Heat transfer performance Calculated single-tube maximum heat flux
5
Flow within kettle reboilers
3
Top of weir
Heat flux, W/m2
2 104
Bundle data
5 3 2 Single tube
Common commercial design method (Kern 1950)
104 Liquid circulation pattern Tube bundle
5 1
Shell
10
100
1,000
Change in saturation temperature, °C
FIGURE 4. This schematic shows the operation of a kettle reboiler, where liquid enters the tube bundle from the base and sides
The cold composite curve In pinch analysis, the concept of “composite curves” is based upon the assumption of counter-current flow. As will be shown below, co-current flow should be used in a thermosiphon reboiler. So, how can such heat demand be represented in a composite curve? The option recommended here is that it is represented as an “isothermal” operation conducted at the vapor exit temperature (Figure 2). (For more on composite curves, see Energy Optimization Using Pinch Analysis, Chem. Eng., November 2011, pp. 36–41.)
Matching of reboiler streams If the stream being used to provide heat to the reboiler is a liquid, then its flow should be co-current with the flow of the stream being vaporized. This is particularly important with vertical thermosiphon reboilers (Figure 3). The mixture being vaporized flows through the tubes of a vertical thermosiphon. It enters the tubes as a subcooled liquid (the liquid in the reservoir will be at saturation temperature, but the surface of the reservoir can be 3–4 m above the lower tube-sheet of the reboiler). The point at which nucleation (the formation of bubbles of vapor) first occurs within the tube is dependant upon both the temperature of the liquid and the temperature of the hot wall. Maximizing the temperature driving force reduces the length of
FIGURE 5. The thermal behavior of kettle reboilers has led the authors to conclude that this type of unit is best avoided when hot liquid is used as the heat source
tube prior to the onset of nucleate boiling. This driving force is maximized if the hot liquid flows co-currently. The heat transfer coefficient on the cold-side of the unit increases rapidly once two-phase flow has been established. Consequently, there is a large variation of heat transfer coefficient along the length of the tube. This variation is “stabilized” if cocurrent flow is used. If the hot liquid flows counter-current to the vaporizing mixture, then its temperature profile is affected by changes in the heat transfer distribution on the coldside. These changes affect the point at which nucleation commences and the heat transfer distribution on the cold-side. So, the use of counter-current flow can result in instability and control difficulties.
Startup and control Startup and control are important considerations that must made when evaluating reboiler integration. While a plant may be designed to operate at a given condition (such as a specific throughput), it will be operated under a range of conditions. This means that reboiler load will be subject to variation, and therefore control will be required. In many situations it will be found that startup and control cannot be undertaken using an integrated unit, and that the overall duty must be un-
dertaken using two separate reboilers: an integrated unit having a fixed duty and an non-integrated unit used for both startup and for control functions. The engineer needs to determine the following two items: 1. The load required for startup purposes (this is set by the minimum loading on the column not the ultimate operating condition); 2. The variation in load required for the control function The design of the non-integrated reboiler will involve identifying a geometry that will provide either the startup performance or the load associated with maximum control load (whichever is the larger) at a point that is at least 20% below the maximum operating condition for the design (set by critical heat-flux conditions). This reboiler needs to be onstream all of the time that the plant is operational. Consequently, its load will never be zero. The minimum value should be set at a value that is around 20% above the lowest stable condition (the minimal sustainable reboiler load). The load for the integrated unit will be the total load minus that required at the minimum throughput minus that needed by the non-integrated unit operating at its minimum load.
Minimum temperature While the reboiler is represented by an isothermal line set at the tempera-
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