Intermediate Reboiler Research Articles

Series versus parallel reboilers in distillation columns

Multiple reboilers are used in distillation columns for several reasons such as implementing heat integration, intermediate reboilers, auxiliary reboilers and vapor recompression. If there is a single source of hot vapor to be used in multiple heat exchangers, the flowsheet can use either a series or a parallel arrangement, depending on the temperature levels of the heat sinks. The purpose of this paper is to point out that the series arrangement can present difficult control problems because of interaction between the units. In the parallel configuration, the heat transferred to each sink can be easily divorced from the others. In the series configuration, more complex methods must be used such as bypassing and flooding to adjust heat transfer in individual units.The controllability of the parallel configuration is demonstrated for a lower-partitioned divided-wall reactive distillation column with vapor recompression.

Improved plantwide control structure for extractive divided-wall columns with vapor recompression

The conventional extractive distillation flowsheet for dehydration of low-concentration bioethanol uses three-columns: a pre-concentrator, an extractive column and a solvent-recovery column. A recent paper studied the control of a process using a single divided-wall column. The complex flowsheet uses both main and intermediate reboilers. Ethanol goes overhead, water is removed as a liquid sidestream and solvent is recovered from the base.The control challenges of such a complex multivariable interacting process are obvious. The published control structure is complex and dynamically very sensitive. Only slow ramp disturbances can be handled. The purpose of this paper is to develop a more simple control scheme that is robust and is capable of handling large step disturbances. The key feature of the modified process is to only compress what is needed in the side reboiler.

New Intensified Heat Integration of Vapor Recompression Assisted Dividing Wall Column

The energy efficiency of dividing wall column (DWC) can be improved by the vapor recompression (VRC) technology, but large temperature difference between the overhead and bottom of the DWC limits the application of VRC. To fully recover the heat generated by the VRC under large compression ratio, new intensified heat integrations of the VRC assisted DWC are provided in this paper. In the intensified configurations, the reboiler condensed liquid is used to vaporize the side liquid stream in an intermediate reboiler (IR), the overhead vapor is preheated by the subcooled liquid. The best IR position can be gotten by the aid of the column grand composite curves (CGCC) of the DWC. Three separation cases with different ESI values are simulated to validity the energy capacity of the intensified configurations, the results show that the intensified heat integration technolgy can significantly improve the energy efficiency of the DWC with large temperature difference between the overhead and the bottom.

Performance Enhancement of Reactive Dividing-Wall Column via Vapor Recompression Heat Pump

In this work, a novel energy-efficient distillation technology combining reactive dividing-wall column (RDWC) with vapor recompression heat pump (VRHP) is proposed. The integrated RDWC with VRHP models with intermediate reboiler or bottom reboiler are designed for reaching better energy-savings and economic performance. The results showed that the reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler is more attractive than the reactive dividing-wall column with vapor recompression heat pump and bottom reboiler in this study. Adding a preheater to the reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler (RDWC-VRHP-IR2) can further reduce the compression ratio and reach better performance. The RDWC-VRHP-IR2 saves 57.06% of total utility consumption, 31.31% of total annual cost with the payback period of 5 years, and 61.52% of CO2 emissions compared with the conventional two-column reactive distillation system.

Thermal integration of vapor recompression in a heat-integrated distillation: Impact of multiple intermediate reboilers

Combining the vapor recompression (VRC) with an intermediate reboiler (VRCIR) and an internally heat integrated distillation column (HIDiC) has emerged as a promising technology. The present work aims at advancing this hybrid HIDiC-VRCIR configuration by introducing multiple intermediate reboilers (IRs). Increasing the number of IR leads to provide a better distribution of internal heat along the column, thereby increasing the energy efficiency. In designing a sample double IR (DIR) based HIDiC-VRC, the location of IRs and their coupling with stripper trays play a crucial role. The optimal position of DIR system is proposed to identify based on two criteria: lowest total annualized cost (TAC) and highest energy savings. Further, aiming to enhance the energetic and economic potential of the HIDiC-VRCDIR structure, it is proposed to choose either the series or parallel mode of compressor applying the same two criteria stated above. By simulating a ternary wide-boiling hydrocarbon system, the promising performance of the sample HIDiC-VRCDIR configuration is evaluated with a couple of unique features. Extending this proposed thermal integration mechanism, one can configure the hybrid HIDiC-VRCIR column with any suitable number of IRs.

Simulation Studies of Process Improvement of Three‐Tower Low‐Temperature Distillation Process to Minimize Energy Consumption for Separation of Produced Gas of CO2‐Enhanced Oil Recovery (EOR)

High concentrations of carbon dioxide (CO2) obtained from the associated gas of CO2‐based Enhanced Oil Recovery (EOR) process need to be recovered and re‐injected into the oilfield in order to improve the amount of extracted crude oil and reduce total operational cost. A three‐tower low‐temperature distillation process is one of the effective technologies for the CO2 recovery process. However, its high energy consumption is a major drawback. In this work, an improved three‐tower low‐temperature distillation process which adds two intermediate reboilers in the stripping section of the CO2 recovery column and uses the overhead vapour of the demethanizer to pre‐cool the feed gas is proposed, in order to reduce the total energy consumption compared with the conventional process. The simulation was performed using ProMax3.2 software. Sensitivity analyses were carried out to evaluate the effects of key parameters (e.g. solvent flow rate, feed inlet stage, solvent inlet stage, reboiler inlet stage, and intermediate reboiler flow ratio) on the energy requirement. The simulation results showed that for a CO2 product purity of 96 %, the methane gas aligned with the GB17820‐2012 natural gas II requirements, and no solid CO2 existed. The reboiler heat duty of the CO2 recovery column of the improved process was reduced by 66.6 % compared with the conventional process. The total equivalent work of the improved process was 1.066 GJ/tCO2,13.4 % lower than the 1.231 GJ/tCO2 of the conventional process.

Improving the Performance of Extractive Dividing-Wall Columns with Intermediate Heating

In spite of the fact that using extractive dividing-wall columns (EDWCs) lead to a considerably reduced reboiler heat duty in comparison with conventional extractive distillation column (CEDC) flowsheets, the involvement of a heavy entrainer frequently requires a relatively high-pressure steam for process heating and incurs consequently adverse steady-state economics. To address the potential deficiency, we proposed, in the present study, to facilitate the EDWC with intermediate heating because the great boiling-point difference between the components in the azeotropic mixture and entrainer results in steep temperature profiles and allows effectively recovering sensible heat from the recycled entrainer and using relatively low-pressure steam. When the extractive separation operation dominates the EDWC, intermediate heating should be arranged in the left side of the dividing wall and feed preheating is frequently the most favorable option. On the other hand, when the entrainer recovery operation dominates the EDWC, intermediate heating should be arranged below the lower end of the dividing wall, i.e., in the stripping section, and an intermediate reboiler is the favorable option. Under both circumstances, the heat recovery from the recycled entrainer should be considered prior to the use of available utilities, thereby permitting great improvement in the steady-state economics. The strategy features simplicity in principle and requires a relatively small number of trial and error searches. It is evaluated in terms of extractive separations of two binary azeotropic mixtures: dimethyl carbonate and methanol (with aniline as the entrainer) and acetone and methanol (with dimethyl sulfoxide as the entrainer). It is found that intermediate heating could substantially enhance the performance of the EDWC with the resultant steady-state economics overwhelmingly above that of the CEDC flowsheet. Even compared with the CEDC flowsheet reinforced with intermediate heating, the EDWC is still likely to yield comparable steady-state economics. These outcomes indicate that intermediate heating should be taken into account in the synthesis and design of the EDWC and the EDWC with intermediate heating should be regarded as a potential option for the extractive separations of binary azeotropic mixtures.

Introducing vapor recompression mechanism in heat‐integrated distillation column: Impact of internal energy driven intermediate and bottom reboiler

A novel combination of internally heat‐integrated distillation column (HIDiC) and vapor recompression column (VRC) with intermediate reboiler (IR) is proposed. Supplying heat at the highest temperature point (i.e., column bottom) of the VRC scheme is not thermodynamically favorable and, therefore, we aim to install the IR for better distribution of heat along the column length, thereby reducing the compressor work. Introducing IR in the combined HIDiC‐VRC system formulates an open‐loop variable manipulation policy to evaluate the comparative impact of internal and external heat sources on bottom liquid reboiling. With internal energy driven bottom reboiler, we further investigate the hybrid HIDiC‐VRCIR column with proposing the two modes of compressor arrangement, namely parallel and series. Finally, a multicomponent distillation system is exampled to show the promising potential of the proposed HIDiC‐VRCIR configurations in improving the energetic and economic performance over the HIDiC‐alone and HIDiC‐VRC schemes with reference to a conventional standalone column. © 2014 American Institute of Chemical Engineers AIChE J, 61: 118–131, 2015

Improving energy efficiency of dividing-wall columns using heat pumps, Organic Rankine Cycle and Kalina Cycle

Distillation is a widespread separation process that accounts for the largest fraction of energy consumed (∼40%) in process industries. It's very low energy efficiency has motivated many researchers to investigate energy reduction methods. Previous research mainly focussed on application of energy reduction techniques such as externally heat integrated designs, dividing-wall columns (DWCs), heat pumps, feed preheat, intermediate reboiler, intermediate condensers and electrical power generation from distillation column condenser, via Organic Rankine Cycle (ORC) and Kalina Cycle (KC). This study investigates further improvement of energy efficiency of 6DWC applications, which are already achieving 31–44% energy savings compared to conventional 2-columns system, through WHR methods: heat pumps, ORC and KC. The technical feasibility and economic viability of each WHR configuration are evaluated. Results show that it is usually favorable to perform WHR on DWC applications, with the exception of applications involving very low Tdist (temperature of distillation overhead vapour stream) and/or very high temperature lift. Applications with Tdist>150°C favor ORC and KC configurations which use cooling water as the cold sink. In locations with cheap electricity, bottoms flashing or vapor recompression is profitable; in other locations with expensive electricity, absorption heat pumps and KC configurations are favored.

A novel multistage vapor recompression reactive distillation system with intermediate reboilers

Most of the published studies have focused on the thermal integration of nonreactive distillation columns. The key limitation of reactive distillation (RD) technology is that the necessary conditions (such as pressure and temperature) for the reaction must match those of distillation. Owing to this constraint, the reaction conversion may be adversely affected at the elevated pressure in the reactive section of an internally heat integrated distillation column (HIDiC). This fact forces us to adopt an external heat integration approach for an industrial heterogeneously catalyzed ethyl tert‐butyl ether (ETBE) RD column. The direct vapor recompression column (VRC) is an external heat integration scheme that is successfully used as an energy efficient scheme for separating a close‐boiling mixture. Interestingly, there exists a large temperature difference between the two ends of the representative ETBE column, and it makes the external heat integration more challenging. Aiming to improve the thermal efficiency of the ETBE column under the VRC framework, various heat pump arrangements with intermediate reboiler(s) (IR(s)) are explored and analyzed with performing a comparative study in terms of energy consumption and economics. To improve further the thermal efficiency, in this contribution, a novel multistage vapor recompression RD column with IRs is introduced addressing a number of practical concerns. An algorithm for the proposed column is formulated showing the sequential steps involved in heat integration. It is inspected that the proposed multistage vapor recompression RD system appears overwhelmingly superior to the classical vapor recompression RD and its conventional stand alone column providing a significant savings in energy as well as cost. © 2012 American Institute of Chemical Engineers AIChE J, 59: 761–771, 2013

Simulated Annealing Approach to the Optimal Synthesis of Distillation Column with Intermediate Heat Exchangers

This article presents a simulated annealing-based approach to the optimal synthesis of distillation column considering intermediate heat exchangers arrangements. The 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.

Reducing Diameters of Distillation Columns with Largest Calculated Diameter at the Bottom

Methods are developed to reduce column diameter for distillation when the calculated diameter is largest at the bottom of the column. This diameter can often be reduced by generating vapor above the reboiler with an intermediate reboiler or with two-enthalpy feed. For a saturated liquid feed that is 50% n-butane and 50% n-pentane the two-enthalpy feed system reduced column volume by 8.2% with no change in energy use. For distillation of dilute acetic acid (5%)−water (95%) mixtures the best system (an intermediate reboiler) resulted in decreases in column volume as large as 53.9% with no increase in energy use. For a 50% acetic acid mixture the two-enthalpy feed system resulted in up to 32.5% reduction in column volume, while vaporizing the entire feed resulted in 31.5% volume reduction. Vapor bypass was effective in concentrated absorbers.

Design and Control of Distillation Columns with Intermediate Reboilers

Distillation columns in which the components being separated have widely different boiling points feature large temperature differences between the condenser and reboiler. If cooling water is used ...

Thermodynamically Efficient Systems for Ternary Distillation

In an earlier work (Agrawal, R.; Fidkowski, Z. T. Eng. Chem. Res. 1998, 37, 3444) it was shown that for the distillation of a ternary liquid mixture into pure component product streams the modified direct or indirect split configurations can have higher thermodynamic efficiencies than the conventional thermally coupled configurations. In this paper, thermally coupled column configurations are modified by incorporating intermediate temperature reboilers and condensers to accept/reject heat at the temperatures of boiling/condensing binary mixtures. The modified side rectifier or side stripper configurations retain the low vapor flow of the unmodified configurations. They have the same total number of reboilers and condensers as do the direct or indirect split but they provide higher thermodynamic efficiencies than the corresponding direct or indirect split. These new configurations also provide an additional degree of freedom to shift heat (cold) duty between the intermediate temperature reboiler (condenser) and the reboiler at the warmest temperature (condenser at the coldest temperature). Finally, a more complete ternary and quarterary superstructure is suggested that contains these new configurations as well as all earlier known superstructures and configurations.

Efficient use of an intermediate reboiler or condenser in a binary distillation

AbstractThe impact of an intermediate reboiler or condenser on the distillation of ideal binary mixtures into pure product streams is studied using a simplified model. The advantage of heuristics derived from this study is that they can quickly tell a process engineer if an intermediate reboiler or condenser is going to be effective in improving the efficiency and, of the two options, which one would be more effective. The heuristics simply states that if the actual fraction of liquid in a given feed is less than that with the maximum thermodynamic efficiency for distillation with no intermediate reboiler or condenser, then an intermediate condenser not only substantially improves the thermodynamic efficiency but is also more effective than an intermediate reboiler. An analogous heuristics exists for the intermediate reboiler when the fraction of liquid in the feed is greater than the optimum. Quick identification of cases that can achieve a substantial improvement in efficiency provides an incentive to search for the proper utilities needed for the intermediate reboiler or condenser. When relatively pure feed streams (concentration of either component greater than 90%) are distilled, the extremely low efficiencies of distillation can be remarkably improved by using an intermediate reboiler or condenser.

Intermediate reboiler and condenser arrangement for binary distillation columns

AbstractThe most thermodynamically efficient configuration for adding or removing heat from an intermediate location of an ideal binary distillation column distilling pure products is derived. The optimal policy requires that preconditioning of the feed be part of the overall decision‐making process. The optimal configuration can be determined through the use of two parameters, αIR and αIC, that are solely functions of feed composition. Simple and readily usable heuristics using these parameters are developed that help instantly identify the most efficient selection among (1) totally vaporizing and returning a side‐draw liquid stream from an intermediate location of the distillation column, (2) partially or totally vaporizing a portion of the given saturated liquid feed, (3) partially or totally condensing a portion of the given saturated vapor feed, and (4) totally condensing and returning a side‐draw vapor stream from an intermediate location of the distillation column.

Prefractionation to reduce energy consumption in distillation without changing utility temperatures

AbstractDistillation systems composed of a prefractionator and a main column are introduced. The columns operate at different pressures, but the temperatures of the utilities are the same as for a single column performing the same separation. The energy integration of the columns is realized by condensing a vapor overhead from the prefractionator in an intermediate reboiler located in the stripping section of the main column or by boiling a bottoms liquid from the prefractionator in an intermediate condenser placed in the rectifying section of the main column.Analysis is performed to determine feasible regions of operational parameters and to minimize vapor flows. A detailed simulation example of the benzene–toluene separation shows that for certain feed compositions it is possible to reduce energy requirements of a conventional binary distillation column by almost one‐third using this new distillation system. This savings is achieved without any change in the temperature of the utilities used.

On the Use of Intermediate Reboilers in the Rectifying Section and Condensers in the Stripping Section of a Distillation Column

Advantages of an intermediate reboiler in the stripping section of a distillation column and an intermediate condenser in the rectifying section are well-known. For highly nonideal mixtures the distillation process can be improved by placing an intermediate reboiler in the rectifying section of the column or an intermediate condenser in the stripping section, which is counterintuitive. In consequence the more expensive heating utility used in the bottom reboiler can be partially replaced with a less expensive heating medium (at a lower temperature) used in the intermediate reboiler. Similarly, a portion of the condensing duty from the top condenser can be replaced with the less expensive condensing duty (at a higher temperature) in the intermediate condenser. This placement of reboilers (condensers) can be used to reduce the total number of stages in the distillation column with a lower (higher) level of utility. Operating and capital costs of the distillation process with an intermediate heat exchanger can be lower than those for a classic column. A simple method providing valuable insights on the possible placement of intermediate heat exchangers along the column is proposed. It is based on calculations of the vapor flow along the height of a reversible, binary distillation column performing the equivalent separation task.

Utilization of Waste Heat Stream in Distillation

Cost of separation can be reduced by utilizing all available energy streams at various temperature levels. In the simplest case a waste energy heat stream can be used to partially vaporize a liquid feed stream. A more beneficial process involves an entire evaporation of a portion of the feed and introducing it into a column below the liquid portion of the feed. One can also use the waste energy stream as a heating medium in an intermediate reboiler in the column. There is, however, a limit to the amount of the waste energy that can be utilized in each case, beyond which this approach is no longer beneficial. Detailed analysis of the waste heat utilization enables one to determine this limit and compare each of these flowsheet options.

Industrial Experience with Double Quality Multivariable Distillation Control Based on Two Separation Indicators and Net Heat Input

Two separation indicators are used for controlling both top quality and bottom quality. The separation performance indicator is the weighted sum of the impurities of top and bottom, which is a measure of the separation performance of the whole column. The separation accent indicator is the weighted difference of these two concentrations, and is a measure of the position of the concentration profile in the column. The separation performance is controlled by the reflux feed ratio. The separation accent is controlled by the heat input via reboiler by calculating the net heat per ton feed to the column. This approach has been applied to a high purity C2-splitter in an industrial steam cracker. A reduction of the standard deviation of the impurities by global a factor 7 and a reduction of the energy consumption of the column with 5% was found compared with the conventional double quality control. In addition the intermediate reboiler could be operated for optimizing purposes without affecting the operation of the column.After four years of experience this control approach turned to be robust and well accepted by the operators. Besides the periods for instrument maintenance the control scheme operates on automatic.