Drying and gas or vapor contact with solids – Process – Diverse types of drying operations
Reexamination Certificate
2000-05-17
2003-03-11
Walberg, Teresa (Department: 3742)
Drying and gas or vapor contact with solids
Process
Diverse types of drying operations
C034S495000
Reexamination Certificate
active
06530160
ABSTRACT:
FIELD OF INVENTION
This invention relates in general to the treating of material with gas or vapor contact, and in particular, to the in-bin drying and conditioning of grain.
BACKGROUND
The Energy Costs of Drying
Due to uncertainty and lack of understanding, most operators continuously heat the grain and only use cooling to reduce spoilage risk when the grain is dry. The energy cost of this conventional practice is huge. It can exceed all the other grain production costs combined (except fertilizer). In 1978, 75% of a seven billion-bushel corn crop required nearly 875 million gallons of LP gas for drying. Today's corn crops exceed 9 billion bushels and require 90% artificial drying. In addition, millions of dollars are lost each year because of grain being cracked from poorly controlled drying.
PROBLEMS IN THE PRIOR ART
The basic approach to drying has remained essentially the same since the 1840's: blowing heated air through the grain followed by ventilation just for cooling. It has been assumed that heating the grain was the only method for accelerating drying. In those instances where researchers did cool their grain to aid drying, they only did so as a one-time procedure.
Relative humidity (RH) and its companions—Equilibrium Moisture Content (EMC) and Equilibrium Relative Humidity (ERH)—have remained the principal control variables. On occasion grain moisture sampling is used for direct feedback. On the surface, these concepts appear quite reasonable; however, our drying research has uncovered the following:
1) Relative Humidity, as a control in itself, is fundamentally flawed.
2) Grain Moisture feedback, as commonly used, is also flawed.
3) The use of constant heat does not produce drying optimization.
4) The standard methods for solar heating are often counter-productive.
5) The ‘wet warming’ of grain during warm fronts followed by the planned use of cooling during the nights of cold fronts, is much more efficient than continuos heating.
OBJECTIVES
It is the objective of this art to advance methods for drying materials in general, and grain in particular, with greater certainty, control, quality, and economy.
The Theory, Methods, and Means to do so:
The basic problem of the prior art is an inadequate expression of the nature of drying.
It was known since the 1930's and proven in 1940 that the driving force for drying is differences in vapor pressure. However, the early researchers directly measured grain moisture and indirectly described these vapor pressure differences in terms of relative humidity (RH) and equilibrium limits. At first this was due to the lack of the needed sensors. However, even when the sensors did become available, the use of comparative measures continued. Without the ability to quantify relationships, optimization is not obtainable.
Theory of Differential Vapor Pressure (dPv):
This instant art advances a method of indirectly measuring the material and directly analyzing dPv's. As I will show, this allows integration of temperature and moisture relationships and provides a method for quantifying the changes. Out of this emerges a better understanding of the needed sensing and sensor placement, data formatting, and the subsequent opportunities for optimization.
Direct computation of differential vapor pressure opens a new method of analyzing drying changes. While these concepts are not obvious and intuitive at the start, the underlying dynamics are simple extensions of classic concepts. With such analysis, the present art can open a whole new paradigm of drying.
Logic.
The key to understanding drying and sensor placement is to understand the role of pressure in drying, particularly that of vapor pressure.
A: Pressure is the only manipulable force for moving fluids.
B: Water, water vapor, and air are fluids, (fluid—Greek, to flow).
C: Drying only occurs if the pressure for the vapor to leave the material is greater than for it to re-enter or remain.
D: The Ideal Gas law is the foundation for understanding Pressure.
In
Drying and Storage of Grains and Oilseeds
, Donald B. Brooker, Fred W. Bakker-Arkema, C. W. Hall, published by Van Nostrand Reinhold, New York, (633.1046/b791ds), write “Under the conditions at which grain drying takes place, the Ideal Gas Law expresses accurately the relationship between the pressure, temperature, and volume for the dry air and associated water vapor”.
The Ideal Gas Law is traditionally written as: PV=nRT.
P=pressure; V=volume; n=moles; R=gas constant; and T=temperature.
With M=mass of moisture replacing the chemical term n=moles, we can rewrite the traditional expression in terms of pressure:
P=M/V*T*R,
If we set V=1 cu ft and regard it as a constant we can omit the V term. Likewise, R being a constant, we can for convenience, omit it also.
Results: P~M*T: Pressure is proportional to a product of Mass (of the moisture) and Temperature (of the moisture).
Proportional=~, for sake of writing with this word processor.
To express vapor pressure differences mathematically requires two opposing pressure equations.
P
(material)−
P
(air)
Omitting the P term on each side of two pressure equations and substituting the mass and temperature variables for air (a) and grain (g) in this instance leaves the following:
Basic Model:
dPv
=[(
Mg*Tg
)−(
Ma*Ta
)]
with M=actual moisture per unit volume and T=temperature.
Key: Each vapor pressure difference is dependent on two opposing multivariate expressions. Four values are needed to precisely define each relationship.
Inductive conclusions of the symbolic model.
1) Looking at the symbolic model, (Mg*Tg)−(Ma*Ta), it is possible to see that constant or continual heating will not produce optimization. When the temperatures have reached equilibrium, the only remaining driving force is the differences in moisture concentrations. While this could be large, it can not be as large as a ‘lowered’ ambient temperature or moisture or both. When the grain and air temperatures have reached equilibrium, the use of naturally cold or chilled air will be more efficient than constant heat.
2) In addition, it is possible to see why drying can be facilitated by the deliberate ventilation of warmer grain with colder, drier air. When air is selected (or conditioned) for low ambient temperature and moisture, its ambient vapor pressure is low. Since drying is driven by difference, this ‘lowering’ of the outside vapor pressure increases the drying potential. Also, the effect of the lower moisture can be dramatic. A two-degree drop in incoming dew point can have the same effect as a twenty-degree rise in grain temperature. Moreover, this increased potential for drying comes minus the cost of expensive fossil fuels and the penalty of the added vapor from the combustion of those fossil gases.
Hidden bias: The Grain as just an Object.
In the past, an almost unconscious attitude developed that grain is just an object to be treated and that all the drying heat comes form the air. It's our finding that having the grain ‘participate’ in the drying not only can lead to greater efficiency, but also higher quality. This participation is in two forms.
First, because grain has an inherently higher specific heat than air, it can act as a solar collector and heat storage means. Instead of having to build solar collectors and thermal storage means external to the bin, we simply ‘store’ heat from warm fronts (or auxiliary heating) in the grain. Conventional use of solar heating can actually be counter-productive. When the air is heated before entering the bin, it can be made ‘too’ dry. The heated air can be so low in moisture that it overdryes the bottom layers and then transports that moisture to the upper layers leaving them too wet.
Second, when outside air is colder and dryer that the grain, running the fans makes the grain the ‘heater’ of that air. This cold, dry air already has a naturally low vapor pressure. However, as the incoming cold air is heated, it expands. This expa
Fastovsky Leonid M
Gookins William L.
Shinjyu Global IP Counselors, LLP
Walberg Teresa
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