Microwave drying device and method

Method and device in which a suspension in air or other gas of liquid droplets or fine particles containing an electromagnetic energy absorbent component is subjected to microwave radiation to apply energy to the droplets or particles as they fall to a collection point.

FIELD OF THE INVENTION 
This invention relates to a method and device for drying liquids and 
particulate materials and particularly to a continuous flow process and 
device for applying energy to such material to remove a portion or all of 
a volatile component of the material. 
BACKGROUND OF THE INVENTION 
It is frequently desirable to remove a portion or all of the water or other 
liquid component of a substance in order to stabilize materials associated 
with the liquid. Examples of such processes are lyophilization and spray 
drying. 
The process of lyophilization meets many of the desired goals. However, it 
requires that the sample be first frozen and then lyophilized in a high 
vacuum. See U.S. Pat. Nos. 3,721,725 and 3,932,943. Because of these 
several steps, the process requires considerable time. Also, the resultant 
product suffers from being inhomogeneous due to the uneven freezing and 
the resultant inhomogeneity of dissolved materials recovered from a frozen 
state. In addition, the product of lyophilization is characteristically 
very hygroscopic. 
The process of spray drying requires the evaporation of water from droplets 
of spray by use of high temperature gases. The result is that heat 
sensitive elements in the treated material may become inactivated, 
denatured or hydrolized. The powder resulting from spray drying, however, 
is normally much less hydroscopic than the product of lyophilization. 
BRIEF DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a method and device for 
uniform, rapid, controlled application of energy to liquid and particulate 
material, particularly for removing a portion or all of a volatile 
component from a nonvolatile. 
In the method of the present invention, a suspension in air or other 
gaseous medium of liquid droplets or fine particles containing an 
electromagnetic energy absorbent component is subjected to electromagnetic 
radiation at a frequency strongly absorbed by that component to vaporize 
the droplets or particles as they fall to a collection point. 
Apparatus, according to another aspect of the present invention, for 
carrying out the method includes a radiation chamber, a source for 
directing electromagnetic energy into the chamber and a material supply 
and spray device to introduce the material suspended in air or other gas 
into the chamber in which the components of the apparatus are constructed 
to coordinate the manner and rate of introducing material to be treated 
into the chamber, the geometry of the chamber, the frequency and power of 
the energy source and other components for cooperation to enable effective 
treatment of the material.

DETAILED DESCRIPTION OF THE INVENTION 
In the method of the present invention, the material to be treated is 
pumped at a controlled rate from a supply reservoir 10 and sprayed as a 
suspension in a gas supplied from pipe 12 by a nozzle 14 or other spray 
device in finely divided or mist like form into the upper portion 16 of 
the radiation chamber 18. The sprayed material then falls through the 
chamber 18 where an electromagnetic wave energy absorbent component of the 
material is engergized by electromagnetic wave energy supplied at the 
resonant frequency of the energy absorbent by the wave generator 20. The 
height through which the sprayed material falls in the radiation chamber 
18 provides a falling time determined by the spray conditions and by 
control and direction of additional air or other gas introduced through a 
port 22 sufficient to effect the desired drying when the material reaches 
a collection point 24. 
The method may be used wherever evaporation of a material containing an 
electromagnetic wave energy absorbent component is required. Thus the 
method may be used to remove volatile components or to effect a reaction 
while the particles or droplets are suspended in a gaseous medium. 
A preferred form of the method is its use to evaporate a portion or all of 
a volatile component from a non-volatile component; and the following 
description will relate primarily to that form of the method. 
Solutions, emulsions or dispersions in which water is the energy absorbent 
volatile component are most common and the description will ordinarily 
refer to such component as water. It is to be understood, however, that 
solutions or dispersions of non-volatile material in energy absorbent 
liquids other than water, such as alcohols, aliphatic and aromatic 
compounds, hetrocyclics such as piperidine or pyridine, or halogenated 
compounds like "freon" and other polar liquids are considered within the 
scope of the invention. Alternatively, materials in which the non-volatile 
component is the energy absorbent component and the volatile material is 
not absorbent may be treated in accordance with the present invention. 
Where the sprayed material is droplets of a solution, emulsion or 
suspension of a non-volatile material in a wave energy absorbent volatile 
liquid such as water, the wave energy is absorbed by the liquid and serves 
to increase enormously the energy of the molecules of the liquid. This 
enables the escape from the surface of the droplets as a vapor and reduces 
the liquid content of the droplets. Due to the absorption of the 
electromagnetic radiation at the surface of the droplets, little energy is 
transmitted to the interior of the droplets and drying is effected with 
little or no heating of the dissolved or suspended non-volatile 
components. Also each tiny droplet sprayed is representative of the whole 
of the solution, emulsion or suspension. Accordingly, there is no 
introduction of inhomogeneity into the dried product as a result of the 
drying process. 
Conditions within the system are controlled with respect to the properties 
and requirements of the material to be treated. Thus, for particularly 
heat sensitive materials, the radiation chamber 18 may be operated at 
reduced pressure to cause volatilization at lower temperatures or higher 
rate. And where the material is readily oxidized, the atomizing system and 
the radiation chamber may be supplied with a non oxidizing gas which is 
inert toward the material, e.g. helium or nitrogen. Conversely, the 
atomizing system and the radiation chamber 18 may be supplied with a gas 
reactive with the material under the conditions existing in the chamber. 
For example, the chamber 18 might be filled with hydrogen sulfide to 
introduce sulfhydryl groups into a material such as creatinine phosphate 
kinase when treating solutions of such materials in order to stabilize or 
protect them. 
The apparatus of the present invention enables efficient practice of the 
method through components constructed to give coordination of the rate and 
fineness of dispersion of the material to be treated in the air or other 
gas, the frequency and strength of wave energy input, the geometry of the 
chamber 18 for resonance at the selected wave frequency and other 
conditions of operation with respect to the requirements of the material 
to be processed. 
The reservoir 10, shown as a liquid reservoir, is provided to store 
material to be processed. Suitable temperature control means (not shown) 
may be provided to ensure that the material remains stable prior to 
treatment. A metering pump 26 is connected to withdraw material from the 
reservoir through the pipe 28 and supply it to the atomizing device, shown 
as the nozzle 14, in the radiation chamber 18 at a controlled rate 
determined by the character of the material, the dimensions of the chamber 
18 and the strength of the electromagnetic source 20. Alternatively, the 
material in the reservoir 10 may be under pressure and this pressure may 
be used to drive material from the reservoir through the pipe 28 to the 
atomizing nozzle 14. At the atomizing nozzle 14, the material is mixed for 
spraying with a flow of gas which is provided at constant pressure by use 
of a pressure regulator 30 and adjusted to the proper flow rate by a 
needle valve 32 disposed in the pipe 12 leading from the gas supply to the 
nozzle. 
The atomizing device or nozzle is chosen relative to the dimensions of the 
radiation chamber 18 to disperse the material as droplets or particles 
sufficiently fine to have a falling time in the chamber to enable the wave 
energy to supply the desired vaporization of the volatile material. The 
radiation chamber 18 has a geometry and dimensions resonant to the 
frequency of electromagnetic wave energy applied by the source 20, such as 
a magnetron, through the wave guide or horn 38 and is proportioned such 
that its vertical dimension provides a length of free fall of the sprayed 
material from the nozzle 14 to its collection area 24 which will give a 
falling time of the droplets or particles required for action of the 
radiation required to energize the sprayed material. The energization 
needed is determined by the necessity of avoiding destructive temperature 
rise in the sprayed material and the time required within this temperature 
limitation to effect the desired physical or chemical change. 
Correspondingly, the electromagnetic wave source 20 is operable at a 
frequency strongly absorbed by a component of the material being 
processed. The strength of the wave source is selected to provide the 
necessary energy input to the falling droplets or particles within the 
temperature limitation imposed by the material being processed. 
Also, the radiation chamber may have one or more additional electromagnetic 
energy sources (not shown) which may be disposed to provide plural zones 
through which the droplets or particles will pass; and where plural 
energy-absorbent components are present in the material to be treated, the 
energy sources may provide radiation at different frequencies matched to 
the different energy-absorbent components. 
Additional gas may be supplied to the radiation chamber at a controlled 
rate by a flow or pressure regulator 34 and flow meter 36 for introduction 
through the port 22 in the wall of the radiation chamber 18 for mixture 
with the sprayed material. The additional gas may provide additional 
capacity for taking up volatile evaporated in the chamber. Also, this gas 
may be introduced in a direction to cause a swirling motion in the chamber 
to increase the time of treatment of material in the chamber. Additional 
materials, such as a powder to prevent caking of the product, solid 
diluents or other additives may be carried in the added gas for mixture 
with the product. 
The material supply metering pump 26 is controlled to supply material at a 
rate determined by the character of the material and the cross section of 
the radiation chamber 18 and the energy supply and absorption factors of 
the system. The rate must not be so high as to cause significant merging 
of droplets nor to overload and cause overheating of the energy supply. A 
temperature monitor (not shown) on the energy supply may be arranged to 
cut off power from the magnetron tube in the event of overheating and to 
cut off supply of material at the same time to prevent introduction of 
liquid material in the absence of electromagnetic radiation. 
In the radiation chamber 18 shown, the lower portion 40 is conical to 
direct the treated material to a discharge conduit 42 through which it 
passes to a cyclone collector 44 which separates the solid product and 
discharges it through the outlet 46. 
Vacuum may be applied to the system through the port 48 in the cyclone 
collector to assist in the evaporation of the volatile component. 
Alternatively, the system may be operated at pressures above atmospheric 
by restricting discharge of gas. 
The following examples are given to aid in understanding the invention, but 
it is to be understood that the invention is not limited to the particular 
procedure, materials, conditions or apparatus employed in the examples. 
EXAMPLE 1. 
The device used had a radiation drying chamber 29 inches in height and a 
diameter of 9 inches discharging into a cyclone collector. A microwave 
generator was connected to the chamber through a wave guide to supply 
electromagnetic radiation at approximately 2,450 mhz. The system was kept 
at reduced pressure by a commercial "Shop Vacuum" connected to the cyclone 
collector. 
100 ml. of coffee solution at room temperature was pumped from the 
reservoir to the nozzle and sprayed as a fine mist in air into the chamber 
at the rate of 3 ml./min. with 2 liters/min. of air. 
The product collected in the cyclone was a fine dry powder having a 
moisture content of only 5.07% and capable of solution in water to reform 
coffee showing no observable deterioration from the original solution. 
EXAMPLE 2. 
Using the same heating device as in Example 1, 135 ml. of human serum was 
sprayed into the radiation chamber at a rate of 1.5 ml./min. with an 
airflow of 4 liters/min. 
The product was collected in the cyclone and discharged as a fine dry 
powder, having a moisture content of 5.2%. No apparent loss in quality was 
observed. 
EXAMPLE 3. 
A 15% Bovine Serum Albumin solution containing Horse Serum Cholinesterase 
was sprayed into the radiation chamber at a rate of 1.5 ml./min. using an 
airflow of 4 liters/min. 
A fine dry powder having a moisture content of only 4.5% and showing no 
apparent degradation was collected. 
EXAMPLE 4. 
A diagnostic reagent for the determination of Lactate Dehydrogenase, 
containing the following: 
5.0.times.10.sup.-2 moles/liter Phosphate buffer 
2.3.times.10.sup.-4 moles/liter Nicotinamide-adenine dinucleotide, reduced 
6.2.times.10.sup.-4 moles/liter Pyruvate 
was prepared in 50 ml. of distilled water containing 7.5 grams of Bovine 
Serum Albumin. 
This solution was introduced into the radiation chamber at the rate of 1.5 
ml./min. with an airflow of 6 liters/min. 
The product recovered was a fine dry powder having a moisture content of 
8.7%. There was no apparent harm to the material. 
EXAMPLE 5. 
Human Whole Blood was sprayed into a radiation drying chamber at the rate 
of 1.25 ml./min. with an airflow of 6 liters/min. 
A fine powdered material showing no observable harm was collected. 
EXAMPLE 6. 
A solution of 15% Bovine Serum Albumin and 10% isopropanol was sprayed into 
the radiation chamber at the rate of 1.5 ml./min. and an airflow of 6 
liters/min. 
A fine dry powder having a moisture content of 5.4% and showing no 
observable degradation was collected.