(87f) Industrial Photo-Chemistry in Agitated Reactors

Agitated photochemical reactors have found their way into the industrial chemistry to produce special polymers and rubbers, additives and intermediates, surface active substances etc. The design of such reactors may deviate from the standard. Their profitable, safe and reliable operation is based on some design principles which are explained below.

Chemical reactions need activation energy to transform reactants into products. Catalysts reduce this energetic barrier, but the catalyst is an additional component which is incorporated into the reaction system. It is often expensive and has to be handled before and removed afterwards from the reaction products, where it bears operational risks by its toxic and sometimes pyrophoric character.

Light is an elegant alternative source of the activation energy. Photochemical reactions can be initiated by irradiation with wave lengths of 200 to 700 nanometers, that is from the ultraviolet to the visible range. The photons get absorbed by the molecules, which increases their energy level, thus initiating the reaction. This allows lower reaction temperatures, milder conditions for the conversion of temperature sensitive molecules and finally less by-products.

Typical examples of industrial photo reactions are chlorinations, sulfochlorinations, sulfoxidations or nitrosylations. Loop reactors have been used in the past as a solution for these reactions with their primary liquid phase. However, if gases or solids are additionally introduced, the loop reactor has massive drawbacks in operation, which restricts its productivity.

The new concept of agitated reactors is ideal for these two to three phase reactions. It combines high productivity and flexibility with a reliable operation of the submerged UV sources. Reactor sizes up to 50 m³ or beyond can be realized with the proven design.

In the lab scale, photo reactors are made of glass: the irradiation is simplified due to the small dimensions. The full volume of the reactor can be covered by the light. Production reactors are made of steel, alloys or titanium, and the UV-sources are mercury or LED lamps submerged into the reaction mass inside a quartz glass tube. The glass tubes pass through nozzles in the tank head, where they are connected to the power supply and the cooling medium. They need a hermetic sealing and monitoring to ensure, that in the unexpected case of a glass breakage this is immediately detected. Special care has to be taken during design and installation of the tubes so as to not creating local stress, allowing for thermal expansion and preventing it from vibration excitation, which can develop due to the intense flow produced by agitation. As the power of the UV-lamps ranges from 5 to 60 kW each, the lamps themselves need to be cooled individually inside the quartz tubes, otherwise high surface temperatures could damage the products. Industrial reactors can be equipped with a number of 4 to 20 of such quartz tubes reaching from the tank head into the liquid.

The penetration of the light into the reaction mass is only a few tens of millimeters. Thus, the primary task of the agitator is to provide high pumping rates and thereby a permanent renewal of the reactants in the high irradiation zone around the tubes. If one of the reactants is a gas as Cl₂, O₂ or SO₂, it must be dispersed into the liquid creating a maximum surface for the gas-liquid mass transfer and dissolution of the gas. Special designs for the primary dispersion of the feed gas and the recirculation of unconverted gas from the head space through the shaft of the agitator, into the slurry, avoids a mass transfer limitation. The homogeneous suspension of solids as, e.g. polymer particles, and the removal of the reaction heat have to be considered as well in the design of the agitator and tank.

The examples above illustrate that often toxic and corrosive reaction components are involved in the industrial photo chemistry. The sealing of the rotating agitator shaft towards the atmosphere is a key element for a safe and reliable operation. Even if temperatures and pressures of photo reactions are low, only double acting mechanical seals or hermetic sealed magnetic drives are applied. The inherent safety of the seal system is based on the monitoring and the automatic refill of seal liquid to compensate for the defined leakage through the seal surfaces.

The presentation provides a concise insight into the potential of industrial photo reactors and proven design principles regarding the irradiation source, agitation and safety aspects.