DE3920999C1 - Nitro:guanidine spherical crystals - obtd. by forming nucleating seeds and crystallising by agitated cascade process - Google Patents

Abstract

Spherical crystals (SC) of nitroguanidine are continuously obtd.: by (A) forming in an externally cooled spiral condenser nitroguanidine seeds from a hot, satd. soln. of nitroguanidine; and (B) effecting further crystallisation by an agitated cascade process. ADVANTAGE - Effective method to obtain the desired SC of nitroguanidine.

Description

The invention relates to a continuous process for Production of spherically crystallized nitroguanidine.

Several methods are already known from the prior art, the subject of the production of spherical nitroguanidine to have. For example, EP-PS 00 02 740 describes a method for the production of compact nitroguanidine by cooling and stirring a hot saturated solution, cooling by pouring a hot nitroguanidine solution into one cold solution is done.

This results in a temperature gradient when cooling of approx. 7 ° Celsius reached per minute (EP 00 02 740). These methods is peculiar, however, that it is only a batch production of nitroguanidine. This batch process finds its limits to those with increasing batch volumes decreasing heat transfer rates, large processing volumes of the sub-steps of dissolving and crystallizing, high Dwell times at elevated temperature, thereby reduced Reusability of residual solutions and limited daily output of product manufacture.

The invention is therefore based on the object of a continuous Specify process because the manufacture of spherical crystallized nitroguanidine enables.  

Surprisingly, it has now been found that the entry Problem solved by the features of the claim will.

By using this method according to the invention continuous operation possible. The cooled solution (Nucleation phase) is this from the outlet of the cooler in a separate device for crystallization (growth phase) passed into a stirring cascade. From there then the mother liquor, e.g. B. by means of a wet screening machine, separated and back to concentration to the cooling device (Nucleation phase).

This process is particularly suitable for the production of spherulitic crystallites. Training closed Spherolites are made in compact form (spherical) only in the metastable area of a solution (primary area of nucleation). If nucleation goes into the secondary area, The number of germs increases like an avalanche and some form or even completely open spherulites, so-called Sea urchin.

One way to keep the number of germs low is e.g. B. the low mechanical excitation of the solution the lowest possible specific stirring power during cooling or the addition of a limited number of bacteria.

A second variant (EP 002 740) for suppressing the transition from primary to secondary nucleation is slow Cooling of the hot solution due to high temperature gradients (e.g. on the cooling wall surface: crust formation) the nucleation rate and thus increase the number of germs. At the same time, the specific stirring energy must be limited stay.  

The cooling rate of a stirred tank content becomes also limited by the heat transfer on the wall (the volume-related Jacket cooling surface falls with increasing container volume) and high cooling rate increasingly requires higher wall temperature gradient, which increases the risk Germ count and crust formation increase.

Leave small volumes (flow streams with a small cross-section) contrast, with a relatively low temperature gradient (cooling wall and flow boundary layer) with short dwell times in Cool down the cooler very quickly, e.g. B. in the snake cooler long cooling pipe section. Nevertheless occur in such Cooling tube high temperature gradient in the inlet to the cooling tube that can lead to crust formation (narrowing of the cross-section and pipe closure) and also the number of bacteria in the Increase the flow current quickly and impermissibly.

The spiral tube with external cooling medium offers here surprisingly to cool a solution quickly but "gently", without impermissibly increasing the number of germs, whereby Cooling speeds of several hundred degrees per minute can be achieved without the solution in the area secondary nucleation. Solution and cooling medium to be cooled are guided in counterflow (e.g. concentric Double tube).

However, another essential one is missing in the straight double tube Property for gentle, targeted germ excitation, turbulence (Cross-mixing). In the spiral pipe (with appropriate Winding diameter) occurs due to centrifugal forces a cross flow to the direction of flow (pipe axis) leading to circulation in the pipe cross-section. This will make it Inside the pipe flow (hot core) with and constantly with the pipe flow near the wall (cold boundary layer) back mixed.  Therefore, only small solutions occur in the flowing medium Temperature gradient on, d. H. it will be in "small steps" cooled down. The cross circulation in the spiral tube also works "like a stirrer", with low stirring energy in small Volume; the flow volume is constant and homogeneously turbulent. In contrast, there is always a wide one in the mixing vessel Turbulence distribution before, very high turbulence near the stirring blade, very little turbulence on the tank wall.

The uniform turbulence of the spiral tube flow causes therefore a narrow grain distribution of the germs (nucleation phase) and thus the product grain, since the germs that form in Spiral tube arise under constant flow conditions and grow evenly, what's in the mixing vessel with increasing Volume becomes less favorable (increasing turbulence differences Center / near the wall).

The spiral tube cooler thus gets sufficiently and approximately germs of the same size in the growth phase (stirring cascade). It is obvious that the turbulence strength (cross-mixing) in the spiral tube is determined by the spiral diameter, Pipe cross-section and flow velocity (axial) in the pipe; the latter, in particular, determine throughput and cooling rate and solution exit temperature.

As is well known, higher heat transfer coefficients are also found in the spiral tube reached than in a straight tube, also a consequence the cross circulation by centrifugal forces. Besides, is the surface / volume ratio for fast heat exchange inexpensive and large pipe lengths (large spec. Heat exchange surfaces) housed in the smallest construction volume will.  

Strong flow diversions, such as B. 90 ° angles are because to avoid the occurring pressure gradient (danger increased nucleation), which is why axial entry and exit of the Spiral is cheap.

Such a cooler ( 1 ) is shown in Fig. 1. Therein means ( 2 ) the cooling medium inlet and outlet, ( 3 ) the entrance of the hot solution and ( 4 ) the exit of the cooled solution with the nuclei.

Several spirals (of the same length) can be operated in parallel throughput, which multiplies the throughput. Also a technical design with layers wound on top of each other Pipes of the same length (but different pitch) is known in deep-freeze technology (spiral tube bundle heat exchanger).

Small pipe cross sections are also used here, to achieve high cooling rates economically, d. H. at not too high temperature gradients.

Here, however, the spiral tube flow comes another one Meaning too. Apart from higher heat transfer coefficients in the spiral tube (compared to straight tubes), as a result Cross circulation perpendicular to the pipe axis (flow direction), caused by centrifugal forces in the spiral flow this cross circulation is a "uniform" turbulence in the cooling (nucleation) phase.

• - In contrast, there is always a wide turbulence distribution in the stirred tank present, leading to a wide range of grain distributions the germ leads.

In contrast, there is a narrow particle size distribution in the spiral tube or germ cluster distribution. The same applies to yourself forming Solvathülle the germ cluster, which as known play a crucial role in spherulitic Play crystal growth.

The dwell time in the pre-cooler is also crucial, since the distribution of the relaxing solvate shells (Relaxation time spectrum) and thus crystal growth determined in the subsequent phase.

There are also spiral heat exchangers with flat, flat flow cross-section, but practically no cross-circulation generate, but only slight shear forces (external / Inner boundary layer). The spiral diameter is included large and steadily increasing (Archimedes spiral). Cooling medium and hot solution can also be countercurrent (cooling and heating gap in layers one above the other).  

Example 1

In a glass spiral tube with an external cooling jacket (Pipe diameter 4 mm inside, pipe length approx. 6 m, spiral diameter approx. 30 mm outside, cooler length approx. 700 mm) was approx. 1.2 l hot DMF / nitroguanidine solution (80 ° C satur.) with 8% water addition in a run from 95 ° C to different outlet temperatures cooled (nucleation phase) and in two downstream Mixing vessels (stirring cascade) of approx. 0.5 l each Content 30 min at constant temperature (growth phase) stirred (special stirring power = 0.1 w / kg, see DE 35 27 200 A1. The throughput time of 1.2 l solution was approx. 9 min, which is one Flow velocity of 0.18 m / s or a dwell time of 33.5 s in the cooler. When cooling from 95 ° C to 20 ° C this means z. B. an average cooling rate of 134 / min (throughput approx. 7 l / h).

The tests were carried out with the same pre-pressure (approx. 0.5 m liquid column) driven, what is the throughput speed varied depending on the cooling temperature in the spiral tube cooler (Viscosity of the solution depends on the temperature). Higher Throughputs are possible at higher forms, whereby of course the achievable outlet temperature of the solution changes.

Tab. 1 shows the results of the average grain size, the Bulk density or tapping density of the product grain and the product yield depending on the pre-cooling temperature (radiator outlet temperature d. Solution) and growth temperature (temperature i. d. Stirring cascade).

• - The product grain size increases with increasing pre-cooling (falling nucleation temperature) at all growth temperatures from.
• - The product grain is at high cooler outlet temperatures coarser than at low outlet temperatures (Pre-cooling).
• - The bulk or knock density of the product grain is only at certain combination of pre-cooling and growth temperature optimal (maximum).

Example 2

Instead of a glass spiral tube cooler, a stainless steel Spiral tube cooler of the same dimensions as in example 1 used. The higher thermal conductivity of stainless steel leads to a different temperature profile over the Pipe length. The corresponding final temperature of the pre-cooling is reached earlier in the stainless steel tube than in Glass tube.

On the other hand, the dwell time also increases in the stainless steel pipe the solution at the pre-cooling end temperature with falling Pre-cooling temperature (greater temperature gradient). this leads to to a shift in the optimal test conditions (Temperature control pre-cooling and growth) for spherulitic Grain growth, s. Tab. 2.

Example 3

Example 3 shows the results for different ones Additional quantities (e.g. water) in a spiral tube of 6 mm Inside diameter, otherwise the same dimensions. The temperature was controlled isothermally here (nucleation temperature = growth temperature), s. Tab. 3. It turns out that in addition to optimal temperature control the additional quantity must be optimized.  

Higher throughput speeds result for larger spiral pipes and higher cooling rates, unless this is limited by the heat transfer, d. H. extremely high temperature gradient between solution and Cooling medium required to have the same outlet temperature to achieve (example 1 to 3). As is known, the co-determining heat transfer takes Boundary layer thickness of a pipe flow with increasing Flow rate.

For example occurs in a glass spiral tube with an inner diameter of 10 mm a throughput speed of 0.4 m / s or one Dwell time of 13.2 s in the cooler of the same length at otherwise the same arrangement as described above. When cooling from Z. B. 95 ° C to 20 ° C, this means a medium Cooling rate of 340 ° / min (throughput approx. 100 l / h).  

Table 1

Experiments in the glass spiral tube cooler (spiral tube 4 mm ⌀), addition: 8% water

Table 2

Experiments in the stainless steel spiral tube cooler (spiral tube 4 mm ⌀), addition: 8% water

Table 3

Experiments in the stainless steel spiral tube cooler (spiral tube 6 mm ⌀), addition: 6, 8 and 12% water

Claims (1)

Continuous process for the production of spherically crystallized nitroguanidine, characterized in that

• a) crystal nuclei are formed in a spiral tube cooler with external cooling medium from a hot, saturated solution of nitroguanidine and
• b) the further crystallization takes place in a subsequent stirred cascade in a manner known per se.