Reactive stripping

Reactions can be combined with other unit operations, as in the example of reactive stripping in the production of hypochlorous acid (HOC1). An RPB was... 

Although considerable studies on the use of centrifugal fields in chemical processing have been reported for lab- and pilot-scale operations, little information is public on scale-up criteria for either performance parameters or equipment design. Three examples of commercial use of centrifugal fields are available for review. These include liquid-liquid extraction, water deaeration, and reactive stripping for hypochlorous acid production. 

Trent D, Tirtowidjojo D, Quarderer G. Reactive stripping in a rotating packed bed for the production of hypochlorous acid. In Green A, ed. 3rd International Conference on Process Intensification for the Chemical Industry. London BHR Group, 1999 217-231. 

Reactive Stripping in Structured Catalytic Reactors Hydrodynamics and Reaction Performance... 

In a number of petrochemical processes, a gas (hydrogen) is present as reactant. In hydrodesulfurization (HDS), hydrocracking (HC), and hydrodenitrogenation (HDN), the reaction products H2S and ammonia, respectively, are known to decrease the catalyst activity, but are partly transferred to the gas phase. Therefore, also these processes profit from reactive stripping. 

Fig. 8.4. Schematic flowsheet for a countercurrent reactive stripping process with liquid recycle.

In principle, any catalyst bed used for reactive distillation or trickle bed operation can also be applied in reactive stripping. The performance will depend mainly on the optimal ratio between catalyst hold-up, the gas-liquid and the liquid-solid interface. However, recycling of the strip gas flow makes a low pressure drop (and therefore a high voidage) especially beneficial. In countercurrent operation, flooding - a well-known problem - must be avoided. The present studies have focused on structured catalyst supports, developed for either reactive distillation or reactive stripping, with a particular emphasis being placed on the use of so-called film-flow monoliths. 

Fig. 8.5. Schematic flowsheet for a co-current/cross-flow reactive stripping process.

The feasibility of using catalyst-coated film-flow monoliths for reactive stripping applications was demonstrated successfully by a set of reactive experiments in a pilot-scale reactor [28]. 

For reactive stripping experiments, ca. 13 L of liquid was used containing cumene as solvent, tetradecane as internal standard, and ca. 12 mol% each of hexanoic acid and octanol-1. For the experiments with elevated acid concentrations, these values were 20 mol% and 11 mol%, respectively. All experiments were carried out at 160 °C and 5 bar absolute pressure. 

Fig. 8.25. Schematic flowsheet for pilot-scale reactive stripping experiments in countercurrent mode.

The conversion of hexanoic acid as a function of time for the three different experiments is plotted in Fig. 8.27. For orientation, the maximally obtainable conversion if no water is removed from the mixture is also plotted (79 % under the chosen conditions). The lowest curve shows a blank experiment where the liquid flows over uncoated monoliths and reacts homogeneously. The second curve represents an experiment in the autoclave where the water removal is suppressed. The comparison of these two curves indicates that, without decreasing the water contents, the catalytic activity of the BEA cannot be used efficiently. The third (highest) curve proves the effect of reactive stripping. The reaction runs much faster and easily continues beyond the equilibrium. 

The esterification of 1-octanol with hexanoic acid is a good system for studying reactive stripping, as it not only represents many industrial processes, but also illustrates the interdependencies of activity, selectivity, and mass transfer in reactive separations. 

Experiments in a pilot-scale plant demonstrated the feasibility of applying coated monoliths and DX packings or katapak-S successfully as catalyst carriers in reactive stripping. It was proven that the water removal increases the catalyst activity and permits a shift" in the equilibrium. In accordance with the kinetics of this... 

Reactive stripping has its own importance, in addition to reactive distillation. Several situations exist in which reactive stripping becomes more interesting for example, in the production of high-boiling esters and ethers, especially, when reactants or products are temperature-sensitive, and also in gas-liquid-solid processes, where product inhibition may play a role. 

The potential of structured packings as catalyst carriers for reactive stripping, film-flow-monoliths, Sulzer DX -packings, both coated with zeolite BEA, and katapak-S , filled with BEA-particles, was explored in cold-flow experiments and under reactive stripping conditions in a pilot-scale plant. 

All reactive stripping experiments showed that reducing the water content level (due to better stripping performance) increases the per-pass conversions, but has a negative effect on selectivity in the chosen model reaction system. Nonetheless, the water contents are the result of a balance between stripping efficiency and catalyst hold-up. As a consequence, the space-time yield was highest for katapak-S , whereas in DX -packings, the excellent separation efficiency optimized the use of catalyst, but decreased the selectivity. For industrial applications, the choice will always depend on the balance between mass transfer performance, the kinetics, the activity of the catalyst, and the process economics. 

In principle, a higher catalyst hold-up in monoliths is possible, but then the mass transfer performance should also be adapted. For this purpose, the remixing of laminar layers in monoliths by intelligent stacking shows promising preliminary results, although this has still to be demonstrated in reactive stripping experiments. 

Among the most important examples of RS processes are reactive distillation, reactive absorption, reactive stripping and reactive extraction. For instance, in reactive distillation, reaction and distillation take place within the same zone of a distillation column. Reactants are converted to products with simultaneous separation of the products and recycle of unused reactants. The reactive distillation process can be both efficient in size and cost of capital equipment and in energy used to achieve a complete conversion of reactants. Since reactor costs are often less than 10% of the capital investment, the combination of a relatively cheap reactor with a distillation column offers great potential for overall savings. Among suitable reactive distillation processes are etherifications, nitrations, esterifications, transesterifications, condensations and alcylations (Doherty and Buzad, 1992). 

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