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Content archived on 2024-04-30

Process intensification : developments, performance studies and integration of multifunctional compact condensers in distillation processes

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The main objective was to study and analyse two-phase flow, under adiabatic conditions, in compact heat exchangers. The particular objective was to study two-phase flow (with emphasis on flooding) in two "compact" geometries, i.e. a plain vertical rectangular channel (5 and 10 mm), and an inclined small diameter tube (7 mm). The latter is considered to simulate a single flow "element" of a compact heat exchanger with corrugated plates. In reflux condensers the major problem, which hinders smooth operation, is flooding. This is associated with choking of the upward moving vapours, and with a sharp increase of pressure drop, that upsets the condenser operation. In the case of compact condensers this problem is aggravated by the narrow flow passages between plates. Therefore, the design engineer needs reliable tools or information to determine the critical conditions leading to flooding as well as the region of condenser operability. The work carried out at CPERI is focused on developing this type of predictive tools. New flooding data in a vertical rectangular channel with 5 and 10mm gap between its main parallel plates are reported. Visual observations and fast recordings are made to determine conditions associated with the onset of flooding in the channel. For the smooth tube, experiments were conducted at 30, 45, 60, 80 and 90 degrees with respect to the horizontal position. At each set of experiments (inclination angle) several liquid flows were set. The gas flow rate was then progressively increased until the flooding point was reached. The main objective of the experiments was to study reflux flow (countercurrent gas-liquid flow) and to investigate the effect of inclination angle on the flooding mechanism. CPERI will use these results as a base for more advanced studies in case of corrugated plates. The studies will be operated by specific contracts with manufacturers of plate heat exchanger. Flow visualisation studies in the experimental apparatus show that for the 10mm gap, flooding in the channel occurs via the following mechanisms: Initially, in the upper part of the channel (at liquid entry), the film is relatively smooth, having waves with small amplitude. As they move downwards, these waves grow, due to the influence of gravity and gas flow shear. A coherent large wave, covering the whole channel width, appears to be momentarily arrested ("wave levitation"), near the liquid outlet, by the counter-currently flowing gas and it is then swept up the channel, passing beyond the liquid inlet and producing a sustained co-current upflow in this region, thus characterising the flooding event. After flooding is established, co-current flow above the liquid injector zone coexists with a counter-current flow below the injector, this counter current flow being at a liquid rate which is insufficient for flooding. The flow below the injector often takes several seconds to calm down to a stable counter current flow after flooding. With regard to the critical flooding velocity, defined as the gas velocity beyond which flooding occurs, one can draw the conclusion from the experimental data that it tends to increase with decreasing liquid velocity, only for relatively low liquid flow rates. In that range a Wallis (1969) type correlation fits the data reasonably well. However, it has been shown that for relatively high liquid flow rates, the critical flooding velocity tends to be independent of liquid velocity. As far as the visual studies made for the 5mm gap channel are concerned, it can be concluded that "wave bridging" is the dominant mechanism responsible for flooding phenomena. In this case, waves on both channel plates grow as they move downwards and block this smaller gap before they reach the middle of the test section. One observes different behaviour depending on the angle of inclination for the pipe: At the vertical position (90 degrees from horizontal) a countercurrent annular flow is readily established at low flow rates. By increasing the gas flow coherent symmetrical waves appear, covering the entire circumference and travelling downwards. Further increase of the gas velocity results in the "levitation" of these waves first, followed by the reversal of the liquid flow. This implies that the gas force exerted on the wave becomes large enough to carry it upwards. In a small diameter tube the formation of waves causes a relatively large reduction of the area available for gas flow, which in turn increases the drag on the wave. As the liquid flow rate is increased, the liquid film thickness tends to increase and thus the area available for gas flow decreases. As a result, the airflow rate necessary for the onset of flooding decreases with increasing water flow rate. In these low liquid rates, the critical gas velocities follow a Wallis type correlation; i.e. the gas flow rate needed to cause flooding is inversely proportional to the liquid flow rate. In the inclined tube (30 to 80 degrees from horizontal) a stratified two-phase flow is first maintained. At relatively small gas flow rates, waves are evident on the liquid surface. Further increase of the gas flow rate causes the liquid from the wave crest to climb up in the circumferential direction and to form coherent "ring" type waves, which move downwards. The area available for the gas flow is consequently reduced and the drag exerted by the gas on the wave surface attains a larger value. The data of critical velocity for the onset of flooding are employed in the form of flow map where the prevailing flow patterns and regimes are marked.
Compact heat exchangers can be used as condensers in industrial processes, but there is no reliable tool for the design of such equipment with process fluids. A Compabloc type heat exchanger has been installed on one the Greth hydrocarbon test rigs, and tests have been performed using various pure and of mixture of hydrocarbons. Overall measurements have been realised for total condensation under actual flow conditions for pressures between 4 and 18 bars. Five sets of tests have been performed: three with pure fluids and two with mixtures. For each fluid, the absolute pressure and the mass flow rate on the condensation side were varied. - Pentane; - Butane; - Propane; - Butane-propane (49%-51% in mass); - Butane-propane (28%-72% in mass). Flooding is an important issue for reflux condensers as it gives the limits for the operating conditions. Experiments have been performed with saturated n-pentane in nitrogen and the inlet parameters, vapour mass fraction, Reynolds number, coolant temperature and coolant mass flow rate have been varied. Tests on reflux condensation have been performed at Greth on a cross-corrugated compact heat exchanger. The tested heat exchanger as a similar geometry than the one tested in downward condensation. Dimensionless local heat transfer coefficients have been determined in terms of the average Nusselt number during reflux condensation of pure R134a as well as of R134a in the presence of the non-condensable gas nitrogen in a small diameter inclined tube. It has been found in the experiments with pure R134a that the inclination angle has a significant effect on the reflux condensation heat transfer coefficient. For pure fluids (butane and propane), two regimes in downward condensation have been identified. For low Reynolds numbers, the heat transfer coefficient in condensation decreases, indicating a laminar regime. For higher Reynolds numbers, the heat transfer coefficient increases gently, indicating a transition to turbulent flows. For mixtures (butane and propane), the behaviour is different. The heat transfer coefficient is much lower than with pure fluids, and increases with the Reynolds number. Then, it remains almost constant or increases slightly. Furthermore, a significant pressure effect is observed., low pressures giving higher heat transfer coefficient. These observations indicate that mass transfer effects affect heat transfer in condensation. Based on this database, a model for pure fluid and mixture condensation has been elaborated. The heat transfer results for reflux condenser indicate that the heat exchanger operates in laminar flow and that mass transfer affects significantly the heat transfer performances. The estimation of the heat transfer resistance of a laminar falling film gives much higher heat transfer coefficients than the one measured, which confirms the importance of mass transfer effects in the gas phase. For increasing equivalent Reynolds numbers, the heat transfer performance increases too. Similar behaviour has been observed for downward condensation of mixture in the same heat exchanger.
Compact heat exchangers are characterised by small hydraulic diameters (1-10 mm) and there is no reliable design method to estimate heat transfer coefficients during condensation in such small passages. In the open literature, condensation of mixtures and of vapour in presence of non-condensables have been studied, but essentially for conventional geometries (plain tubes), and only few results have been published with fluids representative of actual process conditions (hydrocarbons). The nature of the results is on: - Downward condensation. - Reflux condensation. - Modelling and process integration. Condensation occurs in many industrial processes, but rarely with pure fluids. The fluids encountered are mixtures and often non-condensable gases are present, and this makes the condensation process very complex. In the case of mixtures or in the presence of a non-condensable gas, the vapour must diffuse through the gas to the interface before condensing. The use of a reflux condenser in place of a conventional downward condenser has two major advantages. First, the separation of the inert gas from the condensate is realised within the heat exchanger and does not require any further separation equipment. Secondly, the condensate is recovered at a higher temperature that in downward condensers, and therefore increase the thermal effectiveness of the system. The issue of this work was to use the experimental results to write a model, which has then been incorporated into a leading process simulation package (HYSYS). The compact condenser model has been verified against plant data and also used to identify the potential for energy saving in column operation using compact condensers. The evaluation of the performance of the cross flow plate condenser represents the core of the experimental work. HYSYS does not feature plate & frame exchangers as unit operations. To do this a special feature of HYSYS, allowing the user to write his/her own unit operation, is used. A user-defined unit operation can be integrated into HYSYS much like any other unit operations. Its behaviour is defined entirely with Visual Basic compatible code provided by the user.
The use of compact heat exchangers in chemical and process industry as well as in automobile, aviation and domestic sectors has grown significantly in the last decade. This has resulted in an influx of research activities aimed at developing smarter surfaces / channel geometry's for achieving heat transfer enhancements whilst reducing the space requirement. One way of reducing the weight of the heat exchanger is to use polymeric materials as the material of construction. Unfortunately polymers are poor thermal conductors and a wall thickness of 0.5 �1 mm can adversely affect the heat transfer performance. However PIIC at Newcastle has taken advantage of the fact that PEEK (poly-ether-ether ketone), a thermoplastic with a working temperature of 220 ºC and a 100 µm thick film, can be easily corrugated and can withstand a differential pressure of approximately 10 bars. In addition PEEK has attractive chemical resistance properties making it suitable for application in chemical aggressive areas. The design and testing of a PEEK heat exchanger has been undertaken at the University of Newcastle. The main benefits of using a PEEK heat exchanger should be its improved chemical resistance, non-magnetic properties, reduced fouling as it has very low surface energy, high thermal efficiency and very high surface area densities. A specific study of the performance of a PEEK PFCHE for condensation has been carried out and a test facility has been developed. Experimental research has been conducted on a compact polymer film cross-corrugated compact heat exchanger. Experiments centred on the performance of the unit for the condensation of water vapour from a humid air stream under various operating conditions. Performance has been quantified in terms of heat transfer coefficients and effectiveness. Description The matrix used for the acquisition of design data was constructed by bonding the corrugated PEEK sheets. The heat exchanger matrix consists of seven layers of cross-corrugated PEEK sheets (13.5cm x 13.5cm x 100mm), stacked vertically in cross corrugation and housed in a PERSPEX block. The polymer sheets were arranged such that a perfect cross flow (parallel/perpendicular orientation) was obtained. A 45º orientation of the corrugation was tested too. Research activities concentrated on the following areas: -The effect of gas flow rate on heat transfer performance. -The effect of liquid flow rate on heat transfer performance. - The effect of corrugation orientation on heat transfer performance. - Liquid condensate hold-up inside the heat exchanger. Two different methods were investigated to describe the performances of the heat exchanger under condensation duties. The first method was the LMTD (log mean temperature difference) approach and the second method described the heat exchanger in terms of effectiveness and number of transfer units (NTU). The results of the experiments would indicate that hold-up was greater for the configuration in which 45º orientation of the corrugation was used. Compared with the slightly lower heat transfer performance of this configuration this would suggest that the parallel/perpendicular orientation was the better one for the PFCHE. - Heat transfer and pressure drop data have been obtained for the PFCHE operating over a range of gas and liquid flow rates. - Performance results have been given in terms of heat transfer coefficients, effectiveness and NTU using modified methods from standard heat transfer theory. - The results show that these types of values are not wholly appropriate to this configuration due to the additional energy derived from condensation and that absolute temperature and humidity need to also be considered when examining system performance. - Heat transfer is limited by the gas side and typical heat transfer coefficients for the system lie in the region of 50 to 450 Wm{-2}K{-1} when operating with 30-45°C saturated air. - Condensation rates of up to{-1} from saturated air were achieved in this system. - The effect of PEEK corrugation orientation with respect to the direction of flow was inconclusive with an indication that hold-up might be less and performance greater in for the 1st configuration oriented parallel/perpendicular to the flow (as opposed to 45° to the flow). - Condensate hold-up was significant and estimated at around 50% to 90%. The level of liquid in the system was shown to decrease steadily with increasing gas flow suggesting high flow velocities may be preferable to efficiently remove condensation from the packing. - The pressure drop on the condensing side of the exchanger is 25% less for the vertical configuration and the condensate hold-up for the vertical orientation is smaller. These trends are consistent with the fact that the vertical orientation aids the drainage of condensate from the condenser. - The rate of heat transfer in the vertical configuration is not noticeably better than that in the horizontal configuration. The values of the heat transfer coefficients are similar but a comparison of the heat transfer rate per unit area shows that as the gas flow rate increases the performance of the vertical orientation is better. As for the pressure drop, the improvement in performance can be attributed to the condensate being cleared more easily from the vertical channels.

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