Cold-flow models

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. 

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

Literature references and the measurement of the contact time distribution in a large, cold-flow model of a gas-fluidized bed are reported in... 

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, W. C., Revay, D., Anderson, R. G. Chelen, E. J., Keaims, D. L., and Cicero, D. C., Fluidization Phenomena in a Large-Scale Cold-Flow Model, Fluidization, (D. Kunii, and R. Toei, eds.), Engineering Foundation, New York, p.77 (1984b)... 

The particulate removal efficiency of a TSS is difficult to calculate with a single theoretical relationship. The technology licensors have utilized pilot plants and cold flow modeling to improve their removal efficiencies to meet stricter environmental regulations. While there is no theoretical relationship that exactly matches removal efficiencies, the following efficiency relationship from Rosin, Rammler, and Intehnann [2] is often used to understand cyclone fundamentals ... 

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. 

By converting the governing hydrodynamic equations for a particular system into nondi-mensional ones, Horio et al. (1986) and Glicksman (1988) derived the so-called scaling laws for fluidized beds. These laws should be seen as a guide to design small-scale, cold-flow models, which simulate the hydrodynamic behavior of the commercial units (Knowlton et al., 2005). 

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). 

In the development of these processes and their transference into an industrial-scale, dimensional analysis and scale-up based on it play only a subordinate role. This is reasonable, because one is often forced to perform experiments in a demonstration plant which copes in its scope with a small produdion plant ( mock-up plant, ca. 1/10-th of the industrial scale). Experiments in such plants are costly and often time-consuming, but they are often indispensable for the lay-out of a technical plant. This is because the experiments performed in them deliver a valuable information about the scale-dependent hydrodynamic behavior (arculation of liquids and of dispersed solids, residence time distributions). As model substances hydrocarbons as the liquid phase and nitrogen or air as the gas phase are used. The operation conditions are ambient temperature and atmospheric pressure ( cold-flow model ). As a rule, the experiments are evaluated according to dimensional analysis. 

Just as for this model all similar approaches found in the literature suffer from the problem that not all fluid mechanical variables can be precalculated on the basis of the operating conditions. Instead, reasonable estimations or measurements in cold flow models are used... 

Avidan, A. A., Gould, R. M., and Kam, A. Y., Operation of a Circulating Fluid-Bed Cold Flow Model of the 100 B/D MTG Demonstration Plant, Proc. First Intern. Conf. Circulating Fluidized Bed, Halifax, Canada, pp. 287-296 (1985). 

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. 

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). 

This research is being performed in two phases evaluation of pressure drop and flow characteristics of cold flow models of the filter and collection efficiency tests with a pilot-scale filter coupled to a 4.S tonne per day biomass gasifier. In this paper we describe three fluid dynamic design features developed in the cold flow model that improve performance of the Alter. 

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. 

The Mizushima Oil Refinery of Japan Energy Corporation first implemented an operation of vacuum residue hydrodesulfiirization in the conventional fixed bed reactor system in 1980. We have also conducted a high conversion operation to produce more middle distillates as well as lower the viscosity of the product fuel oil to save valuable gas oil which is used to adjust the viscosity. Vacuum residue hydrodesulfurization in fixed bed reactors mvolves the characteristic problems such as hot spot occurrence and pressure-drop build-up. There has been very little literature available discussing these problems based on commercial results. JafiFe analyzed hot spot phenomena in a gas phase fixed bed reactor mathematically, assuming an existence of the local flow disturbance region [1]. However, no cause of flow disturbance was discussed. To seek for appropriate solutions, we postulated causes ofhot spot occurrence and pressure-drop build-up by conducting process data analysis, chemical analysis of the used catalysts, and cold flow model tests. This paper describes our solutions to these problems, which have been demonstrated in the commercial operations. 

We conducted cold flow model experiments in a air-water/glycerin system to investigate a cause of maldistribution in a catalyst bed. The apparatus used was a 30 cm I.D. acrylic column equipped with a liquid distributor at the top and a liquid collector with 33 compartments at the bottom. Bed depth can be varied by combining the pipes. Liquid distribution at a given depth of the bed was estimated by measuring the liquid flow from each compartment of the collector. We examined effects of gas and liquid velocity, liquid viscosity, particle shapes, and ways of catalyst loading on liquid distribution in the bed. An increase in liquid velocity or viscosity slightly improved liquid distribution. However, gas flow rate did not affect liquid distribution. 

A good liquid distributor is necessary to prevent maldistribution. The existing liquid distributor consists of a tray and a number of short chimneys. Liquid is collected on the tray and flows onto the bed from the chimneys through the small holes on the side. A cold flow model test showed that liquid flow rate from the short chimneys was extremely sensitive to the level of the tray. It was also pointed out that liquid dispersion from the short chimneys was poor. Therefore, we developed a new liquid distributor, which improved the defects of the existing one. The new liquid distributor with tall chimneys can achieve uniform liquid distribution, even if the tray is declined. Each chimney also has a feature to well disperse liquid onto the bed. 

A cold flow model test also showed that an effect of a liquid distributor was limited in a certain depth of the bed, which varied with a catalyst size. However, we have expected that a good liquid distributor lower a chance of the maldistribution which is caused by non-uniform deposition of solids on the top of the bed. 

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. 

Turning vanes may be required to ensure distribution of the airflow across the catalyst. Computer modeling (CFD) or cold flow modeling may be required to ensure the proper air distribution and the proper design of the straightening vanes. 

A fluidized-bed MTG concept was concurrently developed by Mobil. The process research went through several stages involving bench-scale fixed fluidized-bed 4-bbl/day and 100-bbl/day cold-flow models, and a 100-bbI/day semiwork plant. 

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