Chemistry Benefit – Heat Transfer Factors
Chemical processing relies on heat transfer. In order for a reaction to take place, the molecules involved must receive sufficient energy to overcome the activation barrier and form a high energy, activated species. As the reaction proceeds, the molecules will release energy to form the products. For exothermic reactions, the energy of the products are lower that the energy of the starting materials and so the overall process gives out heat.
If this heat is not removed by the processing equipment, it can build up and cause unwanted side reactions to occur such as degradation or the formation of impurities. In a chemical reactor, the jacket of the reactor is usually set up to provide heating or cooling as required by the process. For example, by circulating a cooling fluid through the reactor jacket, heat from the bulk fluid will be conducted through the reactor wall and into the cooling fluid.
As this conduction process occurs on the surface, changing the bulk, average temperature of the process fluid then depends on convection and physical stirring. As the volume of a batch reactor increases, the ratio of its heating/cooling surface area to its volume decreases. This results in poorer heat transfer as it takes longer for the heat generated at the centre of the reactor to be convected to the cooling surface of the reactor where it can be conducted away. As a consequence, the difference in temperature between the cooling fluid in the jacket and the process fluid needs to be increased to increase the rate of conduction at the surface and the other processing parameters are often manipulated to slow down the process such that the heat energy it produces matches the amount of heat the reactor can safely remove per unit time.
By intensifying the process and using a much smaller reactor, the ratio the surface area to volume is going to be better in a 7L reactor compared with a 1000L reactor. As a result, the continuous reactor is able to more effectively control the heat within the reactor. The convection path length from the centre of the reactor to the cooling surface of the reactor will be smaller and so the system can extract heat more quickly. This results in processing parameters that are much closer to the intrinsic kinetics of the process – less compromise is required to be able to run the process as fast as the chemistry would allow in an “ideal reactor”.
Chemistry Benefit – Mass Transfer Factors
Mass transfer describes the influence of mixing on chemical processing. Many chemical processes require two chemical entities to collide with one another with sufficient energy to overcome their activation barrier and proceed to complete a chemical reaction. In chemical processing, the chemical entities will often start as pure materials and will need to be dissolved in solvents and/or blended together to create the right atomic and molecular environment for the chemistry to occur.
For some chemical processes, different material may be in different physical states including gases, liquids and solids. For two miscible liquids of similar, low viscosity, the mass transfer rate will be governed by the diffusion rate of the molecules within the two liquids. For immiscible liquids or where two or more physical phases are present, more factors influence the rate of mass transfer. For example, for a metal catalysed hydrogenation of a fatty acid, the reaction can only occur if the hydrogen gas has been transported through the liquid fatty acid phase to make contact with the metal surface where it is absorbed. The fatty acid molecules then need to be transported to the activated surface of the metal where the desired chemical transformation can take place.
In a 1000L vessel, the distances involved in these mass transport processed can be quite large and over 1 metre. As a rule of thumb, the blending time in a batch reactor is taken as the time required for the fluid to travel five times around the fully mixing path. This time is dependent on the design and power of the agitator within the batch reactor. As the volume of the reactor increases, the increase in power required is proportional to the square of the volume and so there comes a limit on how big and powerful the agitator can be. As a consequence, the blending time typically increase as the reactor volume increases which has the effect of slowing down the processing.
In the corresponding 7L continuous reactor, the mixing path will be 1-3 orders of magnitude shorter and the distances will typically be measured in centimetres instead of metres. So fast blending is easier to achieve even with low energy mixing input. This means that a continuous reactor can be design in such a way to ensure that all of the process volume is evenly mixed.
Chemistry Benefit – Steady State Processing
In a batch reactor, the contents of the reactor remain in one place and change over time. This means that over the course of the time spent in the reactor, the starting materials are initially surrounded by other starting material molecules but over time, they become increasingly surrounded by product molecules.
This means that if there are any potential consecutive reactions, the longer the materials stay in the batch reactor, the more likely that some of these consecutive reactions can occur. This becomes a more significant issue as the process is scaled up as the heat and mass transfer factors become limitations to the process parameters of a batch reactor. Both factors lead to the selection of processing parameters that slow down the chemistry process.
So the process is changed and made less optimal in order that it can still be run within the limitations of the batch reactor. This can result in lower yields or more by-products/impurities that have to be removed in subsequent unit operations.
By contrast, a continuous processing reactor keeps the starting materials separate from the products by moving materials from one end of the reactor to the other end of the reactor. The time that the starting materials are given to react is based on how long they stay travelling through the reactor. The ideal reactor would be designed so that two molecules of the starting materials enter the beginning of the reactor together, are given enough energy to react together and any heat of reaction is removed so that they leave the reactor together as product.
For a reaction with a 5 minute reaction time, this means they would stay within the reactor for just 5 minutes compared to the hours they would potentially need to stay within the equivalent 1000L batch reactor. Once operating, a continuous reactor will be operating under steady state conditions – a snap shot of the process parameters at any time would be identical – unreacted materials at the start of the reactor, completed product at the end of the reactor. This means that the product coming out of the reactor is very consistent as the residence time, temperature profile and mixing regime for any molecules entering the reactor will always be the same.