Moisture content solid

Water activity is, thus, an index of water vapor pressure of food or its moisture state (in our case meat) and is a function of (1) the moisture content/solid content of a meat or meat mixture, (2) the components in and composition of a meat or meat mixture, (3) the microstructure of a meat or meat mixture, (4) the temperature, and (5) the state of some component solids (i.e., sugar). 

FIGURE 3.9 Longitudinal parameter distribution for a cross-flow dryer with one-dimensional solid flow. Drying of a moderately hygroscopic solid (a) material moisture content (solid line) and local exit air humidity (broken line) (b) material temperature (solid line) and local exit air temperature (broken line), is wet bulb temperature of the incoming air. 

The two primary factors that influence the efficiency of solid/liquid separation by gravity are the rate of sedimentation and the moisture content (solids concentration) of sediment. The rate of sedimentation should be maximized while the moisture content of the sediment should be minimized. 

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some soflds may leave in the hquid stream, and some Hquid will be entrained with the separated soHds. As emphasis on the separation efficiency of soHds or Hquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, ie, how much of the incoming solids is coUected by the filter. Separation of Hquid usually is measured in how much of it has been left in the filtration cake for a surface filter, ie, moisture content, or in the concentrated slurry for a filter-thickener, ie, solids concentration. 

Dry-weight basis expresses the moisture content of wet sohd as kilograms of water per kilogram of bone-dry solid. 

Moisture content of a solid is usually expressed as moisture quantity per unit weight of the diy or wet sohd. 

Capillary Flow Moisture which is held in the interstices of solids, as liquid on the surface, or as free moisture in cell cavities, moves by gravity and capiUarity, provided that passageways for continuous flow are present. In diying, liquid flow resulting from capiUarity appUes to liquids not held in solution and to aU moisture above the fiber-saturation point, as in textiles, paper, and leather, and to all moisture above the equiUbrium moisture content at atmospheric saturations, as in fine powders and granular solids, such as paint pigments, minerals, clays, soU, and sand. 

Liquid Diffusion The movement of liquids by diffusion in soUds is restricted to the equihbrium moisture content below the point of atmospheric saturation and to systems in which moisture and solid are mutually soluble. The first class apphes to the last stages in the diying of clays, starches, flour, textiles, paper, and wood the second class includes the diying of soaps, glues, gelatins, and pastes. 

When a solid is dried experimentally, data relating moisture content to time are usually obtained. These data are then plotted as moisture content (dry basis) W versus time 0, as shown in Fig. 12-41 7, This curve represents the general case when a wet sohd loses moisture first by evaporation from a saturated surface on the sohd, followed in turn by a period of evaporation from a saturated surface of gradually decreasing area, ancf finally, when the latter evaporates in the interior of the solid. 

In diying solids it is important to distinguish between hygroscopic and nonhygroscopic materials. If a hygroscopic material is maintained in contact with air at constant temperature and humidity until equilibrium is reached, the material will attain a definite moisture content. This moisture is termed the equilibrium moisture content for the specified conditions. Equilibrium moisture may be adsorbed as a surface film or condensed in the fine capillaries of the solid at reduced pressure, and its concentration will vaiy with the temperature and humidity of the surrounding air. However, at low temperatures, e.g., 15 to 50°C, a plot of equilibrium moisture content versus percent relative humidity is essentially independent of temperature. At zero humidity the equilibrium moisture content of all materials is zero. 

Equilibrium moisture content of a solid is particularly important in drying because it represents the limiting moisture content for given conditions of humidity and temperature. If the material is dried to a moisture content less than it normally possesses in equilibrium with atmospheric air, it will return to its equilibrium value on storage unless special precautions are taken. 

Material Physical form Moisture contents, kg/kg dry solid Inlet-air temperature, K Depth of bed, cm Loading, kg product/m Air velocity, m/s X 10 Experimental drying time, s X 10 ... 

Pretreatment Chemicals Even though the suspended solids concentration of the slurry to be tested may be correct, it is frequently necessary to modify the sluriy in order to provide an acceptable filtration rate, washing rate, or final cake moisture content. The most common treatment, and one which may provide improvement in all three of these categories, is the addition of flocculating agents, either inorganic chemicals or natural or synthetic polymers. The main task at this point is to determine which is the most effective chemical and the quantity of chemical which should be used. 

Objective Determine the filter size and vacuum capacity required to dewater and wash 15 mtph of dry solids while producing a final washed cake with a moisture content of 25 wt % and containing 0.10 wt % TDS based on dry cake solids. 

Cake Dryness In dewatering, usually the cake needs to be as diy as possible. Cake diyness is commonly measured by the solids fraction by weight W or by volume ,. The moisture content is measured by the complement of W or ,. The volume fraction of the pores and void in the wet cake is measured by the cake porosity (= 1 — ,) whereas the volume fraction of the liquid in the pores of the cake is measured by the saturation S. For welf-defined solids in the cake with sohd density (bone diy) p, and hquid density Pl, and given that the cake volume... 

Estimating Minimum Sample Quantity for Moisture Measurement Estimates of material quantity for testing moisture content depend on mechanisms of moisture distribution in the material. Moisture is physically retained on particle surfaces, chemically adsorbed on surfaces and within pores of particulate solids, and contained as an internal constituent of solids. Significant internal moisture is most often encountered in organic and agricultural source materials. 

Physical Composition Information and data on the physical composition of solid wastes including (1) identification of the individual components that make up industrial and municipal sohd wastes, (2) density of solid wastes, and (3) moisture content are presented below. 

Moisture content. The moisture content of solid wastes usually is expressed as the mass of moisture per unit mass of wet or diy material. In the wet-mass method of measurement, the moisture in a sample is expressed as a percentage of the wet mass of the material in the diy-mass method, it is expressed as a percentage of the diy mass of the material. In equation Form, the wet-mass moisture content is expressed as follows ... 

Typical data on the moisture content for the solid-waste components are given in Table 25-51. For most industrial sohd wastes, the moisture content will vary from 10 to 25 percent. 

TABLE 25-51 Typical Density and Moisture-Content Data for Domestic, Commercial, and Industrial Solid Waste... 

Adsorption for gas purification comes under the category of dynamic adsorption. Where a high separation efficiency is required, the adsorption would be stopped when the breakthrough point is reached. The relationship between adsorbate concentration in the gas stream and the solid may be determined experimentally and plotted in the form of isotherms. These are usually determined under static equilibrium conditions but dynamic adsorption conditions operating in gas purification bear little relationship to these results. Isotherms indicate the affinity of the adsorbent for the adsorbate but do not relate the contact time or the amount of adsorbent required to reduce the adsorbate from one concentration to another. Factors which influence the service time of an adsorbent bed include the grain size of the adsorbent depth of adsorbent bed gas velocity temperature of gas and adsorbent pressure of the gas stream concentration of the adsorbates concentration of other gas constituents which may be adsorbed at the same time moisture content of the gas and adsorbent concentration of substances which may polymerize or react with the adsorbent adsorptive capacity of the adsorbent for the adsorbate over the concentration range applicable over the filter or carbon bed efficiency of adsorbate removal required. 

Applicability/Limitations Liquid injection incineration can be applied to all pumpable organic wastes including wastes with high moisture content. Care must be taken in matching waste (especially viscosity and solids content) to specific nozzle design. Particle size is a relevant consideration so that the wastes do not clog the nozzle. Emission control systems will probably be required for wastes with ash content above 0.5 percent (particulate control) or for halogenated wastes (acid gas scrubbers). 

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