Patent Application
20150233636
The present invention relates to the microwave drying of extruded honeycomb structures, and in particular relates to systems and methods for efficient microwave drying of extruded honeycomb structures.
Microwave radiation is used for drying honeycomb structures that are formed by extrusion and used for a variety of applications such as engine filters, catalytic converters, and the like. In comparison with conventional heat-based oven drying, microwave drying provides a higher drying rate and is generally faster because the honeycomb structure or “log” is heated directly through the interaction of the microwave energy with the water in the log.
Microwave drying is carried out in a microwave dryer that includes at least one applicator, and that often has a series of applicators, e.g., two or three. A portion of the microwave radiation introduced into a given applicator is absorbed (dissipated) in the log during the drying process. The amount of microwave power dissipation is generally proportional to the water (moisture) content in the log. For example, a wet log (e.g., a newly extruded log) will generally absorb more power than a dry log. During the drying process, the microwave radiation that is not absorbed by the honeycomb structure is either absorbed by other materials in the applicator or reflected back to the generator and therefore does not contribute to the drying process. A large amount of reflected microwave radiation can cause throughput reduction, inefficiency in the manufacturing process, and damage to the microwave radiation source (e.g., magnetron).
To have an efficient microwave process, it is desirable to keep the amount of reflected microwave power within a given applicator to within an acceptable limit or threshold, e.g., less than about 20% of the output power. Toward the end of the drying process when the logs are nearly dry and nearly ready to exit the applicator, the applicator system can reflect a large amount of microwave power. Consequently, to maintain the amount of reflected microwave power within an acceptable limit, the amount of microwave radiation (power) needs to be reduced. While effective, this approach leads to the underutilization of the applicator.
Aspects of the disclosure are directed to systems and methods of efficiently performing microwave drying of honeycomb structures in an applicator. The systems and methods include providing a cross-flow of wet and partially dry honeycomb structures to ensure that both wet and partially dry honeycomb structures are present in the applicator at all times. This arrangement ensures that wet honeycomb structures are present in all applicators, which keeps the reflected power in each the applicator low. This allows the applicator(s) to operate closer to their maximum capacity. The systems include various conveying configurations for providing a good mix of wet and partially dry honeycomb structures to one or many applicators in a typical microwave dryer.
An aspect of the disclosure is a method of efficiently drying honeycomb structures in a microwave dryer having at least one applicator. The method includes conveying first and second sets of at least one honeycomb structure per set in opposite directions through at least one applicator having a cavity, wherein each honeycomb structure has a moisture content MC, and wherein an average moisture content MCA averaged over all of the honeycomb structures within the cavity during drying is between 40% and 60%. The method also includes irradiating the first and second sets of honeycomb structures within the cavity with microwave radiation to effectuate drying. The microwave radiation has an amount of input microwave power PI that creates an amount of reflected microwave power PR from the honeycomb structures, where PR<(0.2)PI.
Another aspect of the disclosure is a method of microwave drying extruded honeycomb structures or “logs” in a batch configuration in a microwave applicator having a cavity. The method includes arranging a plurality of first wet logs in the cavity, and microwave drying the first wet logs for a first drying time at a first input microwave power to form therefrom one or more partially dry logs. The method also includes, after the first drying time, swapping at least one of the partially dry logs for at least one second wet log. The method then includes microwave drying the logs that reside in the cavity at a second input microwave power that is the same or greater than the first microwave input power, and for a second drying time.
Another aspect of the disclosure is a system for microwave drying extruded logs. The system includes one or more applicators each having a cavity. The system also has first and second conveyors configured to convey the first and second sets of logs in opposite directions through each cavity. Each log has the moisture content MC. The logs define the average moisture content MCA averaged over all of the logs within the cavity during drying, wherein 40%≦MCA≦60%. The system also has at least one generation source of microwave radiation operably arranged relative to the at least one applicator and its cavity. The microwave generation source is configured to irradiate the first and second sets of logs within the cavity with microwave radiation to effectuate drying. The microwave radiation has an amount of the input microwave power PI that creates an amount of the reflected microwave power PR from the honeycomb structures, where PR<(0.2)PI.
Another aspect of the disclosure is a method of efficiently drying logs in a microwave dryer having at least a first-end applicator and a second-end applicator having respective first and second cavities. The method includes conveying first wet logs from the first to the second cavity while microwave drying the first wet logs in the first cavity to form first partially dry logs that enter the second cavity, and microwave drying the first partially dry logs in the second cavity to form first dry logs that exit the second cavity. The method also includes conveying second wet logs from the second cavity to the first cavity during microwave drying of the first partially dry logs in the second cavity, thereby forming second partially dry logs that enter the first cavity, and then microwave drying the second partially dry logs in the first cavity during microwave drying of the first wet logs in the first cavity to form second dry logs that exit the first cavity.
Another aspect of the disclosure is a method of microwave drying extruded logs. The method includes arranging first and second sets of wet logs respectively at a first end of a first applicator having a first cavity and at a second end of a second applicator having a second cavity. The method also includes counter-propagating the first and second sets of logs through the first and second applicator cavities while maintaining substantially equal amounts of input microwave power in each cavity. The method also includes outputting the first set of wet logs from the second cavity as a first set of either nearly dry logs or dry logs, and outputting the second set of wet logs from the first cavity as a second set of either nearly dry logs or dry logs.
It is to be understood that both the foregoing general description and the following Detailed Description represent embodiments of the disclosure, and are intended to provide an overview or framework for understanding the nature and character of the disclosure as it is claimed. The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the disclosure and together with the description serve to explain the principles and operations of the disclosure.
Additional features and advantages of the disclosure are set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the disclosure as described herein, including the detailed description that follows, the claims, and the appended drawings. The claims are incorporated into and constitute part of the Detailed Description set forth below.
Additional features and advantages of the disclosure are set forth in the Detailed Description that follows and will be apparent to those skilled in the art from the description or recognized by practicing the disclosure as described herein, together with the claims and appended drawings.
Cartesian coordinates are shown in certain of the Figures for the sake of reference and are not intended as limiting with respect to direction or orientation.
The microwave dryer 10 has first and second ends 12 and 14 that serve as input and output ends, respectively. The microwave dryer 10 includes by way of example three applicators 20, namely, 20-1, 20-2 and 20-3. Generally, one or more applicators 20 are used. The applicator 20 at first end 12 can be referred to as the first-end applicator, and applicator 20 at second end 14 can be referred to as the second-end applicator. In an example, microwave dryer 10 includes at least first-end and second-end applicators 20 (i.e., at least two applicators). The microwave dryer 10 also includes transition housings 30 that connect adjacent applicators 20 and reside at first and second dryer ends 12 and 14 and serve as covers.
The applicators 20 each have a top 22 and an interior cavity (“cavity”) 24, which is sized to accommodate multiple honeycomb structures or logs 110 (introduced and discussed below) and in which the drying of the honeycomb structures or logs takes place. The applicator 20 supports (e.g., at top surface 22) a microwave generating system 40 that includes a microwave source 42 and microwave waveguides 44. The microwave waveguides 44 are operably arranged to introduce microwave radiation (“microwaves”) 50 into applicator cavity 24. In an example, microwaves 50 have a wavelength comparable to the diameter of honeycomb structures or “logs” 110. The microwave waveguides 44 are shown for ease of illustration as residing near top 22 of applicator 20 in cavity 24. However, microwave waveguides 44 are configured relative to cavity 24 to provide a generally uniform distribution of microwave radiation 50 in the region of the cavity through which logs 110 travel while being dried, as discussed below. The microwave dryer 10 includes a conveyor 60 that extends through each applicator 20 from input end 12 to output end 14.
Also shown in
In an example, logs 110 have an internal honeycomb structure. The ceramic-based material used to form logs 110 can be any ceramic-based material known in the art and used to form ceramic articles, such as the aforementioned engine filters, wherein the ceramic-based material has a moisture content MC that can be substantially changed (e.g., by more than 10%) by microwave drying. In an example, the ceramic-based material has a moisture content MC such that logs 110 can be microwave dried to have a moisture content MC≦2%. Example ceramic-based materials that meet the MC≦2% requirement include aluminum-titanate (AT)-based ceramic-based materials and cordierite.
Each log 110 is supported on conveyor 60 by a tray 120. The logs 110 typically have a substantial moisture content upon being extruded, so that such newly extruded logs are referred to herein as “wet logs” 110W. In an example, wet logs 110W have a moisture content in the range of 75%<MC≦100%. Also, logs 110 can be partially dry logs 110P, which in an example have a moisture content in the range 25%≦MC<75%, with MC≈50% being an exemplary value. Further, logs 110 can be nearly dry logs 110N, which in an example have a moisture content in the range 5%≦MC<25%. Further, logs 110 can be dry logs 110D, which in an example have a moisture content in the range 0%≦MC<5%, and in another example have a moisture content in the range 0%≦MC<2%.
In an example, the average moisture content MCA averaged over all of the logs 110 within a given cavity 24 during the microwave drying process is maintained between 40% and 60%.
The wet logs 110W from extruder system 100 are conveyed into cavity 24 of first applicator 20-1 at input end 12 of microwave dryer 10 via conveyor 60 and then conveyed through the applicator cavity. The first applicator 20-1 provides via microwave system 40 microwaves 50 having an amount of input microwave power PI1 that can partially dry wet logs 110W so that they exit the first applicator as partially dried logs 110P. In an example wherein partially dried logs 110P have a moisture content MC of 50%, the logs are referred to as being “half dry.”
The conveyor 60 then conveys partially dried logs 110P to and through second applicator 20-2 and its cavity 24. The microwaves 50 in second applicator 20-2 have a second amount of microwave power PI2 that can further dry the partially dried logs 110P so that they exit cavity 24 as nearly dry logs 110N.
Conveyor 60 then conveys nearly dried logs 110N to and through third applicator 20-3 and its cavity 24. The microwaves 50 in third applicator 20-3 have a third amount of microwave power PI3 that can further dry the nearly dry logs 110PN so that they exit the cavity as dry logs 110D.
During the experiments, it was observed that one 5.66″×8″ AT-based fired log soaked in water such that it picks up 30% weight of water takes 5 minutes to dry to a 95% dryness level at 12 kW of input microwave power PI. A collection of six similar logs took 19 minutes to dry to the same dryness level under similar input power conditions. This experiment showed that increasing the amount of moisture in the applicator via non-dry logs can enhance the drying throughput. In this example, the drying throughput was enhanced by about 35%, with 6 logs being dried in 19 minutes instead of 30 minutes.
The data in
In first applicator 20-1, the corresponding input microwave power PI1 is relatively high at about PI1=90 kW. In second applicator 20-2, the corresponding input microwave power PI2 is reduced to about PI2=65 kW. In third applicator 20-3, the corresponding input microwave power PI3 is reduced to about PI3=15 kW. This reduction in the amount of input microwave power PI3 reduces the efficiency of the log drying process because not all the available microwave input power is being used to dry logs 110.
In the configuration shown in
In an example, microwave dryer 10 of
In an example, logs 110 have an axial length L that is shorter than (e.g., half the axial length of) the logs shown in
The configuration of logs 110 shown in
In an example, two extruder systems 100A and 100B are used in the microwave dryer 10 of
In the example configurations of
These partially dried logs 110P3 are then conveyed by conveyor section 62 to upper conveyor 60B, which conveys these logs back through third conveyor 20-3 in the opposite direction. These partially dry logs 110P3 are then further dried upon their second pass through third applicator 20-3 and exit the third applicator as partially dried logs 110P4 that are yet a bit more dry (e.g., 5/6 dry). These partially dry logs 110P4 are then further dried upon their second pass through second applicator 20-2 and exit the second applicator as nearly dried logs 110N. These nearly dry logs 110N are then further dried upon their second pass through first applicator 20-1 and exit the first applicator at first end 12 on conveyor 60B as dried logs 110D. Thus, at any given time in the drying process, each applicator cavity 24 contains substantially the same average amount of log moisture content MCA by virtue of the logs 110 therein. This in turn allows for substantially the same amount of input microwave power PI to be used for all three applicators 20, i.e., PI1 PI2≈PI3.
The complex dielectric constant ∈ of a material, such as the ceramic-based material used to form logs 110, is expressed as:
∈=∈′+j∈″ (1)
where ∈″ is the imaginary part of the dielectric constant that represents the absorption of electromagnetic radiation and therefore provides an estimate of the amount of dielectric heating that occurs inside the material. The penetration depth of the electromagnetic energy is given by both ∈′ and ∈″. Therefore, to better describe the drying behavior of a log, the real and imaginary parts of the dielectric constant are combined to define the loss tangent:
The dielectric heating (or the power loss) during microwave drying of a single log 110 is given by the dissipated power Pdiss:
P
diss=2πf∈′ tan δ|Erms|2 (3)
where f is the frequency of the electromagnetic radiation and Erms is the root-mean-square of the electric field of microwaves 50, with |Erms|2 representing the intensity of the microwaves.
Equation (3) indicates that the higher the loss tangent, the greater the amount of power dissipation Pdiss inside log 110, and the greater the moisture loss due to boiling. It is therefore desirable to have a high loss tangent to ensure fast and effective log drying.
Electromagnetic simulations were performed to validate the performance of the microwave drying systems and methods disclosed herein.
Table 1 below summarizes the five Examples. Examples 1, 2 and 3 simulate sequential processing in first, second and third applicators 20, wherein wet logs 110W enter first applicator 20-1 and exit as partially dry (50%) logs 110P, which enter second applicator 20-2 and exit as nearly dry logs 110N, which are then processed by third applicator 20-3.
With reference to the trends obtained from the simulation results in
The simulations for Example 3 and Example 4 were based on the counter-flow configuration as discussed above in connection with
The electromagnetic simulations indicate that in a counter-flow drying configuration, all applicators 20 can operate at substantially the same input microwave power PI. The electromagnetic simulation results are plotted in the aforementioned bar graph of
In the example applicator 20 of
With reference to
With reference next to
In
The at least one new wet log 110W can be considered a sacrificial log in the sense that it is used ostensibly to allow the second input microwave drying power PI2 to be at least as great as the first input microwave power PI1 and to avoid having to decrease the second input microwave power due to concerns over the amount of reflected microwave power PR. This allows for faster drying of the remaining non-wet logs 110 formed from the first set of wet logs 110W.
The swapping out of non-wet logs 110P or 110N for sacrificial wet logs 110W is carried out based on the aforementioned total drying time. In an example, wet logs 110W can be swapped into cavity 24 for drier logs 110P, 110N, 110D, or a combination of these logs, one or more times in the course of the total drying time. In an example, the combined first and second drying times add up to less than the total drying time. In other words, non-wet logs 110 that are not swapped out of cavity 24 end up drying faster than if all first wet logs 110W were left in the cavity and dried until they became dry logs 110D.
In an example, at least one microwave-uniformizing device 25, which is configured to provide an increased uniformity to the microwave field distribution for microwaves 50 in applicator cavity 24, is employed. The microwave-uniformizing device 25 may include, for example, mode stirrers or rotating plate 27.
Replacing partially dry logs 110P or nearly dry logs 110N with new wet logs 110W keeps the total log moisture content MC in cavity 24 higher than if all of the logs that started out as wet logs 110W were to be allowed to progress to becoming nearly dry logs 110N. As a consequence, the input microwave power PI can be maintained rather than having to be reduced to keep the amount of reflected microwave power PR low. In
This method continuously moves wet logs 110W into cavity 24 in place of nearly dry logs 110N or dry logs 110D so that the amount of reflected microwave power remains low, allowing for the amount of input microwave power to remain relatively higher than if nearly dry and dry logs were allowed to remain in the cavity as the other logs were drying.
The systems and methods of the disclosure provide cost savings in the form of better use of existing equipment and energy reduction by reducing the amount of reflected microwave power. Other advantages may include better process control and predictability, higher log throughput, increased drying efficiency and improved quality of the ware made from the dried log.
Although the embodiments herein have been described with reference to particular aspects and features, it is to be understood that these embodiments are merely illustrative of desired principles and applications. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13306359 | Nov 2011 | US |
| Child | 14705452 | US |
