HETP values, height equivalent theoretical plate

In real reactive absorption processes, the thermodynamic equilibrium can seldom be reached. Therefore, some correlation parameters such as tray efficiencies or HETP-values (Height Equivalent to One Theoretical Plate) are introduced to adjust the equilibrium-based theoretical description to the reality. However, reactive absorption always occurs in multicomponent mixtures, for which this simplified concept often fails [16, 23, 24]. 

K is a thermodynamic member and corresponds to the median distribution coefficient of two neighbouring coefficients). Assuming almost equal selectivities in the gas and liquid phases and a uniform R value of 0.5 with GC and LC which is common in practice, LC approaches the efficiency of GC, as far as comparable N values or heights of equivalent theoretical plates (HETP) which are closely related to this. 

Thus a very sharp peak, that is, a small amount of dispersion, implies a large number of theoretical plates. Values of several million plates are not uncommon in analytical systems. This plate number is used to define the height of an equivalent theoretical plate (HETP), a parallel to the height of a transfer unit ... 

The height equivalent to a theoretical plate (HETP) may be calculated from the value for N and the column length ... 

Efficiency of the distillation column is measured by the height equivalent to a theoretical plate, abbreviated HETP or simply h. The length of the column is L, thus h=L/n. The h value is independent of L, whereas the n value is dependent on L. 

Dingenen [9], who studied the effect of the mobile phase velocity on the height equivalent to a theoretical plate (HETP) at different temperatures for benzotriazole derivatives, obtained the results shown in Figure 22, which represents the HETP values found for methanol and the hexane-ethanol mixture. Both curves of Figure 22 clearly demonstrate that the kinetic circumstances are less favorable at low temperatures. A slow mass transfer between the two phases clearly determines the band-broadening process at temperatures below 20°C. This... 

The amount of resin to pack in a column, column geometry, flow rates, pressure, column hardware, and wetted materials of construction should all be evaluated in development. Chromatography columns must be properly packed prior to validating the purification process. From a business perspective there should be some criteria other than purification of the product by which the quality of the packed column can be assessed prior to applying the feedstream, which by this time in the process is quite expensive. Height equivalent to a theoretical plate (HETP) and asymmetry determinations can be used to evaluate the quality of column packing, but may have limited value for some types of packed columns... 

Since the value of N depends on column length, a more useful term is plate height, (h), or height equivalent to one theoretical plate (HETP). This term is derived as shown below ... 

A gas chromatographic study (Carrott and Sing, 1990) of the adsorption of a series of hydrocarbons (C2-C6) by the AMOCO AX21 carbon has revealed some unusual features. Thus, values of the height equivalent to a theoretical plate (HETP) over the range 90-l20°C were found to be independent of both the adsorptive and the temperature. Furthermore, over the range of gas flow rate studied, only the initial section of the Van Deemter plot could be observed (in contrast to the behaviour of other microporous carbons). At present, the explanation for this behaviour is not entirely... 

Height Equivalent to a Theoretical Plate. Provided both the equilibrium and operating lines are straight, HETP values may be estimated by combining the HG and HL values predicted by the above correlations and by translating the resulting HQG into HETP by combining equations 47, 51, and 56 with equation 85, which is discussed under bubble tray absorption columns ... 

In equation (6) is the width at the base of the peak, measiued in the same units as tj.. The theoretical plate model assumes the column to be made of a series of plates. The distribution of the analyte between the mobile and stationary phase occurs at each plate. Therefore, the higher the number of plates, the better the separation since more sorption-desorption cycles occur. Coliunn efficiency can also be expressed in terms of Height Equivalent to a Theoretical Plate value (HETP or H-value) ... 

If the comparison between the two techniques were to be based on the criterion of HETP (height equivalent to theoretical plate) values, HPLC can easily be shown to be a more efficient technique than HPTLC. The conventionally packed HPLC columns used in most analytical laboratories are capable of providing approximately 10,000 theoretical plates. Special HPLC columns, such as capillary columns, are capable of even greater efficiency. However, HPTLC gains certain advantages, which are outlined below, because it is an open-bed system while HPLC is a closed-bed system. 

Equilibrium-stage methods are usually adequate for nearly ideal distillation systems when coupled with calculations of plate efficiency to estimate actual trays or, in the case of packed towers, when HETS (height equivalent of a theoretical stage) or HETP (height equivalent to a theoretical plate) values are known from experience or from experiment to enable the estimation of packed height. For absorbers, strippers, and nonideal distillation systems, mass-transfer models are preferred, but their use requires a value for the tower diameter and a tray layout or type and size of packing. Even when mass-transfer models are preferred, initial calculations are usually made with equilibrium-stage models. Also, note that data for reliable mass-transfer coefficients is often difficult to obtain. 

Except for the short introductory Section 1.3. to this point the entire analysis of separation processes has been equilibrium based. Effects of nonequilibrium operation have been lunped into either a stage efficiency f Sections 4.11.10.2.12.5. and 13.51 or to the height equivalent of a theoretical plate (HETP Sections 10.9 and 10.111. We must move beyond an equilibrium analysis if we want to be able to predict values of the stage efficiency and the HETP fChapter 161. to study membrane separators fChapter 171. or to study sorption separations fChapter 181. For all of these situations, we must look at the mass transfer occurring in the separator. This chapter presents the fundamentals of diffusion and mass transfer in sufficient detail so that the analysis in the remaining chapters is understandable. Additional information on mass transfer is presented as needed in Chapters 16 to 18. If you have already studied mass transfer and diffusion, most, but probably not all, of this chapter will be a review, and you will not have to spend much time studying the material. 

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