Cold flow studies

Cold flow studies have several advantages. Operation at ambient temperature allows construction of the experimental units with transparent plastic material that provides full visibility of the unit during operation. In addition, the experimental unit is much easier to instrument because of operating conditions less severe than those of a hot model. The cold model can also be constructed at a lower cost in a shorter time and requires less manpower to operate. Larger experimental units, closer to commercial size, can thus be constructed at a reasonable cost and within an affordable time frame. If the simulation criteria are known, the results of cold flow model studies can then be combined with the kinetic models and the intrinsic rate equations generated from the bench-scale hot models to construct a realistic mathematical model for scale-up. 

Construction of a plexiglass scale model for cold flow studies typically occurs after completion of the CFD analysis. Smoke entrained in an air stream is used to empirically confirm the SCR design. Tuning efficiencies of the guide vanes and mixing ability of the static mixers are some of the design qualities confirmed by the cold low model. Figure 17.15 is a picture of an actual model used for commercial scale-up. 

Dou, S. M., Harandi, X. X Schipper, P. H., and Owen, H. Multi-Solid Fluid Bed Cold-Flow Study, in Circulating Fluidized Bed Technology IV" (Amos A. Avidan, ed.), pp. 484-489. Somerset, Pennsylvania (1993). 

E. K.T. Kam, E. Alper, S. Al-Safadi, F. Abu-Seedo, M. Absi-Halabi and M. Sabri, Cold flow studies on catalyst properties and hydrodynamic characteristics in ebullated-bed model reactor. Technical Report KISR 4169, Kuwait Institute for Scientific Research, Kuwait, 1992. 

In this section we derive the algebraic-slip mixture model equations for cold flow studies starting out from the multi-fluid model equations derived applying the time- after volume averaging operator without mass-weighting [204, 205]. The momentum equations for the dispersed phases are determined in terms... 

This form of the mixture model is called the drift flux model. In particular cases the flow calculations is significantly simplified when the problem is described in terms of drift velocities, as for example when ad is constant or time dependent only. However, in reactor technology this model formulation is restricted to multiphase cold flow studies as the drift-flux model cannot be adopted simulating reactive systems in which the densities are not constants and interfacial mass transfer is required. 

Exelus has developed a novel structured catalytic system that allows one to meet all four criteria in a single catalytic system Hydrodynamic tests reveal that the HyperCat has similar gas hold-up as a slurry bubble column reactor but with a much lower liquid axial-dispersion coefficient. Cold-flow studies appear to indicate that the heat-transfer coefficient of this new system is similar to a bubble column reactor. Catalyst performance tests reveal that the performance of the HyperCat is similar to that of a powder catalyst when used in a plug-flow reactor. 

Gutmark We will compare cold- and hot-flow measurements to gain understanding. The cold-flow and hot-flow facilities are identical and will be run in parallel. We can always learn from cold-flow studies. Next, we will study reacting flow fuel mixedness, flow field, what creates stabilities and their effects, etc. 

Tarmy et al. [9] measured gas holdup under various cold flow conditions in order to predict the holdup for the EDS coal liquefaction process. These authors observed rather low holdup values and churn-turbulent flow in their cold flow studies while, surprisingly, under the actual conditions of the EDS process bubbly flow prevailed and gas holdup was as high as 0.5. 

Chao [21] and Chao et al. [20, 22, 23] made a three-fluid model for a binary particle system with an interstitial gas to simulate particle segregation due to size and weight differences between the two particle types. The first part of the work considered a cold flow study of a binary particle mixture. Later, Chao [21] and Chao et al. [24] made a three-fluid model for a reactive binary particle system with a multi-component interstitial gas. The process investigated was sorption-enhanced steam methane reforming (SE-SMR) which is steam methane reforming (SMR) and a gas-solid adsorption reaction CO2 capture process. 

The engineer is offered a large variety of flow-modeling methods, whose complexity ranges from simple order-of-magnitude analysis to direct numerical simulation. Up to now, the methods of choice have ordinarily been experimental and semi-theoretical, such as cold flow simulations and tracer studies. 

Consider the scaleup of a small, tubular reactor in which diffusion of both mass and heat is important. As a practical matter, the same fluid, the same inlet temperature, and the same mean residence time will be used in the small and large reactors. Substitute fluids and cold-flow models are sometimes used to study the fluid mechanics of a reactor, but not the kinetics of the reaction. 

Figure 11.39 Photographs of cold-flow experiments studying the flow regimes and catalyst suspension in laboratory bubble columns. Left low gas flow middle high gas flow right high gas flow with catalyst suspension.

The cold flow visualization shows the effects of active air forcing and liquid-fuel pulsations. Higher air flow rates, three times higher than those used in the combustion studies, were used for flow visualization. At the higher flow rates, the structures would be imaged more distinctly. The images captured show the... 

Although general treatments of the flow in fluidized beds of coarse particles, in view of the difficulty of the problem, will only evolve slowly, there have been some promising developments. These include the success achieved in correlating heat transfer data (7, 113., 114) and the development of scaling parameters that permit the use of cold flow models to study many of the characteristics of AFBC s (114%115). 

Nearly linear correlations for CFPP versus CP and LTFT versus CP were reported for neat biodiesel and its blends with petrodiesel (Dunn and Bagby, 1995,1996 Dunn et al., 1996). For LTFT, the correlation was essentially LIFT CP, suggesting the labor- and time-intensive LTFT test could be spared by simply measuring CP. A major conclusion from these studies was that development of approaches to improve cold flow properties of biodiesel should focus on technologies that decrease CP. 

As part of the work undertaken by APCI under contract to the DOE, to develop a slurry phase Fischer-Tropsch process to produce selectively transportation fuels, a study of the hydrodynamics of three phase bubble column reactors was begun using cold flow modelling techniques (l ). Part of this study includes the measurement of solid concentration profiles over a range of independent column operating values. 

The burner of Case 1 uses a swirled injector (Fig. 9.1) where swirl is produced by tangential injection downstream of a plenum. A central hub contributes to flame stabilization. In the experiment methane is injected through holes located in the swirler but mixing is fast so that perfect premixing is assumed for computations. Experiments include LDV (Laser Doppler Velocimetry) measurements for the cold flow as well as a study of various combustion regimes. The dimensions of the combustion chamber are 86 mm X 86 mm x 110 mm. 

It is basically a hydrodynamic model, including particle scale effects, which can, therefore, be used to study scale-up and optimization of fluidized bed gasifiers. The hydrodynamic component of the model has been validated through comparison with cold flow visualization data and limited hot flow measurements. 

Especially for multiphase systems flow visualization (Wen-Jei Yang, 1989 Merzkirch, 1987) can provide valuable initial information on the prevailing flow patterns and should at least always be considered as a first step. Of course, in applications that involve extreme conditions such as high temperature and/or pressure it is very difficult if not impossible to apply flow visualization and other techniques should be considered. Here the use of cold flow models which permit visual observation might be considered as an alternative as an important first step to obtain (qualitative) information on the flow regime and associated flow pattern. Of course, multiphase flows exist such as dense gas-solid flows that do not permit visual observation and in such cases the application of idealized flow geometries should be considered. A well-known example in this respect is the application of so-called 2D gas fluidized beds to study gas bubble behavior (Rowe, 1971). 

To study the gas mixing capacity of circulating fluidised bed (CFB) biomass gasifiers, radial and axial gas concentration profiles have been measured and interpreted in both a hot pilot scale biomass gasifier (100 kg/hr fuel) and a cold-flow set-up. The presented pilot plant data are unique in their sort and provide new insight in radial gas mixing in a CFB riser. 

Entrainment and important fluidised bed parameters were measured for a I l /h and a 5 kg/h fast pyrolysis reactor. In addition, a cold-flow model of the Ikg/h rig was built to study fluidisation aspects that were difficult to obtain from the pyrolysis reactor. The cold-flow model was subsequently modified to validate the model s capability to deal with changes in the reactor geometry. The cold flow rig is illustrated in Figure 1. 

If incompletely devolatilized char enters the reduction zone, tar may evolve there and will remain largely unconverted in the gas. The second reason for undesired tar in the product gas may be incomplete conversion of tar in the oxydation zone. To study this problem, three types of experiments have been carried out a) methane tracer injection in the gasifier b) cold flow model tests c) tar production measurement at different geometries. 

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