Of dense gas

A complete analysis of dense gas dispersion is much beyond the scope of this treatise. More detailed references are available (Britter and McQuaid, Workbook on the Dispersion of Dense Gases, Health and Safety Executive Report No. 17/1988, England, 1988 Lees, 1986, pp. 455 61 Hanna and Drivas, 1987 Workbook of Test Cases for Vapor Cloud Source Dispersion Models, AlChE, 1989 Guidelines for Chemical Process Quantitative Risk Analysis, 1989, pp. 96-103). 

Figure 5-13 Britter-McQuaid dimensional correlation for dispersion of dense gas plumes.

For the case of dense-gas heat- and mass transfer, free convective effects are dominant under certain conditions. In engineering work, use is made of these properties. 

When the gas-solid flow in a multiphase system is dominated by the interparticle collisions, the stresses and other dynamic properties of the solid phase can be postulated to be analogous to those of gas molecules. Thus, the kinetic theory of gases is adopted in the modeling of dense gas-solid flows. In this model, it is assumed that collision among particles is the only mechanism for the transport of mass, momentum, and energy of the particles. The energy dissipation due to inelastic collisions is included in the model despite the elastic collision condition dictated by the theory. 

As long as so little is known about the interstellar particle and photon radiation fields which provide the heating of the interstellar gas, one should not put too much faith in estimated equilibrium temperatures of dense gas clouds such as shown in Fig. 9. However, at least qualitatively it appears to be correct that dense condensations in the interstellar gas have low kinetic temperatures. In fact, gas-kinetic temperatures as low as 5 °K have been observa-tionally determined in the center of dark clouds. 

Stephan, K. Lucas, K. Viscosity of Dense Gas Plenum Press ... 

A different mode of operation, characterised as bubble flow, is possible if the continuous phase is the liquid to be extracted. The liquid level is kept constant at the top of the column and the supercritical gas which is introduced at the bottom is bubbled through the liquid for extraction. This method is mainly used if small amounts of dense gas are sufficient for complete extraction, i.e. if the solvent ratio of dense gas/ liquid feed is around 1 kg/kg. Examples are the extraction of flavours from wine and fruit juice where only very small amounts of extract can be expected. 

Meroney, R.N. (1988) Guidelines for Fluid Modeling of Dense Gas Cloud Dispersion, Journal of Hazardous Materials Vol. 17, 23 16. 

Lathouwers D, Bellan J (2001) Modelhng of dense gas-solid reactive mixtures applied to biomass pyrolysis in a fluidized bed. Int Journal of Multiphase Flow 27 2155-2187... 

Lathouwers D, Bellan J (2001) Yield optimization and scaling of fluidized beds for tar production from biomass. Energy Fuels 15 1247-1262 Lathouwers D, Bellan J (2001) Modelhng of dense gas-solid reactive mixtures applied to biomass pyrolysis in a fluidized bed. Proc of the 2001 US DOE Hydrogen Program Review. NREL/CP-570-28890... 

VAN DER HoEF, M. A., VAN SiNT AnNALAND, M., DeEN, N. G. KUIPERS, J. A. M. 2008 Numerical simulation of dense gas-solid fluidized beds a multiscale modeling strategy. Annual Review of Fluid Mechanics 40, Al-10. 

Zhu, C., Dynamic Behavior of Unsteady Turbulent Motion in Pipe Flows of Dense Gas-Solid Suspensions, Ph.D. Thesis, University of Illinois at Urbana-Champaign, 1991. 

The extraction of metals based on a membrane contactor system with conventional solvents is a process widely studied using different configurations, extractants, and extraction solvents. One of the upcoming applications of membrane contactors is supercritical extraction. This process is called porocritical extraction. Porocritical process or porocritical extraction is a commercial supercritical fluid extraction (SFE) technique that utilizes an hollow fiber membrane contactor (HFMC) to contact two phases for the purpose of separation. As an improvement, the extraction of Cu + from aqueous solutions by means of dense gas extraction was achieved by using a hollow fiber membrane contactor device [7]. The authors... 

In another variation, dense gas CO can be used to induce porosity in polymeric biomaterials. As dense gas CO generally has a low solubility in hydrophilic polymers, various attempts have been made to improve the ability of dense gas to diffuse into hydrogel precursor solutions and produce porosity in hydrogel matrices, such as COj-water emulsion templating or the use of cosolvent systems [69]. With this technique, it is difficult to control pore size and ensure pore interconnectivity. 

In what follows the simple model of Van Ulden [2, 30] is described. It gives an impression of the mechanisms of dense gas dispersion. In any case the model according to [27] should be preferred for practical apphcations. 

Mohan M, Panwar TS, Singh MP (1995) Development of dense gas dispersion model for emergency preparedness. Atmos Environ 29(16) 2075-2087... 

All the models proposed for fluidized bed membrane reactors have the same limitations. These models are phenomenological models that make use of closure equations originally derived for fluidized bed without internals. Although it is known that the presence of internals (membranes and permeation of gas through them) may vary the behavior of bubbles, mass transfer and solid circulation inside the bed, reliable closure equations for membrane assisted fluidized bed are not yet available. A way to solve this problem is to use a so-called multi-scale modeling of dense gas-solid systems, where low level... 

Multi-Scale Modeling of Dense Gas-Solid Systems... 

M. Ye, Multi-level modeling of dense gas-solid two-phase flows, PhD dissertation. University of Twente, 2005. 

---