Diffusion units

Liquid phase diffusivity Units in SI system m2/s Dimensions in M. N. L, i T... 

A dry combustion-direct injection apparatus was applied to water samples by Van Hall et al. [51 ]. The carbon dioxide was measured with a non-dispersive infrared gas analyser. Later developments included a total carbon analyser [97], a diffusion unit for the elimination of carbonates [98], and finally a dual tube which measured total carbon by combustion through one pathway and carbonate carbon through another. Total organic carbon was then calculated as the difference between the two measurements [99]. 

These sensors differ fi-om ordinary gas-diffusion units in the fact that the analytical reaction (derivatization of volatile species) and detection take place simultaneously immediately after the continuous separation step. They also differ from (usually electrochemical) sensors using a static internal solution. In this respect, they resemble the sensor reported by DeGrandpe [12], from which they differ in that the sample is not passed through the sensor, even though the reagent is. 

The number of diffusion units is computed from the relation /dT/(Hs -H). It is determined from process conditions imposed on the tower alone, not by the tower s performance. Htu is the only parameter determined experimentally. 

A cooling tower has a cross-sectional area of 25 X 25 ft. The total heat load to the unit is 27,500,000 Btu/hr. The locality has a 5% wet-bulb temperature of 75°F. Water exits the tower with a 12° approach to the wet-bulb temperature (i.e., 87°F).The hot process water enters the tower at a temperature of 125°F, and the water equivalent to this range is 1800 gpm. The systems fan capacity is 150,000 cfm (a) Determine the number of diffusion units that the tower must be capable of performing to meet process requirements (b) the tower manufacturer provided the following data for overload and underload conditions for the tower ... 

A direct-contact gas cooler system operates as follows Approximately 35,000 lb/hr of bone-dry air is passed over hot trays. The air is heated from 150°F to 325°F as it passes over the trays. It exits from the unit with a due point of 105°F. The hot air is sent to a direct-contact cooler, where its temperature is reduced back to 150°F. During the cooling stage, the air is dehumidified with water that is heated frpm 75°F to 105°F. The unit is rated at 3.5 inches of water pressure drop (a) Determine the number of diffusion units needed for this operation and (b) Establish the required dimensions for the direct-contact cooling tower (Hint Use standard low-pressure-drop data from the literature. Some of the older literature give pressure drop data for simple fill. See Sherwood, T. K. and C. E. Reed [6]. 

For problem 6.3, determine how many diffusion units are needed to cool the gas from 700°F to 150°F. Assume same operating conditions. 

Figure 10.9. Continuous system used by Su et al. to implement various anaiyticai methods inciuding a fiow-injection manifoid, a gas-diffusion unit and a buik acoustic impedance sensor. A acceptor, BAWIS buik acoustic wave impedance sensor, C — carrier, GDC — gas diffusion ceii, iV — injection vaive, PP — peristaitic pump, R — reagent, RC — reaction coii, SL — sampie ioop, 1/1/— waste, WB — water bath. (Reproduced with permission of Eisevier, Ref. [98].)...

The thermal conductivity of a material is defined in terms of the transport of heat under steady-state conditions. On the other hand, one is often interested in the transport of heat when a specimen is not at equilibrium so that the flow of heat is transient. The thermal diffusivity a , which is defined by Equation 14.2, describes these time-dependent, non-steady-state aspects of heat flow. The thermal diffusivity is used to calculate the temperature (T) as a function of the position within the specimen (z) and the time (t) under non-steady-state conditions. It is related by Equation 14.3 to the thermal conductivity, the density, and the specific heat capacity. The values of X and a can be measured independently. However, often one of them (usually a) is estimated from the measured value of the other one (usually X) by using Equation 14.3. If X is in J/(K m sec), cp is in J/(g K) and p is in g/cc, then the a value calculated by using Equation 14.3 must be multiplied by 100 to convert it into our preferred diffusion units of cm2/sec. 

FIGURE 8.21 Gas diffusion units with planar (upper) and tubular (lower) membranes. D = donor stream flowing through the unit towards waste A = acceptor stream flowing through the unit towards detection M = membrane (traced lines) a = drilled channel (upper) or concentric tube (lower) b = manifold tube. 

FIGURE 8.22 Classical manifold architectures for implementing in-line gas diffusion involving intermittent (upper) or continuous (lower) sampling. S = inserted sample C = sample carrier stream S/C — continuous inlet of sample or carrier stream R = reagent A = acceptor stream Rci — coiled reactors GD — gas diffusion unit D = detector ... 

FIGURE 8.23 Schematic representation of gas diffusion units involving the classical architecture (left), pervaporation (centre) and membraneless GD (right), a = drilled channel for the donor stream b = drilled channel for the acceptor stream c — air-gap d = air headspace dashed lines — membrane. For details, see text. 

For simultaneous determinations and /or speciation, different separation/ concentration steps can be implemented in the same manifold, as in, e.g., the determination of nitrogen, phosphorus and potassium in fertilisers [314]. The sample was inserted and passed successively through a dialysis unit and a gas diffusion unit to a flow cell for the spectrophotometric determination of phosphate. The dialysed potassium ions and the diffused gaseous ammonia were collected in specific streams and determined by flame photometry and potentiometry, respectively. 

S. Motomizu, K. Toei, T. Kuwaki, M. Oshima, Gas-diffusion unit with tubular microporous poly (tetrafluoroethylene) membrane for flow-injection determination of carbon dioxide, Anal. Chem. 59 (1987) 2930. 

N. Choengchan, T. Mantim, P. Wilairat, P.K. Dasgupta, S. Motomizu, D. Nacapricha, A membraneless gas diffusion unit design and its application to determination of ethanol in liquors by spectrophotometric flow injection, Anal. Chim. Acta 579 (2006) 33. 

M.I.G.S. Almeida, J.M. Estela, M.A. Segundo, V. Cerda, A membraneless gas-diffusion unit — multisyringe flow injection spectrophotometric method for ammonium determination in untreated environmental samples, Talanta 84 (2011) 1244. 

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