Distillation: Research Needs

Abstract

Distillation will undoubtedly continue to be the most-used method for separating liquid mixtures, at any scale of operation. For this reason, and also because of its recognized energy intensiveness, distillation commands continued scrutiny with respect to cost-effective improvements. In this special report the authors suggest fruitful areas of research that can lead to lower cost distillation separations. The areas of research are classified under the headings of phase equilibrium, material and energy balances, mass transfer efficiencies, equipment design, and system energy consumption. For each of the categories a summary is given of the present status of the technology as well as directions that improvement-type investigations might take.     

Gas—solid Chromatographic separations of petroleum compounds and distillates on lithium chloridecoated silica

Gas—solid Chromatographic separation of petroleum-derived compounds and distillates on column packings of lithium chloride-coated diatomaceous silica and porous silica beads have been evaluated. The required operating temperatures and efficiencies of separation for most of the investigated saturated hydrocarbons, aromatics and sulphur- and oxygen-containing compounds on these packings were comparable with those achieved by gas—liquid Chromatographic separations. The retention values are discussed in terms of specific and non-specific interactions to show how compounds of similar boiling points are separated on the lithium chloride-coated silica columns according to chemical structure. These column packings are thermally stable, and can be used for type separations of highboiling petroleum fractions without any of the ‘bleeding’ problems that attend the use of polar stationary liquid phases and which can contaminate further analyses, especially when mass spectrometry is used. Examples of such separations, prior to mass spectrometry, are given.     

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.     

Effect of operating conditions on energy efficiency for a small passive direct methanol fuel cell

Energy conversion efficiency was studied in a direct methanol fuel cell (DMFC) with an air-breathing cathode using Nafion 117 as electrolyte membrane. The effect of operating conditions, such as methanol concentration, discharge voltage and temperature, on Faradic and energy conversion efficiencies was analyzed under constant voltage discharge with quantitative amount of fuel. Both of Faradic and energy conversion efficiencies decrease significantly with increasing methanol concentration and environmental temperature. The Faradic conversion efficiency can be as high as 94.8%, and the energy conversion efficiency can be as high as 23.9% if the environmental temperature is low enough (10 °C) under constant voltage discharge at 0.6 V with 3 M methanol for a DMFC bi-cell. Although higher temperature and higher methanol concentration can achieve higher discharge power, it will result in considerable losses of Faradic and energy conversion efficiencies for using Nafion electrolyte membrane. Development of alternative highly conductive membranes with significantly lower methanol crossover is necessary to avoid loss of Faradic conversion efficiency with temperature and with fuel concentration.     

Detailed mathematical modelling of membrane modules

Membrane technology is used for a wide range of separations from particle-liquid separations to gaseous and liquid–liquid separations. In this paper, we introduce a detailed model that describes a general membrane separation. The model disregards many common assumptions such as plug flow; constant temperature; constant pressure; binary mixture; steady-state conditions; and constant physical properties. Our approach is applicable to any membrane separation and in this paper we demonstrate its application to both liquid mixture separation (pervaporation) and gas separation in hollow-fibre modules. Both cases are seen to exemplify the need for a detailed model.     

Economic benefits of membrane technology vs. evaporator

European Union food ingredients production operations face operating cost challenges. Ingredient production steps frequently incorporate product recovery steps employing centrifugation, rotary vacuum filtration, evaporation and spray drying demanding high energy inputs.Product and process waste streams contain small molecular weight components that can be concentrated, desalted or, in some applications, fractionated with membrane filtration technology.Substantial capital and operating cost benefits together with selective separations can be derived by use of low energy consumption membrane technology as a complete stand alone process step or in conjunction with evaporation enabling product recovery and water for reuse.Reverse osmosis membrane technology can offer greater than 75% reduction in operating cost when compared with 5 multi-effect evaporation with thermal vapour re-compression process. Mechanical vapour recompression evaporators offer the lowest operating cost, similar to membrane technology, but with a considerably higher capital investment.Many companies which employ evaporation as a standard de-watering step in the manufacture of products such as dairy food ingredients, sugars, sweeteners, beverages, organic acids, renewable source bio-fuels, etc., can potentially reduce their carbon foot print with membrane technology.     

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.     

Effects of Solids Concentration and Underflow Diameter on the Performance of a Newly Designed Hydrocyclone

Solid‐liquid and liquid‐liquid separations in a hydrocyclone are versatile and require low maintenance costs. The demand for process improvement and cost reduction has motivated numerous optimization studies. The performance of a newly designed hydrocyclone is evaluated. The proposed device was obtained by application of differential evolution techniques and is called optimized thickening hydrocyclone (OTH). The OTH provides promising results, leading simultaneously to low Euler numbers and high solids concentrations of the underflow streams. Compared to the conventional Rietema hydrocyclone, it shows higher efficiency and better thickening power. The effects of solids concentration and underflow diameter on the performance of the OTH are quantified and a design equation for this device is proposed.     

The Potential of Energy Saving by Gas‐Phase Adsorption Processes

In large chemical plants multicomponent fluid mixtures are separated in fractions or in pure components. Dealing with gaseous mixtures, separations can be carried out by cryogenic distillation, adsorption followed by temperature swing adsorption (TSA) or pressure swing adsorption (PSA) or membrane processes, as gas permeation or pervaporation. When separation is carried out with membrane processes, the purification efficiency is often not sufficient if ultrapure products are the objective of the process. In the case of cryogenic distillation and adsorption product purity is not a problem. The investment cost of membrane as well as cryogenic separation processes are high in comparison to adsorption separation plants but the increase of investment cost versus capacity is smaller for cryogenic plants compared to membrane and adsorption units [1]. Therefore, the installation of a cryogenic process may be advantageous in very large plants designed for ultrapure products whereas adsorption is often the domain for plants with medium capacity. The future of membrane separation depends on the development of more efficient membranes.     

The Radiation Chemistry of Ionic Liquids: A Review

Ionic liquids have received increasing attention as media for radiochemical separations. Recent literature includes examinations of the efficiencies and mechanisms of the solvent extraction of lanthanides, actinides, and fission products into ionic liquid solutions. For radiochemical applications including as potential replacement solvents for nuclear fuel reprocessing, a thorough understanding of the radiation chemistry of ionic liquids will be required. Such an understanding can be achieved based upon a combination of steady-state radiolysis experiments coupled with post-irradiation product identification and pulse-radiolysis experiments to acquire kinetic information. These techniques allow for the elucidation of radiolytic mechanisms. This contribution reviews the current ionic liquid radiation chemistry literature as it affects separations, with these considerations in mind.     

Progress in membrane gas–liquid reactors

Gas–liquid reactions are crucially important in chemical synthesis and industries. In recent years, membrane gas–liquid reactors have attracted great attentions due to their high selectivity, productivity and efficiency, and easy process control and scale‐up. Membrane gas–liquid reactors can be divided into three categories: dispersive membrane reactor, non‐dispersive membrane reactor and pore flowthrough reactor. The progress in membrane gas–liquid reactors, including features, applications, advantages and limits, is briefly reviewed. © 2012 Society of Chemical Industry     

Global minimization of total exergy loss of multicomponent distillation configurations

The operating cost of a multicomponent distillation system comprises two major aspects: the overall heat duty requirement and the temperature levels at which the heat duties are generated and rejected. The second aspect, often measured by the thermodynamic efficiency of the distillation system, can be quantified by its total exergy loss. In this article, we introduce a global optimization framework for determining the minimum total exergy loss required to distill any ideal or near-ideal multicomponent mixture using a sequence of columns. Desired configurations identified by this new framework tend to use milder-temperature reboilers and condensers and are thus attractive for applications such as heat pump assisted distillation. Through a case study of shale gas separations, we demonstrate the effectiveness of this framework and present various useful physical insights for designing energy efficient distillation systems.     

Liquid film flow on structured wires: Fluid dynamics and gas‐side mass transfer

The liquid film flow on different structured wires and chains is observed experimentally to assess the suitability of a structured packing consisting of vertical wires. The results show that liquid beads as they appear on cylindrical wires are inhibited by certain chain geometries. This increases the flooding gas load up to F = 12 Pa^0.5. As the stabilized film shows no liquid bead motion, the liquid velocity at the interface is less which results in lower gas‐side mass‐transfer coefficients. An estimation of the packing characteristics for different chain geometries with an assumed wire packing density of 40,000 wires/m² is made. The interfacial area, mass‐transfer coefficients, and consequently the separation efficiency strongly depend on the liquid load. However, the proposed gas‐side separation efficiencies are slightly lower compared to common structured packings but the advantages are higher load limits, a better liquid distribution, and lower pressure drop. © 2012 American Institute of Chemical Engineers AIChE J, 59: 295–302, 2013     

An industrial perspective on membrane distillation processes

Process intensification has led to significant developments in both distillation and membrane technology. Membrane distillation (MD) is an emerging technology for fluid separations that are typically performed by conventional separation processes, such as distillation or reverse osmosis (e.g. water desalination, or water removal). Compared with other membrane technologies, the driving force in MD is the difference in vapor pressure across the hydrophobic membrane, rather than the total pressure. MD can be a cost effective separation process, especially when more sustainable alternative sources of energy (e.g. geothermal and solar) or waste heat sources are used. Many review papers on MD are available in the open literature, but most of them focus on the membrane characteristics (e.g. material aspects). This industrial perspective paper assesses the MD technology and reports on relevant issues by offering a concise overview of MD technology, addressing different MD configurations, current major applications, operating parameters and their effect on the MD process, commercially available membranes, as well as cost estimations to determine the feasibility of MD processes. While successfully applied in desalination and a few other niche applications, MD has failed to make a strong industrial impact in other areas still dominated by distillation. A key message is that membrane distillation is still a growing technology for separation and purification processes, but it needs further exploration and optimization to become a mature technology applicable to more industrial processes in the chemical process industry. © 2018 Society of Chemical Industry     

Membrane extraction of a new antibiotic (Shengjimycin): equilibrium and mass transfer analysis

BACKGROUND: During the last two decades, separations based on liquid membrane technology have been demonstrated to be a potentially attractive process for a large number of industrial separations. The advantages of this technology over the current separation processes allowed its commercialization in the area of chemical/environmental applications (for the removal of toxic metals). Efforts are being devoted to biotechnological processes where membrane extraction has an added advantage of improving the productivity by removing inhibitory product during its production cycle. In this article the applicability of a membrane extraction technique based on hollow‐fibre membrane modules is investigated to extract a new antibiotic, Shengjimycin (SJM). SJM is produced in a multicomponent fermentation broth, from which the main components required to be separated selectively. RESULTS: From equilibrium experiments, sunflower oil, a cheap and less toxic solvent, was found to be good for the extraction of SJM at its natural pH of 7.2–7.8. Addition of a small amount (1.5%) of Amberlite LA‐2 (a carrier) in the solvent could intensify the process to achieve a high distribution coefficient. The process using this organic phase (Amberlite LA‐2 in sunflower oil) gave good extraction (ca 70%) within 3–4 h in a pilot‐scale hollow‐fibre membrane module. This new system is preferred over the toxic solvents being used or tried because of its cost, less toxicity, low environmental impact and operator‐friendliness. CONCLUSIONS: Because of the above‐mentioned favourable characteristics this membrane extraction method has the potential to be sustainable and effective as it has shown selective separation of the desired component from a multicomponent mixture. Copyright © 2011 Society of Chemical Industry     

中粮(安徽)非粮乙醇技术改造能量投入/产出分析

中粮(安徽)15万吨/年粮食燃料乙醇的非粮原料技术改造取得了预期效果。本文在概述非粮原料改造基础上,采用能量投入/产出分析方法,对以木薯和玉米为原料的燃料乙醇生产过程的能量效率进行了分析,对原料变化产生的影响进行评价,最后展望今后采用非粮路线发展生物质液体燃料的前景。结果表明,生产环节改造对能量效率变化具有重要影响。玉米乙醇的净能量值为1.77MJ/L,木薯乙醇为7.82MJ/L,以木薯为原料的燃料乙醇具有较高的能量效率。但综合考虑能量效率、碳排放及土地利用等因素,继续拓展生物质原料应以农林废弃物为主,木薯原料的使用规模不宜轻易扩大。     

Pressure-induced diffusion of organic liquids through highly swollen polymer membranes

The transport of twelve organic liquids through a highly swollen rubbery membrane has been studied. The transport was caused by a pressure applied to the liquid above the membrane (reverse osmosis). The flux was found to be a highly nonlinear function of the driving pressure. Detailed thermodynamic and diffusion theories are proposed to describe the transport in terms of the concentration gradient of the swelling liquid within the membrane induced by the applied pressure. The data and the theory appear to be in very good agreement. The diffusion coefficients deduced from the data are explained in terms of a hydrodynamic mechanism of diffusion. Highly swollen membranes can yield very high liquid fluxes at moderate pressure and consequently may have applications for performing certain separations.     

A study using classical or membrane separation in the biodiesel process

Biodiesel is a clean burning biofuel produced from renewable resources (straight vegetable oil, animal oil/fats, tallow and waste cooking oil), which can be blended at any level with petroleum diesel to create a blend of biodiesel.The EU has adopted a series of directives to promote and to represent some of the most important renewable energy sources out of biofuels also covering biodiesel as well.The main processing stages currently applied for biodiesel technology are represented by transesterification, neutralization of mixture, phase separation, biodiesel and glycerine purifications. The reaction, generally occurring in a two-stage mixer-settler unit, arises some difficulties for clear cut separations.A new alternative technology, using hydrophobic porous membranes, can be used to prevent bulk mixing of the two phases and facilitate contact and mass transfer of species between the two phases.The glycerine side stream (roughly representing 10% of biodiesel) typically contains a mixture of many components, which are generally difficult to separate. Current methods for glycerine purification are complicated and conducted with higher costs.In this case, the new technology provides an economical solution for the purification of crude glycerine stream combining the high efficiency of electro-dialysis and nano-filtration processes.A comparative cost approach based on available information is sketched. Also, some examples sustain the aim of the study.     

Controlling thickener underflow rheology using a temperature responsive flocculant

Continuous solid–liquid separations with the temperature responsive flocculant poly (N‐isopropylacrylamide) (PNIPAM) were conducted in a pilot‐scale thickener for the first time, using fine quartz as the feed slurry. The performance in continuous operation was compared to batch sedimentation. The increase in sediment consolidation on cooling in batch sedimentation was less significant in the continuous operation due to kinetic limitations of the deeper sediment bed and shorter residence times in the pilot‐scale thickener. The reduction in underflow rheology which results when using the temperature responsive polymer as flocculant is significant. Paste‐like behavior results when underflow is discharged at 50°C, whereas low viscosity, near Newtonian behavior results when the underflow is discharged at 20°C. Compared to conventional polyacrylamide‐based flocculants, PNIPAM produces higher concentration underflow but lower clarity overflow and most importantly, significantly reduced underflow rheology (viscosity and yield stress). Temperature responsive flocculants have significant potential to reduce underflow pumping energy and cost for mineral tailings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2940–2948, 2014     

Thermodynamic efficiency of membrane-assisted distillation processes

Membrane processes are considered as comparably mild separation processes offering the potential for significant energy savings compared with azeotropic distillation processes. Despite higher investment and material costs, they are of particular interest for improving the energy efficiency in the chemical industry. However, energy savings of more than 20%–30% are rarely reported and even a general superiority can be disputed. To further elucidate this controversial, the current study pursues a quantitative assessment of the thermodynamic efficiency of pervaporation and vapor permeation processes with stand-alone distillation and hybrid membrane-assisted distillation processes for the separation of azeotropic mixtures. The results confirm the case-dependent potential of distillation processes to outperform membrane-assisted separations in terms of energy efficiency, considering proper heat integration. Although energy efficiency is becoming significantly important, it should be considered in the context of economic performance to determine an optimal trade-off and to select the best process alternative during conceptual process design.