Loop reactor

A loop reactor is a process engineering apparatus . It consists of a tubular reactor and a material recirculation system. By changing the size of the tube of the reactor and the proportion of the recycled material flow, the reaction volume and the residence time can be set separately from one another. A loop reactor can be used to conduct the reaction with the properties between a tubular reactor and an ideally mixed stirred tank reactor . It can work on the same principle as an airlift reactor or it can be operated with a pump.

The main difference is that in the loop reactor there is a spatial separation between the upward flow direction and the downward flow. The gas is only in the Aufstromsäule ( English : guided "riser"). The gas bubbles out of the medium in the head area of the upflow column. The gas-free medium flowing in prevents the gas-free medium from falling back into the upstream column, so that the gas-free medium returns to the starting point through the downstream column ( English : "downcomer").

A particularly effective mixing, as it is, for. B. is necessary for gas / liquid reactions, can be done by introducing a Venturi nozzle in the head of the reactor. This nozzle has the advantage that it optimally disperses the reaction gas and enables the reaction gas to be recirculated through a differential pressure between the reactor and the nozzle head. This type of construction, which has become known as the loop reactor, is particularly used in large-scale hydrogenations.

Physical background

The circulation of the medium is based on a pressure difference ( ) between the upstream and downstream columns. Due to the introduction of gas into the upflow column, the density of the liquid is apparently reduced, since it is now a dispersion (the density of the gas is negligibly small in this case): ${\ displaystyle \ Delta p}$

${\ displaystyle \ rho _ {\ mathrm {Disp}} = \ rho _ {\ mathrm {Medium}} \ cdot (1- \ varepsilon _ {\ mathrm {Gas}})}$ .

Strictly speaking, there is also a dispersion of gas and medium in the downstream column. However, as the gas bubbles out in the head space, the proportion of gas is greatly reduced and thus the density of the dispersion in the downstream column is significantly higher than that in the upstream column.

The pressure difference as the driving force is therefore based on the difference in the gas components in the respective dispersions:

${\ displaystyle \ Delta p = h \ cdot g \ cdot \ rho _ {\ mathrm {Medium}} (\ varepsilon _ {\ mathrm {Gas \ Upstream}} - \ varepsilon _ {\ mathrm {Gas \ Downstream}}) }$

Designs

A wide variety of designs and materials are possible in the area of loop reactors, depending on the dimensions, purpose and framework conditions. A characteristic of a loop reactor is its slim, tall design (height: diameter ratio 10: 1 and larger).

Furthermore, two basic types of loop reactors can be distinguished:

1. External loops:
The spatial separation is clearly visible here, as the upstream and downstream columns stand next to each other as cylinders and are connected by a pipe at the top and bottom. They are mostly used in the research area and in small dimensions, since a separate observation of the downcomer and riser is possible here.

2. Internal loops:
Here is a smaller cylinder in the center of a cylinder with a larger diameter. The connection between the two cylinders is made possible because the smaller cylinder is slightly shorter at both ends than the surrounding cylinder. This design is used particularly in larger dimensions because it saves space.

literature

Peter Cramers: Hydrodynamics and Mass Transfer Characteristics of Liquid Driven Jet Ejectors. Dissertation ETH Zurich