Process And Apparatus For Drying Porous Material

Abstract

Porous material, for example, spongy polyvinyl acetal article, containing volatile liquid such as water is uniformly dried by charging the porous material into a closed drying chamber, blowing drying air, preferably, of a temperature higher than room temperature onto the porous material, and directing microwaves of very high or ultra high frequency onto the porous material without deterioration in quality.

Description

The present invention relates to a process and apparatus for drying porous materials, particularly, relates to a process and apparatus for uniformly drying porous material containing volatile liquid such as water within a short time without deterioration in quality of the porous material.

Broadly speaking, it is difficult to uniformly quickly dry a porous material having numerous continuous pores containing therein volatile liquid by the conventional heat-drying process. For instance, polyvinyl acetal porous articles are usable as filter material and scrubbing material due to numerous continuous fine pores capable of containing a large amount of liquid such as water. Such porous articles are produced by the process wherein a mixture of polyvinyl alcohol, starch as a pre-forming agent, aldehyde compound and sulfuric acid as a reaction catalyst in water is heated to convert polyvinyl alcohol to polyvinyl acetal. Usually, by the process, 60 to 85 percent by mol of hydroxyl groups in the polyvinyl alcohol are converted to acetal groups. The product of the reaction is insoluble in water and has numberless continuous pores of a size of several to several hundred micron and a porosity of 70 to 95 percent. The product thus produced is washed and dried. The resultant polyvinyl acetal porous article has a high hydrophilic property derived from the hydroxyl groups of the non-converted polyvinyl alcohol and a high resistance to water and chemicals derived from the acetal groups of the polyvinyl acetal. In the process for producing the polyvinyl acetal porous article, the reaction product is washed and then dried by the conventional drying method wherein a high temperature heating medium such as hot air, comes into contact with the porous article. However, it is difficult to complete the drying within a short time. The reason for the difficulty will become apparent by reading the following description.

In the conventional heat-drying method, water located in the outermost portion of the porous article is evaporated by being imparted latent energy for vaporization from the heating medium, and then, water located in the inner portion of the porous article migrates to the outermost portion so as to make the distribution of water in the porous article uniform throughout. The migrated water comes into contact with the heating medium, and evaporates. By the above-stated proceedings, the water in the porous material is successively evaporated.

However, due to the high hydrophilic property of the hydroxyl groups remaining in the polyvinyl acetal porous article, a portion of water is maintained in the inner portion of the porous article so as to resist the migration and evaporation during drying. In order to forcibly migrate and evaporate the maintained water, it is necessary to raise the temperature of the inner portion.

However, owing to a relatively high specific heat of water and low heat-conductivity of the polyvinyl acetal, the velocity of heat conduction through the porous article is very low. Further the rapid evaporation of water located in the outermost portion results in shrinkage of the outermost portion. This shrinkage obstructs the migration of water from the inner portion to the outermost portion. That is, in the conventional heat-drying the polyvinyl acetal porous article can be dried rapidly only in the earlier stage of drying, but the drying velocity decreases rapidly with the lapse of time of drying in latter stage. Additionally, the conventional heat-drying process tends to a non-uniform distribution of water in the porous article. This results in non-uniform quality of the dried article, for example, in porosity and size of pore.

It is well-known that electromagnetic microwaves of very high frequency (VHF) and ultra high frequency (UHF) are utilizable as a heating medium for cooking and welding. In this case, the subject matter of the cooking or welding is heated to a high temperature within a short time. However, the microwaves are not practical for use in drying the porous material such as the polyvinyl acetal porous articles, because the electromagnetic energy imparted to the porous material is consumed not only to evaporate the volatile liquid such as water, but to raise the porous material itself to a high temperature up to the boiling point of water, 100.degree.C at which the porous material is deteriorated in quality.

Further, it is known that in the drying or heating method using microwaves, it is difficult to accurately control the temperature of the porous material lying under the radiation of microwaves.

Particularly, in the case of the polyvinyl acetal porous article, the microwave drying process tends to change in quality, because the polyvinyl acetal has a high plasticity and a low resistance to hot water. That is, if quickly dried using the microwave, the polyvinyl acetal porous articles often tend to partly fuse or dissolve in water contained in the porous article itself. This causes a change of the porosity, size, configuation and number of the pores. It should be noted that even if the microwaves are imparted with a very small amount of energy, the above-mentioned disadvantages generally occur. Also, such radiation of a small amount of microwave energy results in a very low efficiency of drying of the porous material. In order to eliminate the above-stated disadvantages, periodical radiation of the microwaves has been attempted. However, such attempts have resulted in failure because of difficulty in controlling the temperature of the porous material to be dried.

The inventors, as a result of long study, have found the facts as detailed below. In the case where the microwaves are imparted to the porous material to dry it, the vapor of the volatile liquid such as water covers the surface of the porous material under a high partial pressure so as to obstruct the sequent evaporation of the volatile liquid, and the porous material is raised to a high temperature. In order to protect the porous material from the elevation of temperature, it is necessary to enhance the velocity of evaporation of the volatile liquid. This enhancement is accomplished by promoting diffusion of the vapor around the surface of the porous material and lowering partial pressure of the aqueous vapor in the porous material. Such promotion can be accomplished by blowing air onto the surface of the porous material so as to blow away the vapor covering the surface. That is, it was discovered that by blowing air onto the surface of the porous material, almost all the imparted microwave energy can be used in evaporating the volatile liquid in the porous material quickly, and therefore, the porous material can be uniformly dried with a practically no elevation of temperature of the porous material itself.

The present invention was completed on the ground of the above discovery.

An object of the present invention is to provide a process and apparatus for uniformly drying porous material containing a volatile liquid therein within a short time.

Another object of the present invention is to provide a process and apparatus for quickly drying a porous material containing volatile liquid therein without change in quality of the porous material.

The above objects can be accomplished by the process and apparatus of the present invention.

According to the process of the present invention, air is blown onto a porous material containing volatile liquid therein, for example, a polyvinyl acetal porous articles containing water therein, preferably, in at least the later stage of the air blowing, electromagnetic microwaves are imparted to the porous material, whereby the porous material is uniformly dried without change in quality thereof.

Also, according to the present invention, the apparatus for drying porous material containing volatile liquid such as water therein comprises a closed drying chamber, for containing porous material to be dried, means for blowing drying air into the drying chamber and means for directing electromagnetic microwaves into the drying chamber.

The objects, features and advantages of the present invention will become apparent by reading the following description while referring to the accompanying drawings, wherein;

FIG. 1 is a diagram showing relationship between moisture content of porous material and drying time,

FIG. 2 is a schematic view of an embodiment of the apparatus of the present invention,

FIG. 3 is a schematic view of another embodiment of the apparatus of the present invention,

FIG. 4 is a schematic plane view of further embodiment of the apparatus of the present invention,

FIG. 5 is a schematic plane view of a part of the apparatus of FIG. 4 showing a method of radiating microwaves onto carriages containing a porous material of large volume, or an accumulation of numerous small amounts of porous material,

FIG. 6 is a schematic side view of a part of the apparatus of FIG. 4 showing another radiation method of microwaves onto trucks containing a porous material of large volume,

FIG. 7 is a schematic plane view of a still another embodiment of the apparatus of the present invention,

FIG. 8 is a schematic side view of a truck of the apparatus of FIG. 7 showing a circulating mechanism for a plurality of porous materials,

FIG. 9 is a schematic front view of still another embodiment of the apparatus of the present invention having a douser plate for preventing leakage of microwaves, and

FIG. 10 is a schematic side view of a conveyer chain containing gutter capable of preventing leakage of microwaves therethrough.

Referring to FIG. 1, curve A shows a relationship between drying time and the amount of water contained in the porous material when the porous material is dried by the conventional hot air drying method. It is obvious that curve A approaches an equilibrium percentage of water in the porous material in the form of a hyperbola and thus, the hot air drying requires a long time to complete. In such drying procedure, there is large difference in the amount of aqueous vapor between the outermost portion and the inner portion of the porous material. Therefore, a large differential pressure is produced between the outermost and inner portions. The differential pressure causes non-uniformity of the dried porous material in quality, for example, porosity and size of pores.

The hot air drying of the porous material is carried out in accordance with the following Equation 1:

W = (W.sub.o - W.sub.b).epsilon..sup.-.sup.t/T + W.sub.b (1)

wherein W is a percentage of water with respect to the weight of the porous material during drying, W.sub.o is a percentage of water initially contained in the porous material, W.sub.b is a percentage of water in the equilibrium condition, T is a time constant which refers to a time long enough for the porous material to have reached the equilibrium condition if the drying had proceeded at the initial drying rate, t is a drying time, and .epsilon. is the base of a natural logarithm. From Equation 1, it is evident that the value of W varies as a negative exponential function of t, and therefore, a long time is necessary to complete the drying. Generally, provided W .apprxeq.W.sub.b, the value of t is as large as about four times that of T which is a time constant.

When the porous material is dried using the hot air and the electromagnetic microwaves, the relationship between W and t is shown by Equation 2:

W = (W.sub.o - W.sub.b).epsilon..sup.-.sup.t/T + W.sub.b - K.sup.. t (2)

wherein W, W.sub.o, W.sub.b, t, and T are as defined above, and K is a capacity of drying by the electromagnetic microwaves. That is, Equation 2 includes a primary proportional factor K.sup.. t with respect to t. This indicates that in the drying process using the hot air and electromagnetic microwaves, the electromagnetic energy imparted is consumed at an early short stage of drying for elevating the temperature of the porous material and volatile liquid, and thereafter, for evaporating the volatile liquid.

A drying capacity dw/dt of the porous material at an early stage of drying hot air is defined by Equation 3:

-dw/dt = (W.sub.o - W.sub.b)/T (3)

when the drying capacity of the microwave drying for the porous material is m times that of the hot air drying capacity, Equation 2 is modified to Equation 4:

W = (W.sub.o - W.sub.b)(.epsilon..sup.-.sup.t/T - m t/T) + W.sub.b (4)

From Equation 4, provided m = 1/2, t = 0.85T, t is the time necessary for the amount of water in the porous material to reach the equilibrium condition, that is, the time necessary for the value (.epsilon..sup.-.sup.t/T - m t/T) to reach 0.

That is, when electromagnetic microwave radiation is utilized with a drying capacity of one-half times that of hot air, the drying time becomes about one-fifth that of the hot air.

In order to shorten the drying time to one-half of that of the hot air, it is necessary that electromagnetic microwave radiation, with a drying capacity of 0.07 times that of the hot air, be added to the hot air drying.

The amount of water evaporated by the microwave radiation is as follows:

m(W.sub.o - W.sub.b)2T/T = 2m(W.sub.o - W.sub.b).

Also, the amount of water evaporated by the hot air is as follows:

(1 - 2m)(W.sub.o - W.sub.b)

Therefore, the ratio of the former to the latter is as follows:

(1 - 2m)/2m.

Provided m = 0.07, the ratio is 0.86/0.14.apprxeq.6. That is, the microwave radiation has a much higher effectiveness for drying than the hot air. In the other words, the drying time can be shortened by the combination of hot air blowing and electromagnetic microwave radiation.

In the process of the present invention, it is preferable that the drying air has a temperature not lower than room temperature, more preferably, 40.degree. to 100.degree.C, particularly, 40.degree. to 60.degree.C. The hot air is prepared by using the conventional heater such as a gas burner, electric heater, steam heater or heat exchanger. Also, it is preferable that the drying air is blown at a velocity of 0.5 to 5 m/sec, more preferably, 1 to 2m/sec. The air can be blown by the conventional blower.

In the process of the present invention, the electromagnetic microwave radiation can be effected by using the conventional radiation device, for example, vacuum-tube type separately excited and self-oscillators for radiating microwaves of very high frequency of 30 to 300 MHz, and magnetron oscillator for generating ultra microwaves of an ultra high frequency of 300 to 3,000 MHz. The drying capacity of the microwave radiation device depends on the output power thereof. The necessary drying capacity can be obtained by selecting an oscillator suitable for the purpose and adjusting voltage of an electric source device to a pertinent value. For example, the oscillators of 2,450 MHz and 5 KW output, or of 915 MHz and 20 KW output, can be utilized for the process of the present invention. In the process of the present invention, the radiation of the electromagnetic microwaves may be begun at a stage prior to, simultaneously with or after the beginning stage of blowing the drying air. However, it is important that the radiation is effected at at least the later stage of the hot air drying. Generally, the radiation of the microwaves cause a rapid elevation of temperature quite uniformly throughout the porous material, i.e., the outermost and inner portions. This rapid elevation of temperature may result in a change of quality of the porous material. Therefore, it is preferable that the radiation is begun at a stage simultaneous to or after the beginning stage of blowing the drying air. By this process, the vapor around the surface of the porous material is blown away by the drying air so as to promote the migration of the volatile liquid from the inner portion to the outermost portion. The migrated volatile liquid is rapidly evaporated at the surface of the porous material while maintaining the partial pressure of the vapor from the volatile liquid at a low level. Almost all the radiation energy of the electromagnetic microwave is converted to latent heat for evaporation of the volatile liquid, the drying of the porous material can be successively carried out at an approximately constant rate. The drying capacity of the radiation may be larger than that of the drying air. However, the large energy of the radiation may result in fusion, dissolution or deterioration in quality of the porous material such as a polyvinyl acetal porous article. Therefore, it is preferable that the drying capacity of the radiation is the same as or smaller than that of the drying air. The radiation of the microwaves may be carried out continuously or periodically. In periodical radiation, it is preferable that total capacity of the microwave radiation for drying is smaller than that of the drying air.

In the process of the present invention, the drying may be applied to the porous material in a stationary or dynamic state. Also, the porous material may move continuously or intermittently through a drying apparatus.

The apparatus of the present invention comprises a closed drying chamber to contain the porous material to be dried, means for blowing drying air into the drying chamber and means for imparting electromagnetic microwaves to the porous material. The drying chamber must be surrounded by a material capable of shielding the electromagnetic microwaves and ultra microwaves. The shielding material can prevent the leakage of the microwaves from the drying chamber by reflecting the microwaves so that all the radiation is absorbed by the porous material charged into the drying chamber. The shielding material is selected from metal plate and punched metal plate or net having pores of a size very much smaller than the wave length of the imparted microwaves. The metal plate and punched metal plate or net may consist of iron, preferably of a non-magnetic and high electroconductive metal such as copper and aluminium. The drying chamber may be formed by the above-stated metal plates themselves. Also, the drying chamber may be formed by an inner wall made of the punched metal plate or metal net and an outside wall made of wood or metallic material.

FIG. 2 shows an apparatus of the present invention for drying a porous article in a stationary state. Referring to FIG. 2, several porous articles 1 are charged into a drying chamber 2 by opening a door 3 and hung using hangers 4 and rings 5 in a stationary state. A magnetron oscillator 6 is disposed outside the drying chamber 2, and the output of the oscillator is controlled by an electric source device (not shown in the drawing). The oscillator 6 is connected to the drying chamber 2 through a waveguide 7 and a radiation opening 8. The electromagnetic microwaves oscillated by the oscillator 6 are conduced to the opening 8 through the waveguide 7 and directed into the drying chamber 2 through the opening 8. A motor 9, a fan 10, engaged with the motor 9, and a heater 11, located above the fan 10, are arranged in a lower chamber 2a formed beneath the drying chamber 2. The lower chamber 2a is connected to the drying chamber 2 through a punched plate or net 12 and provided with partitions 2b for forming paths 2c, through which drying air circulates, a thermometer 13, inserted into the path 2c, and a punched plate or net 16 for connecting the lower chamber 2a to atmosphere. A blower 15 is disposed beneath the oscillator 6 to cool the oscillator 6. A cooling air flow generated by the blow 15 passes through the oscillator 6 while cooling it and, thereafter, is introduced into the drying chamber 2 through the waveguide 7 and the opening 8.

When the fan 10 is rotated by the motor 9, air in the lower chamber 2a is circulated, as drying air flow, through the heater 11, in which the drying air is heated to a desired temperature, the punched plate or net 12, the drying chamber 21 and the paths 2c, along the circulation paths as shown by arrows 14. A part of the drying air is exhausted into the atmosphere through the punched plate or net 16. The oscillator 6 is actuated, preferably, at at least the later stage of the drying process. In this case, the oscillator preferably has a drying capacity not higher than that of the drying air flowing through the drying chamber. The drying operation of the apparatus of FIG. 2 is carried out in the condition that the drying chamber 2 is closed so as to prevent leakage of the microwaves.

FIG. 3 shows an apparatus of the present invention for continuously drying the porous material in a dynamic state. Referring to FIG. 3, a porous material 17 is charged into a drying chamber 18 through an entrance 27 at a predetermined velocity. The drying chamber 18 contains a plurality of ducts 19 having nozzles 20 opening to the porous material 17 and connected to a drying air supply (not shown in the drawing). The drying air may be conditioned at a predetermined temperature by a heat exchanger (not shown) and at a predetermined pressure by a blower (not shown). The porous material 17 is supported by rotatable rollers 21 while advancing through the drying chamber.

A magnetron oscillator 22 is disposed above the drying chamber 18 and connected to the inside of the drying chamber 18 through a waveguide 23 and a radiation opening 24. The opening 24 is located close to the exit 28 so as to direct the microwave to the porous material at a later stage of drying. The opening 24 may be located close to the entrance 27 or at a middle portion of the drying chamber 18. A rotatable scattering plate 26, which is rotated by a motor 25, is located in front of the opening 24 so as to uniformly scatter the microwave directed into the inside of the drying chamber 18 through the opening 24. The entrance 27 and exit 28 are each formed by a pair of long projections spaced a small distance from each other in order to prevent leakage of the microwaves from the drying chamber.

The length of porous material 17 continuously enters into the drying chamber 18 through the entrance 27 and advances along the path formed by the rotatable rollers 21, while being subjected to the blown drying air ejected through the nozzles 20 of the ducts 19. At a later stage of the advance, the porous material is exposed to the radiation of microwaves which are regenerated by the oscillator 22, conduced through the waveguide 23 and the opening 24 and scattered by the rotatable plate 26. After completing the drying, the porous material is continuously delivered through the exit 28.

The drying may be carried out periodically using the apparatus as shown in FIGS. 4, 5 and 6. In FIGS. 4 and 5, a floor conveyor 41 advances through a drying chamber 40 along a closed path (not shown). The conveyor 41 is provided with a plurality of joint apertures (not shown) for fastening therein joint members 43 of carriages 42a, 42b and 42c. The carriages 42a, 42b and 42c containing a large amount of porous materials are drawn by the conveyer 41 along the closed path. The joint members 43 of the carriages 42a, 42b and 43b can be automatically removed from the joint apertures of the conveyor 41 when a stopping device 44, disposed at a suitable position of the closed path, is actuated so as to stop the advance of the carriages, or the carriages contact the foregoing carriage. Also, the joint members 43 can be automatically fastened with the joint apertures when the stopping device 44 releases its actuation so as to advance the carriages along the closed path or a carriage is separated from the foregoing carriages.

The drying chamber 40 is provided with three pairs of microwave radiation devices 45a, 45b and 45c located at positions A, B and C, respectively. When the carriage 42a stops at a position A by actuating the stopping device 44, the carriages 42b and 42c following the carriage 42a automatically stop at positions B and C. Each pair of the micro radiation devices 45a, 45b, 45c, impart microwaves horizontally to the porous materials contained in the carriages 42a, 42b and 42c, respectively, at right angle to the direction of the advance of the carriages.

The microwave radiation devices 45a, 45b and 45c have a plurality of radiation openings 46a, 46b and 46c facing the block portions of the carriages, respectively, and extending vertically. The radiation openings 46a, 46b and 46c are located in a distribution wherein all the porous material in each carriage can be imparted uniform radiation of the microwaves.

For example, referring to FIG. 5, the carriage is subjected to three radiations of the microwaves at positions C, B and A, successively. In this case, the radiation openings are located so that the carriage is imparted, at position C, the radiation at the hatched portion thereof, at position B at another hatched portions thereof at which the carriage has not been imparted the radiation at position C, and at position A at a further hatched portion thereof at which the carriage has not been exposed to the radiation at either positions C and B. By the above-mentioned arrangement of the radiation openings, the porous material in the carriage can be uniformly dried.

When the carriages 42a, 42b and 42c stop in the drying chamber 40, doors 47 and 48 close the drying chamber so as to prevent leakage of the microwaves, and a supplementary carriage 49 stops at a predetermined position D outside the drying chamber 40 by action of a stop device 50. After the radiation is completed, the doors 47 and 48 are opened, the carriage 42a leaves the drying chamber 40 and the carriage 49 enters into the drying chamber 40.

The drying chamber 40 is provided, at positions A, B and C, with three pairs of blowers 51a, 51b and 51c for drying air. When the three carriages stop in the drying chamber by the action of the stop device and the doors close, the blowers are automatically actuated. That is, the blowers, microwave radiation devices, doors and stop device are controlled on and off by a control device (not shown in the drawings) in accordance with a predetermined operation program. The above-stated operations are repeated in accordance with the program.

Referring to FIG. 6, the openings of the radiation devices (not shown in the drawing) extend horizontally and are located in positions facing the hatched portions of the carriages 42a, 42b and 42c in the drawing. The porous material contained in each carriage can be uniformly imparted the radiation at positions C, B and A, while being dried by drying air generated by blowers 51a, 51b and 51c. That is, the carriage is imparted, at position C, the radiation of the microwaves at the hatched portion thereof, at position C at another hatched portion thereof at which the carriage has not been imparted the radiation at position C, and at position A at a further hatched portion thereof at which the carriage has not been imparted the radiation at both positions C and B.

The periodical apparatus as shown in FIGS. 4 to 6 is valuable for drying a porous material having a relatively large volume or an accumulation of numerous amounts of small porous materials both of which need a long time to complete drying.

FIGS. 7 and 8 show another embodiment of the apparatus of the present invention for periodically drying the porous material.

Referring to FIGS. 7 and 8, the carriages 42a, 42b and 42c each having a mechanism for circulating the porous material to be dried therewithin, a frame 75 supporting the circulating mechanism and wheels 76 carrying the frame 75 along a closed path formed by the floor conveyer 41.

Referring to FIG. 7, the carriage 42a, 42b and 42c have rotatable shafts 71a, 71b and 71c disposed on the frames 75 and have electromagnetic clutch joints 73a, 73b and 73c provided at their ends, respectively. The drying chamber 40 is provided with motors 72a, 72b and 72c have electromagnetic clutch joints 74a, 74b and 74c provided at ends of the rotating shafts of the motors, respectively. Further the drying chamber 40 has electromagnetic microwave oscillators 45a, 45b and 45c and blowers 51a, 51b and 51c located at positions A, B and C. The carriages can be engaged with and removed from the floor conveyer 41 in the same manner as that of the apparatus of FIGS. 4 through 6.

Referring to FIG. 8, the shaft 71a is fastened on the uppermost part of the frame 75 and the clutch joint 73a is fixed to an end of the shaft 71a. A pair of chain wheels 77 are fastened at both the ends of the shaft 71a. In the same way, other shafts 78 through 84 are disposed parallel to the shaft 71a as shown in the drawing, and each having a pair of chain wheels 86 through 91. The chain wheels 77 and 86, 87 and 88, 89 and 90, and 91 and 92 are respectively connected to each other by chains 85. The rotatable shafts 78, 79, 81 and 84 have a pair of the other chain wheels 92 through 96 fastened at both the end portions thereof, respectively. The chain wheels 93 through 96 are connected with a pair of chains 97. A plurality of tie bars 99 are bridged between the chains 97, and a plurality of hangers 98 for the porous materials 1 are attached to the tie bars 99.

In FIGS. 7 and 8, when the carriages 42a, 42b and 42c enter the drying chamber 40 and stop at the positions A, B and C, respectively, the joints 73a, 73b and 73c engage with the joints 74a, 74b and 74c so as to connect the shafts 71a, 71b and 71c to the motors 72a, 72b and 72c, and then the motors are actuated. By the rotation of the motor, the circulation mechanism of each carriages is driven so as to circulate the porous materials hung on the tie bars 99 along the path of the chain 97. In the above-mentioned system, during the circulation of the porous materials, the microwave drying and the air drying are effected for the porous materials. By the circulation in the apparatus as shown in FIGS. 7 and 8, the porous materials are uniformly dried.

The periodical apparatus as shown in FIGS. 7 and 8 is valuable for simultaneously drying numerous porous materials each having a relatively small volume.

The apparatus of the present invention may have a device for preventing leakage of microwaves through the path of the conveyer chain, as shown in FIG. 9. If the leaked microwaves are absorbed by the conveyer chain the absorption results in undesirable over-heat of the conveyer chain.

In FIG. 9, the carriage 42a has a bottom plate 100 effective for preventing leakage of the microwaves imparted to the porous materials 1 in the carriage 42a, through the bottom plate 100. In order to prevent the leakage of the microwaves through the space between the lower surface of the bottom plate 100 and the upper surface of the bottom of the drying chamber 40 and a duct 101 containing the conveyer chain 41, a shield plate 103 extends from the lower surface of the bottom plate 100 toward the bottom of the drying chamber 40. Also, the shield plate 103 covers the joint member 43. The shield plate 103 can reflect the microwaves directed to the conveyer chain 41 in the duct 101. The reflected microwaves are further repeatedly reflected on the inside surface of the drying chamber 40 so as to result in complete absorption thereof by the porous material in the carriage. That is, the shield plate 103 is effective for preventing the overheating of the conveyer chain.

Also, the leakage of the microwaves can be avoided by filling the gutter 101, containing the conveyer chain 41, with water or other liquid capable of absorbing the microwaves even if no shield plate is used. The duct 101 may be partly filled with water as shown in FIG. 10. Referring to FIG. 10, a part 102 of the duct 101 in a position A in the drying chamber, at which the carriage is exposed to the microwave radiation, is formed at a lower level than that of other parts. Thus the low level part 102 of the duct 101 is filled with water to prevent the leakage of the microwaves therethrough. The microwaves are attenuated by repeatedly reflecting against the inside surface of the duct part 102, and absorbed by the water. This is effective for preventing the absorption of the microwave by the conveyer chain and thus, the over-heat thereof.

The features and advantages of the process and apparatus of the present invention are further described by the following examples which are not intended to limit the scope of the present invention.

EXAMPLE 1

Cylindrical spongy polyvinyl formal articles, each having a diameter of about 80 mm and a length of 250 mm and containing about 100 percent of water based on the weight of the article, were dried using the apparatus as shown in FIG. 2. The drying apparatus has a magnetron oscillator of a maximum output of 600 watts capable of diverging microwaves of a frequency of 2,450 MHz, and means for blowing drying air.

The drying chamber was charged with the spongy articles and hermetically closed. The drying air having a temperature of 75.degree.C was circulated at a flow rate of about 1 m/sec at parts around the spongy articles. The oscillator was actuated at outputs of 100 and 200 watts.

The drying capacity of the drying air was 22 percent /hour and those of the microwaves of 100 and 200 watts were 9 and 17 percent /hour, respectively.

When the drying was carried out only by the drying air at 75.degree.C, the spongy articles were dried along curve A in FIG. 1. That is, the spongy articles could reach a moisture content of about 3 percent after drying for a long time of about 18 hours. However, in the case where the hot air drying and microwave drying at an output of 100 watts were simultaneously effected for the spongy articles, the drying was completed in about 5 hours. Further, in the case where the microwave drying at an output of 200 watts was effected simultaneously together with the hot air drying, the spongy articles were completely dried in about 3 hours.

The drying capacities of the 100 watt microwaves and 200 watt microwaves are respectively in ratios of 0.41 and 0.78 with respect to the drying capacity of the hot air of 75.degree.C. In spite of the small drying capacities of the 100 and 200 watt microwaves, it was extremely pleasing that the simultaneous use of the hot air flow with 100 watt microwaves having a drying capacity ratio of 0.41 to that of the hot air flow resulted in a short drying time of 5 hours, and; further, that the simultaneous utilization of hot air flow with 200 watt microwaves of a drying capacity ratio of 0.78 resulted in a very short drying time of 3 hours.

In the case where the 100 watt microwaves were used together with the hot air, the spongy article was raised to a maximum temperature of 77.degree.C in the inner portion thereof. In the case of 200 watt microwaves, the maximum temperature of the inner portion of the spongy articles was 79.degree.C. In these cases, the hot air was of a temperature of 75.degree.C. In both the cases, in spite of high heat-sensitive property of the polyvinyl formal, the dried spongy articles had a uniform quality.

However, the comparison drying by only hot air of 75.degree.C, resulted in a large difference of about 30 percent in density, which depends on the pore size and porosity, between the outermost and inner portions of the spongy article.

For further comparison, the spongy articles were dried by the 200 watt microwaves only for about 3 hours. In this case, the inner parts of the spongy articles rose to a temperature of about 100.degree.C and fused. This resulted in articles of no value commercially.

From the results as stated above, it is obvious that the process and apparatus of the present invention is effective for uniformly evaporating water within the spongy article at a substantially constant rate throughout drying stage, and therefore, results in a uniform quality of the dried spongy article.

In the apparatus of FIG. 2, the hanger disposed on the ceiling of the drying chamber may be rotated so as to rotate the spongy articles hung from the hangers. This is effective for uniformly exposing the spongy articles to the microwave radiation. Further, a plurality of radiation openings may be formed on the ceiling of the drying chamber and connected to the oscillator through the corresponding waveguides branched from a main waveguides. This is effective for uniformly directing the microwave into the drying chamber.

EXAMPLE 2

A length of spongy polyvinyl formal sheet having a width of 1 m and a thickness of 5 mm and containing 100 percent of water based on the weight of the sheet was continuously dried using the apparatus shown in FIG. 3. The oscillator generated microwaves of a frequency of 2,450 MHz at an output of 1 KW. Drying air of a temperature of 95.degree. to 97.degree.C was blown onto the spongy sheet at a velocity of 1.4 m/sec. The drying capacities of the microwave and the hot air were 0.5 percent/min and 0.21 percent/min, respectively. Accordingly, the ratio of the drying capacity of hot air to that of the microwave was 1 : 0.42.

The spongy sheet could be dried to a moisture content of 3 percent by advancing through the drying chamber at a velocity of 4.8 m/min for 5.5 minutes.

For comparison, the spongy sheet was dried by the apparatus of FIG. 3 without using the oscillation. When advanced at a velocity of 1 m/min for 28 minutes through the drying chamber, the spongy sheet was dried to a moisture content of 5 percent based on the weight of the sheet.

That is, the combination of the hot air drying with the microwave drying having a drying capacity of 0.42 times that of the hot air could shorten the drying time to about one-fifth that of the hot air only, and remarkably increase efficiency of production of the spongy sheet.

In the apparatus as shown in FIG. 3, numerous pieces of the porous material may be successively carried by an endless conveyer made of an organic material having a small absorption for electromagnetic microwaves and a high thermal stability, for example, a polyolefin or natural or synthetic rubber belt, or of non-magnetic metal, for example, a copper or aluminium chain, net or belt. Further, the apparatus may have a plurality of microwave radiation openings connected to the oscillator.

The process and apparatus of the present invention have the advantages as detailed below.

1. Shortened Drying Time

The porous material can be quickly dried by the process and apparatus of the present invention at a drying velocity of several times that of the conventional process and apparatus using hot air only.

2. Uniform Quality of the Dried Porous Material

Even in the case where an organic substance having a low thermal stability, for example, polyvinyl acetal is rapidly dried, there is no change in quality, because the volatile liquid is evaporated at a constant rate and there is substantially no elevation of temperature of the inner portion.

3. Low Cost

Almost all of the expensive microwave energy is effectively consumed for evaporating the volatile liquid in the porous material, but not for substantially elevating the temperature of the porous material itself.

Claims

What we claim is:

1. A process for uniformly drying a porous material consisting essentially of polyvinyl acetal, which comprises

i. charging a water containing porous material consisting essentially of polyvinyl acetal into a closed drying chamber;

ii. blowing drying air onto the porous material; and

iii. directing electromagnetic microwaves of a very high or ultra high frequency having a drying capacity not higher than that of the drying air onto the porous material at a stage after the beginning stage of blowing the drying air.

2. A process as set forth in claim 1, wherein the drying air is of a temperature higher than room temperature.

3. A process as set forth in claim 1, wherein the temperature of drying air is higher than 40.degree.C.

4. A process as set forth in claim 1, wherein the drying air is blown at a velocity of 0.5 to 5 m/sec.

5. A process as set forth in claim 1, wherein the microwaves have a very high frequency of 30 to 300 MHz.

6. A process as set forth in claim 1, wherein the microwaves have an ultra high frequency of 300 to 3,000 MHz.

7. A process as set forth in claim 1, wherein the porous material is in a dynamic state during drying.