Liquid Split Ratio Research Articles

Impact of liquid split ratio on separation performance of a multiple dividing wall column pilot plant

Multiple dividing wall columns are a promising intensified process to reduce energy consumption of conventional distillation sequences by up to 55 %. Their operation is of course more complex, in particular with regard to the internal split ratios of the liquid and vapor flows at the dividing walls. Theoretical studies have shown that the split ratios may not need to be at a specific value but in a certain range, which would simplify operation. However, until recently no experimental setup existed to prove this observation in reality. This work presents the first comprehensive experimental study in this regard obtained from an approximately 10 m high multiple dividing wall column pilot plant. The plant has two dividing walls, so the column consists of a prefractionator, a middle and a main column. The results validate the theoretical observations. With the chosen mixture in particular the middle column is found to be operable in a certain range. Temperature profiles prove that internally different separations are performed, while the resulting product compositions are unaffected.

Thermodynamic performance analysis of a novel integrated energy cascade system of liquid air energy storage and two-stage organic Rankine cycles

Liquid air energy storage can enhance the absorptive capacity for renewable energy due to its high energy storage density and extensive application scenarios. This paper proposes an integrated cascade energy system including liquid air energy storage, two-stage organic Rankine cycle, organic Rankine cycle, liquid natural gas regasification and absorption heat pump/chiller to use waste heat and liquid natural gas's cold energy fully and improve the round-trip efficiency. The waste heat of liquid air energy storage and the cold energy of liquid natural gas could increase the temperature and condense the working fluid of organic Rankine cycles. The system's thermodynamic model in design and off-design conditions are established. The charge and discharge minimum loads of liquid air energy storage are 82.5 % and 33.5 %, respectively. The round-trip efficiency of liquid air energy storage obtains a maximum of 49.6 % and a minimum of 29 % in the load ranges. Based on the load changes, the effects on the organic Rankine cycle are discussed, and four control strategies are proposed to absorb the waste heat and cold energy efficiently under off-design conditions. Besides, the organic Rankine cycles are affected by the mass flow of liquid natural gas and split ratio, which are discussed, and its underlying causes are analyzed. The results show that the integrated cascade energy system's round-trip efficiency reaches a maximum of 80.3 % in design conditions and a minimum of 62.7 % in off-design conditions, increasing by 61.9 % and 116.2 %, respectively.

Design optimization and control of dividing wall column for purification of trichlorosilane

• TA C appr . is proposed as a novel objective function for DWC parameter optimization. • Optimal energy control structure can be achieved by manipulating liquid split ratio. • MPC-PI hybrid structure balances both control performance and safety performance. • Simulated annealing algorithm is employed to optimize the weights of MPC controller. Polysilicon quality and energy consumption are directly affected by the purification process of trichlorosilane. In this work, the dividing wall column (DWC) as a promising energy-saving technology is utilized for trichlorosilane purification, with the design optimization carried out using steady-state simulation. Compared with conventional distillation process, DWC can reduce total annual cost (TAC), CO 2 emissions and exergy loss ( El ) by 35.81%, 53.56% and 54.10% on average, respectively. Established on the characteristics of superfractionator, four control structures are proposed in this work, two of which are multi-loop proportional-integral (PI) control schemes including condenser duty ( C ), distillate flow rate ( D ), bottom flow rate ( B )/reflux flow rate ( R ), side flow rate ( S ), reboiler duty ( V ) control structure and optimal energy control structure. The other two are model predictive control (MPC) schemes including standard MPC structure and MPC-PI hybrid structure. The weights of MPC controller are optimized using the simulated annealing (SA) algorithm. Dynamic simulation results demonstrate that MPC schemes achieve improved closed-loop performance in terms of minor overshoot, shorter transition time and reduced oscillation than PI control schemes. The integral absolute error (IAE) is introduced to further quantitatively evaluate MPC schemes. The simulation results demonstrate that the standard MPC structure achieves the best control performance and the MPC-PI hybrid structure enhances process safety while maintaining the desired control performance.

Optimal operation of a dividing wall column using an enhanced active vapor distributor

During dividing wall column (DWC) operation, the liquid and vapor split ratios must be actively tuned to maximize the energy-saving potential for the desired separation process. However, no active vapor split control devices have been reported to be installed in existing industrial DWCs. Therefore, the enhanced active vapor distributor (EAVD®) has been developed to overcome the existing problems in controlling the vapor split ratio in DWCs. In the EAVD, the opening area for the vapor flow is tuned by altering the liquid level of a modified chimney tray. The liquid level can be adjusted by operating the control valve in each section of the DWC. In this study, a laboratory-scale EAVD was tested under a number of experimental conditions to validate its reliability for controlling the vapor flow associated with its pressure drop behavior in a DWC. The results showed that the use of an EAVD allowed the pressure drop differences in the above mass transfer section to be compensated, and the desired vapor split ratio could be achieved during operation. Furthermore, modification of the existing DWCs can be easily accomplished by substituting the current chimney tray with the EAVD, improving the operability and flexibility of DWCs.

Modeling/Simulation of the Dividing Wall Column by Using the Rigorous Model

Dividing wall column (DWC) is an atypical distillation column with an internal, vertical WE partition wall that effectively accommodates two conventional distillation columns into one to improve the thermodynamic efficiency. In previous studies, different equivalent models by combining conventional columns are adopted to approximate the DWC modeling, which may not well describe the integration of the DWC; moreover, the computational cost increases when multiple columns are implemented to represent one DWC. In this paper, a rigorous mathematical model is proposed based on the mass balance, the energy and phase equilibrium of the DWC, where decision variables and state variables are equally treated. The model was developed in the general process modeling system (gPROMS). Based on the rigorous model, the influences of liquid split ratio and vapor split ratio are discussed, and it is shown that the heat duty is sensitive to changes on the liquid and vapor split ratio. Inappropriate liquid and vapor split ratio will increase the mixing effects at both ends of the dividing wall, and adversely affect the thermodynamic efficiency. Hence, the degree of mixing is defined to characterize the column efficiency. Furthermore, the middle component split ratio at the top of the pre-fractionator has an optimal point for better energy saving with certain liquid and vapor split ratios, and can be used as an indicator for the energy performance. Finally, the model was tested and validated against literature data by using the ternary benzene–toluene–xylene mixture system as a case study.

A novel kind of multiple steady states characteristics in the dividing wall column

Abstract In this article, one kind of multiple steady states (MSS) phenomenon was investigated for a dividing wall column (DWC). The four-section model constructed in Aspen Plus was employed to simulate two DWC cases: mixture of n-hexane, n-heptane and n-octane; system of methanol, ethanol and n-propanol. It can be seen that there is a range of vapor split ratio in which multiple solutions of reflux ratio exist for fixed DWC configuration with the same feed and product streams. The width and the curve shapes of the MSS region, and the number of solutions change with the liquid split ratio. This MSS phenomenon was further explained using the component recovery around the prefractionator and the component recycling flow inside the DWC. This MSS phenomenon is helpful for DWC design by knowing the probable existence of multiple solutions in advance.

Optimal Operation of Dividing Wall Column using Enhanced Active Vapor Distributor

Due to its outstanding energy and capital cost-saving potential, a dividing wall column (DWC) has gained importance for the last two decades. Unlike conventional distillation columns, the DWC has more design and operation variables, such as controlling liquid split and vapor split ratios. Liquid split and vapor split ratios need to be actively tuned during the operation to obtain optimal performance of DWC. However, it is not yet reported any active vapor split control device implemented in any existing industrially applied DWCs. Recently, Benit M has successfully developed an enhanced active vapor distributor (EAVD®) to overcome the existing problems of DWC associated with the vapor split control. In the EAVD®, the window opening area of the vapor flow path is hydraulically adjusted by changing the liquid level of a modified chimney tray. The adjustment of liquid level can be done by operating the control valve in each section of DWC. The EAVD® was tested under different operating conditions to demonstrate its reliability. The results showed that the EAVD® was able to actively adjust the desired vapor split ratio during operation along with the liquid split ratio variations. Currently, despite its great potential to notably reduce costs, only hundred DWCs are being operated in the industry. The EAVD® considerably aims to close this gap. Without any risk, the modification of existing DWCs also could be done easily by substituting the conventional chimney tray with the EAVD® to improve its operability and flexibility to maximize the benefit obtained by the advantages of DWC.

Separation of azeotrope (2,2,3,3-tetrafluoro-1-propanol + water) via heterogeneous azeotropic distillation by energy-saving dividing-wall column: Process design and control strategies

To separate the azeotrope of 2,2,3,3-tetrafluoro-1-propanol (TFP) and water, the energy-saving heterogeneous azeotropic dividing-wall column (DWC) is proposed using chloroform as an azeotropic agent. Compared with the conventional design, the total condenser duty reduction of 44.57%, total reboiler duty reduction of 42.66% and TAC reduction of 37.79% by the DWC design are obtained. Based on the simulation results, the energy-saving in the DWC design is due to the thermal coupling of the conventional design and the separation of water by the decanter with the purity of 99.5mol%, rather not to the removal of remixing effect for the column sequence. The control strategy performance for the conventional and DWC designs are satisfactory, since TFP and water are separated with high purity, despite the disturbances of the flow rate of fresh feed, fresh feed composition and liquid split ratio.

Effects of Thermal Feed Quality on the Performance of a Divided Wall Distillation Column

The present work deals with the effect of thermal feed quality on the performance of a divided wall distillation column (DWC). The thermal condition of the feed alters the pressure profile across the two sides of the dividing wall, thereby affecting not only the mass transfer characteristics but also the hydrodynamics of a DWC. It was observed that the natural (feasible) vapor split ratio does not depend on the liquid split ratio and the reflux flow rate when the feed is saturated liquid or sub-cooled liquid (q ≥ 1). However, for q < 1, that is, for two phase (vapor-liquid), saturated vapor, or superheated vapor feed, the liquid split ratio and the reflux flow rate have profound effect on the feasible vapor split ratio, and the pressure profiles are altered significantly. For the stable operation of a DWC, the feed should be either saturated liquid or sub-cooled liquid or the feed quality may be manipulated to adjust the vapor split ratio.

Experimental and simulation of the reactive dividing wall column based on ethyl acetate synthesis

Abstract Reactive dividing wall column (RDWC) is a highly integrated unit which combines reaction distillation (RD) with dividing wall column separation technology into one shell, and it realized the chemical reaction and the separation of multiple product fractions simultaneously. In this paper, the reaction of esterification with acetic acid and ethanol to produce ethyl acetate was used as the research system, experiments and simulations of the RDWC were carried out. This system in the traditional process mostly used the homogeneous catalyst ( e.g. sulfuric acid). However, in view of the corrosion of the equipment caused by the acidity of the catalyst, we used the heterogeneous catalysts – iron exchange resins – Amberlyst15 and proposed a novel catalyst loading method. Firstly, the reliability of the model of the simulation was verified by the experimental study on the change of liquid split ratio and reflux ratio. After that, the four-column model was established in Aspen Plus to analyze the effects of the amount of azeotropic agent, reflux ratio and acetic acid concentration. Finally, for a fair comparison, the economic analysis was conducted between traditional RD column and RDWC. The results showed that RDWC can save 34.7% of total operating costs and 18.5% of TAC.

Design of a Dividing‐Wall Column Considering its Multiple Steady State Characteristic

AbstractA dividing‐wall column (DWC) may have more than one solution of liquid and vapor split ratio for the same feed and product streams under a fixed number of stages and reflux ratio, so the multiple steady states (MSS) of a DWC in the design stage should be considered. An improved design method of a DWC is proposed by studying its MSS characteristics. Additional steps are inserted after optimization to check if MSS exist for the obtained optimal solution of vapor and liquid split ratio, and further to select the best candidate. Finally, two cases of three‐product DWCs are described to verify this method. The results indicate that multiple solutions exist for these cases, which confirm the necessity of this method and provide an important guidance for the flowing design of a DWC.

Transient split features of slug flow at an impacting micro-T-junction: A numerical study

The transient split behavior of slug flow units at an impacting micro-T-junction is studied by the VOF method. The micro-mechanism for the phase split of slug flow at the impacting micro-T-junction is disclosed. The effects of the characteristics of slug flow units on the split of gas bubbles and liquid slugs are studied. These characteristics include the Ca number, the length/length ratio of the gas bubble to the liquid slug, as well as the surface tension. The split of a gas bubble at the T-junction shows obstructing, non-obstructing modes, or a combination of them. It is found that the length of the gas bubble and the Ca number of the inlet flow play key roles in determining the split modes. The split ratio of the continuous flow shows a notable impact on the split of a gas bubble as its length is 2 diameters less than the tube. At a critical liquid split ratio, the bubble shifts from a fissionable one to a non-fissionable one. A longer gas bubble requires a higher liquid split ratio to be shifted from a fissionable state to a non-fissionable state, which then would lead to a wider linear region on the phase split curve. The length ratio of the gas bubble to the liquid slug slightly affects the split behavior of the unit, by causing a merge of the consecutive gas bubbles at the junction and thus the change of split mode. A lower surface tension facilitates the split of the gas bubbles and thus reduces the maldistribution of the slug unit. Some of these transient features have been directly reflected on the averaged phase split data obtained from our previous experiments.

Novel Procedure for Assessment of Feasible Design Parameters of Dividing-Wall Columns: Application to Non-azeotropic Mixtures

Dividing wall columns (DWCs), as a subset of fully thermally coupled distillation systems (FTCDS), is considered as one of most appealing distillation technologies to the chemical industry, because it can bring about substantial reduction in the capital investment, as well as savings in the operating costs. This study targets on how to improve the energy efficiency of DWCs by achieving their well-designed feasible parameters. Two methods are applied to study the effect of liquid and vapor split ratios including a shortcut method and a method of systematic calculations by using differential equation profiles. In the latter approach, differential composition profiles in each column section are obtained by considering feasible key design parameters. The finding of pinch points for each section profiles allowed determining the limiting values of the operating parameters. The intersections of these profiles are used to get well-designed feasible parameters of the liquid and vapor split ratios in an attempt to obtain the desired purities of the top, bottom, and side-stream products. The obtained parameters are validated by rigorous simulations. Three types of case studies involve the separation of hydrocarbons (n-pentane, n-hexane, n-heptane), aromatics (benzene, toluene, p-xylene), and alcohols (ethanol, propanol, butanol).

Design and Control of Dividing-Wall Column for tert-Butanol Dehydration System via Heterogeneous Azeotropic Distillation

In this Article, a three-column model of azeotropic dividing-wall column (A-DWC) for heterogeneous azeotropic distillation (HAD) is investigated by demonstrating the design and control of tert-butanol (t-butanol) dehydration using cyclohexane as entrainer. Significant energy consumption savings of 23.80% and a minimal total annual cost (TAC) reduction of 19.93% between the optimum A-DWC design and the original two-column design can be obtained, respectively. Sensitivity analysis of whether the liquid split ratio (βL) can be a manipulated variable to maintain the product purities is conducted before the control structure for the A-DWC is established. Meanwhile, with two reboiler duties (QRs) still preserved in the A-DWC, the proposed control structure can solve disturbance issues and maintain the product purities very close to the set points with small deviation and short settling time.

Design of a dividing wall column for fractionation of biodiesel

This study presents an efficient design method for DWC which can fractionate palm methyl esters (PME, biodiesel) into three more valuable product groups: a mixture of methyl laurate and methyl myristate as light-cut, pure methyl palmitate (≥99.5%) as middle cut, and the mixture of the remaining methyl esters (biodiesel), which has good low-temperature operability to such an extent as to come close to cold filter plugging point (CFPP) 0 °C, as heavy cut. The first step of the design was to determine numbers of stages for four sub-sections of DWC, liquid split ratio, and initial reflux ratio by the shortcut design, based on the component net flow model and the method of Fenske, Underwood, and Gilliland (FUG method). Secondly, optimal reflux ratio, vapor split ratio, locations of stages for feed and side product were found out by sensitivity analysis in rigorous simulation. The results from the simulation model developed by the method show that the reboiler duty of a single DWC is about 24% less than that of two simple columns in direct sequence and about 25% less than in indirect sequence. These energy saving ratios are almost close to 30%, which is popularly known as a typical value for energy saving of DWC.

Energy-Saving Mechanism in Heat Transfer Optimization of Dividing Wall Column

Currently most of the dividing wall column (DWC) designs are based on a Petlyuk Column in which the heat transfer process across the dividing wall is ignored. This paper proposes a heat transfer model of DWC, in which additional virtual intermediate heat exchangers are introduced between the prefractionator and the main column, and thus the heat transfer process among the virtual intermediate heat exchangers will produce the thermal-coupled effect. This effect can be explored by fixing the beneficial heat transfer region and a corresponding algorithm of heat transfer. Compared with the designs based on a Petlyuk Column, this DWC model can further reduce the exergy loss and energy consumption. Besides, the optimal operating zone is enlarged, and the influence of liquid and vapor split ratio fluctuation on the minimum energy consumption of DWC is lowered. This model provides a guidance for the energy-saving design of DWC.

Model reduction and optimization of a reactive dividing wall batch distillation column inspired by response surface methodology and differential evolution

Carrying out reaction and separation simultaneously in a reactive dividing wall batch distillation column batch RDWC in the case of ethyl acetate synthesis provides the possibility of separating both products and increasing the equilibrium reaction conversion. Overcoming the known azeotrope conditions, high purity for ethyl acetate and decreasing the batch time compared to simple reactive batch distillation are the advantages of this configuration. The corresponding dynamic simulation is carried out by simultaneously solving the model-associated system of differential and algebraic equations. In this study, the optimal values of the vapour and liquid split ratios are considered as the decision variables in order to maximize the amount of ethyl acetate accumulated during batch time. The optimization strategy is implemented inspired by response surface methodology in which an optimal surface is fitted to the collected data set using differential evolution (DE). The optimal surface relevant algebraic equation is then considered as the reduced form of the complex model and the optimal values are obtained using the DE method.

Reformate 분리공정에서의 열복합 증류탑 제어구조에 관한 연구

Reformate 분리 공정에 대한 열복합 증류탑의 가능한 제어 구조를 조사하고 그 제어 성능을 검토하기 위하여 동적 모사를 수행하였다. Aspen HYSYS를 사용하여 <TEX>$3{\times}3$</TEX> 제어구조를 적용하여 여러 가지 제어구조로 동적 모사한 결과 Reformate 분리 공정 주탑의 상부, 중간, 하부 제품 조성은 각각 reflux flow rate, 주탑에서 전처리탑 상부로 흐르는 liquid draw rate, vapor boilup rate로 제어되는 구조가 가장 운전 안정성이 확보됨을 확인하였다. A dynamic simulation of a fully thermally coupled distillation is conducted for the design of a possible operation scheme, and its performance is examined with an example process of reformate fractionation process. The outcome of the dynamic simulation indicates that the column can be operated by using a <TEX>$3{\times}3$</TEX> control structure. The structure consists of three controlled variables of the compositions of overhead, side products and bottom and three manipulated variables of the flow rate of reflux, liquid split ratio between a main column and a prefractionator and steam.

Development of a new type high-efficient inner-cone hydrocyclone

Performance of a hydrocyclone can be influenced by many factors, such as structural type, geometric parameters and operation parameters, among which the structural type plays a very important role. The separation principle of a typical hydrocyclone was introduced. Focused on a gas–liquid separation, numerical simulation of the typical hydrocyclone was carried out. The gas phase fraction distribution was analysed. It is shown that the separation effect was not satisfactory. A revising idea was thus proposed and developed step by step, so a new inner-cone hydrocyclone (ICH) was designed. The inner-cone structure was thought to provide a more stable flow field for phase separation. It functions like a gas carrier that is beneficial for radially separated gas congregating on and growing into larger gas bubbles. It also produces an upward pushing force to gas bubbles, so as to enhance the gas–liquid separation performance, although the ICH has lower inner tangential velocities than the typical hydrocyclone. Numerical simulation and experimental study verified the analysis. Pressure drop of the ICH is much lower than that of the typical hydrocyclone; and the ICH has a wider scope for the change of liquid split ratio or inlet gas–liquid ratio. Development of the ICH would provide a new thought for the design of other separators.

Importance of pressure drop in divided wall distillation column

Pressure drop calculations are extensively used for finding tray spacing at the time of designing a column, but these are rarely performed for the simulation purposes as a minor change in pressure (from condenser to reboiler) has insignificant effect on the vapour–liquid equilibrium. However, pressure difference plays an important role in deciding vapour and liquid flow rate across a tray (or trays), particularly in case of interconnected or divided wall distillation column (DWC). In case of a DWC, the feasible vapour split ratio below the divided wall section is naturally adjusted in such a way that the pressure drops in the two parallel branches of the DWC are equal. In the present work, simulation of a DWC using the MultiFrac model of aspen plusTM (Aspen Technology, Inc., Burlington, MA, USA) was carried out to study the feasible vapour split as a function of reflux ratio and liquid split ratio. The results are interesting in that they show a little effect of liquid split ratio on the feasible vapour split ratio and significantly different pressure profiles in two sides of the DWC. It was also found that the variation in vapour flow rate leads to a large variation in the concentration profile. These results indicate that the pressure drop may be one of the key parameters that should be considered for the optimal design and control of a DWC. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.