Column bottom temperature

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

The unit operating philosophy and its apparent operating limits often dictate unit constraints. For example, limitations on the main column bottoms temperature, the flue gas excess oxygen, and the slide valve delta P often constrain the unit feed rate and/or conversion. Unfortunately, some of these limits may no longer be applicable and should be reexamined. Some of them may have resulted from one bad experience and should not have become part of the operating procedure. 

There is a condition on this rule of thirds—it is to be limited to areas of latent heat and not to the tube area dedicated to sensible heat. Most reboilers of this type comply to the requirement for very little sensible heat because the large flywheel of circulating liquid is near or exactly equal to the fractionator column bottoms temperature. Therefore, only latent heat applies, and this rule of thirds also applies. 

Raises column bottom temperature. This increases chemical degradation, polymerization, and fouling. 

The operating conditions in the cold separation section arc essentially shown in Fig. Z17. The choice of the gradation in the decreasing operating pressures between the demethanizer and the debutanizer is designed to ensure that the corresponding column bottom temperatures are suf dently low to prevent any undesirable polymerization. 

Column bottom temperature, from Antoine equation for styrene... 

The EB recycle column separates the unconverted EB for recycle to the dehydrogenation reactors. Recent EB recovery columns use high efficiency packing to obtain minimum pressure drop through the column. This allows the column bottoms temperature to be maintained below 100°C. This is an important aspect of the design as styrene polymerization becomes significant at temperatures higher than approximately 100°C. 

For preliminary design, column operating pressure and type condenser can be established by the procedure shown in Fig. 12.4, which is formulated to achieve, if possible, reflux drum pressures Pp between 0 and 415 psia (2.86 MPa) at a minimum temperature of 120°F (49°C) (corresponding to the use of water as the coolant in the overhead condenser). The pressure and temperature limits are representative only and depend on economic factors. Both column and condenser pressure drops of 5 psia are assumed. However, when column tray requirements are known, more refined computations should allow at least 0.1 psi/tray for atmospheric or superatmospheric column operation and 0.05 psi/tray pressure drop for vacuum column operation together with a 5 to 2 psia condenser pressure drop. Column bottom temperature must not result in bottoms decomposition or correspond to a near-critical condition. A total condenser is used for reflux drum pressures to 215 psia. A partial condenser is used from 215 psia to 365 psia. A refrigerant is used for overhead condenser coolant if pressure tends to exceed 365 psia. 

Reducing product column bottom temperature to avoid an undesirable re>... 

In services where the difference between column top and column bottom temperature is small, it may be difficult to find any point where temperature is sufficiently sensitive to composition. Sometimes the... 

Setting Operating Targets with Column Bottom Temperature... 

Because of high column pressure, the column bottom temperature increases, which will have some impact on the reboiler heat input. In case steam or hot oil is used as the heat source, the LMTD (log mean temperature difference) of the reboiler is reestimated. Because of the increase in bottom temperature, the LMTD and reboiler heat duty would reduce. In case of a fired heater, reboiler heat duty is assumed to be the same. 

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