Vertical Thermosiphon Reboiler Research Articles

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).

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.

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.

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%.

The determination of superheated layer thickness and wall superheat in vertical tube natural circulation reboiler

The characteristics of incipient boiling for vertical thermosiphon reboiler are studied in detail. The liquid film adjacent to the heating surface, the superheated layers thickness, δ*, must attain a threshold value so that the critical bubble nuclei with radius r c can further grow to the point of detachment. Thus, the relation between δ* and r c at the threshold of post-critical growth is of primary importance in determining the onset of nucleate boiling. In the present study a semi-empirical equation for wall superheat is proposed which includes the effect of submergence for the experimental data available in the literature. The data for water and organic fluids having wide range of thermophysical properties were correlated with a single correlation having mean absolute deviation of 14.73%. The value of δ*/ r c for each fluid and unified value have also been also proposed.

Conditions of Onset of Boiling in a Vertical Thermosiphon Reboiler

An analysis of the incipience of nucleate boiling for a vertical thermosiphon reboiler was developed as a modification and extension of earlier analysis. The maximum superheat attained around the onset of boiling was taken from the wall-temperature distributions and correlated with heat flux, submergence, and physical properties of the test liquids. The results predicted from theoretical analysis are consistent with the experimental data that are available in the literature. All of the data for the eight fluids tested, each having different thermophysical properties, were correlated with a single correlation having an average absolute relative error of 21.35%. Nomenclature a1, b 1= constant used in equation of Agrawal 13 a2, b 2= constant used in equation of Ali and Alam 15 a3, b 3= constant used in equation of Kamil et al. 17 C1 = constant used in equation of Davis and Anderson 5 (C1 = 1, θ = 90 deg) C P = heat capacity, kJ/kg K hfg = latent heat of vaporization, J/(kgK) k = thermal conductivity, W/m K N = number of experimental data P = pressure, psia PrL = Prandtl number of fluid q = heat flux, W/m 2 R =g as constant, Nm/kg K r = radius, m rmax = maximum cavity radius, m rtan =c avity radius based on the tangency criterion, m S = submergence, % T = temperature, ◦ C � Ts =d egree of superheat (Tw − Ts), ◦ C � Tsub =d egree of subcooling (Ts − TL ), ◦ C y = distance perpendicular to the heated wall, m Z = distance along the test section, m δ ∗ = superheated layer thickness, m µ = viscosity, Ns/m 2 ρ = density, kg/m 3 σ = surface tension, N/m ψ =e xponent used in Eq. (11) Subscripts av g= average B = boiling b =b ubble

Effect of submergence on boiling incipience in a vertical thermosiphon reboiler

An analysis of the incipience of nucleate boiling has been developed as a modification and extension of the previous analyses for the wall superheat including the effect of submergence. The minimum degree of wall superheat required for the onset of fully developed boiling of liquid was related to the thermophysical properties of test liquids. The model prediction was made with the experimental data available in literature for nine organic liquids including water. The results predicted from the theoretical model were found to be consistent with the data. An effort was also made to obtain a unified correlation for nine different fluids, together covering wide ranges of heat flux, submergence and inlet liquid subcooling. It was observed that the majority of the data points lie within a maximum error of ±22% and mean absolute deviation of 16 percent.

Operation of a Vertical Thermosiphon Reboiler Experimental Facility at Reduced Pressures

Experimental data were obtained for the evaporation of water in a vertical thermosiphon reboiler having 50 tubes of 3 m length and 25.4 mm outer diameter. The effect of step changes in heat load was noted for operation over a pressure range from 0.2 to 1.0 bar. The reboiler behaviour exhibited three ‘operational modes’, and a definite transitional point that is independent of process pressure was observed between operational modes one and two. The influence of reduced pressures on the vertical thermosiphon reboiler sub-cooled heating length was significant. Overall heat transfer coefficients at 0.2 bar were about 60% of those at 1.0 bar and circulation rates were approximately halved. Two important considerations for the reboiler operation analysis were (a) the possible non-uniform behaviour of the reboiler tubes and (b) the change of phase and processes occurring on the reboiler shell as well as the tube side.

Prediction of circulation rates in vertical tube thermosiphon reboiler

Prediction of circulation rates in vertical tube thermosiphon reboiler

Heat transfer to boiling liquids in a single vertical tube thermosiphon reboiler

An experimental investigation of heat transfer to boiling liquids has been carried out on a single tube natural circulation reboiler. Experiments were performed with distilled water, methanol, benzene, toluene, and ethylene glycol to cover a wide range of boiling points and thermophysical properties. The heat transfer section consisted of an electrically heated stainless steel tube 25.56 mm I.D. and 1900 mm long. Uniform heat flux values were used in the range of 4.1 to 43.0 kW/m 2 while inlet liquid subcooling was varied from 0.5 to 11.6°C. Liquid submergence levels were maintained around 100, 75, 50, and 30%. All the data were generated with increasing heat flux at atmospheric pressure. The effect of heat flux and liquid submergence on the variation of wall and liquid temperatures have been graphically presented and discussed. The heat transfer coefficients in the nonboiling and boiling regions for all the test liquids have been correlated by two separate correlations in terms of dimensionless groups. The experimental results are compared with different correlations given in the literature.

Predicting the onset of nucleate boiling in a vertical thermosiphon reboiler

Predicting the onset of nucleate boiling in a vertical thermosiphon reboiler

Heat transfer to boiling liquid mixtures in a vertical tube thermosiphon reboiler

Heat transfer to boiling liquid mixtures in a vertical tube thermosiphon reboiler