Cold models

The draft-tube airlift bioreactor was studied using water-in-kerosene microemulsions [263], The effect of draft tube area vs. the top-section area on various parameters was studied. The effect of gas flow rates on recirculation and gas carry over due to incomplete gas disengagement were studied [264], Additionally, the effect of riser to downcomer volume was also studied. The effect of W/O ratio and viscosity was tested on gas hold-up and mass transfer coefficient [265], One limitation of these studies was the use of plain water as the aqueous phase in the cold model. The absence of biocatalyst or any fermentation broth from the experiments makes these results of little value. The effect of the parameters studied will greatly depend on the change in viscosity, hold-up, phase distribution caused due to the presence of biocatalyst, such as IGTS8, due to production of biosurfactants, etc., by the biocatalyst. Thus, further work including biocatalyst is necessary to truly assess the utility of the draft-tube airlift bioreactor for biodesulfurization. 

Another approach to scale-up is the use of simplified models with key parameters or lumped coefficients found by experiments in large beds. For example, May (1959) used a large scale cold reactor model during the scale-up of the fluid hydroforming process. When using the large cold models, one must be sure that the cold model properly simulates the hydrodynamics of the real process which operates at elevated pressure and temperature. 

Fitzgerald et al. (1984) measured pressure fluctuations in an atmospheric fluidized bed combustor and a quarter-scale cold model. The full set of scaling parameters was matched between the beds. The autocorrelation function of the pressure fluctuations was similar for the two beds but not within the 95% confidence levels they had anticipated. The amplitude of the autocorrelation function for the hot combustor was significantly lower than that for the cold model. Also, the experimentally determined time-scaling factor differed from the theoretical value by 24%. They suggested that the differences could be due to electrostatic effects. Particle sphericity and size distribution were not discussed failure to match these could also have influenced the hydrodynamic similarity of the two beds. Bed pressure fluctuations were measured using a single pressure point which, as discussed previously, may not accurately represent the local hydrodynamics within the bed. Similar results were... 

Differential pressure measurements were made between several vertical elevations within the bed. The probability density function of the cold model and combustor gave very close agreement (Fig. 35). The solid fraction profiles were obtained from the vertical pressure profile with a hydrostatic assumption. The cold model solid fraction profile showed very close agreement with data taken from pressure taps in two different locations within the combustor (Fig. 36). The solid fraction shows a... 

Figure 35. Comparison of the Tidd PFBC and cold model based on simplified...

Figure 36. Solid fraction profile comparisons for Tidd PFBC and cold model based on simplified scaling laws. (From Glicksman and Farrell, 1995.)...

Almstedt, A. E., and Zakkay, V., An Investigation of Fluidized-bed Scaling-capacitance Probe Measurements in a Pressurized Fluidized-bed Combustor and a Cold Model Bed, Chem. Eng. Sci., 45(4) 1071 (1990)... 

Glicksman, L. R., Yule, T., Dymess, A., and Carson, R., Scaling the Hydrodynamics of Fluidized Bed Combustors with Cold Models ... 

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. 

Yang et al. (1995) described the application of this scale-up approach. Comprehensive testing programs were performed on two relatively large-scale simulation units for a period of several years a 30-cm diameter (semicircular) Plexiglas cold model and a 3-m diameter (semicircular) Plexiglas cold model, both operated at atmospheric pressure. 

Yang, W. C., and Keaims, D. L., Recirculating Fluidized Bed Reactor Data Utilizing a Two-dimensional Cold Model, AIChE Symp. Series, 70(141) 27 (1974)... 

In addition, changes in the flow rate of the substrate stream in turn cause complex alterations in the flow pattern within these reactors, which may lead to consequent unexpected effects upon the conversion rate. The most useful tool to solve such problems in fluidized beds is the employment of cold-flow models. Thus, it is not surprising that most work in fluidized beds has been focused on the cold model behavior and thus on their hydraulic behavior. 

Whatever the reason, the Synthesis Team chose the most extreme temperature model when it chose the CGCM1. For balance, then, it could have used an analogously cold model, such as the new version from the U.S. National Center for Atmospheric Research8 for all applications that didn t require daily data, such as mean seasonal or annual temperature or precipitation changes. 

Example 5.4 Design a geometrically similar laboratory-scale cold model fluidized bed to simulate the hydrodynamics of a large-scale fluidized bed combustor. Also specify the operating conditions for the cold model. The combustor is a square cross section column with a width of 1.0 m and a height of 6 m. The fluidized bed combustor is operated at a temperature of 1,150 K, a superficial gas velocity of 1.01 m/s, and a bed height of 1.06 m. Particles with a density of2,630 kg/m3 and a diameter of677ptm are used for the combustor. The cold model is operated at a temperature of 300 K. Air is used for both the cold model and hot model fluidized beds. The physical properties of air are... 

In terms of the preceding scale factors, the operating conditions for both the hot and cold models can be obtained as given in Table E5.2. 

Table E5.2. The Operating Conditions for the Hot Model and Cold Model Fluidized Beds...

In addition, early in the program, process design engineering for commercial-scale plants was initiated. Cold models were also used effectively to develop the pilot-plant design and then to prove out elements of the commercial-scale design. 

A problem arises, e.g., when model (laboratory, bench-scale) measurements are to be performed in a so-called cold model , but the industrial plant operates at high temperatures (petrochemicals T = 800 - 1000 °C). How can we ascertain that the laboratory model system behaves hydrodynamically similarly to that in the industrial plant Here, different temperature dependence of physical properties (viscosity, density) may cause problems. 

At this point the remarks given in the introduction to this chapter should be recalled Water is of no use to investigate the performance of a petrochemical plant in a cold model . 

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