Lab Instruments

Trickle bed reactors consist of a vessel filled with solid catalyst particles over which gas and liquid flow. In classic trickle bed operation, the gas and liquid flow co-currently downward over the particles, however, upward flow of the gas or both liquid and gas is also used.

Trickle bed reactors are used throughout the chemical industry, but especially for hydrogenation and oxidation reactions in the petrochemicals industry.

Trickle beds are attractive reactors as liquid flow patterns tend to produce high conversions, catalyst losses are minimal, they can be operated at high temperature and pressure, and are scaled up with relative ease. The catalyst effectiveness in trickle bed reactors is low owing to the large particle size needed to achieve a reasonable pressure drop, liquid maldistribution can lead to decreased performance, side reactions can lead to fouling, and heat transfer is poor.

Industrial trickle bed reactors tend to be large, on the order of 10-m diameter. However, scale-up of these reactors can be achieved using parallel small diameter columns contained in a single process vessel and hydrodynamic features tend to be dominated by particle-scale phenomena that are largely scale invariant. As a result, the challenge for tomographic measurements of trickle bed reactors is not so much to determine how best to scale up these reactor designs but rather to identify how to operate the reactors most efficiently, regardless of scale. Research has tended to focus on either hydrodynamic studies in cold-model trickle beds or on the interaction of operating conditions with the chemical conversion.

The core part of any fixed-bed reactor is the solid catalyst where the reaction takes place. A large variety of catalyst structures are applied in practice. One class of structures, used in randomly packed beds, consists of catalyst pellets of different shapes. A second class comprises regularly arranged structures like monoliths with flow channels of different shape.

With regard to application and design, it is convenient to differentiate between fixed-bed reactors for adiabatic and nonadiabatic operation. Since temperature control is one of the most important means of influencing a chemical reaction, adiabatic reactors are used primarily where the adiabatic temperature change during the reaction is small or where there is only one major reaction pathway. In these cases no adverse effects on selectivity or yield due to the adiabatic temperature development are expected. In adiabatic reactors the catalyst is present in the form of a fixed bed which is surrounded by an outer insulating jacket.

If the reaction temperature must be maintained within a specified range, multistage adiabatic reactors can be used, whereby between each stage the temperature can be influenced by heat exchange or by cold/hot gas injection.

Reactions with a large heat of reaction and reactions that are very temperature sensitive are usually carried out in reactors in which heat of reaction is provided to or removed from the fixed bed via a circulating heat-transfer medium. Since in most cases the task of the heat-transfer cycle is to maintain the temperature in the fixed bed within a specific narrow range, this concept is frequently described as "isothermal fixed-bed reactor". The most common arrangement for isothermal reactor operation is the multitubular fixed-bed reactor, in which the catalyst is arranged in the tubes, and the heat carrier circulates externally around the tubes. Since isothermal reaction control does not necessarily provide optimum selectivity or yield, heat-exchange sections with changing temperatures of the heat carrier can be designed to establish an optimal temperature profile along the flow path.

Since the reactor feed must be heated to the ignition temperature of the catalytic reaction before reaction starts, the hot reactor effluent is often used to heat the cold reactor feed. This causes a thermal feedback which results in socalled autothermal reactor concepts.

Fixed-bed reactors for industrial syntheses are generally operated in a stationary mode (i.e., under constant operating conditions) over prolonged production runs. Design therefore concentrates on achieving optimum stationary operation. Unstationary operation, however, is unavoidable during startup and shutdown as well as during load change or in the case of automatic control actions. In particular, fixed-bed reactors with a strongly exothermic reaction exhibit an, at times, surprising dynamic behavior which can affect operational safety.