Exchange of material between a gas and a liquid plays a very important part in many chemical processes. This transfer forms the basis of the unit operations of distillation, absorption and humidification. In these cases there is a physical transfer of material across the phase boundary controlled by the physical properties of the system. In some instances, physical transfer may he followed by a second step, chemical reaction, the unit operation then becomes gas-liquid reaction. The rate and extent of the reaction and the choice of equipment is influenced not only by the physical transfer of material but also by the chemical kinetics.
Garg produces a range of equipment specially designed for gas-liquid reactions. The range includes continuous and batch stirred tank reactors, bubble columns with and without internal fittings, wash, spray and jet columns. The Garg units, together with other items such as heat exchangers, pumps and valves, and our process engineering capability in the fields of distillation, absorption, extraction, etc. enable us to design complete systems for laboratory, pilot plant or production use.
Gas-liquid reactions may be either spontaneous or initiated thermally, photochemically or catalytically. They include processes such as oxidation, halogenation, hydration, nitration and sulphonation, and are widely used throughout the chemical industry from petrochemicals to pharmaceuticals. The complexity of the processes that occur in gas-liquid reactors, involving not only mass-transfer but also chemical reaction, means that on many occasions pilot plant studies must be carried out before the production plant design can be finalised. The use of glass is an asset at both the pilot plant and production scale.- The reacting materials are frequently aggressive. The wide corrosion resistance of glass makes it an ideal material of construction. Transparency is a major advantage. When photochemical reactions are involved, the light source can be mounted externally and the effect of varying light intensity can be more readily examined. In addition to this, the various reaction parameters such as size of gas bubbles, gas distribution, changes in colour etc. can be easily observed. Glass is a modular material of construction and the plant can be easily modified during pilot plant experiments so that an optimal unit arrangement is achieved.
Selection of Reactor Type
With almost all gas-liquid reactions, parallel or series side reactions take place which lead to unwanted by-products. This can frequently have a decisive effect on the process econornies and the correct choice of reactor type and informed process design is essential.
In order to achieve the aim of an optimal production unit the process design engineer needs to know
- The process chemistry involved including possible side reactions. - The mechanisms, kinetics and thermodynamics of the reaction occurring.- The mass-transfer characteristics of the system.- The characteristics of the reactor and particularly its hydrodynamics in terms of gas and liquid flows, retention times and distribution.
In many cases this information is not fully available and laboratory and pilot plant tests are required using small scale continuous or batch systems. These tests can be carried out by our engineers, using customer materials, in experimental units we have available at our headquarters. Alternatively units are available to borrow in order for customers to carry out their own experimental work.
The following information gives an initial guide to the preferred reactor type.
- The stirred tank reactor has a long retention time for the liquid phase.- The bubble column has a long retention time for the liquid phase.- The wash and spray columns have short retention times for the liquid phase. - The jet column has very short retention times for both the liquid and gas phases.
In addition to retention time the initial choice must also be influenced by the following considerations.- The gas-liquid exchange surface available which is determined by bubble size, gas distribution and hold-up.
The gas-liquid ratio, which can be adjusted to a desired value by recycle if this does not adversely affect the chemical kinetics.
The supply or removal of heat, whether an isothermal or adiabatic reaction cycle is desirable and the influence of the reactor temperature distribution on the desired product spectrum.
Stirred Tank Reactor
The figure shows a stirred tank reactor which is capable of being operated in several modes:
- batch liquid/continuous gas- batch liquid/recycle gas- recycle liquid/continuous gas- recycle liquid/recycle gas- continuous liquid/continuous gas- continuous liquid/recycle gas.
The gas may be dispersed in two ways, either using a stirrer or using a Garg gas-mixing valve which is described in detail Heat may be supplied or removed from the system through the jacket of the reactor or in a heat exchanger in the recycle line. To enable photochemical reactions to be carried out submersible lamps may be put into the reactor. The stirred tank reactor is suitable fora wide range of applications. It is particularly useful for carrying out gas-liquid reactions which require a solid catalyst. The laboratory scale unit can be used for the determination of kinetic data required for the exact design of production units.
The bubble column is predominantly used for continuous operations, in contrast to the stirred tank reactor it has no moving parts. The unit consists of an unpacked column section (except that a short packed section is provided at the top to remove scum). Gas is introduced at the base of the column through a gas manifold, or alternatively using a Garg gas-mixing valve, the gas may be recycled if required. The liquid can also be recycled via an external heat exchanger. Photochemical reactions can be carried out by fitting lights around the column. Columns of this form have similar retention times to that of a continuous stirred tank reactor. However, the continuous stirred tank is a thoroughly mixed system whereas the column is a dispersed plug flow system. It is possible to improve the retention time of the liquid phase by including short packed sections in the column, this however leads to an increase in back mixing and a reduced gas loading.
Multi-stage bubble columns allow greater variation of the retention time of the liquid phase. The simple construction of the columns enables a cascade mode of operation to be used, with a well defined flow path and no remixing between stages. Multi-stage bubble columns can be supplied with constant or variable bubble formation layer heights. The column with constant bubble formation height is based on a filter plate column with fixed overflow heights and external liquid transfer lines. In the case of the column with variable bubble layer height, the liquid outlets are completely flooded and provided with restrictions which make it possible to adjust the heights of the bubble formation layers independently of the gas and liquid loadings . This enables a mean liquid retention time to be maintained under load changes. If required heat exchange coils can be fitted in the column and the temperature profile controlled. Multi-stage bubble columns are an advantage when an undesirable side reaction must be controlled and they are particularly suitable for carrying out physical absorption processes which require stringent control of the purity of discharge gases.
The figure shows this very simple unit. Wash columns are commonly used for purely physical absorption processes, but they are also suitable for gas-liquid reactions if a short retention time is required for the liquid phase.Wash columns are normally operated counter currently. Co-current operation is possible if the first step in the reaction is rapid and irreversible. Co current operation also means that the column can be operated with higher loadings. This unit, shown in Figure 1 1 is also used for physical absorption it is suitable for chemical reactions which require a very short retention time for the liquid phase.
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