Hold-up time

If oil buffered seals are used on the compressors, the seal leakage toward the process side of the compressor must be carefully measured, as it is (and should be) a small value. While five gallons per day doesn t sound too small, in a four-hour run, this is less than two pints, making the hold-up time at the inner seal chamber and in the lines to the drain pots a significant value. This makes exact measurement quite difficult. 

Polymerization to Cg and CJj olefins is the chief side reaction. Polymerization increases with extraction temperature and with the hold-up time in the extraction section. It limits the temperature used to obtain high extraction rates. 

Continuous polymerization processes for PA-6,6 have been reported for over 30 years.5,6,28 Prepolymerization in tubular (Fig. 3.21) or baffled reactors is particularly well suited to continuous polymerization. The polymerization of prepolymers to high-molecular-weight materials in a continuous process is more difficult to control as small differences is molecular weights result in large differences in viscosities. Viscosity differences result in different hold-up times in die reactor and thus nonhomogeneous products. 

For maximum effectiveness the periods of the oscillation were chosen so as to be relatively long with respect to the hold-up time of the reactor (see Figure 1). A control policy was selected so that the following also pertained. 

Discussion. It is apparent from Table II and Figure 3 that even though the reactor has been subjected to very severe oscillatory conditions where the frequency of oscillation was low with respect to the hold-up time and the amplitudes of the functions large, the MWD s of the resulting polymers differ very little from those produced in the steady-flow steady-state. 

The influence of changes in these other variables on MWD in a homopolymerization has not yet been tested, but whatever perturbations are introduced to the feed in a radical polymerization in a laboratory-scale CSTR, they are unlikely to introduce dramatic changes in the MWD of the product because of the extremely short life-time of the active propagating chains in relation to the hold-up time of the reactor. This small change in MWD could be advantageous in a radically initiated copolymerization where perturbations in monomer feeds could give control over polymer compositions independent of the MWD. This postulate is being explored currently. 

A rough estimate of the decanter volume required can be made by taking a hold-up time of 5 to 10 min, which is usually sufficient where emulsions are not likely to form. Methods... 

The liquid level will depend on the hold-up time necessary for smooth operation and control typically 10 minutes would be allowed. 

A horizontal separator would be selected when a long liquid hold-up time is required. 

In the design of a horizontal separator the vessel diameter cannot be determined independently of its length, unlike for a vertical separator. The diameter and length, and the liquid level, must be chosen to give sufficient vapour residence time for the liquid droplets to settle out, and for the required liquid hold-up time to be met. 

Hold-up time = liquid volume/liquid flow-rate... 

Size the vessel to restrict the carryover of liquid droplets. The liquid hold-up time need not be considered, as the liquid level will be a function of the thermal design. 

Intermediate storage tanks will normally be needed to smooth out fluctuations in column operation and process upsets. These tanks should be sized to give sufficient hold-up time for smooth operation and control. The hold-up time required will depend on the nature of the process and on how critical the operation is some typical values for distillation processes are given below ... 

The adjusted retention time provides a measure of the strength of intermolecular interaction between the analyte and the stationary phase, with stronger interactions giving a longer time. The gas hold-up time is derived from the flow rate and the column dimensions and is often measured by injecting a non-retained compound. The retention factor, which represents a ratio of the mass of analyte dissolved in the stationary phase to the mass in the mobile phase, can be calculated from the adjusted retention time and the gas hold-up time. 

The average volumetric flow rate (ml/min) is calculated from the gas hold-up time, the column length (cm) and the column radius (cm). 

Systems with electronic pneumatic control use pressure transducers at the inlet and outlet, the column dimensions and physical properties of the carrier gas to determine the gas hold-up time and the flow rate. 

The carrier gas viscosity is given as r, L the column length, pQ the outlet pressure, P the ratio of inlet pressure to outlet pressure and dc the column diameter. Analysts should take care to be sure that the methods used for determining the flow rate are consistent from in-strument-to-instrument and from method-to-method. Otherwise it will be difficult to compare any data that have the flow rate, gas hold-up time or linear carrier gas velocity as a component. 

Using the retention data and the chromatogram shown in Fig. 14.8, tabulate the following for each peak retention time ( r), adjusted retention time (t K), retention factor (k), partition coefficient (Kc) and number of theoretical plates (N). The column phase ratio was 250 and the gas hold up time ( m) was 0.995 min. 

Fig. 4. 33 The AKUFVE solvent extraction apparatus Efficient mixing is achieved in the separate mixing vessel, from which the mixture flows down into the continuous liquid flow centrifugal separator (the H-centrifuge, hold-up time <1 s). (From Refs. 83a,b.) The outflow from the centrifuge consists of two pure phases, which pass on-line detectors, AMXs, for on-line detectors or continuous sampling. (From Refs. 80a-80d, 81.)...

Oumada, R Z., Roses, M., and Bosch, E. Inorganic salts as hold-up time markers in C18 columns, Talanta, 53(3), 667-677, 2000. 

Calculate the apparent capacity factor as kg = (tg - t0)/t0, where t is the retention time and t0 is the mobile phase hold-up time. 

The major disadvantage of batch reaction now is the hold-up time between batches. Although the actual reaction time necessary to process a given amount of feed may be substantially less than for a time-averaged reactor such as a CSTR, when the hold-up time is added, the total process time may be greater. Other disadvantages of the batch reactor are dependent on the particular type of reaction being considered, such as whether the reaction is in parallel or series. 

Let us consider some of the special problems encountered in the operation of a radioisotope detector and the compromises that must be considered. Like any chromatographic detector, a carbon-14 detector should have a small volume and a short hold-up time in order to minimize band spreading and loss of resolution. Unfortunately radioisotopes are measured with an inherent time factor - disintegrations per minute. Therefore, the smaller the cell and the shorter the hold-up, the lower will be the sensitivity, a circumstance which is totally at odds with the first requirement. In practice, we have found that a U-tube with a cross-section diameter of 2mm is generally satisfactory. This gives a cell with a void volume of 200-300 yl, which is high compared to the 2-10 yl volumes of many UV flow cells, and may introduce some band spreading when used with the best new HPLC columns. 

The capacity parameters do not affect the selectivity (a), but they do have an effect on the capacity factor (fc) and hence on the resolution (Rs see eqn.1.22). The physical parameters only affect the resolution through the efficiency (N). They also have an effect on the retention time through the hold-up time t0 (see eqn.1.6). 

In order to estimate k (and hence f(fc)), an estimate for the hold-up time t0 is required. However, this can be avoided if N is expressed as a function f( KR) of the retention volume. 

In this equation kin is the capacity factor, which the solute would show under isocratic conditions (i.e. an elution at a constant mobile phase composition) corresponding to the composition at the inlet of the column at the time t that has elapsed since the start of the gradient. ka is the capacity factor at the start of the gradient (t = 0),b the gradient steepness parameter, and t0, as usual, the hold-up time of the column. 

The mobile phase composition at the column inlet, at a time twice the value of the hold-up time before the elution of a sample component from the column, may be expected to yield a capacity factor of three for that component under isocratic conditions. 

The two applications shown here concern the optimization of the mobile phase composition in RPLC. However, the method may easily be adapted to other problems. It is most practical if straight retention lines can be obtained. It should be noted that this is not usually the case for retention as a function of mobile phase composition in RPLC. In fact, Colin et al. [555] adapted the value of the hold-up time (t0) such as to obtain straight lines. The fact that they succeeded in doing so for all of 11 solutes considered at the same time is remarkable, but it may not always be possible. In any case, adapting t0 in order to linearize the retention lines will be an awkward practice. 

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