Computer-Aided Exergy Sensibility Analysis of Nitrobenzene Production through Benzene Nitration Using an Acid Mixture

Nitrobenzene is widely produced via benzene nitration to be applied in several industries such as pharmaceutical, textile, and agricultural. In this work, an exergy sensibility analysis was performed with the aim of identifying possible opportunities of process improvements. 1e irreversibilities, exergy of wastes, and efficiency were calculated per stage through exergy balance. 1e simulation software Aspen plus V10.1 provided the physical exergies of process streams while chemical exergies were found in literature. A sensibility analysis was also carried out in order to study the effect of efficiency of some stages (polyfunctional reaction and cooling 1) on global exergy efficiency. 1is analysis reveals that nitrobenzene production from benzene is an efficient process from an exergy viewpoint (88%). 1e total irreversibilities, total exergy of wastes, and exergy of utilities-inlet were calculated in 41,647,341.85MJ/h, 5,537,487.3MJ/h, and 18,137,363.71MJ/h, respectively. 1e results obtained from sensibility analysis suggested that heat flow of heat exchangers HE-201, HE-303, and HE-301 could provide energy requirements for heating.

e nitration of aromatics is an important electrophilic substitution reaction widely used in the large-scale industrial process of nitroaromatic valuable chemicals such as nitrobenzene, nitrotoluene, and 2,4-dinitrotoluene [4,5].Nitrobenzene is commercially produced via nitration of benzene with a mixture of sulfuric and nitric acids, which is an excellent example of reactions in the liquid phase [6,7].e sulfuric acid acts as a homogeneous nitration catalyst and at the same time as a dehydrating agent protonating the HNO 3 molecules, while nitric acid acts as the main nitration agent [8,9]. is reaction occurs quickly and is highly exothermic; hence, temperature must be controlled to avoid formation of more by-products, decomposition of nitric acid, and even explosion [10].
e nitration of benzene is commonly used because of its high efficiency; however, it is not environment friendly due to the huge amount of acid wastes generated in this hazardous industrial process, which include diluted sulfuric acid (0.9 ton wastes per 1 ton of commercial products) obtained after reaction and formation of water and nitrophenols formed as by-products [8,11,12].To overcome these drawbacks, many efforts have been made on developing improvements involving lower energy consumption and various clean nitration approaches [13].To date, exergy analysis tool has not been employed to identify process stages that need to be enhanced from an energy viewpoint.
is work is focused on performing exergy sensibility analysis to nitrobenzene production through benzene nitration as the heterogeneous liquid-liquid catalyzed reaction using sulfuric and nitric acids solution in order to analyze global irreversibilities and propose a novel alternative to increase global exergy efficiency.

Process Description.
e nitrobenzene is produced via nitration of benzene using a mixture of concentrated acids [14].e sulfuric acid medium acts as a catalyst and dehydrating agent that bonds with water to ensure that all of the nitric acid is employed for nitration [8,9].Some of the reactives (streams 1, 2, and 4) are transported from storage tanks to the adiabatic mixer using a pump.During the mixing, an exothermic reaction takes places; the rise in temperature is due to the enthalpy of mixing, and the output stream is increased to 120 °C, which is constituted by the components listed in Table 1.A heat exchanger is used to cool this stream (stream 6) to a target temperature of 80 °C.
e process block diagram for the nitrobenzene manufacturing plant via benzene nitration is shown in Figure 1.As can be observed, this process involves 18 stages that are described as follows: the catalyzed liquid-liquid reaction is carried out in four adiabatic reactors achieving conversions of 91% (stream 9), 98% (stream 10), 99% (stream 11), and 99.5 % (stream 12) for reactors 1, 2, 3, and 4, respectively.e catalyst is in a different phase as benzene is an organic reactant and the mixed acid is in the aqueous phase; hence, it is a heterogeneous catalyst.e high purity benzene (stream 8) enters the first reactor in excess in order to react with all of the nitric acid.
is stage requires continuous stirring to keep the benzene, nitric acid, and sulfuric acid well mixed [15].e nitric acid acts as the main nitration agent, and the sulfuric acid protonates the HNO 3 molecules generating active electrophilic species that attack the aromatic ring.e nitration reaction is described as follows [9]: As can be analyzed from this reaction, the nitronium ion (NO 2 + ) is obtained by dissociation of nitric acid in the presence of sulfuric acid reaction [16].e overall reaction scheme is shown in Figure 2.
e stream leaves the fourth reactor at 135.75 °C and is fed into a heat exchanger to cool it until 80 °C (stream 13).A separator is employed to remove the sulfuric acid solution (stream 14) from the organic phase (stream 19).e sulfuric acid solution is purified using an evaporator in order to send it back to the mixer.e organic phase is sent to a centrifugal extractor, in which sodium carbonate is used as the extracting agent (stream 22).e resulting wastes (stream 23) are sent to the environmental treatment unit.e stream leaving the extractor passes through a stripping column that is employed to separate benzene from nitrobenzene with steam.e water content in the benzene stream is removed in a liquid-liquid separator.e purified benzene is cooled and sent back to the first reactor.

Exergy Analysis.
e exergy analysis represents a powerful tool to overcome the limitations of the first law of thermodynamics, allowing to identify the main sources of energy loss and improve process efficiency [17,18].In addition, this analysis has been widely used to measure the sustainability of industrial processes [19].Exergy is defined as the maximum theoretical useful work obtained when a system is brought into thermodynamic equilibrium with its environment [20].
e exergy transfer by heat flow at temperature T( _ Ex heat ) and exergy by work flow ( _ EX work ) was calculated by the following equation: ( e mass exergy component is divided into physical exergy ( _ Ex phy ), chemical exergy ( _ Ex che ), potential exergy ( _ Ex pot ), and kinetic exergy ( _ Ex kin ), as shown in the following equation: e potential and kinetic exergy were considered negligible.e chemical exergy was calculated in the following equation: where _ Ex 0 ch−i is the chemical exergy of each elemental compound, ΔG 0 f is the Gibbs free energy of formation for the compound, and v i is the number of atoms of each element contained into the stream.e chemical exergy of process streams was determined as follows: where y i is the molar fraction of component i, T 0 is the reference temperature, and R is the gas constant.In this work, chemical exergy of some compounds was found in literature and used to perform exergy analysis [21].e physical exergy is defined as For an ideal gas with constant heat capacity, the physical exergy can be also calculated by equation (7).For a solidliquid mixture, it is defined by equation (8), where C p is the heat capacity, v m is the molar volume, T is the operating temperature, P 0 is the reference pressure, and P is the operating pressure: e physical exergy for each process stream was provided by Aspen plus V10.1 simulation software considering operating conditions as pressure, temperature, and composition.
e UNIQUAC model and Peng-Robinson equation of states were used to simulate thermodynamic properties of compounds.
e output of exergy is attributed to products or residues generated during the manufacturing process, which is shown as follows: A global balance around the system allows to calculate the process irreversibilities that indicate the unused potential work: e exergy e ciency of the process and the contribution of each stage to exergy loss were calculated by the following equations, respectively: Assumptions.e following assumptions were considered to successfully perform exergy analysis: (i) A polyfunctional reactor was incorporated to synthesize the four reactors, a heat exchanger, and a separator, as shown in Figure 3 (ii) Heat ow was supplied to the mixer in order to have zero irreversibilities (iii) e evaporator was combined with a heat exchanger and represented as a new single stage

Exergy Analysis.
e exergy balance around the system provides information concerning the e ciency of process stages and allows to identify promising improvements.Table 2 summarizes the chemical and physical exergy of process streams, which were calculated using both mathematical equations and simulation software.
e exergy analysis revealed that the condenser (C-301-Condensation 1) employed after the extraction process exhibited the highest exergy of residues because of the amount of water used to cool stream 17. e extraction stage also reported a high value for exergy of wastes followed by the separation stage, which was expected due to the amount of sodium carbonate, sulfuric acid, and water that are removed from the main stream (stream 19).e results in the separation stage were attributed to the 4155 kg/h of water considered as wastes and removed to obtain pure benzene.Figure 4 shows the irreversibilities, percentage of destroyed exergy, exergy e ciency, and exergy of wastes in each process stage.As can be observed, the highest percentage of destroyed exergy (45.6%) was achieved in the cooling 1 stage (HE-201-202) incorporated for reducing the temperature of the mixed acid stream, followed by polyfunctional reaction with 32.36%.Many stages showed exergy e ciency around 100% including mixing, evaporation, extraction, stripping, and separation.Hence, improvements are required in the other stages in order to increase exergy e ciency.International Journal of Chemical Engineering Figure 5 shows global exergy e ciency, total exergy of wastes, total irreversibilities, and total exergy of utilities.As can be observed, the nitrobenzene production process exhibited a global e ciency of 88%, indicating a good conversion of the raw material into the desired products.e total irreversibilities and exergy of wastes were calculated in 41,647,341.85 and 5,537,487.3MJ/h, respectively.In order to improve this process from an exergetic viewpoint, some streams of wastes could be used.e composition of stream 18 is 0.98 of water and 0.02 of a mixture constituted by nitric acid, benzene, and nitrobenzene.Hence, water content can be recovered and used for cooling or heating requirements.

Sensibility
Analysis.An exergy sensibility analysis was performed in order to evaluate the e ect of irreversibilities and exergy e ciencies of stages on global process e ciency.As is shown in Figure 6, the global exergy e ciency increased by 4% as exergy e ciency of the polyfunctional reaction stage varied from 68% to 95%. Figure 7 shows an increase in global exergy e ciency as the e ciency of heat exchangers HE-201 and HE-202 increased from 54% to 95%.
In order to identify how process e ciency may be improved, it was assessed how the conversion in each of the four reactors may a ect the exergy e ciency of the stage.Hence, the exergy of products leaving the reactors was calculated and plotted along with the cumulative conversion.As shown in Figure 8, the exergy of streams leaving the reactors decreased as the number of reactors increased, i.e., both variables have an inverse relationship, which is attributed to the lower exergy value for products compared to the exergy of reactives.
e exergy of products also  e sensibility analysis in Figure 6 shows a 4% increase in global exergy e ciency based on an increase in the exergy of the polyfunctional reaction stage.However, this stage is composed of four reactors, heat exchanger, and separator, and the contribution of these unit operations to the stage exergy e ciency needs to be assessed.Figure 9 presents the contribution of each unit operation to the irreversibilities of the polyfunctional reaction stage.It was found that the separation unit a ects signi cantly the irreversibilities of the stage, representing 96% of total irreversibilities, followed by cooling (3.9%) and reaction (0.8%).According to these results, there is a need to improve the separation unit in this stage.is contribution is directly related to the separation e ciency because a change in separation e ciency a ects the exergy of wastes.On the contrary, contributions of the cooling unit are given by the lack of equipment for heat recovery and use (heat integration).Finally, the low contribution of the reaction stage is given by the e cient use of energy through the use of adiabatic reactors which allow to take advantage of the heat released by the reaction for heating the outlet stream in each reactor increasing conversion.
Figure 10 shows the heat ow in heat exchangers HE-201, HE-301, HE-303, HE-501, and HE-502, which were employed in the stages of cooling 1, polyfunctional reaction, evaporation, cooling 2, and cooling 3, respectively.As is observed, the HE-201, HE-303, and HE-301 exhibited the highest output heat ow that could be used to supply the heating requirements of other process streams.

Conclusions
An exergy analysis was carried out by the nitrobenzene production process via benzene nitration in order to identify stages with high irreversibilities and propose suitable alternative to improve global exergy e ciency.e global e ciency was calculated in 88% attributed to the few wastes streams as a result of good conversion of raw materials into desired products.e highest irreversibilities were found in the cooling 1 stage, followed by the polyfunctional reaction stage.
is process reported total irreversibilities, total exergy of wastes, and exergy of utilities-inlet of 41,647,341.85MJ/h, 5,537,487.3MJ/h, and 18,137,363.71MJ/ h, respectively.It was proposed to use the heat ow of heat exchangers HE-201, HE-303, and HE-301 to provide energy requirements for heating.International Journal of Chemical Engineering

Figure 3 :
Figure 3: Simpli ed block diagram of nitrobenzene production via benzene nitration.

Figure 9 :Figure 10 :
Figure 9: Contribution of unit operations to the irreversibilities of the polyfunctional reaction stage.

Table 1 :
Composition of the mixer output stream.

Table 2 :
Chemical and physical exergy of process streams.