Static mixers as heat exchangers in supercritical fluid extraction processes

https://doi.org/10.1016/j.supflu.2007.07.015Get rights and content

Abstract

The performance of a Kenics static mixer as a heat-transfer device for supercritical carbon dioxide (CO2) flow is studied and compared with conventional tube-in-tube heat exchangers. Measurements were carried out at pressures ranging from 8 to 21 MPa, temperatures from 283 to 323 K, and mass flowrates from 2 to 15 kg/h. The corresponding Reynolds and Prandtl numbers, at bulk conditions, ranged between 103 and 2 × 104 and between 2 and 7, respectively. The temperature increase experienced by the supercritical CO2 stream varied between 10 and 35 K. The heat fluxes obtained with the static mixer are one order of magnitude higher than the ones observed with a tube-in-tube heat exchanger for the same set of operating conditions. The heat-transfer enhancement is caused by the cross-sectional mixing of the fluid and to a lesser extent by conduction across the metallic mixing elements. Heat-transfer is also affected by temperature-induced variation of physical properties, especially in the pseudocritical region of the fluid. From the experimental data, a correlation was developed for convective heat-transfer to supercritical CO2 in terms of the Nusselt number.

Introduction

Supercritical fluid extraction (SFE) is a process that uses fluids at supercritical conditions to selectively extract substances from solid or liquid mixtures. By changing pressure and temperature, the solvating power of the fluid (usually carbon dioxide) can be adjusted to obtain good solubilities, selectivities and transport properties. SFE technology has the potential to enhance product quality, while at the same time being an environmentally clean extraction technology. Heat-transfer to supercritical fluids (SCF) is important when dealing with technological applications of these fluids, especially for extraction, reaction or particle formation processes. Every SFE unit has several heat exchangers (HXs) in its flowsheet; their purpose can be, among others, to preheat the supercritical fluid before being fed to the high pressure vessel, to cool down the SCF before compression, or to change temperature conditions of the high pressure flow before separation of the solubilized solutes from the SCF solvent takes place. Conventional heat exchangers, such as double-pipe or shell-and-tube, have been used in SFE. They present important advantages (established design and manufacturing), but also show some undesirable features, such as relatively high surface areas to attain desired thermal exchange and heat-transfer inefficiencies due to non-negligible laminar film build-up on the tube walls.

Static mixers have been considered credible alternatives to conventional HXs, because they provide very high and uniform heat-transfer rates. They are currently being used in a wide range of applications, such as blending of gases or miscible liquids in laminar or turbulent flow, continuous co-current liquid/liquid or gas/liquid dispersions, heat exchangers, and interphase mass transfer between immiscible phases [1], [2], [3].

A static mixer consists of a contacting device with a series of internal stationary mixing elements of specific geometry, inserted into a pipe. The added effects of momentum reversal and flow division due to the internal elements of the mixer contribute to a maximization of mixing efficiency. The benefits of dispersion efficiency, short residence times, and low flow resistance, are advantages for the use of static mixers in mass and heat-transfer applications [4]. Recently, the use of static mixers in SFE processes has been proposed as, e.g., alternative contacting devices [5], [6], equilibrium cells for high-pressure thermodynamic studies [7], [8] and mixers of powder coatings and additives with SC-CO2 to make fine, coated particles from gas-saturated solutions [9]. Static mixers have advantages over conventional equipment for use at SC conditions. These include lower capital costs at a large scale of operation, no possibility of flooding even if there is a low density difference between the phases, short residence times, and minimal space requirements for location in a SFE plant.

In this work, the performance of a Kenics static mixer to heat a SC-CO2 stream is studied and compared with conventional tube-in-tube HXs. The Kenics mixer is comprised of a series of mixing elements aligned at 90°, each element consisting of a short helix of one and a half pipe diameters in length. Each helix has a twist of 180° with right-hand and left-hand elements being arranged alternatively in the pipe (Fig. 1a). The internal mixing elements direct the flow of material radially toward the pipe walls and back to the center. Additional velocity reversal and flow division results from combining alternating right- and left-hand elements, thus increasing mixing efficiency. All material is continuously and completely mixed, eliminating radial gradients in temperature in the bulk fluid. This in turn increases the thermal gradient near the hot wall and, consequently, the heat-transfer rate into the fluid.

Measurements are carried out at pressures ranging from 8 to 21 MPa, temperatures ranging from 288 to 323 K, and mass flowrates ranging from 2 to 15 kg/h. The effect of the density of the supercritical fluid, the heat flux, and the fluid flowrate, on the heat-transfer performance of the static mixer at high-pressure conditions is examined. Based on the experimental data, a correlation is developed for convective heat-transfer to SC-CO2 on the basis of a Nusselt number.

The design and optimization of HXs for supercritical fluids have an additional difficulty when compared with normal liquids or gases. Near its critical point, a fluid exhibits strong variations of the physical properties with temperature, especially near the pseudocritical point, Tpc (that is, the temperature at which the specific heat reaches a maximum for a given pressure), that strongly influences the heat-transfer. Therefore, the effect of these strong variations on the thermal efficiency of the static mixer is also investigated.

Section snippets

Experimental apparatus and procedure

The experimental apparatus used in the experiments is shown schematically in Fig. 1b; it is essentially the same as that used for vapor–liquid equilibrium measurements at high-pressure conditions [8]. A Kenics static mixer (model 37-04-065 from Chemineer, Inc.) was used in these experiments. The main characteristics of this mixer are an internal diameter of 4.623 mm, an overall length of 178 mm, and 21 helical mixing elements with a length-to-diameter ratio of 1.7.

Carbon dioxide, which was

Results and discussion

In the series of heat-transfer experiments reported here, CO2 is circulated through the static mixer at different conditions of mass flow, pressure and temperature. The experiments were carried out at four pressures (8, 9, 15 and 21 MPa) and three heating tape temperatures (313, 333 and 353K). The CO2 mass flowrate varied between 2 and 15 kg/h. For each run, the outlet wall temperature, the inlet and outlet temperatures, the gas flowrate, as well as the inlet pressure and pressure drop across the

Heat-transfer correlations

For heat-transfer involving supercritical fluids in horizontal or vertical heated tubes, a Dittus–Boelter type correlation has been proposed by many authors [11], [12]:Nub=0.023Reb0.8Prb0.4where the Nusselt, Reynolds and Prandtl numbers are all evaluated at bulk conditions. As suggested by van der Kraan et al. [15], Eq. (7) can be used to estimate SCF heat-transfer coefficients when the temperature difference between tube wall and bulk conditions is small. In that case, physical properties

Conclusions

The efficiency of a Kenics static mixer for heating SC-CO2 was studied and compared with data for conventional tube-and-tube HXs. For the range of operating conditions studied, the static mixer provides heat fluxes one order of magnitude higher than the ones obtained in conventional tube-tube heat exchangers. The influence of the variation of physical properties, in particular, the specific heat, with the temperature nearby the pseudocritical region of carbon dioxide was found to be

Acknowledgements

Financial support by Fundação para a Ciência e Tecnologia, under project grant number POCTI/EME/61713/2004 is gratefully acknowledged.

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