Reboilers condensate stabilizers

The tower operates in the same manner as a condensate stabilizer with reflux. The inlet liquid stream is heated by exchange with the gas to approximately 30 F and is injected in the tower at about the point in the tower where the temperature is 30 F. By adjusting the pressure, number of trays, and the amount of reboiler duty, the composition of the bottoms liquid can be determined. 

A total reflux startup is best performed with a mixture whose composition resembles the feed composition. Alternatively, total reflux operation can be carried out using one or more of the feed components. The column, reboiler, condenser, and reflux system are stabilized at... 

Controlling heat input by a control valve in the reboiler condensate outlet line may adversely affect reboiler stability, especially in vacuum reboilers. This valve varies condensate level in the heating side of the reboiler. It therefore varies the point at which heating is first applied to the process side. This is equivalent to operating the process side at a fluctuating reboiler sump level. Further details of this and alternative control schemes are in Sec. 17.1.2. 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

The condensate outlet scheme can destabilize thermosiphon reboiler operation. The condensate-side level fluctuations inherent in this scheme are just as detrimental to thermosiphon reboiler stability as process-side level fluctuations (Section 15.3). This is most troublesome in vacuum services. 

In the distillation of products from naturally occurring raw materials, the feed may contain a small percentage of very low-boiling compounds that are not desired in the distillate product. A separate column could be used to stabilize the feed by removal of these very low boilers before distillation in the main column. This stabilizer would require additional energy to operate the reboiler and condenser. However, it may be possible to install a pasteurization section at the top of the main column to remove these very low boilers. 

The stabilizer contains 20 trays and a total condenser. Feed enters at tray 10. The normal column overhead pressure is 700 kPa and there is a 20 kPa pressure difference that is evenly distributed between the condenser and the reboiler. Each tray is 2.0 m in diameter with a 0.10 m weir, which is 1.6 m long. 

Kiw-pressure stabiliser Reflux condenser Rich gas from crude Rich stabilizer gas Reboiler exchanger Heat exchanger Gas separator... 

In designing a stabilizer, the conditions at the top (Example 22-9) and at the bottom are computed, the number of plates and reflux ratio are adopted from examining commercial operations (Table 16-10), and means-are provided to condense the reflux at the top and to supply reboiler heat at the bottom. 

---