Virtual Emergy Flow - American Chemical Society

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Virtual Emergy Flow: A New Concept for Emergy Analysis of Feedback Structures Hanfeng Mu,† Xiao Feng,*,‡ Jingyao Liu,§ and Khim Hoong Chu† †

Department of Chemical Engineering, Xi’an Jiaotong University, Xi’an 710049, China College of Chemical Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China § School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China ‡

ABSTRACT: Emergy analysis is useful for evaluating the resource utilization and environmental performance of industrial systems. However, emergy analysis in its current state can produce erroneous results when it is applied to systems with feedback structure. It is shown that traditional emergy algebra, if not applied properly, can cause violations of the tenet of total inheritance, which is one of the core notions in emergy theory. To overcome this deficiency, this paper proposes a new concept, called virtual emergy flow, for calculating the emergy of feedback structures. The utility of the virtual emergy flow concept is illustrated by the emergy analyses of two different types of feedback structures commonly found in chemical process systems as well as by a case study based on a power and methanol multiproduct system.

1. INTRODUCTION Sustainable development has become an important concept for long-term development of industries and effective environmental protection. Frosch and Gallopoulos1 initiated so-called “industrial metabolism” research by simulating biological metabolism and ecosystem regeneration cycles in the industry context. Various analytical tools are now available for assessing the sustainability of industrial systems. Examples include net energy analysis,2 life cycle assessment (LCA),3 exergy analysis,4 cumulative exergy theory,5 and emergy analysis.6 The lastmentioned method, first developed by Odum,6 is especially appealing due to its ability to compare different qualities and types of energy or material in terms of a common energetic basis and to account for the contribution of ecosystems to economic activity. The process industry is by nature complex and varied, typified by complex networks with dramatic convergence of material, energy, and information flows. Although emergy analysis has already been applied successfully to a number of natural and agriculture ecosystems7−9 as well as industrial systems,10−12 this paper illustrates that the existing rules of emergy algebra require some refinement and further testing when applied to chemical process systems. The improvement of these rules will also help clarify the state of emergy flows within the internal network of a chemical process system. In general, a network can be decomposed into four basic structures: series, parallel, loop circuit, and feedback. Series structure can be handled easily by the existing rules of emergy algebra.13 Cao and Feng14,15 studied the parallel and loop circuit structures and proposed improved methods of emergy calculation. The application of emergy algebra to these structures has been discussed in considerable detail elsewhere,13−17 so a description here is unnecessary. However, applying emergy algebra to feedback structure is not straightforward and has remained a challenge. As first pointed out by Lazzaretto18 in 2009, according to the existing rules of © 2013 American Chemical Society

emergy algebra, the input of emergy is not equal to the output of emergy in feedback structure. The work of Lazzaretto18 raises a perplexing conundrum in the emergy analysis of feedback structure that remains unsolved. This difficulty is likely to restrict the practical application of emergy analysis to chemical processes that invariably feature different types of feedback structures. In this paper, we propose a new concept, called virtual emergy flow, which can be used to ensure correct emergy analysis of various feedback structures commonly found in chemical process systems. Rectifying the abnormalities of emergy algebra will help promote its use in the analysis of chemical process sustainability. For example, emergy analysis can be integrated with other well-established analytical tools such as pinch analysis when evaluating resource utilization by chemical processes.19

2. FEEDBACK STRUCTURE AND EMERGY ALGEBRA Feedback structure is a typical basic structure in the chemical process industry and may include more than one process unit. It is mainly used to enable the recycle of material and energy of a system, that is, the partial material or energy flow, originated from the main flow, that returns to the upstream input position in the main flow. Two basic feedback structures involving two process units are shown in Figure 1. The scheme of Figure 1a illustrates a one product and split feedback structure. In this scheme there is only one product (material or energy) and partial product is fed back to the process to improve production efficiency or increase concentration of the product. Figure 1b depicts a scheme showing a multiproduct and one partial feedback structure where there is Received: Revised: Accepted: Published: 2330

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In Figure 2c, there are two emergy input flows: 400 sej/yr from source S to unit A and 100 sej/yr from source F to unit B. Thus, the emergy output of unit B must equal 500 sej/yr. It is assumed that three-fifths of this emergy output, i.e. 300 sej/yr, is sent back to unit A via the feedback flow. Because unit A receives 400 sej/yr of emergy from source S and 300 sej/yr of emergy from the feedback flow, the emergy output of unit A should be 700 sej/yr. However, a total of 240 sej/yr of emergy, which is equivalent to three-fifths of the source S emergy, out of the 300 sej/yr of emergy in the feedback flow is originally from sources S. Accordingly, this amount of emergy must be subtracted from the feedback flow emergy to avoid double counting. Therefore, the correct output of emergy from unit A is 460 sej/yr (400 sej/yr from source S + (300 − 240) sej/yr from feedback). Similarly, unit B receives two emergy input flows: 100 sej/yr from source F and 460 sej/yr from unit A. On the basis of the same reasoning, 60 sej/yr of emergy must be subtracted from the emergy output of unit A because this amount of emergy comes from the feedback flow that goes through unit A. Thus the emergy output of unit B is 500 sej/yr, in which 400 sej/yr (460 − 60) is from source S and 100 sej/yr from source F. As stated above, it is assumed that three-fifths of the emergy output of unit B, i.e., 300 sej/yr comprising 240 sej/ yr from source S and 60 sej/yr from source F, is fed back to unit A. The final output of emergy from this system is 200 sej/ yr and it is composed of 160 sej/yr of emergy from source S and 40 sej/yr of emergy from source F. In this way, the individual emergy flows in the system with feedback structure are now fully accounted for according to the rules of emergy algebra, avoiding double-counting errors. Besides the attribute of double counting, a different tenet of emergy theory that requires attention is total inheritance. A product of a system will inherit all resources that have been supplied to the system. This characteristic has two meanings: one is that the total outputs of a system will inherit all of the system inputs; the other is that the products of a multiproduct system will follow the rules of emergy algebra and inherit all of the system inputs. This characteristic is a fundamental aspect of emergy theory and any violations should thus be avoided. It is obvious that violating the tenet of total inheritance will result in erroneous analysis. For example, the total emergy input for the scheme depicted in Figure 2c is 500 sej/yr, but the total emergy output is only 200 sej/yr. It can be seen that 300 sej/yr of emergy is unaccounted for. The emergy analysis of Figure 2c as presented above avoided double-counting errors but violated the tenet of total inheritance. In addition, any violation of total inheritance in emergy analysis will lead to ambiguous conclusions when comparing series and feedback structures. For example, the series and feedback (taken from Figure 2c) structures shown in Figure 3 have the same total emergy input (500 sej/yr). Violating the tenet of total inheritance in the emergy calculations for the feedback structure suggests that the series structure has a higher emergy output than the feedback structure (500 sej/yr versus 200 sej/yr). One may therefore conclude that the feedback structure is inferior to the series structure. This conclusion contradicts the well-known fact that a system with feedback structure is able to improve its production efficiency or enhance its product concentration. Interestingly, erroneous calculation of emergy flows in feedback structures is not uncommon. In addition to the case of Brown and Herendeen17 described in Figure 2c, other notable examples can be found in Odum16 and Lazzaretto.18

Figure 1. Basic feedback structures in chemical process systems: (a) one product and split feedback structure and (b) multiproduct and one partial feedback structure.

one product without and another with partial split, which is sent back to the process. This structure is quite common in systems with waste treatment and reaction units. Feedback structure has been studied using emergy algebra by some researchers, albeit not in the chemical process context.13,16,17 Although avoiding a common error in emergy analysis known as double counting can be readily achieved for some feedback structures, complying with a fundamental tenet of emergy theory called total inheritance is often ignored. Nonetheless, Li et al.20 found that double-counting errors can still easily occur when estimating the emergy in feedback structure and multiproduct systems and so proposed an improved matrix method to simplify the calculation of transformities. Lazzaretto18 pointed out that the use of traditional emergy algebra in the analysis of feedback structure was not correct because the total inheritance tenet was violated.

3. ANALYSIS OF FEEDBACK STRUCTURE USING EXISTING EMERGY ALGEBRA As noted earlier, some researchers have applied emergy algebra to certain types of feedback structures. For example, Brown and Herendeen17 provided a detailed description of the state of emergy flows in the feedback structure shown in Figure 2c. Figure 2 depicts a classic feedback structure, which is illustrated in terms of energy flows, transformities, emergy flows and a feedback flow from unit B to unit A. We start by analyzing this feedback structure in some detail.

Figure 2. Two-compartment system with feedback:17 (a) energy flows, (b) transformities, and (c) emergy flows. 2331

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of total inheritance is violated. Here we demonstrate how the concept of virtual emergy flow can be used to rectify this shortcoming. Figure 6 shows a simple system comprising two process units with a single emergy source S and a feedback flow from unit 2

Figure 3. Comparison of (a) series structure and (b) feedback structure (units: sej/yr).

For example, the case from Lazzaretto18 is shown in Figure 4. In Figure 4, the emergy input of the system is 100 000 sej/yr.

Figure 6. Improvement of emergy calculation for feedback structure: (a) emergy calculation using traditional emergy algebra and (b) emergy calculation incorporating the concept of virtual emergy flow.

to unit 1. In the figure, “a” denotes the value of emergy input to the system from source S, while “b” designates the value of feedback flow emergy. Figure 6a shows the individual emergy flows in the feedback structure according to the traditional rules of emergy algebra. To avoid double counting, the feedback flow with emergy b is hidden in the main flow. At point A, the main flow is split into two: the feedback flow with emergy b and the output flow with emergy (a-b). According to this calculation, the input emergy (a) is higher than the output emergy (a-b). It is clear that in this case the tenet of total inheritance is violated. Figure 6b shows the individual emergy flows in the feedback structure calculated according to the concept of virtual emergy flow. The virtual emergy flow with emergy b is indicated by the dashed lines, while the main flow with emergy a is given by the solid lines. When the virtual emergy flow and the main flow arrive at point A, the former becomes the actual feedback flow with emergy b, while the latter becomes the output flow with emergy a. It is obvious that, in this case, the input and output flows have the same emergy (a), satisfying the requirement of the total inheritance tenet. This approach also makes it easier to avoid double counting because the emergy of the feedback flow is not hidden in the main flow. Next, we illustrate how the concept of virtual emergy flow can be incorporated in emergy calculation to ensure adherence to the tenet of total inheritance and avoidance of double-counting errors. The two types of feedback structures depicted in Figure 1 are used as examples and they are reproduced in Figure 7. Let us assume that the emergy of source S for both structures is 1000 sej/yr, the emergy of the feedback flow in Figure 7a,b is 300 sej/yr. In Figure 7a, according to one of the rules of emergy algebra (i.e., byproducts, when reunited, cannot be added to equal a

Figure 4. The case from Lazzaretto18 (units: sej/yr).

The emergy flow leaving unit C is split into different flows. Two feedback flows, each with 2702.73 sej/yr of emergy, are fed back to units A and B. After accounting for the emergy of the feedback flows, the emergy output of the system is reduced to 94 594.94 sej/yr. As in the case of Figure 2c, the emergyaccounting procedure avoided double-counting errors but at the same time violated the tenet of total inheritance. In view of this incongruity, the traditional rules of emergy algebra for calculating the emergy of feedback structure require modification. In the next section, a new concept is proposed to bring about this improvement.

4. VIRTUAL EMERGY FLOW: A NEW CONCEPT FOR EMERGY ANALYSIS OF FEEDBACK STRUCTURE A new concept, termed virtual emergy flow, is defined in this paper to overcome the defect of traditional emergy algebra in analyzing feedback structure. According to this concept, a virtual flow is added to the interior of a system, which joins up with the actual feedback flow to form a flow loop, as described below. As shown in Figure 5, the dashed and solid lines given by ANMBA form an emergy flow loop. This flow loop starts at

Figure 5. Virtual emergy flow (dashed lines).

point A, goes through points N and M, feeds back to point B to enter the main flow, and ends at point A, forming a complete circulation flow. Note that the parallel dashed and solid lines in the system interior are the same flow. This circulation flow is in essence a semivirtual flow. As noted above, in the case of Figure 2c, the emergy of the feedback flow is neglected to avoid double counting, causing the emergy output of the system to be less than the emergy input of the system. As a result, the tenet

Figure 7. Application of the virtual emergy flow concept to different types of feedback structures (units: sej/yr): (a) one product and split feedback structure and (b) multiproduct and one partial feedback structure. 2332

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of virtual emergy flow in the emergy calculation makes the final emergy output (500 sej/yr) equal the sum of the emergy inputs from source S (400 sej/yr) and purchased emergy F (100 sej/ yr).

sum greater than the source emergy from which they were derived),17 the total emergy output of this structure is max{product1, product2} in order to avoid double counting, which is 1000 sej/yr. The same reasoning applies to Figure 10b. Given that the one product and split feedback structure of Figure 7a is identical to the feedback structure of Figure 6 analyzed previously, applying the virtual emergy flow concept to Figure 7a is straightforward. The emergy flow coming from source S enters units 1 and 2. Part of the emergy of the output flow of unit 2 (300 sej/yr) is fed back to unit 1. Then, this flow goes though unit 1 and unit 2, as indicated by the dashed lines, and merges into the feedback flow. This pathway forms an emergy flow loop. The input and output flows have the same emergy (1000 sej/yr), thus satisfying the condition imposed by the tenet of total inheritance. The feedback flow is used to improve the quality of the product or enhance the energy utilization ratio of the system. The one partial feedback structure shown in Figure 7b has two products. According to one of the rules of emergy algebra, i.e., byproducts from a process have the total emergy assigned to each pathway,17 each product will inherit the entire emergy of the input (1000 sej/yr). Because product 1 is associated with the feedback flow, a virtual emergy flow with an emergy of 300 sej/yr is included in this part of the structure. In this way, the emergy of the output remains at 1000 sej/yr. The output of product 2, which has no feedback flow, assumes a value of 1000 sej/yr. Therefore, the emergy output of each product is 1000 sej/yr. The emergy flows in the feedback structure of Figure 2c are recalculated using the proposed method, as shown in Figure 8.

5. CASE STUDY The final section of this work uses a case study founded on a coal-based multiproduct system to illustrate the utility of the virtual emergy flow concept in emergy calculation. This system was simulated using the commercial process simulator Aspen Plus and the relevant process information reported in the original study.21 The simulated material flow data were then converted to emergy flow data using the appropriate transformity parameters. Figure 9 shows a schematic diagram of the coal-based multiproduct system. The system combines power generation and methanol production from syngas (CO and H2) and is based on the partial-recycle methanol synthesis scheme without a CO/H2 ratio adjustment process. Coal, water, and O2, which comes from the ASU (air separation unit) unit, enter the gasification unit and react to produce a crude gas stream containing syngas and acid gases (H2S and CO2). The hightemperature crude gas stream is first cooled in the heat recovery steam generation unit and then purified in the clean up and acid gas removal unit. Subsequently, the cleaned syngas is pressurized and enters the methanol synthesis tower, producing a gas−liquid mixture. The gas−liquid mixture from the methanol synthesis tower is separated into a gas stream (unreacted syngas) and a liquid stream (crude methanol) in the gas−liquid separator. The crude methanol is sent to the distillation unit for purification, and the unreacted syngas is split into two separate streams at point S: recycle syngas and nonrecycle syngas. The recycle syngas is fed back to the methanol synthesis tower. Before entering the methanol synthesis tower, the recycle syngas is mixed with the cleaned syngas from the clean up and acid removal unit at point M. The nonrecycle syngas is sent to the combined cycle unit for use as fuel to generate electricity. It can be seen that the methanol synthesis tower, the gas− liquid separation unit, splitting point S, and mixing point M form a multiproduct and one partial feedback structure (see Figure 1b). The two products are the nonrecycle syngas sent to the combined cycle unit and the crude methanol sent to the distillation unit. According to the original study,21 the syngas input stream at point S is split as follows: 80% recycle syngas and 20% nonrecycle syngas. Figure 10a shows the individual emergy flows calculated for this multiproduct and one partial feedback structure using traditional emergy algebra. Note that F is purchased emergy representing equipment, construction, and labor. Care has been

Figure 8. Application of the virtual emergy flow concept to the feedback structure of Figure 2c (units: sej/yr).

The emergy input from source S (400 sej/yr) and the feedback emergy flow (300 sej/yr) enter unit A. Accordingly, a total of 700 sej/yr of emergy from unit A enters unit B, which also receives another emergy input from purchased emergy F (100 sej/yr), giving a grand total of 800 sej/yr of emergy output. This grand total emergy output flow is split into two when it leaves unit B, one of which with an emergy of 300 sej/yr returns to unit A as the feedback flow and forms a virtual emergy flow with the same amount of emergy. The other flow with an emergy of 500 sej/yr departs from the system as a product output. It may be seen that incorporating the concept

Figure 9. A coal-based multiproduct system combining power generation and methanol production. 2333

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emergy flow concept should result in it being of wide practical use.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-10-8973-3991. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Basic Research Program of China (973 Program: 2012CB720500) and the National Natural Science Foundation of China under Grant 50876079 is gratefully acknowledged.

Figure 10. Enlarged schematics showing emergy calculations for the feedback structure of the case study (units: ×1018 sej/yr): (a) traditional emergy calculation and (b) emergy calculation incorporating the concept of virtual emergy flow.

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REFERENCES

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taken to avoid double-counting errors in the calculation procedure. The total emergy input is 508.7 × 1018 sej/yr (502.0 × 1018 sej/yr from the clean up and acid removal unit plus 6.7 × 1018 sej/yr from purchased emergy). The emergy output calculated for one of the two products, crude methanol, is identical to the total emergy input. However, this is not the case for the other product: nonrecycle syngas. The emergy of the nonrecycle syngas leaving point S and entering the combined cycle is calculated as 101.7 × 1018 sej/yr, accounting for 20% of the syngas entering point S. It is evident that it is not possible to account for the emergy of the recycle syngas (80% of the syngas input at point S) in the feedback flow in the calculation of the nonrecycle syngas emergy. Consequently, in this multiproduct system the total emergy input (508.7 × 1018 sej/yr) is not equal to the emergy output of one of the products (nonrecycle syngas; 101.7 × 1018 sej/yr). The calculation procedure based on the existing rules of emergy algebra therefore violates the total inheritance tenet of emergy theory. Next, we show how the violation of total inheritance identified above can be rectified by using the concept of virtual emergy flow. In Figure 10b, setting point S as the starting point, a virtual emergy flow is formed by the loop SBMAS. The calculated emergy of the feedback flow (recycle syngas) is 407.0 × 1018 sej/yr, accounting for 80% of the emergy input at point S. Due to the existence of the virtual emergy flow, the calculated emergy of the nonrecycle syngas entering the combined cycle is 508.7 × 1018 sej/yr. It is clear that there is now quantitative agreement between the emergy output of the nonrecycle syngas and the total emergy input. As before, the emergy output of the crude methanol agrees with the total emergy input. Incorporating the concept of virtual emergy flow in the emergy calculation therefore ensures adherence to the tenet of total inheritance and avoidance of double-counting errors.

6. CONCLUSION The application of emergy algebra to different types of feedback structures has been analyzed in this study. For systems with feedback structure, it is shown that emergy calculation based on the existing rules of emergy algebra can cause total inheritance violations. A new concept proposed in this study, termed virtual emergy flow, can be used to overcome this pitfall of traditional emergy algebra. The utility of the virtual emergy flow concept was illustrated using a case study based on a power and methanol multiproduct system. The simplicity of the virtual 2334

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(21) Huang, H.; He, F.; Li, Z. Integrated Gasification Combined Cycle Economic Estimation Model of China. J. Power Eng. 2008, 28, 633−638 (in Chinese).

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