Side-stream Flow Rate Research Articles

Dynamic control analysis of energy-efficient side-stream extractive distillation process for separating close-boiling mixture

To separate the close-boiling mixture comprising heptane and toluene, this study introduces an environmentally-friendly method of side-stream extractive distillation by utilizing aniline as the separating agent. This approach modifies traditional extractive distillation by adding a side rectifier, converting two-way vapor and liquid communication into one-way liquid transfer using an additional distillation section along with corresponding reboiler. Our present study investigates the dynamic control behavior for a strongly non-ideal system without and with heat integration. It focuses on achieving stable and reliable regulatory control by closely keeping the dual-product composition to their original design standards. Key control loops are pinpointed, with a specific emphasis on those centered around adjusting reboiler heat duty and side-stream flow rate within extractive column C1 to regulate the temperatures of Stages 48 and 36. Furthermore, control effectiveness is enhanced by introducing a feedforward action of reboiler heat duty-to-feed flowrate. By developing a new and robust control architecture, high feed flowrate and composition disturbances can be managed efficiently, solving the issues of oscillation and large transient response.

Optimal design and control of an energy-efficient triple-side-stream quaternary extractive distillation process

Dynamic control of extractive distillation process with multiple liquid side-streams is challenging although it has an energy-saving potential. In this work, the optimal design and control of a high-efficient triple-side-stream quaternary extractive distillation process (TSQED) is presented for separating acetone/methanol/butanone/tert‑butanol. All operating parameters are optimized by a simulated annealing based systematic method, resulting in the reduction of total annual costs by 9.37%, energy consumption by 18.76%, and CO2 emissions by 33.17% as compared with conventional extractive distillation process. Subsequently, three control structures (CS1, CS2, and CS3) are proposed and demonstrated according to different side-stream flowrate control strategy. In the CS1, the side-stream flowrate is proportional to feed flow. In the CS2, the side-stream flowrate is proportional to reboiler vapor flow. In the CS3, and temperature-sensitive plate in distillation column is identified and cascaded by side-stream flowrate. By contrast, CS3 is far better than CS1 and CS2 in terms of dynamic responses and integral performance criteria. This attributes to the virtuous effect of the inferential temperature control on side-stream flowrate in the whole process control of TSQED. Finally, the feedforward control of QR/F is added in the CS4 to further improve the effectiveness of CS3.

Fractionation of oleochemical fatty acid using vacuum dividing wall column: A controllability analysis

Dividing wall column (DWC) provides a good alternative for oleochemical fractionation. However, the internal configuration and multiple input multiple output (MIMO) system of DWC leads to complexity in operation and control. This work aims to analyze the controllability of fractionating oleochemical fatty acid using vacuum dividing wall column (VDWC). To achieve this, Aspen Plus and Aspen Dynamics were used to develop a rigorous steady state and dynamic model of the column. Five manipulated variables (MVs) were considered namely reflux flowrate (L), distillate flowrate (D), bottom flow rate (B), side-stream flowrate (S) and vapor boilup (V) while controlled variables (CVs) were the product compositions. Pairing of MV and CV to determine the best 3×3 control configuration was performed using relative gain array (RGA) and singular value analysis (SVA). The selected control structure was tested on PID controllers for several regulatory and servo problem. The results of RGA and SVA shows that DSV was the best control configuration. Performance analysis was found to be successful in rejecting the disturbances as well as obtaining good set point tracking. However, distillate and bottom composition shows poor controllability compare to middle composition.

Dynamic controllability investigation of an energy-saving double side-stream ternary extractive distillation process

The investigation of dynamic controllability for the double side-streams ternary extractive distillation (DSTED) is necessary due to its advantages in energy-efficient. However, the control strategy of the energy-saving DSTED scheme is unique and complex issues owing to the wobbly of side-stream flow rate. Herein, control strategy of the energy-saving DSTED scheme is explored for separating ternary azeotropic mixtures acetonitrile/methanol/benzene. Inspiring from the control strategy of the ordinary triple-column extractive distillation scheme, the fundamental control structure CS1 is firstly designed. The control scheme CS2 with feed/side-stream (F/SS) ratio and control structure CS3 with feedforward and F/SS are then investigated to achieve higher efficiency of process control. Finally, a robust control strategy CS4 with the ratio of reboiler duty and feed flow rate and without F/SS ratio is proposed to well maintain the product purities. Moreover, additional ±15% feed flow rate and composition disturbances are added in the CS4 to assess the stability and controllability of the energy-saving DSTED. Dynamic performances illustrate that the structure CS4 can handle as well as that of confronted ±10% disturbances when the large disturbances are occurred in the real chemical process.

Side stream control in semicontinuous distillation

The idea to reduce cycle time (T), by controlling the side stream flow rate using a feedforward control model – the ideal side draw recovery arrangement (ISR) – was standard in most semicontinuous distillation studies. However, its effect particularly on ‘T’ and more broadly on the system dynamics was not clearly understood. In the current study, we compare the performance of using a modified form of ISR model with the status quo based on the criteria T and separating cost (SC) on different case studies. Results show that the modified control model performed better with a 10-20% reduction in SC while maintaining product purities. Furthermore, the side stream flow rate trajectory that minimizes SC was found by using dynamic optimization and it did not differ a lot from the trajectory generated by the modified control model. The improvement in SC was at most 2%.

Control of an energy-saving side-stream extractive distillation process with different disturbance conditions

Abstract It is important to study the dynamic controllability for the side-stream extractive distillation due to its superiority of energy saving. Control structures of the side-stream extractive distillation process are special and complex due to the instability of side-stream flow rate. In this work, the dynamic control of the side-stream extractive distillation was explored for separating azeotropic mixture of acetone and methanol. The detailed control structures were used to investigate the control strategies of side-stream extractive distillation. During the whole design process, the control of flow rate on side stream is a key factor for achieving this process efficient control. A new control structure combining a component controller and a side-stream throughput valve was proposed to achieve good dynamic performance for the side-stream extractive distillation process when ±10% disturbances were introduced, but it is difficult to control the ±20% feed disturbances. In addition, the side-stream extractive distillation takes about a longer time to reach a steady state while maintaining the purity of products, compared with conventional process. Research on control performance is of great significance to the development of energy saving technology for side-stream extractive distillation.

Optimal design of an intensified column with side-reactor configuration for the methoxy-methylheptane process

In an effort to diminish energy consumption and improve economic performance, a distillation column coupled with an adiabatic side-reactor column configuration is proposed. The production of 2-methoxy-2-methylheptane is examined to show the promising potential of process intensification using the proposed side-reactor column configuration. In order to design a side-reactor that restricts the temperature of catalyst activity to 423K, the pressure of the column and the flow rate of the feed stream withdrawn for the side-reactor are examined. An economic steady-state flowsheet is developed that minimizes the total annual cost by optimizing the column pressure, side-stream flowrate, side-stream withdrawn location, and reactor effluent reentry location. For the 2-methoxy-2-methylheptane process, the proposed side-reactor column configuration appears overwhelmingly superior in terms of capital, energy, and total annual cost compared to that of a conventional energy-intensive scheme. In addition, compared to a reactive distillation process, the proposed configuration reduces total capital investment by decreasing column diameter with a smaller catalyst load. Furthermore, additional benefits such as less expensive hardware requirement, easy catalyst removal/regeneration could be achieved. Finally, a multi-effect heat integration sequence is suggested to improve the energy and economic performance of the proposed configuration.

Simulation-Based Artificial Neural Network Predictive Control of BTX Dividing Wall Column

For the separation of ternary liquid mixture, use of dividing wall column is one of the nonconventional techniques in the field of liquid separation by thermal process. Benzene, toluene, and o-xylene have been selected as a ternary system for this study. As it is difficult to control the product purity directly due to delay time in the composition analyzer, temperatures of the appropriate trays were selected as the controlled variables. Reflux rate, sidestream flow rate, and reboiler heat duty were selected as manipulated variables to control sixth tray temperature of rectifying section, 11th tray temperature of the main column, and 12th tray of stripping section. Back-propagation algorithm was used as a training algorithm to tune the connection weights for the function of each neuron. The control performance of ANNPC was investigated for ±10 % load changes in feed flow rate, feed composition, and liquid split factor. The control performance was analyzed by using performance criteria indexes and performance parameters such as IAE, ITAE, ISE, ITSE, rise time, and settling time. It is observed that these performance parameters are less for ANNPC as compared to PID control. The settling time in case of PID varies from 2.77 to 4.55 h, significantly higher than that in ANNPC (0.37–0.66 h). The rise time is 0.66–1.20 h for PID and 0.03–0.27 h for ANNPC. These results indicate that ANNPC performs better than PID controller.

Modeling and model predictive control of dividing wall column for separation of Benzene–Toluene-o-Xylene

In this paper, dividing wall column (DWC) has been chosen for a BTX (Benzene–Toluene–o-Xylene) system. A MATLAB® program has been written for nonlinear unsteady-state DWC, which is used in Simulink environment for control of the system by Model Predictive Control (MPC). Compositions of the three products (benzene, toluene, and o-xylene) are indirectly controlled by controlling the corresponding temperatures of the respective tray due to requirement of online analyzer. The temperature of uppermost tray in the rectifying section, stage temperature in the main column corresponding to the side stream withdrawn, and the bottom stage temperature in the stripping section have been chosen in order to maintain the compositions of the three products. The manipulated variables are reflux rate (L0), side-stream flow rate (SSRF), and reboiler heat duty (QB). It has been observed that MPC shows good performance even in the presence of ±10% change in the feed flow rate, feed composition, and liquid split factor in comparison with conventional controllers. The MPC has less settling time (almost 1.5 h) compared with the PI controller (approximately 3–4 h).

Control of an Isomerization Column/Reactor Process

The control of a distillation column with a side reactor is studied for the conversion of n-butane into isobutane for use in the alkylation process. Fresh feed of a mixture of propane, isobutane, n-butane, and isopentane is fed to a distillation column. A vapor sidestream is withdrawn in the stripping section of the column and fed to an isomerization reactor in which some of the n-butane is converted to isobutane. Reactor effluent is fed back into the column in the rectifying section. Isopentane is removed in the bottoms, and moderate-purity isobutane is removed in the distillate. The control of the purity of the isobutane product can be achieved in two alternative ways: manipulating sidestream flow rate to the reactor or manipulating reflux ratio. The latter method is demonstrated to be better because manipulating sidestream flow rate presents potential control problems due to the existence of multiple steady states.

Temperature Control of the BTX Divided-Wall Column

The control of a divided-wall column is more difficult than the control of a conventional two-column separation sequence for the separation of ternary mixtures because there is more interaction among control loops. In a previous paper, a control structure using four composition loops was shown to provide effective control of the purities of the three product streams and also achieve minimum energy consumption for both feed flow rate and feed composition disturbances. The numerical example studied the separation of benzene, toluene, and o-xylene. The four manipulated variables were reflux flow rate (R), side-stream flow rate (S), reboiler heat input (QR), and liquid split (βL) at the top of the wall. In this paper we explore the use of temperatures to avoid expensive and high-maintenance composition analyzers. Two types of temperature control structures are studied. In the first, three temperatures located in the main column and one temperature on the prefractionator side of the wall are used to adjust the...

Improvement in Performance of Thermal Diffusion Columns on Heavy Water Enrichment under Sidestream Operations and Flow-Rate Fraction Variations

ABSTRACTDevelopment of the separation equation in a classical Clusius-Dickel column with sidestream operations for the heavy water enrichment has been accomplished theoretically. Performance of the modified Clusius-Dickel column with sidestream operations and its corresponding maximum separation was determined with the sidestream flow-rate ratio at the feed position and top flow-rate fraction as parameters. The effect of the sidestream operation of a modified Clusius-Dickel column on separation efficiencies for the H2O-HDO-D2O system has been investigated theoretically and experimentally. The theoretical predictions were in good agreement with the experimental results. The theoretical results indicated that the maximum separation efficiency increases with the side stream flow-rate fraction but decreases with feed flow rate and are represented graphically and compared with that in a classical Clusius-Dickel column. Considerable improvement in the device performance is obtained by employing a modified Clusius-Dickel column under sidestream operations and flow-rate fraction variations, instead of using a classical Clusius-Dickel column.

Liquid-chromatographic separation and on-line bioluminescence detection of creatine kinase isoenzymes.

Abstract We separated isoenzymes of creatine kinase by anion-exchange chromatography, with use of an elution of gradient containing lithium acetate (0.1 to 0.6 mol/L). A stream splitter was used to divert a 5% side stream of column effluent, which was subsequently mixed with the reagents necessary for bioluminescence assay of the separated isoenzymes. The use of the stream splitter greatly decreased the rate of consumption of reagent and, when combined with a peristaltic pumping system, permitted independent control of the side-stream flow rate. Thus both the residence interval in a delay coil in which the ATP reaction product is formed and the bioluminescence response could be increased. Bioluminescence emission was monitored in a flow-through fluorometer without use of an external light source or filters. Separation and detection of the isoenzymes of creative kinase were rapid, sensitive, and highly selective. The incremental decrease of bioluminescence response owing to inhibition by the ions in the eluent was less than 31% across the entire gradient.

Liquid-chromatographic separation and on-line bioluminescence detection of creatine kinase isoenzymes.

We separated isoenzymes of creatine kinase by anion-exchange chromatography, with use of an elution of gradient containing lithium acetate (0.1 to 0.6 mol/L). A stream splitter was used to divert a 5% side stream of column effluent, which was subsequently mixed with the reagents necessary for bioluminescence assay of the separated isoenzymes. The use of the stream splitter greatly decreased the rate of consumption of reagent and, when combined with a peristaltic pumping system, permitted independent control of the side-stream flow rate. Thus both the residence interval in a delay coil in which the ATP reaction product is formed and the bioluminescence response could be increased. Bioluminescence emission was monitored in a flow-through fluorometer without use of an external light source or filters. Separation and detection of the isoenzymes of creative kinase were rapid, sensitive, and highly selective. The incremental decrease of bioluminescence response owing to inhibition by the ions in the eluent was less than 31% across the entire gradient.