Document 0qXyYy3xMEgV67N9nokE5k6nd

sbsssssssw 128 CHAPTER 9 1965 Guide And Data Book Table I .... Maximum Moisture Content for Safe Direct Methods Storage for One Year Three oven methods are commonly used for the determinv Motrturq tion of moisture content Cr-- Coetmnt, % Air Oven. Duplicate samples of finely ground grains (2-3 Cora Wheat (hard red winter) Wheat (soft red winter) Oats Grain Sorghum grams each) are dried for one hour at 130 C. If the initial 12* 13? moisture exceeds 13 percent, a two-stage method is used. A weighed sample of whole grain is partially dried to a mots- ture content of 13 percent or below and then ground and coo. 13 pletely dried as in the one-stage method. The moisture lost in Soy Beans * -Rough-Rice Flaxseed Hay 13* , ' 20 both stages is considered in determining the initial moisture " , content The air-oven is the .basic method specified for d&- > : termining the moisture content of grains in the official grain ' standards of the United States.1 , Another commonly used air-oven method for the determi- nation of moisture in grain and stock feeds is presented as an official method by the Association of Official Agricultural Chemists. This method, which gives slightly higher results, characteristic fermented odor. Bmrilarf crops are not generally*' uses a ground sample dried at 135 C for 2 hr. considered marketable through the matal channels, nor are One of the most widely accepted direct methods is based on they useful for human food; however, ensiled products make *- the use of a vacuum to increase the rate of drying. Duplicate excellent livestock feed. two to three gram samples of finely ground grain are dried at Groin Condition 90-100 C for approximately five hours at a pressure of 25 mm or less of mercury. This method is also one of the official raetb- The condition of grain as a result of its previous history-is an important, though not easily defined, factor in its stonw bility. Grain in its original state apparently possesses some natural resistance to deteriorative changes, perhaps due'in part to the seed coat. Mechanical or heat damage, mold iiV vasion, and insect invasion tend to decrease this natural re sistance. ods of AOAO and gives reasonably good agreement with the one hour air-oven method. The Brown-Duval method is the most common distillation procedure (or determining moisture content. A weighed sample of whole grain is heated in a special apparatus and the vapor evolved is condensed and collected in a cylinder cafc brated directly in moisture content. The Brown-Duval method, as with the oven methods, is somewhat empirical. MOISTURE MEASUREMENT Proper operating procedure and accurate calibration are ] j 5 / I s f i T \ f i \ | s ? \ ) ^ * | ; \ 1 Moisture content can be expressed on either the wet or dry basis; however, the wet basis is used exclusively by fanners and in the grain trade. Unless otherwise noted, moisture con tents indicated in this chapter are calculated on a wet basis. The dry basis moisture content is used by research workers and in mathematical expressions dealing with moisture con1 tent and drying. The moisture content on a wet basisiscal- culated by dividing the weight of water in the material by the total weight The percent moisture on a dry basis is'calculated by dividing the weight of water by the weight of dry matter'. Percent moisture (wet basis) 100wm Wm + Wo ; (1). Percent moisture (dry bams) -- Mo ~ "(2) token Wm ~ weight'd water, pounds. Wo " weight of dry matter, pounds. Percent moisture on a wet basis (Mw) may be converted to percent moisture on a dry basis and vice versa :ijy use of/the following equations: 100(Af.) Mo - (100 - if.) Af. - lOO(Afo). (100 + Md) . (3) (4) . Methods used for determining moisture content can < be >.i<uarifipid as either direct or indirect. In the direct methods the water, is driven from the product and the lora in product weight or the amount of vapor evolved is used to determine the moisture content.1 Indirect methods involve the measure ment ofproperties of the material which are a function of the moisture content. Indirect Methods The simplicity and speed of measuring moisture content by electrical methods have made these, the most commonly used for measuring moisture in practical applications. All electrical meters must be calibrated for each product by one of the direct methods. They are also sensitive to temperature changes and the calibration must include a temperature correction fac-. tor. ' A practical limit on the range of moisture.oontent that can be measured by conductive'meters is approximately 7-23 percent Recently dried grain will give low readings' and re cently wetted grain'will give high readings as a result of the unrepresentative surface conditions. Mixed wet and dry grain and grain out of condition will also resultin erroneous read ings with the conductance type meters. The dielectric properties of products depend to a large degree oh the moisture content The capacitance meter utilizes this relationship by introducing grain as the dielectric in a capacitor in a high frequency electrical circuit. Although tire capacitive reactance is the primary portion ofthe overall impedance measured, the resistive component is also signifi cant in many of the capacitance meters. At higher frequencies and in instruments with insulated electrodes the relative effect of the resistance is reduced. The reduction in the re sistive effect.is important in reducing errors introduced by unusual product surface conditions. ! Meters of the capacitance type are generally less subject to errors resulting from uneven moisture distribution. The range r of moisture content that can be measured is wider than for the conductance meters. - Moisture measurement by capacitance meters is sensitive to temperature, product weight and product density. To reduce' Physiolog'^1 Factors in.Drying ond Storing Farm Crops: 129 sources of error a weighed sample is introduced into the rnrasuring- cell, by some' reproducible mechanical means. pjforfttio" including temperature correction is required. Recent developments have been aimed at overcoming the need for a given sample size and density. However, no instru ments are commonly available which incorporate these im provements. rjkg humidity of the air surrounding a product m a closed ^nfoi'nw will come to equilibrium with the product. This-is achieved when the vapor pressure of the moisture in .the product is equal to the vapor,pressure of the air. This equilib rium relative humidity may be used as an-indication of the moisture content It has been proposed that this' method could indicate the storabUity of grain independent of the ac tual moisture content since the equilibrium relative humidity of the air in grain, to a large extent, controls mold growth. ' ffay Moisture Determination. The`determination of hay moisture content has not received the consideration that has been devoted to grains. The oven methods have been used rather extensively. A rapid oven'method of drying- samples has been developed- by Dexteri .using the exhaust .from a tractor. A weighed sample is placed in a cylinder and attached * to the exhaust. An improved version of this exhaust oven was developed by Isaacs and Wiant* in which outside air was rpiTpd with,the exhaust air. This reduced the drying tempera ture and increased the airflow rate to prevent burning of the ; EftlTiplftElectrical meters have been applied to some extent on forage crops; however, the extreme variability of.the moisture and density of the material tested leads to great variability in the readings obtained. A reasonable indication of the average moisture content of a mass of hay can be obtained if a large number (25 or more) measurements are taken and averaged. An electrical scanning device for making and' averaging a large number of readings automatically was developed by Isaacs and Wiant.* DRYING. THEORY Drying in its ordinary applications is a heat and a mass transfer process involving the vaporization of water in the liquid state, miring the vapor with the drying air, and removal of the vapor by mechanically carrying away the mixture. Sufficient heat for vaporisation of the moisture in the product must be supplied, about 1000 Btu per pound of water. This amount must be supplied by reducing the sensible heat of the drying air or by applying heat directly to the product by con duction, radiation, dielectric beating or some other means. Utilizing the-sensible heat content of the air is by far the most common mode of drying. Drying by this method can be described from the standpoint of the drying air on the psychrometric chart- The drying process is generally assumed to be adiabatic, i.e., all of the sensible heat lost by the air. is utilized for moisture vaporization and is converted to the latent heat of water vapor in drying air. The total heat content of the air does not change during its participation in the drying process. Therefore, the state point of the air may be considered to move up the adiabatic saturation lines on the psychrometric chart. (See Fig. 1.) If in contact with the product long enough, the air will exhaust at equilibrium with the moisture in the material. That is to say that the partial pressure of the water vapor in the air is equal to the partial pressure of the water vapor in the ma terial. There being no longer any potential for moisture trans fer, no more drying takes place. The relative humidity of the air at equilibrium with material of a giveri moisture content is known as the equilibrium relative kumiditif'The moisture con tent of hygroscopic material in contact with air of a given relative humidity for"an indefinite period of time is known as the equilibrium moisture content The relationship between relative humidity and moisture content at'equilibrium may be presented graphically as in fig. 2, for different materials and at various temperatures.1 Henderson* expresses' the equilibrium relationship mathe matically with the following equation: 1 . (5) where . >. V * relative humidity, expresed as a decimal, e -- natural (Naperian) base of logarithms. T -- absolute temperature,- Ranking degrees. M. equilibrium moisture content in percent dry basis. e * empirical constant;" . ' n = empirical constant. Some values of the constants c and n for various materials are given in Table 2. ; : - ' - Referring to Fig. l, if drying air at temperature. T. and relative humidity <tn reaches equilibrium with material at rela- time humidity 4>i, the exhaust;air temperature will be 7V The moisture gained by each pound of drying air is the'difference in the humidity ratio (H% -- TFO- This does not reveal what the condition of the exhaust air is if the air is not in contact with' the product'long enough to reach equilibrium, nor does it say what the drying rate of the product will be under such conditions. --.................................................. Several methods of analyses are available for predicting drying rates of porous'materials.such as grain under stated conditions, of air temperature, humidity/ original moisture content, and gram depth.*-10-11 An analytis of grain drying by Hnlrill* will be presented here. - - - The maTimnirt rate :(dM/dt) at which a granular hygro- - scopic materiaTsuch as grain win transfer moisture to or from air may be described by the following differential equation: dM ' - --------- C(P,-P.) (6) where__; C TM constant, reptesenting the vapor conductivity of the kernel and air film around it' Pt -- partial pressure of the water vapor,-in the grain. ; P, -- water vapor pressure in the drying air. If P, > Pa, drying talcM place. If P9 =* P,, moisture.equl- librium"exists andnomoisture trahsfer occurs; 7f Pi < P,, wetting occurs. r . i- --