Batch manufacturing methods are common throughout the chemical process industries (CPI) for specialized compounds and reactions. However, typical batch-wise manufacturing processes are often time-consuming due to large volumes and long loading and unloading times added to the batch processing time, which can cause delays between batches. Continuous reactors, such as tubular reactors (plug-flow reactors), continuous stirred-tank reactors (CSTRs) and modular configured microreactors, can help processors realize a host of efficiency and productivity benefits over batch processes, including improved control over reaction conditions, more consistent product quality and opportunities for larger product output with a smaller footprint (Figure 1) [1]. In particular, thoughtfully constructed tubular reactors with static mixers can have positive implications for sustainability, safety, process intensification and product quality.
FIGURE 1. Continuous processes, such as those using a CSTR or tubular reactor, have advantages over batch processes
Tubular reactor advantages
Tubular reactors containing static mixing elements can achieve thorough mixing with precise control over residence time and can tighten residence-time distributions in continuous reactions. This type of tubular reactor allows improved control over reaction conditions and temperature, while still maintaining excellent heat and mass transfer. Compared to empty tube reactors, tubular reactors with static mixers have advantages with respect to product yield and selectivity, as well as improved heat transfer (Figure 2).
FIGURE 2. Reactors with static mixers have improved temperature profile compared to empty tubes
The mixing elements inside the reactor tube continuously split and recombine the fluid streams, and help homogenize concentrations and temperatures across the pipe cross-section. This leads to a more homogeneous residence-time distribution, less reactor wall effects and minimized stagnant boundary layers. Overall, tubular reactors with static mixers promote optimal renewal of the concentration and temperature next to the reactor wall, with positive effects on product quality.
In an example of the potential impact of mixing elements, a leading pharmaceutical company saved $9 million/yr via improved product yields by changing from an empty tube reactor to a tube reactor with static mixing elements.
Safety
Tubular reactors containing static mixers provide superior safety control through better product consistency and the avoidance of adverse reactions. Smaller reaction volumes at any given time minimize the dangers associated with hazardous or exothermic reactions. Monitoring the process with sensors, such as temperature or pressure probes, allow for constant process-parameter monitoring and real-time quality control.
Scale up
Although continuous reactors can have real benefits, process adaptation is a critical aspect in transitioning from batch processes to continuous ones. Several factors, such as reaction rate, reaction kinetics, rate constant and order of reaction, play a vital role in determining ideal reactor sizes and operating conditions. Reliable scale up — possibly in steps — and ongoing process optimization during the transition, including small-scale testing before growing to industrial-scale production, allow a high degree of success in the changeover.
Test skids or components are available for rent to conduct such tests in users’ own laboratories. With the data collected, scaleup can be carried out using internal calculation tools and scaleup rules backed up by computational fluid dynamics (CFD) simulations. Normally, the test size is around 1–6 L/h [2]. Therefore, a kilogram-scale laboratory test after the initial studies would make sense in general. During the scale-up procedure, attention should be paid to heat development during the reaction. Especially in larger pipe diameters, the available exchange surface and heat increase must be kept in mind.
Also, because of the static nature of the mixing, complications like fouling — especially when solid catalysts are present or precipitation takes place — or clogging need to be considered in more detail.
References
1. Baumann, M., T.S. Moody, M. Smyth, S. Wharry, A Perspective on Continuous Flow Chemistry in the Pharmaceutical Industry, Organic Process Research & Development, 2020.
2. Al Azr, N. and others, Batch-to-Continuous Transition in the Specialty Chemicals Industry: Impact of Operational Differences on the Production of Dispersants. Chemical Engineering Journal, Vol. 442, 1 October 2022, 136775.
Editor’s note: The content for this column was assembled by Dr. Kishor Kulkarni, sales and application manager for mixers at Sulzer Chemtech Ltd. (Winterthur, Switzerland; www.sulzer.com), and Marcel Suhner, global product and application manager for mixers at Sulzer Chemtech.