Application of a thermally coupled distillation column with separated main columns to gas concentration process

A modified fully thermally coupled distillation column (FTCDC) for operability improvement is utilized in a gas concentration process. The column consists of a prefractionator and two separated main columns having high distillation efficiency and flexible control structure. The operability of the proposed column is evaluated by examining the open-loop dynamic responses of step input variations with the HYSYS simulation. The simulation result indicates that the modified system can give better control than the original FTCDC. The energy saving and reduction of construction cost are discussed, and the ease of vapor flow manipulation and the elimination of a compressor in the vapor transfer are also evaluated as possible improvements.     

Approximate design of fully thermally coupled distillation columns

An approximate design procedure for fully thermally coupled distillation columns (FTCDCs) is proposed and applied to example ternary systems. The procedure gives a fast solution of structural and operation design for a preliminary study of the FTCDC. The structural information resolves the design difficulty, caused from the interlinking streams of the column, which is encountered when a conventional design procedure is implemented. The design outcome explains that how the thermodynamic efficiency of the FTCDC is higher than that of a conventional two-column system and how the system of a separate prefractionator is different from a dividing wall structure. From the design result of three example systems with three different feed compositions, the useful performance of the proposed scheme is proved. In addition, the structural design of the FTCDC gives better understanding of the system and leads to high efficiency design of the column.     

Internally heat-integrated distillation system for quaternary separation

A new internally heat-integrated distillation column for quaternary separation modified from a conventional three-column system is proposed, and its performance is examined here. Two sets of heat integration between the rectifying section and the stripping section of two adjacent columns are placed in the conventional three-column system. The proposed system has been applied two example processes, the hexane and BTX processes, for the performance evaluation of energy saving and reduction of entropy production. In the hexane process, the duty reductions in reboilers and condensers are 28.5% and 30.5%, respectively, and the entropy production is reduced by 12.2% compared with the conventional system. In the BTX process, the duty reductions are 27.8% and 31.6%, respectively, and the entropy production is decreased 9.8%. The compressor utilized in the existing internally heat-integrated distillation column is not used in the proposed system leaving no difficulty of its operation and maintenance. Also, the structural similarity of the new system to the conventional system gives the column operation as easy as the conventional system.     

Design of a fully thermally coupled distillation column for hexane process using a semi-rigorous model

An industrial scale hexane process is designed for the implementation of a fully thermally coupled distillation column (FTCDC). A semi-rigorous material balance and Peng-Robinson equilibrium relation are utilized in the structural design. The operational design is conducted with a commercial design program, the HYSYS. The design outcome of the structural design indicates it to be comparable with the practical system of a conventional two-column arrangement in field operation, which shows the effectiveness of the design procedure implemented here. The procedure is good for the system of many components found from actual field applications. In addition, an investigation of the energy requirement of the FTCDC and a conventional system shows that an energy saving of 34.1% is available with the FTCDC.     

Logic hybrid simulation-optimization algorithm for distillation design

In this paper, we propose a novel algorithm for the rigorous design of distillation columns that integrates a process simulator in a generalized disjunctive programming formulation. The optimal distillation column, or column sequence, is obtained by selecting, for each column section, among a set of column sections with different number of theoretical trays. The selection of thermodynamic models, properties estimation, etc. is all in the simulation environment. All the numerical issues related to the convergence of distillation columns (or column sections) are also maintained in the simulation environment. The model is formulated as a Generalized Disjunctive Programming (GDP) problem and solved using the logic based outer approximation algorithm without MINLP reformulation. Some examples involving from a single column to thermally coupled sequence or extractive distillation shows the performance of the new algorithm.     

A Novel Strategy for Simulating the Main Fractionator of Delayed Cokers by Separating the De-superheating Process

Delayed coking is an important process in refinery to convert heavy residue oils from crude distillation units (CDUs) and fluid catalytic cracking units (FCCUs) into dry gas, liquefied petroleum gas (LPG), gasoline, diesel, gas oils and cokes. The main fractionator, separating superheating reaction vapors from the coke drums into lighter oil products, involves a de-superheating section and a rectifying section, and couldn't be simulated as a whole column directly because of non-equilibrium in the de-superheating section. It is very important to correctly simulate the main fractionator for operational parameter and energy-use optimization of delayed cokers. This paper discusses the principle of de-superheating processes, and then proposes a new simulation strategy. Some key issues such as composition prediction of the reaction vapors, selection of thermodynamic methods, estimation of tray efficiency, etc. are discussed. The proposed simulation approach is applied to two industrial delayed cokers with typical technological processes in a Chinese refinery by using PRO/II. The simulation results obtained are well consistent with the actual operation data, which indicates that the presented approach is suitable to simulate the main fractionators of delayed cokers or other distillation columns consisting of de-superheating sections and rectifying sections.     

Energy saving and thermodynamic efficiency of a double-effect distillation column using internal heat integration

The existing internally heat-integrated distillation column with the problem of utilizing a compressor is modified to propose a new heat-integrated distillation column without the compressor. Two identical columns of a conventional binary distillation are implemented to the heat integration. The energy used in the reboiler is recovered by the internal heat integration between the stripping section of one of the columns at lower pressure and the rectifying section of the other higher pressure column. The heat integration is similar to double-effect distillation, but internal heat integration requires less pressure elevation. The performance of energy saving and thermal efficiency improvement of the proposed system is evaluated with the two examples of the benzene-toluene and methanol-ethanol processes. The performance comparison indicates that the proposed system requires 17.4% less of reboiler duty for the benzene-toluene process and 15.8% less of heating duty for the methanol-ethanol process. The thermal efficiencies are 16.3% and 23.8% for the benzene-toluene and methanol-ethanol processes, respectively. Elimination of the compressor makes the column operation easy and the separate reboilers and condensers for the two columns in the proposed system provide flexible control, when the controllability of the proposed system is compared with that of the existing internally heat-integrated distillation column.     

外部热耦合技术在BTX分离中的应用

外部热耦合式复合精馏塔将热量从高压塔的精馏段传向低压塔的提馏段,降低了分离操作的不可逆性,从而提高系统的热力学效率,降低系统所需的能耗。目前国内苯/甲苯/二甲苯（BTX）的分离技术仍然与国外先进水平相差很远,主要表现在系统的能耗高、工艺落后等方面。本文将外部热耦合技术应用到苯/甲苯/二甲苯的分离过程中,并用M athem atica软件对系统的设备投资和操作费用进行了仿真计算。结果表明,使用外部热耦合结构,系统节能幅度可达到37.94%,年度总费用降低达18.84%。     

Design and Control of a Fully Thermally Coupled Distillation Column Modified from a Conventional System

A modified fully thermally coupled distillation column is proposed, with utilization of the existing distillation columns of the conventional system, and its control scheme is suggested here. The proposed distillation system is applied to a benzene‐toluene‐xylene (BTX) separation process, of which the system design and control performance evaluation are conducted using the HYSYS software. The performance of the suggested 3 × 3 control is examined in the set‐point tracking of product specification and the regulation for the changes of feed flow and composition. The pairings of three proportional‐integral control loops are the reflux flow and the specification of overhead product, the prefractionator vapor flow and that of the side product, and the vapor boil‐up rate and that of the bottom product. The multi‐variable controllability using various indices is investigated for the proposed control scheme, and the controllability is compared with that of the cross‐pairing between the control loops of the side and bottom products.     

A novel multi-effect methanol distillation process

Crude methanol distillation is an energy-intensive separation process and contributes significantly to the cost of methanol production. Although a number of energy-efficient distillation systems have been proposed, there is potential for energy savings in methanol distillation. To further reduce the energy consumption of methanol distillation, a novel five-column multi-effect distillation process is proposed in this work, which is essentially an improved version of an existing four-column scheme. The four-column scheme is made up of a pre-run column, a higher-pressure column, an atmospheric column and a recovery column. The new five-column scheme adds a medium-pressure column after the original higher-pressure column. In this way, the load of the original higher-pressure and atmospheric columns can be decreased by about 30%. The five-column arrangement creates a multi-effect distillation configuration involving efficient heat integration between higher-pressure and medium-pressure columns, atmospheric and recovery columns, and recovery and pre-run columns. Steady-state process simulation results indicate that temperature differences at two sides of each heat exchanger are appropriate, allowing effective heat transfer. Economic analysis shows that the energy consumption of the five-column scheme can be reduced by 33.6% compared to the four-column scheme. Significant savings in operating costs can therefore be achieved, resulting in an economically viable process for methanol distillation.     

Industrial application of an extended fully thermally coupled distillation column to BTX separation in a naphtha reforming plant

Aromatic compounds are yielded from naphtha reforming in a petrochemical plant, and the products are separated with binary distillation columns for benzene, toluene, xylene and heavy components in sequence. In this study, the first three columns of the fractionation process in the naphtha reforming unit are replaced with an extended fully thermally coupled distillation column (EFTCDC) also known as the extended Petlyuk column. An industrial-sized application of the EFTCDC is examined to compare the performance of the column with a conventional system. From a structural design giving the optimum structure of the column, a practical column structure is derived and used in the HYSYS simulation to find the optimal operation condition for a given set of product specifications. The EFTCDC gives an energy saving of 9.7% over a conventional three-column process. In addition, it is proved that the design procedure is good for an industrial process of 18 components.     

间壁塔萃取精馏制取植物油抽提溶剂的研究

针对植物油抽提溶剂萃取精馏系统存在的萃取剂结焦问题展开研究,分析了萃取剂结焦的原因,提出了间壁塔萃取精馏制取植物油抽提溶剂的思路,并应用工艺模拟软件对比了间壁塔萃取精馏工艺和常规精馏工艺。模拟结果表明,间壁塔主塔合适的理论塔板数为30,侧线塔塔板数为10,剂油比为1.1(体积比),侧线采出位置为第27块板。与常规两塔精馏相比,再沸器热负荷降低约10%,冷凝器热负荷降低15%,且设备投资也有所减少。     

乙醇胺精馏过程五塔流程模拟分析

用ASPEN11.1化工流程模拟软件,对乙醇胺分离工艺中的蒸氨塔、脱水塔、一乙醇胺塔、二乙醇胺塔、三乙醇胺塔进行了模拟计算,采用非随机双液体(NRTL)热力学计算模型,并进行了热力学参数修正,通过调整各塔的理论板数、进料位置和回流比,以及后二塔的塔顶回路质量流量等操作参数,得出各塔的最佳工艺条件。在最佳工艺条件下,分析了此分离过程的能耗问题。模拟结果表明:五塔流程分离得到的乙醇胺均能达到产品质量要求,工艺流程合理、可靠,对工程设计和工艺操作具有较强的指导作用。     

Experimental evaluation of a modified fully thermally coupled distillation column

A modified fully thermally coupled distillation column replacing the conventional distillation system is introduced, and its performance is experimentally evaluated for the stable operation of the column. The existing distillation system is modified to an energy-efficient distillation column for the reduction of investment cost and energy requirement. The experiment is conducted for the separation of methanol, ethanol and n-propanol mixture using a 4-in sieve tray column. The temperatures at seven different locations of the column are measured to monitor the column operation, and the measurements indicate that the column is stably operable. The stable operation proves that neither compressor nor pump is necessary for the vapor or liquid flow between tray sections. The experimental result is compared with that of the HYSYS simulation to show how satisfactory the separation is. The modeling of temperature variation provides a successful prediction of the temperature variation.     

Thermodynamic optimisation of distillation columns

In this paper a methodology for thermodynamic analysis and distillation column ‘targeting’ is presented, with emphasis on the use of side condensers and side reboilers. Research in the past has been towards the establishment of a heat distribution curve, showing the way in which heat can be added or extracted across the different column sections. One major disadvantage of these profiles is that they refer to reversible columns, and cannot be used effectively to target for modifications in a real column.The main feature of the proposed methodology is the introduction of a minimum driving force, defined in terms of exergy loss distribution of the existing column, to set realisable targets for side reboiling/condensing in real columns, resulting in considerable energy savings. In addition to providing realisable targets, the new approach also provides the design engineer with information about the best location to place a side exchanger, and the required additional column modifications. The methodology can be applied using conventional column models in commercial process simulation programs, but can be significantly simplified by using reboiled and refluxed absorber models in a bespoke program. Simulation results for modified designs set by the new approach, for binary and multicomponent separations, verify the feasibility of the targets. This contrasts with previous approaches, which result in temperature shifts and heat load penalties after placing side reboilers/condensers, thus requiring additional simulation time and experienced judgement.     

Analysis of a hybrid distillation-pervaporation column in a single unit: Intermediate membrane section in the rectifying and stripping section

Distillation-pervaporation in a single unit (DPSU) column can perform separations that are not possible in conventional distillation by overcoming distillation boundaries. Unlike conventional hybrid distillation-pervaporation columns, in a DPSU system the pervaporation membrane is located inside the column. The separation by distillation and pervaporation is carried out simultaneously inside the same column section. In a previous work, a simplified model was used to design and analyze distillation-pervaporation in a single unit (DPSU) systems with a hybrid rectifying-pervaporation section, where the membrane constitutes the whole section. In this study, this simplified model is applied to DPSU columns where the membrane partially constitutes the rectifying or the stripping sections, including the model derivation of the stripping section and the operation leaves. The simplified model is applied for the separation of two mixtures with different Serafimov's topology classifications: acetone-isopropanol-water (topology type 1.0-2) and ethyl acetate-ethanol-methanol (topology type 2.0-2b). Thermodynamic limitations are identified for the separation of the ethyl acetate-ethanol-methanol mixture. Multiple operation leaves are produced depending on the liquid composition at the beginning of the membrane section, hindering the conditions that help to overcome the distillation boundary through a DPSU column. For some conditions, a section that is partially constituted by a membrane performs better than if the membrane constitutes the whole section.     

Evaluation of control configurations for a depropaniser

Distillation columns are highly coupled and non-linear, and have major impact on the utilities consumption and product quality. By proper design of their controls, energy consumption, product variability, operator intervention and equipment downtime can be reduced. Thus, selection of proper controls for distillation columns is both challenging and critical. Although extensive literature have been published on various aspects of distillation control, viz., level controller tuning, ratioing manipulated variables and turndown operation, there is no comprehensive study on control evaluation considering all aspects and rigorous simulation. In particular, turndown operation has received little attention in control research. This work deals with the composition control of distillation columns considering depropaniser as an example and using rigorous simulation. Effect of level controller tuning, ratioing the manipulated variable and turndown operation on the performance of several control structures to reject step disturbances in feed flow rate and composition, and sinusoidal disturbance in feed composition, is studied. Both single- and dual-ended composition control of the depropaniser are considered. Results of this study show the need and importance of a comprehensive and rigorous analysis including column operation far away from the design conditions, for optimal design of column control.     

苯乙烯装置塔系热集成

精馏作为过程工业中最重要和最常用的分离手段, 是耗能最大的单元操作。精馏塔一般从再沸器输入热量, 从冷凝器取走热量, 利用某些塔高温位的冷凝热加热其他塔的再沸器, 并将单塔节能技术与过程集成相结合, 实现塔系的热集成, 可充分挖掘系统内部的节能潜力, 达到减少公用工程消耗的目的。本文通过对某化工厂的苯乙烯装置精馏塔系的分析, 通过各个塔的温焓图之间的关系, 提出了精馏塔系内部热集成的措施, 包括直接热集成、调压热集成和双效精馏与间接热集成耦合等3种方案。对于后两个热集成方案, 采用Aspen Plus模拟改造后精馏塔的变化并验证了方案的可行性。结果表明, 苯乙烯装置采用该热集成措施能明显节省高品位蒸汽的消耗, 降低能量费用。     

An Energy‐Efficient Crude Distillation Unit with a Prefractionator

An energy‐efficient crude distillation unit (CDU) was proposed, and its performance was evaluated and compared with that of a conventional CDU. The problem of the large energy demand associated with the single‐column operation of a conventional CDU was solved with a two‐column operation – a prefactionator and a main column – in the proposed CDU. The two‐column operation reduces feed‐tray mixing, and thus raises the thermodynamic efficiency of the CDU. The proposed unit has reduced heating and cooling duty compared with that of the conventional unit. The proposed unit also has fewer investment and utility costs. Vapor and liquid flow connections between the two columns are readily available by adjusting the difference in column operating pressure.     

A generalized method for the synthesis and design of reactive distillation columns

Owing to the combination between the reaction operation and the separation operation involved, it is extremely difficult to determine in advance the optimum configuration of a reactive distillation column and this makes process synthesis and design a great challenging task. Currently, no easy-to-use and yet effective methods are available to guide process synthesis and design, restricting considerably the applications and therefore the impacts of reactive distillation columns to the chemical process industry. In this paper, a generalized method is proposed for the synthesis and design of reactive distillation columns in terms of the insights from process intensification. The method is initiated from a simple process design with all feeds of reactants at the middle of the process and all stages as reactive ones. In terms of an economical objective function, it can be evolved into the optimum process design via sequential structure adjustments, including reactive section arrangement, feed stage relocation, feed splitting, and catalyst redistribution. The generalized method proposed is characterized by great simplicity in principle, the capability to tap the full potentials of process intensification, and the high robustness to the initial guess of process configuration as well as the thermodynamic properties of the reacting mixtures separated. Four example systems are employed to evaluate the generalized method proposed and the obtained outcomes demonstrate its effectiveness and applicability to the synthesis and design of various reactive distillation columns.