al. [29] experimentally proved that the efficiency of the cyclone separator could be improved by combining components to the cyclone separator. The combined cyclone separator includes components such as a steady flow element, leaf grind element, and folding plate element, which increases the efficiency of gas-liquid separation by more than 95%. Chemical looping is one of the methods of splitting the H2O and CO2 in the exhaust gas. The exhaust, which consists of H2O and CO2 undergoes a chemical reaction with the metal oxide used in chemical looping and produces different components. Farooqui et al. [30] state the process of chemical looping with cerium oxide (CeO2). The H2O and CO2 are pressurized, and the temperature is raised up to 500 °C by compression. By integrating chemical looping, oxidation occurs with CeO2, which splits H2O into Hydrogen and CO2 into carbon monoxide. The separated components from the exhaust of the chemical looping is further used for dimethyl ether (DME) production. This is considered one of the methods for CCU technology using chemical looping. Wotzka et al. [31] presented the possibility of separating CO2 and water with the application of a microporous membrane. The separation of carbon dioxide and water using an MFI zeolite membrane treated with amine is analyzed experimentally and with molecular simulation. For experimental purposes, the liquid water is heated up to 120 °C, mixed with CO2 in an evaporator, and further sent to separation. The performance of membrane separation is analyzed under different factors.
When considering direct contact condensers, researchers were directed to several topics. The first one covers various physical aspects of the direct contact condensation phenomenon in different construction variants, as in downcommerless trays for the steam–water system, direct contact condensation in a moving steam-water interface, in the case of the steam jet in subcooled water flow in a rectangular mix chamber or in a vertical square cross-section pipe. Additionally, visualization studies involving high-speed cameras were presented. There can also be distinguished direct contact condensation in the presence of non-condensable gas.
Other studies were devoted to analyzing heat transfer coefficient or volumetric heat transfer in direct contact condensers. Then, various construction aspects and their impact on condenser performance were analyzed. Finally, a few cases of DCC performance in the presence of non-condensable gases were given. Their short description, with emphasis on applied fluids (liquids and gases), is shown in Table 1.
In the following paragraphs, experimental studies on direct contact heat exchangers are presented. They are grouped according to the previous section into the parallel flow, counter flow, and ejector flow condensers.
The Thermochemical Power Group (TPG) at the Polytechnic School of the University of Genoa developed and implemented the contact condenser test rig [36,37,38,39] (Table 2). It was intended for studies on the humid air turbine cycle where water introduction in a gas turbine circuit is provided by a pressurised saturator (i.e., humidification tower or saturation tower).
Changing input variables at the levels given in Table 3, the authors performed 162 tests in total. Data analysis was provided using two types of correlations for the non-dimensional outlet air temperature, i.e., based on polynomial correlation:
T a d i m = c 0 + c 1 x 1 + c 2 x 1 2 + c 3 x 1 3 + c 4 x 2 + c 5 x 2 2 + c 6 x 2 3 + … ,and applying non-dimensional parameters, as non-dimensional temperature (T*), mass flow ( M* ) and the Reynolds number of inlet air ( Re* ):
