Dual-flow Trays Research Articles

Determining heat and mass exchange kinetic in a column heat exchanger with direct phases contact

The object of this study was the processes of heat and mass transfer in a column heat exchanger with direct phases contact. The investigated problem is related to the need to determine the estimates of the kinetic characteristics of heat exchange during the concentration of solutions in a contact heat exchanger equipped with dual-flow trays. In particular, it was assumed that the determination of the influence of the gas velocity in the apparatus and the density of liquid irrigation of the plates, as well as the geometry of the plates, on the kinetic coefficients would make it possible to reveal the patterns of heat and mass exchange between the liquid and the air in contact with it in the column apparatus. It was determined that to increase the intensity of mass transfer in the gas and heat transfer in the liquid, it is necessary to increase the gas velocity and irrigation density. Then the gas velocity and irrigation density have approximately the same effect on the intensity of mass transfer in the gas and heat transfer in the liquid. When studying the effect of the geometry of the plate on the kinetic coefficients of heat transfer in the liquid and mass transfer in the gas, it was established that the value of the portion of the free cross section of the plate has a decisive influence on the value of the considered kinetic coefficients. A generalized equation was built, which makes it possible not only to calculate the enthalpy transfer coefficient during the interaction of sodium chloride solution with air but also to determine the limiting stage of this process. The results could be used to design a unit for concentrating a hot solution of sodium chloride by evaporating water during blowing with air in a contact heat exchanger. This would make it possible to obtain crystalline sodium chloride using secondary energy resources and other non-traditional sources of thermal energy

Experimental Fluid Dynamics Study of a New Fractal Dual-Flow Tray

Countercurrent flow in conventional dual-flow trays results in segregation in liquid and vapor flow, reducing tray efficiency. Therefore, a new dual-flow tray with fractal geometry was experimental...

Comparison of Performance Characteristic Prediction of Sieve Tray with and without Down comer

A sieve tray is a type of tray that is widely used in separation in industry. While sieve tray without down comer, frequently called dual flow sieve tray is commonly used to increase the effective area, so it can increase tray capacity. Dual-flow sieve tray operations can be classified into two categories, it is froth and transition based on gas/vapour velocity, so that there are parts of holes that are only flown by gas/vapour and at the same time there are parts of holes that are only flushed with liquid. Performance characteristics studied include clear-liquid height, dry and wet pressure drop, efficiency, and entrainment. The study aims to compare performance characteristic of sieve tray with and without down comer. The sieve tray without down comers showed almost 50% higher capacity compared to the sieve tray with down comers. However, sieve tray without down comers has 40% lower pressure drop compared to that with down comers. The results can ensure that the trays performance prediction for 298 mm diameter with open area in the range of 77-80%, 5 mm diameter holes, 12.5 mm hole spacing and 35 cm tray spacing satisfying and optimistic can be utilized in the distillation process.

Hydrodynamics and mass-transfer characteristics analysis of vapor–liquid flow of dual-flow tray

In the present study, the behavior and details of vapor–liquid contact on the dual-flow trays (DFTs) were investigated using a 3D CFD model within the two-phase Eulerian framework. The simulation provided good agreement with experimental results, which verified the reliability of the model. Firstly, the operation range was divided into four regimes having different characteristics of flow phenomena, and most attention was paid to the hydrodynamic behavior of froth regime and fluctuating regime due to its importance in operation and structure improvement. Then, the liquid flow characteristics of these two regimes were revealedwith the analysis of velocity profiles and liquid phase distribution contour. Meanwhile, some in-depth picturing of local events of hydrodynamics and mass-transfer performance was also presented through the discussion of cyclohexane liquid volume fraction distribution and vapor-phase mole fraction distribution. The froth regime was proved to have higher and homogeneous point efficiency at bulk zone around the plate center area than the fluctuating regime. Thus, the improvement of DFT should be focused on the restriction of “free-flow” to prolong the froth regime and delay the formation of vortex flow with circulation cells. Finally, an orthogonal wave tray (OWT) was designed and compared with DFT for evaluation of the effect of proposed modifications on the enhancement of tray hydraulics, mass-transfer efficiency and stability.

Experimental study of two phase flow characteristics on the dual-flow tray

This paper addresses an experimental investigation in the hydrodynamic behavior of a new type of dual-flow fixed valve tray. Water/air system was used at atmospheric pressure and ambient temperature. The dry pressure drop, total pressure drop and clear liquid height were measured and the comparisons between different tray geometry were made. The results show that the total pressure drop and clear liquid height have the same trend, which is increased with the decreased center distance of holes, increased holes diameter and decreased open area. Correlation for clear liquid height is proposed, and the agreement of the experimental and calculated data is demonstrated in the paper.The results show that gas and liquid flow counter-currently through the tray hole area and four main hydrodynamic regimes are distinguished at different gas/liquid load. A characteristic parameter named the fraction of holes passing gas is defined and deduced. The calculated results show that the fraction of holes passing gas is reduced by the decreased center distance of holes and holes diameter, while the increased open area leads to the less fraction.

Hydrodynamics and Mass-Transfer Analysis of a Distillation Ripple Tray by Computational Fluid Dynamics Simulation

A 3D two-phase computational fluid dynamics model in the Eulerian–Eulerian framework was developed to predict the hydrodynamics, mass-transfer behaviors, and tray efficiency of dual-flow trays: ripple trays. Interaction between the two phases occurs via interphase momentum and mass transfer. Mass-transfer coefficients were estimated using the Higbie penetration theory model. The simulated results were compared with the experimental data obtained from distillation of cyclohexane and n-heptane at total reflux. The results show that vapor and liquid flow countercurrently through the tray holes and four main hydrodynamic regimes are distinguished at different vapor/liquid loadings (Fs factor). It was found that the mass transfer of the spray zone above the froth was also significant, especially at lower loadings. In addition, the results indicated that the efficiency of a ripple tray was a strong function of the open hole area and Fs factor.

Overall efficiency evaluation of commercial distillation columns with valve and dualflow trays

AbstractThe main objective of this work is to establish appropriated ways for estimating the overall efficiencies of industrial distillation columns with valve trays with downcomer and dualflow trays. The knowledge of efficiencies has fundamental importance in the design and performance evaluation of distillation columns. Searching in the literature, a tree of alternatives was identified to compose the tray efficiency model, depending on the mass transfer models, the liquid distribution and vapor flow models on the tray, the liquid entrainment model, the multicomponent mixture equilibrium model, the physical properties models, the height of froth on the tray model and the efficiency definition. In this work, different methods to predict the overall efficiency of distillation columns with valve and dualflow trays were composed and compared with data from three commercial distillation columns under different operating conditions. The models were inserted in the Aspen Plus 12.1 simulator, in Fortran language, together with tray geometrical data, fluid properties and operating data of the distillation columns. For each column, the best thermodynamic package was chosen by checking the temperature profile and overhead and bottom compositions obtained via simulation against the corresponding actual data of industrial columns. A modification in the fraction of holes evaluation that is jetting parameter of the Garcia's hydraulic model of dispersion above the tray was proposed. This modification produced better results than the original model to predict the fraction of holes that are jetting and in the efficiency of dualflow trays and similar results to Garcia model in the efficiency evaluation of valve trays. © 2010 American Institute of Chemical Engineers AIChE J, 2010

Study on vapor-rise-up through hanging downcomers

A common problem in high capacity trays is the difficulty of establishing a liquid seal of hanging or truncated downcomers since the hanging or truncated downcomers came out. When the downcomer seal is lost, the vapor rises up through the downcomer, which may cause tray instability and poor separation. For the downcomer unseal or vapor-rise-up, it was found that there exist two regimes of free vapor-rise-up regime and dynamic vapor-rise-up regime. The mechanism of free vapor-rise-up is lower liquid height than the tray pressure drop, whereas the mechanism of dynamic vapor-rise-up is the driving force of the tray pressure drop. Using the hydraulic model of dualflow tray, the vapor-rise-up model in dynamic regime was developed. The experiments were carried out in a 500 mm-diameter transparent plexiglass column with air–water system. From the experimental data, the vapor-rise-up correlation was obtained. The free vapor-rise-up regime converts into dynamic regime with the increase of the vapor rate, and the transition point is a certain real velocity of rise-up vapor. For preventing the vapor-rise-up, a seal line is required. With the definition of the rise-up vapor being limited to a certain ratio of rise-up vapor rate to liquid rate, the seal line is qualitatively obtained from the vapor-rise-up model. The seal line has the minimum liquid rate, which is in accord with the seal line by Li et al. [Li, Y.M., Yu, X.M. and Yao, K.J., 2004, Dynamic seals on hanging downcomers. Chem Eng Res Des, 82: 1].

Development of a Novel Basket Impeller Dualflow Tray Catalytic Distillation Reactor

A new catalytic distillation column which combines the superior gas-to-liquid mass transfer attributes of a dualflow tray with the higher reaction rates found in a rotating basket slurry reactor has been investigated in this study to improve conversion and yield for thermodynamically limited reaction systems possessing product-inhibited kinetics using tert-butyl alcohol dehydration over Amberlyst 15 as model reaction. Optimal conditions for nonreactive separation efficiency were also the most favorable for catalytic distillation. Compared to conventional fixed bed catalytic distillation, the new basket impeller column offered a 70% yield improvement in TBA dehydration. Moreover, unlike the common nonrotating catalytic distillation reactor, the spinning basket impeller column can handle reactions characterized by both low equilibrium constant and low Damkohler number.

Use of Structured Packing as a Dualflow Tray in Distillation

A novel dualflow tray design has been studied. This new tray type makes use of a thin slice of structured packing and, as such, is named the structured packing tray (SP-tray). The SP-tray was found to have a 44% higher capacity compared with the dualflow sieve tray using high liquid density systems. However, the capacity and turndown of the SP-tray are much more dependent on the liquid density of the system than the dualflow sieve tray. It was found that the efficiency of the SP-tray remains fairly constant over a wide range of vapour loadings. In regions where vapour loadings were low enough that no froth exists on the surface of the tray, the SP-tray had efficiencies 100% greater than the dualflow sieve tray. The occurrence of the additional vapour–liquid contact time present within the SP-tray itself leads to a nearly infinite turndown ratio.

Parametric Study of a Novel Basket Impeller Column

AbstractThe Basket Impeller Column combines the mass transfer advantages of a spinning basket reactor with the practicality of a dual flow tray column to achieve controllable liquid holdup, as well as a potential solution to many of the problems plaguing current catalytic distillation internals. Sieve plate design, impeller size and speed, along with water and air, flow rates were explored in order to establish a feasible design. An optimal set of specifications was determined via statistical evaluation. Additional experiments were then conducted to obtain correlations for liquid holdup, gas holdup, froth height, pressure drop and power in terms of the significant hydraulic variables.

Distillation Sieve Trays without Downcomers: Prediction of Performance Characteristics

A large amount of performance data on larger-scale trays without downcomers has become available recently, and the data have been examined with a view toward understanding and modeling the contacting mechanisms under distillation conditions. The objective of this paper is to describe new predictive models for efficiency, pressure drop, and flooding of trays without downcomers and to show how they can be applied to distillation separations. Such an effort has not been reported previously. The contacting devices are assigned the generic name of dual-flow trays. The models are shown to give reasonable estimates of the performance of these trays in larger columns with open areas in the range of 15−25%, hole diameters in the range of 12−25 mm, and tray spacings in the range of 0.3−0.6 m.

Column hydraulics: system limit/ultimate capacity

The concept of system limit or ultimate capacity has been around for over 40 years. During the period 1959–1961 Tek [F.R.I. Topical Report 21, Special Collections Section, Oklahoma State University Library, 1959; F.R.I. Topical Report 25, Special Collections Section, Oklahoma State University Library, 1961] combined the entrainment flood approach of Souders and Brown [I&EC 26 (1) (1934) 98] with the drop stability work of Hinze [Appl. Sci. Res. A1 (1949) 273; AIChE J. 1 (3) (1955) 289] and came up with the concept that in a turbulent field there is a maximum capacity of counter flow devices (trays or packings) that is a function of vapor velocity and system properties and is independent of the hardware parameters. He developed a model using data obtained from what was believed to be the highest capacity device known—high open area dualflow (DF) trays (29% hole area) at spacings of 24 to 96 in. A revised model was developed by Stupin [F.R.I. Topical Report 34, Special Collections Section, Oklahoma State University Library, 1965] in 1965 based on additional data, primarily at high pressure, and concepts developed by Levich [Physiochemical Hydrodynamics, Prentice-Hall, Englewood Cliffs, NJ, 1962]. At that time it was recognized that the relatively few experimental data points at low liquid loads tended to fall below model projections. This was not considered to be a problem since the vapor rates in question are normally only encountered in vacuum operations and in 1965 tray pressure drop was the governing factor in vacuum. Recent experimental work with modern, high capacity, low pressure drop devices confirm that at low liquid rates there appears to be a capacity ceiling which is independent of liquid loading. It should be noted that the liquid loading effect was empirical in all versions of the model. This was necessary since the theoretical development was based on the stability and/or deformation of a single drop.

Separation Tray without Downcomers

The Dualflex tray is a fractionation device without downcomers but with mobile valve plates and top weirs positioned above the perforations of the tray deck, which allows liquid and vapor or gas to pass through. According to the results of systematic experimental investigations, this mass transfer device can be considered as a high capacity tray in comparison with conventional trays; its separation efficiency is also higher than that of a comparable Dualflow tray under identical test conditions. It can be recommended not only for installing in new separation plants, but also for revamps in cases where increasing throughput in existing columns are required. The aforementioned efficiency and capacity studies were carried out in a pilot plant of 600 mm column diameter.

Froth Heights on Dual-Flow Trays with a Heterogeneous Binary Azeotropic System and a Heterogeneous Ternary System with a Homogeneous Azeotrope

Visual froth height measurements and the clear liquid heights on 20% free area dual-flow trays, operated at total reflux, have been obtained on the homogeneous system of water−steam, on the heterogeneous systems of ethyl acetate−water and ethyl acetate−ethanol−water, and in the homogeneous region near the homogeneous ternary azeotrope of that ternary system. The experimental results were obtained under distilling conditions at a total pressure of 101.3 kPa for a range of liquid and vapor densities and vapor velocities. The churn flow conditions on the dual-flow trays at total reflux were found to have the characteristic of constant volume fraction liquid in the froth for all systems investigated: ΦL = 0.291; hf < 0.300. A correlating equation for the froth height that contains the impact pressure was found for all systems investigated to be hf = 2.625(1/2ρVu2) with hf < 0.300. A dimensionless correlating form, based on a modified Froude number, was found to be suitable for all systems: FrM = 280.0 × 10-6(ρV/ρL)-1/2 with hf < 0.300. The clear liquid height was found for all systems to follow hCL = ΦLhf with hf < 0.300. The dimensionless correlation for froth heights for all systems could be converted to a flooding vapor velocity relationship at total reflux for dual-flow trays by ufl = 0.0283√Z(ρV/ρL)-3/4 with Z < 0.300.

Froth Heights on Dualflow Trays with a Ternary Azeotropic System of Ethyl Acetate−Ethanol−Water

Experimental froth height measurements have been obtained on nine dualflow trays in a 150 mm diameter glass column. The froth heights can be predicted from Azbel theory, given the height of clear liquid. A comparison of predicted and experimental froth heights at the boiling point temperature in the water−steam system showed constant clear liquid height over a restricted vapor velocity range. Froth heights observed when distilling a ternary azeotropic mixture of ethyl acetate−ethanol−water showed considerable scatter for nine dualflow trays over an F number range from 0.5 to 2.0. The minimum capacity on dualflow trays was observed at an F number of 0.75 for the water−steam system. Turndown ratios, covering a given range of froth heights, were high for the ternary system at 3.60. New economic conditions in the competitive global environment require a reexamination of dualflow trays for low cost, low tray spacing, and good turndown ratios.

Removal of acetic acid from water by catalytic distillation. Part 1: Experimental studies

AbstractCatalytic distillation experiments were carried out in a 100 mm diameter column for the removal of dilute acetic acid from water. The column was installed with a novel internal composed of alternating a dualflow tray and a catalyst basket. Amberlyst 15 was used as a catalyst to accelerate the esterification of acetic acid with methanol. The effects of various operating parameters on the acid removal were investigated. For the feed which contains 2.5 to 9.9 wt% of acetic acid in water, more than 50 wt% of acetic acid can be recovered as methyl acetate in the 1.5 meter high test column.

Efficiency of dualflow trays in distillation

AbstractThe mass transfer efficiencies of dualflow trays with open hole areas of 20, 28 and 37% were experimentally investigated under total reflux conditions in a 300‐mm diameter distillation column using methanol‐water and methanol‐isopropanol systems. The results indicate that the efficiency of a dualflow tray is a strong function of the open hole area, the vapor/liquid load, and liquid properties such as surface tension and density. A fundamental model was developed to predict tray efficiency. The prediction was found to agree with the measured data to within 15.3%.

EFFICIENCY OF A CONTROLLED-CYCLE EXTRACTOR

ABSTRACT Experimental studies were performed with a 4.0 in. (10.2 cm.) diameter extraction column containing ten dualflow type trays and operated in a controlled cycling mode. The systems methyl isobutyl ketone/acetic acid/water and toluene/acetone/water were used. The overall stage efficiency was found to be a function of the ratio of the volume of a phase transferred during a cycle to the volume of that phase held up on the tray, of the feed-to-solvent ratio, and of the phase throughput rates. A non-equilibrium model was developed to represent the experimental data. For the HIBK/acetic acid/water system, overall stage efficiencies were in the range of 30 to 38 percent, corresponding to HETS values of 0.46 to 0.55 meters. For the toluene/acetone/water system, overall stage efficiencies were in the range of 20 to 31 percent, corresponding to HETS values of 0.55 to 0.95 meters.