Mass transfer modelling for GS heavy water plants: Part 1: Point efficiency on GS sieve trays

A model that takes into account the gas-phase and liquid-phase resistance to mass transfer has been developed, where the overall mass transfer coefficient (K_OGa) is expressed as a function of the equivalent Sauter-mean bubble diameter. This parameter was back calculated from mass transfer measurements made at a pilot plant on single pass sieve trays of 0.311 m diameter. Hydraulic parameters were measured for these trays as well. Mean bubble diameters were then correlated as a function of active area F-factor and dispersion height for various tray geometries, and these correlations are used to predict point efficiencies on production plant trays up to 8.5 m in diameter.     

Mass transfer modelling for GS heavy water plants: Part II: Tray efficiency on GS sieve trays

A mass transfer model to predict tray efficiency on industrial GS process sieve trays is presented. Tray efficiencies are predicted from point efficiencies on large trays up to 8.5 m in diameter, by taking into account effects of liquid flow distribution, weeping, entrainment, gas mixing and liquid mixing in the downcomers. Predictions by the model agree with tray efficiencies measured at the heavy water plants.     

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

The 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     

Effect of two-pass trays on distillation efficiencies

Two important structural factors that affect distillation efficiencies, the outlet weir heigh and the liquid flowpath length, are investigated. Performance and efficiency data of an industrial scale i-butane/n-butane distillation column equipped with two-pass trays are used as a basis for the calculations. A mathematical development for a new method for predicting the numbers of vapor and liquid phase transfer units is given. This method together with some other NTU calculation methods is used to assess the effect of the outlet weir height on efficiencies. The effect of outlet weir height on the Murphree tray efficiencies is investigated using the observed point efficiencies and different point efficiency to the Murphree tray efficiency relation methods. The effect of varying liquid flowpath lengths on efficiencies is studied by calculating the Murphree tray efficiencies for one-pass and two-pass trays. The results obtained using the NTU calculation method presented in this study show that a certain outlet weir height point efficiency reaches its maximum. Most of the other methods give opposite results giving a minimum for point efficiency at a certain outlet weir length. The results also show that the Murphree tray efficiencies for one-pass trays are higher than for two-pass trays. Obviously, this is caused by the longer liquid flowpath length of one-pass trays. The Murphree tray efficiencies are also calculated for an industrial-scale MTBE purification column. The column is equipped with two-pass trays in the stripping section and with one-pass trays in the rectifying section. The Murphree tray efficiencies of one-pass trays are considerably higher than the two-pass tray Murphree tray efficiencies.     

Mass transfer in a single-hole tray column

A theoretical model for deuterium exchange in the H₂S/H₂O system was derived to describe mass transfer in the bubble formation, bubble displacement and bubble breakup regions above a single-hole tray in an 8 cm diameter transparent column. The model was applied to data measured at 31°C in the pressure range of 790 to 2000 kPa and in a wide range of gas and liquid flows. High speed cinematography was used to measure bubble diameters, bubble formation times and bubble contact times in the liquid, all of which are parameters in the mass transfer models. Results show that for single-hole trays and the major part of mass transfer occurs in the upper surface of the froth where bubbles disengage from the liquid. However, when the same model is applied to pilot plant sieve trays, it is shown that the bulk of mass transfer takes place in the region where bubbles are ascending in the froth. The tray efficiencies predicted by the model agree well with measured values.     

Efficiencies of Sieve Tray Distillation Columns by CFD Simulation

A 3‐D two‐fluid CFD model in the Eulerian‐Eulerian framework was developed to predict the hydrodynamics and heat and mass transfer of sieve trays. Interaction between the two phases occurs via interphase momentum and heat and mass transfer. The tray geometries are based on the large rectangular tray of Dribika and Biddulph and FRI commercial‐scale sieve tray of Yanagi and Sakata. In this work a CFD simulation is developed to give predictions of the fluid flow patterns, hydraulics, and mass transfer efficiency of distillation sieve trays including a downcomer. The main objective has been to find the extent to which CFD can be used as a design and prediction tool for real behavior, concentration and temperature distributions, and efficiencies of industrial trays. Despite the use of simple correlations for closure models, the efficiencies obtained are very close to experimental data. The results show that values of point efficiency vary with position on the tray because of variation of affecting parameters, such as velocities, temperature and concentration gradients, and interfacial area. The simulation results show that CFD can be used as a powerful tool in tray design and analysis, and can be considered as a new approach for efficiency calculations and as a new tool for testing mixing models in both phases. CFD can be used as a “virtual experiment” to simulate tray behavior under operating conditions.     

Predicting valve tray efficiency

Existing literature models for predicting the mass transfer efficiencies of binary hydrocarbon, distillation columns employing moving valve trays are evaluated. Only four models for predicting valve tray efficiencies exist in the open literature. All of these models use data from valve trays. The last theoretical model was published in 1972, 42 years ago. By comparison, sieve tray efficiency models are numerous and recent. Sieve tray models were developed from large databases. There are no valve tray equivalents to the fundamental mechanistic models available for sieve trays. Despite the differences between valve and sieve trays, many of the phenomena on sieve and valve trays are similar. Consequently, sieve tray models can be employed to provide estimates for valve trays. In this work, using public FRI data on round moving valves, the performance of the Chen and Chuang sieve tray mechanistic model is compared to the performance of four valve tray models. It appears that, in the absence of fundamental (and qualified) valve tray models, the sieve tray models present a potential alternative for valve tray efficiency predictions.     

Parallel-flow slotted sieve tray efficiency

Parallel-flow trays incorporating slotted sieve tray decks were originally developed for air separation applications. They are still used for that purpose and also in a wide range of chemical and petrochemical separations. The present paper outlines the main features of the trays, including the liquid flow-path arrangement and the slotting pattern that is typically used. The results of an experimental study are presented in which the tray efficiency was measured by using a 6.1 m diameter air-water tray simulator. The system used was acetone stripped from water by air. This is a convenient system because the slope of the equilibrium line is close to unity and so the results are relevant to distillation systems. For purposes of comparison, trays were tested both with and without a slotted tray deck. In the latter case, severe liquid maldistribution was evident. The measured tray efficiencies of the slotted and unslotted trays were found to be similar when no outlet weirs were used, but the slotted tray had a lower froth height and pressure drop. Theory was used to predict the point efficiencies for the two trays and to show that the slotted tray had a larger enhancement of tray efficiency over point efficiency than the unslotted tray. The measured tray efficiencies divided by the calculated point efficiencies were compared with theoretical predictions and reasonable agreement was demonstrated.     

The second law optimal state of a diabatic binary tray distillation column

A new numerical procedure to minimize the entropy production in diabatic tray distillation columns has been developed. The method was based on a least square regression of the entropy production at each tray. A diabatic column is a column with heat exchangers on all trays. The method was demonstrated on a distillation column separating propylene from propane. The entropy production included contributions from the heat transfer in the heat exchangers and the mass and heat transfer between liquid and vapor inside the distillation column. It was minimized for a number of binary tray distillation columns with fixed heat transfer area, number of trays, and feed stream temperature and composition. For the first time, the areas of heat exchange were used as variables in the optimization. An analytical result is that the entropy production due to heat transfer is proportional to the area of each heat exchanger in the optimal state. For many distillation columns, this is equivalent to a constant driving force for heat transfer. The entropy production was reduced with up to 30% in the cases with large heat transfer area and many trays. In large process facilities, this reduction would ideally lead to 1-2 GWh of saved exergy per year. The most important variable in obtaining these reductions is the total heat transfer area. The investigation was done with a perspective to later include the column as a part in an optimization of a larger process. We found that the entropy production of the column behaved almost as a quadratic function when the composition of the feed stream changed. This means that the feed composition is a natural, easy variable for a second law optimization when the distillation column is a part of a process. The entropy production was insensitive to variations in the feed temperature.     

Trays for gas-liquid contacting

A type of tray for gas-liquid contacting is described, in which the free area for gas flow through the tray is variable with and controlled by the gas flowrate. The property through which control of the free area is effected is the surface tension of the liquid phase. The advantages are the same as those of ballast and valve trays; namely, reduced pressure drop at high gas rates resulting in increased column capacity, and reduced free space at lower gas rates resulting in high mass transfer efficiency at low and medium vapor loadings as well. Tooling and production costs for the trays of the present investigation, however, are considerably lower than for ballast or valve trays or any other types which require machining operations in their fabrication, and are as low or lower than for sieve trays. Initial experimental comparisons with conventional trays demonstrated that the trays of the present investigation, although designed primarily for operation at high vapor loadings, were indeed capable of yielding high mass transfer efficiencies in the low-to-medium range of vapor loading     

Efficiency of dualflow trays in distillation

The 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%.     

新垂直筛板塔汽体不混合时的板效率数学模型

本文在新垂直筛板双池模型基础上,建立了汽体不混合时板效率数学模型,给出了逐板逐池数值计算方法,导出了液体并流塔板的板效率解析式,将液体并流及逆流塔板的板效率结果分别与假定汽体完全混合时的板效率值进行了比较。     

Effect of a variation in perforation size on performance of sieve tray absorbers

A six inch diameter sieve plate absorption column was used to assess the effect of perforation size on the performance of sieve trays under otherwise identical conditions. Four separate test trays with equal gas flow areas but perforations of 1/8-inch, 1/4-inch, 1/2-inch and 1-inch respectively, were used with the liquid phase absorption-controlled system of carbon dioxide — water at 77°F and approximately 830 mm. mercury. Comparisons of overall efficiencies indicated that the trays with 1-inch diameter perforations had efficiencies similar to, or slightly lower than those of 1/8-inch diameter. Concentration profiles within the froth were measured, with distinct profiles being observable in both the horizontal and vertical directions. This indicated that neither the plug flow nor the completely mixed models accurately described the actual transfer process. Accordingly, comparisons of volumetric mass transfer coefficients computed from both models were made. Comparison of volumetric mass transfer coefficients showed that for most gas and liquid flowrates the plates with 1-inch perforations performed as well as plates with smaller holes. For this system it was concluded that trays with large holes would be much better suited to fouling services and, indeed, they would be only marginally less attractive than trays with smaller holes for all services.     

B型导向浮阀塔板实验研究

为增大导向浮阀塔板对液体流动的推动力,在1600 mm×400 mm实验塔内,利用空气-水系统,对B型导向浮阀塔板进行实验研究。与导向浮阀塔板相比,它具有更好的流体力学性能和更大的液体负荷适用范围。实验研究和工业应用表明,B型导向浮阀塔板具有处理能力约提高20%、塔板效率约提高10%、操作弹性约提高30%等突出特点,用于蒸馏、吸收、汽提等传质过程,可获得良好效果。     

筛板不均匀漏液的规律及其影响

筛板不均匀漏液的规律及其影响曾爱武刘富善许松林黄洁余国琮（天津大学化学工程研究所，天津３０００７２）关键词筛板不均匀漏液塔板效率１引言塔板漏液是一种导致塔板效率降低的纵向返混。在工程设计中通常以相对漏液１～１０％来决定气相负荷下限，但值得指出的...     

A new method for determining the efficiency of tray columns with downcomers

A new method for determining the efficiency of columns with sieve, sieve-valve, valve, bubble cap and tunnel trays is introduced, taking into consideration entrainment, weeping and mixing of liquid on the tray. The presented calculation concept is valid for any stripping factors and any quality of liquid mixing on the tray. A comparison of the new calculation concept with experimental data from literature resulted in an average relative deviation of approx. 8% between the sieve tray efficiencies. The comparison of calculated values with experimental data for sieve-valve, valve and tunnel trays also produced a satisfactory agreement.     

垂直筛板塔流体力学与传质研究状况

对我国在新型垂直筛板塔的流体力学性能 ,诸如塔板干板压降与湿板压降、板孔漏液、塔板过量雾沫夹带、板上帽罩单元的液体提升能力与液体循环程度的研究及其传质性能 ,诸如塔板传质效率模型、塔板空间气相中液体大小与分布、板间空间立体传质作用的研究等进行了综合 ,并予以分析论述 ,从中看出我国对新型垂直筛板塔已进行了较全面深入的研究工作 ,并取得了新进展。     

Understanding maldistribution in 3-pass trays

Understanding 3 pass trays is the key to design of 6 pass trays, such as those in the large towers expected in carbon-capture services. This paper presents a hydraulic distribution model for 3 pass trays with no liquid or vapor equalization. The model is verified by comparison with FRI's test data and with one operating tower. Our paper predicts good distribution for the 3 pass trays tested at FRI, and shows that the poor efficiencies in some of the tests were due to losing the downcomer seals and not tray maldistribution. The model predicts good distribution at maximum rates in the operating tower, but finds that at lower rates, where valves open and close, there is an extreme sensitivity of vapor distribution to small pressure drop variations. This sensitivity precludes reliable modeling of distribution in this region and may define an unstable region in the operation of 3 pass trays.     

Tray-to-tray method for estimation of the number of trays in gas–liquid columns in case of intensive entrainment

The paper deals with the tray-to-tray method for determining the required number of trays in gas–liquid columns in case of intensive entrainment. The equations for estimation of the tray efficiency are derived for top, bottom and intermediate trays, and they incorporate the number of gas and liquid phases transfer units, stripping factor, Peclet number of mixing and fractional entrainment. These equations can be simplified to the well-known one-parametric diffusion model explained in Bubble Tray Design Manual (1958) [Bubble Tray Design Manual, 1958, AIChE, New York] for case of negligible entrainment. The algorithm of tray-to-tray method is based on tray efficiency calculated for each tray separately and it enables the calculation of the number of required trays, starting from the bottom tray. Given example shows how the excessive entrainment decreases of the driving force of the process.     

浮阀塔板最新应用研究进展

浮阀塔是一种应用极为广泛的汽液传质设备,本文介绍了国内研究开发的新型浮阀塔板。这些浮阀塔板是在F1型浮阀塔板的基础上开发而成的,相比于F1型浮阀塔板,具有压降低、雾沫夹带量小、泄漏量小、处理量大等优点。本文以塔板的开发年代和塔板类型为主线,对这些浮阀塔板的结构特点、流体力学、传质性能、优缺点等进行了概括总结,对每个系列浮阀塔板的设计开发理念进行了总结概括,同时介绍了导向浮阀塔板在齐鲁石化公司丁二烯装置中应用的成功实例,很大程度地提高了塔板负荷率和产品质量;并简单介绍了一些常用塔板在工业生产中的应用情况,阐述了这些新型浮阀塔板的发展思路,即浮阀形状以条形为主,并且大部分浮阀塔板都开有导向孔;最后指出了今后塔板技术的研究和发展动向。