17 Mar 2020

Application Note AN19-05 Acetic Acid Production

Find how Specac can aid your industrial process adding continuous monitoring to your flow reaction.
Acetic acid is an important intermediate chemical for the manufacturing of solvents like ethyl acetate and for key ingredients in paints like vinyl acetate monomer. It can also be used for the production of acyl chloride, which is widely used in the synthesis of pharmaceuticals and pesticides and acetic anhydride via the ketene route.

The annual production of acetic acid is about 1.2 Mtons [1]. If the throughput of the plants can be increased by just 1%, then at an average price of about $500 per ton of acetic acid, extra product worth more than $6m can be produced each year if the plant runs continuously. As a result, any installation to monitor the process in real time would pay itself back within months rather than years.

The benefits of in-line analysis are:
NIR Process flow cells custom designed to your specific requirements ensuring quality and reliability from inside to out:
  • Durable sapphire optics
  • Double-sealed optical assemblies
  • Fibre-supporting gland and shielding for environmental protection.
  • Cleaning ports as standard
  • Optional seal bypass tell-tales
Online Analysis of the Acetic Acid Production Line
The manufacturing of acetic acid involves high temperatures, pressures and toxic chemicals including methyliodide, carbon monoxide and methanol. Historically samples of the reactor had to be taken manually to measure the key components of the carbonylation reaction off-line. The sample was then analysed in a laboratory for the concentration of key components. Not only did this require operators adhere to stringent safety control to allow safe removal of this sample, but also resulted in a significant delay between the time the sample was taken and the results being available to the reactor control system.

NIR spectroscopy can be used to monitor the key components of an acetic acid production process on-line [1], including: methyl acetate (MeOAc); acetic acid (AcOH); methyl iodide (MeI); water (H2O); and potassium iodide (KI). Concentrations used in the study by Chung et ai. were those typically encountered in an acetic acid production process, even though they were not actual process samples. Standard errors of prediction of around 0.2 wt. % coupled with the excellent precision and accuracy of NIR spectroscopy mean that process parameters can be fine-tuned to optimize production.

At Specac similar measurements have been carried out to assess the suitability of our Vortex Pro cells for the measurement of key components in the acetic acid production process. The concentration of KI in distilled water ranged from 0 to 50 % and a 1 mm pathlength cell was used due to the strong absorbance of water. Resulting spectra are shown in Figure 1. As expected, a clear shift in the peak absorbance wavelength can be observed with increasing concentrations of KI. 
A chemometric PLS-1 model was built to demonstrate how varying concentrations of KI can be measured with NIR spectroscopy, shown in Figure 2. The spectra (1100-1900 nm) were pre-processed by calculating the first derivative, followed by an area normalisation. An excellent regression coefficient could be achieved with only 1 factor, demonstrating again the feasibility of NIR for the measurement of key components.

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