Often an appropriate option to reduce the energy demand of an entire process is use of the heat of an exothermal reaction. This can be done directly in the process or in other processes on the same site (see Section 7.10). In the latter case this is realized in general by means of heating utilities, for example steam.
There are manifold options for the recovery of reaction heat. The energy can be used to preheat feed streams, to generate steam or to support the subsequent separation.
An example for the recovery of reaction heat is the nitration process as shown in Figure 7.25. In the presence of nitric acid and sulfuric acid, large quantities of heat are generated in the reactor as the exothermal reaction proceeds. In a conventional isothermal process, the heat of reaction is removed from the system by cooling water. After product separation, water (also water coming from nitric acid and the reaction) is separated from diluted sulfuric acid in an energy intense
260 | 7 Energy Efficient Unit Operations and Processes Isothermal Nitration
Nitric Acid ft
Figure 7.25 Comparison of isothermal and adiabatic nitration.
manner. Such concentrated sulfuric acid is sent back to the reactor. The heat generated in the reaction is essentially wasted in this process. For this reason efforts have been made to modify the process in a way that would enable the valuable heat of exothermal reaction to be recovered. The result of this work is a new adiabatic nitration, in which the heat of reaction instead of being wasted by cooling water is used in concentration of sulfuric acid, consequently lowering the energy demand of the entire system. For a Bayer plant in Leverkusen, it was possible to reduce the steam demand by 240000t/a, which translates to 40000t/a of C02 emissions .
If a number of hot and cold streams exists within the plant, between which heat exchange is possible, one of the classical methods of use of available energy in the plant is optimal design of heat exchanger network. Most chemical plants have numerous heat sources and heat sinks at different temperature levels, that are satisfied by different steam and cooling medium networks. The design of the heat exchanger network focuses on identification of profitable and feasible combinations of heat exchanger network that would minimize the amount of energy imported from outside, through heat transfer between two process streams. Pinch analysis is the most established tool used to identify the optimal combination of hot and cold streams. The development of the pinch principle by Linnhoff et al. has provided engineers with a scientific design methodology which has achieved outstanding results across the range of process industries . This methodology is discussed in detail in Chapter 6.
The resulting heat exchanger network is characterized by a minimum energy requirement that needs to be imported. However, the balance between energy savings and capital costs, as well as operability and safety considerations for the network needs to be taken into account.
Pinch analysis is an established tool in the design phase for new plants. In addition, retrofitting of the heat exchanger network of an existing plant is highly recommended. Since energy prices are rising in the long term and many older plants were not designed considering energy minimization, and often have been de-bottienecked, there is usually a large optimization potential to be identified.
A possibility to improve heat integration is intensification of heat transfer. A reduced temperature difference between the hot and the cold process streams of the heat exchanger network allows increased energy recovery. The possibilities to improve heat transfer are increased heat transfer coefficients or increased heat transfer surfaces. Heat transfer coefficients can be increased for instance by turbulence inducing elements in pipes or by structured heat exchanger surfaces. Plate heat exchangers or microstructured equipment (see Section 7.9.6) can help to increase heat transfer surfaces.
Besides thermal coupling by heat exchange within the process it is also worth considering the use of waste heat to generate other types of energy carrier. An example is the increasing usage of waste heat at a temperature of approx. 120 °C to 150) C to generate chilled water of approx. 5) C by an adsorption chiller. An additional example is the usage of low pressure steam in a steam turbine to generate electricity.
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