Liquid drop, excess pressure

Flooding may also be brought on by increasing the liquid rate while holding the gas rate constant. Excessive liquid flow can overtax the capacity of downcomers or other passages, with the ultimate result of increased liquid inventoiy, increased pressure drop, and the other characteristics of a flooded column. 

The pressure spike introduces a disruption in the flow. Depending on the local conditions, the excess pressure inside the bubble may overcome the inertia of the incoming liquid and the pressure in the inlet manifold, and cause a reverse flow of varying intensity depending on the local conditions. There are two ways to reduce the flow instabilities reduce the local liquid superheat at the ONB and introduce a pressure drop element at the entrance of each channel, Kandlikar (2006). Kakac and Bon (2008) reported that density-wave oscillations were observed also in conventional size channels. Introduction of additional pressure drop at the inlet (small diameter orifices were employed for this purpose) stabilized the system. 

Carbonated beverages illustrate what happens when a dissolved gas undergoes a rapid drop in pressure. Soft drinks, soda water, and champagne are bottled under several atmospheres pressure of carbon dioxide. When a bottle is opened, the total pressure quickly falls to 1 atm. At this lower pressure, the concentration of CO2 in the solution is much higher than its solubility, so the excess CO2 forms gas bubbles and escapes from the liquid. As the photo shows, this process can be dramatic. 

The most difficult problem to solve in the design of a Fischer-Tropsch reactor is its very high exothermicity combined with a high sensitivity of product selectivity to temperature. On an industrial scale, multitubular and bubble column reactors have been widely accepted for this highly exothermic reaction.6 In case of a fixed bed reactor, it is desirable that the catalyst particles are in the millimeter size range to avoid excessive pressure drops. During Fischer-Tropsch synthesis the catalyst pores are filled with liquid FT products (mainly waxes) that may result in a fundamental decrease of the reaction rate caused by pore diffusion processes. Post et al. showed that for catalyst particle diameters in excess of only about 1 mm, the catalyst activity is seriously limited by intraparticle diffusion in both iron and cobalt catalysts.1... 

The main requirement of a tray is that it should provide intimate mixing between the liquid and vapour streams, that it should be suitable for handling the desired rates of vapour and liquid without excessive entrainment or flooding, that it should be stable in operation, and that it should be reasonably easy to erect and maintain. In many cases, particularly with vacuum distillation, it is essential that the drop in pressure over the tray should be a minimum. 

With this model, it is predicted that there should be a liquid drop size which, if too large, will result in long delay times and excessive numbers of new embryos to vapor blanket the surface. Thus, small drops (or thin layers) are more prone to escape termination by vapor blanketing. Also, if experimental variables are modified so as to reduce the growth rate of embryos from A to B, e.g., by increasing pressure, again one would expect a lower probability of escalation. These two predictive conclusions appear to be substantiated by experiment. 

The packed-bed gas-liquid reactors use non-porous, large-size packings, so that they can be operated at high gas and liquid flow-rates without excessive pressure drop. The shape of the... 

Equations (12.48) and (12.49) apply to a filled drop. The excess pressure within a bubble is twice this value because both the inner and outer surfaces of the thin liquid film make a contribution given by equation (12.48)."... 

Determine the type of flow that exists. Flow is laminar (also termed viscous) if the Reynolds number Re for the liquid in the pipe is less than about 2000. Turbulent flow exists if the Reynolds number is greater than about 4000. Between these values is a zone in which either condition may exist, depending on the roughness of the pipe wall, entrance conditions, and other factors. Avoid sizing a pipe for flow in this critical zone because excessive pressure drops result without a corresponding increase in the pipe discharge. 

The countercurrent-flow fixed-bed operation is often used for gas-liquid reactions rather than gas-liquid-solid reactions. Examples of reactions using this type of reactor are given by Danckwerts.29 A comparison between a gas liquid-solid (catalytic) fixed-bed reactor and a gas-liquid-solid (inert) fixed-bed reactor is shown in Table 1-7. The major difference between packed-bed gas-liquid reactors and gas-liquid-solid catalytic reactors is in the nature and size of the packing used and the conditions of gas and liquid flow rates. The packed-bed gas-liquid reactors use nonporous, large-size packing, so that they can be operated at high gas and liquid flow rates without excessive pressure drop. The shape of... 

Valves should be installed vertically as shown in Fig. 3. Care must be taken that the internal diameter of the pipe fixing is equal to or greater than the valve bore and the connector is short enough to limit pressure loss between the reservoir and bore to 3% of set pressure. If the pipe-fixing diameter is too small then the valve may not discharge at full capacity. Moreover, instability may be induced because of excessive pressure drop in the pipe connector. Rapid and repetitive rise and fall of a valve disk is referred to as valve chatter. It can also be induced by misapplication of a valve intended for gas service on liquid service. 

For tray colnmns, liquid flow constrictions are normally found in the tray downcomers. Example causes are lack of adequate froth collapse time, excessive pressure drop for flow under the downcomer baffle, and foreign materials being left in the downcomer by construction people. For packed columns, clogged support plates, broken packings, or growth of fouling deposits can cause liquid flow restrictions. 

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