B

Figure 6.18 Cross-pinch heat transfer.

In principle, the region above the pinch requires only hot utility, and that below the pinch only the cold one. An ideal design would reflect this arrangement.

Following the logic of this separate heat balances above and below the pinch, the design rules instruct that no heat should be transferred from a hot stream placed above the pinch point to a cold stream placed below the pinch point. If such heat is transferred from the hot side of the pinch to its cold side, the hot and cold targets would not be met. Instead, this cross-pinch heat would need to be replaced by an equivalent amount of hot utility above the pinch, and, consequently, the consumption of the cold utility below the pinch (air, cooling water, etc.) would be increased by the same amount (Figure 6.18a).

As shown in Figure 6.18b, it is possible to use a hot stream to perform some low temperature heating, and indeed many sub-optimal designs feature such heat recovery. However, a careful designer would not allow the transfer of heat from the region below the pinch to the region above the pinch, because it would violate the chosen AT^.

To meet the hot and cold utility targets, suitable matches for this heat must be found on the hot side of the pinch rather than on the cold side. There are three rules for achieving the minimum energy target for a process.

• Heat must not be transferred across the pinch;

• There must be no cold utility used above the pinch;

• There must be no hot utility used below the pinch.

Violating any of these rules will lead to 'cross-pinch' heat transfer, which increases the energy requirement beyond the target. The 'pinch equation' defines this rule:

A = Existing (or actual) energy consumption T = Target energy consumption XP = Cross pinch

In other words, the difference between current energy use and the target is the sum of all cross-pinch inefficiencies.

Eliminating any cross-pinch heat transfer would be the focal point of many revamping projects of existing exchanger networks.

Network Design

Network designs are usually and conveniently carried out and presented in a 'grid' diagram. The hot streams are shown as horizontal lines running from left to right, and cold stream running from right to left (Figure 6.19).

A heat exchanger transferring heat between two process streams is shown as a vertical line joining the circles that are placed on the two matched streams. Heater and coolers are shown as single circles placed on the relevant streams. The pinch division is clearly shown in the grid diagram, and consequently any cross-pinch heat transfer would be conveniently indicated as an exchanger stretching across the pinch division line.

The network design procedure [5, 6] can briefly be summarized as follows:

• To ensure that no cross-pinch heat transfer occurs, the design starts at the pinch, by placing the 'pinch' exchangers first. These are the exchangers that bring a stream precisely to the pinch temperature.

• After these exchangers have been correctly placed and any cross-pinch is thus prevented, the remaining exchangers are placed according to designer's preferences and convenience.

• Ensure that the heat is properly 'cascaded', meaning that the high temperature hot streams are used against high temperature cold stream, that is, that the heat transfer is 'vertical' when observing it in the composite curves.

- Criss-crossing, when it happens on the same side of the pinch does not incur energy penalty, but would be wasteful of the temperature driving forces, and thereby of exchanger area.

Figure 6.19 Grid diagram.

G Heat Integration and Pinch Analysis 6.5.3

Network and Process Design Interaction 6.5.3.1 Process Modifications

The minimum energy requirements set by the composite curves are based on a given process heat and material balance. The targets and the optimum heat exchanger configurations identified using the composite and grand composite curves are based on these fixed process conditions. The proposed configuration will not have any effect on the fundamental energy requirements and the process temperatures.

In this context, process modifications are changes to these conditions. As the designer modifies the heat and material balance, the composite curves are changed, and consequently the heat recovery. Appropriate process changes can 'bend' the curves in a way that actually improves the heat recovery.

There are several important parameters that the designer may want to review. These include distillation column operating pressures and reflux ratios, feed vaporization pressures, column side cooling (pumparound) duties and flowrates,

For example, increasing the heat removal duty in pumparounds (side coolers) of distillation units 'flattens' the hot composite and increases heat availability above the pinch, thus reducing the use of hot utility.

Figure 6.20 shows a distillation column with four products and two pumparounds (P/A-l and P/A-2). Increasing the bottom pumparound duty and reducing the overhead condenser duty correspondingly changes the shape of the hot composite and reduces the hot utility target at constant AT^.

Pumparound maximization strategy is often employed when designing or revamping distillation columns for improved energy performance. It is to be noted that the increase in pumparound duty reduces the column reflux, which in turn affects the fractionation sharpness. A careful designer will take this into consideration when optimizing the combined yield and the energy performances of the unit.

Distillation Tower Pumparound
Figure 6.20 Process modification changes the shape of the composite curves.
Figure 6.21 Adjustment column pressure to improved heat integration.

Another example of manipulating process design parameters in order to improve heat integration is the adjustment in condenser temperature by changing the column pressure.

The set of composite curves in Figure 6.21 shows a reboiler and a condenser of a distillation column. The cold composite has another flat part in it, representing another vaporizer. If the column pressure is sufficiently increased, its condenser temperature may become high enough to preheat the flat part (the vaporizer) of the cold composite, and the hot utility requirement would be greatly reduced.

6.5.3.2 The Plus/Minus Principle

The number of choices and potential modifications is large. An exhaustive search to identify the three or four such parameters that could be changed to the overall benefit of the process would be time consuming. However, thermodynamic rules based on pinch analysis can be applied to identify the key process parameters that can have a favorable impact on energy consumption. In general, the hot utility target will be reduced by:

• increasing hot stream (heat source) duty above the pinch;

• decreasing cold stream (heat sink) duty above the pinch.

Similarly, the cold utility target will be reduced by:

• decreasing hot stream duty below the pinch;

• increasing cold stream duty below the pinch.

The set of four simple rules, described in the four dot-points above, is termed the '+/-principle' for process modifications [2]. This simple principle provides a reference for any adjustment in process heat duties, and indicates which modifications would be beneficial and which would be detrimental.

As shown in the example Figure 6.21, it is sometimes possible to change temperatures rather than the heat duties. It is clear from the composite curves that temperature changes that are confined to one side of the pinch will not have any effect on energy targets. However, temperature changes across the pinch can change them. The pattern for shifting process temperatures can be therefore summarized as follows:

• shift hot stream from below the pinch to above;

• shift cold streams from above the pinch to below.

6.5.3.3 Integration Rules forVarious Process Equipment

In terms of their 'appropriate placement' relative to the process pinch point, the following equipment items deserve special attention:

• Distillation columns: Although not always possible, the most energy efficient integration of a distillation column is on one side of the pinch, so that either its condenser duty can be used to heat up the background process (if the column is placed above the pinch), or its reboiling duty can be supplied by the process (if the column is placed below the pinch).

• Heat pumps: The appropriate placement of a heat pump is across the pinch, meaning that heat is taken from below the pinch and rejected above the pinch. A similar principle applies to refrigeration systems.

• Gas turbines: Gas turbines should be integrated above the pinch, so that the exhaust heat is fully used. This is usually the case, as the hot flue gas is at high enough temperature to generate HP steam.

Revamping Heat Exchanger Networks

Retrofitting existing networks is a much more complex task than designing the new ones.

If an engineer were to start a heat exchanger network design with a blank sheet of paper and a good knowledge of pinch technology, the resulting minimum energy design would, almost always, be appreciably different from the existing design for the same process.

Revamping an existing network to achieve a 'pinch design' can therefore be quite costly, considering the changes that would have to be made to existing equipment. Because of this, different rules have been developed for retrofits to obtain the best energy savings from the fewest modifications to the existing design-a pragmatic approach that recognizes the limits concerning the changes that can be economically made to an existing network. It is also recognized that the final design will rarely be identical to the ideal grassroots design obtained from Pinch principles.

Area Efficiency Method

The minimum heat exchanger network surface area is obtained when all heat flows from hot streams to cold streams are vertical when analyzed against the

Figure 6.22 The driving force plot.

composite curves. One approach to retrofitting existing networks is to check the individual exchangers and see if their surface areas are used effectively [7].

This introduces the concept of the 'driving force plot' (DFP), which is a graph of ideal temperature differences (between hot and cold streams) across the whole temperature range. It is derived from the composite curves (Figure 6.22).

Once this diagram is prepared, the 'fit' of individual heat exchangers to the ideal profile can be assessed. If the matches closely follow the DFP, this results in a vertical heat transfer that leads to minimum area designs.

In Figure 6.23 the two highlighted heat exchangers are well-designed from basic pinch principles, and show a good fit with the DFP.

In most existing networks, individual heat exchangers will not follow the ideal temperature profiles (even if there is no cross-pinch heat transfer) and, consequently, the surface area required is more than the minimum required, thermodynamically. This tends to be because exchangers may have a larger than ideal temperature difference at one end, the benefit of which is outweighed by a smaller than ideal temperature difference at the other end. Overall, the heat transfer per unit area is less than ideal. An experienced pinch engineer may use the DFP to ensure that each exchanger in the network fits closely to the ideal plot, and hence contributes to the minimum surface area of the overall network.

Modern Retrofit Techniques

Modern, pragmatic approaches to network improvement seek to squeeze the best performance out of the existing exchangers and minimize the need for new exchangers. Typical retrofits may involve surface area enhancement equipment, such as tube inserts and twisted tube exchangers, and often one new exchanger or exchanger shell, but will avoid extensive changes to the network.

The advanced design techniques include the use of loops' and 'paths' within a network (Figure 6.24), because these provide a degree of freedom to adjust the heat flows. Paths are the flow trails within the network. They connect the cold and the hot utilities, and because of this any improvement in the heat recovery along a path can reduce the consumption of both utilities (Figure 6.25). A loop is a closed energy path within the network.

Heat-load loop

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