Conventional Distillation Column Research Articles

Optimal start-up strategies for a conventional distillation column using simulated annealing

The start-up process for a distillation column presents a challenging control problem in the chemical industry. To address this problem, reducing wasting time and duty consumption during the transient period are crucial issues. In this study, the separation of the dichloromethane, chloroform, and carbon tetrachloride system (CH2Cl2 + CHCl3 + CCl4) was investigated as a start-up example. The single column was used to obtain 83.1% CHCl3 and 99% CCl4. The distillation start-up steps were divided into the following four parts for investigation: feed control, bottom product discharge time, total reflux switch time, and medium temperature heating operation method of the reboiler duty. The feeding time depended on the sump level for safety, and the bottom product was discharged until reaching purity near the specification to avoid producing excess undesired products. Switch total reflux policies were presented using the minimum temperature difference or waiting until the top composition CHCl3 reached 50 wt%. Furthermore, the simulated annealing (SA) algorithm was applied to determine the optimal heating step. The objective function (j) and a set of variables during optimization (d) were subsequently designed, and a flow chart for the SA approach was developed and connected to Aspen Plus Dynamic software. Following the SA calculation, the modified cases with optimal heating steps exhibited better behavior than did the example case. These rules enabled the complicated evaluation process to be simplified for designing a start-up strategy.

Feasibility of integrated batch reactive distillation columns for the optimal synthesis of ethyl benzoate

The synthesis of ethyl benzoate (EtBZ) via esterification of benzoic acid (BeZ) with ethanol in a reactive distillation is challenging due to complex thermodynamic behaviour of the chemical reaction and the difficulty of keeping the reactants together in the reaction zone (ethanol having the lowest boiling point can separate from the BeZ as the distillation proceeds) causing a significant decrease in the conversion of BeZ in a conventional reactive distillation column (batch or continuous). This might be the reason of not reporting the use of reactive distillation for EtBZ synthesis although the study of BeZ esterification reaction is available in the public literature. Our recently developed Integrated Conventional Batch Distillation (i-CBD) column offers the prospect of revisiting such reactions for the synthesis of EtBZ, which is the focus of this work. Clearly, i-CBD column outperforms the Conventional Batch Distillation (CBD) column in terms of product amount, purity and conversion of BeZ and eliminates the requirement of excess use of ethanol. For example, compared with CBD column, the i-CBD operation can yield EtBZ at a much higher purity (0.925 compared to 0.730) and can convert more benzoic acid (93.57% as opposed to only 74.38%).

Using hot-vapor bypass for pressure control in distillation columns

Abstract Distillation column control is widely explored in literature due to its complexity and importance in chemical and petrochemical industries. In this process, pressure represents one of the most important variables to be controlled. However, there are few studies about how pressure affects the dynamic behavior of distillation columns and most research on distillation column control involve direct manipulation of cooling fluid through the condenser. Nevertheless, such an approach demands constant changes in cooling fluid flowrates that are commonly by the order of tons per hour, which can be difficult to work or even unfeasible in a real plant. Furthermore, this strategy is usually avoided, as it can cause fouling and corrosion acceleration. The hot-vapor bypass strategy fits well as a solution for these issues, eliminating the need to dynamically manipulate cooling fluid flowrates in the condensation unit. This work presents the modeling and simulation of a conventional distillation column for the separation of water and ethanol, in which a comparative study between a conventional pressure control and a control using hot-vapor bypass was performed. The main results were obtained through dynamic simulations which considered various disturbances in the feed stream, and demonstrated superior performance by the hot-vapor bypass system over the usual scheme proposed in literature, while evaluating the Integral Absolute Error (IAE) norm as the control performance index.

Internally heat integrated batch distillation: Vapor recompression and nonlinear control

Abstract This work proposes a vapor-recompressed heat integration scheme in batch distillation column and its nonlinear control. For this, first a thermal coupling is made to form a heat integrated batch distillation column (HIBDC). To further improve energetic and economic potential, a mechanical heat pump system is introduced in the HIBDC, which gives rise to the vapor-recompressed HIBDC (VRHIBDC). For a ternary hydrocarbon system, this VRHIBDC exhibits superiority in the aspects of energy savings and economic performance over the HIBDC with respect to the conventional batch distillation column (CBDC). Aiming to achieve a constant product composition and to collect higher amount of distillate from the VRHIBDC column, an extended generic model controller (EGMC) is formulated. To obtain the state information required for this model-based controller simulation, two nonlinear observers, namely high gain observer (HGO) and extended Kalman filter (EKF), are devised and then coupled with the EGMC, yielding the EGMC-HGO and EGMC-EKF, respectively. To avoid the design complexity and computational load, both the observers are designed based on only the component mole balance equation around the reflux drum. The open-loop performance of the EKF shows its superiority over the HGO for the same example system. Subsequently, the comparative closed-loop performance is evaluated between the EGMC-HGO and EGMC-EKF with respect to a traditional proportional-integral (PI) controller. It is investigated that the EGMC-EKF shows the best result followed by EGMC-HGO.

Simplifying and synthesizing practical four-product dividing wall column configurations

The fully thermally coupled distillation column for four components mixture separation is shown to be energy efficient compared to conventional distillation column sequence. However, it has not been implemented in practice as the form of four-product dividing wall column (FPDWC) because inherent three pairs of vapor and liquid splits increase the complexity and operation difficulty of this equipment. Thus it is necessary to reduce the vapor and liquid split number in order to promote its industrial application. In the present work, three heuristics were proposed to simplify the internal structure of FPDWC, which could be used separately or simultaneously. In order to provide a criterion for energy requirement comparison, minimum vapor flow diagram (Vmin) was employed and shown to be an effective tool to analyze the feasibility of each simplified configuration. Compared to the complex Petlyuk arrangement, these derived practical configurations use less partition walls without energy penalty. To summarize, a flowchart was developed to synthesize configuration with least design and operation effort by employing the proposed heuristics. Two industrial mixtures were taken as examples to validate its effectiveness.

Towards energy efficient styrene distillation scheme: From grassroots design to retrofit

The current state-of-the-art commercial styrene distillation schemes, featured by conventional distillation columns to purify styrene, are introduced with energy, exergy and economic analyses. Amongst all the procedures the separation of ethylbenzene/styrene, the critical close-boiling system, accounts for ∼65% of the total energy requirement. To improve the energetic efficiency, double-effect distillation (DED) and heat pump distillation (HPD) are suggested as competitive improvements on conventional distillation schemes (CDSs), which give birth to advanced distillation schemes (ADSs). In addition, sensitivity analysis is carried out to determine the optimal operational parameters of columns in styrene distillation process. Taking the CDSs as benchmark processes, the ADSs with DED and HPD can lower operating costs by up to 30% and 40%, respectively. The synergistic effect makes retrofit proposals' payback period very attractive, through considerably energy costs reduction and uttermost equipment reuse. In the view of total annualized cost (TAC), the ADSs can cut a corner of ∼35–40% from the CDSs. Specifically, the ADS using HPD slightly outperforms its DED counterpart in TAC comparison. Despite energetic or monetary advantage, the ADSs also show their environmental drawback of higher exergy losses than the CDSs.

Quick Decision-Making for Close-Boiling Distillation Schemes

Separating close-boiling components using distillation is very common in industry. Considering the higher capital and energy intensity of the task, schematic selection of optimal distillation strategies becomes a significant decision of both industrial and methodological importance. In this sense, this paper introduces a reliable shortcut method of simplicity and robustness for optimizing the target of total annualized cost (TAC). In detail, selective analyses are carried out among four schematic candidates for three close-boiling systems. The schemes are conventional distillation column, mechanical vapor recompression (MVR), double-effect distillation, and distillation with a recycle process. The mixtures to be separated are methyltrichlorosilane/dimethylchlorosilane, methylcyclopentane/cyclohexane, and isobutanol/n-butanol. After the first round evaluation, hydraulic calculations through rigorous simulations are worked out to size the equipment, which is necessary for TAC analyses. In the second round c...

Synthesis of Methyl Decanoate Using Different Types of Batch Reactive Distillation Systems

Methyl decanoate (MeDC) is a fatty acid methyl ester (FAME) and is an important chemical compound with global production of 31 million tons per year. However, synthesis of methyl decanoate (MeDC) via esterification of decanoic acid (DeC) with methanol by reactive distillation is operationally challenging due to the difficulty of keeping the reactants together in the reaction zone; methanol being the lightest component in the mixture can separate itself easily from the other reactant deteriorating significantly the conversion of DeC using either a conventional batch or continuous distillation column. This is probably the main reason for not applying the conventional route to MeDC synthesis. Whether a semibatch distillation column (SBD) and the recently developed integrated conventional batch distillation column (i-CBD) offer the possibility of revisiting such chemical reactions for the synthesis of MeDC is the focus of this paper. The minimum energy consumption (Qtot) as the performance measure is used to evaluate the performances of each of these reactive column configurations for different ranges of methyl decanoate purity and the amount of product. It is observed that the use of i-CBD column provides much better performance than the SBD column in terms of the production time and the maximum energy savings when excess methanol is used in the feed. However, the SBD column is found to perform better than the i-CBD column when both reactants in the feed are in equal amounts. Also, the optimization results for a given separation task show that the performance of a two-reflux-interval strategy is superior to the single-reflux interval in terms of operating batch time, and energy usage rate in the SBD process at equimolar ratio.

Study of thermally coupled distillation systems for energy-efficient distillation

Distillation is one of the most widely used separation unit operations in process industries, although it is quite energy intensive. In many cases, the enormous energy requirements for distillation make it economically infeasible to carry out the separation. Thermally coupled distillation system (TCDS) is an advanced distillation method that provides significant energy savings of about 30% as compared with conventional distillation column sequences. The most well-known TCDS sequence, the Petlyuk configuration, has some operational challenges due to bidirectional vapour flow, which makes its implementation difficult in two-column mode. To overcome these limitations, a number of unidirectional vapour flow configurations have been proposed in the literature. The work on simulation analysis for such configurations is limited. In this paper, simulation models for two such configurations are developed, analyzed and compared with the Petlyuk and conventional distillation column sequences for separation of equimolar mixture of benzene–toluene–ethylbenzene.

Intensification for the Silane Production Involving Economic and Safety Objectives

The use of conventional reactive distillation columns for silane production has the disadvantage of large consumption of refrigerant because the silane is produced in the dome of the column and this must be condensed at about −78 °C. The incorporation of intercondensers may improve the cost and safety of the system; however, a proper study for the system is needed to determine the associated effects. Therefore, in this paper is presented an optimization approach for the silane production accounting simultaneously for economic and safety issues incorporating intercondensers. The presented approach is based on a software link between Microsoft Excel (for implementing the optimization algorithm based on differential evolution), Aspen Plus (for simulating the process), and SCRI (for quantifying the associated risk). The results show that the addition of intercondensers minimizes the total annual cost, because the amount of needed refrigerant is reduced. The safety analysis also shows that the use of intercond...

Design and analysis of an ethyl benzene production process using conventional distillation columns and dividing-wall column for multiple objectives

Downstream separation in the ethyl benzene (EB) production is energy intensive due to the use of multiple distillation columns. One technology to achieve significant energy and capital cost savings for this separation, is the use of dividing-wall column (DWC) to replace conventional columns. In the present study, a typical industrial EB process including reaction, separation and heat exchangers, is first simulated rigorously and modified to improve the efficiency and performance of both individual units and the overall plant. DWC is integrated into the EB process, and its performance is compared with the base case of utilizing only conventional columns for downstream separation. All simulations are performed using Aspen HYSYS, and design data used is appropriately validated for realistic simulation. Subsequently, sensitivity analysis is performed on both EB designs for a number of design variables. Objectives selected for sensitivity analysis are net present value, total capital investment and benzene loss. Results show that integrating DWC is not only viable but also offers potential to reduce capital investment, decrease benzene loss and increase profit compared to a conventional column design for the EB process.

Integrated batch reactive distillation column configurations for optimal synthesis of methyl lactate

Although batch reactive distillation process outperforms traditional reactor-distillation processes due to simultaneous reaction and separation of products for many reaction systems, synthesis of Methyl lactate (ML) through esterification of lactic acid (LA) with methanol in such process is very challenging due to difficulty of keeping the reactants together when one of the reactants (in this case methanol) has the lowest boiling point than the reaction products. To overcome this challenge, two novel reactive distillation column configurations are proposed in this work and are investigated in detail. These are: (1) integrated conventional batch distillation column (i-CBD) with recycled methanol and (2) integrated semi-batch and conventional batch distillation columns (i-SBD) with methanol recovery and recycle.Performances of each of these configurations are evaluated in terms of profitability for a defined separation task. In i-SBD column, an additional constraint is included to avoid overflow of the reboiler due to continuous feeding of methanol into the reboiler as the reboiler is initially charged to its maximum capacity. This study clearly indicates that both integrated column configurations outperform the traditional column configurations (batch or semi-batch) in terms of batch time, energy consumption, conversion of LA to ML, and the achievable profit.

Dividing‐Wall Column for Fractionation of Natural Gas Liquids in Floating Liquefied Natural Gas Plants

AbstractThe development of floating liquefied natural gas (FLNG) plants has resulted in a focus on reducing the weight and size of the topside processing facilities for these units. The conventional fractionation of natural gas liquids (NGL) in LNG plants implies a direct sequence of three or more conventional distillation columns requiring different levels of refrigeration. The results of a feasibility study are described, indicating that a packed three‐product dividing‐wall column (DWC) could replace conventional de‐ethanizer and depropanizer columns. This could provide significant energy, hardware, weight, and footprint benefits, but, very likely, at the expense of an unaffordable cold utilities demand.1)

Dividing wall column: Improving thermal efficiency, energy savings and economic performance

This work aims at investigating the performance improvement of a dividing wall column (DWC) for the separation of a ternary system. It is true that for fractionating a ternary mixture, at least a sequence of two conventional distillation columns is required. To improve energetic and economic potential, and reduce space requirement, two columns are proposed to merge into one shell with a dividing wall. For developing the mathematical model of a distillation column, we consider the effect of heat transfer through the metal wall placed at an intermediated position inside the cylindrical column. The simulated DWC model is verified using the Aspen Plus flowsheet simulator with a wide variety of phase equilibrium models. The superiority of this proposed heat integrated configuration is shown for a ternary hydrocarbon system over a conventional distillation sequence (CDS) in terms of mainly three performance indexes, namely thermal efficiency, energy savings and total annual cost (TAC). It is investigated that the dividing wall distillation scheme can secure a 37.5% energy efficiency, and a 22.6% savings in energy consumption and 23.23% in TAC. The promising performance can also be quantified in terms of a reasonably low payback period of 2.11years.

Selecting suitable energy-saving distillation schemes: Making quick decisions

More and more frequently, chemical engineers need to make quick decisions when selecting suitable energy-saving distillation schemes, for example, choosing between double effect and heat pump. Having endured this situation for years, the authors try to introduce, in this paper, shortcut methods to help choose between heat pump-assisted distillation and multi-effect distillation by evaluating energy efficiency in comparison with a conventional distillation column without any heat integration. To verify the accuracy of the shortcut methods, five binary systems with different relative volatilities, namely, methyltrichlorosilane-dimethyldichlorosilane, 2-methylbutane-n-pentane, benzene-toluene, methanol-water and cyclohexane-cyclohexanol, are studied using the shortcut methods with the aid of rigorous simulations. The energy efficiency results are presented in terms of standard oil versus atmospheric relative volatilities for both double-effect distillation and mechanical vapor recompression assisted distillation. Systems with lower relative volatilities perform best with the heat pump option, while systems with higher relative volatilities perform better with heat integration. The aim of the shortcut methods—to simplify and speed up the decision using only easily accessible data such as relative volatility and utility prices—is achieved.

Systematic design of the integrating heat pump into heat integrated distillation column for recovering energy

Many developed energy efficient configurations based on heat-integrated distillation column (HIDiC) have been proved to be attractive separation alternatives for saving energy. The key limitation of heat integrated technology is how to build splendid schemes for the heat integrated distillation column. In this work, the overall economic evaluation of conventional distillation column (CDiC), basic heat-integrated distillation column (basic HIDiC) and vapor recompression column (VRC) were executed systematically. Based on the comparison of results, by coupling the basic HIDiC with heat pump technology, a new distillation process model was developed as an intensified HIDiC (int-HIDiC) configuration for propylene-propane splitter. By this enhanced heat integration, the reboiler and the low pressure steam were no longer needed in the process. Meanwhile, the heat distribution proportion and the corresponding operating conditions of this int-HIDiC were optimized through vast simulations and computations. Finally, based on the above optimization results, we developed a simplified HIDiC configuration with two separated columns. The results show the better economic and energy performances than all the other energy-saving schemes including VRC.

Enhanced recovery of PGME and PGMEA from waste photoresistor thinners by heterogeneous azeotropic dividing-wall column

Propylene glycol monomethyl ether (PGME) and propylene glycol monomethyl ether acetate (PGMEA) are representative photoresistor thinners used extensively and generated as waste during the display and semiconductor material manufacturing processes. Although the waste thinner is normally retrieved by distillation, the azeotropes of these two thinner components with water limit the distillation performance. In this paper, an extensive design study of enhanced distillation processes was carried out to determine a favorable path for waste thinner recovery. Appropriate thermodynamic models for the design of a waste thinner recovery process were obtained through the regression and validation of experimental vapor-liquid-liquid equilibrium data. An optimal direct sequence using three conventional distillation columns with a decanter was introduced as a base design to overcome the distillation boundary by azeotropes. Several advanced distillation configurations were examined to further improve the energy efficiency of the conventional recovery process. A novel heterogeneous azeotropic dividing-wall column was developed based on process intensification and integration. The proposed enhanced recovery process reduced the energy requirement for waste thinner recovery significantly by 33.1%. The advanced distillation configuration can be an attractive option for improving the economic and environmental efficiency of the commercial waste thinner recovery and recycling processes.

Novel retrofit designs using a modified coordinate descent methodology for improving energy efficiency of natural gas liquid fractionation process

Tough environmental regulations, intense competition, expensive fossil energy use, and the strong growth predictions of the natural gas market have prompted efforts to retrofit the existing purification processes to reduce their energy requirements. The important goals of retrofit design are to analyze, evaluate and propose suitable technologies to improve the energy efficiency and/or increase the capacity. This paper reports the results of a techno-economic feasibility study to retrofit a natural gas liquid (NGL) fractionation process. A novel hybrid system, side reboiler and heat pump-assisted, thermally-coupled distillation sequence to maximize the energy efficiency, was proposed. Fractional utilization of the area was used as a hydraulic performance indicator to determine if a bottleneck occurs in a retrofit design. A modified coordinate descent methodology was employed to solve the optimization problem. As a result, the modified coordinate descent methodology was successful in finding the optimal proposed sequence structure and the operating variables, which resulted in operating cost savings of 44.55% compared to the representative base case. The short payback period of 14 months and reduced CO2 emissions of up to 42.05% showed that the proposed sequence is an attractive option for retrofitting in industrial implementation. This sequence can be employed for both grass-root and retrofit designs. This study also showed that even the heat pump can reduce the energy requirements significantly, and may have higher exergy loss than the existing conventional distillation columns.

Rigorous modeling, simulation and optimization of a conventional and nonconventional batch reactive distillation column: A comparative study of dynamic optimization approaches

In this paper we address the optimization of two batch distillation columns: a conventional batch distillation column (BDC) and a nonconventional dividing wall batch distillation column (DWBDC) with and without chemical reaction. The simultaneous solution of these systems of differential and algebraic equations is performed using two different approaches: pure equation oriented approach (EOA) based on orthogonal collocation over finite elements implemented in GAMS (24.2.2), and control vector parameterization (CVP) as implemented in gPROMS (3.7.1). In order to accomplish this objective, we describe and propose for the nonconventional batch configuration, two dynamic models that involve stage-by-stage calculations for the time varying column profiles. Case studies with and without reaction are presented in order to compare the two discretization approaches and to demonstrate possible benefits of dividing wall columns in batch separations.

A new design method and operation of fully thermally coupled distillation column

From economic point of view, the fully thermally coupled distillation columns have shown benefits in terms of energy and capital cost savings when compared with conventional distillation columns. However, the design of such column systems is a topic of ongoing research because there are more degrees of freedom to deal with during the design phase. In this paper, a design method is presented and used to determine the structures of conventional two-product and Petlyuk columns, and the results show that the proposed design method works well. The steady state models are developed using HYSYS software, and the results show that the proposed design method provides good initial values for rigorous simulation. According to controllability analysis, the steady state models present better theoretical control properties; consistent with controllability results, when the steady state model is studied under closed-loop using PI controllers good dynamic responses for load rejection are obtained.