Reboiler temperature control

This result suggests the following pairings (1) fl-< > (the imbalance in the holdups controlled by the imbalance in the heat duties of the two reboilers), (2) AT-F (the reboiler temperature controlled by the total heat duty), and (3) i f-AB (the total holdup controlled by the bottoms flow rate). These control loops largely respond independently of each other (there is a small one-way interaction between the second and third loops), and are referred to as decoupled. ... 

A simple recalibration of the reboiler temperature controller rectified this problem. Incidentally, operating a triethylene glycol reboiler at 375°F-400°F does not necessarily result in a noticeable increase in glycol degradation. The trick is to keep the glycol filters in good repair. Dirty glycol fouls the reboiler heat-transfer tube. This in turn causes hot spots on the heat-transfer surface, which accelerates thermal decomposition. 

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. 

Abnormal Heat Input From Reboiler - Reboilers are designed with a specified heat input. When they are new or recently cleaned, additional heat input above the normal design can occur. In the event of temperature control failure, vapor generation may exceed the process system s ability to condense or otherwise absorb the buildup of pressure, which may include noncondensibles due to overheating. 

The hydroformylation reaction is highly exothermic, which makes temperature control and the use of the reaction heat potentially productive and profitable (e.g, steam generation). The standard installation of Ruhrchemie/Rhone-Poulenc s aqueous-phase processes is heat recovery by heat exchangers done in a way that the reboiler of the distillation column for work-up of the oxo products is a falling film evaporator... 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

The statement that the mass, or weight flow of vapor through the trays, increases as the refluxed rate is raised is based on the reboiler being on automatic temperature control. If the reboiler were on manual control, then the flow of steam and the reboiler heat duty would remain constant as the reflux rate was increased, and the weight flow of vapor up the tower would remain constant as the top reflux rate was increased. But the liquid level in the reflux drum would begin to drop. The reflux drum level recorder controller (LRC) would close off to catch to falling level, and the overhead product rate would drop, in proportion to the increase in reflux rate. We can now draw some conclusions from the foregoing discussion ... 

An increase in reflux rate, assuming that the reboiler is on automatic temperature control, increases both the tray weir loading and the vapor velocity through the tray deck. This increases both the total tray pressure drop and the height of liquid in the tray s downcomer. Increasing reflux rates, with the reboiler on automatic temperature control, then will always push the tray closer to, or even beyond, the point of incipient flood. 

In case 2, the shell-side reboiler temperature rises from 240 to 280°F (one reason for such a rise in temperature could be an increase in tower pressure). Now AT = (320°F - 280°F) = 40°F. Looking at the equation above, it looks as if Q will drop in half to 5000 lb/h (which is about the same as 5,000,000 Btu/h). Thus, the flow of steam to the reboiler has been cut in half, even though the control valve position has not moved. 

On temperature control of the vapor leaving the reboiler or at some point in the tower,... 

Remark 7.2. 77 vapor flow rate V depends through a constitutive relation on the reboiler temperature V = V (Tg), thus V cannot be set independently for control purposes, but only through manipulating the reboiler heat duty Qb-... 

Step 5. Reboiler steam controls product purity. Now the fresh feed Ftf cannot be used to control pressure. The purge stream is so small that effective pressure control is unlikely. Reactor cooling water flow is used to control reactor temperature. Therefore, the logical choice for pressure control is the cooling water flow to the condenser CWc. The controller gain of this loop was empirically set at 25 (with a pressure transmitter span of 3000 kPa). 

Temperature controller not operating correctly Carryover of water from inlet separator Inaccurate reboiler thermometer... 

A liquid stream of hydrocarbons with a flow rate of 500 kmol/h is sent to the top of a reboiled stripper column. The column has the equivalent of seven equilibrium stages and a reboiler in a configuration similar to Figure 12.6. The column and reboiler pressure may be assumed uniform at 1000 kPa. The feed temperature to the column is controlled such that the average column temperature is 95°C. The heat duty to the reboiler is controlled such as to maintain the reboiler temperature at 120°C. The feed stream is given below, along with average component K-values in the column and reboiler, evaluated at the pressure and temperatures provided. Determine by the modular column section method the rates and compositions of the stripper column products. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

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