Hot streams


The analysis of the heat exchanger network first identifies sources of heat (termed hot streams) and sinks (termed cold streams) from the material and energy balance. Consider first a very simple problem with just one hot stream (heat source) and one cold stream (heat sink). The initial temperature (termed supply temperature), final temperature (termed target temperature), and enthalpy change of both streams are given in Table 6.1.  [c.160]

Consider the simple flowsheet shown in Fig. 6.2. Flow rates, temperatures, and heat duties for each stream are shown. Two of the streams in Fig. 6.2 are sources of heat (hot streams) and two are sinks for heat (cold streams). Assuming that heat capacities are constant, the hot and cold streams can be extracted as given in Table 6.2. Note that the heat capacities CP are total heat capacities and  [c.161]

Instead of dealing with individual streams as given in Table 6.1, an overview of the process is needed. Figure 6.3a shows the two hot streams presented individually on temperature-enthalpy axes. How these hot streams behave overall can be quantified by combining them in given temperature ranges. The temperature ranges in question are defined where an alteration occurs in the overall rate of change of enthalpy with temperature. If heat capacity is constant, then alterations will occur only when streams start or finish. Thus in Fig. 6.3 the temperature axis is divided into ranges defined by the supply and target temperatures of the streams.  [c.162]

Figure 6.2 A simple flowsheet with two hot streams and two cold streams. Figure 6.2 A simple flowsheet with two hot streams and two cold streams.
Figure 6.3 The hot streams can be combined to obtain a composite hot stream. Figure 6.3 The hot streams can be combined to obtain a composite hot stream.
Specifying the hot utility or cold utility or AT m fixes the relative position of the two curves. As with the simple problem in Fig. 6.2, the relative position of the two curves is a degree of freedom at our disposal. Again, the relative position of the two curves can be changed by moving them horizontally relative to each other. Clearly, to consider heat recovery from hot streams into cold, the hot composite must be in a position such that everywhere it is above the cold composite for feasible heat transfer. Thereafter, the relative position of the curves can be chosen. Figure 6.56 shows the curves set to ATn,in = 20°C. The hot and cold utility targets are now increased to 11.5 and 14 MW, respectively.  [c.165]

All cold streams All hot streams  [c.175]

First, determine the shifted temperature intervals T from actual supply and target temperatures. Hot streams are shifted down in temperature by and cold streams up by AT J2, as detailed  [c.175]

Now cascade any surplus heat down the temperature scale from interval to interval. This is possible because any excess heat available from the hot streams in an interval is hot enough to supply a deficit in the cold streams in the next interval down. Figure 6.18 shows the cascade for the problem. First, assume that no heat is supplied to the first interval from a hot utility (Fig. 6.18a). The first interval has a surplus of 1.5 MW, which is cascaded to the next interval. This second interval has a deficit of 6 MW, which reduces the heat cascaded from this interval to -4.5 MW. In the third interval the process has a surplus of 1 MW, which leaves -3.5 MW to be cascaded to the next interval, and so on.  [c.178]

Next, calculate the shifted interval temperatures. Hot streams are shifted down by 2.5°C, and cold streams are shifted up by 2.5°C (Table 6.5).  [c.179]

The grand composite curve is obtained by plotting the problem table cascade. A typical grand composite curve is shown in Fig. 6.24. It shows the heat flow through the process against temperature. It should be noted that the temperature plotted here is shifted temperature T and not actual temperature. Hot streams are represented ATn,in/2 colder and cold streams AT iJ2 hotter than they are in practice. Thus an allowance for ATj in is built into the construction.  [c.185]

Example 7.1 For the process in Fig. 6.2, calculate the minimum number of units given that the pinch is at 150°C for the hot streams and 140°C for the cold streams.  [c.215]

HOT STREAMS I COLD STREAMS J  [c.217]

I = total numher of hot streams in enthalpy interval k J = total numher of cold streams in enthalpy interval k K = total number of enthalpy intervals  [c.218]

Enthalpy interval AT m Hot streams 2(q,/, )A Cold streams A,  [c.221]

Enthalpy interval Hot streams 2 iQi/hi)k Cold streams A  [c.231]

Another way to relate these principles is to remember that heat integration will always benefit by keeping hot streams hot and cold streams cold.  [c.322]

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams.  [c.329]

If changes to the reactor design are possible, then the simple criteria introduced in Chap. 12 can be used to direct those changes. Heat integration will always benefit by making hot streams hotter and cold streams colder. This applies whether the heat integration is carried out directly between process streams or through an intermediate such as steam. For example, consider the exothermic reactions in Fig. 13.1a. Allowing the reactor to work at higher temperature improves the heat integration potential if this does not interfere with selectivity or catalyst life or introduce safety and control problems, etc. However, if the reactor must work with a fixed intermediate cooling fluid, such as steam generation, then the only benefit will be a reduced heat transfer area in the reactor. The steam becomes a hot stream available for heat integration after leaving the reactor. If the pressure of steam generation can be increased, making a hot stream hotter, then there may be energy or area benefits when it is integrated with the rest of the process.  [c.338]

If the reactor is not appropriately placed, then it is more likely that the rest of the process would be changed to bring about appropriate placement rather than changing the reactor. If changes to the reactor design are possible, then the simple criterion of making hot streams hotter and cold streams colder can be used to bring about beneficial changes.  [c.339]

Figure 16.4a shows the grid diagram with a CP table for design above the pinch. Cold utility must not be used above the pinch, which means that hot streams must be cooled to pinch temperature by recovery. Hot utility can be used, if necessary, on the cold streams above the pinch. Thus it is essential to match hot streams above the pinch with a cold partner. In addition, if the hot stream is at pinch conditions, the cold stream it is to be matched with must also be at  [c.366]

The design in Fig. 16.5a can now be completed by satisfying the heating and cooling duties away fi"om the pinch. Cooling water must not be used above the pinch therefore, if there are hot streams above the pinch for which the duties are not satisfied by the pinch matches, additional process-to-process heat recovery must be used. Figure 16.56 shows an additional match to satisfy the residual cooling of the hot streams above the pinch. Again, the duty on the unit is maximized. Finally, above the pinch the residual heating duty on the cold streams must be satisfied. Since there are no hot streams left above the pinch, hot utility must be used (Fig. 16.5c).  [c.368]

Turning now to the cold-end design, Fig. 16.6a shows the pinch design with the streams ticked off. If there are any cold streams below the pinch for which the duties eu e not satisfied by the pinch matches, additional process-to-process heat recovery must be used, since hot utility must not be used. Figure 16.66 shows an additional match to satisfy the residual heating of the cold streams below the pinch. Again, the duty on the unit is maximized. Finally, below the pinch the residual cooling duty on the hot streams must be satisfied. Since there are no cold streams left below the pinch, cold utility must be used (Fig. 16.6c).  [c.369]

Example 16.1 The process stream data for a heat recovery network problem are given in Table 16.1. A problem table analysis on these data reveals that the minimum hot utility requirement for the process is 15 MW and the minimum cold utility requirement is 26 MW for a minimum allowable temperature diflFerence of 20°C. The analysis also reveals that the pinch is located at a temperature of 120°C for hot streams and 100°C for cold streams. Design a heat exchanger network for maximum energy recovery in the minimum number of units.  [c.371]

If there had been more cold streams than hot streams in the design above the pinch, this would not have created a problem, since hot utility can be used above the pinch.  [c.374]

Had there been more hot streams than cold below the pinch, this would not have created a problem, since cold utility can be used below the pinch.  [c.375]

Figure 16.11 If the number of hot streams above the pinch at the pinch is greater than the number of cold streams, then stream splitting of the cold streams is required. Figure 16.11 If the number of hot streams above the pinch at the pinch is greater than the number of cold streams, then stream splitting of the cold streams is required.
If the number of hot streams at the pinch above the pinch is greater than the number of cold streams, the cold streams must be split to satisfy the constraint. If the number of cold streams at  [c.397]

Consider now the possibility of transferring heat between these two systems (see Fig. 6.76). Figure 6.76 shows that it is possible to transfer heat from hot streams above the pinch to cold streams below. The pinch temperature for hot streams for the problem is 150°C, and that for cold streams is 140°C. Transfer of heat from above the pinch to below as shown in Fig. 6.76 transfers heat from hot streams with a temperature of 150°C or greater into cold streams with a temperature of 140°C or less. This is clearly possible. By contrast. Fig. 6.7c shows that transfer from hot streams below the pinch to cold streams above is not possible. Such transfer requires heat being transferred from hot streams with a temperature of 150°C or less into cold streams with a temperature of 140°C or greater. This is clearly not possible (without violating the ATmin constraint).  [c.167]

Analogous effects are caused by the inappropriate use of utilities. Utilities are appropriate if they are necessary to satisfy the enthalpy imbalance in that part of the process. Above the pinch in Fig. 6.7a, steam is needed to satisfy the enthalpy imbalance. Figure 6.86 illustrates what happens if inappropriate use of utilities is made and some cooling water is used to cool hot streams above the pinch, say, XP. To satisfy the enthalpy imbalance above the pinch, an import of (Q mjj,+XP) is needed from steam. Overall, (Qcmin+AP) of cooling water is used.  [c.168]

Figure 6.9a shows a design corresponding to the flowsheet in Fig. 6.2 which achieves the target of Q/fmin = 5 MW and Qcmin = 10 MW for ATnjjn = 10°C. Figure 6.96 shows an alternative representation of the flowsheet, known as the grid diagram. The grid diagram shows only heat transfer operations. Hot streams are at the top running left to right. Cold streams are at the bottom running right to left. A heat exchange match is represented by a vertical line joining two circles on the two streams being matched. An exchanger using a hot utility is represented by a circle with a H. An exchanger using cold utility is represented by a circle with a C. The importance of the grid diagram is clear in Fig. 6.96, since the pinch, and how it divides the process into two parts, is easily accommodated. Dividing the process into two parts on a conventional diagram such as that shown in Fig. 6.9a is both difficult and extremely cumbersome.  [c.169]

Although composite curves can he used to set energy targets, they are inconvenient because they are based on a graphic construction. A method of calculating energy targets directly without the need for graphic construction can be developed. The process is first divided into temperature intervals in the same way as was done for construction of the composite curves. Figure 6.14a shows that it is not possible to recover all the heat in each temperature interval, since temperature driving forces are not feasible throughout the interval. This problem can be overcome if, purely for the purposes of construction, the hot composite is pretended to be T iJ2 colder than it is in practice and the cold composite is pretended to be ATmin/2 hotter than it is in practice (see Fig. 6.146). The shifted composite curves now touch at the pinch. Carrying out a heat balance between the shifted composite curves within a shifted temperature interval shows that heat transfer is feasible throughout each shifted temperature interval, since hot streams in practice are actually ATmin/2 hotter and cold streams T iJ2 colder. Within each shifted interval the hot streams are in reality hotter than the cold streams by just  [c.174]

Figure 16.45 shows the grid diagram with a CP table for design below the pinch. Hot utility must not be used below the pinch, which means that cold streams must be heated to pinch temperature by recovery. Cold utility can be used, if necessary, on the hot streams below the pinch. Thus it is essential to match cold streams below the pinch with a hot partner. In addition, if the cold stream is at pinch conditions, the hot stream it is to be matched with also must be at pinch conditions otherwise, the AT in constraint will be violated. Figure 16.45 shows a design arrangement below the pinch that does not use temperature differences smaller than ATmin-  [c.367]

By contrast, now consider part of a design below the pinch (Fig. 16.12a). Here, hot utility must not be used, which means that all cold streams must be heated to pinch temperature by heat recovery. There are now three cold streams and two hot streams in Fig. 16.12a. Again, regardless of the CP values, one of the cold streams cannot be heated to pinch temperature without some violation of the constraint. The problem can only be resolved by splitting a hot (a)  [c.374]

It is not only the stream number that creates the need to split streams at the pinch. Sometimes the CP inequality criteria [Eqs. (16.1) and (16.2)] CEmnot be met at the pinch without a stream split. Consider the above-pinch part of a problem in Fig. 16.13a. The number of hot streams is less than the number of cold, and hence Eq. (16.3) is satisfied. However, the CP inequality also must be satisfied, i.e., Eq. (16.1). Neither of the two cold streams has a large enough CP. The hot stream can be made smaller by splitting it into two parallel branches (Fig. 16.136).  [c.376]

Figure 16.12 If the number of cold streams below the pinch at the pinch is greater than the number of hot streams, then stream splitting of the hot streeuns is required. Figure 16.12 If the number of cold streams below the pinch at the pinch is greater than the number of hot streams, then stream splitting of the hot streeuns is required.

See pages that mention the term Hot streams : [c.163]    [c.163]    [c.164]    [c.175]    [c.175]    [c.187]    [c.229]    [c.322]    [c.373]    [c.374]    [c.377]    [c.380]    [c.388]    [c.395]    [c.396]   
Chemical process design (2000) -- [ c.160 ]