Diabatic Distillation Column Research Articles

THERMAL STUDY OF COOLANT CIRCUITS OF BUILT-IN DEPHLEGMATORS OF DIABATIC DISTILLATION COLUMN

Diabatic distillation by using additional heat exchangers in the stages can significantly increase the energy efficiency of the column compared to adiabatic distillation. The paper proposes a calculation method for the condensation of ascending vapors on the tube surface of dephlegmators that are internally integrated on the trays of a multistage distillation column. The influence of parallel and series circuits of the coolant water supply to the dephlegmators on the technological parameters of the diabatic distillation unit is evaluated. For three types of coolant circuits, the following parameters are determined: thermal power, coolant temperature at the outlet of the tube dephlegmator, wall temperature on the condensate side, convective heat transfer coefficients, overall heat transfer coefficients, and thermal resistances on each tray of the distillation column. The results show that the parallel circuit to supply coolant water to the internal dephlegmators is the most suitable, providing the control of water flow rate on each tray and obtaining of the required temperature profile for height of the column to achieve the maximum efficiency of the separation process. For the parallel circuit, the heat transfer resistance can be concentrated in the heat transfer from both the coolant water to the wall of the dephlegmator tubes and the liquid condensed on the outer surface of these tubes.

Intensification of Heat and Mass Transfer in a Diabatic Column with Vortex Trays

We used vortex contact devices that we developed and investigated to make a new design of an alcohol diabatic distillation column with heat exchange pipes (as the reflux condenser) passing through concentrating section trays. In the column, ascending vapors partially condensed on the surface of vertically installed heat exchange tubes, forming a reflux. The reflux was then mixed with the draining liquid flow in the vortex contact devices placed on the trays. Heat was removed from the column through the boiling of the draining water film along the inner surface of the heat exchange pipes. We compared both diabatic and adiabatic columns fitted with the developed vortex contact devices on the trays. The proposed innovative contact system allows increasing productivity, reducing column dimensions and steam- and heat-transfer medium consumption, and increasing separation efficiency. Dependences for calculating the gas content, hydraulic resistance, and interphase surface required for designing the vortex contact devices of the proposed unit trays are presented.

A residential absorption chiller for high ambient temperatures

Abstract A standalone, compact, thermally driven ammonia-water absorption chiller designed to deliver a cooling capacity of 10.5 kW at ambient temperatures greater than 40 °C is presented. Heat and mass exchangers with novel microscale features and geometries are employed to minimize the physical size of the system. A diabatic distillation column based desorber and a shell-and-tube absorber with microscale features are used. Thermodynamic cycle modeling and heat and mass exchanger design methodologies are discussed. The performance of the packaged unit chiller is demonstrated experimentally over a wide range of operating conditions. The system delivered 10.6 kW of cooling at a COP of 0.63 at design conditions in a compact 0.7 m x 0.9 m x 1.0 m envelope. This work validates the scalability of microchannel heat exchanger technology from a proof-of-concept laboratory scale device to a residential-scale absorption heat pump.

New distributed-action control strategy with simultaneous heating and cooling in trays of a pilot-scale diabatic distillation column

In order to minimize operation transient times, a new distributed control strategy was performed in a distillation unit. Acting simultaneously by heating in tray 11 (stripping section), and cooling in tray 3 (rectifying section) of a distillation column, the control strategy was implemented using Aspen Hysys® with experimental validation. The new distributed control strategy showed a reduction of 0.32h (19min) in the transient time when a negative disturbance in the feed temperature was applied (−14°C), and 0.37h (22min) when a positive disturbance in the feed temperature was applied (+14°C), compared to conventional control. Variations in internal flow rates, as well as temperatures and compositions, were punctual (just where there is control action) and did not affect the steady-state after the disturbance rejection, when compared to conventional control. Distributed-action approach reduced heat amount transferred in the reboiler and reflux flow rate, due to the association with the heat added/removed in trays, evidencing the soft distribution of the control action along the entire column height. Thus, this new distributed control strategy with simultaneous corrective action between top and bottom of the distillation unit allows a significant reduction in transient times, improving plant productivity.

Externally Heat-Integrated Multiple Diabatic Distillation Columns (EHImxDC): Basic Concept and General Characteristics

This paper introduces a new type of distillation column structure that is externally heat-integrated in multiple diabatic distillation columns (EHImxDC). By matching the temperatures of different column sections (from the same distillation column or different distillation columns), heat integration of different distillation column sections is carried out to improve the thermodynamic reversibility of the distillation operation. First, the structure of the column shell is proposed, which may exist in this kind of distillation column. Second, the mathematical model describing EHImxDC is established, and a concise method is proposed to construct the EHImxDC structure. At last, based on two conventional distillation columns for the separation of a hexane–heptane system and a hexane–octanane system, the heat integration in two diabatic distillation columns (EHImtDC) is established. Through simulation, the results show that the reversibility of EHImtDC is significantly higher than that of the conventional distillation column. By comparing the annual operating costs of the EHImtDC and the double-effect distillation process, it is found that the EHImtDC saved about 10% of the annual operating cost compared to the double-effect distillation process. It can be seen that EHImxDC can greatly improve the second law energy efficiency of the distillation column and reduce the energy consumption of the distillation operation.

Heat and mass transfer in microscale diabatic distillation columns for ammonia–water desorption and rectification

An experimental investigation of heat and mass transfer in novel desorber and rectifier components based on the diabatic distillation principle is conducted. Heat and mass transfer coefficients measured here are compared with predictions of correlations from the literature. Temperature profiles are compared with component-specific heat and mass transfer models. Good agreement between model predictions and experimental results is shown. While heat and mass transfer correlations from the literature provide reasonable prediction of the data, further improvement is achieved with proposed correlations for liquid mass transfer and binary mixture correction coefficients. The results from this study show the potential for these novel designs to be implemented in small-capacity ammonia–water absorption systems.

Design and modeling of a microscale diabatic distillation column for small capacity ammonia–water absorption systems

The concept of diabatic distillation is applied to desorption and rectification in an ammonia–water absorption system. Results of a hydrodynamic investigation of novel components for these processes are used to develop a non-equilibrium model to predict heat and mass transfer performance. A modular modeling approach is proposed whereby physical consistency of various heat and mass transfer processes is achieved while reducing model complexity. A target vapor generation rate of 3 g s−1 with an ammonia mass fraction of 0.9985 is specified. It is shown that highly compact, lightweight desorption and rectification components can be realized. Heat source temperature and mass flow rate control are used to optimize system performance, and performance predictions are made over wide range of operating conditions. These results are also used to obtain reduced-order heat and mass transfer performance measures that facilitate incorporation of system performance into system level models and dynamic models.

Thermodynamic considerations for optimal thermal compressor design

A unified framework for the modeling of the thermal compressor of a single-effect ammonia–water absorption system is developed. An optimal thermal compressor configuration is found through a comparative assessment of various desorption and rectification configurations, and pertinent figures of merit are defined. The diabatic distillation column configuration is shown to achieve the highest thermal compressor performance. Parameters relevant to quantitative thermal compressor characterization are identified. Two optimal operating temperatures are identified, for the optimization of energetic and exergetic objectives, respectively. Statistical regression and artificial neural network analyses from thermodynamic simulations over a wide range of ambient and evaporator temperatures provide accurate prediction of thermal compressor performance and are compared. These results facilitate the assessment of absorption technology through simple predictive tools at the cycle design stage. This framework could also yield a design tool and could guide system design, optimal control, and flexible operation.

Design and control of energy-efficient distillation columns

Distillation is the best option for the separation of hydrocarbon mixtures, unless the boiling points of the constituents are close together. Despite being widely utilized in field applications, the high energy demand of distillation calls for efficient columns in order to save energy. The efficient divided wall column (DWC), diabatic distillation column, and internally heat-integrated distillation column (HIDiC) are introduced here, and the design and control of the columns are briefly reviewed. The practical applications of the columns in the processes of natural gas production from raw gas drawn from underground and benzene separation from naphtha reformate are presented to show the energy-saving performance of the energy-efficient distillation columns. The side-rectifier DWC reduced the heating duty of the conventional system by 5.9%, and provided a compact construction, replacing the three-column conventional system with a single column suitable for offshore application. Moreover, the controllability of DWC was improved by utilizing the side-rectifier. The benzene removal process utilizing the extended DWC lowered the heating duty of the whole conventional process by 56.8%.

Experimental and optimization studies of diabatic membrane-based distillation columns

Abstract Membrane-based distillation column (MDC) which utilizes hollow fibers as structured packings provides the advantages of large and fixed interfacial area and independent control of fluid flow rate without the constraints of flooding and loading. Diabatic distillation columns can approach the thermodynamic reversible operation, which is energy-efficient. MDC can be easily transformed into diabatic columns (D-MDC), i.e. with internal heat exchange, by adding a jacket as the flow channel for heating or cooling media. For the methanol-water system, the experimental results show noteworthy reduction of HTU (height of a transfer unit) under diabatic operation when compared to adiabatic operation. For the benzene-toluene system, the optimization of the internal heat exchange rates for D-MDC was conducted by a nested loop optimization algorithm, which incorporates the equipartition of entropy production (EoEP), equipartition of heat exchange rate (EoQ) or linear distribution of heat exchange rate (LQ) approximation, and an evolutionary approach. The optimal solutions of EoEP, EoQ and LQ provide significant improvements in specific exergy loss, exergy loss per NTU (number of transfer unit) and variance of driving force to adiabatic columns. The optimal solutions obtained from nested loop algorithm are superior to the solution from evolutionary approach.

Entropy production and economic analysis in diabatic distillation columns with heat exchangers in series

Distillation columns with adiabatic stages inherently entail high entropy production because heat is only supplied to the reboiler and removed from the condenser. DDC (Diabatic distillation columns) can reduce the entropy production and they can be realized through the installation of heat exchangers in series at the stages in the column by circulating a cooling fluid in the rectifying section from the condenser and by circulating a heating fluid in the stripping section from the reboiler. Although the diabatization of stages can effectively reduce the entropy production, it can cause an increase in the equipment cost because more heat exchanges are installed and in the operation cost because the total heat load supplied or removed in the diabatic column is higher than that in the adiabatic column. In this work, the entropy production and economic (i.e., total annual cost) criteria are assessed for diabatic distillation columns. A solution procedure that combines numerical analysis and rigorous simulations is proposed. For the presented examples, the existence of a trade-off between entropy production and total annual cost was demonstrated.

Optimal design of cryogenic distillation columns with side heat pumps for the propylene/propane separation

Propylene is one of the most important products in the petrochemical industry, which is used as raw material for a wide variety of products. The propylene/propane separation is a very energy-intensive process because their boiling points are quite similar. In addition, at atmospheric conditions, their boiling points are −47.6°C and −42.1°C, respectively. To separate this mixture conventional columns which operate at high pressure and cryogenic distillation columns which operate at low pressure have been used, however, this approaches are still energy-intensive. This work presents energy-efficient and intensified distillation columns which are adiabatic such as the vapor recompression column (VRC) or diabatic such as columns with heat-integrated stages. A design and optimization procedure, which minimizes the energy consumption in the propylene/propane separation is presented. Conceptual design, superstructure representation, rigorous simulations and mathematical programming techniques are effectively combined to assess all the candidate distillation structures, refrigeration cycles, and heat integration possibilities simultaneously. Results showed that VRC and diabatic distillation columns with heat-integrated stages can reduce the energy consumption between 58 and 75% when compared with conventional distillation at high pressure. Furthermore, the proposed synthesis procedure derived simplified optimal distillation structures with few heat-integrated stages and still attained important energy savings.

Energy-Efficient Diabatic Distillation Using a Horizontal Distillation Column

A conventional distillation column has two limitations causing heavy energy demand: short residence time of liquid flow and excessive vapor flow due to the external reflux flow. These are originated from the vertical structure and adiabatic operation of the column. By utilizing a horizontal distillation column with diabatic operation, the reduction of energy demand of the conventional distillation process was proposed. The separation performance of the proposed column was examined with the distillation experiment, and the column model was developed to calculate the composition profiles of vapor and liquid and the energy consumption. The experiment and simulation of the horizontal distillation column demonstrate that the intrinsic problems associated with the conventional distillation column can be solved to improve its energy efficiency. The computed energy requirement of the horizontal diabatic distillation column was 25.7% less than that of the conventional distillation column calculated from the HYSYS ...

A new approach to entropy production minimization in diabatic distillation column with trays

Previous approach to direct numerical minimization of entropy production in diabatic distillation column in order to determine heat quantity to be exchanged at trays was based on temperatures on trays as control variables and it was applied only to simple binary columns. Also, previously developed theoretical models for determining optimal exchanged heat profile were determined only at such columns and while they were approximated they produced worse results than numerical minimum of entropy production. In this paper, as control variables for minimization, exchanged heat on the trays is used. It enables application to complex multicomponent diabatic columns. Ishii-Otto global method, based on model linearization and iterative solution by Newton-Raphson technique, is applied for solving column mathematical model. Needed thermodynamical properties for ideal systems are calculated using Lewis-Randall ideal solution model, and for non-ideal slightly polar systems they are calculated using Soave equation of state. Five direct methods are used for numerical optimization. Applied approach is successfully demonstrated at frequently used example of distillation of benzene and toluol mixture by using for these purposes specially written program. Simplex method appeared to be the most convenient optimization method for the considered problem.

Performance optimization of a diabatic distillation-column by allocating a sequential heat-exchanger inventory

A diabatic distillation model is improved and the effects of the allocation of a heat-exchanger inventory on the diabatic-distillation performance are taken into account. Minimizing the entropy-production rate of the diabatic distillation column is taken as the optimization objective and the allocations of the heat-exchanger inventory on each tray in the diabatic distillation column are optimally redistributed under the condition of the fixed total heat-exchanger inventory. The optimal performance of the diabatic distillation column is obtained. The diabatic distillation model is meaningful for the design and optimization of diabatic distillation plant.

Optimization of a Diabatic Distillation Column with Sequential Heat Exchangers

Diabatic distillation is a separation process in which heat is transferred on the trays inside the column as opposed to classical adiabatic columns where heat is only supplied to the reboiler and extracted from the condenser. Such diabatic columns dramatically reduce the exergy needed to perform the separation. One implementation, particularly suitable for retrofitting applications, uses a single heating fluid circulating in series from one tray to the next below the feed tray and a single cooling fluid circulating in series above the feed tray. The optimal design of these sequential heat exchangers, minimizing the overall rate of entropy production in the separation process, is a difficult optimization problem because traditional algorithms for optimization invariably get stuck. However, an algorithm based on physical intuition for adjusting the temperature profile can find the optimum. The resulting column operation is compared to the optimal operation with independent heat transfer to each tray (the co...

Comparison of Entropy Production Rate Minimization Methods for Binary Diabatic Distillation

The purpose of this study is to compare two analytical methods with two numerical methods for minimizing the entropy production rate in diabatic distillation columns (i.e., with heat exchangers on all trays). The first analytical method is the equal thermodynamic distance method. The second uses Lagrange minimization on a model derived from irreversible thermodynamics. The numerical methods use Powell's and a Monte Carlo algorithm and gave the same results. Both analytical methods agreed well with the numerical ones for two columns with low separation per tray, while they did not agree well for a column with large separation per tray.

The Influence of Heat Transfer Irreversibilities on the Optimal Performance of Diabatic Distillation Columns

A distillation column with the possibility of heat exchange on every tray (a fully diabatic column) is optimized in the sense of minimizing its total entropy production. This entropy production counts the interior losses due to heat and mass flow as well as the entropy generated in the heat exchangers. It is observed that the optimal heating distribution, i.e. the heat exchange required on each tray, is essentially the same for all trays in the stripping and rectification sections, respectively. This makes a column design with consecutive interior heat exchanger and only one exterior supply for each of the two sections very appealing. The result is only slightly dependent on the heat transfer law considered. In the limit of an infinite number of trays even this column with resistance to transfer of heat becomes reversible.

Numerically optimized performance of diabatic distillation columns

Recently, the concept of equal thermodynamic distance (ETD) has been proposed to minimize entropy production in a distillation process using a diabatic column. ETD gives the optimal temperature profile to first-order in N −1, where N is the number of trays. ETD, however, does not generally give the true minimum for distillation columns with few trays. We therefore apply a fully numerical, multidimensional optimization routine to determine minimum entropy production. Since this method does not depend on an underlying theory, we expect a true minimum to be revealed. We then compare the performance of ETD and numerical optimization by varying the number of trays and the purity requirements. Our results show a surprisingly good agreement between the ETD results and those obtained numerically.