What is continuous processing and why is it important?
Batch processing and continuous processing are two different ways to manufacture chemicals. The top 30 petrochemicals and most of the top 300 organic chemicals are manufactured at large scale using dedicated, continuous processing plants which run for many years without shutdown. As bulk chemicals, there is significant pressure on the price and so the cost of manufacture of the chemical must be minimised. As it is expected to be produced for many years, a significant amount of development work to fully understand the chemistry and chemical engineering requirements can be justified. This information informs the design and installation of a bespoke chemical plant highly tuned to the desired chemistry process with minimal waste of energy and materials. The need to provide as efficient a plant as possible leads to the selection of continuous processing technologies.
The feed of raw materials into the continuous reactor are tightly controlled, as is the residence time within the reactor and the temperature profile that the reacting materials are exposed to. As a result, the product flowing from the reactor tends to be more consistent as the reaction parameters are better controlled than in a batch operation whose reaction parameters must change over time. The capital costs per tonne of product of a continuous reactor are significantly lower that for a batch reactor system.
Batch reactors are, however, very process and product flexible. A batch reactor can be reconfigured from operating one process to operating another. The process is inherently inefficient but for low volume, high value chemicals, this has historically been an acceptable compromise.
To fully understand the impact on the process and economics of running a batch reactor, consider the following typical scenario. For a process A + B -> C, the reaction time may be only 5 minutes, but when running at pilot plant scale, say 1000L, the following sequence of process steps is not uncommon:
- Begin preparing reactor for use t = 0
- Seal and inert reactor 30 mins
- Introduce solvent 30-60 mins
- Add reagent A 30 mins
- Heat to near reaction temp 2 hours
- Add reagent B 30 mins
- Reaction complete 5 mins
- Cool to room temperature 2 hours
- Discharge product 2 hours
- Clean reactor 12 hours
For a 5 minute reaction time, using a batch reactor results in a process cycle time of anywhere from 8 to 24 hours.
For a continuous reactor, if we want to process the same quantity of material in 12 hours with a 5 minute residence time, the reactor would be 7L in size:
7L processed every 5 minutes = 1.4 L per min = 84 L per hour = 1008 L in 12 hours
So, we can compare the costs of installing, running and maintaining a 1000L batch reactor with the costs of installing, running and maintaining a 7L continuous reactor to see the first benefits of continuous processing. As the continuous reactor is less than 1% the size of the batch reactor, the peak loading on the utilities (heating, cooling) is going to be less. The facilities required to install the system can be smaller, saving on HVAC systems or allowing greater utilisation of the space available.
For hazardous chemistry, especially highly energetic processes, using a much smaller reactor results in much lower inventories of unstable intermediates which greatly reduces the risk and in turn the safety controls needed to managed the residual risk. These “process intensification” factors all result in a more energy efficient and safer manufacturing process. In addition, there are potential chemical process improvement factors arising from better heat transfer, better mass transfer and running the process under steady state conditions. The factors all add up make the economic benefits of continuous manufacture clear.