Supersaturation profiles of L-sorbose water solutions in the cooling batch crystallization process
Advancing the chemical engineering fundamentals
Crystallization (T2-9P)
Keywords: L-sorbose, batch crystallization, supersaturation, DT MSMPR crystallizer, cooling rate
L-sorbose is a second, intermediate product in L(+)-ascorbic acid synthesis according to Reichstein procedure. An industrial scale crystal product should be composed of, according to the assumed technology yield, 97.0 – 98.8 mass % of L-sorbose. This required chemical purity results the most often from the subsequent batch mass crystallization from its postprocessing water solutions.
The characteristic feature of this purification step is that water solutions of L-sorbose tend to form stable supersaturated systems, characterized by high values of maximum supercooling: from ca. DT = 10 up till 50 K, depending on saturation temperature of initial solution in a batch crystallizer, as well as a cooling rate applied. It reveals a direct influence both on mass crystallization process yield and crystal product quality (crystal size distribution, their uniformity, shape and chemical purity).
The experimental results concerning time–distribution of supersaturation in the seeded water solutions of L-sorbose during batch mass crystallization process with adjusted cooling rate value providing linear decrease of temperature with the process time are presented. The experiments were performed in a laboratory DT (Draft Tube) MSMPR (Mixed Suspension Mixed Product Removal) crystallizer with internal circulation of suspension equipped with a propeller agitator. The laboratory test stand performance (temperature reduction regime) was controlled by means of PC computer system.
The crystallizer was fed with water solutions of L-sorbose of concentration adjusted to fit within the 55 – 75 mass % range (corresponded to saturation temperature range of Teq = 324.5 – 368.5 K), cooled with linear cooling rate selected individually from the RT = (1.39 – 8.33)E–3 K/s range.
On the basis of laboratory test results it can be concluded, that with the decrease of saturation temperature of the batch solution introduced into the crystallizer (lower initial concentration values) the supersaturation in the solution reaches during crystallization process only slightly higher values in a whole time range tested. In a final process temperature (Tf) for two extreme cases under study (Teq = 324.5 and 368.5 K) the difference between final supersaturation values in these two solutions was a 1.9 mass % only. The discharge of supersaturation, continuously generated while the process, runs smoothly in all water solutions of L-sorbose under investigation.
Supersaturation value in a process final temperature (Tf) is also crystallization time, tcr, dependent (where crystallization time is defined as tcr = (Tcr – Tf)/RT and Tcr is a temperature value corresponded to spontaneous crystallization event within the solution). For the solution characterized by Teq = 324.5 K (corresponded to concentration of 55 mass % of L-sorbose) relatively short batch crystallization time (tcr = 1920 s resulting from the highest value of RT = 8.33E–3 K/s) is responsible for the fact, that the remaining supersaturation in the solution in the process final temperature is as much as 5.0 mass %. Contrary, lower values of linear cooling rate applied correspond to higher crystallization process yields. For example, in case of RT = 1.39E–3 K/s thus tcr = ca. 2E+4 s, the undischarged residual supersaturation in a postprocessing mother solution reaches only a 0.3 mass %, what is profitable from the mass balance (thus economic) point of view.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 13:30 to 15:00, in session Crystallization (T2-9P).
