Application of Internal Seeding and Temperature Cycling for Reduction of Liquid Inclusion in the Crystallization of RDX

Abstract

In the cooling crystallization of cyclotrimethylene trinitramine (RDX) from γ-butyrolactone, an internal seeding method combined with temperature cycling, was applied to suppress liquid inclusion and improve particle size and size distribution for RDX at initial saturation temperatures of 60À80 °C. The process parameters including initial cooling rate, the range of the temperature cycle, and recooling rate during crystal growth period were determined based on direct measurements of size and amount of liquid inclusion of RDX crystals. Crystal size was measured by a dynamic light scattering particle size analyzer. Liquid inclusion was characterized in mass and area measurement based content by gas chromatography and matching refractive index method combined with image analysis. In the present work, RDX particles with average crystal sizes of 169.0À322.0 μm and inclusion area contents of 4.4À5.3% were obtained. Those values imply that the crystal quality of RDX is much improved compared with those of commercial RDXs such as Hanwha-RDX (conventional grade; average crystal size of 158.0 μm and inclusion area content of 9.3%) and I-RDX (reduced sensitivity grade; average crystal size of 218.0 μm and inclusion area content of 5.0%). For instance, the average crystal size of RDX obtained from a saturated solution at 70 °C by the present method is 290.3 μm, which was much larger than that of I-RDX, but the inclusion area content was about 4.6%, which was even lower than that of I-RDX.

Evaluation of nucleation rate by in‐situ focused beam reflectance measurement in an unseeded batch cooling crystallization

“…From the CLD data, the number of chord length measurements for the entire size range per unit second, i.e . total counts/s, was calculated by the FBRM software as follows: where Ch is the number of channels in FBRM and N Ch,i is the number of counts measured in an i th channel .…”

“…In recent years, advanced analytical methods are developed for in‐situ determination of crystal size and CSD . In particular, a method using focused beam reflectance measurement (FBRM) is widely applied for the estimation of crystallization kinetics , the in‐situ control of industrial crystallization , or monitoring solvent‐mediated transformation of polymorph .…”

Partial Seeding Policy for Controlling the Crystal Quality in Batch Cooling Crystallization

“…In the crystallization process, seeding is often utilized to obtain products with the desired crystal size distribution (CSD) , . Several seeding policies such as internal seeding , , full seeding , , and partial seeding have been investigated to optimize the operating conditions for the crystallization step. In internal seeding, no seed crystal is added to the solution, and primary nuclei are treated as seed crystals.…”

“…In internal seeding, no seed crystal is added to the solution, and primary nuclei are treated as seed crystals. Then, the extra nuclei are dissolved by temperature cycling, and the preset number of nuclei grows to yield the product , . In internal seeding, the solution cannot be contaminated by seed crystals, but the temperature cycling may consume more time and energy than in external seeding, in which the seed crystals are added.…”

“…In partial seeding, a small quantity of seed crystals is added to induce secondary nucleation and to inhibit primary nucleation, and seed‐originated crystals such as grown daughter nuclei and grown granddaughter ones are yielded as products . In the production of added‐value crystals, addition of a large quantity of seed crystals is avoided due to the impurities derived from the seed crystals that have been filtrated, dried and sieved , , and so partial seeding is preferable. In addition, Lee et al pointed out that full seeding was not necessarily performed successfully due to the slow growth kinetics of the added dry seeds, and that partial seeding deserves more attention because it may contribute to a high reproducibility.…”