Intensification of Heat Transfer Processes

New challenges in efficient heat recuperation arise when integrating renewables, polygeneration and combined heat and power (CHP) units with traditional sources of heat in industry and the communal sector, as it is shown by Klemeš et al. (2010). Heat transfer enhancement is an efficient technique to increase energy saving when retrofitting heat exchangers or designing a new heat transfer system. By implementing intensified techniques in existing exchangers, higher heat transfer coefficients can be achieved, leading to higher heat exchange duties allowing a reduced size of heat transfer equipment and the associated benefits (especially improving heat transfer performance). Intensification techniques provide: (i) Reduction in size of a heat exchanger for a given duty; (ii) Increase in capacity of an existing heat exchanger; (iii) Reduction in approach temperature difference; or (iv) Reduction in pumping power. Conventional enhancement techniques include tube-side enhancements (i.e. enhanced surface tubes, internal tube fins, coatings, fluid additives, mechanical mixing devices, twisted-tape inserts, coiled-wire inserts, etc.); shell-side enhancements (i.e. externally enhanced surface tubes, external tube fins, coatings, fluid additives, helical baffles, etc.). The compact heat exchangers such as tube-fin, plate-fin and plate heat exchangers are using heat transfer intensification and offer significant reduction in size, weight and cost of heat recuperation equipment. Developments in mini- and micro- channel heat exchangers are offering new possibilities of heat transfer intensification in channels of very small hydraulic diameters. Recently such intensification has been widely studied in the process industry from the point of view of individual heat exchangers. Combining several enhancement techniques can achieve higher energy savings when compare to implementing a single technique. It is difficult to identify which intensification technique is more suitable in a certain design, or which combinations of enhancement techniques are expected to contribute the most in compound augmentation applications. This work will survey current practices and review recent advances in enhancement techniques from an economic and performance standpoint

A Multi-Level Mathematical Model of the CO Catalytic Conversion Process

This paper presents a three-level modelling approach to the catalytic carbon monoxide oxidation in a temperature range between 400 K – 800 K. The first level involves the description of the chemical kinetics for the exothermic reactions on the catalyst surface. The second level models the thermal and hydrodynamic processes in the boundary diffusion layer between the catalyst surface and the reactive stream. Finally, the third modelling level focuses on the representation of the hydrodynamic and thermal properties for the bulk multi-component gas flow at various gas velocity and temperature ranges. The dynamic behaviour of the reactive system has been studied through simulated runs

Accounting for Thermal Resistance of Cooling Water Fouling in Plate Heat Exchangers

The Plate Heat Exchanger (PHE) is one of the most efficient types of modern heat exchangers. Heat transfer enhancement is one of the main features of PHEs, and lower fouling tendencies render them even more advantages for the use in different applications. The effects on fouling accumulation rate of process parameters in PHE channels of intricate geometry are studied in this article. The asymptotic behavior of the water fouling on heat transfer surfaces is examined. The fouling accumulation rate is described as a difference between the fouling deposition term and the fouling removal term. On comparison with data for fouling on different heat transfer surfaces available in literature it is shown that asymptotic fouling thermal resistance inversely proportional to wall shear stress. The proportionality coefficient in this relation is determined for a number of considered cases. To calculate the wall shear stress the equation for PHE channel main corrugated field is used, which accounts for corrugations geometrical parameters. It is shown that for given fouling properties of water this coefficient is constant and can be determined by monitoring fouling behavior of any item of heat exchangers working on specific enterprise. After that all other heat exchangers of that enterprise can be calculated using that data and developed Equation for accounting of fouling in their design

Mitigation of Fouling in Plate Heat Exchangers for Process Industries

Among different methods for mitigation of fouling the use of enhanced heat transfer surfaces is one of the major categories. Heat transfer enhancement is one of the main features of Plate Heat Exchanger (PHE) and mitigation of fouling render even more advantages to the use of this type of heat transfer equipment in different applications at process industries. In present study the period of fouling deposit formation in PHEs and influencing it factors are investigated. To estimate development of fouling thermal resistance with time the Equation is proposed. The pictures of fouling deposit distribution along the plate of PHE for fresh water heating are analyzed. The conclusion is, that for scaling fouling there exist some threshold conditions on wall shear stress, wall temperature and salt content, after which fouling deposition starts. The expression of fouling deposition rate proposed for the tubes with heat transfer enhancement by Yang and Crittenden is used. Comparison with available in literature experimental data have shown good agreement with proposed model, when one its parameter is adjusted. For certain cooling water circuit of a big industrial enterprise this parameter can be determined by data about fouling in one heat exchanger. After that the model can be used for prediction of cooling water fouling development with a time in all heat exchangers of this circuit

Generalised semi-empirical correlation for heat transfer in channels of plate heat exchanger

The analogy of heat and momentum transfer in turbulent flow modified for channels of Plate Heat Exchanger (PHE) is proposed. The effects of channel geometry, flow velocity and fluid properties on heat transfer are accounted in the resulting equation, which permits the calculation of film heat transfer coefficients using the generalized correlation for friction factor at the main corrugated field of the interplate channel. The results of calculations are compared with data from experimental study. The good accuracy of film heat transfer coefficients prediction is shown. In the case when the corrugations direction is parallel to the flow direction, the calculations results are quite close to the predicted by the Equation published in the literature for straight pipes. The Prandtl number influence on heat transfer is discussed and semi-empirical Equation for its evaluation is proposed. The comparison with experimental data available in the literature confirmed the accuracy of the heat transfer prediction. The proposed Equation is recommended to be used for optimization of PHEs channels geometry for different conditions in the process industries. It can be employed also for optimizing PHEs heat exchange networks and also to determine PHEs heat transfer area targets when process integration methodology is employed

The Modified Analogy of Heat and Momentum Transfers for Turbulent Flows in Channels of Plate Heat Exchangers

The modification of Von Karman analogy for turbulent flow in channels of Plate Heat Exchangers (PHEs) is proposed. The resulting equation enables to calculate film heat transfer coefficients in PHE channel on a data of hydraulic resistance of the channel main heat transfer field, accounting for the influence of channel geometry, flow velocity and fluid properties. The comparison with experimental data for water flow in models of PHE channels main corrugated fields is presented. It is shown the good accuracy of prediction for film heat transfer coefficients. In the limiting case, where corrugations are parallel to plate axis, the results of calculations by proposed Equation are in excellent agreement with Equation published for straight tubes and channels by Gnielinski in 1975. The analysis of the Prandtl number influence on heat transfer is performed. It explains the difference of Pr powers, which varies from 0.6 to 0.3 at correlations reported in different experimental papers on heat transfer. The proposed Equation can be used for modelling of PHEs heat transfer performance in a wide range of different applications in process industries

Optimal design of plate-and-frame heat exchangers for efficient heat recovery in process industries

The developments in design theory of plate heat exchangers, as a tool to increase heat recovery and efficiency of energy usage, are discussed. The optimal design of a multi-pass plate-and-frame heat exchanger with mixed grouping of plates is considered. The optimizing variables include the number of passes for both streams, the numbers of plates with different corrugation geometries in each pass, and the plate type and size. To estimate the value of the objective function in a space of optimizing variables the mathematical model of a plate heat exchanger is developed. To account for the multi-pass arrangement, the heat exchanger is presented as a number of plate packs with co- and counter-current directions of streams, for which the system of algebraic equations in matrix form is readily obtainable. To account for the thermal and hydraulic performance of channels between plates with different geometrical forms of corrugations, the exponents and coefficients in formulas to calculate the heat transfer coefficients and friction factors are used as model parameters. These parameters are reported for a number of industrially manufactured plates. The described approach is implemented in software for plate heat exchangers calculation

The Choice of the Optimal Retrofit Method for Sections of the Catalytic Reforming Unit

Hydrotreating section and catalytic reforming section of catalytic reforming unit L-35-11/600 were examined in this paper. This unit was designed for processing large fraction of naphtha by catalytic reforming in order to obtain components of gasoline with an octane number 78-85 points. Pinch diagnostics for these sections was carried out. Comparative economic analysis of their effectiveness after the proposed retrofit was performed for each section separately and for the total their flowsheet. The implementation of the Pinch Method for hydrotreating section will reduce energy intensity by 2.2 MW. Energy consumption for catalytic reforming section reduced by 6.4 MW. Energy consumption for joint integration of hydrotreating section and catalytic reforming section reduced by 11.4 MW. Therefore, it was concluded that Pinch design for these sections of catalytic reforming unit L-35-11/600 is most advisable to carry out for the two sections together

Integration Processes of Benzene-toluene-xylene Fractionation, Hydrogenation, Hydrodesulphurization and Hydrothermoprocessing on Installation of Benzene Unit

The heat exchanger network (HEN) of the unit of benzene production at petrochemical plant was inspected and the obtained data were analyzed for possible plant retrofit targeting the minimal energy consumption and increasing of plant efficiency. The benzene-toluene-xylene fractionation, hydrogenation, hydrodesulphurization and hydrothermo processing units of the plant were analysed and the data of existing HEN flowsheets were extracted. The minimum temperature difference for the retrofit was determined by analyzing the cost parameters of the proposed modifications as well as the energy cost. Using Pinch Analysis methodology the Composite Curves and Grid Diagram were obtained for the integrated processes of these plant units. The new flowsheet of the HEN of the regarded units was developed; and the possible application of heat transfer equipment was analyzed and proposed

Computer Aided Design of Plate Heat Exchangers

The computer aided design of plate heat exchanger with mixed grouping of plates is considered. It is formulated as the mathematical problem of finding the minimal value for implicit nonlinear discrete/continues objective function with inequality constraints. The optimizing variables include the number of passes for both streams, the numbers of plates with different corrugation geometries in each pass, the plate type and its size. To estimate the value of objective function in a space of optimizing variables the mathematical model of plate heat exchanger is developed. To account for thermal and hydraulic performance of channels between plates with different geometrical forms of corrugations, the exponents and coefficients in formulas for heat transfer coefficients and friction factors calculation are used as model parameters. The procedure and software for numerical experiment to identify model parameters by comparing the calculation results with those obtained with free available in web computer programs of plate manufacturers is developed. The sets of such parameters are obtained for a number of industrially manufactured plates. The described approach is implemented as software for plate heat exchangers calculation