This curve will be drawn between T* calculated before during the temperature interval
diagram construction which is the average temperature of hot and cold side each interval
boundary and the corresponding surplus heat/enthalpy from each interval.
Constructing the Grand Composite Curve (G.C.C.) | Energy Targeting
These data are shown below for a generic balanced cascade diagram that can be developed for any application such as plant retrofit or new grassroots facility.
Drawing these data as T* versus Enthalpy results in the following diagram that can be used to define different levels of utilities mix that can be used to satisfy the process heating utility requirement for instance, as shown below.
Multiple Utility Selection Using Grand Composite Curve (GCC)
Upon maximizing heat recovery in the heat exchanger network, those heating duties and
cooling duties not serviced by heat recovery must be provided by external utilities.
The most common utility is steam. It is usually available at several levels. High
temperature heating duties require furnace flue gas or a hot oil circuit. Cold utilities might be refrigeration, cooling water, air cooling, furnace air preheating, boiler feed water preheating, or even steam generation at higher temperatures.
Although the composite curves can be used to set energy targets, they are not a suitable tool for the selection of utilities. The grand composite curve drawn above is a more appropriate tool for understanding the interface between the process and the utility system.
It is also, as will be shown in later modules, a very useful tool in studying of the interaction between heat-integrated reactors, separators and the rest of the process.
The GCC is obtained via drawing the problem table cascade as we shown earlier.
The graph shown above is a typical GCC. It shows the heat flow through the process against temperature. It should be noted that the temperature plotted here is the shifted temperature T* and not the actual temperature. Hot streams are represented by ΔT_min/2 colder and the cold streams ΔT_min/2 hotter than they are in the streams problem definition. This method means that an allowance of ΔT_min is already built into the graph between the hot and the cold for both process and utility streams. The point of “zero” heat flow in the GCC is the pinch point. The open “jaws” at the top and the bottom represent QH_min and QC_min respectively.
The grand composite curve (GCC) provides a convenient tool for setting the targets for the multiple utility levels of heating utilities as illustrated above.
The graphs below further illustrate such capability for both heating and cooling utilities.
The above figure (a) shows a situation where HP steam is used for heating and refrigeration is used for cooling the process. In order to reduce utilities cost, intermediate utilities MP steam and cooling water (CW) can be introduced. The second graph (b) shows the targets for all the utilities. The target for the MP steam is set via simply drawing a horizontal line at the MP steam temperature level starting from the vertical axis until it touches the GCC.
The remaining heat duty required is then satisfied by the HP steam. This maximizes the MP steam consumption prior to the remaining heating duty be fulfilled by the HP steam and therefore minimizes the total utilities cost. Similar logic is followed below the pinch to maximize the use of the cooling water prior the use of the refrigeration.
The points where the MP steam and CW levels touch the GCC are called utility pinches since these are caused by utility levels. The graph (C) below, shows a different possibility of utility levels where furnace heating is used instead of HP steam. Considering that furnace heating is more expensive than MP steam, the use of the MP steam is first maximized. In the temperature range above the MP steam level, the heating duty has to be supplied by the furnace flue gas.
The flue gas flowrate is set as shown in the graph via drawing a sloping line starting from the MP steam to theoretical flame temperature Ttft.
If the process pinch temperature is above the flue gas corrosion temperature, the heat available from the flue gas between the MP steam and pinch temperature can be used for process heating. This will reduce the MP steam consumption.
In summary the GCC is one of the basic tools used in pinch technology for the selection of appropriate utility levels and for targeting for a given set of multiple utility levels. The targeting involves setting appropriate loads for the various utility levels by maximizing cheaper utility loads and minimizing the loads on expensive utilities.
Normally, we have a choice of many hot and cold utilities and the graph below shows some of our options. Generally, it is recommended to use hot utilities at the lowest possible temperature while we generate it at the highest possible temperature. And for the cold utilities it is recommended to use it at the highest possible temperature and generate it at the lowest possible temperature. These recommendations are best addressed systematically using the grand composite curve.