Encyclopedia of Renewable Energy. James G. Speight
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Название: Encyclopedia of Renewable Energy
Автор: James G. Speight
Издательство: John Wiley & Sons Limited
Жанр: Физика
isbn: 9781119364092
isbn:
BTU
The Btu (British thermal unit) is a unit of measurement for energy and is the amount of heat that is necessary to raise the temperature of one pound of water by 1 degree, Fahrenheit.
BTX
BTX is an acronym for benzene, toluene, and xylene, which are components of the lower-boiling liquid fuels.
Benzene, the simplest aromatic, is carcinogenic, and its level in gasoline is severely restricted. Toluene and xylene have benzene rings which are attached to one or two methyl (CH3) groups, respectively; xylene has three isomers, with the methyl groups adjacent on the ring (ortho), separated by one carbon (meta), or separated by two carbons (para):
Bubble Point
The bubble point is the temperature at which incipient vaporization of a liquid in a liquid mixture occurs, corresponding with the equilibrium point of 0% vaporization or 100% condensation. The bubble point is based on the fact that liquid is held in the pores of the filter by surface tension and capillary forces. The minimum pressure required to force liquid out of the pores is a measure of the pore diameter. The pressure required to force liquid out of a liquid-filled capillary must be sufficient to overcome surface tension and is a direct measure of an effective tube diameter.
The bubble point pressure (Pb) is the pressure at which saturation will occur in the liquid phase (for a given temperature) and is the point at which vapor (bubble) first starts to come out of the liquid (due to pressure depletion). The bubble point temperature is usually lower than the dew point temperature for a given mixture at a given pressure.
Since the vapor above a liquid will probably have a different composition to the liquid, the bubble point (along with the dew point) data at different compositions are useful data when designing distillation systems and for constructing phase diagrams as a means of studying phase relationships. As pressures are reduced below the bubble point, the relative volume of the gas phase increases. For pressures above the bubble point, a crude oil is said to undersaturated. At or below the bubble point, the crude is saturated.
A bubble point test is a test method that is designed to determine the pressure at which a continuous stream of bubbles is initially seen downstream of a wetted filter under gas pressure. The bubble point test is a practical, nondestructive test used for estimating the pore size of microporous filters and confirming the integrity of sterilizing membrane filters and filter systems. It is the most widely used non-destructive integrity test.
To perform a bubble point test, gas is applied to one side of a wetted filter, with the tubing downstream of the filter submerged in a bucket of water. The filter must be wetted uniformly such that water fills all the voids within the filter media. When gas pressure is applied to one side of the membrane, the test gas will dissolve into the water, to an extent determined by the solubility of the gas in water. Downstream of the filter, the pressure is lower. Therefore, the gas in the water on the downstream side is driven out of solution. As the applied upstream gas pressure is increased, the diffusive flow downstream increases proportionally. At some point, the pressure becomes great enough to expel the water from one or more passageways establishing a path for the bulk flow of air. As a result, a steady stream of bubbles should be seen exiting the submerged tubing. The pressure at which this steady stream is noticed is referred to as the bubble point.
Bubbling Fluidized Bed Gasifier
A bubbling fluidized bed (BFB) consists of fine, inert particles of sand or alumina, which are selected based on their suitability of physical properties such as size, density, and thermal characteristics. The gas flow rate is chosen to maintain the bed in a fluidization condition, which enters at the bottom of the vessel.
The dimension of the bed at some height above the distributor plate is increased to reduce the superficial gas velocity below the fluidization velocity to maintain inventory of solids and to act as a disengaging zone. A cyclone is used to trap the smaller size particle that exit the fluidized bed, either to return fines to the bed or to remove ash rich fines from the system.
Biomass is introduced either through a feed chute to the top of the bed or deep inside the bed. The deeper introduction of biomass in to the bed of inert solids provides sufficient residence time for fines that would otherwise be entrained in the fluidizing gas. The biomass organics pyrolytically vaporize and are partially combusted in the bed. The exothermic combustion provides the heat to maintain the bed at temperature and to volatilize additional biomass.
The bed needs to be preheated to the startup temperature using hydrocarbon resources such as natural gas, fuel oil, either by direct firing or by indirect heating. After the bed reaches the biomass ignition temperature, biomass is slowly introduced into the bed to raise the bed temperature to the desired operating temperature which is normally in the range of 700 to 900°C (1,290 to 1,650°F). Bed temperature is governed by the desire to obtain complete devolatilization versus the need to maintain the bed temperature below the biomass ash fusion temperature. The advantages of the fluidized bed gasifiers are (i) yield of a uniform product gas, (ii) able to accept wide range of fuel particles sizes, including fines, (iii) a near-uniform temperature distribution throughout the reactor, and (iv) a high rate of heat transfer between inert material and biomass, aiding high conversion, with low tar. The disadvantage is formation of large bubbles at higher gas velocities, which bypass the bed reducing the high rate of heat /mass transfer significantly.
See also: Biomass – Gasification.
Bulk Density
The bulk density is the mass of an assembly of coal particles in a container divided by the volume of the container, assuming that the container is full and represents the volume occupied by the solid. The bulk density depends on true density, particle size and size distribution, particle shape, surface moisture, and degree of compaction. The parameter is often used in the design of coal handling, transportation, and storage systems.
Bulk density is a property of powders, granules, and other “divided” solids, especially used in reference to coal and similar solids. The total volume includes particle volume, inter-particle void volume, and internal pore volume.
The bulk density is not an intrinsic property of a material; it can change depending on how the material is handled. For example, powdered coal poured into a cylinder will have a particular bulk density, but if the cylinder is disturbed, the coal particles will move and usually settle closer together, resulting in a higher bulk density. For this reason, the bulk density of powders is usually reported both as freely settled and tapped density – the tapped density refers to the bulk density of the coal powder after a specified compaction process, usually involving vibration of the container.
The test method (ASTM D29l) for determining bulk density concerns the compaction of crushed coal to determine either its compacted or uncompacted weight, for purposes such as charging coke ovens. In addition to the character of the coal itself, moisture content and size distribution of the coal are the two main factors that affect the cubic foot weight. A moisture determination and sieve analysis of the coal should be reported along with the cubic foot weight СКАЧАТЬ