Thermosiphon Reboiler Research Articles

Evaporation of Binary Mixtures in a Pillow Plate Thermosiphon Reboiler

AbstractPillow plate heat exchangers show promising performance in extending the operating range of thermosiphon reboilers. Here, the evaporation of binary mixtures during natural circulation evaporation in a pillow plate apparatus was investigated. The experimental results show that the thermal performance is dominated by mixture effects and a component system‐dependent minimum is found. The model approach according to Bennett and Chen has proven to reliably estimate the product‐side heat transfer coefficient for pure substances and mixtures with a low boiling point distance. For wide‐boiling mixtures and low pressures the approach must be adjusted.

Simulation of a pillow plate thermosiphon reboiler using an equivalent tube bundle geometry

Pillow plate thermosiphon reboilers are innovative heat exchangers due to their compactness, flexible design options as well as high temperature and pressure resistance. Furthermore, thanks to their design they stabilize the recirculation flow and thus extend the operating range. However, due to their special geometry, available simulation tools do not allow for a quantitative rating or sizing. In this study, the geometry of a pillow plate apparatus is converted to an equivalent shell and tube geometry. It is investigated whether experimentally obtained results in a pillow plate thermosiphon reboiler can be reproduced by using that geometric equivalent in two industrial heat exchanger simulation tools. Eight experimental cases with varied operating pressures, submergences, and driving temperature differences are studied and compared to simulation results based on the geometrical equivalent. Both simulation tools provide acceptable results with deviations between +15% and −30% compared to the experimental determined circulation and heat flow. Additionally, they illustrate a special feature of the pillow plate thermosiphon reboiler: the pinch point appears at a shorter axial length than in a typical shell and tube thermosiphon reboiler, providing a larger area for evaporation.

Influence of surfactants on thermosiphon reboiling

Surfactants can increase two-phase heat transfer in aqueous solutions by depressing the surface tension and activating new bubble nucleation sites. Yet, boiling experiments are mainly conducted on a laboratory scale and hardly ever conducted under flow conditions which complicates the transfer of the results to larger scales. The present study demonstrates that the addition of an anionic secondary alkylsulfonate surfactant has a profound impact on the fluid dynamics and thermal performance of a single-tube vertical thermosiphon reboiler. At low heat flux conditions, heat transfer was enhanced by up to 26% and circulation rate was increased by up to 65% depending on the surfactant concentration. The increase in two-phase heat transfer also led to a stabilisation of the natural circulation and thus to an extension of the lower operating range of the reboiler. All experiments were conducted in the turbulent flow regime and at surfactant concentrations around the critical micelle concentration (CMC).

Operational Performance of a Thermosiphon Reboiler under Foaming Conditions

AbstractThe evaporation of aqueous solutions with varying surfactant concentrations was investigated with regard to foam formation and performance impairment of the reboiler. Foam formation was induced by partial evaporation in the reboiler and increased with rising driving temperature difference and decreasing operating pressure. Compared to the evaporation of pure water, the addition of surfactants resulted in a reduction in circulation velocity at higher heat fluxes. Temperature profiles in the reboiler tube and area‐averaged heat transfer coefficients were determined to characterize the thermal performance. Only at the lowest surfactant concentration a slight decrease in heat transfer was observed.

Machine Learning Supports Robust Operation of Thermosiphon Reboilers

AbstractThe analysis of process and equipment operational data in chemical engineering regularly requires a high level of expert knowledge. This work presents a Machine Learning‐based approach to evaluate and interpret process data to support robust operation of a thermosiphon reboiler. By applying an outlier detection, potentially interesting and unstable operating conditions can be identified quickly. A multidimensional regression allows to forecast the circulating mass flow. The results obtained fit well into the current state of research and manual evaluation of thermosiphon reboilers.

Use of Wire Matrix Inserts in Thermosiphon Reboilers in the Lower Operating Range

AbstractTurbulators are applied to increase the thermal efficiency of heat transfer units. The additional pressure drop can be challenging for the self‐circulation of thermosiphon reboilers. Thus, the effects of hiTRAN®‐wire matrix inserts for the boiling of water and a water/glycerol mixture were investigated herein, using the performance of a thermosiphon reboiler with plain tubes as a reference. The reboiler was operated at sub‐atmospheric pressures, small driving temperature differences, and under flooded conditions. Favorable and unfavorable operating conditions for using inserts were specified. Especially for the water/glycerol mixture, significant improvements of self‐circulation and heat transfer up to six times compared to the plain tube reference were observed, allowing an operation of thermosiphon reboilers at smaller driving temperature differences under sub‐atmospheric pressures.

Modelling and simulation of a pillow plate thermosiphon reboiler

Thermosiphon reboilers are the most widely used evaporators in the process industry. Experimental studies suggested a superior performance of pillow plate thermosiphon reboilers over conventionally used tubular designs. This contribution presents a comprehensive model to simulate fluid dynamic as well as thermal performance of pillow plate thermosiphon reboilers. Experimental heat transfer coefficients are extracted from measurements at a thermosiphon reboiler test rig. With this information, correlations for the single phase heat transfer coefficient and the heat transfer coefficient with nucleate boiling as well as convective boiling are presented. A correlation for the heat transfer coefficient for film condensation inside the pillow plate and the two-phase pressure drop between the pillow plates is presented. These correlations are implemented in a modular structured simulation program. Good agreements were shown for the heat transfer with water experiments. The fluid dynamic could be described satisfactory with larger uncertainties at small circulation rates.

Troubleshooting a Thermosiphon Reboiler – Why New Is Not Always Better than Old

During an ethylene plant turn-around, a 38-year-old tube bundle of a vertical thermosiphon reboiler was replaced with a new, seemingly identical one. However, when the plant returned to full operation after the turn-around, the reboiler was not able to reach the same duty as the old tube bundle.Systematic troubleshooting using detailed column and thermosiphon simulation, neutron backscatter, plant data reconciliation and flow regime analysis ruled out possible reasons such as thermosiphon inlet line plugging, or lowered heat transfer due to steam superheat, or the presence of non-condensables in the steam, leaving reduced heat transfer on the tube side as the strongest suspect.To avoid plant shut-down, the issue was temporarily solved by switching from low pressure (1.6 barg) to medium pressure (7.2 barg) steam as heating medium. With the higher steam pressure and increased ?T across the exchanger the original duty was successfully recovered.To investigate the reduced heat transfer, light, laser and electron microscopy studies revealed that the surfaces of old and new reboiler tubes were very different in roughness and cleanliness, causing differing heat transfer characteristics. The gradual degradation of the tube surface, which occurred over the years, is believed to have increased the maximum heat transfer duty of the old reboiler. This theory is supported by a report from the old tube bundle’s first year of operation when similar capacity problems occurred.

Analysis of artificial neural network in prediction of circulation rate for a natural circulation vertical thermosiphon reboiler

The present study has been carried out on the applicability of Artificial Neural Networks (ANNs) for predicting circulation rate in a vertical thermosiphon reboiler for different liquids with wide variation in thermo-physical properties and operating parameters. The different architecture of feed forward artificial neural network with back-propagation error technique is studied. The experimental data from the literature including the author’s own work were used for training and testing the different networks. It has been shown that the trained network can predict the circulation rate better than the empirical correlations from the literature. Effect of the different structures of the network on its performance has also been studied. In most of the cases it has been noted that by increasing the nodes and hidden layers network performance improved up to certain limit. Five different training algorithms have been applied with different hidden layers and nodes to train the network. However it is seen that SCG and BFGS training algorithm is more efficient with respect to CPU time in comparison to other algorithms like GDA, GDM and LM. A comparison of the ANN-based model was made with the empirical correlations reported in the literature. It was observed that the proposed network based model significantly showed a better prediction.

Enlarged operation ranges for thermosiphon reboilers using pillow plates

Thermosiphon reboilers are widely used as reboilers in distillation columns and as evaporators in the chemical and petrochemical industry. The main reasons are the low investment and operating costs. Heat transfer and fluid dynamics are strongly linked in thermosiphon reboilers. Therefore the operation range of these evaporators is limited.Pillow plates offer a very good heat transfer with a low pressure drop. Therefore great potential is expected for their use in thermosiphon reboilers. Evaporation experiments with a pillow plate thermosiphon reboiler were carried out with water and a water–glycerol mixture. The total pressure, the submergence and the temperature difference between heating and evaporation side were varied. Circulation rate and heat transfer were measured. Stable natural circulation was seen over a wide range of influence parameters. Even at low total pressures and at low submergences the natural circulation was stable.Compared to literature data for tubular thermosiphon reboilers the operating limits for pillow plate thermosiphon reboilers can be increased, especially at low total pressures, low submergences and low temperature differences. The measured heat transfer is increased compared to tubular stainless steel thermosiphon reboilers.

Novel application of support vector machines to model the two phase boiling heat transfer coefficient in a vertical tube thermosiphon reboiler

The prediction of the rate of heat transfer is one of the primary requirements for the design of thermosiphon reboilers. For the first time, a unified data-driven model for the prediction of boiling heat transfer coefficient in a thermosiphon reboiler, using support vector regression (SVR) as the modeling method, has been proposed. Different single component liquids with wide variation in thermophysical properties and operating parameters have been employed for the purpose. The SVR model for boiling heat transfer coefficient (in the form of Nusselt number) has been developed in terms of dimensionless parameters, namely PeB, Xtt, PrB and σwater/σL. 300 experimental runs from literature including the author's own work for the boiling of acetone, benzene, ethanol, ethyl acetate, ethylene glycol, propan-2-ol, toluene and distilled water have been utilized for the development and validation of the SVR model. The data was randomly divided into a training set and a test set in the ratio of 8:2. The former was used for training the SVM while the latter was used to test the generalization ability of the model. The optimal values of the SVR model hyper parameters viz., C and ɛ, and the RBF kernel parameter γ, were obtained by applying the grid search methodology with 10-fold cross-validation on the training data set. The values were: C=256, ɛ=0.145, and γ=0.35, respectively. An AARE of 8.43%, R of 0.9836, RMSE of 0.1103, SD of 0.0649, Q2LOO of 0.9672, and MRE of 0.0852 on the training data were obtained while the corresponding values for the test data were: 9.18% AARE, 0.9775 R, 0.1136 RMSE, 0.0581 SD, 0.9555 Q2ext, and 0.0946 MRE.The said SVR-based model has shown high prediction accuracy in terms of model evaluation indices, both for the training data as well as for the virgin test data. This model has scored highly over the other models from literature in terms of a remarkably enhanced prediction and generalization performance with an AARE of 8.43% on the training data and 9.18% on the test data.

Thermosiphon Reboilers with Enhanced Tubes

AbstractReliable heat exchangers capable of transferring heat at low driving temperature differences are key to energy efficient processes. In this paper, heat transfer and fluid dynamics of hydrophobic nanoparticle (HBNP) coated tubes are investigated in a steam‐heated single‐tube thermosiphon reboiler and compared to experimental results of standard stainless steel tubes. In comparison to the bare tube, the coated tube showed higher heat transfer and circulation rates as well as an improved stability of the circulating flow. The stability of the coating was verified in a long‐term stability test.

Fouling Monitoring in Thermosiphon Reboiler

A method to monitor the fouling resistance of a heat exchanger, especially the thermosiphon reboiler, is presented. The historical hourly data of reboiler and distillation column are used to analyze fouling factor. The initial value of overall heat transfer coefficient and the current overall heat transfer coefficient are needed to calculate fouling resistance. Since the initial heat transfer coefficient is not constant but is affected by operating conditions such as shell/tube flow rate and temperature, a relationship between initial heat transfer coefficient and these operating conditions has to be developed to calculate the fouling factor. However, sometimes important data such as tube-side flow, inlet/outlet temperature of hot/cold fluid, and liquid level in shell/tube, which are needed to bring out the fouling condition of equipment, are missing. With such insufficient information, the fouling behavior can be analyzed by building a new equation based on measurable data. Dimensional analysis determines the structure of the function; parameters of each term are obtainable using short-term plant data after cleaning, assuming that there is no fouling during this period. As it is not easy to measure actual fouling factor temporal change in the heat exchanger tube, the calculated fouling resistance is compared with the relative amount of solid material removed from the equipment during cleaning, and it is evaluated qualitatively. The comparison shows good agreement. In addition, the evolution of the fouling resistance of some thermosiphon reboilers over a period of a few months or even years can be matched using this model.

Development of support vector regression (SVR)-based model for prediction of circulation rate in a vertical tube thermosiphon reboiler

The hydrodynamics and heat transfer in a thermosiphon reboiler interact with each other making the process very complex. Prediction of the rates of heat transfer and thermally induced flow are the primary requirements for the design of thermosiphon reboilers. The objective of this study was to develop, for the first time, a unified data-driven model, for the prediction of circulation rate in a thermosiphon reboiler for different pure components with wide variation in thermo-physical properties and operating parameters, using support vector regression (SVR)-based modeling technique. In the present work, 148 experimental data points from accessible sources, including the author's own study were used. First, a multiple regression (MR) model for circulation rate (in the form of Reynolds number) was developed as a function of dimensionless parameters namely, Peclet number for boiling ( Pe b ), Subcooling number ( K sub ), and the Lockhart–Martinelli parameter ( X tt ), followed by the formulation of an SVR-based model. Statistical analysis revealed that the proposed generalized SVR-based model had high prediction accuracy with an average absolute relative error (AARE) of 3.82%, root mean square error (RMSE) of 0.0717, leave-one-out cross validation ( Q 2 LOO ) of 0.9975 and mean relative error (MRE) of 0.0288 on the training data. Corresponding values of 6.11% AARE, 0.0816 RMSE, 0.9991 leave-one-out cross validation on test data ( Q 2 ext ) and 0.0541 MRE were obtained for the test data. A comparison of the SVR-based correlation was made with the MR model and with some selected empirical correlations in the literature. It was observed that the proposed SVR-based model significantly exhibited an enhanced prediction and generalization performance.

Circulation rates of boiling liquids in a vertical tube

An experimental study of heat transfer to boiling liquids under natural convective flow has been carried out in a single tube vertical thermosiphon reboiler to investigate the effect of heat flux and submergence on circulation rates. The test liquids used were distilled water, various concentrations of propan-2-ol in water and their azeotrope. The test section was an electrically heated stainless steel tube of 25.56 mm i.d., 28.85 mm o.d. and 1900 mm long with twenty-one spot welded copper constantan thermocouples to measure the variations in wall temperature along its axis. The uniform heat fluxes in the range of 3.5–42.4 kW/m 2 were employed while inlet liquid sub-cooling varied from 0.2 to 30.7 °C. The liquid submergence levels were maintained in the range 30–100%. The typical experimental data has been graphically presented and discussed. An empirical correlation has been developed from the experimental data of the present study through regression analysis.

Effect of Bubble Turbulence and Submergence on Boiling Incipience in Vertical Thermosiphon Reboiler

A detailed study has been carried out to clarify the conditions of boiling incipience phenomena for natural flow in vertical tube thermosiphon reboiler. A semi-empirical model has been proposed that considers the effect of turbulent eddies and submergence for the data available in literature. Boiling incipience in liquid films is influenced both by turbulent eddies and submergence, and their effect is dominant with increasing heat flux. The superheat for boiling incipience was determined experimentally for different types of organic liquids including water, covering a wide range of physical properties. The predicted results from the proposed model and experimental data available in literature show a consistency for all fluids investigated in this study. It has an average absolute relative error of 15% for the proposed unified correlation of all fluid systems together, which have a wide range of thermophysical properties.

Onset of Nucleate Boiling of Binaries in a Vertical Thermosiphon Reboiler

Onset of Nucleate Boiling of Binaries in a Vertical Thermosiphon Reboiler

Prediction of temperature profiles using artificial neural networks in a vertical thermosiphon reboiler

The objective of this work is to use artificial neural networks (ANNs) for predicting the temperature profiles as well as temperatures at various operating conditions in a vertical thermosiphon reboiler. The experimental data from the literature were used for training of feed forward artificial neural network with error back propagation technique. Gradient descent methods of optimization have been applied for training the network. It was observed that the predicted temperature profiles were very close to the actual experimental data. As the number of nodes increased, the training time decreased and the training was faster initially then it slowed down asymptotically. The prediction of ANN results was very close to the experimental values with a mean absolute relative error less than 4.3%.

Onset of Nucleate Boiling of Binaries in a Vertical Thermosiphon Reboiler

In the present study a semi empirical equation has been developed as a modification and extension of the previous analyses of pure fluids for the prediction of wall superheat for six binary liquid mixtures including the effect of liquid submergence. The wall superheat required for the onset of nucleate boiling of binary mixtures was related to thermo physical properties, heat flux and liquid submergence. The results predicted from the theoretical analysis are consistent with the experimental data available in the literature. The results were compared with the available correlations, indicating that the correlations based on the submergence are in better qualitative agreement. A unified correlation for the binary mixtures covering wide ranges of heat flux, submergence, inlet liquid subcooling and surface material was also obtained with enhanced deviation of 20.37%.

Intensification of fluiddynamic and thermal performance of thermosiphon reboilers

The potential benefit of hiTRAN ® tube inserts for fluiddynamic as well as heat transfer intensification of thermosiphon reboilers was investigated experimentally. Circulating flow expressed as single phase tube inlet velocity is regarded as primary criterion for fluiddynamic behaviour while product side heat transfer coefficient represents heat transfer performance. Pure water and a 29 mol% glycerol/water mixture were used as test fluids. Fluiddynamic as well as thermal performance of the bare tube vs. five inserts with different loop volume fractions were determined in a single tube lab-scale thermosiphon reboiler. It was found that most beneficial use appears for the glycerol/water system at low driving temperature difference between service and product side employing inserts with a loop volume fraction around 3%. In best cases a more than 50% increase of the product side heat transfer coefficient was obtained.