Thermodynamic analysis, simulationand optimization on energy savings of ideal internal thermally coupleddistillation columns

Internal thermally coupled distillation columns (ITCDIC) are the frontier of distillation energy saving research. In this paper, a novel energy saving model of ideal ITCDIC and a simulation algorithm are presented,upon which a series of comparative studies on energy savings with conventional distillation columns are carried out. Furthermore, we present an optimization model of ideal ITCDIC, which can be used to achieve the maximum energy saving and find the optimal design parameters directly. The binary system of benzene-toluene is adopted for the illustrative example of simulation and optimization. The results show that the maximum energy saving of ITCDIC is 52.25% (compared with energy consumption of conventional distillation under the minimum reflux ratio operation); the optimal design parameters are obtained, where the rectifying section pressure and the feed thermal condition are Pr=0.3006 MPa and q=0.5107 respectively.     

Soft-sensing modeling and intelligent optimal control strategy for distillation yield rate of atmospheric distillation oil refining process

It is a challenge to conserve energy for the large-scale petrochemical enterprises due to complex production process and energy diversification. As critical energy consumption equipment of atmospheric distillation oil refining process, the atmospheric distillation column is paid more attention to save energy. In this paper, the optimal problem of energy utilization efficiency of the atmospheric distillation column is solved by defining a new energy efficiency indicator — the distillation yield rate of unit energy consumption from the perspective of material flow and energy flow, and a soft-sensing model for this new energy efficiency indicator with respect to the multiple working conditions and intelligent optimizing control strategy are suggested for both increasing distillation yield and decreasing energy consumption in oil refining process. It is found that the energy utilization efficiency level of the atmospheric distillation column depends closely on the typical working conditions of the oil refining process, which result by changing the outlet temperature, the overhead temperature, and the bottom liquid level of the atmospheric pressure tower. The fuzzy C-means algorithm is used to classify the typical operation conditions of atmospheric distillation in oil refining process. Furthermore, the LSSVM method optimized with the improved particle swarm optimization is used to model the distillation rate of unit energy consumption. Then online optimization of oil refining process is realized by optimizing the outlet temperature, the overhead temperature with IPSO again. Simulation comparative analyses are made by empirical data to verify the effectiveness of the proposed solution.     

在高压下规整填料中的流动实验研究

Both distillation performance and hydrodynamic study for backmixing by tracer technique were carried out in a high-pressure packed column with 0.15 m inner diameter over a wide range of operating conditions. Isobutane and n-pentane are employed as test mixture in the distillation experiment and air/water is used for the hydrodynamic study. The column is installed with Mellapak 350Y structured packing and the total packing height is 2.0 m. With the increasing operating pressure, the separation efficiency increases slightly while the F-factor corresponding to the maximum efficiency at each pressure is descending. It is noted that, at all operating pressures, with the increase of F-factor, the packing efficiency is slightly higher up to the flooding point. The application of SRP model to high-pressure distillation gives much lower values of HTUOG than those obtained experimentally. An additional term, the height of mixing unit, is introduced to correct the SRP model and improve its accuracy at high pressure. From the tracer experiments, the height of mixing unit for gas phase was found to be larger than that for the liquid phase. From this viewpoint, it is believed that the gas phase backmixing gives more unfavorable influence on the separation efficiency in comparison with liquid phase.     

A systematic approach for synthesizing a low-temperature distillation system

In this paper, by combining a stochastic optimization method with a refrigeration shaft work targeting method, an approach for the synthesis of a heat integrated complex distillation system in a low-temperature process is presented. The synthesis problem is formulated as a mixed-integer nonlinear programming (MINLP) problem, which is solved by simulated annealing algorithm under a random procedure to explore the optimal operating parameters and the distillation sequence structure. The shaft work targeting method is used to evaluate the minimum energy cost of the corresponding separation system during the optimization without any need for a detailed design for the heat exchanger network (HEN) and the refrigeration system (RS). The method presented in the paper can dramatical y reduce the scale and complexity of the problem. A case study of ethylene cold-end separation is used to il ustrate the application of the approach. Compared with the original industrial scheme, the result is encouraging.     

苯氯化侧反应精馏过程的系统设计与控制(英文)

The distillation column with side reactors (SRC) can overcome the temperature/pressure mismatch in the traditional reactive distillation, the column operates at temperature/pressure favorable for vapor-liquid separation, while the reactors operate at temperatures/pressures favorable for reaction kinetics. According to the smooth operation and automatic control problem of the distillation column with side reactors (SRC), the design, simulation calculation and dynamic control of the SCR process for chlorobenzene production are discussed in the paper. Firstly, the mechanism models, the integrated structure optimal design and process simulation systems are established, respectively. And then multivariable control schemes are designed, the controllability of SRC process based on the optimal steady-state integrated structure is explored. The dynamic response performances of closed-loop system against several disturbances are discussed to verify the effectiveness of control schemes for the SRC process. The simulating results show that the control structure using conventional control strategies can effectively overcome feeding disturbances in a specific range.     

γ-戊酮酸丁酯反应精馏过程的逐步优化方案设计

Butyl-levulinate has been identified as a promising fuel candidate with high oxygen content. Its combustion in diesel engines yields very low soot and NOx emissions. It can be produced by the esterification of butanol and levulinic acid, which themselves are platform chemicals in a biorenewables-based chemical supply chain. Since the equilibrium of esterification limits the conversion in a conventional reactor, reactive distillation can be applied to overcome this limitation. The presence of the high-boiling catalyst sulfuric acid requires a further separation step downstream of the reactive distillation column to recover the catalyst for recycle. Optimal design specifications and an optimal operating point are determined using rigorous flowsheet optimization. The challenging optimization problem is solved by a favorable initialization strategy and continuous reformulation. The design identified has the potential to produce a renewable transportation fuel at reasonable cost.     

Using hot-vapor bypass for pressure control in distillation columns

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.     

Simultaneous optimization of heat-integrated crude oil distillation systems☆

Crude oil distil ation is important in refining industry. Operating variables of distil ation process have a critical ef-fect on product output value and energy consumption. However, the objectives of minimum energy consumption and maximum product output value do not coordinate with each other and do not lead to the maximum eco-nomic benefit of a refinery. In this paper, a systematic optimization approach is proposed for the maximum an-nual economic benefit of an existing crude oil distil ation system, considering product output value and energy consumption simultaneously. A shortcut model in Aspen Plus is used to describe the crude oil distillation and the pinch analysis is adopted to identify the target of energy recovery. The optimization is a nonlinear program-ming problem and solved by stochastic algorithm of particle warm optimization.     

START-UP PROCESS DYNAMICS OF PLATE DISTILLATION COLUMNS

A mathematical model for discribing the start-up period of a plate distillation column operating undertotal reflux is presented.Based on the differential-difference equation expressing the material balance onthe trays in a complete column with holdups in the reboiler,trays and condenser,the present model isderived and an analytical solution is obtained.The existence of a constant concentration point during thestart-up period is proved mathematically.Experimental work on start-up period is carried out in a seven sieve tray distillation column of 0.17meter in diameter with a water-acetic acid system.Good agreement is found between the experimentaldata and the calculated results.The experimental results also show the evidence of a constant concentra-tion point in the column during start-up period.     

模拟退火方法用于带有中间换热器精馏塔的优化综合

This article presents a simulated annealing-based approach to the optimal synthesis of distillation column considering intermediate heat exchangers arrangements. T-he number of intermediate condensers and/or intermediate reboilers, the placement locations, the.operating pressure of column, and the heat duties of intermediate heat exchangers are treated as optimization variables. A novel coding procedure making use of an integer number series is proposed to represent and manipulate the structure of system and a stage-to-stage method is used for column design and cost calculation. With the representation procedure, the synthesis problem is formulated as a mixed integer nonlinear programming （MINLP） problem, which can then be solved with an improved simulated annealing algorithm. Two examples are illustrated to show the effectiveness of the suggested approach.     

内部热耦合空分塔的节能优化分析

内部热耦合精馏是迄今为止所提出的节能效果最好,而唯一没有商业化的节能技术。将内部热耦合技术用于空分塔,可以带来良好的节能效果。根据低温空气分离过程以及三组分精馏的特点,提出了一种新的内部热耦合空分精馏塔优化模型。在优化模型基础上,对热耦合塔进行了深入的节能优化与分析,并且与常规空分仿真结果进行了比较分析,压缩机能耗下降20.75%,产值增加17.46%,单位产值能耗下降32.53%。内部热耦合空分塔的提取率以及能耗均优于常规热耦合空分塔,节能效果显著。     

Modeling,Control, and Optimization of Ideal Internal Thermally Coupled Distillation Columns

Internal Thermally Coupled Distillation Columns (ITCDIC) are the frontier of energy saving distillation research. In this paper, the ideal ITCDIC is considered. A novel mathematical model and a related simulation algorithm are proposed. The dynamic responses of open‐loop, PID controllers and the responses of closed‐loops are carried out. The results show that the ITCDIC is a self‐balance process and could be operated smoothly with two PID controllers; the steady‐state optimization met the need of ITCDIC optimization. Furthermore, a steady‐state optimization model of the operation parameters is presented, which can be used to directly obtain the optimal operation parameters simultaneously guaranteeing not only the product quality and the maximum energy savings but also the dynamic operability and controllability. The benzene‐toluene system is studied as an illustrative example.     

4组分隔板塔热耦合精馏节能技术

对丙醇/丁醇/3-甲基丁醇/2-乙基丁醇组成的4组分物系的分离从完全热耦合原理出发,详细阐述了该物系精馏过程的建立模型到模拟计算,讨论了热耦合过程的自由度和隔板塔的最佳塔段数,指出了操作变量和完全热耦合在最佳热力学状态下应满足的条件,通过化工流程模拟软件Aspen Plus 11.1对该物系的分离工艺从简捷法到严格法计...     

Adaptive generalised predictive control of high purity internal thermally coupled distillation column

Internal thermally coupled distillation column (ITCDIC) is a frontier in the energy‐saving distillation researches. One of the bottlenecks to prevent the column from being commercialised is the difficulty in control design due to the high sensitivity to disturbances, strong asymmetric nonlinearity and inverse response especially under high purity. An adaptive multivariable generalised predictive control (AM‐GPC) strategy of ITCDIC process is proposed to solve the difficulties in high purity. The simulation results of AM‐GPC are compared with single input and single output GPC (S‐GPC), multivariable GPC (M‐GPC), modified IMC (M‐IMC) and traditional PID control in detail. The performances confirm the accuracy and validity of AM‐GPC for the high‐purity ITCDIC process. © 2011 Canadian Society for Chemical Engineering     

Adaptive Robust Generic Model Control of High‐Purity Internal Thermally Coupled Distillation Column

The internal thermally coupled distillation column (ITCDIC) is a frontier in energy‐saving distillation research. The process inside a high‐purity ITCDIC is of great nonlinear dynamics, which trouble the conventional control schemes. A multivariable adaptive robust generic model control (ARGMC) is presented to solve the difficulties, where an ARX model is derived and a recursive least squares estimation (RLSE) method is introduced. The benzene/toluene system is studied as an illustrative example. The results of ARGMC are compared with the robust generic model control (RGMC) and traditional PID control in detail. The performances in both servo control and regulatory control confirm the accuracy and validity of ARGMC for the high‐purity ITCDIC process.     

热泵精馏在三氯氢硅提纯过程中的模拟

运用化工模拟软件Aspen Plus,选用NRTL-RK物性模型和RADFRAC精馏模型,对三氯氢硅精馏塔的两种热泵流程进行了模拟计算,分别是塔顶气体直接压缩式和塔釜液体闪蒸再沸式热泵精馏。对比热泵精馏流程和常规精馏流程,结果表明：对三氯氢硅提纯而言,塔釜液体闪蒸再沸式热泵流程更有利。本研究采用双塔串行流程提纯三氯氢硅,运用塔釜液体闪蒸再沸式热泵精馏技术,优化后的主要操作参数为：T1塔回流比20,节流阀压力180 kPa,压缩机出口压力309 kPa;T2塔回流比5,节流阀压力227 kPa,压缩机出口压力310 kPa。优化后三氯氢硅的一次收率为88.75%,纯度超过99.9999%;在处理量相同情况下,与常规精馏相比,能耗费用节约82%。     

丙酮-水精馏过程模拟和优化

丙酮是一种广泛应用的有机溶剂,丙酮回收具有重要的经济和环境意义。丙酮回收过程中,需采用精馏单元操作进行分离提纯,精馏塔的模拟优化有着重要意义。文章采用Aspen Plus过程模拟软件,对精馏工艺进行了模拟核算,发现实际操作中精馏塔中存在着恒浓区,恒浓区的存在会带来精馏塔基建费用和能耗的增加。文章结合经济分析对丙酮精馏塔进行了优化,消除了恒浓区;同时,探讨了不同丙酮进料质量分数对精馏的影响,指出提高进塔丙酮质量分数是节能关键。     

Optimal Design and Optimal Operation of Separate Heat Pump Distillation

For a distillation system, in which relative volatility is large in low composition range and small in high composition range, using separate heat pump distillation (SHPD) can markedly save energy. In this paper, vapour flow rate to compressor is proposed as a parameter for system optimization, from the viewpoint of system energy balance. This parameter can be used for both optimal design and optimal operation of SHPD. Mathematical models for optimal design and optimal operation are formulated to minimize total annual cost and annual operating cost respectively. Optimal design of SHPD is performed and evaluated through process simulation, for a typical case study for the distillation of ethanol‐water system. The results show that SHPD has notable energy saving and economic benefit when compared with conventional distillation. Optimal operation of SHPD is also evaluated for changes in feed composition which is an operating variable.     

Implementation of Ethanol Dehydration Using Dividing-Wall Heterogeneous Azeotropic Distillation Column

In this article, the design and optimization procedures of a dividing-wall column for heterogeneous azeotropic distillation (DWC-A) using cyclohexane as an entrainer for ethanol dehydration are investigated. The proposed procedures can detect the optimal values of the design variables and thereby guarantee the minimum energy requirements, which is related to the minimum CO₂ emissions and the lowest total annual cost (TAC). Since ethanol and water form an azeotrope under atmosphere pressure, a conventional heterogeneous azeotropic distillation sequence (CHADS), including an azeotropic column and a recovery column, is usually used to perform the ethanol dehydration process. However, due to high energy requirements and equipment investments of CHADS, the TAC is at a relatively high level. DWC-A can be used to eliminate the condenser of the second column and decrease the degree of back-mixing. Both CHADS and DWC-A are simulated with Aspen Plus®, and the results show that DWC-A has an energy saving of 42.17% and the TAC reduction of 35.18% along with higher thermodynamic efficiency and reduction in greenhouse gas emissions.