Column Temperature Control

It follows that, at least for SEC, column temperature control can be important. An example of a commercially available column oven is shown in figure 17. The available temperature range varies a little from instrument to instrument but the model shown above has an operational range from 10°C to 99°C. One of the problems associated with the temperature control of ovens is the high thermal capacity of... 

Since the column temperature controls both the overall flow rate and the retentions of the individual compounds, a programmed temperature gradient can be used to shorten the CEC run times and optimize the selectivity [151]. 

The HPLC system consist of an HP 1100 series binary gradient pump, a vacuum degasser, and a column temperature controller (all from Hewlett Packard), connected to a Gilson 231 XL autosampler (Gilson). 

Another advantage of this scheme is that it lends itself readily to the application of feedforward control in order to maintain the D/F ratio for measured changes in feed flow. This feedforward signal would be trimmed by the addition of a feedback signal from the column temperature controller. 

Supercritical fluid chromatography is a very important chromatographic technique still underestimated and underutilized. It presents characteristics similar to both GC and HPLC, although having its own characteristics. Whereas the column temperature control is the way to achieve a good separation in GC and the solvating power of the mobile phase is controlling factor in HPLC, in SFC the density of the fluid is the major factor to be optimized. Both packed (LC-like) and capillary (GC-like) columns have been used in this technique, which has found applications in practically all areas in which GC or HPLC has shown to be the selected separation technique. 

SEC was performed according to the method of Chin et al. (1994). A Shodex KW802.5 SEC column (Waters Corp., Milford, MA., USA) was used and a Waters liquid chromatography system consisting of the following components was used for the analysis Waters 501 high pressure pump. Waters 717 autosampler, InterAction column temperature control oven, Waters 484 UV/VIS detector, and Waters Millenium 2.0 computer software package. 

Column temperature control is perhaps the most popular means of controlling product composition. The control temperature is used as a substitute to product composition analysis. A change in control temperature represents a corresponding variation in the concentration of key components in the product. For instance, a rise in top section control temperature represents a rise in the concentration of the heavy key component in the top product. 

A related limitation impairing the compatibility of configuration 19.66 with schemes 16.4a, 6, and e is the strong interaction between the accumulator level and column temperature control. Consider a rise in column temperature. The temperature controller will decrease boilup (schemes 16.4a and e) or increase reflux (scheme 16.46) to com-... 

Indirect MB control can be troublesome where ambient variations are a prime source of disturbances. A non-MB control can perform well where fluctuations in feed rate and composition are minor. Column temperature control may be ineffective when its range of variation is narrow. 

Column temperature controls were m-able to prevent periodic oif-qjec product resulting from feed fluctuations. Replacing temperature controls by analyzer oontrois eliminated problem and gave smooth, ti t composition control. 

In the manipulated distillate scheme, a column temperature controller manipulates a control valve in the distillate line. In other words, the column temperature is the process variable that is controlled to a setpoint, and the controller output is... 

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

The manipulated boilup scheme is still used in many cases and especially for top-fed stripping columns that have no reflux stream and no rectification section above the feed. A column temperature controller manipulates the steam flow rate to the reboiler. The MRT point for a stripper is often the overhead vapor temperature leaving the column. If the overhead vapor temperature is close to the bottoms temperature, then an overhead vapor flow rate controller can be used to manipulate the steam flow rate. 

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