MELT CRYSTALLIZATION OF CAPROLACTAM IN A CONTINUOUS CRYSTALLIZER WITH FINES REMOVAL

S. Misztal

Faculty of Mechanical and Power Engineering, Wroclaw University of Technology, Wroclaw, Poland (slawomir.misztal@pwr.wroc.pl)

Keywords: novel crystallization equipment, modelling, population balances The aim of this research was to obtain new data on the suspension melt crystallization of caprolactam in a new type of continuous crystallizer with fines removal and to determine the effect of its operating parameters on the size of produced crystals and on its capacity.

A diagram of the experimental set-up is shown in Figure 1. Its main part is the specially developed crystallizer (1) for suspension melt crystallization, with a working volume of 3.6.10-3 m3. [1] The crystallizer is equipped with a draft-tube heat exchanger (2) and a special agitator consisting of a three-bladed propeller (3) which is connected to two external (4) and internal scrapers (5). A special system of baffles is mounted in the upper part of the crystallizer to create a settling zone. The system consists of six large vertical baffles (6) and an additional cone baffle (7). Such a design makes possible the removal of fines from the crystallizer. A slurry stream is removed from its bottom part while a stream containing fines flows out from the settling zone. The stream with fines passes to a heat exchanger (25) where the fines are remelted. The fines stream is then cooled in another heat exchanger (27), combined with the feed liquid, additionally cooled in still another heat exchanger (26), and it flows into the bottom part of the crystallizer. The feed liquid is pumped by a pump (9) from a storage tank (10). Caprolactam containing about 1.85 wt.% water was used in the experiments. The crystal size distributions were determined by sieve analysis.

During the experiments (15 runs) the operating conditions were changed as follows: a) mean residence time τ of the product crystal slurry from 3028 to 4623 s, b) crystal slurry density Mt from 194.61 to 316.13 kg/m3 , c) stirrer speed ω from 3.100 to 3.933 s-1 (and calculated specific power input ε from 88.26 to 170.52 W/m3), d) crystallization temperature T from 60.80 to 61.95 ºC, e) fines destruction rate r ( ( ) PPF VVVr &&& /+= , where: FV& - fines volumetric flow rate, PV& - product volumetric flow rate) from 2.789 to 7.635. It was found that median crystal size L50 changed from 0.948.10-3 to 1.633.10-3 m and crystallizer capacity Pa (calculated as the ratio of production rate and draft-tube heat exchanger surface area) changed from 13.2 to 24.6 kg/(m2h). On the basis of the results the following equations were formulated:

3399.02285.02703.02619.04

50 102658.1 rML t ⋅⋅= −− ετ (1)

(with correlation coefficient R = 0.965)

0566.01658.01814.11809.15414.761 rMP ta

−−= ετ (R = 0.984) (2)

1 – crystallizer; 2 – draft-tube heat exchanger; 3 – three-bladed propeller; 4, 5 – scrapers; 6 – baffle (six); 7 – cone baffle; 8 – baffle (six); 9, 32, 33 – pumps; 10, 16 – storage tanks; 11 –remelting tank; 12, 13, 14 – flowmeters; 15 – centrifuge; 17 – computer; 18÷24 – thermostats; 25, 26, 27 – heat exchangers; 28, 29 – safety-valves; 30, 31 – manometers; 34 – control valve.

Figure 1. Diagram of experimental set-up.

In order to determine the effect of fines removal on the median crystal size, an additional experiment was carried out for the MSMPR configuration of the crystallizer working in similar conditions (τ = 4248 s, ω = 3.950 s-1, Mt = 251.43 kg/m3). It was found that the increase in the median crystal size in the crystallizer with fines removal was by about 80% higher than that in the MSMPR crystallizer (Figure 2). This increase is much higher than the one achieved in the previous investigation [1], which is owing to the different operating conditions and changes in the fines destruction loop. For most runs, the semilogarithmic plots (typical for the crystallizer with fines removal, with two different CSD straight line segments [2-4]) of crystal population densities as shown in Figure 3 were obtained. Using the plots and the following relations [3,4] for population density:

( ) ⎟

⎠⎞

⎜⎝⎛−=<

τGrLnLLn o

F exp (3)

( ) ⎟

⎠⎞

⎜⎝⎛−⎥⎦

⎤⎢⎣⎡ −

=>ττ G

LG

LrnLLn FoF exp)1(exp (4)

Figure 2. Comparison of cumulative mass distributions of caprolactam crystal sizes obtained in respectively MSMPR crystallizer and crystallizer with fines removal.

nuclei population densities on , growth rates G and fines cut sizes LF (the maximum fines sizes) were obtained.      

            

Figure 3. Exemplary caprolactam CSD produced in crystallizer with fines removal.

For the above parameters slurry density 21 ttt MMM += can be calculated from the equations:

dLL

GrLnfM

FLo

svt3

01 exp ⎟

⎠⎞

⎜⎝⎛−= ∫ τ

ρ                                                           (5)

dLL

GL

GLrnfM F

L

osvt

F

32 exp)1(exp ⎟

⎠⎞

⎜⎝⎛−⎥⎦

⎤⎢⎣⎡ −

= ∫∞

ττρ                                      (6)

where: fv – a volumetric shape factor, ρs – crystal density. Similarly, the concentration of crystals in slurry M1 for size range 0–LF can be expressed by equation (5), applying limits of integration from 0 to L. Also concentration

of crystals in slurry M2 for size range LF–L can be expressed by equation (6), applying limits of integration from LF to L. Taking into account the equations, cumulative mass distributions Q(L) were calculated from the relations:

( ) F

tt

LLMM

MLQ ≤<+

= 021

1 (7)

( ) ( )

21

21

tt

F

MMMLMLQ

++

=                   L > LF (8)

Figure 4 shows exemplary calculated and experimental cumulative mass distributions of caprolactam crystal sizes in the crystallizer with fines removal for: r = 5.440, LF = 0.302·10-3 m, τ = 3458 s, on = 4.773·1014 m-4, G = 8.895·10-8 m/s. For most of the runs the calculated cumulative mass distributions are in good agreement with the experimental ones (R = 0.9908 - 0.9989).

Figure 4. Exemplary calculated and experimental cumulative mass distributions of caprolactam crystal sizes in crystallizer with fines removal (R = 0.9989).

The presented equations can be used for the simulation of the effect of fines removal on the size of produced caprolactam crystals and in crystallizer design. References: [1] Misztal, S.; Verdoes, D. Investigation into methods to increase the crystal size in suspension melt crystallization of caprolactam. Proceedings of the 14th Symposium on Industrial Crystallization, Cambridge, UK, 1999. [2] Randolph, A. D.; Larson, M. A. Theory of particulate processes, 2nd ed.; Academic Press Inc.: New York, 1988. [3] Juzaszek, P.; Larson, M. A. Influence of fines dissolving on crystal size distribution in an MSMPR crystallizer. AIChE J. 1977, 23, 460-468. [4] Rojkowski, Z.; Synowiec, J. Crystallization and crystallizers (in Polish), WNT: Warsaw, 1991.