Bubble column reactors, long

Low feed rates are suitable for trickle bed reactors where for gas-liquid-solid mixing, the gas and the liquid are fed into the top of the reactor. This gives long gas residence times but short liquid residence times. Such a configuration is often used in hydrogenation reactions. When the gas-liquid is fed into the bottom of the reactor, it is known as a bubble reactor. Here the gas residence times are short but the liquid residence times are relatively long. This is commonly used in oxidation reactions. Heat transfer can be a major problem with both trickle and bubble reactors and in such cases a slurry bubble column reactor can be employed. 

The simplest form of a bubble column is a vertical tube in which a gas distributor is placed at the bottom packed or plate bubble columns are also used. The gas bubbles rise through the liquid phase, which may flow through the column either cocurrent or countercurrent to the gas. As a result of the short residence time of the gas bubbles in the liquid phase, bubble column reactors are preferred for reactions which require a short gas and a long liquid reaction time. Therefore the residence time distribution of the liquid phase is a characteristic factor for the design of the reactor. The dependence of the residence time distribution upon the column diameter has to be known for any scale-up of bubble columns. 

Even in the case of standard reactors such as stirred tanks and bubble columns, lack of knowledge in this area limits our ability to use particle stress as a selection criterion. The reasons for this lack of knowledge are, on the one hand, that the velocity fields in the reactors, which would allow a certain prediction, can only be obtained by sophisticated measurements and measurement techniques, and on the other hand, the stress on particles becomes evident only as an integral result of a long term process. 

In cases when the feed stream is a liquid, which requires rather long residence times, the suspension bubble column or an agitated tank reactor is used (Figures 1.2(b) and 1.2(c)). Here, in the reactor exit, quite elaborate filtering systems are required to remove the catalyst from the liquid stream. In these reactors a gas generally is supplied, because these suspension reactors are mostly used for hydrogenations and oxidations. 

The stirred tank with a jacket for heating or cooling (also called autoclave or digester) is the workhorse of the pharmaceutical, fine-chemicals, mineral and paper industries. It is used as a reactor, mixer, decanter, heater and cooler. The bubble and slurry-bubble columns too are akin to the STR. It is used as a batch reactor or semi- or fed-batch reactor for those reactions requiring a large residence time, as the tubular reactor would be too long and unwieldy. The STR is bulky, and the yield and selectivity could be low. 

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