Reboiler temperature pinch

Reboiler temperature pinches. As column pressure increases from normal operating pressure to the relieving pressure, column temperature rises. This may reduce the temperature difference (or create a temperature "pinch ) in the reboiler. The pinching that can be relied on to occur can be credited toward the relief requirement. Each situation must be anal3rzed separately. Some common situations are ... 

Fig. 14.1a. The background process (which does not include the reboiler and condenser) is represented simply as a heat sink and heat source divided by the pinch. Heat Qreb is taken into the reboiler above pinch temperature and rejected from the condenser at a lower temperature, which is in this case below pinch temperature. Because the process sink above the pinch requires at least Q min to satisfy its...

When the column is reboiled by a fired heaters, temperature pinches rarely (if at all) occur (60). 

Increasing column mechanical design pressure This is one of the most effective techniques in cases where credit can be taken for a temperature pinch (see Sec. 9.4). When a close-boiling mixture is distilled, and the reboiler operates at a relatively small AT, a modest increase in column pressure can drive the reboiler AT to zero... 

Temperature pinch. A temperature pinch is process temperature approaching the heating medium temperature. It therefore occurs only when the reboiler AT is small. A temperature pinch lessens reboiler heat transfer. Two mechanisms are usually responsible for temperature pinches ... 

Pinching is sometimes difficult to distinguish from fouling, because most s3unptoms are similar. Recording reboiler temperature difference (outlet minus inlet, process side), while experimenting with a temporary increase of the lights content in the column bottom or with circulation rates, can help identify a pinch. Alternatively, a computer simulation of the reboiler can often detect a temperature pinch (372). 

Figure 1S.S Temperature pinches in a reboiler, (a) Induced by pressure drop, single-component (b) induced by pressure drop, wide boiling range (c) induced by composition, wide boiling range.

Distribution. With tray towers, liquid distribution to the reboiler is uneven. With wide-boiling mixtures, this may prompt composition pinches. Further, light components can be depleted near the inlet or the top of the bundle, causing temperature pinches. In most cases, however, distribution is not a major problem because the bundle is small and internal reboiler AT is large. 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Consider now the consequences of placing simple distillation columns (i.e. one feed, two products, one reboiler and one condenser) in different locations relative to the heat recovery pinch. The separator takes heat Qreb into the reboiler at temperature Treb and rejects heat Qcond at a lower temperature Tcond There are two possible ways in which the column can be heat integrated with the rest of the process. The reboiler and condenser can be integrated either across, or not across, the heat recovery pinch. 

The first few plates above the reboiler show sharp changes in the concentrations of n-pentane and n-butane, similar to that shown by ethane near the tpp of the column, and this results in a maximum concentration of f-butane at plate 34. The concentration of n-pentane is nearly constant from plate 35 to the feed plate because of a pinch, and changes slowly as the temperature gradually decreases. The -butane does not show such a plateau because its volatility is about 0.8 that of i-butane, the heavy key. 

Use the Fenske-Underwood-Gilliland shortcut method to determine the reflux ratio required to conduct the distillation operation indicated below if NIN , = 2.0, the average relative volatility = 1.11, and the feed is at the bubble-point temperature at column feed-stage pressure. Assume external reflux equals internal reflux at the upper pinch zone. Assume a total condenser and a partial reboiler. 

Either tjrpe of pinching can also be alleviated by increasing the heating medium temperature or by injecting sparge gas such as nitrogen at the reboiler bottom. Sparge gas iiyection, however, may adversely affect overhead condenser action or top product purity. 

The temperature profile decreases monotonically from the reboiler to the condenser. This is illustrated in Figure 5-3 for the same computer simulation. This is again similar to the behavior of binary systems. Note that plateaus start to form where there is litde temperature change between stages. When there are a large number of stages, these plateaus can be quite pronounced. They represent pinch points in the column. 

The temperature recorder controller (TRC) holding the reboiler outlet temperature signaled the steam-inlet control valve to pinch back. 

Figure 7 shows the temperature profiles for the adiabatic, diabatic thermally optimal, and diabatic one side-reboiler VCM column design. For the diabatic thermally optimal column design, the temperature or concentration (not shown) profiles are almost linear between the pinch points at both ends of the column. For the diabatic column with one side-reboiler the profiles are also closer to being linear than for the adiabatic design. The critical factors for obtaining such "near linear" profiles are the location and size of the side-reboiler as well as... 

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