Mixed-phase vapor/liquid flow

In the process (Figure 9-12), the feedstock (a vacuum residuum) is mixed with recycle vacuum residue from downstream fractionation, hydrogen-rich recycle gas, and fresh hydrogen. This combined stream is fed into the bottom of the reactor whereby the upward flow expands the catalyst bed. The mixed vapor liquid effluent from the reactor, either goes to flash drum for phase separation or the next reactor. A portion of the hydrogen rich gas is recycled to the reactor. The product oil is cooled and stabilized and the vacuum residue portion is recycled to increase conversion. 

The mass balances [Eqs. (Al) and (A2)] assume plug-flow behavior for both the gas/vapor and liquid phases. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the reactor performance, for example, due to back-mixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for CD processes, experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK are performed (168). 

Computational fluid dynamics based flow models were then developed to simulate flow and mixing in the loop reactor. Even here, instead of developing a single CFD model to simulate complex flows in the loop reactor (gas dispersed in liquid phase in the heater section and liquid dispersed in gas phase in the vapor space of the vapor-liquid separator), four separate flow models were developed. In the first, the bottom portion of the reactor, in which liquid is a continuous phase, was modeled using a Eulerian-Eulerian approach. Instead of actually simulating reactions in the CFD model, results obtained from the simplified reactor model were used to specify vapor generation rate along the heater. Initially some preliminary simulations were carried out for the whole reactor. However, it was noticed that the presence of the gas-liquid interface within the solution domain and inversion of the continuous phase. 

Picket fence weirs are used in low-liquid-rate applications (Fig. 8). Picket fence weirs can serve two purposes at low liquid rates. First, they reduce the effective length of the weir for liquid flow increases the liquid height over the weir. This makes tray operation less sensitive to out-of-level installation. Second, pickets can prevent liquid loss (blowing) into the downcomer by spraying. This occurs at low liquid rates when the vapor is the continuous phase on the tray deck. Picket fence weirs should be considered if the liquid load is less than 1 gpm per inch of weir (0.0267 ft /sec/ft, 0.00248 m /sec/m). At liquid rates lower than 0.25 gpm per inch of weir (0.00668 ft / sec/ft, 0.000620 m /sec/m) even picket fence weirs and splash baffles have a mixed record in improving tray efficiency. Operation at liquid rates this low strongly favors the selection of structured packing. 

This article first describes the ideal reactor types, namely batch, plug flow, and completely mixed reactors. Then, the petroleum reactors are discussed based on whether the reaction occurs in the vapor, liquid, or mixed vapor-liquid phase. More specifically, the naphtha-processing reactors are examined first, then gradually moving to heavier hydrocarbons, like kerosene and distillate, that react partially in the liquid and gas phases, and finally ending with a discussion on reactors processing heavy hydrocarbons like petroleum residuum, which reacts completely in the liquid phase. 

In this method, a mixed A -value is defined as the ratio of the mole fraction of a component in the vapor to its mole fraction in the mixed liquid phase (Schuil and Bool, 1985). Applied to an equilibrium stage or a flash drum, the phase equilibrium is solved using the mixed /f-valucs instead of the usual vapor-liquid X-values to determine the flow rates and compositions of the vapor and the total liquid. The liquid phase split is then calculated on the basis of A -values for each liquid phase to determine the flow rates and compositions of the two liquid phases. An energy balance may also be included to determine the temperature or the heat transfer for the unit. 

A column operator can control the column performance by manipulating the reboiler and condenser duties. Consider starting up a column with a mixed-phase feed introduced at some intermediate tray between the condenser and reboiler. With no condenser or reboiler duties, the liquid flows down the column and out as bottoms, and the vapor flows up the column and out as overhead. The column thus acts as a flash drum. 

Batch reactors are not common in experimental catalysis since flow systems are so simple. They are most often found with liquid phase reactors using slurried catalysts. Good mixing is essential and checks should be made to eliminate external diffusion problems. Also it is necessary to vary catalyst loading during the experiment as a means of detecting vapor-liquid interfacial effects. ... 

Care should be taken in feeding mixed vapor-liquid streams to the column, The reboiler return is or this character, and its flow should be directed away from the bottom seel pen so as not to hinder liquid flow from that pan (see Fig. 5.7-17). Various baffling aimngemenis are possible for seperaiing the liquid and vapor for a mixed-phase fsed si mam. 

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