Figure 6.13 Using low pressure steam instead of high pressure.
As will be discussed later, when the pinch rules are introduced, an ideal process does not transfer heat through the pinch point. Because of this, the grand composite curve, representing the heat flows, has that characteristic shape of touching the y-axis at the pinch point.
There is a small, mathematical adjustment to make to the composite curves prior to converting them to the grand composite curve. The separate hot and cold composites are 'shifted' by moving them down (hot curve) and up (cold curve), each by half the ATmin until they touch at the pinch point. The resulting composite curves are referred to as shifted curves and have no real physical meaning but are merely a step in the construction of the grand composite curve. This ensures that the resulting grand composite shows the required zero heat flow at the pinch point.
Figure 6.15a shows a grand composite curve for a case where the HP steam is used for heating and refrigeration is used to cool the process. In order to reduce the utility cost, the intermediate utilities are introduced: LP steam, medium pressure (MP) steam and cooling water (CW).
The target for LP steam (the least cost hot utility) is first set by plotting a horizontal line at the LP steam temperature from the y-axis until it touches the grand composite curve (Figure 6.15b). The MP steam target then follows in a similar way. The remaining heating duty is finally satisfied by HP steam. This minimizes HP consumption in favor of LP and MP steam and thus minimizes total utility cost. A similar construction below the pinch maximizes the use of cooling water ahead of refrigeration.
The points where the LP, MP and CW levels touch the grand composite curve are called 'utility pinches'. Similarly to the process pinch, the existence of the utility pinch sets a limit to how much heat can be exchanged between a particular utility and the process.
Total Site Integration
The overall process design of a chemical plant is a complex task that may start with designing the reactor and the separation system, followed by heat exchanger network and finally the design of the utility systems.
In the 'total site' approach, the designer seeks to take advantage of the interaction between these four parts of the system, particularly in the choice of utilities that would be best suited to the rest of the process systems .
Where appropriate, total site analysis can be used to extend the pinch analysis to site-wide integration of a number of processes by means of the utility system.
A direct integration of two or even three processes is common. However, to heat-integrate more units, or across the whole site, while potentially beneficial is in most cases impractical. A wider integration may pose operational problems, and is in most cases infeasible, considering the physical distances between the units. However, an indirect site-wide integration can be achieved by using the utility system as the intermediary. For example steam can be generated in a unit that has heat in excess, and the steam can then be used in a unit that requires heating. Matching these heat sources and heat sinks is central to total site analysis. It is applied as follows:
• In grass-roots designs the choice of utility levels, such as steam header pressures, is almost unlimited. Total site technique is used to match the overall site sources and sinks and determine the most appropriate utility levels. The site profiles are developed by using the grand composite curves of each process. The designer sums up the utility requirements of all processes above the pinch thus creating the site 'sink profile', and below the pinch, creating the site 'source profile' (Figure 6.16). The source and the sink profiles are used to determine how much heat and at what temperature can be exchanged between the processes, by using the steam network as the site - s energy transmission system.
Process' Grand Composite Curves
In revamps-total site techniques identify the inefficiencies in utility use across a site and find opportunities for exploiting the potential that is there. In many cases it is possible to balance steam supply to consumers while taking advantage of existing steam turbines to generate power.
Steam and Power System and Efficient Power Generation
Steam and power systems tend to be large and complex in process industries. They consume substantial amounts of fuel, and are therefore subjected to extensive study and optimization. Good understanding of what improves the steam system's efficiency provides an insight into its use for effective 'total site' integration.
Power can be produced from heat using various cycles and power engines. Due to the very nature of this process, some heat will always be rejected as low grade energy, and will not be converted to power.
Any industrial facility that consumes heat or steam provides a heat sink which enables generation of power at high efficiency. While conventional power plants using a condensing steam turbine cycle (the 'Rankine' cycle) may achieve power generation efficiencies of no more than about 40%, a backpressure turbine would generate power essentially at the boiler efficiency (i.e., 90% or higher). This is because the condensing cycle sends the rejected heat from the turbine outlet to waste, while the steam at the outlet of a backpressure turbine is used by the industrial process (Figure 6.17). Maximizing backpressure power generation would be a clear objective of an energy-conscious process designer.
The steam system of an energy efficient industrial plant would therefore include HP steam boilers, typically >40bar. The steam demand at lower pressures will be met by passing the HP steam through efficient backpressure turbo-generators. The following set of additional rules will further guide efficient design:
Condensing cycle Backpressure cycle r| = 25-40% n = 90%
Figure 6.17 Power generation cycles.
Figure 6.17 Power generation cycles.
• Consume steam at the minimum pressure that the user's temperature profile allows.
• Minimize the use of small process turbine drivers, and maximize the use of electric motors.
• When possible, avoid generating LP steam from waste heat-instead use any waste heat available to preheat feeds to process units, or to preheat furnace combustion air.
• For high power consuming sites consider the use of gas turbines coupled with their waste heat recovery boilers.
The above features of an efficient steam system will also direct the total site integration. For example, the designer will need to ensure that the source profile is used to generate the highest pressure steam possible, because this would maximize power generation. There will be incentive to replace HP steam consumers with the lower pressure ones, and save on the relative costs, etc.
Options for Low Grade Heat Use
With the total site approach the whole plant becomes heat integrated. Whatever the extent of this integration, however, there will almost always be some low grade heat that is in excess and remains unused, that is, wasted to air or cooling water. The amount of the waste may be substantial, and questions naturally arise as to how this 'free' energy could be used. Obviously, an additional useful heat sink has to be found. It can be located inside or outside the facility that produces the excess heat. A 'useful' sink is either an existing consumer whose energy requirement incurs cost (e.g., a low temperature consumer that uses steam), or a new consumer that is unprofitable to run at the actual energy prices (e.g., refrigeration for additional process cooling).
Potential uses of the low grade heat are:
• Install a hot water circuit as a new utility level. It would recover heat from waste heat sources which are too low in temperature to generate LP steam. The hot water could then be used for the following purposes:
- Heating low temperature heat sinks to save LP steam
- To drive an absorption refrigeration unit to produce chilled water. This can be used to reduce power demand from conventional refrigeration units or air conditioning units.
• District heating. Exporting heat 'across the fence' for either municipal heating or to a neighboring industrial consumer. This will be further discussed in Section 6.7.1.
In addition, there are two technologies that allow power generation from waste heat: the organic Rankine cycle and the Kalina cycle. However, they both have limited application due to the relatively high investment cost, and very low efficiency (less than 10%) of conversion of heat to power in the range of 110-140 °C, which would be considered for a hot water circuit or LP steam.
The targeting procedure of pinch technology sets the energy consumption targets ahead of design. One can sensibly question weather it is always possible to design the process that meets those predetermined targets. The answer is affirmative-pinch technology provides the methodology for designing heat exchanger networks that would invariably produce configurations featuring the targeted utility consumption.
This process of configuring the heat exchanger network is called 'synthesis'. In the formative days of pinch technology, a manual procedure was used. This is well documented [5, 6] and the interested reader may find it useful to familiarize with the methodology. Today, however, heat exchanger network design is almost fully automated, and the advanced software tools that combine stream analysis, targeting and network design are extensively used.
Either manual or automated, the synthesis methodology is built around several basic pinch rules.
The Pinch Rules
The existence of the pinch point divides the heat recovery region, defined by the composite curves, into the hot and the cold sections. They are in heat balance with their respective utilities.
Cross-Pinch Heat Transfer is Possible
Cross-Pinch Heat Transfer is Possible
.^^Cross-Pinch Heat ^^ Transfer is not allowed
UCmin (ATwould be < ATmjn)
Was this article helpful?
Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.