Height of a transfer unit, HTU

With this value, the height of a transfer unit, (HTU)oc can be estimated from ... 

A well-established concept in the design of stripping operations in the chemical processing industries is the use of equations in which the height of the unit required is expressed as the product of two terms the number of transfer units (NTU), a measure of the difficulty of the operation to be carried out, and the height of a transfer unit (HTU), a measure of the height required in order to carry out an operation of unit difficulty. The advantage of this procedure is that the effectiveness of a particular unit is directly reflected in the value of the HTU. On the other hand, the advan-... 

In order to permit sizing a tower, data must be available of the height of a transfer unit (HTU). This term often is used interchangeably with the height equivalent to a theoretical stage (HETS), but strictly they are equal only for dilute solutions when the ratio of the extract and raffinate flow rates, E/R, equals the distribution coefficient, K = xE/xR (Treybal, 1963, p. 350). Extractor performance also is expressible in terms of mass transfer coefficients, for instance, KEa, which is related to the number and height of transfer units by... 

The integral can be found from the operating and equilibrium lines (Fig. 6) for the process and is known as the number of transfer units (NTU). It is a unitless measure of the degree of separation required. The term in parentheses in Eq. (17) is the height of a transfer unit (HTU), which is a characteristic of the flow conditions and mass transfer effectiveness of the contactor. The more effective conditions correspond to smaller values of the HTU whereby a shorter contactor is required for a given NTU. If the equilibrium line and the operating line are approximately parallel, the NTU is approximately equivalent to the number of ideal stages required. 

Technological efficiency involves the following features (a) efficiency characterized by the height of a transfer unit (HTU) or the height equivalent of the theoretical plate (HETP) (b) specific productivity and the possibility of operating over a wide range of phase rates (c) ease of putting into and out of operation, reliability. 

An atmospheric packed tower air stripper is used to clean contaminated groundwater with a concentration of 100 ppm trichloroethylene (TCE). The stripper was designed such that packing height is 13 ft, the diameter (D) is 5 ft, and the height of a transfer unit (HTU) is 3.25 ft. Assume Henry s law applies with a constant (N) of 324 atm at 68°F. Also, at these conditions the molar density of water is 3.47 Ibmol/ft and the air-water mole ratio (G/L) is related to the air-water volume ratio G" jL") through G" jL" = 130 G/L, where the units of G" and L" are ft /(s ft ). 

An alternative approach to the HETP method for calculating the column packing height, also discussed in this chapter, is based on the height of a transfer unit (HTU) and the number of transfer units (NTU). In this approach the packing height is calculated by a theoretical analysis of mass transfer phenomena across the liquid and vapor phases. 

In continuous differential extractors (columns) it has been convenient to think in terms of a height equivalent to a theoretical stage (HETS), and to correlate HETS as a function of system and equipment variables. Alternately, correlations may be obtained on the basis of the height of a transfer unit (HTU), which is more amenable to calculations which separately include the effects of backmixing.l H ... 

The expression in from or the integral has the units of length and is called the height of a transfer unit (HTU) or, for this cuse, the height of a gas-phase transfer unit, H0. which is defined as... 

From the definition of the height of a transfer unit HTU, given by Eq. (13-33), it is evident that the multiplication of Eq. (13-35) by the total depth of packing yields the following relationship used by Bolles and Fair in their analysis... 

Figure 13.11 shows the principal characteristics of Sulzer CY packing for water distillation service [M6]. The optimum throughput is said to be at 75 percent of flooding, at which the F factor is 1.7. At this load, the gas-phase pressure drop is about 4 Torr/m, the Uquid holdup is about 6 percent of the packed volume, and the observed height of a transfer unit (htu) has been found to be between 6.5 and 12 cm. The observed variations in htu are attributed to variations in the wetting of the packing, which is impaired by traces of oil and other hydrophobic impurities in the water. 

In Fig. 14.19 composition is plotted horizontally as hi yjyp, to bring out another difference from an ideal cascade. A plot of distance versus In y in an ideal cascade with constant height of a transfer unit (htu) would be a straight line. In this centrifuge with variable circulation rate, the htu from Eq. (14.186) varies from 18.4 cm at the feed elevation to 3.8 cm at the top and bottom. This causes In to change more rapidly with z at the top and bottom than at the feed elevation. 

Height of a Transfer Unit, HTU Number of Transfer Units, NTU ... 

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