Flow instabilities columns

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

The mechanisms by which freely rising bubbles interact with each other in relatively low-viscosity liquids and, specifically, how they approach, contact, and coalesce or break up are important aspects of multi-phase flow. Coalescence and breakup can control the interfacial area and mass transfer rate in bubble columns and gas-sparged chemical and biological reactors. Bubble interaction is fundamental in two-phase flow instability that plagues boilers and oil and gas wells. But bubble interaction remains a relatively mysterious area. 

To overcome slug flow instability, some distillation columns are equipped with dual risers. A small-diameter riser is used at low throughputs. At higher rates, the larger diameter riser is put into service. 

From a hydraulics standpoint, a reboiler and column base function crudely as a U-tube manometer whose natural period in seconds is approximately V//1.3 where / is the length in feet of the liquid column. Manometer theory says that damping is increased as the liquid flow restriction is increased. There may be a connection between this and choked-flow instability for a period of 3 seconds, / = 15 feet, which is fidrly typical for industrial installations. 

A full opening valve or variable orifice should be able to restrict flows of liquid into the bottom of the reboUer so that the instability in the liquid in the column will not be direcdy introduced into the inlet of the reboUer. Generally, the liquid inlet nozzle size should be about 50% in the inlet tube flow cross-section area. A large line is sometimes used, but a restricting provision should be provided to to stabilize operations. 

Slagt et al. [134] have stated that because of their thermal instability and reactivity sultones could not be easily analyzed by gas chromatography. They studied the two methods published by Martinsson and Nilsson using a Carlo Erba Fractovap G1 equipped with a flame ionization detector and a glass column (length 0.65 m OD 1/4 in.) filled with 10% OV 1 on Chromosorb W-AW (80-100 mesh). The column temperature was 230°C and the injector/de-tector temperature 275°C. The gas flow rates were N2 25 ml/min (carrier gas), H2 25 ml/min, and air 250 ml/min. One microliter of sample was injected. 

Mathematical methods for determining the gas holdup tine are based on the linearity of the plot of adjusted retention time against carbon number for a homologous series of compounds. Large errors in this case can arise from the anomalous behavior of early members of the homologous series (deviation from linearity in the above relationship). The accuracy with which the gas holdup time is determined by using only well retained members of a homologous series can be compromised by instability in the column temperature and carrier gas flow rate [353,357]. The most accurate estimates... 

As mentioned in the previous -section, the existence of a liquid film of uniform thickness around the base of a slug was pointed out by Nicklin, Wilkes, and Davidson (N4). In an extension of this work, Nicklin and Davidson (N3) have carried the analogy between this situation and a wetted-wall column to the instability point known to occur in wetted-wall columns at high gas-flow rates. 

At low liquid rates, the onset of instability occurs at a constant value of the total superficial velocity, and is predictable from holdup and flooding data for wetted wall columns. As liquid flow rates increase, Nicklin and Davidson predict that unstable flow begins at lower values of the gas flow rate. For high liquid flow rates, however, the slug length becomes important, and the unstable flow will begin at higher values of gas flow rate. Therefore, a definite liquid flow rate exists at which an unstable flow pattern appears with a minimum gas flow rate. 

Flooding is an excessive accumulation of liquid inside a column. Flood symptoms include a rapid rise in pressure drop (the accumulating liquid increases the liquid head on the trays), liquid carryover from the column top, reduction in bottom flow rate (the accumulating liquid does not reach the tower bottom), and instability (accumulation is non-steady-state). This liquid accumulation is generally induced by one of the following mechanisms. 

Despite the breakthrough associated with Merrifield s approach, there are several limitations such as the discontinuous nature of the reaction, the need for large excesses of reagent and the mechanical instability of the polymer matrix. An early solution to the restrictions imposed by Merrifield s polystyrene supported batch process was the use of commercially available benzyl alcohol-functionalized silica (used for H PLC columns). This was initially derivatized with the first member of the peptide chain to be propagated. The synthesis of a tetrapeptide in flow was completed in half the time required for the equivalent batch mode assembly and required significantly smaller excesses of the solution-phase reagent [92],... 

In order to obtain a commercial loading of the near-critical extractant, the extraction is sometimes carried out at enhanced pressures in the droplet regime. In such cases the liquid phase does not flow downwards as a film adhering to the packings of a column as is usually assumed, rather it falls down as a swarm of droplets. On the basis of the separation of a mixture of partial glycerides the behavior of packed columns in the droplet regime (instable flowing films) the efficiency of different column installations are compared. A mixture of 55 wt.% propane and 45 wt.% carbon dioxide is used as an extractant. 

The principal advantages in the use of superheated water are that it is relatively easy to attain and the back-pressures required on the column are small. Thus even a modest length of narrow bore tubing can be employed to provide sufficient resistance to prevent boiling in the column and at these pressures many conventional spectroscopic flow cells can be used. Because of the high temperatures, there have been concerns about the thermal stability of the analytes, but of the numerous examples, there have been few reports of instability or a tendency for accelerated hydrolysis or oxidation, of the reported examples, only aspirin has hydrolyzed. Compounds which might be expected to be labile to oxidation or hydrolysis, such as the paraben antioxidants, have chromatographed without problems even up to 200°C [59]. 

The katherometer detector is a mgged, very forgiving detector that can be quite seriously abused in operation and still provide accurate quantitative results. This, of course, is a direct result of its relatively low sensitivity. The main problem with the katherometer is instability due to poor control of operating and ambient conditions. The katherometer is very sensitive to temperature changes and changes in columns flow rate (the reasons have already been discussed). Consequently, the flow controller and the sensor oven controls must be well maintained to ensure a precise column flow rate and a precise... 

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