In Chapter 9 we saw how to estimate the mass and energy required to make products through chemical processes. The energy required is mostly thermal or mechanical, with the latter provided by electricity in a great number of cases. These requirements translate into heating requirements and electricity use, respectively. At the same time, heat has to be removed from the system, thereby creating cooling requirements.
Green Chemistry and Engineering: A Practical Design Approach, By Concepcio´n Jimenez-Gonzalez and David J. C. Constable
Copyright2011 John Wiley & Sons, Inc.
519
To satisfy the electricity, heating, and cooling requirements, other processes have to take place to provide the energy sources or sinks. These energy processes will have material inputs and outputs that can readily be traced back to their extraction from the Earth (e.g., natural gas, coal, potential energy of water). A representative example of a heating requirement is the process of producing steam, and a representative example of a cooling requirement is the use of a cooling tower. Figure 18.1 is a graphical representation of energy production for a chemical process.
As can be seen in the figure, different energy requirements are fulfilled through the use of different direct energy sources (i.e., the energy used directly in chemical processes) and include such sources as petroleum fuels, steam, electricity, and heat transfer fluids. There are also miscellaneous direct energy sources, such as the use of coal or biomass combustion, but these are not significant contributors to direct energy sources at this time. These direct energy sources are produced by different means, and each means of producing the energy will have a different environmental impact profile. For example, producing 1 MJ of steam has a different set of emissions and resource requirements than those producing 1 MJ of electricity. In the same vein, the environmental impacts from energy production will vary depending on the efficiency of the energy generation and/or distribution processes, or on the typical energy sources used in a specific region. For example, in 2005, the production of 1 kWh of electricity in Alaska came from about 9.5% bituminous coal, in contrast with Texas electricity production, where 37% of the energy was derived from bituminous coal. On average, about half of the electricity in the United States comes from bituminous coal. These figures include line-loss factors during energy distribution.1,2See Figure 18.2 for a general illustration of the 2005 electricity mix in the United States according to the North American Electric Reliability Corporation.3This type of energy mix information can significantly affect the outcome of an LCA, so it is critical to obtain representative data, especially when dealing with averages.
Oil
Coal Nuclear
Hydro Others
Heating:
Reactors, Heaters,
Dryers, Evaporators, Distillation columns,
etc.
Electricity Steam Gas
Heat transfer
fluids Cooling:
Reactors, Chillers, Cooling tower,
Refrigeration, etc.
Transport:
Conveyors, Pumps,
etc.
Energy Recovery
Primary Energy Carriers
FIGURE 18.1 Potential energy paths in a chemical process. Primary energy carriers are shaded.
Example 18.1 In Example 16.2 we provided gate-to-gate and cradle-to-gate summaries for the production of 3-pentanone. Table 18.1 summaries the energy requirements for that process.
(a) Where does the energy to satisfy the heating requirements come from?
(b) How would you satisfy the cooling requirements?
(c) What might you do with any excess energy from the process?
Solution Table 18.2 shows the direct energy sources to be used for the process. All conveyance equipment (e.g., pumps, vacuum systems) will use electricity. Steam or heat exchange fluid will be used for heating. For this particular example we propose using DowTherm for heating to higher temperatures, and the vacuum distillation will require the use of some steam. The distillation condenser will probably take cooling water (or brine) at around 20C. Any excess energy could be used to reduce the DowTherm requirement for reactor heating and in the first heat exchanger.
Additional Points to Ponder How was the potential energy recovery estimated? How do you know that a vacuum distillation is taking place?
Since both mass and energy are normally included in a typical LCA, that is, we normally tally both total energy requirements and crude oil for energy production in LCIs, one question that may come to mind is: Are we double counting? In essence, from the viewpoint of the inventory, this is not the case, as it is equivalent to reporting the results for two different units. Nonetheless, one should be mindful of the potential for double counting during the FIGURE 18.2 Contributions to the U.S. electricity grid for 2005 by geographical zone. SERC, Southeast Electric Reliability Council.
assessment phase, so no additional weight is given to energy, except for weighting that is desired expressly as part of the value judgment. Another potential for a double-counting error is when a portion of the direct energy (e.g., MJ of steam) is reported along with the primary energy carrier (e.g., MJ of fuel oil to produce steam). As noted in Chapter 17, the key to ensuring that this does not happen would be in maintaining the transparency of the LCI data.
Besides avoiding potential double counting when reporting the energy and mass associated with energy production, there are several good reasons to provide separate figures for the resources required for energy production and consumption in general: that energy is normally well understood and it is easy to communicate, measure, and monitor (e.g., it is easier to obtain measured energy requirements than measured emissions).
Another interesting point regarding energy is that some raw materials can be used to produce energy or products. An example of this is when petroleum products are used to produce plastics. The total energy embedded in the materials being manufactured is called feedstock energyand the energy content for energy production is not included in feedstock energy. One must be careful when using feedstock energy in LCIs, as it is easy to double count if it is not used appropriately.4
When feedstock energy values are reported, it is important to specify the type of heat value that is used. It is common to see either the high heat value (HHV) or the low heat value (LHV), or both, reported. To avoid double counting one must know the distinction between these two values. The HHV represents the energy content as the full intrinsic energy, that is, the gross calorific value, and captures the combustion energy plus the phase transfer heat for the water formed in the combustion reaction. The LHV does not capture the phase-transfer heat of the water formed in the combustion reaction. For example, the HHVof natural gas is in general between 53 and 54 MJ/kg of natural gas, while its LHV is about 48 MJ/kg of natural gas. The TABLE 18.1 Summary of Energy Requirements
Energy Input (MJ/batch) Cooling Requirements (MJ/batch)
Unit
Energy Input (MJ/1000 kg
product) T0(C) Unit
Energy
Loss Teff(C)
Recovery Efficiency/
Energy Recovered
Pump 1 0.102 Heat exchanger 2 2046 370 60%/1227
Heat
exchanger 1
1312 300 Distillation
condenser 1
493 25.0 0
Reactor 1 516 370
Pump 2 2.23103
Pump 3 0.174
Pump 4 6.66104
Pump 5 1.34106
Pump 6 4.86104
Distillation reboiler 1
493 25.0
Pump 7 0.206
Pump 8 3.86104
Pump 9 4.50107
Vacuum pump 1
54.0 0
Unit
Energy Input (MJ/1000 kg
Product) T0(C)
Direct Energy
Type Unit
Energy
Loss Teff(C)
Recovery Efficiency/Energy
Recovered
Direct Energy Type
Pump 1 0.102 Electricity Heat exchanger 2 2046 370 60%/1227 Heat exchangers
Heat exchanger 1 1312 300 DowTherm Distillation
condenser 1
493 25.0 0 Cooling
water
Reactor 1 516 370 DowTherm
Pump 2 2.23103 Electricity
Pump 3 0.174 Electriciy
Pump 4 6.66104 Electricity
Pump 5 1.34106 Electricity
Pump 6 4.86104 Electricity
Distillation reboiler 1 493 25.0 Steam
Pump 7 0.206 Electricity
Pump 8 3.86104 Electricity
Pump 9 4.50107 Electricity
Vacuum pump 1 54.0 0 Electricity
523
difference between these two figures is the latent heat of the water formed in the combustion reaction.
18.1.1 Energy Requirements in a Chemical Plant
In a chemical plant, energy is required for the reaction (reactor, preheater, compressor, cooler, etc.) or for the separation train (filtration, dryer, heater, cooler, distillation, etc.). An article published by Kim and Overcash analyzed the gate-to-gate process energy for 86 chemical manufacturing processes,5where the energy requirements were estimated using a design-based methodology6following general chemical engineering principles such as those described inPerry’s Chemical Engineers’ Handbook.7In this paper it was found that the net energy used in half of the organic chemicals analyzed was between 0 and 4 MJ/kg, while the net energy requirement for inorganic chemicals was between1 and 3 MJ/kg.
The zero or negative net energy values were due to recovering the surplus energy (e.g., exothermic reactions) from steam production or direct heating and were regarded as an energy credit. In addition, the study found that about half of the process energy used in the organic and inorganic chemical processes was used for separation and purification processes and indicates the potential for general improvements. Table 18.3 shows the contrast between energy requirements for organic and inorganic chemical production found in the study.
As can be seen from the table, all these data have large associated standard deviations and suggest that reliable estimations for chemical production process energy is not possible at this time. However, these values could be used in screening life cycle assessments to establish if a specific chemical process contributes significantly to the cumulative energy demand when no primary information is available. One would, of course, want to do a sensitivity analysis to determine the impact of the estimations on the overall LCI.
Example 18.2 Is the energy required to produce 3-pentanone, as found in Example 16.2, a typical example of the production of this type of chemical?
Solution It will all depend on what “typical” means. Table 18.4 summarizes the energy requirements from Examples 16.2 and 18.1. From Table 18.1 we see that the electricity required and the potential energy recovery are within the expected ranges but just below average. However, given the large variation in energy requirements, this would be expected. Steam requirements seem to be at the lower end of the spectrum, but since
TABLE 18.3 Energy Requirements for Gate-to-Gate Production of Chemicals Direct Energy
Organic Chemicals Average Standard Deviation (MJ/kg)
Inorganic Chemicals Average Standard Deviation (MJ/kg)
Electricitya 0.60.98 1.95.1
Steam 7.714 3.68.2
Heating fuel 0.150.5 1.53.2
Potential recovery 1.6 1.9 2.0 5
Total (range) 0 to 4 1 to 3
Source:ref. 5.
aHeat transfer fluid is not included, due to its infrequent use.
steam use is so variable, it is still within the standard deviation for organic chemical production processes.
Additional Points to Ponder From an LCA viewpoint, why would it be important to know if the energy requirements are within certain ranges? Would the conclusions be different if the process did not have any energy integration?