Particulate drying system

Details Particulate drying system Particulate drying system

2000-03-06

2001-06-26

Wilson, Pamela (Department: 3749)

Drying and gas or vapor contact with solids

Apparatus

For diverse operations on treated material

C034S079000, C034S181000, C034S210000, C122S00700A, C122S422000, C432S091000

Reexamination Certificate

active

06249988

ABSTRACT:

SUMMARY
The present invention, as exemplified by a number of embodiments described herein, has particular applicability to the drying of particulate materials, such as sawdust. Sawdust refers to small wood particulate materials generated from sawing, grinding or otherwise processing logs, lumber and wood and may also include particulate materials generated by sanding operations. Sawdust typically has a particulate size varying from about 0.0625 in to about 0.125 in in cross-sectional dimension. The term particulate materials includes larger materials such as wood flakes and chips, although such larger materials are excluded from the definition of sawdust. According to a specific embodiment of the invention, the particulate materials to be dried are sized to pass through a 1 ½ in square screen.
According to embodiments of the invention, a blending chamber for use in a system for drying particulate materials such as sawdust or other particulate materials uses, as its primary source of heat, excess heat or exhaust heat from a heat source used for purposes other than particulate drying.
For example, relatively hot exhaust gas from a boiler or other heat source can be used as a heat input to the blending chamber. Additional heat input to the blending chamber can be derived by heating ambient air with a heat exchanger through which steam generated for another operation is circulated. Such steam may also be produced by the same boiler that produces the exhaust gas. The boiler preferably is the primary source of heat for a process other than particulate drying. Thus, excess or waste heat is desirably used from the boiler rather than a dedicated heat source for particulate drying.
If necessary, these one or more “hot” inputs to the blending chamber, e.g., the exhaust gas from the boiler and the heated air, can be cooled to provide an output stream at an appropriate temperature for a particulate drying operation. For example, relatively cool air, such as ambient temperature air (from the exterior environment outside of the blending chamber, i.e., a “cold” input) may be added to the hot gas inputs before, simultaneously with, or after mixing the hot inputs together. There may be applications in which the “hot” inputs are the appropriate temperature, and a “cold” input is not required.
Particulate material to be dried may be added to the output stream exiting the blending chamber and carried by the blended output stream to a dryer. After the material is dried in the dryer, the output stream may carry the now at least partially dried particulates to a separator, wherein the dried material is separated from the output stream. As an alternative to this continuous drying process, a batch drying approach, although less desirable, may be used.
The output stream temperature may be monitored for desired drying performance. A feedback-type control arrangement may be used in which the amounts of the hot and cold streams are varied with respect with each other to achieve a desired output stream temperature. In one specific example, the mass flow rate of gas in the output stream is maintained substantially constant. In this case, an increase in the amount of the hot streams blended into the output stream is accomplished by a corresponding decrease in the amount of the cold stream blended into the output stream, and vice versa.
The blending chamber preferably uses excess heat, and thus is relatively inexpensive to operate. Further, the drying process may take place at relatively low temperatures, and may be controlled to limit thermal degradation of the product being dried. In the case of drying sawdust and other wood particulates, if low temperature drying is used, the production of volatile organic compounds is virtually eliminated.
With the drying system, the moisture content in the dried product can be substantially controlled, such as to within 1½% by weight. Also, in the case wherein the drying system is attached to a boiler, the drying process need not interfere with the draft on the boiler.
These and other features and advantages of the embodiments will be apparent from the drawings and following detailed description. The invention is directed to new and non-obvious features of systems, components and methods both alone and in combination with one another as set forth in the claims below. Not all advantages need be present in an embodiment for the embodiment to be included in the present invention.


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