PROCESS FOR PROVIDING A HOMOGENOUS SLURRY CONTAINING PARTICLES

Abstract

The present invention is concerned with a process for providing a homogeneous particle-containing slurry comprising the steps of: (a) providing a vessel comprising at least one impeller rotating around a vertical axis of the vessel, wherein a rotational speed n.sub.1 of the at least one impeller is higher than n.sub.min according to equation (1), the vessel further comprising an inlet and an outlet; (b) introducing a particle-containing slurry into the vessel or introducing components forming the particle-containing slurry into the vessel; (c) rotating the at least one impeller at least around the vertical axis for homogenizing and/or maintaining a homogeneous particle distribution within the slurry; (d) withdrawing the homogeneous particle-containing slurry via the outlet; (e) reducing the rotational speed n.sub.1 of the at least one impeller to a reduced rotational speed n.sub.red, whereas n.sub.red is lower than n.sub.1 and higher or equal gas inlet than n.sub.min according to equation (1):

[00001]$\begin{array}{cc}{n}_{\min }=\frac{S{v}^{0.1}{D_p^{0.2}\left(g\frac{\Delta \rho }{{\rho }_f}\right)}^{0.45}{B}^{0.13}}{D_a^{0.85}}& \left(1\right)\end{array}$

Claims

1. Process for providing a homogeneous particle-containing slurry comprising the steps of: (a) providing a vessel comprising at least one impeller rotating around a vertical axis of the vessel, wherein a rotational speed n.sub.1 of the at least one impeller is higher than n.sub.min according to equation (1), the vessel further comprising an inlet and an outlet; (b) introducing a particle-containing slurry into the vessel or introducing components forming the particle-containing slurry into the vessel; (c) rotating the at least one impeller at least around the vertical axis for homogenizing and/or maintaining a homogeneous particle distribution within the slurry; (d) withdrawing the homogeneous particle-containing slurry via the outlet; (e) reducing the rotational speed n.sub.1 of the at least one impeller to a reduced rotational speed n.sub.red, whereas n.sub.red is lower than n.sub.1 and higher or equal than n.sub.min according to equation (1): $\begin{array}{cc}{n}_{\min }=\frac{S{v}^{0.1}{D_p^{0.2}\left(g\frac{\Delta \rho }{{\rho }_f}\right)}^{0.45}{B}^{0.13}}{D_a^{0.85}}& \left(1\right)\end{array}$ with n.sub.min=minimum rotational speed of the impeller, S=shape factor of the impeller, v=kinematic viscosity, D.sub.p=average particle size in the slurry, g=gravitational constant, Δρ=difference between particle density ρ.sub.p and liquid density ρ.sub.f in the slurry (ρ.sub.p−ρ.sub.f), β.sub.f=density of the liquid phase of the slurry, B=100*solid weight/liquid weight, D.sub.a=diameter of the impeller.

2. The process according to claim 1, wherein in step e) n.sub.red is n.sub.min.

3. Process according to claim 1, wherein the at least one impeller has a vertically upper end with respect to the vertical axis of the at least one impeller and a vertically lower end with respect to the vertical axis of the vessel and wherein the reducing of the rotational speed of the impeller is effected when the level of the slurry during withdrawal is within a range from 0.5*D.sub.a to 0.1*D.sub.a above the vertically upper end of the at least one impeller and is continued until the level of the slurry is within a range from 0.1*D.sub.a to 0.05*D.sub.a below the vertically lower end of the at least one impeller.

4. Process according to claim 1, wherein the vessel comprises more than one impeller at different height levels with respect to the vertical axis of the vessel and wherein the rotational speed n.sub.1 of one or more of said impellers is reduced according to step (e).

5. Process according to claim 1, wherein the rotational speed of each impeller can be reduced independently from the remaining impellers.

6. Process according to claim 1, wherein the at least one impeller extends at least partially into the horizontal plane being orthogonal to said vertical axis and further extends at least partially into the direction of said vertical axis.

7. Process according to claim 1, wherein the process is automatically controlled.

8. Process according to claim 7, wherein the process comprises a control system comprising a controller, a level sensor and an inferential control system.

Description

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0023] The inventors have found a process for providing a homogeneous particle-containing slurry comprising the steps of: [0024] (a) providing a vessel comprising at least one impeller rotating around a vertical axis of the vessel, wherein a rotational speed n.sub.1 of the at least one impeller is higher than n.sub.min according to equation (1), the vessel further comprising an inlet and an outlet; [0025] (b) introducing a particle-containing slurry into the vessel or introducing components forming the particle-containing slurry into the vessel; [0026] (c) rotating the at least one impeller at least around the vertical axis for homogenizing and/or maintaining a homogeneous particle distribution within the slurry; [0027] (d) withdrawing the homogeneous particle-containing slurry via the outlet; (e) reducing the rotational speed n.sub.1 of the at least one impeller to a reduced rotational speed n.sub.red, whereas n.sub.red is lower than n.sub.1 and higher or equal than n.sub.min according to equation (1):

[00003]$\begin{array}{cc}{n}_{\min }=\frac{S{v}^{0.1}{D_p^{0.2}\left(g\frac{\Delta \rho }{{\rho }_f}\right)}^{0.45}{B}^{0.13}}{D_a^{0.85}}& \left(1\right)\end{array}$ [0028] with [0029] n.sub.min=minimum rotational speed of the impeller, [0030] S=shape factor of the impeller, [0031] v=kinematic viscosity, [0032] D.sub.p=average particle size in the slurry, [0033] g=gravitational constant, [0034] Δρ=difference between particle density ρ.sub.p and liquid density ρ.sub.f in the slurry (ρ.sub.p−ρ.sub.f), [0035] ρ.sub.f=density of the liquid phase of the slurry, [0036] B=100*solid weight/liquid weight, [0037] D.sub.a=diameter of the impeller.

[0038] A definition of the shape factor of the impeller S can be found in ‘Comparing Impeller Performance for Solid-Suspension in the Transitional Flow Regime with Newtonian Fluids’, Chem. Eng. Res. Des., Vol. 77(8), November 1999, pp. 721-727.

[0039] Preferably, the reducing is effected when the level of the slurry during withdrawal is close to the vertically upper end of the impeller until the lower end of the impeller with respect to the vertical axis is above the level of the slurry.

[0040] For simplicity reasons, the impeller could also just be stopped. However, giving the fact that pumping the slurry out of the vessel is generally taking place at slow rates, the time of stopping the impeller needed to let the level of the slurry pass is quite high. Hence, the possibility of sedimentation of the particles during stopping of the impeller is increased. This is in particular relevant, if there are exceptionally big particles that might be accidentally formed.

[0041] Furthermore, the minimum rotational speed that ensures that no sedimentation will take place can be determined using the Zwietering correlation (equation (1)), which is described e.g. in ‘Unit Operations of Chemical Engineering’, McCabe, W., Smith, J., Harriott, P., and Mcgraw-Hill, 1993. p. 265.

[0042] This invention discloses a process for operating a continuously stirred tank reactor, e.g. feeding vessels, resulting in minimum or even elimination of splashing of slurry on the wall of the vessel. This is achieved by the process as described above, in which the rotational speed of the impeller is reduced to a minimum value. Such a process has the advantage of ensuring minimum material accumulation on the wall and consequently avoiding blockage of the vessel outlet or failure of the process in general. Moreover, this process ensures that the quality of the slurry is not deteriorated because of particles segregation and sedimentation. In particular this process further suits operating strategies that require slurry withdrawal over an extended period of time.

[0043] It is particularly preferable to reduce the rotational speed of the impeller when the level of the slurry is close to the vertically upper end of the impeller with respect to the vertical axis of the impeller. This ensures that the reducing of the rotational speed of the impeller happens when the splashing would occur. Such a process additionally allows for improved homogenization at reduced splashing.

[0044] Even more preferably, in the process according to the present invention the reducing of the rotational speed of the impeller is effected when the level of the slurry during withdrawal is within a range from 0.5*D.sub.a (diameter of the impeller) above the vertically upper end of the impeller with respect to the vertical axis of the impeller. Furthermore, it is preferred that the reducing is stopped when the level of the slurry is 0.05*D.sub.a below the vertically lower end of the impeller with respect to the vertical axis of the impeller.

[0045] The process of the present invention generally works with vessels having any dimensions.

[0046] An impeller as comprised in the vessel according to the present invention preferably extends at least partially into the horizontal plane being orthogonal to said vertical axis and further extends at least partially into the direction of said vertical axis. Thereby an up- or down-pumping effect can be achieved depending on the orientation of the horizontal plane alongside the direction of the vertical axis. If the plane orientates alongside the direction of the vertical axis towards the upper part of the vessel, an up-pumping impeller is provided. On the other hand, if the plane orientates alongside the direction of the vertical axis towards the lower part of the vessel, a down-pumping impeller is provided. In the process according to the present invention, down-pumping impellers are preferred.

[0047] The vessel in the process according to the present invention can comprise only one impeller or preferably more than one impeller at different levels of height with respect to the vertical axis of the vessel.

[0048] If more than one impeller is comprised by the vessel, these impellers can be connected to the rotatable vertical axis in a way that all connected impellers have the same rotational speed. This might be achieved e.g. in that there is only one rotatable axis comprised in the vessel. In such an embodiment, all impellers will have reduced rotational speed if one impeller has reduced rotational speed to avoid splashing of the slurry.

[0049] In another even more preferred embodiment more than one impeller can be present in the vessel which all can be driven independently of each other by their own rotational speed. Each of their axes could be driven by separate engines allowing for individual rotational speeds of the impellers. In such an embodiment, only one impeller could have reduced rotational speed to avoid splashing of the slurry, while the other impellers still maintain their original rotational speed. Preferably, in such an embodiment, the reducing of the rotational speed of only one impeller is effected when the level of the slurry during withdrawal is within a range from 0.5*D.sub.a (diameter of the impeller) above the vertically upper end of said impeller to 0.05*D.sub.a below the vertically lower end of said impeller with respect to the vertical axis of said impeller.

[0050] In the process according to the present invention in step (e) the rotational speed of the impeller is reduced to the minimum rotational speed n.sub.min according to equation (1). This process is in particular useful for lower slurry withdrawal speeds. It ensures that the time for the slurry surface to pass the level of the impeller can be arbitrarily chosen. Hence, also embodiments with very slow slurry withdrawal can be processed. Furthermore, it might be the case in some embodiment that the slurry also rises again or stays at the level of the impeller for an unpredicted longer time. Also in these situations the second process ensures significantly reduced splashing occurrence.

EXAMPLES

Example 1

[0051] This example is implementing the minimum rotational speed n.sub.min without causing sedimentation of the particles in the vessel. The minimum rotational speed is estimated using the Zwietering correlation (equation (1)). In this case, the minimum rotational speed has been calculated for different particles diameters as shown in Table 1 below.

TABLE-US-00001 TABLE 1 Calculated minimum rotational speeds for different particle diameters for a shape factor S of 6.13, a kinematic viscosity v of 2.1 × 10.sup.−5 m.sup.2/s, a density of the liquid phase of the slurry ρ.sub.f of 907 kg/m.sup.3, a density of the particles of the slurry ρ.sub.p of 1300 kg/m.sup.3, a solid weight/liquid weight ratio B of 30%, and a diameter of the impeller D.sub.a of 0.5 m. D.sub.p [μm] n.sub.min [rpm] 70 62 80 63 90 65 100 65

[0052] Considering e.g. an average particle diameter in the slurry of 90 μm, the minimum rotational speed required to prevent sedimentation is 65 rpm. As shown in FIG. 2, in a first operational mode the respective minimum rotational speed can be adjusted at a time before the level of slurry reaches the upper level of the first impeller in vertical direction. This minimum rotational speed is maintained until the level of slurry has passed the lower level of the last impeller in vertical direction.

[0053] Such an operational mode is in particular advantageous if the observation of the level of slurry is difficult and/or generally the process should be run in a very simplistic manner.

[0054] FIG. 3 shows an operational mode, in which the reduction of rotational speed is reverted, if the level of slurry is in between two impellers. This Figure further shows the decrease of the level of the slurry inside the vessel during withdrawal of the feeding vessel at a slow rate. In such an operational mode the homogenization is further improved while withdrawing the slurry from the reactor. Moreover, the risk of sedimentation is further reduced. Hence, this operational mode ensures both minimum sedimentation and minimum splashing when the level is close to the impellers.

[0055] The best operational mode is achieved if the rotational speed is reduced when the level of the slurry is 0.5*D.sub.a (diameter of the impeller) above the vertically upper end of the impeller and 0.05*D.sub.a below above the vertically lower end of the impeller. If the rotational speed is reduced at levels of the slurry of <0.5*D.sub.a above the vertically upper end of the impeller and/or again raised at levels of the slurry of <0.05 D.sub.a below above the vertically lower end of the impeller, splashing is not completely avoided. On the other hand, if the rotational speed is reduced at levels of the slurry of >0.5*D.sub.a above the vertically upper end of the impeller and/or again raised at levels of the slurry of >0.05 D.sub.a below above the vertically lower end of the impeller, the overall operation is significantly slowed down and it even might result in particles sedimentation.

[0056] Such an operation mode is especially advantageous in case of vessels having more than one impeller, all of which are connected to the vertical axis and, hence, cannot be driven with differing rotational speeds. In cases where each impeller can be driven at its own rotational speed it is also the best mode to achieve the reducing of the rotational speed for each impeller in said interval.

Example 2

[0057] Manual operation of the proposed methodology can have negative effects on the efficiency of the process. Such negative effects could arise e.g. from variation in withdrawal speed of the slurry or even unexpected stops of the withdrawal. In such cases, the reducing of the rotation speeds of the impeller must be adapted. Thus, it is preferred to apply a controlled process providing a homogeneous particle-containing slurry.

[0058] Such a controlled process is depicted in FIG. 4 including a control system comprising: (a) a level sensor, e.g. ultrasound level sensor; (b) an inferential control system to estimate the set point for the controller and (c) a controller, e.g. proportional-integral-derivative controller (PID), to regulate the rotational speed of the impeller.

[0059] From system properties such as average particles size, viscosity, density and stirrer design parameters the minimum rotational speed is determined using the Zwietering correlation (equation (1)) and sent to the controller as a set-point for motor rotational speed. During operation of the vessel, the level of the slurry inside the vessel is measured by the level sensor and also sent to the controller.

[0060] The controller automatically enables reduced rotational speed of the impeller, which is close to the level of the slurry, if the slurry passes a certain predetermined trigger point monitored by the level sensor. Hence, at normal operation, the process follows a pre-defined pattern as that depicted in FIG. 3.

[0061] However, if a disturbance affects the process, the control algorithm adjusts the trigger times of the reducing of the rotational speed of the impeller based on the data provided by the sensors to the control system. In FIG. 5 a case is depicted, where a sudden stop of withdrawal of the slurry happens (between 45 and 60 min). The control system reacts and shifts the reduction of the impeller rotational speed by the time delay needed to re-establish the normal withdrawal rate.

[0062] Most of e.g. preparations of polymerization catalysts are long-time processes. Hence, running these processes manually would be cost intensive and prone to errors. Automating the variation of rotational speed modification is vital to ensure efficient operation over a long time. Moreover, automation can further be advantageously used to gather information about the operability of the system. Such information could be used to further optimize the process.