Molecular Distillation of Palm Oil Distillates: Evaporation Rates,Relative Volatility,and Distribution Coefficients of Tocotrienols and other Minor Components

Abstract

The effects of feed flow rate and temperature of distillation on extraction of minor components from PFAD were studied in terms of concentration, distribution coefficient and relative volatilities. The order of volatilities for tocotrienols, based on the distribution coefficients, was desmethyl tocotrienols (δ‐T3)>γ‐tocotrienols (γ‐T3)>α‐tocotrienols (α‐T3). The separation of tocotrienols from FFA approached maximum values only at low temperatures and fell drastically as the temperature increased. FFA and squalene evaporate at lower temperatures than tocotrienols, α‐tocopherol, and sterols. Based on these properties, it was possible to separate the tocotrienols, α‐tocopherol, and sterols from FFA by molecular distillation using low temperatures.     

热耦合精馏工艺的模拟

为了进一步了解热耦合精馏工艺的适用范围,同时寻找其中经济性较优的工艺流程,本文利用Aspen Plus流程模拟软件,针对相对挥发度依次递增的四组二元物系(苯-氟苯、苯-正庚烷、苯-甲苯、苯-氯苯)分别开展了常规精馏和4种热耦合精馏工艺(热泵精馏塔、理想内部热耦合精馏塔、内部热耦合精馏塔简化构型、差压热耦合精馏塔)的模拟研究,过程优化的目标函数为年均总费用最低。通过优化结果的对比可知,热耦合精馏工艺在分离相对挥发度较小的物系时能耗相对较低,经济性相对较好,其中又以热泵精馏塔和差压热耦合精馏塔的效果最为显著;而在相对挥发度较大的物系分离中,常规精馏则是较为合适的工艺选择。此外,随着投资回收期的减小,热耦合精馏工艺相对于常规精馏工艺的优势也有所下降。     

Effects of feed tray locations to the design of reactive distillation and its implication to control

The effects of feed locations to the design of reactive distillation are explored. In this work, ideal reactive distillation systems are used to illustrate the advantage of feed trays optimization in design and control. Process parameters such as relative volatilities between reactants, relative volatilities between products, column pressure, activation energies, and pre-exponential factors are varied to seek possible generalization. For all systems studied, the percentage of energy saving ranges from 6% to 47%, and this is obtained by simply rearranging the feed locations. Finally, the idea of optimal feed trays is extended to the operation/control of reactive distillation systems. First, steady-state analysis is carried out to find the optimal feed trays as measurable load variable varies. Then, a control structure is proposed to rearrange the feeds as the disturbance comes into the system. The results indicate that, again, substantial energy can be saved by feed rearrangement via the coordinated control structure.     

Nonlinear model based control of two-product reactive distillation column

Nonlinear feedback control scheme for reactive distillation column has been proposed. The proposed control scheme is derived in the framework of Nonlinear Internal Model Control. The product compositions and liquid and vapor flow rates in sections of the reactive distillation column are estimated from selected tray temperature measurements by an observer. The control scheme is applied to an example reactive distillation column in which two products are produced in a single column and the reversible reaction A+B=C+D occurs. The relative volatilities are favorable for reactive distillation so that the reactants are intermediate boilers between the light product C and the heavy product D. Ideal physical properties, kinetics, and vapor-liquid equilibrium are also assumed. It is shown that the proposed control scheme keeps tight product composition control.     

Distillation of Reactive Methyl Glyoxylate-Methyl Glycolate Mixtures

Abstract

A process for refining methyl glyoxylate produced via air-oxidation of methyl glycolate is described. This is a unique separation as methyl glyoxylate reacts reversibly with several impurities in the reaction effluent (i.e., water, methanol and methyl glycolate). Separating water and methanol is accomplished using differences in volatilities and reaction equilibria. Azeotropic distillation is of particular importance in reducing water to specification levels. Separating methyl glycolate utilizes the effect of system pressure on volatility and reaction equilibrium. Recovery of methyl glycolate for recycle to oxidation is accomplished by passage through low-retention time stills.     

A Short Note About Energy‐Efficiency Performance of Thermally Coupled Distillation Sequences

In this work, we present a comparative study of the energy‐efficiency performance between conventional distillation sequences and thermally coupled distillation arrangements (TCDS) for the separation of ternary mixtures of hydrocarbons under the action of feedback control loops. The influence of the relative ease of separation of the feed mixture and its composition was analyzed. The feedback analysis was conducted through servo tests with individual changes in the set points for each of the three product streams. Standard PI controllers were used for each loop. The results show an apparent trend regarding the sequence with a better dynamic performance. Generally, TCDS options performed better for the control of the extreme components of the ternary mixture (A and C), while the conventional sequences offered a better dynamic behaviour for the control of the intermediate component (B). The only case in which there was a dominant structure for all control loops was when the feed contained low amounts of the intermediate component and the mixture had similar relative volatilities. The Petlyuk column provided the optimal choice in such case, which contradicts the general expectations regarding its control behaviour. In addition, the energy demands during the dynamic responses were significantly lower than those observed for the other distillation sequences. TCDS options, therefore, are not only more energy efficient than the conventional sequences, but there are cases in which they also offer better feedback control properties.     

Method and Apparatus for Precision In‐Line Sampling of Distillate

Abstract

In this short note we present a method and describe a simple apparatus for use with simple, fractional, and steam distillation operations in which the operator requires a precise measurement of instantaneous distillate composition as the distillation proceeds. There are many examples in which such a measurement will play a critical role. One such example is in the measurement of boiling or distillation curves, where one desires an explicit measure of distillation process temperatures as a function of distillate volume fraction. The ability to augment temperature‐volume distillation curves with composition information is very helpful. The optimization of critical distillation conditions for the separation of sensitive solutes is another example in which instantaneous distillate composition is of value. The purification of reactants before a reaction is still another example. In all of these cases, it is the instantaneous composition that emerges from the condenser that is needed, rather than the integrated composition of the distillate that accumulates in the receiver. In this note, the apparatus and method are described, and an example is given with a mixture of n‐decane/n‐tetradecane.     

Study on a Continuous Heat Integrated Distillation Column

Abstract

A novel continuous heat integrated distillation column called a concentric column has been studied using computer simulation. It is shown that a concentric column is a novel implementation of a thermodynamically reversible distillation column and has a lower energy loss than an ordinary distillation column. Our studies show that a concentric column has reduced column height, since the stripping section is configured concentrically around the rectifying section, and uses less utilities than a conventional column.     

萃取精馏分离苯-环己烷三元混合溶剂的效应

引言环己烷是一种性能较好的溶剂,可用来取代苯作脱油脂剂及脱漆剂等,因其一般通过苯催化加氢或汽油分馏来制备,在其产物中通常存在苯和环己烷的混合物,由于二者沸点相近,能形成最低共沸混合物,所以如何对苯和环己烷混合物系进行分离,许多研究人员对此进行了研究。     

Comparison of Energy Usage for the Vacuum Separation of Acetic Acid/Acetic Anhydride Using an Internally Heat Integrated Distillation Column (HIDiC)

Abstract

Energy savings for an internally heat-integrated distillation column (HIDiC) and a vapor recompression column for the vacuum separation of acetic acid/acetic anhydride was theoretically analyzed and compared to the simulation of a reference column configuration of the Eastman Chemical Company using ASPEN Plus. In these simulations, the design and operating variables were defined and optimized to minimize total energy used. The effects of design variables such as quantity and location of the heat integration stages, reflux ratio, and rectifying section absolute pressure on energy consumption and product purity revealed that one HIDiC configuration had 62% energy savings over the reference column. The distillation column using vapor recompression was evaluated as a benchmark for comparing the HIDiC configurations and the reference column. The VRC column simulation predicted both increased product purity and an energy savings of 91% over the reference unit.     

物系和分离要求对最优三组元精馏结构选择的影响

针对三组分混合物的分离,选取6种不同分离指数的物系,根据多种进料组成和分离要求,对传统的直接分离序列和间接分离序列及其热集成方式、部分热耦合精馏中的侧线精馏和侧线提馏、完全热耦合结构（即隔板塔）进行了模拟与优化设计,并以年度总费用最低为目标选出各种情况下的最优精馏结构。结果表明,分离指数、进料组成及分离要求都对精馏结构的优化选择都有显著影响,热集成精馏、部分热耦合以及完全热耦合精馏结构分别在不同条件下各具有优势。根据分析结果,对影响最优三组元分离精馏结构的因素进行归纳。     

FIBONACCI SEARCH FOR OPTIMAL FEED LOCATION

Abstract

A Fibonacci search technique is used in conjunction with a rigorous mullicomponenl distillaiion computer module to find the optimal feed location within a section of a distillation column. The function lo be minimized can be one of the following:

?key component ratio difference

?reflux ratio or reboiler ratio

?condenser duty or reboiler duty

This technique has been used successfully in the relocation of feed stages of many existing distillation columns and thus saved energy consumption. We shall describe this technique with a sample problem.     

Towards further internal heat integration in design of reactive distillation columns—part I: The design principle

The thermodynamic efficiency of a reactive distillation column involving reactions with a highly thermal effect could sometimes be improved substantially through seeking further internal heat integration between the reaction operation and separation operation. Prudent arrangement of the reactive section and deliberate determination of feed location are the two effective methods that can complement internal heat integration within a reactive distillation column. The reactive section is suggested to properly superimpose onto both the stripping section for exothermic reactions and the rectifying section for endothermic reactions. Feed location should be determined so that the effect of internal heat integration can be maximized between the reaction and separation operations. Two reactive distillation systems, involving a highly exothermic reaction and a highly endothermic one, are employed to evaluate the proposed design philosophy and the results obtained confirm its feasibility and effectiveness. In addition, sensitivity analysis is conducted with respect to the amount of catalyst employed, relative volatilities of reacting mixtures, and thermal condition of feeds. The applicability and potentials of the design principle proposed are highlighted.     

Reducing the energy demand of continuous distillation processes by optimal controlled forced periodic operation

In a continuous distillation process, it is generally believed that steady-state conditions represent the optimal operation mode from the viewpoint of required energy supply and that any disturbance of steady-state conditions (e.g. an oscillating profile of reflux ratio or reboiler heat rate) will cause a higher energy demand if the product specifications (expressed as average values) have to be fulfilled. However, this assumption has been disproven by some theoretical and experimental studies in the past, where the distillation process was studied under unsteady-state conditions, to reduce the energy demand. In these studies, both the reflux and the vapor flow rates have been controlled periodically. In the present paper, dynamic optimization methods are applied to find the optimal profiles for all possible control variables (flow rates of feed, products, reflux and vapor). Two examples are discussed. In the first example, an ideal ternary mixture is separated into two fractions, and in the second example, the same mixture is separated into three fractions using a column with a sidestream. For both examples, it is demonstrated that appropriate control profiles for the main control variables can reduce the required energy supply compared with steady-state conditions. The results were obtained on the basis of a simple mathematical model of a distillation column with the assumption of constant relative volatilities and equimolar evaporation and condensation. To generate the control profiles, a new software package for the solution of large-scale optimal control problems was used. The optimal control profiles represent a decoupling of feed supply, removal of distillate product, removal of bottom product, and, in the second example, withdrawing of the sidestream.     

Pervaporation with Reactive Distillation for the Production of Ethyl tert-Butyl Ether

Abstract

The combined process of pervaporation with reactive distillation was studied for the production of ethyl tert-butyl ether (ETBE) from ethanol (EtOH) and tert-butyl alcohol (TBA). on an ion-exchange catalyst. An apparatus consisting of a stirred batch reactor, a distillation column, and a pervaporation membrane was used to test this technique. The permeation flux and selectivity of water in the membrane were investigated in water-alcohols system. The etherification was performed in the liquid phase by using a batch reactor. The reactive distillation was examined with and without pervaporation at the boiling point of the reactant mixture under atmospheric pressure. When pervaporation was not conducted, two layers formed in the top products because of a higher concentration of water. However, these phenomena were not observed when pervaporation was added. It was revealed that pervapovation might be effective for removing water from the bottom and that a higher fraction of ETBE could be obtained as a top product.     

Extension of a Method for the Design of Divided Wall Columns

For most separations, a fully thermally coupled distillation column or divided wall columns are thermodynamically more efficient than conventional columns. This paper develops a method for the design of divided wall columns consisting of multi‐component mixtures (more than three components) with constant relative volatilities and a relatively sharp separation between the key components.     

Practical Sulfur Isotope Separation by Distillation

Abstract

Sulfur isotopes were separated in 1.07 and 1.50-m experimental packed distillation columns at cryogenic temperatures under total reflux. The ³²S/³⁴S ratio at column top was as much as 19% different from the bottom ratio. The ³²S/³³S separations were the square root of ³²S/³⁴S separations. Typical relative volatilities of ³²S/³⁴S are 1.0023 for H₂S, 0.9978 for SF₄, 0.9985 for SF₆, 1.0006 for COS, and 1.0011 for CH₃SH. A ¹²C/¹³C volatility of 0.9982 was also seen in COS. Sulfur separations in SF₄ and SF₆ and the carbon separation in COS are reverse isotope effects.

Distillation is a feasible method for sulfur isotope separation. Hydrogen sulfide is the best candidate compound for a practical sulfur isotope separation. Existing distillation columns could economically produce as much as 50 kg/yr of contained ³⁴S at 15% enrichment. Smaller amounts of more highly enriched isotopes could also be produced.     

Design and control of extractive distillation for the separation of methyl acetate-methanol-water

The azeotrope of methyl acetate methanol and water was isolated using extractive distillation with water as entrainer. The pressure-swing extractive distillation (PSED) process and vapor side-stream distillation column (VSDC) with the rectifier process were designed to separate the methyl acetate, methanol and water mixture. It was revealed that the VSDC with the rectifier process had a reduction in energy consumption than the PSED process. Four control schemes of the two process were investigated: Double temperature control scheme (CS1), Q_R/F feedforward control of reboiler duty scheme for PESD (CS2), Q_R/F feedback control scheme for VSDC (CS3), the feedback control scheme of sensitive plate temperature of side-drawing distillation column to dominate the compressor shaft speed (CS4). Feed flow and composition disturbance were used to evaluate the dynamic performance. As a result, CS4 is a preferable choice for separation of methyl acetate-methanol-water mixture. A control scheme combining the operating parameters of dynamic equipment with the control indicators of static equipment was proposed in this paper. It means using the sensitive plate temperature of side-drawing column to control the compressor shaft speed. This is a new control scheme for extractive distillation.     

Ionic liquid effects on mass transfer efficiency in extractive distillation of water–ethanol mixtures

The relatively high viscosities of ionic liquids could reduce the mass transfer efficiency of the extractive distillation process. The rate-based model was adopted to analyze this phenomenon since it predicted the performance of an extractive distillation pilot plant using ionic liquids as solvent. For the water–ethanol separation, three ionic liquids: 1-ethyl-3-methylimidazolium chloride, 1-ethyl-3-methylimidazolium acetate and 1-ethyl-3-methylimidazolium dicyanamide and the organic solvent ethylene glycol were used for the analysis. Simulations were conducted for sieve trays and Mellapak^® 250Y. The results indicate that relatively high viscosities affect the mass transfer efficiency. However, the improvements in relative volatilities obtained from the ionic liquids help to overcome this effect. However, with high solvent viscosities (>65 mPa s at T = 353.15 K) it was not possible to overcome the reductions. Additionally, at higher distillate rates high relative volatilities yielded negative effects on mass transfer efficiency because of a decrease in vapor velocity.     

Enhancing mass and energy integration by external recycle in reactive distillation columns

The synthesis and design of reactive distillation columns separating reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the heaviest and lightest components with the products being the intermediate ones) are described. The unfavorable thermodynamics poses great difficulties in combining the reaction operation and the separation operation involved and limits severely the potential of reactive distillation columns in the reduction of capital investment (CI) and operating cost. To remove the limitation, we propose two strategies for facilitating the synthesis and design of this kind of reactive distillation columns in this article. One is to arrange prudentially the reactive section so as to strengthen internal energy integration between the reaction operation and the separation operation involved; that is, while the reactive section should be placed at the bottom of the reactive distillation columns separating exothermic reactions, it should be at the top of the reactive distillation columns separating endothermic reactions. The other is to introduce an external recycle flow between the two ends of the reactive distillation columns to reinforce internal mass integration and internal energy integration between the reaction operation and the separation operation involved; that is, whereas the external recycle flow should be directed from the top to bottom of the reactive distillation columns separating exothermic reactions, it should be from the bottom to top of the reactive distillation columns separating endothermic reactions. Separation of four hypothetical ideal (i.e., two quaternary and two ternary systems, respectively) and two real nonideal (i.e., two quaternary systems) reacting mixtures is chosen to evaluate the proposed strategies. The results show that they can considerably lower energy requirement besides a further reduction in CI. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2015–2032, 2013