Reverse Flow Problems

Flow of material into or out of a column in the opposite direction to that intended often takes place during startups, shutdowns, and normal operation. In many cases, reverse flow through a line is of no consequence or is even desirable, but there are also several instances where it is troublesome or hazardous. The consequences of reverse flow through each line connecting to the column should be analyzed under normal operation as well as startup and shutdown conditions. Special attention should be paid to outages of rotating 

Rgur 12.9 Effect of changing column pressure on the startup stability region (From Henry Z. Kister, excerpted by special permission from Chemical Engineering, April 6,1981 copyright by McGraw-Hill, Inc., New York, N Y10020.) 

The author is familiar with a case where caustic from a wash tower backflowed into an entirely different section of the plant. The other section was normally at a higher pressure, but not at startup. The caustic attacked an aluminum heat exchanger in that section, resulting in a fire. Piping was later changed to prevent recurrence. 

In a third case (125), reverse flow in an absorber-desorber system (sim- 

In a fourth case (18), ammonia recovery storage tanks were damaged by overpressure due to gas entering their liquid rundown line (Fig. 12.11). The gas backflowed through the upper leg of an absorber pumparound circuit following pump failure at startup. The rundown line branched off the pumparound circuit at an elevated position the accident could possibly have been prevented had this line branched near grade (Fig. 12.11). The absorber piping and control system were modified to prevent recurrence (18). 

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It is important to avoid jumping to obvious solutions —all possible recommendations should be considered. For example, the obvious recommendation to do with a reverse flow problem could be to install a check valve. However, it may be possible to change the process to prevent the possibility of reverse flow from taking place. It is also important to side step some of the more common objections to proposed solutions. These include the following ... 

The value of root cause analysis is that conclusions are reached that may address a much broader range of issues than those immediately to do with the event being investigated. For example, the initial evaluation of the reverse flow problem in the standard example finds that the check valve failed. Further examination identifies a root cause that all check valves purchased from the manufacturer of that particular valve are unreliable and need to be replaced. This conclusion may, in turn, suggest weaknesses to do with the company s overall procurement system. 

Similarly, in the standard example the check valve failed to close when reverse flow occurred. If this incident turns out to be a single instance of check valve failure, then a radical overhaul of the maintenance and purchasing procedures to do with check valves is hardly justified. The root cause for the reverse flow problem lies elsewhere. 

Air Recirculation Recirculation of air which has been heated as it crosses the tube bundle provides the best means of preventing operating problems due to low-temperature inlet air. Internal recirculation is the movement of air within a bay so that the heated air which has crossed the bundle is directed by a fan with reverse flow across another part of the bundle. Wind skirts and louvers are generally provided to minimize the entiy of low-temperature air from the surroundings. Contained internal recirculation uses louvers within the bay to control the flow of warm air in the bay as illustrated in Fig. 11-47. Note that low-temperature inlet air has access to the tube bundle. 

In the actual case of a boundary-layer flow problem, the fluid is not brought to rest reversibly because the viscous action is basically an irreversible process in a thermodynamic sense. In addition, not all the free-stream kinetic energy is converted to thermal energy—part is lost as heat, and part is dissipated in the form of viscous work. To take into account the irreversibilities in the boundary-layer flow system, a recovery factor is defined by... 

As outlined above, supramolecular binding offers new possibilities in this regard. Solids functionalized with a single acceptor motif can be used in more than one application, and the effective cost of the synthesis of the support is reduced. After (partial) catalyst decomposition, the catalyst can be removed easily, and the support can be reused and the catalyst regenerated. Leaching of immobilized catalysts remains the key problem, even without decomposition the leached catalyst can be handled by applying reverse-flow techniques in an "oversized bed. However, no applications of this approach have been reported, but it can be improved. 

In summary, periodic reverse-flow SCR seems advantageous for NO abatement in the specific cases of low-temperature streams with relatively high NO concentrations. Moreover, the problems associated with the treatment of SO2- and dust-containing gases have not been addressed so far. Accordingly, the present features of this technology do not fit power plants applications. 

For most multiphase reactive flow problems, it is not possible to analyze all the operators in the complete solution method simultaneously. Instead the different operators of the method are analyzed separately one by one. The working hypothesis is that if the operators do not possess the desired properties solely, neither will the complete method. Unfortunately, the reverse is not necessarily true. In practical calculation we can only use a finite grid resolution, and the numerical results will only be physically realistic when the discretization schemes have certain fundamental properties. The usual numerical terminology employed in the CFD literature is outlined in this section [141, 202, 49]. 

To see that this is true, we simply note that we can obtain the detailed flow problem for configuration (a) directly from (7-11) and (7 12) by simply reversing the sign of the undisturbed flow at infinity. In particular, if the solution for configuration (b) is denoted as (u, p), the solution for configuration (a) is just... 

Our simulation showed that the reverse-flow operation makes it possible to solve this problem Once ignited, the bed of monolith supported oxidation catalyst is able to maintain high temperatures over all spectrum of engine operation modes, from idle to full load, with adequate methane destmction. Other hydrocarbons and CO are also completely removed. 

Following Figure 5.4 it was stated that the attempt to model micromixing in a sequence of CSTRs with reverse flows was not particularly fruitful (at least for the configuration shown) because of the complexity of the resulting equation. However, a variant on the problem would include only one feedback loop (recycle), as shown below. 

This paper examines theoretically the continuous flow extraction by emulsion globules in which the transferring solute reacts with an internal reagent. The reversible reaction model is used to predict performance. These results are compared with advancing front calculations which assume an Irreversible reaction. A simple criterion which indicates the Importance of reaction reversibility on performance is described. Calculations show that assuming an irreversible reaction can lead to serious underdesign when low solute concentrations are required. For low solute concentrations an exact analytical solution to the reversible reaction problem is possible. For moderate solute concentrations, we have developed an easy parameter adjustment of the advancing front model which reasonably approximates expected extraction rates. 

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