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FLOW CHEMISTRY
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Figure 3- Typical loop reactor set-up for production of alkyl nitrites r--� I I I I I
Off gas
Static mixer
LI
Heat exchanger
Pump
Phase separation
Alkohol NaN02 40 % HCI 33 %
In other cases, particularly when product variety or lower volumes are required, batch processes offer the necessary flexibility.
Arxada has also implemented hybrid process designs within its hydrogen cyanide( HCN) network. HCN is industrially produced through continuous gas-phase reactions( Figure 2, 6) such as the Andrussow or BMA process, both of which involve high temperatures and high potency reagents. Continuous operation is essential both for efficiency and to minimise HCN inventory and exposure risk.
Downstream products like cyanogen chloride( ClCN, 7) are also produced in continuous systems due to their extreme potency and reactivity. ClCN, when reacted with acetonitrile, leads to malononitrile( 8) via a hightemperature gas-phase reaction. While batch processes starting from cyanoacetates are possible, they typically produce more waste and are less efficient than the continuous method employed at Visp.
Another advanced application is the continuous production of chlorosulfonyl isocyanate( CSI, 9), synthesised from ClCN and sulfur trioxide. The reaction involves highly corrosive and reactive conditions, necessitating a continuous set-up with specialised materials and design.
CSI is then used for downstream products such as bis( chlorosulfonyl)- imine, a compound whose synthesis is greatly improved by the thermal control
and short residence times afforded by flow chemistry. In batch, the required reaction temperature is near the system’ s decomposition threshold; in flow, this limitation is overcome safely.
Variable set-ups
The above examples highlight that flow is an extremely powerful option to provide safe and efficient access to highly potent and / or very reactive basic building blocks. Flow is the dominant technology in dedicated production of high-volume chemicals. Historically, certain products could only be industrialised after continuous processes were available to produce them.
A nice example is ammonia, which has been continuously produced for nearly a century at Visp. Once these building blocks are transformed into more complex specialities, volumes become smaller and chemistry more diverse and complex. For these cases we use multi-purpose production facilities that offer a variety of different construction materials and technologies to flexibly adapt to the needs of the process.
The real challenge emerges when a process requires both. A typical example is processes featuring steps with long reaction times combined with handling suspensions followed by highly exothermic and safety critical steps, such as nitrosation or nitration.
For this reason, Arxada uses a wide range of reactor technologies to meet the unique needs of each process,
Aqueous waste
Product alkyl nitrite
including reaction loops, continuous tubular reactors and plug flow reactors for flexible scale-up and intensification of homogeneous liquid-phase reactions. The former, when applicable, allow an efficient way to integrate flow in a batch set-up.
Other than one time pass through set-ups, the buffer capacity to decouple the batch from the flow process are reduced in such a set-up, as in case of an interruption in the preceding batch mode steps the loop can be set in cycle mode and restarted once the interruption is resolved. Figure 3 shows an example. ●
Dr Ulrich Mayerhöffer
HEAD, TECHNICAL EVALUATION
ARXADA AG, NCE CDMO k + 41 27 560 67 95 J ulrich. mayerhoeffer @ arxada. com j www. arxada. com
56 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981