A study of the influence regularity of caprolactam on the crystallisation of ammonium sulphate in aqueous solution

The crystal size of ammonium sulphate, a by-product of caprolactam production, is generally less than 1 mm. In order to increase the crystal size, a detailed study of the crystallisation process is required. In this paper, we investigated the effects of crystalline species, rotational speed, cooling rate and caprolactam addition on the crystallisation process of ammonium sulphate. The kinetic equation of ammonium sulphate nucleation and the kinetic equation of ammonium sulphate growth were obtained by fitting the optimised ΔL law model. The results showed that B = 1.0334 × 10⁻¹⁴τ^0.3071, G = 2.1997τ^−0.4792 with a rotational speed of 450 r min⁻¹, a cooling rate of 0.25 K min⁻¹, the absence of crystal seed and without the addition of caprolactam. When caprolactam was added at 1.04%: B = 1.5946 × 10⁻²¹τ^2.1423, G = 5.097τ^−0.4792 while other conditions remained the same. When ammonium sulphate solution contains caprolactam, the nucleation rate of ammonium sulphate increases while the growth rate decreases. The nucleation rate of ammonium sulphate increases gradually with the increase of the mass fraction of caprolactam and the growth rate decreases slowly with the increase of the mass fraction of caprolactam.