Use of glow discharge in fluidized beds

Static charges and agglomeration of particles in a fluidized bed system are minimized by maintaining in at least part of the bed a radio frequency glow discharge. This approach is eminently suitable for processes in which the conventional charge removing agents, i.e., moisture or conductive particle coatings, cannot be used. The technique is applied here to the disproportionation of calcium peroxide diperoxyhydrate to yield calcium superoxide, an exceptionally water and heat sensitive reaction.

TECHNICAL FIELD 
This invention relates to the elimination of static charges in fluidized 
bed processes, and more particularly, in those where the introduction of 
moisture or of extraneous conductive particles is not desirable. 
THE PRIOR ART 
The problem of electrostatic charge in a fluidized bed is widely recognized 
and various approaches have been tried or suggested in order to eliminate 
the undesirable effects of this phenomenon. Yet, only two of the methods 
mentioned in the art seem reasonably effective in eliminating 
agglomeration, one of the effects of static charge on non-conductive 
fluidized powders with particle size ranges suitable for the types of 
chemical reactions commonly carried out in fluidized bed systems. These 
particular methods are (1) the use of high humidity in the fluidizing gas 
stream and (2) the addition of an agent, such as graphite, which coats the 
particles with a conductive surface layer [Bafrnec and Bena, Chem. Eng. 
Sci, 27, pp. 1177-81 (1972)]. The use of grounded and conductive walls in 
the apparatus has not seemed very effective [Shikov et al, Khim prom. 9, 
No. 1, pp. 57-9 (1977)], and neither have experiments with electron 
bombardment of particles agglomerated by electrostatic charges [Ithara, M. 
S. thesis, Dept. Chem. Eng., M.I.T. (1960)]. As to the application of high 
potential electric fields to various fluidized beds, e.g., glass beads, 
alumina powder, etc., it has produced agglomeration of the particles, 
presumably due to the enhancement of van der Waal forces with "surface 
polarization charges" and "induced mutual attraction of particles." Free 
charges from corona and associated effects were assumed to be negligible 
[Katz and Sears, Can. J. Chem. Eng. 47, pp. 50-53 (1969); U.S. Pat. No. 
3,304,249]. 
Various types of radiation have been used to control or eliminate 
undesirable electrostatic charges on flat surfaces and in flowing liquids. 
For instance, U.S. Pat. No. 4,057,071 discloses an electrostatic charge 
remover wherein the charge produced by a non-conductive liquid flowing 
through a pipeline is reduced by gamma radiation. Okamoto et al, U.S. Pat. 
No. 3,851,230, remove residual static charge in a transfer printing 
process by exposing the appropriate surface to light. In U.S. Pat. No. 
3,863,108, on the other hand, there is disclosed the control electric 
charge density on a surface, paper or thin polymer sheets, by means of an 
ion current generated by a corona discharge. Neither the method nor the 
apparatus involved, designed as they are for the reduction of charge on 
fast moving sheets of non-conductive material, can be applied to the 
reduction of powders in a flowing gas stream. In particular, no way is 
suggested to sense the surface charge and apply a bias voltage to 
particulate agglomerates, as the technique requires. 
As to methods that have been described earlier as somewhat effective to 
reduce charges in fluidized beds, it should be kept in mind that there are 
numerous conventional treatments or uses of powders in which either 
contact with a humid gas stream or addition of particle coating material 
would be deleterious to the objective to be achieved by the fluidization 
process. 
The principal object of the present invention, therefore, is to eliminate 
static charge agglomeration of particles suspended in a flowing gas stream 
for the purpose of undergoing or causing chemical or physical reactions, 
especially in situations where the gas stream functions as a means of 
thermal control and as a vehicle to convey vapor reactants or products to 
or away from the surface of the suspended particles. 
STATEMENT OF THE INVENTION 
The objects of this invention have been accomplished by carrying out a 
fluidized bed process in a fluidization apparatus equipped with radio 
frequency (RF) glow discharge or "plasma" generating components with 
adequate power to maintain a glow discharge in the fluidization chamber. 
During operation, a glow discharge is induced in the flowing gas which 
becomes a conductive medium capable of dissipating the charges on the 
suspended particles that cause the undesirable agglomeration.

DETAILED DESCRIPTION OF THE INVENTION 
The removal of electric charges from solid particles suspended in a 
fluidized bed can be accomplished with a variety of apparatus that can be 
selected to accommodate the particular chemical or physical process that 
is being carried out. For the purpose of the present disclosure, however, 
the invention shall be described in terms of the preparation of calcium 
superoxide by the disproportionation of calcium peroxide diperoxyhydrate. 
Although both process and equipment shall involve parameters and design 
features intended to optimize this disporportionation reaction, it should 
be remembered that other reactions which can benefit from this fluidized 
bed technique may require adaptation of the present apparatus and 
technique that remain within the principles of the invention as claimed. 
With this in mind, the apparatus used shall now be described in detail. 
An embodiment of a useful apparatus employed for the disproportionation of 
calcium peroxide diperoxyhydrate is illustrated schematically in the 
drawing. It consists of three main parts: a funnel-shaped reaction chamber 
11, a small sample-transfer chamber 14, and a powder-knockout chamber 26. 
The reaction chamber 11 is connected to the sample-transfer chamber 14 by 
attaching means 12 and the assembly is provided with porous plates, e.g., 
fritted glass plates 15 and 16. The scale of the drawing is too small to 
show the porous nature of the plates. The assembled vessels 11, 14 and 26 
are evacuated by means of a cryopump and a mechanical pump (not shown) 
through conduits 24 and 28, and bellows seal valve 25. Thermocouple probe 
41 with tip submerged in fluidized solid particles 17 on plate 16, is 
connected via lead 42 to a reference junction and a temperature recorder 
(not shown). During operations, dry nitrogen gas is lead to chamber 11 
through heat transfer coil 55, valve 53, and conduit 51. The gas then 
passes through porous plate 16 t create the turbulence necessary to 
suspend the particles of diperoxyhydrate and form a fluidized bed (17). 
The configuration of chamber 11 in terms of increasing internal diameters 
in an upward direction, is such that the velocity of the nitrogen stream 
is sufficiently decreased as it moves upwards so that a very small 
fraction of the suspended particles will be entrained into the 
powder-knockout chamber 26. The powder particles that reach chamber 26 
will hit baffle 27 and settle on the floor of the chamber. Another gas 
conduit 50 with valve 52 and heat transfer coil 54 may be provided to 
deliver a minor portion of nitrogen into chamber 11 above porous plate 16. 
The lower portion of chamber 11 is submerged in cooling bath 18 which is 
generally maintained at a temperature of 20.degree. C. A pressure port 21 
is connected to a pressure gauge (not shown) through valve 22. Finally, 
transfer chamber 14 is connected to a nitrogen purge line 19 closed by 
valve 20. 
The RF power necessary to carry out the neutralization of particle charges 
is supplied by an RF generator 30 connected by lead 31 to high voltage 
electrode 13 and grounded at 32. A power meter (not shown) and an 
impedance matching network (not shown) complete the RF generating system. 
The high voltage electrode used here has the form of an o-ring joint clamp 
13 which also holds the upper and lower sections of reaction vessel 11 
together. Liquid 18 in bath 19 serves as the ground electrode and is 
connected to earth by lead 33. 
When operating, the level of liquid 18 in cooling bath 19 is kept just 
above porous plate 16 in the lower part of reactor 11. When RF is used, 
nitrogen gas is introduced only through inlet 51, inlet 50 being closed by 
valve 52. 
In other runs that have not proved successful, a Tesla coil (not shown) has 
been used to ionize some nitrogen introduced through inlet 50, with the 
bulk of the nitrogen still being introduced to lower inlet 51. 
The generation of an RF glow discharge can be accomplished by any 
commercial or privately made RF glow discharge or "plasma" generating 
equipment that has adequate power to bring about a glow discharge in the 
fluidization chamber. The electrode configuration which introduces the RF 
power to the chamber can also differ significantly from the arrangement 
shown in the drawing. One possibility involves a "glass cross" reaction 
chamber, as shown in FIG. 1 of U.S. Pat. No. 4,101,644, in which flat 
circular or rectangular electrodes are spaced in capacitative relationship 
and placed in such a manner that part of the fluidized bed is between said 
electrodes. 
In addition to having a "glass cross" or a funnel shape, the reaction 
chamber may be cylindrical or have any other geometrical configuration 
that will allow a powder to be suspended in a flowing stream of gas. Size 
is not critical, but for a given process operation, practical limits are 
set by the volume of flowing gas needed and the size and power of the RF 
generator necessary to bring about a glow discharge in the given reaction 
chamber. It should also be noted here that although the terms 
"fluidization" and "fluidized bed" are used in describing the preparations 
disclosed in this specification, such preparations involved a partially 
fluidized bed, with much of the powder suspended in the gas stream. In any 
event, the use of RF discharge to reduce static agglomeration of small 
particles is applicable to the entire range of conditions in which the 
small particles are out of contact with each other part or all of the time 
due to buoyant effect of a flowing gas stream. 
Although the RF power employed in the disporportionation of calcium 
peroxide diperoxyhydrate has been at the level of about 5 watts, higher 
power levels may be more effective in preventing agglomeration, especially 
in processes carried out at higher pressures. For the present application, 
it was desired to keep the particles isothermal at a temperature 
controlled by the temperature and flowrate of the incoming gas. 
The pressure of the flowing gas in the process used for illustrative 
purposes was in the range of about 0.5 to 1.5 mm Hg. However, the RF glow 
discharge may be applied to suspended or fluidized particulate beds at 
higher pressures, up to, at best, atmospheric pressure. A pulsed discharge 
generator capable of supplying the power for an Rf glow discharge at 
atmospheric pressure has been described elsewhere [Donohoe et al., I & EC 
Fundamentals 16, No. 2, pp. 208-215 (1977)]. 
Any number of gases may be employed fo the particle bed fluidization 
process involving an RF glow discharge minimization or elimination of 
static agglomeration of suspended particles. For example, relatively inert 
gases such as helium, neon, and argon are suitable flow media. More 
reactive gasses may be employed for particular purposes, especially when a 
reaction between the dispersed powder and the gaseous species is 
anticipated or desirable. A partial list of such gasses includes oxygen, 
hydrocarbons, substituted hydrocarbons, metal vapors, metal-organic 
compounds, as well as mixtures of such gases with or without other gases. 
Gaseous streams may contain water vapor in the lower humidity ranges below 
those effective in alleviating agglomeration due to static charges, 
provided that the water vapor either does not harm the process or is 
desirable as a reactant. Higher humidity levels, where usable, preempt the 
use of RF glow discharge solely for the purpose of preventing static 
agglomeration of suspended particles. The selection of a fluid gas media 
should also ensure that the equilibrium vapor pressure of the flowing gas 
species be high enough at the temperature of the fluidizing chamber that a 
pressure can be maintained sufficient to support the suspended particles 
by the momentum from collisions with the flowing stream, high enough also 
the allow the initiation of the glow discharge, and high enough finally to 
preclude undesirable condensation. However, mixtures of gases and vapors 
may be employed to meet these operating requirements by increasing the 
total pressure of the fluid gas media and decreasing the partial pressure 
of a component. 
As mentioned earlier, examples of the process of the invention shall now be 
given to illustrate the advantages of the use of an RF glow discharge in a 
fluidized bed operation. The examples all deal with the disproportionation 
of calcium peroxide diperoxyhydrate to yield calcium superoxide according 
to the following equation: 
EQU 2 CaO.sub.2 2H.sub.2 O.sub.2 .fwdarw.Ca(OH).sub.2 +Ca(O.sub.2).sub.2 
+3/2O.sub.2 +3H.sub.2 O 
This process has been disclosed in great detail in U.S. Pat. No. 4,101,644, 
which is incorporated into the present specification by reference. 
Scale-up of the patented process by means of the fluidized bed technique 
gave rise to various difficulties which have been solved by the 
application of an RF glow discharge to the system. 
EXAMPLES 1 to 7 
Several disproportionation runs were made with calcium peroxide 
diperoxyhydrate using the apparatus described in the drawing. In Example 1 
, no RF power nor high voltage field was used. In Examples 2 and 3, a 
Tesla coil was brought into contact with inlet 50 through which a minor 
proportion of the nitrogen, i.e., about 30%, was introduced into reactor 
11. In the last four runs, Examples 4 to 7, all the nitrogen was 
introduced through inlet 51 and then subjected to a 5 W 13.56 MHz RF glow 
discharge by passage through activated electrode 13. Other variables of 
lesser significance for the purpose of this invention are noted in the 
following table along with the results obtained for the runs described. 
All runs were carried out at 20.degree. C. for 3 hours, with nitrogen used 
as the suspending gas. When RF power was applied, it was turned on 
immediately after starting the nitrogen flow and turned off after stopping 
the nitrogen flow. 
__________________________________________________________________________ 
CONDITIONS AND RESULTS FOR DISPROPORTIONATION OF 
CaO.sub.2 . 2H.sub.2 O.sub.2 IN REACTION CHAMBER SHOWN IN DRAWING 
Linear Gas.sup.1 
Gas Particle 
Reactant 
Product 
RF Glow 
Flow Velocity 
Pressure 
Diameter 
Loading 
Purity 
Example 
Discharge 
cm/sec mm Hg 
mm g/cm.sup.2 
% Ca(O.sub.2).sub.2 
__________________________________________________________________________ 
1 none 310 1.0 .ltoreq.0.30 
2.1 .times. 10.sup.-2 
.sup.3 
2 none.sup.2 
206 2.0 0.15 -0.30 
4.1 .times. 10.sup.-2 
55.6 
3 none.sup.2 
260 3.0 0.15-0.30 
2.3 .times. 10.sup.-2 
.sup.3 
4 5W 186 1.0 0.30-0.50 
5.6 .times. 10.sup.-2 
63.5 
5 5W 186 1.0 0.30-0.50 
1.0 .times. 10.sup.-1 
59.9 
6 5W 186 0.5 0.30-0.50 
1.0 .times. 10.sup.-1 
55.8 
7 5W 186 1.5 0.30-0.50 
1.0 .times. 10.sup.-1 
57.1 
__________________________________________________________________________ 
.sup.1 Average linear velocity based on crosssectional area of plate 16. 
.sup.2 Tesla coil used on inlet 50. 
.sup.3 No product collected for analysis due to agglomeration and adhesio 
to reactor walls. 
In a previous disporportionation process which does not comprise any 
fluidization, RF, or Tesla component, a maximum purity of 67% 
CA(O.sub.2).sub.2 could be obtained with a flowing nitrogen stream through 
a bed of reactant on a fritted glass disc at initial reactant loading of 
about 9.times.10.sup.-3 gram per square centimeter of disc surface U.S. 
Pat. No. 4,101,644). When fluidization of the bed of reactant was 
attempted by increasing the nitrogen flow and the reactant loading in 
order to scale up the disporportionation process, the reaction observed 
was vigorous and, as indicated by the results of Example 1 in the table, 
no sample could be obtained for analysis because the material remained 
stuck to the reactor walls. As examples 2 and 3 have shown, attempts to 
use a Tesla coil to prevent agglomeration of particles and adherence of 
the products to the walls of the reactor did not succeed, although in one 
case (Example 2) sufficient material could be obtained for analysis. When 
an RF glow discharge was introduced into the system, however, the clumps 
of powder broke up and the particles became completely suspended into the 
medium. At the end of the reaction period, all the product could be easily 
moved to the transfer chamber and it did not appear to be statically 
charged. On analysis, the product showed a purity of 63.5% (Exmple 4) and 
this for a reactant loading 6 times as large as the best run of U.S. at. 
No. 4,101,644, i.e., 5.6.times.10.sup.-2 as apposed to 9.times.10.sup.-3 
g/cm.sup.2. Examples 5 to 7 illustrate the effect of variations in reactor 
gas pressures at a higher loading, namely 1.0.times.10.sup.-1 g/cm.sup.2. 
In all these cases, no particle agglomeration was observed and all the 
product could be recovered. 
To summarize these results, it has been established that the application of 
an RF glow discharge to a very heat and water sensitive chemical process 
carried out in a fluidized bed of reactant has eliminated the static 
charges that cause detrimental agglomeration of particles and adherence of 
reactant and product to the reactor walls. The results also indicate the 
feasibility of further increases in reactant loading, as well as the 
attainment of purity levels equalling and possibly surpassing those 
obtained by the static processes of the art. 
Finally, from the observations made in the course of this work and because 
of the flexibility of the parameters and equipment design that are 
involved in the application of RF power to a reactor, it is believed that 
the man skilled in the art can utilize this technique in other systems, 
including fluidized beds, in which agglomeration due to electrostatic 
charges remains a problem.