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Adipic Acid Crystallisation using A spinning Disc Reactor (SDR)

 

Introduction

The spinning disc reactor (SDR) technology is based on the surface rotation technique and is aimed at intensifying rates of processes which present heat and mass transfer limitations in conventional processing equipment [1].  The creation of high acceleration fields by rotation causes reacting fluids introduced on the surface to flow in the form of thin, intensely mixed films.  The thin‑film characteristics of the SDR fulfil the major requirements of an ideal reactor: excellent heat transfer, mass transfer and mixing characteristics even in quite viscous reaction media. Apart from utilisation of polymerisation [2] and chemical reactions [3], SDR Technology can be applied to control particle size in crystallisation [4], including nanoparticle formation. Applications in which the solid content of a process fluid often poses a number of problems with regard to fouling in conventional devices, can be handled by the rotating equipment.  The rotating action in itself provides a scraping or ‘self‑cleaning’ mechanism strong enough to shift any solid deposit away from the surface of revolution, thereby ensuring maximum exposed area at all times during operation [5].

 

Crystallisation experiments were performed in a SDR using adipic acid (HO2C‑(CH2)4‑CO2H; hexanedioic acid or 1,4‑butanedicarboxylic acid) which is a valuable intermediate in a wide range of applications, including the following:

 

  • As a monomer in nylon, paper additives, copolyamides, terpolymers and unsaturated polyester resins (UPRs);

  • In polymer additives for epoxy curing agents and plasticizers;

  • As a chemical intermediate in synthesis;

  • In other applications such as solvents, lubricants, electronics, soil conditioners, glass protection agents, briquetting agents, leather tanning agents, food additives and cleaning aids.

 

Adipic acid was originally made for the production of nylon, but is now increasingly used in a wide array of industries for a broad range of applications and is currently a key raw material of polyamides and polyurethanes.

 

The crystals produced through a crystallisation process have a critical influence on the downstream processing.  For that reason, the particle size distribution (PSD) should be reproducible in each operation and as regular as possible.  It is therefore very important that the variables that affect the crystallisation process be known and easily controlled, in order to satisfy the requirements concerning the final product quality and the production demand.  The main purpose of this investigation is to propose and analyse ways to improve the performance of adipic acid crystallisation processes.

 

Experimental Results

Cooling crystallisation

Solutions of adipic acid in water at range of temperatures (30 ‑ 600C) were transferred onto a spinning disc reactor, with the disc surface temperature kept at 200C.  The solution temperature was kept constant before the liquid hit the surface, to prevent crystallisation to occur before the disc stage, by dipping the delivery tube in a constant temperature (coinciding with the temperature of the solution) water bath (Figure 1).  After being processed in the SDR, the crystals were filtered, dried and analysed for PSD, using Malvern Mastersizer S.  Furthermore, all samples were analysed using Scanning Electron Microscopy (SEM).  Solubility diagram of adipic acid in water is presented in Figure 2 (a) [6].  Figure 2 (b) presents solubility diagrams of adipic acid in various solvents.

 

Fig. 1 - Experimental set up

 

(a) (b)
Fig. 2 - Solubility diagrams for adipic acid; (a) in water, (b) in various solvents [7]

 

Parameters varied in this study were: adipic acid solution temperature (ranging from 300C to 600C), liquid solution flow rate (ranging from 130 cm3/min to 540 cm3/min) and disc rotational speed (ranging from 500 to 2500 rpm).

 

SEM images of adipic acid before re‑crystallisation can be seen in Figures 3 (a) and 3 (b).  Supersaturation ratios were kept at 1.89 (for the 600C solution) and 1.62 (for 400C solution).

 

(a) (b)
Fig. 3 - Adipic acid crystals before re-crystallisation; (a) Magnification: 50x, (b) Magnification: 200x

 

Fig. 4 - PSD trends compared to starting material

 

Comparison between adipic acid crystals before and after processing on the SDR is shown in Figure 4 and sample SEM images for the product of disc run performed at 2500 rpm and 200 cm3/min are presented in Figure 5.

 

(a) (b)
Fig. 5 - Disc speed: 2500 rpm, flow rate: 200 cm3/min; (a) Magnification: 35x, (b) Magnification: 750x

 

Anti-solvent crystallisation

Anti-solvent crystallisation (or drown out crystallisation), widely used in the pharmaceutical industry, was accomplished by adding a miscible anti-solvent into a mixture of solute and solvent, effectively reducing the original solubility of the solute in the solvent, increasing the supersaturation and thus, causing the crystallisation of the solute.

 

Anti-solvent used must be miscible/partially miscible with the original solvent over the ranges of concentrations encountered, and the solute must be relatively insoluble in it.  Additionally, the final solvent/anti-solvent mixture must be readily separable.

 

Systems used in this study included various alcohols (solvent)/ethyl acetate (anti‑solvent), THF/ethyl acetate, acetone/ethyl acetate, alcohols/water, THF/water, acetone/water, however, crystallisation on the SDR did not happen except in cases of isopropanol/water where some crystals were observed and ethanol/ethyl acetate where more adequate results were accomplished.

 

The results for ethanol/ethyl acetate are presented in Figures 6 and 7 for different flow rate ratios between streams of solvent (ethanol) and anti‑solvent and various disc rotational speeds.  It can be seen (in Figure 6) that, as far as the particle size is concerned, ratio between flow rates is very important and having more solvent in the system reduces the average crystal size down to 5 mm.  Disc rotational speeds did not have as much influence, nevertheless a shift towards larger size crystals can be easily distinguished with the decrease of rotation speed (Figure 7).

 

 Fig. 6 - PSD using anti-solvent crystallisation for various flow rate ratios between solvent and anti-solvent at 2000 rpm

 

Fig. 7 - PSD using anti-solvent crystallisation for various disc rotational speeds with solvent/anti-solvent flow rate ratio of 5:1

 

Conclusions

Adipic acid crystallisation experiments performed in a spinning disc reactor by cooling crystallisation have shown that crystal size and PSD can be easily controlled by adjusting parameters such as supersaturation ratio, disc rotational speed and liquid flow rate.

 

Drown out crystallisation was also tried in a SDR using various systems, but viable results were only obtained using ethanol/ethyl acetate system.  From the obtained results it was obvious that the size of the crystals was extremely dependent on the ratio between the streams of solvent and anti‑solvent.

 

Using these crystallisation techniques, average particle sizes of around 15 mm were obtained in the SDR and the range of particles from less than a micron to around 70 mm were attained by varying the experimental conditions (supersaturation, disc rotational speed, feed flow rate).

 

 
 

Acknowledgements

 

 

 

References

  1. Jachuck, R. J. J. and Ramshaw, C. (1994) Heat Recovery Systems & Chp, 14, 475-491.

  2. Boodhoo, K. V. K. and Jachuck, R. J. (2000) Applied Thermal Engineering, 20, 1127-1146.

  3. Vicevic, M., Jachuck, R. J. J., Scott, K., Clark, J. H. and Wilson, K. (2004) Green Chem., 6, 533-537.

  4. Jachuck, R. J. J., Hetherington, P. and Scalley, M. J. (2001) In 4th International Conference on Process Intensification for the Chemical Industry Brugge, Belgium.

  5. Ramshaw, C. (1993) Heat Recovery Systems & CHP, 13, 493-513.

  6. Mullin, J. W. (1972) Crystallisation, Butterworth & Co. (Publishers) Ltd.

  7. http://adi-pure.invista.com/e-trolley/page_11546/index.html

Contact:

Dr Kamelia Boodhoo

 

 

 

 Last modified: 02-Jun-2017