Distributor effects near the bottom region of turbulent fluidized beds

The distributor plate effects on the hydrodynamic characteristics of turbulent fluidized beds are investigated by obtaining measurements of pressure and radial voidage profiles in a column diameter of 0.29 m with Group A particles using bubble bubble-cap or perforated plate distributors. Distributor pressure drop measurements between the two distributors are compared with the theoretical estimations while the influence of the mass inventory is studied. Comparison is established for the transition velocity from bubbling to turbulent regime, Uc, deduced from the pressure fluctuations in the bed using gauge pressure measurements. The effect of the distributor on the flow structure near the bottom region of the bed is studied using differential and gauge pressure transducers located at different axial positions along the bed. The radial voidage profile in the bed is also measured using optical fiber probes, which provide local measurements of the voidage at different heights above the distributor. The distributor plate has a significant effect on the bed hydrodynamics. Owing to the jetting caused by the perforated plate distributor, earlier onset of the transition to the turbulent fluidization flow regime was observed. Moreover, increased carry over for the perforated plate compared with the bubble caps has been confirmed. The results have highlighted the influence of the distributor plate on the fluidized bed hydrodynamics which has consequences in terms of comparing experimental and simulation results between different distributor platesPublicad

Performance of a LiBr water absorption chiller operating with plate heat exchangers

This paper studies the performance of a lithium bromide water absorption chiller operating with plate heat exchangers (PHE). The overall heat transfer coefficients in the desorber, the condenser and the solution heat recoverer are calculated using the correlations provided in the literature for evaporation, condensation and liquid to liquid heat transfer in PHEs. The variable parameters are the external driving temperatures. In the desorber, the inlet temperature of the hot fluid ranges from 75°C to 105°C. In the condenser and the absorber, the inlet temperature of the cooling water goes from 20°C to 40°C. The coefficient of performance (COP) obtained ranges from 0.5 to 0.8 for cooling duties ranging from 2 kW to 12 kW. The chiller response to different hot fluid temperatures and circulated mass flow rates is also presented. The performance and the internal parameters of the chiller at part load are, therefore, calculated. A higher efficiency results when the solution pumped from the absorber to the desorber decreases. The heat transfer analysis of the PHEs is also presented. The overall heat transfer coefficient in the desorber, equal to 790 W/m²K at the design conditions, is also analysed at part load. The condenser performance can be represented by a similar relationship found in conventional air cooled condensersThis work was partially funded by CICYT Ministerio de Ciencia y Tecnología (CLIMABCAR project no. DPI 2003-01567)Publicad

Fluidization of Group B particles with a rotating distributor

A novel rotating distributor fluidized bed is presented. The distributor is a rotating perforated plate, with 1% open-area ratio. This work evaluates the performance of this new design, considering pressure drop, Δp, and quality of fluidization. Bed fluidization was easily achieved with the proposed device, improving the solid mixing and the quality of fluidization. In order to examine the effect of the rotational speed of the distributor plate on the hydrodynamic behavior of the bed, minimum fluidization velocity, Umf, and pressure fluctuations were analyzed. Experiments were conducted in the bubbling free regime in a 0.19 m i.d. fluidized bed, operating with Group B particles according to Geldart's classification. The pressure drop across the bed and the standard deviation of pressure fluctuations, σp, were used to find the minimum fluidization velocity, Umf. A decrease in Umf is observed when the rotational speed increases and a rise in the measured pressure drop was also found. Frequency analysis of pressure fluctuations shows that fluidization can be controlled by the adjustable rotational speed, at several excess gas velocities. Measurements with several initial static bed heights were taken, in order to analyze the influence of the initial bed mass inventory, over the effect of the distributor rotation on the bed hydrodynamics.Publicad

Parametric study of operating and design variables on the performance of a membrane-based absorber

A plate-and-frame microchannel H2O&-LiBr absorber using a microporous membrane as contactor between the vapour and the solution is simulated. The heat and mass transfer equations, describing the absorption of the vapour phase into the solution, are solved for different membrane properties and for variable design and operating conditions. The parametric study evaluates the sensitivity of the ratio between the cooling capacity of the chiller and the absorber volume (rqV) to changes in the following parameters: width and height of the solution and cooling water channels; concentration, temperature and mass flow rate of the solution; temperature and mass flow rate of the cooling water; porosity, pore diameter, thickness and thermal conductivity of the membrane; thickness and thermal conductivity of the interface wall between the solution and the cooling water; and temperature, pressure and mass flow rate of the vapour. At the design stage of the membrane absorber, the parameters that can be optimised to maximise rqV are porosity, pore diameter, solution channels depth and membrane thickness. The thickness of the interface wall between the solution and the cooling water, as well as the solution channels width should be also taken into account. For a good performance during the operation of the absorber, special care should be taken to select the adequate vapour pressure and solution inlet temperature and concentration.The financial support of this study by the Ministerio de Economía y Competitividad of Spain through the research grant ENE2013- 43131-R is greatly appreciated

Experimental characterisation of a novel adiabatic membrane-based micro-absorber using H2O-LiBr

In the interest of reducing the size of absorption chillers, a novel adiabatic membrane-based micro-absorber prototype is experimentally studied. Water-lithium bromide solution is used as the working fluid flowing through 50 rectangular microchannels of 0.15mm height, 3 mm width and 58 mm length. In the present study, a laminated microporous PTFE membrane of 0.45 µm pore diameter, separating the solution from the vapour, is tested. It incorporates a supporting layer of polypropylene. Different operating parameters were tested, including the inlet solution mass flow rate, temperature and concentration and the pressure potential for absorption. The measured concentration and temperature of the solution at the absorber outlet are used to evaluate the mass transfer characteristics of the micro-absorber. It is demonstrated that the process is controlled by the solution mass transfer resistance. Calculated results of the absorption rate and the absorption ratio show the advantages of the proposed design considering its compactness. The cooling power of a hypothetical chiller equipped with the tested micro-absorber of 73.7 cm3 effective volume, for the range of variables considered in this study, is 41 W. The modular configuration of the absorber allows to easily scale-up the cooling capacity

Modeling and performance analysis of an absorption chiller with a microchannel membrane-based absorber using LiBr-H2O, LiCl-H2O, and LiNO3-NH3

In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5-cm length and 9.5cm(3) in volume provided with a porous membrane is presented for 3 different solution-refrigerant pairs: LiBr-H2O, LiCl-H2O, and LiNO3-NH3. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625kW/m(3) for the LiNO3-NH3, 552kW/m(3) for the LiBr-H2O, and 318kW/m(3) for the LiCl-H2O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO3-NH3 and the LiBr-H2O solutions provide the largest cooling effects. The LiNO3-NH3 can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr-H2O system. The lowest cooling effect and coefficient of performance are found for the LiCl-H2O solution, but this mixture allows the use of lower temperature heating sources (below 70 degrees C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature

Experimental performance comparison of three flat sheet membranes operating in an adiabatic microchannel absorber

A microchannel absorber working adiabatically with the H2O-LiBr pair was tested experimentally using three different nanofibrous flat membranes separating the vapour from the solution. Pore diameters of the membranes were 1 and 0.45 mu m, and thicknesses vary from 25 to 175 mu m. The experimental absorption rates varied from 1.5.10(-3) to 2.6.10(-3) kg/m(2) s varying linearly with the solution mass flow rate circulating through the channels. The reduction in pore diameter from 1 mu m to 0.45 mu m induced the need for higher pressure potential or solution mass flow rate to obtain similar performance. Relationships between changes in diameter pore and membrane thickness from previous models were used to quantify the effect of these membranes characteristics on the absorption ratio. The analytical results compared well with our experiments. In the present design, the solution film thickness was 150 mu m and the solution mass transfer resistance dominated the process. The experimental overall resistances, compared with calculated values from correlations used in previous models, showed differences below 30%

Plastic fiber-optic probes for characterizing fluidized beds in bubbling regime

Bubble measurements on a fluidized bed in bubbling regime using optical fibre probes (OFP) are reported. Comparisons between commercial pressure transducers (PT) measurements and OFP have also been carried out. OFP are able to detect smaller bubbles than the PT and reflect more localized phenomena in the bed.Publicad

Energy and Exergy analysis in an asphalt plant's rotary dryer

In this paper, energy and exergy analyses of a rotary dryer employed in a Hot Mix Asphalt (HMA) plant for heating and drying of the aggregates in the mixture is presented. In the analysis, the exergy method in addition to the more conventional energy analysis, is employed to identify and evaluate the ther modynamic losses. The results show that, at design conditions, the plant performs with energy and exergy efficiencies of 0.89 and 0.18, respectively. The energy losses are mainly due to the flue gases. The exergy distribution indicates that the combustion and the heat transfer at different temperatures in the burner yield the highest exergy destruction in the process. A parametric study is conducted for the plant under various operational production parameters, including different humidities of the aggregates and filler content in aggregates, working temperatures and ambient conditions, in order to determine the parameters that affect the plant performance. It is shown that the solids humidity has a great impact on energy requirements. A better and sustainable use of the heat source employed in the dryer is proposed to avoid the high irreversibilities found. Furthermore, operating corrections in the mix or in the exhaust gas temperature are proposed to optimize the performance of the plantThis work has been has been partially funded by the Spanish Government, Project DPI2009 10518 and the Fenix project Research on new concepts for more safe and sustainable roads (CENIT 2007 1014). Also, the authors aknowledge the support provided by Intrame S.APublicad

Standard deviation of absolute and differential pressure fluctuations in fluidized beds of group B particles

This work describes the behaviour of the standard deviation of pressure fluctuations in fluidized beds for group B particles in the bubbling regime. An empirical–theoretical function, which depends on the gas velocity, is proposed for predicting the pressure signal fluctuations, and the corresponding values of are calculated. The differences in the standard deviation of pressure fluctuations obtained for absolute or differential sensors are analyzed and compared to experimental values corresponding to different bed dimensions, pressure probe positions and particle properties.This work has been supported by the National Energy Programme of the Spanish Ministry of Education under the project number ENE2006-01401 and by the Universidad Carlos III de Madrid (CCG06-UC3M/ENE-0764).Publicad