Page 185 - CW E-Magazine (16-4-2024)

Special Report

equipment. At the heart of it is a funda- tions, n-butyl lithium-based couplings, recovery units, which can decrease uti-
mental shift in the philosophy and culture or Grignard synthesis in ~10-kL stirred lity consumption by at least an order
of manufacturing, with a heavy emphasis vessels is a cumbersome, slow (since of magnitude compared to a batch
on first-principles modelling, quantitative feed starvation is the primary means of process. Consider the transesterifica-
analysis, planning, forecasting, and pre- temperature control), and highly hazar- tion of a triglyceride with glycerol,
ventive maintenance. It further requires dous (constant danger of reaction run- which is carried out at ~270°C. In a
continuous, unfettered, and respectful away) process with industrial incidents batch, the raw material (at ~30°C)
communication between members of a occurring with a daunting frequency. would have to be heated to 270°C and
multi-disciplinary team who are constan- The operators of such transformations cooled down again after the batch,
tly educating each other in their respec- take their lives into their own hands resulting in a temperature swing of
tive fields to develop integrated solutions every day of their working lives. Other 240°C. In flow, a cross-heat exchanger
to presented and anticipated problems. reactions, such as amidations of fatty can be installed to use the hot effluent
acids, need to be carried out at high tem- stream to heat the cold feed stream.
When should you go for the flow? peratures and hence concomitantly high If the driving force for heat transfer
The flippant answer to this ques- pressures (~100-bar) to contain the often is ~10K, the hot utility needs only
tion is “As soon as possible.” There is volatile, amine reactants. Operating a raise the temperature of the feed from
a degree of truth to that. To maintain 10-kL vessel at such pressures is not just 260°C to 270°C, and similarly, the
market competitiveness, manufacturers capital intensive, but a significant hazard cold utility needs only to cool the
will eventually be forced into continu- due to the quantum of potential energy effluent from 40°C to 30°C.
ous processing, and there is no point stored in the vessel, not to mention the
fighting the inevitable. However, the inventory of the toxic amine. The utility consumptions thus drop
transition needs must happen in stages, by a factor of 27!
prioritizing those processes that would The same processes in flow are far
benefit most from the flow. safer to operate primarily because of All fluid systems with low residence times
the low in-process inventory inflow due Since pumping fluids is far easier
Good candidate processes for flow to the much lower volume of the flow to execute with standard, off-the-shelf
have one or more of the following reactors, which significantly mitigates technology than pumping solids or
characteristics: the risk of containment failure. The slurries, those processes that involve
lower volume also means that the flow pumpable fluids as reactants are far
Inherent chemistry advantages of flow reactors can be built more robustly at a easier to transition to flow and should
An exemplary flow process would similar cost as the stirred tanks and can be prioritised to benefit early from the
intrinsically benefit from flow process- be put behind containment walls and transition. A lower residence time is
ing over batch processing. An excellent shields cost-effectively. also beneficial because it can reduce the
example of such a reaction is nitration size of the flow reactor.
without sulphuric acid(5). This occurs at Process automation
higher temperatures where the nitric acid Flow processes can readily be Process intensification with temperature
spontaneously forms the nitronium ion. designed to preclude routine manual If a process could be intensified by
However, the residence times of the order intervention completely. The operators’ operating at higher temperature or pres-
of a few minutes would only be possible job is primarily to monitor and flag sure, it is a great candidate for transition
to effect in a stirred tank where heating issues for preventive maintenance. With to flow. For example, n-butyl lithium-
and cooling operations are 30-60 mins. the advent of the Internet of Things, based synthesis (normally carried
Furthermore, a flow reactor can be con- a flow process can be monitored out at cryogenic conditions in batch)
figured with extended surfaces to provide remotely, facilitating communication can readily and safely be conducted
the heat transfer area necessary to remove between operators and plant managers. at 0-5°C in a flow reactor with a very
the reaction exotherm – which can only If a process has multiple points of human short residence time (10-30s). Similarly,
be done clumsily in a large batch reactor intervention, it might be an excellent nitrations that are normally carried
(e.g., using cooling coils). candidate to transition to flow. out at 0-5°C to mitigate the exotherm,
can be affected at ambient or higher
Process safety reasons Utility conservation temperatures with shorter residence
It is well known that carrying out A less appreciated advantage time, greater selectivity and greater
fast, exothermic reactions like nitra- of flow is the ability to install heat atom efficiencies.

Chemical Weekly April 16, 2024 185

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