Applying the Grand Composite Curve for Energy Integration

The graph below shows that utility pinches (circles in graphs) are formed according to the Number of utilities used.

Applying the Grand Composite Curve for Energy Integration

Each time a utility is used a “utility pinch” is created. It also shows that the GCC right
noses, sometimes known as “pockets”, are areas of heat integration/energy recovery and
hence does not need any external utilities. These right noses/pockets represent another
possibility of heat integration among hot and cold process streams at ΔT higher than the
ΔT_min (minimum hot and cold streams approach temperature).

The GCC curve can be used by engineers to select the best match between the utility profile and process needs profile. For instance, the steam system shown below needs to be integrated with the process demands to minimize low pressure steam flaring and high or medium pressures steam let downs. In addition, it helps in selecting steam header pressure levels and loads.

Before closing this part of GCC and its use in selecting process requirements of utilities mix, I need to mention one important fact here about one rule in pinch technology that is generally accepted by many people in the industrial community. This rule says that: “do not use cold utility above the pinch and hot utility below the pinch” to avoid buying extra heating and cooling utilities.

With the aid of GCC we might find situations in which it is better from work generation or work avoidance point of view to use cold water above the pinch and produce steam, then produce electricity and use exhaust for process heating than doing process to process integration with a ΔT_min among hot and cold process streams much higher than the minimum one.

Steam System Example

 

The Grand Composite Curve can also be utilized to select not only best mix in a steam system but also the load and return temperature of hot oil circuits, best integration between process and furnaces exhaust and process refrigeration levels as well as the synthesis of the multiple-cycle refrigeration systems [1,3].

Combined Heat and Power

A more complex utility is the combined heat and power system or the new term coined in industry as cogeneration. In such a case, simply, the heat rejected by a heat engine such as a steam turbine, gas turbine or diesel engine is used as the hot utility.

According to the first law of thermodynamics, heat and power are related. Thus, it should not be surprising that the energy integration concepts can be applied for heat and power integration. According to pinch technology and assuming that we will not be having any change in the location of the problem pinch point location due to the integration of the process unit, which in many cases is not the case, fundamentally there are three possible ways to integrate a heat engine exhaust.

Anyway, before starting to discuss these three possibilities let us first examine heat engines. In this book, here we mean with heat engines, steam turbines and gas turbines. A heat engine takes in heat (Q1) from a heat source at a very higher temperature and rejects heat (Q2) at a lower temperature into a heat sink while producing electricity. The power generation, in form of work (W), equals the difference between heat input (Q1) and heat rejection (Q2), as per the strict definition of the first law of thermodynamics.

Thermodynamic imply that the efficiency of a heat engine is always less than 100% since some of the heat output will always be wasted. If we can utilize this waste heat in another process task to satisfy certain process requirement such as the heating of process streams then we are enhancing the efficiency of the heat engine via its integration with other process units.

A systems approach will now lead to a different conclusion from the consideration of a particular unit in isolation from the rest of the process. The challenge now is how we integrate the heat engine with other process units systematically, without enumeration and in a way that enhances the thermal efficiency of the heat engine.

Integration of the Heat Engine

As we mentioned earlier, there are three possible ways of such integration with process units: above the pinch, below the pinch or across the pinch.
Let us first consider the integration of the heat engine exhaust across the pinch.

Integration across the Pinch

The heat engine takes in QH_min units of heat from the external utility above the pinch, produces W units of work and rejects Q_HE minus W heat into the process below the pinch, which is rejected into cooling water. Such process still needs QH_min and the cooling utilities needs will be increased by Q_HE minus W amount of heat and the heat engine will perform no better than if it was a standalone one.

It takes QH_E and produces W work while it produces Q_HE minus W waste heat. As a matter of fact this integration will be counterproductive from both the capital investment and energy point of views. More capital investment is required since the cascaded heat Q_HE minus W is rejected to the cooling water; therefore, the process will need more heat exchangers. More energy is also required since the level of work extraction from the Q_HE supplied heat was not as high as it should have been by taken the expansion in the turbine/Heat engine all the way down to the condensation level to generate more power. Therefore, we can conclude that heat engines should not be integrated across the pinch since this will be counterproductive from both the energy consumption point of view and the heat engine efficiency as well.

Now, let us examine the other two possible cases of integrating the heat engine: above the pinch or below the pinch. In both cases, the integration of the heat engine with the process will be beneficial.

Integration above the Pinch

Let the first case to study is the effect of integrating the heat engine with the process unit above the pinch point. If we integrate the heat engine above the pinch, then we are rejecting the Q_HE minus W exhaust heat to the process which is a heat sink above the pinch and already needs such an external utility.

This mechanism means less external utility by the amount of Q_HE minus W or all the thermal energy input to the heat engine Q_HE is utilized; part as work and the waste heat Q_HE minus W, as a heating medium for the process. In other words, we take in extra units of energy Q_HE which is more than the QH_min required by the process and which we are going to take any way for a standalone heat engine and utilize it all via turning it into shaft work and process heating medium achieving 100% efficiency for the heat engine as if we are having a 100% conversion of heat to power using the first law efficiency definition.

Integration below the Pinch

Now, if we integrate the heat engine below the pinch we are not integrating the heat engine exhaust but we are taking waste heat from the process, which is a heat source below the pinch, and giving it to the heat engine to produce work.

In such a case, we are getting double benefits, first, we are not getting external Q_HE for the heat engine nor are we not increasing the hot utility requirements. Secondly, we are also decreasing the cooling utility requirement by Qc minus Q_HE. In other words, we are again getting to a situation that we are using the process waste heat below the pinch to generate electricity and saving cooling utility.

This situation of a system approach, compared to the unit based approach, shows that when we put our boundary lines around both the process plant and the utility plant and consider a bigger picture than we normally do/see, we get into a new system in which the process and the heat engine are acting as one system.

In such a new situation, apparently the conversion of heat to work is happening with 100% efficiency. Now, let us take a closer look at the two most commonly used heat engines, the steam turbine and the gas turbine to see the efficiency that they can achieve in practice. Let us examine one simple rule before we start talking about steam and gas turbine, which is the grand composite curve for the process should be used to make any quantitative assessment of any heat and power scheme integration and the heat engine exhaust shall be treated as any other utility.

Steam Turbine Integration

The graph below shows a steam turbine expansion on an enthalpy-entropy plot.
It shows the ideal expansion process and the real expansion one and what is meant by
isentropic efficiency or second law (of thermodynamic) efficiency.

In an ideal turbine, steam with an initial pressure P1 and enthalpy H1 expands
“isentropically” to pressure P2 and enthalpy H2. In such a hypothesis, the ideal work
produced is equal to H1-H2. Because of the losses in the turbine real expansion process
due to frictional effects in the turbine nozzle and blade passages, etc., the exit enthalpy is greater than it would be in the ideal process. Therefore, the actual work output will be less and determined by the H2* value as shown in the above graph. The actual work output is equal to H1-H2*.

The turbine isentropic efficiency

The output from the turbine might be superheated or partially condensed. If the exhaust steam is to be used for process heating, ideally it should be close to saturation condensation. If the exhaust steam is significantly superheated, it can be de-superheated by direct water injection of the boiler feed water which vaporizes and cools the steam.

However, if saturated steam is fed to a steam main, which results with a significant potential heat loss from the main, then it is desirable to retain superheat than to de-superheat. Since the heat losses will cause excessive condensation in the main, this is not desirable. On the other hand if the exhaust steam from the turbine is partially condensed, the condensate is separated and the steam is used for heating.

Fuel supplied to the boiler is used to produce high pressure steam for process heating and for power generation in the turbine. Rejected heat from the turbine is also used for process heating. The process ia carried out as follows, heat Q_HP is taken into the process from the high pressure steam. The balance of the hot utility demand Q_LP is taken from the steam turbine exhaust. The flash steam is recovered after pressure reduction of the high-pressure steam condensate.

Heat Q-fuel is given to the boiler and an overall energy balance gives:
Q-fuel = Q-HP steam+ Q-loss
Q-HP steam = Q_HP+ Q_LP + W; hence,
Q-fuel= (Q_HP+ Q_LP+ W) + Q-loss
The process needs Q_HP+ Q_LP to satisfy its enthalpy imbalance above the pinch.
In addition to boiler losses there are losses from the turbine and sometimes significant one from the steam distribution system.

Gas Turbine Integration

Combustion gas turbine presented in the fourth chapter is essentially a rotary compressor mounted on the same shaft of a turbine. Air enters the compressor where it becomes compressed before entering the combustion chamber where the combustion process increases its temperature. The mixture of the combustion gases, including air, is expanded in the turbine. The energy input to the combustion chamber is enough to drive the compressor and produce useful work (Electricity).

The expanded gas may be discharged to the atmosphere directly or may first be used to preheat the air to the combustion chamber or to produce high pressure steam. Gas turbines are normally used for relatively large-scale applications.

The overall efficiency of conversion of heat into power depends on the turbine exhaust profile, the pinch temperature and the shape of the process grand composite curve.

It is instructive to mention here that reciprocating engines, combined cycles, etc., all have heat sources and heat sinks therefore they can be treated similarly.

Since we have many different types of heat engines to choose from, we need the grand composite curve to help us determine the most suitable heat engine for a given process.
The example in the graph below shows that for a process that has such a type of grand composite curve the best type of utility will be low pressure steam. This result is not easy to obtain via intuition without drawing the process grand composite curve due to the fact that the process extends to high temperatures.

The grand composite curve in the above case indicates a little bit of a “flat” temperature profile for demand which exhibits a good fit between low pressure steam and the process. If high pressure steam is available we can utilize the energy in it via producing work using a steam turbine to recover the shaft work available between the high pressure (HP) and low pressure (LP) mains as shown in graph below.

In other situations where the process grand composite curve slope is far from being flat,
integration of steam turbines with the process might not be appropriate and a gas turbine or hot oil system heated by gas turbine exhaust might be better as shown in the graph below.

The graph shows that a gas turbine is more appropriate because the sloping profile of the GCC is not suitable for a constant-temperature utility like steam. The gas turbine exhaust in this example has a sloping utility profile which better fits the process GCC since several steam levels would be required to make a steam turbine efficient.

Most of the time there is a trade-off between capital and energy costs. In the selection of steam or gas turbines that best integrate with the process utility requirements we have a trade-off between gas turbine efficiency, heat exchanger network (HEN) capital cost and stack heat loss to ambient.

For instance, in order to closely match the process grand composite curve with the gas turbine exhaust, the minimum approach temperature between the process and the hot utility exhaust gas will be small resulting in high capital cost for the HEN. In addition, it will result in increased heat loss to the ambient. On the other hand higher stack temperature means less an efficient gas turbine and lower flowrate-specific heat availability to satisfy the process needs.

Heat Pump Integration

Heat pumps are the opposite of heat engines. We put work into a heat pump to raise the temperature level of the available heat. A heat pump is a device that absorbs heat at low temperatures in the evaporator, consumes shaft-work when the working fluid is compressed and rejects heat at higher temperatures in the condenser.

The condensed working fluid is expanded and partially vaporized, then, the cycle repeats. In many cases, the working fluid is a pure component which means that the evaporation and condensation take place isothermally.

The figure below is a schematic of a simple vapor compression heat pump.

Schematic of a Simple Vapor Compression Heat Pump

Just as with heat engine integration there are appropriate and inappropriate ways to
integrate heat pumps. Essentially, there are two ways to integrate heat pumps with the
process; either across the pinch or not across the pinch. Not across the pinch means the
heat pump will be placed above the pinch or below the pinch point. Let us first examine the case where the heat pump is placed or integrated with the process above the pinch point.

In this configuration the process imports W, shaft-work and saves W, hot utility. In such a configuration the system converts power into heat which normally is never economically
worthwhile considering since as we all know from thermodynamics that work is more
valuable than heat. Another possible integration not across the pinch is to integrate the heat pump with the process below the pinch. Again, the result of integration in such a case is worse economically. In such a case power is turned into waste heat that then needs to be rejected to a cooling utility.

The last possible way of integration of the heat pump with the process is across the pinch as shown in the figure below.

This arrangement brings about a true energy saving. As shown in the graph it reduces the process heating utility import by (W + QHP) and decreases the process cold utility import by QHP. It also makes overall sense because heat is pumped from the part of the process which is an overall heat source to the part of the process which is an overall heat sink.

Thus, the appropriate placement of heat pumps in the process context is that they should be placed across the pinch.

In the case of utility pinch, which is formed upon the introduction of utility to the process, the heat pumps shall also be placed across it.

The graph below shows a heat pump integrated properly across the pinch and the use of the grand composite curve in defining the best place to integrate this heat pump. The (A) graph depicts the heat balance but the (B) graph of the GCC illustrates how the grand composite curve can be used to size the heat pump.

It is instructive to mention here that in order to examine how a heat pump performs we need to check its coefficient of performance (COP). The coefficient of performance for a heat pump (COP) generally can be defined as the useful energy delivered to the process divided by the shaft-work expended to produce this useful energy.

COP for Hp = (QHp + W)/ W

Where COPHp is the heat pump coefficient of performance, QHp is the heat absorbed at low temperature and W is the shaft-work consumed. For any given type of heat pump, a higher COPHp leads to better economics. Having a better COPHp and hence better economics means working across a small temperature lift with the heat pump.

The smaller the temperature lift, the better is the COPHp. For most allocations, a temperature lift greater than 25ºC is rarely economical. Attractive heat pump applications normally require a lift much less than 25ºC.

Using the grand composite curve, the loads and the temperatures of the cooling and heating duties and hence the COPHp of integrated heat pumps can be readily assessed. To target the thermal design of the heat pump, we can use the GCC. The graph below shows that the GCC can help define the W needed for the heat pump if the process cooling utility “Qc” and the desired temperature differences, which can bring up value, are defined. Since the W needed in the heat pump is a function of the temperature difference; iteration will be needed to decide best temperature difference. In the graph below it is clear that the hot side of the heat pump is limiting since the desired process heating utility “Qh” from the heat pump is limiting.

Using the Grand Composite Curve for Determining Heat Pump Placement

Let us now use the COP as a measure to differentiate between two options a heat pump and try to see in numbers how it is a function of temperature difference.

We know that the shaft-work requirement of a heat pump increases with ΔT. If the heat pump operates over small ΔT the shaft-work requirement is lower than working at high ΔT and consequently as shown above the COP is relatively becomes high and in other words better for application.


 

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