Process for continuous supercritical drying of aerogel particles

Abstract

Processes for drying gel particles, in particular for producing aerogels, involve providing a suspension containing gel particles and a solvent, introducing the suspension into a column where carbon dioxide flows in countercurrent, and removing dried aerogel particles from the column. The suspension is introduced in the top region of the column and dried aerogel particles are removed in the lower region. Pressure and temperature in the column are set such that the mixture of carbon dioxide and solvent is virtually supercritical or is supercritical. The aerogel particles can be discharged via discharge vessels or continuous decompression. Aerogel particles can be obtained by such a process and the aerogel particles can be used for medical and pharmaceutical applications, as additive or carrier material for additives for foods, as catalyst support, for cosmetic, hygiene, washing and cleaning applications, for production of sensors, for thermal insulation, or as a core material for VIPs.

Claims

1-9. (canceled)

10: A process for drying gel particles or for producing aerogels, the process comprising: (i) providing a suspension comprising gel particles and a solvent, (ii) introducing the suspension into a column through which carbon dioxide flows in countercurrent, and (iii) removing dried aerogel particles from the column, wherein the suspension is introduced in a top region of the column and the dried aerogel particles are removed in a lower region of the column, wherein the pressure and temperature in the column are set such that the mixture of carbon dioxide and solvent is virtually supercritical or is supercritical, and wherein aerogel particles obtained are removed continuously in a valve-free manner.

11: The process according to claim 10, wherein the gel particles sediment in countercurrent.

12: The process according to claim 10, wherein a CO.sub.2 mass flow rate is set such that dried aerogel particles are obtained.

13: The process according to claim 10, wherein the gel particles have an average diameter in the range from 20 μm to 1000 μm.

14: The process according to claim 10, wherein the gel particles have an average pore diameter in the range from 2 to 100 nm.

15: The process according to claim 10, wherein the solvent is selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol and hexanol.

16: Aerogel particles, obtained by the process according to claim 10.

17: A composition, comprising: the aerogel particles according to claim 16, wherein the composition is a medical or pharmaceutical composition, an additive or carrier material for additives in foods, a catalyst support, a cosmetic composition, a hygiene composition, a washing composition, a cleaning composition, a sensor, thermal insulation, or a core material for VIPs.

Description

EXAMPLES

I. Configuration Examples

[0121] A number of rough calculations and a configuration example are given below. In the following configuration, the operating conditions (120 bar, 50° C.) and the particle properties (particle porosity E=0.93 and tortuosity r=2.5) are assumed to be constant. Hereinafter, the throughput is also assumed to initially be very low, meaning that the fluid phase is approximately described with pure CO.sub.2.

[0122] 1. Calculation of Drying Times [0123] The drying times for particles of various diameters were simulated under the assumption of a one-dimensional mass transfer. The physical variables of the fluid (CO.sub.2 and ethanol) were modeled using the Peng-Robinson equation of state with appropriate mixing rules. The particles are described via a particle porosity of ε=0.93 (corresponding to V.sub.pores=8 cm.sup.3/g) and a tortuosity of r=2.5.

[0124] 1.1 Diffusion in the Particle [0125] The following drying times (table 1) were calculated under the assumption that the rate-determining step was the diffusion within the particle. The mass transfer from the particle to the surrounding fluid phase was assumed to be very large. The drying times for 5 μm-large particles are only a few milliseconds and in the range up to 100 μm are less than a second.

TABLE-US-00001 TABLE 1 Calculated drying times of alginate aerogel particles at 120 bar and 50° C. Particle diameter [μm] 5 25 50 100 200 300 500 calculated drying 0.0019 0.0479 0.1916 0.7664 3.0655 6.897 19.159 time [s]

[0126] 1.2 Consideration of Mass Transfer [0127] The drying times collated in table 2 took the mass transfer from the particle to the surrounding fluid phase into consideration. The mean mass transfer coefficients were calculated on the basis of an Sh correlation for a single sphere. Interestingly, the mass transfer coefficient is a function only of the particle diameter and not of the flow speed, since the relative speed and hence Re is constant and only changes once the particles are discharged.

TABLE-US-00002 TABLE 2 Calculated drying times of alginate aerogel particles at 120 bar and 50° C. taking the mean mass transfer coefficients into consideration Particle diameter [μm] 5 25 50 100 200 300 500 β [m/s] 8.11E−03 2.05E−03 1.40E−03 1.11E−03 9.62E−04 8.98E−04 8.28E−04 calculated drying 0.0028 0.0644 0.2382 0.8765 3.2914 7.255 19.609 time [s] [0128] The relative change in the drying times when taking the mass transfer into consideration compared to the assumption of an infinite mass transfer coefficient is greatest for small particle diameters and becomes smaller as the particle diameter increases. However, there is relatively little change in the absolute drying time and the drying times remain within the same orders of magnitude.

[0129] 2. Discharge of Particles: CO.sub.2 Mass Flow Rate and Particle Diameter [0130] Besides the drying time, another critical aspect of the continuous supercritical drying in a countercurrent column is the fluid dynamics and associated residence time of the particles. The descent velocity of particles can be described via a relationship between the Archimedes number and Reynolds number. For the transition region between the Stokes and Newtonian regions the following applies according to MARTIN:

[00001]$\mathrm{Re}={18\left[\sqrt{1+\frac{1}{9}\sqrt{Ar}-1}\right]}^2$ [0131] The (descent) velocity calculated from the Re number represents the relative speed between particle and surrounding fluid and depends on the particle diameter. Depending on the magnitude of the upwardly directed CO.sub.2 stream, therefore, the absolute descent velocity of the particles is reduced or particles are discharged with the CO.sub.2 stream via the top of the column. [0132] For a column with an internal diameter of d.sub.i=20.57 mm and hence a free cross section of A=3.32E-04 m.sup.2 and a column height of 500 mm, the calculated residence times of alginate aerogel particles of various diameters are collated in table 3 for various CO.sub.2 mass flow rates. This was based on an average apparent density of the particles consisting of wet and completely dried particles.

TABLE-US-00003 TABLE 3 Calculated residence times of alginate aerogel particles with ε = 0.93 CO.sub.2 mass flow Particle diameter [μm] rate [kg/h] 5 25 50 100 200 300 500 0.5 discharged discharged 286.6 73.4 29.2 19.0 12.0 1 discharged discharged 756.5 84.6 30.7 19.6 12.2 1.5 discharged discharged discharged 100.5 32.4 20.3 12.5 2 discharged discharged discharged 125.1 34.3 21.0 12.7 3 discharged discharged discharged 274.2 39.0 22.6 13.3 [0133] The porosity of the aerogel particles has a great influence on the theoretical descent velocity and residence time of the particles in the CO.sub.2 stream. For relatively low porosities, as can be seen from table 4, smaller particles can also be dried without being discharged and/or higher CO.sub.2 flow rates and hence higher particle throughputs can be chosen for the same column height.

TABLE-US-00004 TABLE 4 Calculated residence times of alginate aerogel particles with ε = 0.85 CO.sub.2 mass flow Particle diameter [μm] rate [kg/h] 5 25 50 100 200 300 500 0.5 discharged 971,566186 136.7 44.6 19.5 13.2 8.6 1 discharged discharged 176.5 47.9 20.1 13.4 8.7 1.5 discharged discharged 256.5 51.8 20.7 13.7 8.8 2 discharged discharged 526.6 56.5 21.4 14.0 8.9 3 discharged discharged discharged 69.3 22.9 14.6 9.2

[0134] 3. Influence of the Mass Flow Rate on the Ratio of Required Drying Time/Residence Time [0135] For configuring the column length, the ratio of residence time to drying time should understandably be >1. Table 5 reports the ratios of residence time/drying time for a column height of 500 mm. The particles having a diameter of 500 μm would not be completely dried for the CO.sub.2 mass flow rates shown of 0.5 kg/h to 3 kg/h. This could be counteracted by a further increase in the CO.sub.2 mass flow rate, which would however result in smaller particles being discharged. Alternatively, lengthening of the column height leads to a proportional increase in the residence time and hence to a proportional increase in the residence time/drying time ratio. Doubling the column height to for example 1 m would lead to a doubling of the residence time/drying time ratio, and hence also to a drying of particles having the diameter d=500 μm, without smaller particles being discharged.

TABLE-US-00005 TABLE 5 Ratio of residence time/drying time for 0.5 m column height with an infinite mass transfer coefficient CO.sub.2 mass flow Particle diameter [μm] rate [kg/h] 5 25 50 100 200 300 500 0.5 discharged discharged 1495.89 95.81 9.51 2.76 0.63 1 discharged discharged 3948.27 110.38 10.01 2.84 0.64 1.5 discharged discharged discharged 131.10 10.56 2.94 0.65 2 discharged discharged discharged 163.27 11.18 3.04 0.67 3 discharged discharged discharged 357.75 12.72 3.28 0.69

TABLE-US-00006 TABLE 6 Ratio of residence time/drying time for 1 m column height with an infinite mass transfer coefficient CO.sub.2 mass flow Particle diameter [μm] rate [kg/h] 5 25 50 100 200 300 500 0.5 discharged discharged 2991.77 191.61 19.03 5.51 1.25 1 discharged discharged 7896.54 220.76 20.01 5.69 1.28 1.5 discharged discharged discharged 262.20 21.12 5.88 1.30 2 discharged discharged discharged 326.54 22.37 6.09 1.33 3 discharged discharged discharged 715.50 25.45 6.55 1.39

[0136] 4. Influence of Particle Loading on Drying [0137] In the previous rough calculations, the assumption was made that almost pure CO.sub.2 is also present at the outlet, that is to say that only very low particle loadings are operated with. In industrial application, the aim in the countercurrent operation is to withdraw at the top of the column a CO.sub.2 stream which is as highly loaded with ethanol (EtOH) as possible in order to reduce the CO.sub.2 use per kg of aerogel. For this, the lengthened drying time has to be compensated for by a corresponding increase in the column height. For example, the drying shown in table 6 of 1 l/h of 500 μm particles in a 1 m tall column is not achieved with CO.sub.2 mass flow rates of 1 kg/h and less. (Cf. table 7) With an increase of the column to 3 m, however, drying is also achieved with 1 kg/h of CO.sub.2 and the high outlet proportion by mass of EtOH of 64% (w/w) can be maintained.

TABLE-US-00007 TABLE 7 Drying 1 l/h of 500 μm aerogel particles with different CO.sub.2 mass flow rates in countercurrent in a column with a height of 1 m CO2 mass Proportion by Proportion by flow rate mass of mass of EtOH [kg/h] EtOH in CO.sub.2 [—] in particles [—] 0.50 0.97 0.24 0.75 0.80 0.06 1.00 0.63 0.02 2.00 0.32 0.00 3.00 0.21 0.00 4.00 0.16 0.00

II. Examples

[0138] The principle feasibility of the drying process could be demonstrated on a pilot plant. [0139] A suspension of alginate gel particles having diameters of 50-300 μm (14% (v/v)) in ethanol was conveyed out of a storage vessel at 23 ml/min into the top of a column with a length of 0.5 m and an internal diameter d.sub.i=20.6 mm. At an operating pressure of 120 bar and an operating temperature of 50° C., supercritical CO.sub.2 was conveyed at 40 g/min in countercurrent to the particle stream. The collecting vessel was depressurized after the experiment. [0140] The gel particles sediment counter to the CO.sub.2 stream, while the free ethanol, despite the likewise higher density of the ethanol-CO.sub.2 mixture than CO.sub.2, is discharged via the top in the CO.sub.2-ethanol mixed stream. The comparatively short residence time of the particles compared to a moving bed surprisingly led to complete drying of the aerogel particles. The particles that had fallen into the collecting vessel have, with pore volumes of 9.4 cm.sup.3/g and BET surface areas of 500 m.sup.2/g, similar properties to the aerogel particles produced by the same gelation process and dried in batches.