Graphical Method for Integrating Purification Processes in Hydrogen

Article pubs.acs.org/IECR

Graphical Method for Integrating Purification Processes in Hydrogen Systems with Constraints of Flow Rate and Concentration Minbo Yang,† Xiao Feng,*,‡ Khim Hoong Chu,§ and Guilian Liu† †

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249 China § Honeychem, Nanjing Chemical Industry Park, Nanjing 210047, China ‡

ABSTRACT: Introducing purification devices into hydrogen systems can enhance the extent of hydrogen reuse. The economic performance of a purification device depends on its appropriate placement within a hydrogen system. This work presents an improved version of a previously published graphical method for integrating purification processes in hydrogen systems. A mathematical method is deduced for calculating the maximum hydrogen utility savings potential of a hydrogen system with purification reuse. The improved graphical method is able to handle the constraints of concentration and flow rate of a purification process when targeting the maximum hydrogen utility savings. The proposed method can be used for analyzing purification processes with any feed concentration. The graphical method is tested on two case studies. steps. Bandyopadhyay12 presented a methodology to reduce waste generation through process modification. In addition, approaches using superstructure-based models have also been proposed for optimizing hydrogen systems.13,14 Introducing purification devices into hydrogen systems can improve the extent of hydrogen reuse substantially, and this option is now widely implemented in refineries.14,15 However, the economic performance of purification devices depends on their appropriate placement within a hydrogen system. In 2011, based on the systematic graphical method proposed by Zhao et al.,10 Zhang et al.16 developed a graphical optimization methodology for targeting the minimum utility consumption of hydrogen networks with purification reuse. In their method, the product concentration and recovery rate of the purification process are treated as process variables and the optimization of these purification parameters is combined with the integration of the hydrogen network. Subsequently, Lou et al.17 incorporated algebraic equations into the graphical method of Zhang et al.,16 creating somewhat complicated models that can be used to find the initial location of purifiers and improve the composite curves shifting procedure. However, a drawback of the method is that the feed streams of purification devices are restricted to hydrogen sources with the lowest hydrogen concentration. In 2013, Liu et al.18,19 proposed another systematic graphical method for targeting the maximum purification feed flow rate and the optimal purification feed flow rate of a hydrogen network with specified purification feed, purified product, and the hydrogen recovery, based on the graphical method of Alves and Towler.6 In this method, the feed concentration is regarded as a variable but the possible use of the tail gas from the purification device is not considered. Based on the pinch concept, Liao et al.20,21 introduced a

1. INTRODUCTION In petroleum refining processes, hydrogen is an essential resource used in many processes, such as hydrotreating and hydrocracking. A number of factors including increasingly strict environmental regulations, demands for increasingly highquality fuels, and use of high-sulfur crude oil and heavier oil, have compelled refineries to process crude oil more deeply, causing hydrogen consumption to rise sharply.1−3 As a result, hydrogen supply becomes a critical problem for many refineries, which turns hydrogen into an expensive utility. Therefore, effective utilization of hydrogen can offer significant economic benefits for the petrochemical industry. To reduce hydrogen consumption, Bollinger et al.4 proposed different optimization methods for individual hydrogen purification devices. With the development of systems engineering, a variety of methods have been developed to optimize hydrogen networks as a whole. These methods are essentially based on those developed for optimizing heat exchanger networks.5 In 2002, Alves and Towler6 proposed a graphical method for analyzing hydrogen distribution systems based on the concept of hydrogen surplus, which can determine the minimum supply of fresh hydrogen and identify the pinch point. However, their approach requires iterative calculation. In 2003, El-Halwagi et al.7 introduced a different graphical method for rigorously targeting minimum consumption of hydrogen utility. A drawback of this method is that the utility must be pure hydrogen. Subsequently, Almutlaq et al.8 generalized the method of El-Halwagi et al. to systems with any utility concentration. Agrawal and Shenoy9 proposed a unified conceptual approach for hydrogen networks design, using the nearest neighbors algorithm (NNA). In 2006, similar to the methods proposed by El-Halwagi et al.7 and Almutlaq et al.,8 Zhao et al.10 presented a simple graphical method for determining the minimum hydrogen demand. Foo et al.11 introduced the concepts of property surplus diagram (PSD) and property cascade analysis (PCA) for targeting minimum fresh usage and waste discharge, which eliminate the iterative © 2014 American Chemical Society

Received: Revised: Accepted: Published: 3246

August 23, 2013 January 22, 2014 January 30, 2014 January 30, 2014 dx.doi.org/10.1021/ie402785q | Ind. Eng. Chem. Res. 2014, 53, 3246−3256

Industrial & Engineering Chemistry Research

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rigorous mathematical method for optimizing hydrogen networks under scenarios both with and without a purification unit. Moreover, mathematical programming models were also constructed for optimizing hydrogen systems with purification reuse.14 In this work, the graphical method of Zhang et al.16 is simplified and improved by moving the hydrogen composite curve with the constraint of the pinch point. Then, the maximum hydrogen savings potential of a hydrogen system is deduced based on this graphical method. The improved graphical method can analyze the purification reuse process with any feed concentration. By imposing the constraints of concentration and flow rate on purification processes, this improved graphical method is more in line with actual industrial applications. Furthermore, it is noteworthy that the hydrogen concentrations of purified product and tail gas are specified in the proposed method and the tail gas of a purification process is treated as a hydrogen source with the lowest hydrogen concentration.

Figure 2. Polygon rule of a purification process with three feeds.

the minimum hydrogen utility consumption without purification.

2. IMPROVED GRAPHICAL METHOD 2.1. Material Balance and Triangle Rule of a Purification Process. The mass balances of a purification process can be described as follows:16 Fin = Fpur + Ftail

(1)

FinC in = FpurCpur + FtailC tail

(2)

R=

FpurCpur FinC in

(3)

where F and C represent the flow rate and hydrogen concentration, respectively; subscripts in, pur, and tail denote the feed, purified product, and tail gas, respectively; and R represents the hydrogen recovery rate of the purification process. Following the method of Zhang et al.,16 eqs 1 and 2 can be expressed in terms of the hydrogen load versus flow rate diagram, as shown in Figure 1. If the feed is a mixture of several streams, the triangle rule also can be generalized to a polygon rule, as shown in Figure 2.16 2.2. Graphical Method of Purification Reuse. In the work of Zhang et al.,16 the general procedure of their graphical method for identifying the purification polygon and the maximum hydrogen utility savings is described in considerable detail. For a given hydrogen system, the hydrogen source and sink composite curves are shown in Figure 3. Line AM denotes

Figure 3. Hydrogen source and sink composite curves.

As shown in Figure 3, the part of the hydrogen system below the pinch point can be taken as a net hydrogen source with hydrogen surplus, while that above the pinch point can be taken as a net sink, which needs to consume hydrogen utility. Therefore, only when the purified product is above the pinch point can it reduce the hydrogen utility consumption as well as waste.16 When only the relationship between Cpur and Cutility (utility concentration) is considered, three possible cases exist: Cpur = Cutility, Cpur > Cutility, and Cpur < Cutility. In these cases, Ctail is treated as a known parameter. 2.2.1. Case 1: Cpur = Cutility. As shown in Figure 4, straight line BD with Cpur as its slope is constructed to represent the purified product, and similarly GF with Ctail as its slope representing the tail gas. To determine the location of GF, GF is shifted along the vertical direction until it intersects the sink composite curve and the whole sink composite curve lies above GF. It can be seen in Figure 4 that GF intersects the source composite curve at H and BD at E. Thus, the BNHE enclosure is the final purification polygon, in which BNH is the feed, and BE and EH represent the purified product and the tail gas, respectively. For Cpur = Cutility, BE can replace the hydrogen utility directly. AI represents the hydrogen utility savings which equals BE while CE denotes the final waste discharge. 2.2.2. Case 2: Cpur > Cutility. Similarly, line GF with Ctail as its slope represents the tail gas, as shown in Figure 5a. Straight lines BD and AQ with Cpur as their slope can be constructed which represent the purified product. For Cpur > Cutility, BE cannot be used to replace the hydrogen utility directly. To

Figure 1. Triangle rule of a purification process. 3247

dx.doi.org/10.1021/ie402785q | Ind. Eng. Chem. Res. 2014, 53, 3246−3256


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mentioned. We can easily transform these cases to the adjacent cases introduced above. So these cases are not presented here. Comparing Figure 5 and Figure 3, we can see that the source composite curve above the pinch point is far away from the sink composite curve, and the source composite curve below the pinch point is close to the sink composite curve during the sliding process in Figure 5. Therefore it is possible for the new pinch point to appear below the original pinch point. The situation in Figure 6 is opposite to that in Figure 5, where the appearance of the new pinch point above the original pinch point is possible. Comparison of the three cases in this section indicates that if the slope of GF is the same in Figures 4−6, the tail gas concentration will be the same in all three cases. We can see that the hydrogen utility savings and the final waste gas in Figures 4−6 are also equal to each other, although the purified product concentration is different. For general hydrogen systems, if the new pinch point appears before the termination condition of the sliding process is reached, the source composite curve should slide continuously along the new pinch point. However, if the new pinch appears at the concentration of purified product, the sliding process should be terminated early.

Figure 4. Identification of the purification polygon and utility savings (Cpur = Cutility).

make the final hydrogen systems feasible, the whole source composite curve should slide upward (direction of the arrow) along the pinch point P, as shown in Figure 5b. AQ intersects A′M at point I. During the sliding process, B′E′ is decreased and AI is increased. The sliding process stops when B′E′ = AI, and B′N′HE′ is the final purification polygon, as shown in Figure 5b. A′I represents the hydrogen utility savings, and CE′ represents the final waste discharge. The dotted curve in Figure 5b is the initial source composite curve. It can be seen that BB′ = AA′ and BB′ parallels AA′. Also, given that B′E′ = AI and BE′ parallels AI, we can thus see that triangles BB′E′ and A′AI are congruent triangles and BE′ = A′I. It is noteworthy that BE′ represents the hydrogen utility savings when Cpur = Cutility. 2.2.3. Case 3: Cpur < Cutility. Similar to Figure 5, guides BD and GF are constructed as shown in Figure 6a. For Cpur < Cutility, MQ is constructed below AM by the ranking rule, which is parallel to BD. As in case 2, BE also cannot be used to replace the hydrogen utility directly. The source composite curve below MQ slides downward (direction of the arrow) along the pinch point P until B′E′ = M′I, making the final hydrogen system feasible, as shown in Figure 6b. Thus, B′N′HE′ is the final purification polygon in Figure 6b. MI represents the hydrogen utility savings, and CE′ represents the final waste discharge. Given that BB′ equals and parallels MM′ and B′E′ equals and parallels M′I, similar to Figure 5b, we can get BE′ = MI in Figure 6b. In the work of Zhang et al.,16 cases in which the purified product concentration is not adjacent to that of the utility are

3. MAXIMUM HYDROGEN UTILITY SAVINGS POTENTIAL Referring to section 2, when the tail gas concentration is specified, we can calculate the maximum utility savings potential offered by purification processes of a hydrogen system. This will be illustrated using the simplest case of Cpur = Cutility, as described below. A general hydrogen system with m sources and n sinks is shown in Figure 7. First, we construct guides BD and GF to represent the purified product and the tail gas, respectively, and assume that GF intersects the sink composite curve at C. The hydrogen sinks above C are SK1, SK2, ..., SKk and the sinks below C are SKk+1, SKk+2, ..., SKn (k ≤ n). We also assume that point B is (0, 0) in the X−Y coordinates. Accordingly, A is located at (0, yA) and C at (xC, yC), Because points A and C are located at the same Y coordinate, yA = yC. We can now derive eqs 4 and 5: m

yA = yC =

k

∑ (FC i i) − ∑ (FjCj) i=1

j=1

(4)

Figure 5. Identification of the purification polygon and utility savings (Cpur > Cutility). 3248



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Figure 6. Identification of the purification polygon and utility savings (Cpur < Cutility). yE = FpurCpur m

=

k

m

k

[∑i = 1 (FC i i) − ∑ j = 1 (FjCj)] − (∑i = 1 Fi − ∑ j = 1 Fj)C tail Cutility − C tail

Cutility

(9)

Finally, the flow rate of the final gas discharge (FD) can be calculated from eq 10: m

FD =

n

∑ Fi − ∑ Fj i=1

−

m

k

∑ Fi − ∑ Fj i=1

(5)

j=1

where Fi represents the flow rate of hydrogen source i, Fj is the flow rate of hydrogen sink j, Ci denotes the hydrogen concentration of hydrogen source i, and Cj represents the hydrogen concentration of hydrogen sink j. Straight lines BD and GF are defined by eqs 6 and 7, respectively. yBD = Cutilityx

k

m

k

− ∑ j = 1 (FC j j)] − (∑i = 1 Fi − ∑ j = 1 Fj)C tail Cutility − C tail

(10)

In the hydrogen load versus flow rate diagram, the value of xE represents the flow rate of the purified product and also gives the maximum hydrogen utility savings potential. From eqs 8 and 9, it may be seen that for a given hydrogen system the only variable is Ctail. In other words, if Ctail is given, the maximum hydrogen utility savings potential of the system can be calculated from eq 8. In some cases, if Fpur is larger than Futility, the utility production processes may be replaced by purification processes. It is noteworthy that the flow rate of the final gas discharge cannot be less than zero. If eq 10 indicates that FD ≤ 0, as illustrated in Figure 8 for another hydrogen network, the maximum hydrogen utility savings potential should be

Figure 7. Calculation of maximum hydrogen utility savings potential.

xC =

j=1

m [∑i = 1 (FC i i)

(6) m

k

yGF = C tailx − (∑ Fi − i=1

m

k

∑ Fj)Ctail + [∑ (FC i i) − ∑ (FC j j)] j=1

i=1

j=1

(7)

In Figure 7, BD intersects GF at E (xE, yE) and it follows that xE = Fpur and yE = FpurCpur. Combining eqs 6 and 7 gives xE and yE: x E = Fpur m

=

k

m

k

[∑i = 1 (FC i i) − ∑ j = 1 (FC j j)] − (∑i = 1 Fi − ∑ j = 1 Fj)C tail Cutility − C tail

Figure 8. Calculation of maximum hydrogen utility savings potential (FD = 0).

(8) 3249



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Figure 9. Identification of the purification polygon and utility savings with concentration constraint (feed is below the pinch).

Figure 10. Identification of the purification polygon and utility savings with concentration constraint (feed is below the pinch).

calculated from eq 11. In this case, the flow rate of the final gas discharge is zero. m

xE =

concentration is given, the maximum hydrogen utility savings can be easily identified as BE. By taking the feed concentration into consideration, we can assume that the feed is above point N. The identification of the purification polygon procedure is as follows. (1) Slide line BN upward until point B is located on GF and then slide along GF continuously until BN intersects the sink composite curve and the sink composite curve lies above BN. The final location of BN is shown in Figure 9b as B1N1. (2) Construct guide ND1 with Cutility as its slope and MQ as an extension of AM, as shown in Figure 9b,c. (3) Slide NM downward along the parallel lines ND1 and AQ until N1 is located on NM, as shown by N2M1 in Figure 9c. (4) Construct another tail gas line, G1F1, across N1. G1F1 intersects ND1 at E1. Thus, N1N2E1 is the final purification polygon. In Figure 9c, N1N2 represents the feed which lies below the original pinch point P. N2E1 and E1N1 represent the purified product and the tail gas, respectively. For NN2 = MM1, the hydrogen utility savings is given by NE1. Adding the condition under which BB1 equals and parallels NN1, it can be seen that triangles BB1E and NN1E1 are congruent triangles. This means that NE1 is equal to the maximum hydrogen utility savings BE and B1E = E1N1. Thus, AM1N1B1E is the source composite curve after introducing the purification process. Figure 10a shows another possible hydrogen system, and the feed of the purification process is also above N. After sliding

n

∑ Fi −

∑ Fj

i=1

j=1

(11)

4. GRAPHICAL METHOD FOR PURIFICATION REUSE WITH CONCENTRATION CONSTRAINTS. At present, commonly used purification processes include pressure swing adsorption (PSA), membrane technology, and cryogenic process. These purification processes rely on different separation principles and have different operating characteristics.22−24 The feed concentration of a purification process also has a prescribed minimum value. Otherwise, the purification process may become infeasible.14 Here, we describe an improved graphical method for targeting the maximum hydrogen utility savings which takes into account the feed concentration of the purification process. The proposed graphical method will be illustrated using the case of Cpur = Cutility analyzed in section 3. The location of the feed stream relative to the original pinch point gives three possible scenarios: below the pinch, across the pinch, and above the pinch. The other two cases of Cpur > Cutility and Cpur < Cutility will be briefly analyzed. 4.1. Scenario 1: Feed Stream below the Pinch. For the hydrogen system shown in Figure 9a, if the tail gas 3250



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Referring to Figures 9−12, the general procedure of the graphical method can be summarized as follows. (1) Determine the concentration of the tail gas of purification processes and the feed point (N). The feed point (N) divides the source composite curve into two parts: curve I below N and curve II above N (excluding the utility line). (2) Construct the tail gas line (GF) and identify its location. Construct the purified product guide (ND1) across the feed point N and extend the utility line (MQ). ND1 and MQ are parallel lines. (3) Move curve I upward, and let the bottom of curve I sit on the tail gas line (GF). Then slide curve I along GF continuously until it intersects curve II and lies below the sink composite curve. If the top of curve I is located outside of the area constructed by the parallel lines MQ and ND1, as shown in Figure 12b, slide curve I downward along the intersection of curve I and the sink composite curve until the top of curve I is located on the utility guide (AQ). In this way, the location of curve I is determined. (4) To determine the location of curve II, slide curve II downward along the parallel lines MQ and ND1 until the top of curve I is located on curve II. (5) Construct guides of tail gas through the intersection of curve I and curve II to determine the purification polygon and the final source composite curve. 4.4. Case of Cpur > Cutility. A typical case of Cpur > Cutility is shown in Figure 13. The purification polygon and the final source composite curve are determined according to the procedures described above. Assuming that the feed is above point N, curve I represents the source composite below N, and curve II represents the source composite above N. The procedure for analyzing this case is summarized as follows. (1) Construct lines BD and AQ with Cpur as their slope to represent the purified product and GF with Ctail as its slope to represent the tail gas, as shown in Figure 13a. (2) Referring to section 2.2.2, slide the source composite curve upward along the pinch point P until B1E1 = AI. Figure 13b shows the source and sink composite curves when the feed are the sources with the lowest hydrogen concentration. (3) Construct N1D2 with slope of Cpur as a guide line in Figure 13b. (4) Determine the location of curve I following step 3 of section 4.3. This is shown as B2N2 in Figure 13c.

BN as in step 1, the location of BN is shown in Figure 10b as B1N1. To make the final hydrogen system feasible, B1N1 should be slid along GF continuously until N1 is located on NM, as shown by B2N2 in Figure 10c. NN2E1 is the purification polygon, in which NN2 represents the feed and NE1 denotes the purified product. The utility savings equals NE1, and NE1 = BE. 4.2. Scenario 2: Feed Stream across the Pinch. A hydrogen system is illustrated in Figure 11a with the feed above

Figure 11. Identification of the purification polygon and utility savings with concentration constraint (feed is across the pinch).

point N. According to the identification procedure introduced in section 4.1, AM1N1CE is the source composite curve after introducing the purification process and N2N1E1 is the final purification polygon in Figure 11b. N2N1 represents the feed, which lies across the original pinch point P. This means that the feed can be seen as a mixture of the sources below the pinch and the sources above the pinch. N2E1 represents the purified product but NE1 denotes the hydrogen utility savings and equals BE. 4.3. Scenario 3: Feed Stream above the Pinch. Another hydrogen system is illustrated in Figure 12a, in which the feed is above point N. The source composite curve after introducing the purification process is shown in Figure 12c as curve AN 1 PB 2 E 2 . N 2 N 1 E 1 is the purification polygon. N 2 N 1 represents the feed, which lies above the original pinch point P. N1E1 denotes the purified product. The hydrogen utility savings is N1E1, which is equal to BE2 but is less than BE because the feed above P is not enough to obtain the maximum utility savings. N1E1 represents tail gas, and B2E2 = N1E1.

Figure 12. Identification of the purification polygon and utility savings with concentration constraint (feed is above the pinch). 3251



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Figure 13. Identification of the purification polygon with concentration constraint (Cpur > Cutility).

Figure 14. Identification of the purification polygon with concentration constraint (Cpur < Cutility).

(5) Slide curve II downward along the guide line N1D2 until the top of curve I (N2) sits on curve II, as indicated by curve N3M2 in Figure 14c. (6) Construct the tail gas guide (G1F1) through N2 which intersects N1D2 at E2. N3N2E2 represents the final purification polygon. This work mainly discusses hydrogen systems in which the utility concentration is the highest concentration. Nonetheless, the proposed graphical method in combination with those introduced by Zhao et al.10 and Zhang et al.16 is also applicable to hydrogen systems in which the utility concentration is not the highest concentration.

(5) Slide curve II downward along the guide N1D2 until the top of curve I (N2) sits on curve II, as shown by curve N3A2 in Figure 13c. (6) Construct the tail gas guide (G1F1) through N2 which intersects N1D2 at E2. N3N2E2 represents the final purification polygon. 4.5. Case of Cpur < Cutility. The determination of the purification polygon and the final source composite curve for a typical case of Cpur < Cutility are depicted in Figure 14. Note that there is a small difference between this case and the case of Cpur > Cutility; i.e., curve II for this case does not include the utility line. The procedure for analyzing this case is described as follows. (1) Similar to the case of Cpur > Cutility, construct BD and MQ to represent the purified product and GF to represent the tail gas, as shown in Figure 14a. (2) Referring to section 2.2.3, slide the source composite curve below point M downward along the pinch point P until B1E1 = M1I. Figure 14b shows the source and sink composite curves when the feed are the sources with the lowest hydrogen concentration. (3) Construct N1D2 with slope of Cpur as a guide line in Figure 14b. (4) Execute step 3 of section 4.3 to determine the location of curve I, which is indicated by B2N2 in Figure 14c.

5. CASE STUDIES The proposed graphical method is tested on two case studies. 5.1. Case Study 1. The main aim of this case study is to substantiate the conclusions of sections 2 and 3. Table 1 shows data for a hydrogen network that has been analyzed by Zhang et al.16 The pinch concentration is 0.70, and the minimum utility consumption is 200 mol/s. Table 2 shows the results considering purification reuse reported by Zhang et al.16 as well as the maximum hydrogen utility savings potential offered by purification reuse which has been calculated using the proposed method described in section 3. The data in rows 1 and 2 are the purification parameters. In scenario 1, Cpur = Cutility, in scenario 3252



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Table 1. Source and Sink Streams of a Hydrogen Network (Case Study 1)16 stream

flow rate (mol/s)

hydrogen concn (mol %)

Table 3. Source and Sink Streams of a Literature Hydrogen Network (Case Study 2)18

hydrogen load (mol/s)

hydrogen concn (mol %)

flow rate (mol/s)

hydrogen load (mol/s)

SR1(utility) SR2 SR3 SR4 SR5 SR6 SR7 SR8 SR9 SR10 SR11

95 86 80 73 64 56 46 35 33 28 23

400 (max) 90 80 109 121 118 127.5 94 76 133 198

77.4 64 79.57 77.44 66.08 58.65 32.9 25.08 37.24 45.54

SK1 SK2 SK3 SK4 SK5 SK6 SK7 SK8 SK9

90 78 71 60 51 40 30 25 20

131 143 133 115 124 136 127 118 122

stream

source

source SR1(utility) SR1 SR2 SR3 SR4 SR5

95 93 80 75 73 70

400 (max) 623.8 587.1 1940.5 346.5 457.4

580.13 469.68 1455.38 252.95 320.18

SK1 SK2 SK3 SK4 SK5

80.61 78.85 78.57 75.14 30

2495.0 180.2 554.4 720.7 50

2011.22 142.09 430.05 541.53 15.00

sink

sink

Table 2. Comparison of Results (Case Study 1) result for given scenario purified product concn (mol %) tail gas concn (mol %) utility savings16 (mol/s) utility savings (this work) (mol/s)

1

2

3

4

95 32 123.7 123.5

99 37 116 116.5

94 28 127.2 126.9

92 24 128.6 128.5

117.9 111.54 94.43 69 63.24 54.4 38.1 29.5 24.4

data in Table 3, the minimum utility consumption without purification reuse can be determined from the hydrogen load versus flow diagram, as shown in Figure 15. P is the pinch point, where the concentration is 64%. The minimum utility consumption is 67.7 mol/s, and the minimum waste discharge is 65.2 mol/s. When purification reuse is integrated into this hydrogen system, the minimum consumption of the utility will decrease. The purification parameter requirements must of course be

2, Cpur has the biggest value of 0.99 which is larger than Cutility, and in scenarios 3 and 4, Cpur < Cutility. Furthermore, these four scenarios have different Ctail values. The data in row 3 show the utility savings obtained from the graphical method of Zhang et al.,16 while those in row 4 show the maximum utility savings calculated from eq 8. For scenario 1 with a Ctail value of 0.32, we can get k = 4 from the data in Table 1. Inserting the data of hydrogen sources, sinks, and Ctail into eq 8, as shown in eq 12, gives the maximum hydrogen utility savings potential as 123.5 mol/s. The maximum hydrogen utility savings potential for the other scenarios can be found in the same manner. 5

∑ (FC i i) = 0.95 × 200 + 580.13 + 469.68 + 1455.38 + 252.95 i=1

+ 320.18 = 3268.32 4

∑ (FC j j)=2011.22 + 142.09 + 430.05 + 541.53 = 3124.89 j=1 5

∑ Fi = 200 + 623.8 + 587.1 + 1940.5 + 346.5 + 457.4 = 4155.3 i=1 4

∑ Fj = 2495.0 + 180.2 + 554.4 + 720.7 = 3950.3 j=1

Fpur =

(3268.32 − 3124.89) − (4155.3 − 3950.3) × 0.32 = 123.5 0.95 − 0.32

(12)

The close agreement between the data in rows 3 and 4 confirms that, for a specified Ctail value, the maximum hydrogen utility savings potential remains constant for different values of Cpur. Small differences between the two sets of results are due to resolution problems associated with graphical methods. 5.2. Case Study 2. This case study illustrates and validates the effectiveness of the proposed graphical method with a literature hydrogen network (see Table 3).18 The hydrogen network has 11 source streams and nine sink streams. Using the

Figure 15. Source and sink composite curves of the hydrogen network in case study 2. 3253



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satisfied. Taking the feed concentration as a variable, the purification reuse situation is analyzed under three different scenarios with different feed concentrations, as shown in Table 4. Table 4. Three Scenarios with Different Feed Concentrations (Case Study 2) scenario

feed concn (mol %)

purified product concn (mol %)

tail gas concn (mol %)

1 2 3

≥23 ≥35 ≥80

95 95 95

20 20 20

Figures 16−18 show the source and sink composite curves of the hydrogen system with purification reuse for the three

Figure 17. Composite curves of the hydrogen system with purification reuse (feed concentration ≥ 35%).

hydrogen source with the lowest hydrogen concentration. These results have been obtained by using the method of Zhang et al..16 Comparison of the results for scenarios 1 and 2 indicates that both scenarios achieve essentially the same maximum hydrogen utility savings. However, the tail gas flow rate of scenario 1 is larger than that of scenario 2, indicating that the impurity load for the purification process in scenario 1 is larger than that in scenario 2. In other words, scenario 2 may be more cost-effective than scenario 1. In scenario 3, due to a shortage of hydrogen sources that can meet the specified feed concentration, the hydrogen utility savings is much lower than those of scenarios 1 and 2. In the work of Liu et al.,18 the role of purification process is analyzed with specified values of purification feed, purified product, and hydrogen recovery. SR6 is taken as the purification feed and Cpur and Ctail are 0.73 and 0.36, respectively. Five conditions with different flow rates of feed are studied. When Fin = 10 mol/s, the hydrogen utility savings is the largest among the five conditions, amounting to 1.56 mol/s. When Fin > 99.56 mol/s, the hydrogen utility is negative. The method proposed here can achieve larger hydrogen utility savings than the method of Liu et al.18 for two reasons. First, the purification process in the proposed method is based on the whole hydrogen system to target the maximum hydrogen utility savings. The second reason is that the proposed method considers the possible utilization of the tail gas from the purification process. From these two case studies, it can be seen that a purification process can be placed across or above the original pinch point. With purification reuse, both the utility consumption and waste discharge are reduced. It is also observed that a lower tail gas concentration will lead to more hydrogen utility savings.

Figure 16. Composite curves of the hydrogen system with purification reuse (feed concentration ≥ 23%).

scenarios of Table 4, and the final purification polygons are identified using the procedures described in sections 4.1−4.3. For scenario 3, the identification process shown in Figure 18a reveals that the top of curve B1N1 is located outside of the area constructed by the parallel lines MQ and ND1. To construct the final purification polygon, slide B1N1 downward along the intersection of B1N1 and the sink composite curve until its top sits on MQ. During the sliding process, another intersection occurs at point P1. B2N2 intersects the sink composite curve at two points, B1 and P1, simultaneously, as shown in Figure 18a. Next, slide B2N2 downward along P1 until another intersection appears at point P2. In the same way, slide B2N2 downward along P2 continuously until it reaches its final location at B3N3, as shown in Figure 18b. B3N3 intersects the sink composite curve at point P2 while N3 sits on MQ. Next, slide NM downward along ND1 until N3 is located on it, as shown by N4M1. Finally, construct line G1F1 with slope of Ctail through N3, which intersects ND1 at E1. Thus, N4N3E1 represents the final purification polygon. Calculated results for the three scenarios are summarized in Table 5. Note that in scenario 1 the purification feed is the

6. CONCLUSION A previously published graphical method has been improved and simplified by taking the hydrogen pinch point as a criterion 3254



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the purification process. A lower tail gas concentration will result in more hydrogen utility savings. The improved graphical method has been tested on a number of cases with different purification reuse scenarios. The results indicate that a purification process can be designed across or above the original pinch point. With the maximum hydrogen utility savings as the optimization objective, the flow rate of the tail gas decreases with increasing feed concentration and then remains unchanged. This means that a feed with high concentration for a purification process may be more costeffective. For a purification reuse process, all variables in the mass balance equations can be treated as optimizable variables in this graphical method. By considering the constraints of concentration and flow rate, the proposed graphical method offers a high level of practical relevance for industrial applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 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: Grant 2012CB720500) and the National Natural Science Foundation of China under Grant No. 21276204 is gratefully acknowledged.

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Figure 18. Composite curves of the hydrogen system with purification reuse (feed concentration ≥ 80%): (a) the identification process; (b) the final results.

Table 5. Results for the Three Scenarios of Table 4 (Case Study 2) flow rate (mol/s) scenario

feed concn (mol %)

feed

tail gas

product

utility savings (mol/s)

1 2 3

≥23 ≥35 ≥80

273.7 286.9 158.6

256.7 158.6 24.9

17.0 128.3 133.7

17.0 16.9 7.9

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NOTATION C = hydrogen concentration of stream (mol %) Cin = hydrogen concentration of feed (mol %) Cj = hydrogen concentration of sink j (mol %) Cpur = hydrogen concentration of purified product (mol %) Ctail = hydrogen concentration of tail gas (mol %) Cutility = hydrogen concentration of utility (mol %) Ci = hydrogen concentration of source i (mol %) F = stream flow rate (mol/s) FD = final gas discharge flow rate (mol/s) Fin = feed flow rate of purification device (mol/s) Fpur = purified product flow rate (mol/s) Ftail = tail gas flow rate (mol/s) Futility = utility flow rate (mol/s) k = number of hydrogen sinks m = number of hydrogen sources n = number of hydrogen sinks R = recovery rate of hydrogen in purification process SRi = hydrogen source i SKj = hydrogen sink j REFERENCES

(1) Yuan, Z.; Chen, B. Process synthesis for addressing the sustainable energy systems and environmental issues. AIChE J. 2012, 58 (11), 3370−3389. (2) Fonseca, A.; Sa, V.; Bento, H.; Tavares, M. L.; Pinto, G.; Gomes, L. A. Hydrogen distribution network optimization: A refinery case study. J. Cleaner Prod. 2008, 16 (16), 1755−1763. (3) Wake, H. Oil refineries: A review of their ecological impacts on the aquatic environment. Estuarine, Coastal Shelf Sci. 2005, 62 (1), 131−140. (4) Bollinger, W. A.; Long, S. P.; Metzger, T. R. Optimizing hydrocracker hydrogen. Chem. Eng. Prog. 1984, 80 (5), 51−56.

and moving the composite curve with the constraint of the hydrogen pinch point. By analyzing the effects of incorporating purification reuse, an algebraic method for calculating the maximum hydrogen utility savings potential of a hydrogen system is developed. It indicates that, for a given hydrogen system, the maximum hydrogen utility savings varies with the tail gas concentration of 3255



Article

(5) Linnhoff, B.; Hindmarsh, E. The pinch design method for heat exchanger networks. Chem. Eng. Sci. 1983, 38 (5), 745−763. (6) Alves, J. J.; Towler, G. P. Analysis of refinery hydrogen distribution systems. Ind. Eng. Chem. Res. 2002, 41, 5759−5769. (7) El-Halwagi, M.; Gabriel, F.; Harell, D. Rigorous graphical targeting for resource conservation via material recycle/reuse networks. Ind. Eng. Chem. Res. 2003, 42, 4319−4328. (8) Almutlaq, A. M.; Kazantzi, V.; El-Halwagi, M. M. An algebraic approach to targeting waste discharge and impure fresh usage via material recycle/reuse networks. Clean Technol. Environ. Policy 2005, 7 (4), 294−305. (9) Agrawal, V.; Shenoy, U. V. Unified conceptual approach to targeting and design of water and hydrogen networks. AIChE J. 2006, 52 (3), 1071−1082. (10) Zhao, Z.; Liu, G.; Feng, X. New graphical method for the integration of hydrogen distribution systems. Ind. Eng. Chem. Res. 2006, 45, 6512−6517. (11) Foo, D. C. Y.; Kazantzi, V.; El-Halwagi, M. M.; Abdul Manan, Z. Surplus diagram and cascade analysis technique for targeting propertybased material reuse network. Chem. Eng. Sci. 2006, 61 (8), 2626− 2642. (12) Bandyopadhyay, S. Source composite curve for waste reduction. Chem. Eng. J. 2006, 125 (2), 99−110. (13) Zhou, L.; Liao, Z.; Wang, J.; Jiang, B.; Yang, Y.; Hui, D. Optimal design of sustainable hydrogen networks. Int. J. Hydrogen Energy 2013, 38 (7), 2937−2950. (14) Liu, F.; Zhang, N. Strategy of Purifier Selection and Integration in Hydrogen Networks. Chem. Eng. Res. Des. 2004, 82 (10), 1315− 1330. (15) Liao, Z.; Wang, J.; Yang, Y.; Rong, G. Integrating purifiers in refinery hydrogen networks: A retrofit case study. J. Cleaner Prod. 2010, 18 (3), 233−241. (16) Zhang, Q.; Feng, X.; Liu, G.; Chu, K. H. A novel graphical method for the integration of hydrogen distribution systems with purification reuse. Chem. Eng. Sci. 2011, 66 (4), 797−809. (17) Lou, J.; Liao, Z.; Jiang, B.; Wang, J.; Yang, Y. Pinch sliding approach for targeting hydrogen and water networks with different types of purifier. Ind. Eng. Chem. Res. 2013, 52, 8538−8549. (18) Liu, G.; Li, H.; Feng, X.; Deng, C.; Chu, K. H. A conceptual method for targeting the maximum purification feed flow rate of hydrogen network. Chem. Eng. Sci. 2013, 88, 33−47. (19) Liu, G.; Li, H.; Feng, X.; Deng, C. Novel method for targeting the optimal purification feed flow rate of hydrogen network with purification reuse/recycle. AIChE J. 2013, 59 (6), 1964−1980. (20) Liao, Z.; Rong, G.; Wang, J.; Yang, Y. Rigorous algorithmic targeting methods for hydrogen networksPart I: Systems with no hydrogen purification. Chem. Eng. Sci. 2011, 66 (5), 813−820. (21) Liao, Z.; Rong, G.; Wang, J.; Yang, Y. Rigorous algorithmic targeting methods for hydrogen networksPart II: Systems with one hydrogen purification unit. Chem. Eng. Sci. 2011, 66 (5), 821−833. (22) Ribeiro, A. M.; Grande, C. A.; Lopes, F. V.; Loureiro, J. M.; Rodrigues, A. E. A parametric study of layered bed PSA for hydrogen purification. Chem. Eng. Sci. 2008, 63 (21), 5258−5273. (23) Lu, G.; Diniz da Costa, J.; Duke, M.; Giessler, S.; Socolow, R.; Williams, R.; Kreutz, T. Inorganic membranes for hydrogen production and purification: A critical review and perspective. J. Colloid Interface Sci. 2007, 314 (2), 589−603. (24) Zhang, D.; Kodama, A.; Goto, M.; Hirose, T. Recovery of trace hydrogen by cryogenic adsorption. Sep. Purif. Technol. 2004, 35 (2), 105−112.

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Graphical Method for Integrating Purification Processes in Hydrogen

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