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     

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.     

Heat transfer modelling of sieve trays

A heat transfer model that accounts for both gas and liquid phase resistance was developed for sieve trays where the overall heat transfer coefficient is expressed as a function of Sauter-mean bubble diameter. These diameters, obtained from pilot plant data, were correlated and used to predict point efficiencies on production plant trays. Applying the appropriate liquid flow distribution model allows the prediction of overall tray efficiencies. Predicted temperature profiles have agreed well with measured plant profiles and tray changes at the production plant have resulted in steam savings that were adequately predicted by the model.     

FOULING OF SIEVE TRAYS

Fouling due to the deposition of solids on sieve trays was studied in a 153 mm diameter column and a 90 mm diameter simulator. Tests were carried out with various gas-liquid-solid systems to determine the effect of the fouling mechanism on tray performance. For an air-water system containing saturated NaHCO₃, fouling from crystallisation occurs due to water evaporation by dry air at the tray holes. For a CO₂-water system containing K₂C0₃ and saturated KHC0₃ the reaction between C0₂ and K₂CO₃ causes the precipitation of KHCO₃ at the gas-liquid interface. For the air-water system containing insoluble solids, flour and Ca(OH)₂, no fouling was observed for a 48 hour test period. Interesting results on tray performance were also obtained. Test results indicate that (1) trays with larger holes have higher resistance to fouling, (2) fouling does not occur when trays are operated under weeping conditions, (3) fouling rate increases with increasing gas loading, (4) a bed of mesh packing on the sieve tray accelerates the fouling rate. A mathematical model has been developed for predicting the effect of crystallisation fouling on tray pressure drop as a function of time.     

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%.     

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.     

Performance of combined mesh packing and sieve tray in distillation

The hydraulic and mass transfer performance of a combined knitted mesh packing and sieve tray has been measured for the distillation of methanol-water mixtures. It was found that by adding a shallow bed of packing, the Murphree tray efficiency increased by 40–50% over a wide range of concentrations and flow rates. This increase in tray efficiency can be attributed to a much smaller and more uniform bubble formation on the packed tray. Hydraulic measurements have shown that the packed trays have lower weeping and entrainment, as well as slightly higher froth heights and pressure drops.     

导向浮阀塔板在醋酸生产中的应用

导向浮阀塔板是在F1型浮阀塔板的基础上开发成功的，较早应用于炼油装置的常减压蒸馏塔，在提高轻油的收率上获得了较大的经济效益，在化工产品的分离中应用较少。该项目在醋酸装置的精馏工序中，采用先进的导向浮阀塔板改造原有的F1型浮阀塔板，达到节能降耗，提高醋酸收率，增加醋酸产量，提高经济效益的目的。     

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.     

半椭圆固定阀塔板性能研究

应用空气–富氧水系统,在直径1 200 mm不锈钢塔内,对半椭圆固定阀塔板进行了实验室研究,研究结果表明:半椭圆固定阀塔板的压降略高于筛孔塔板,比F1浮阀塔板小得多,雾沫夹带率低于筛孔塔板和F1浮阀塔板,泄漏率比筛孔塔板低,比F1浮阀塔板高,传质效率优于筛孔塔板和F1浮阀塔板,是一种综合性能优异的塔板。     

Structured Packings in Liquid-Liquid Extraction

Abstract

Extractors equipped with structured packing are becoming more important in the chemical process industries. These devices provide high mass transfer efficiency and capacity relative to random packings and sieve trays. At the present time, many sieve tray extractors are being retrofitted with structured packings to enhance mass transfer efficiency and capacity. This paper will present a comparison of the performance of structured packing with sieve trays, some background on the commercial development of structured packings, and fundamental models required to design a liquid/liquid extractor equipped with structured packing.     

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.     

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.     

垂直筛板和T形排列条形浮阀塔盘的初步研究

在φ300mm的实验塔中,用水-空气系统对“T”形排列条形浮阀塔盘及“VST”塔盘的流体力学性能及氧解吸效率做了实验研究。得出了塔板压降、泄漏、夹带、液层阻力降以及降液管中清液层高度的初步关联式。 在相同实验条件下与F_1型浮阀塔盘的数据对比、分析的基础上,发现了VST和T形排列条形浮阀塔盘的分离效率和板压降与F_1型浮阀塔盘相当,而操作范围要比F_1型浮阀宽。     

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     

Hydraulics and efficiency of screen trays for distillation

Screen trays are proprietary devices similar to sieve trays which are used in distillation and absorption applications. The pressure drop, liquid holdup and entrainment are measured for screen trays in an air/water column and compared to results obtained with sieve trays. The effciencies of both types of trays were compared in a distillation column using systems of various physical properties (methanol/water, acetic acid/water and cyclohexane/n-heptane mixtures). It was found that the screen tray has a lower dry pressure drop, higher liquid holdup, lower entrainment and weeping, and higher jet flooding capacity than the sieve tray. However, the effciencies of both trays are similar for the systems studied. A model was developed for predicting total pressure drop for screen trays using an air/water column. The model was tested using results for methanol/water, cyclohexane/n-heptane and acetic acid/water systems at total reflux. The deviations are within +/- 20% for 90% of the data points. The effect of a bed of mesh packing on the screen tray was also studied. It was found that the packing led to a higher tray effciency, but resulted in a higher tray pressure drop.     

HYDRAULICS AND EFFICIENCY OF SCREEN TRAYS FOR DISTILLATION

Screen trays are proprietary devices similar to sieve trays which are used in distillation and absorption applications. The pressure drop, liquid holdup and entrainment are measured for screen trays in an air/water column and compared to results obtained with sieve trays. The effciencies of both types of trays were compared in a distillation column using systems of various physical properties (methanol/water, acetic acid/water and cyclohexane/n-heptane mixtures). It was found that the screen tray has a lower dry pressure drop, higher liquid holdup, lower entrainment and weeping, and higher jet flooding capacity than the sieve tray. However, the effciencies of both trays are similar for the systems studied. A model was developed for predicting total pressure drop for screen trays using an air/water column. The model was tested using results for methanol/water, cyclohexane/n-heptane and acetic acid/water systems at total reflux. The deviations are within +/- 20% for 90% of the data points. The effect of a bed of mesh packing on the screen tray was also studied. It was found that the packing led to a higher tray effciency, but resulted in a higher tray pressure drop.     

筛板精馏塔的板效率研究

在直径为0.75 m,塔高4m的不锈钢热模塔内,以正庚烷—环己烷为物系、在常压、全回流条件下,进行筛板塔传质效率的研究,选取孔径为13mm和6 mm,开孔率为3.9％和6.4％的塔板,测试了这些塔板的气液相默弗里板效率,通过排列组合,考察了孔径、不同塔板位置和出口堰高对传质效率的影响,并进行了传质效果对比.此外还运用A...     

计算传质法预测精馏塔效率(英文)

A computational mass transfer model is proposed for predicting the concentration profile and Murphree efficiency of sieve tray distillation column. The proposed model is based on using modified two equations formulation for closing the differential turbulent mass transfer equation with improvement by considering the vapor injected from the sieve hole to be three dimensional. The predicted concentration distributions by using proposed model were checked by experimental work conducted on a sieve tray simulator of 1.2 meters in diameter for de-sorbing the dissolved oxygen in the feed water by blowing air. The model predictions were confirmed by the ex-perimental measurement. The validation of the proposed model was further tested by comparing the simulated re-sult with the performance of an industrial scale sieve tray distillation column reported by Kunesh et al. for the strip-ping of toluene from its water solution. The predicted outlet concentration of each tray and the Murphree tray effi-ciencies under different operating conditions were in agreement with the published data. The simulated turbulent mass transfer diffusivity on each tray was within the range of the experimental result in the same sieve column re-ported by Cai et al. In addition, the prediction of the influence of sieve tray structure on the tray efficiency by using the proposed model was demonstrated.     

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。