Minimization of Thermal Oil Flow Rate for Indirect Integration of

Article pubs.acs.org/IECR

Minimization of Thermal Oil Flow Rate for Indirect Integration of Multiple Plants Mukund H. Bade and Santanu Bandyopadhyay* Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India S Supporting Information *

ABSTRACT: Energy integration of multiple plants may be carried out indirectly through thermal oil. Flow rate of thermal oil is related to the heat to be transferred, and it affects the capacity and power requirement of the pumps and the piping size. In this paper, a linear programming formulation is proposed to minimize the flow rate of thermal oil. The proposed formulation considers the minimum total utilities requirement for indirectly integrated multiple plants as a constraint. For the special case of two plants integration, a graphical methodology, based on the principles of Pinch Analysis, is also developed. The proposed methodologies are illustrated with examples, and possible heat exchanger networks are developed.

1. INTRODUCTION Energy conservation is one of the important aspects for various industries (chemical, oil and gas, food processing, etc.) due to increasing cost of fuel and improving greener footprints. Maximum thermal energy conservation for individual plants can be targeted using techniques of Pinch Analysis, such as problem table algorithm (PTA)1 or modified problem table algorithm (MPTA).2 Additional energy conservation may be possible by integration of multiple plants. Energy integration of multiple plants may be achieved through direct or indirect integration. Indirect integration employs intermediate fluids such as steam or thermal oil for heat transfer, while in direct integration, hot streams from a plant directly transfer heat to cold streams of other plant via heat exchangers and higher energy savings can be achieved. However, direct integration involves complex heat exchanger network (HEN) with multiple streams crossing the individual battery limits, topological disadvantages, and chemical and other safety hazards, as well as less operational flexibility and controllability of the overall plant.3 On the other hand, use of intermediate fluids offer greater advantages of chemical and other safety, flexibility, and process control but with reduced energy conservation opportunities.3 Various methods (based on mathematical optimization as well as Pinch Analysis) are developed for targeting the minimum total utility requirements in direct and indirect integration of multiple plants, and practical applications are demonstrated with industrial case studies. Morton and Linnhoff4 proposed a graphical methodology by overlapping Grand Composite Curves (GCCs) of two plants to identify energy saving potential. Ahmad and Hui5 extended the work of Morton and Linnhoff4 for direct and indirect integration of multiple plants to find optimal HEN. An interactive software program was developed for Total Site Analysis and emission targeting.6 The integration of gasification technology to an overall refinery was accomplished by Sadhukhan and Zhu7 with a four stage optimization strategy. Varghese and Bandyopadhyay8 presented targeting the minimum number of fired heaters in direct integration of fired heater into the total site. Bandyopadhyay et al.3 developed a methodology to target © 2014 American Chemical Society

the minimum utility requirements of multiple plants for direct and indirect integration using modified Grand Composite Curve (MGCC). Zhang et al.9 proposed a MILP model to minimize the total hot and cold utilities required for integrated multiple plants. Kralj10 presented a simple graphical method to target total utility requirement for energy integration between multiple plants. Laukkanen et al.11 minimized total cost (capital and energy cost) using MINLP formulation for integrated multiple plants. Varbanov et al.12 developed a methodology for integration of multiple plants with various minimum approach temperatures. Varghese and Bandyopadhyay13 proposed a graphical method to target a minimum number of fired heaters to be integrated with a plant for direct and indirect integration. Sieniutycz and Jezowski14 applied Pinch Analysis to target minimum utility requirements for energy integration of fuel cell with multiple plants. Steam is the commonly used heat transfer fluid for indirect integration of multiple plants. Several methodologies have been proposed for Total Site Analysis using steam as intermediate fluid. Dhole and Linnhoff15 proposed a methodology of total site integration to set targets for steam generation and utilization for multiple plants by site source and site sink profiles. In total site integration methodology15 to generate site source and site sink profiles, cutting of pockets from GCC of an individual plant and appropriately shifting of remaining stream segments are proposed. Hui and Ahmed16 developed graphical methodology for cost optimization of total site integration. Kralj et al.17 proposed MINLP formulation to minimize utility requirements for retrofit problems of multiple plants integration through steam. Gorsek et al.18 presented modified site sink source profiles to determine energy saving potential by integration of existing plant with other plants. The graphical and mathematical techniques for targeting and network synthesis were presented by Coetzee and Majozi19 to reduce Received: Revised: Accepted: Published: 13146

May 20, 2014 July 5, 2014 July 26, 2014 July 27, 2014 dx.doi.org/10.1021/ie502059f | Ind. Eng. Chem. Res. 2014, 53, 13146−13156

Industrial & Engineering Chemistry Research

Article

steam flow rate without compromising the duty requirements of the process heat exchangers. Matsuda et al.20 demonstrated improvement in energy savings potential via integration of multiple plants using Total Site Analysis for a group of industries. Hackl et al.21 used Total Site Analysis for a cluster of chemical industries to determine probable energy efficiency. Chen and Lin22 presents a systematic methodology for the synthesis of an entire energy system typically of chemical plants, which explores the interactions between the steam network and the heat recovery networks of process plants. Chew et al.23 evaluated the impact of problems in practical implementation of total site integration and suggested guidelines for practical implementation. Zhang et al.24 proposed a mixed integer nonlinear program (MINLP) model to integrate process plants and utility systems; the objective is to minimize the energy costs to meet the requirements of the process operations and to maintain a steam balance in the total site. Liew et al.25 extended a numerical algorithm of Total Site Analysis for addressing the effects of plant layout such as pressure drop and heat loss to the minimum multiple utility targets. Although steam is generally used as heat transfer fluid in industry as steam systems are already in use, it is not preferred at high temperature applications. At high temperature applications, pressure required is high for steam systems, which makes the system bulky or impracticable. There are also other issues such as blow-down loss, makeup water treatment, corrosion, etc. Furthermore, steam being point utility (i.e., only the latent heat typically used for heat transfer), it requires multiple pressure levels to recover complete energy savings potential. On the other hand, thermal oil as heat transfer fluid is working in liquid phase at or near atmospheric pressure without the need of pressurized systems. Rodera and Bagajewicz26 proposed linear programming (LP) formulation to target energy savings for direct and indirect integration of multiple plants and showed that, in certain problems, cutting of pockets from GCC reduces energy savings opportunity. In addition to this, a MILP formulation is used for locating intermediate fluid circuits in two plants integration.26 In subsequent work, Rodera and Bagajewicz27 extended mathematical models proposed for two plants to multiple plants integration. HEN is designed for standalone as well as integrated operation of two plants to show operational flexibility of two plants.28 In a successive work of multiple plants integration, Bagajewicz and Rodera29 proposed a MINLP formulation to design a minimum number of oil circuits for interplant heat transfer satisfying minimum total utility requirement constraint. Bade and Bandyopadhyay30 developed graphical methodology for integration of multiple heat demands with fired heater via thermal oil. Dowling and Biegler31 proposed bilevel nonlinear programming (NLP) problems formulation for simultaneous heat integration and flow sheet optimization. Smith et al.32 discussed the role of reduced models (i.e., simplified and statistical models) in the optimization of energy integrated processes. Hipólito-Valencia et al.33 proposed a new superstructure for heat integration of an ecoindustrial park. In this work a proper reuse of the waste heat at low temperature is done by employing a set of organic Rankine cycles (ORCs) inside the ecoindustrial park. Interplant heat transfer is a product of mass flow rate, heat capacity rate, and temperature difference of thermal oil. The reduction in mass flow rate of thermal oil reduces capacity and power required for pumping and piping size. In this paper, a mathematical model is proposed to minimize the flow rate of thermal oil for energy integration of multiple plants. For the

special case of two plants, a graphical methodology, based on the principles of Pinch Analysis, is also proposed. It may be noted that the proposed methodology complements the Total Site Analysis to target the minimum flow rate of thermal oil for energy integration among multiple plants.

2. PROBLEM DEFINITION AND MATHEMATICAL FORMULATION Consider plants P1, P2, P3, P4, ..., as shown in Figure 1. Inlet, outlet temperatures, and heat capacity rate for each stream of a

Figure 1. Indirect energy integration of multiple plants using thermal oil.

plant is given. The minimum approach temperatures for intraplant and interplant energy integration are also provided. Figure 1 shows the energy integration of multiple plants transferring heat among them using thermal oil at minimum total hot and cold utilities requirements. The objective is to determine the minimum total mass flow rate of circulating thermal oil to transfer interplant heat at minimum total utility requirements. Average value of specific heat capacity over the temperature interval is assumed to be constant, and the product of specific heat capacity and mass flow rate of thermal oil is denoted by the heat capacity rate (CP). Therefore, the minimization of mass flow rate is equivalent to the minimization of the heat capacity rate of thermal oil. Energy saving capability of individual plants is determined using PTA1 or MPTA2 at a given minimum approach temperature. Additionally, application of PTA1 or MPTA2 results in Pinch temperatures of individual plants. If individual Pinch temperatures of all plants are the same, then integration of all plants cannot reduce the total utility requirements, and total utility requirements are the addition of utility requirements of individual plants.26 For plants with different Pinch temperatures, total utility requirements can be reduced by energy integration of multiple plants. In multiple plants integration, heat can be transferred from a hot stream of a plant to thermal oil and from thermal oil to a cold stream of another plant. Therefore, the equivalent minimum approach temperature (minimum temperature difference between hot stream of a plant and cold stream of another plant) required to transfer interplant heat is double of the minimum approach temperature of an individual plant (ΔTmin) as shown in Figure S1. In case, the plants having different 13147

dx.doi.org/10.1021/ie502059f | Ind. Eng. Chem. Res. 2014, 53, 13146−13156


Article

minimum approach temperatures, equivalent minimum approach temperature for indirect integration is the sum of the individual minimum approach temperatures of plants. To account for the equivalent minimum approach temperature for indirect integration, methodology proposed by Bandyopadhyay et al.3 is used in this paper. Following this methodology,3 hot segments of a GCC are shifted down by ΔTmin/2, and cold segments of GCC are shifted up by ΔTmin/2, where ΔTmin is the minimum approach temperature, with which the GCC is generated. The overlap regions of shifted segments after intersection points represent insufficient driving potential for interplant heat transfer, so they are eliminated. After elimination of such portions, remaining portions of the shifted segments of GCC is called modified Grand Composite Curve (MGCC). It may be noted that shifting of stream segments of a GCC results in incorporating the minimum approach temperature entirely on the stream side, hence actual temperatures of thermal oil (i.e., no additional shifting is required) can be used for targeting. Each segment of MGCC of a plant is assumed to be a pseudostream of that plant. Consider pseudostreams of a plant exchanging heat with thermal oil using a countercurrent heat exchanger at zero additional temperature driving force represented as a thermodynamically equivalent heat exchanger (see Figure S2). As discussed in the previous paragraph, temperature driving forces for all streams are incorporated in the stream side, and no additional temperature potential is required in the thermal oil side. Inlet to this thermodynamically equivalent heat exchanger of thermal oil may be considered as a demand, and similarly, the outlet from this thermodynamically equivalent heat exchanger may be considered as a source of thermal oil (see Figure S2). The concept of a thermodynamically equivalent heat exchanger is introduced to determine appropriate sources and demands of thermal oils and thereby to develop suitable targeting framework.2 It may be noted that these thermodynamically equivalent heat exchangers do not represent the final HEN of the overall plant. Let NSk and NDk represent the number of sources and demands in plant k. Let CPSik be the heat capacity rate of the ith source of the kth plant at temperature TSik, and CPDjl is the heat capacity rate of the jth demand of the lth plant at temperature TDjl . The heat capacity rate of the ith source of the kth plant transferred to the jth demand of the lth plant is represented as CPikjl. Figure 2 shows schematic representation of sources and demands for integrated multiple plants. Solid lines in Figure 2 represent direct heat transfer from a hot stream to cold streams of the same plant through heat exchangers and not via thermal oil. On the other hand, dotted lines in Figure 2 show interplant indirect heat transfer through thermal oil. Balances of the heat capacity rate for sources are written as

Figure 2. Schematic representation of sources and demands for indirect energy integration of multiple plants via thermal oil.

Energy balances for demands are written as NkS

∑ ∑ CPikjlTikS − Q jlCU k

+ Q jlHU = CP jlDT jlD ∀ j ∈ {1, 2, ...NlD} l ∈ {P1 , P2 , P3 , ...}

l

It is noted that all variable negative:

QCU jl ,

and CPikjl are non(5)

The objective is to minimize the interplant heat capacity rate and can be expressed as NkS

minimize R =

and

NlD

∑ ∑ ∑ ∑ CPikjl i=1

l

j=1

(6)

The objective is to minimize the total interplant heat capacity rate (R) of thermal oil (eq 6) subject to constraints given by eqs 1−5. All constraints and the objective function are linear, and hence, this is an LP problem.

(1)

3. INDIRECT ENERGY INTEGRATION OF TWO PLANTS - A SPECIAL CASE A special methodology based on Pinch Analysis is developed for two plants integration through physical insight. The segments of GCC of a plant with positive and negative slopes represent heat available and heat requirement, respectively.

NkS

∀ j ∈ {1, 2, ···NlD}

QHU jl ,

CPikjl , Q jlCU , Q jlHU ≥ 0

Similarly, heat capacity rate balances for demands are written as and

i=1

l ∈ {P1 , P2 , P3 , ...}

(4)

j=1

l≠k

k ∈ {P1 , P2 , P3 , ...}

k

∑ ∑ Q jlHU l

j=1

∑ ∑ CPikjl = CPjlD

(3)

NlD

Q HU =

k

∀ i ∈ {1, 2, ···NkS}

and

where QHU and QCU jl jl are hot and cold utilities requirements, respectively, to satisfy the jth demand of the lth plant. Minimum hot and cold utilities can be determined using Pinch Analysis3 or using a mathematical optimization technique.26 As the difference between total hot and cold utilities requirement always remains constant, it means both hot and cold utility requirements are dependent on each other. Therefore, only one constraint to match the minimum utility requirement for the overall plant is sufficient.

NlD

∑ ∑ CPikjl = CPikS

i=1

(2) 13148



Article

Simultaneous availability and requirements of heat can be represented for direct integration of two plants using GCC5 and indirect integration of two plants using MGCC.3 The minimum heat capacity rate is a linear function on temperature versus heat duty plot, and there are only two variables viz. the heat capacity rate transferred from plant P1 to plant P2 and vice versa from plant P2 to plant P1. Therefore, energy integration and determination of the minimum heat capacity rate of two plants is possible to show using a graphical approach based on Pinch Analysis. Reflected MGCC (RMGCC) is nothing but a mirror image of MGCC about the temperature axis as shown in Figure 3 for plant P1. To observe simultaneous availability and requirement of heat duty, both RMGCC of plant P1 and MGCC of plant P2 are drawn on the same temperature-heat duty diagram (see Figure 3). Horizontal distances at the top and at the bottom of

To determine the minimum heat capacity rate, temperature intervals and corresponding heat recovery regions should be determined first. Interplant heat transferred by thermal oil from one plant to another can be represented by a line in every heat recovery region. A feasible interplant heat transfer line must be contained within both these MGCCs without intersecting with any of these MGCC. Intersection of interplant heat transfer line with any of the MGCCs will lead to infeasibility (i.e., violation of the second law of thermodynamics). The absolute value of the inverse slope of any line on temperature versus heat duty plot represents the heat capacity rate of thermal oil. Therefore, a line with maximum slope gives the minimum heat capacity rate of thermal oil, provided it is within the feasible region. Figure 4 represents RMGCC of plant P1 and MGCC of plant

Figure 3. Graphical energy integration between two plants.

Figure 4. Targeting the minimum heat capacity rate of thermal oil from RMGCC of plant P1 and MGCC of plant P2.

these MGCCs represent the total hot and cold utilities requirements, respectively (see Figure 3). Any of these MGCCs may be translated toward the other to reduce total utilities requirements. In Figure 3, MGCC of plant P2 is translated toward RMGCC of plant P1. At the minimum utilities requirement, these two MGCCs can touch each other, without crossing. Crossing of MGCCs violates the second law of thermodynamics. The point where these two MGCCs touch each other represents the site Pinch (see Figure 3). The overlapping regions of RMGCC and MGCC represent heat recovery between two plants by interplant heat transfer. Lengths of these heat recovery regions along the x-axis represent interplant heat duty (Figure 3). The relative location of segments (above/below) specifies direction of interplant heat transfer as heat has to be transferred from higher temperature to lower temperature. It may be noted that separate oil circuits are required to transfer heat from plant P1 to P2 and from plant P2 to P1. For indirect integration of two plants P1 and P2 shown in Figure 3, there are three heat recovery regions. These heat recovery regions are specified by temperature intervals b1-ps (heat transfer from plant P2 to P1), a21-b4 (heat transfer from plant P1 to P2), and b51-a5 (heat transfer from plant P2 to P1) shown in Figure 3. Therefore, three circuits of thermal oil are required for the complete interplant heat transfer in energy integration of two plants P1 and P2.

P2 at the minimum total utilities requirement. Consider the first heat recovery region, specified by temperature intervals b1-ps, as shown in Figure 4. In this interval, interplant heat is being transferred from plant P2 to plant P1. The interplant heat transfer line L1 from point b1 (qb1, Tb1) to ps (0, Tc) is in a feasible region with maximum slope, thus it represents the minimum heat capacity rate of thermal oil. Line L1 transfers heat duty ‘qb1’ from plant P2 to plant P1 through thermal oil with the minimum heat capacity rate of CP1 (see Figure 4). Similarly, the interplant heat transfer line L5 from point b51 to a5 represents the minimum heat capacity rate of ‘CP5’ for the heat recovery region represented by the temperature interval b51-a5 (transferring interplant heat from plant P2 to plant P1). For the next heat recovery region (specified by temperature intervals a21-b4), a single line joining a21 (0, Tc1) and b4 (qb4, Tb4) crosses MGCC of plant P2 (see Figure 4) and hence, infeasible. Therefore, to make the line segment feasible, it has to be a piecewise linear curve that originates from a21 (0, Tc1) and terminates at b4 (qb4, Tb4) as well as contained within two MGCCs. One such feasible piecewise linear curve represented by interplant heat transfer line is shown in Figure 4. Interplant heat transfer line L34 (a21-b3-b4) is the union of two segments L3 (a21-b3) and L4 (b3-b4). These two segments, L3 and L4, represent interplant heat transfer from plant P1 to P2 with individual heat capacity rates of CP3 and CP4, respectively. As 13149



Article

Figure 5. Flowchart to determine the minimum heat capacity rate for entire oil circuit for integration of two plants.

Table 1. Limiting Streams Data for Plants P1 and P2 of Example 126 plant P1

plant P2 ΔTmin = 20 K

ΔTmin = 10 K

streams

CP (kW/K)

Ts (K)

Tt (K)

heat duty (kW)

streams

CP (kW/K)

Ts (K)

Tt (K)

heat duty (kW)

H11 H12 C13 C14

2 8 2.5 3

423 363 293 298

333 333 398 373

180 240 262.5 225

H21 H22 C23 C24

8.79 10.55 7.62 6.08

433 522 333 389

366 411 433 533

589 1171 762 876

allocation networks, following two theorems are proposed. The first theorem is to establish the feasible interplant heat transfer line, and the second theorem is selecting the minimum net heat capacity rate line out of feasible lines to determine the net minimum heat capacity rate. Proof of the following theorems, Theorems 1 and 2, can easily be derived based on the proof discussed in Sahu and Bandyopadhyay34 and not discussed here due to brevity. Theorem 1: Any piecewise linear curve, contained within two MGCCs without crossing, represents the feasible interplant heat transfer line. Theorem 2: Inverse of minimum slope of segment contained in any feasible interplant heat transfer line with the least length represents the minimum net heat capacity rate.

both these line segments L3 and L4 are transferring heat from plant P1 to P2 in continuous temperature intervals, the heat capacity rate CP3 of L3 is possible to be reused in subsequent interplant heat transfer (i.e., L4). Therefore, the net heat capacity rate for a piecewise linear interplant heat transfer line L34 is CP3, which is the maximum heat capacity rate between CP3 and CP4. In general, the net heat capacity rate for a piecewise linear interplant heat transfer line is the maximum heat capacity rate of a segment among all segments. It should be noted that any feasible piecewise linear curve need not represent the interplant heat transfer line with the minimum net heat capacity rate. Adopting the results reported by Sahu and Bandyopadhyay34 for the minimum interplant resource interaction between two integrated resource 13150



Article

4. ILLUSTRATIVE EXAMPLES The applicability of the proposed mathematical model and graphical methodology to determine the minimum heat capacity rate to transfer interplant heat is demonstrated through illustrative examples. 4.1. Illustrative Example 1: Two Plants Integration. Limiting operational data of streams for two plants example26 are tabulated in Table 1. Note that ΔTmin for plant P135 is 20 K, while that for plant P236 is 10 K for intraplant and interplant integration. Using MPTA,2 hot and cold utilities are determined for plant P1 as 107.5 and 40 kW, respectively, with Pinch at 353 K (shifted temperature) and similarly for plant P2 as 127.68 and 250.14 kW, respectively, with Pinch point at 517 K (shifted temperature). MGCCs for both plants P1 and P2 are generated from GCC of each plant, and MGCC of plant P1 is reflected. Equivalent minimum approach temperature for indirect energy integration is 30 K. RMGCC of plant P1 and translated MGCC of plant P2 shown in Figure 6 are matched at the minimum total utility requirements. There is only one heat recovery region transferring heat from plant P2 to plant P1, and, hence, it requires only one oil circuit. In the heat recovery region of MGCC and RMGCC, a feasible interplant heat transfer line with a maximum slope, po-a1, is drawn (see Figure 6). The absolute value of the inverse of the slope of line po-a1 is the minimum net heat capacity rate of 0.72 kW/K. The minimum total heat capacity rate is the same as the minimum net heat capacity rate, due to a single oil circuit. Therefore, the overall minimum heat capacity rate for the entire oil circuit is 1.44 kW/K (double of the total minimum heat capacity rate). This matches the overall minimum heat capacity rate for the entire oil circuit (1.44 kW/K) obtained through mathematical optimization and the result reported by Rodera and Bagajewicz.26 It may be interesting to note that the primary objective of Rodera and

The total minimum heat capacity rate is the summation of the minimum net heat capacity rates of each heat recovery region. The overall minimum heat capacity rate of the entire oil circuits for the thermally integrated two plants is equal to twice the total minimum heat capacity rate. As there are supply as well as return of thermal oil in each circuit (i.e., recirculation of thermal oil in loops), a multiplication factor of 2 should be used. It may be noted that the total minimum heat capacity rate of thermal oil depends only upon critical segments having the maximum heat capacity rate of thermal oil in each oil circuit. Therefore, it is possible to generate multiple interplant heat transfer lines for the same minimum total heat capacity rate of thermal oil by keeping the slopes of these critical segments constant and varying heat capacity rates of other segments. This gives multiple oil circuits for the same overall minimum heat capacity rate of thermal oil as an additional flexibility to the designer. It may further be noted that Theorem 2 guarantees existence of at least one such interplant heat transfer line. The methodology discussed in the previous paragraph is shown as a flowchart in Figure 5.

Figure 6. Integration of two plants using graphical methodology for example 1.

Figure 7. HEN with oil circuits for example 1. 13151



Article

Table 2. Summary of Utilities Requirements for Individual Plants plants

hot utility (MW)

cold utility (MW)

shifted Pinch temp (K)

plant P1 plant P2 plant P3

0 1.5 25

35 19.35 36.5

threshold 768 468−473

Table 3. Sources and Demands of Each Plant for Multiple Plants Integration plants

S.N.

source temp (K)

heat capacity rates (MW/K)

demand temp (K)

heat capacity rates (MW/K)

plant P1

1 2 3 4 5 6 7 8 9 10 11 12 13

513 398 763 412 773 303 463 388 382 293 478 398 463

0.5 0.5 0.5 3.5 5.5 2.5 0.2 2.45 2.7 0.2 2.5 0.05 0.2

458 383 523 387 783 387 435 383 289 291.7 488 435 435

0.5 0.5 0.1 0.15 0.15 0.1 0.2 2.45 2.7 0.2 2.5 0.05 0.2

plant P2

plant P3

Bagajewicz26 is not to minimize the heat capacity rate of thermal oil for indirect integration. One of the feasible HENs with the entire oil circuit, satisfying the overall minimum heat capacity rate, is shown in Figure 7 at minimum total utility requirements. Dotted lines show thermal

Figure 8. (a) Energy integration of two plants using graphical methodology for example 2. (b) Enlarged view of integrated portion of two plants.

Figure 9. HEN with oil circuits for example 2. 13152



Article

Figure 10. HEN for three plants integration for example 3.

single oil circuit is easy to operate and control through a central utility system, and requirements of pump and other accessories are less than two oil circuits. One of the feasible HENs with a single oil circuit is shown in Figure 9. Dotted lines show thermal oil streams used to transfer interplant heat from one plant to another (Figure 9). 4.3. Illustrative Example 3: Three Plants Integration. The limiting operational data of hot and cold streams of plants P1, P2, and P3 are given in Table S2.16 For individual plants, hot and cold utilities requirements with shifted Pinch temperatures determined using MPTA2 are given in Table 2. The Pinch point temperatures are different so that energy integration of three plants results in reduction of utility requirements for combined plants. MGCC is generated for each plant by appropriately shifting each segment of GCC to consider the driving potential required for interplant integration. The equivalent minimum approach temperature for indirect energy integration is 20 K. For each pseudostream of each plant, data in the form of sources and demands are extracted and shown in Table 3. Total hot and cold utilities requirement for indirect integration of plants P1, P2, and P3 are determined using MPTA2 as 1.5 and 65.85 MW, respectively, at site Pinch temperature (shifted) of 773 K for the equivalent minimum approach temperature of 20 K. There is 94.3% and 28% reduction in hot and cold utilities, respectively, due to the indirect integration compared to the sum of the minimum utility target of individual plants. Using LP formulation, the minimum heat capacity rate

oil streams used to transfer interplant heat from plant P2 to plant P1 (Figure 7). 4.2. Illustrative Example 2: Two Plants Integration. This example26 consists of a crude distillation unit (plant P1) and a fluid catalytic cracking plant (plant P2). Stream data and the minimum approach temperature, ΔTmin, are given in Table S1. Using MPTA,2 hot and cold utilities are determined for plant P1 as 69.04 and 9.98 MW, respectively, with Pinch at 416.5 K (shifted temperature) and similarly for plant P2 as 5.08 and 33.31 MW, respectively, with Pinch at 618.4 to 746.9 K (shifted temperature). RMGCC of plant P1 and MGCC of plant P2 shown in Figure 8 are matched at the minimum total utilities requirements. The equivalent minimum approach temperature for indirect energy integration is 11.2 K. In the heat recovery region of MGCC and RMGCC, a piecewise linear curve with the least length represented by a feasible interplant heat transfer line a-b-c-ps-d is drawn as shown in Figure 8. The minimum net heat capacity rate is 0.192 MW/K. Therefore, the overall minimum heat capacity rate for an entire oil circuit is equal to 0.383 MW/K. The same solution is also obtained through mathematical optimization. It is interesting to note that Rodera and Bagajewicz28 reported the overall minimum heat capacity rate 0.514 MW/K, 34.1% higher than the solution obtained by proposed methodology, with two oil circuits to transfer the same interplant heat duty. In addition to this, proposed methodology requires a single oil circuit instead of two as proposed by Rodera and Bagajewicz28 for maximum heat recovery. A 13153



Article

Table 4. Summary of Individual Plants Utilities Requirement plants 1 fluidized catalytic cracking unit (FCCU) 2 crude/vacuum distillation unit (CDU/VDU) 3 visbreaker (thermal cracking) (VBU) 4 platformer (reformer) (PLAT) 5 naphtha hydrotreater (NHT) 6 diesel hydrotreater (DHT) 7 kerosene hydrotreater (KHT)

HU (MW)

CU (MW)

ΔTmin (K)

shifted Pinch temp (K)

0

20.46

6.7

threshold

54.94

33.03

11.1

539.45

6.37

10.62

8.3

604.95

18.07

8.37

5.6

349.6

7.67 0 0.73

6.33 2.76 4.19

8.3 5.6 5.6

437.75 threshold 446.9

by transferring it through e-f with plants P2 and P1. The overall minimum heat capacity rate of circulating thermal oil is increased to 0.234 MW/K, which may slightly increase pumping power and piping size. In addition to this, there is reduction of one splitter (used to split oil flow) and one heat exchanger. A modified thermal oil network is shown in Figure 11b. Further simplification of HEN is possible by elimination of sub loop c-h, by transferring it through e-f as shown in Figure 11c. This forms a single circuit of thermal oil without a sub loop by a further increase in the overall minimum heat capacity rate to 0.351 MW/K, and there is no reduction in the heat exchanger. 4.4. Illustrative Example 4: Seven Plants Integration. Applicability of the proposed methodology is demonstrated with an example of seven plants in a refinery38 (for brevity plants data are not reproduced). Summary of hot and cold utilities requirement, minimum approach temperature (ΔTmin), and shifted Pinch temperature of individual plants are given in Table 4. For combined plants, total hot and cold utilities required are determined using MPTA2 as 66279 kW and 64242 kW, respectively, which is 24.5% and 25.1% lower compared to total hot and cold utility requirements of an individual plant. Using LP formulation, the minimum heat capacity rate of thermal oil at minimum total utilities requirement is determined to be 702.7 kW/K using GAMS version 24.2.237 with MINOS solver. The processor used is Intel (R) core 2, 1.88 GHz and 2 GB RAM. The formulation consists of 641 equations and 17,150 variables.

Figure 11. Simplified oil networks for example 3: (a) equivalent oil network as shown in Figure 10, (b) evolved oil network after removal of one sub loop, and (c) evolved oil network after removal of both sub loops.

5. CONCLUSIONS Methodologies are proposed to minimize the heat capacity rate of thermal oil for indirect energy integration of multiple plants. The heat capacity rate of thermal oil is directly related to pumping power and piping size. To determine the minimum heat capacity rate of thermal oil, LP based formulation is developed. As the formulation is LP, global optimality can be guaranteed. Proposed methodology is applied to a realistic problem involving energy integration of seven plants. For a special case of integration between two plants, a graphical methodology based on Pinch Analysis is developed to determine the minimum heat capacity rate of thermal oil. For example 1, the overall minimum heat capacity rate for an entire oil circuit is 1.44 kW/K, which is determined by graphical methodology as well as mathematical optimization. This is identical to the solutions reported by Rodera and Bagajewicz26 through MILP formulation. In example 2, the overall minimum heat capacity rate for an entire oil circuit is

of thermal oil at the minimum total utilities requirement is determined using GAMS version 24.2.237 with MINOS solver. The processor used is Intel (R) core 2, 1.88 GHz, and 2 GB RAM. The overall minimum heat capacity rate of an entire oil circuit determined is 0.231 MW/K. One of the feasible HENs with oil circuits is shown in Figure 10. Figure 11a shows a simplified oil network with interacting plants, heat duty, heat capacity rate, and temperatures of thermal oil. Each plant interacts with streams of thermal oil through heat exchangers, and thermal oil is recirculated using piping networks. A network of thermal oil may be modified either by eliminating sub loops of pipes and/or heat exchangers. The heat capacity rate of thermal oil through sub loop d-g is small (0.002 MW/K), which may be eliminated. An elimination of a sub loop of oil represented by d-g (Figure 11a) is possible 13154



Article

Subscripts

determined to be 0.38 MW/K with a single oil circuit. This is significantly better than the overall minimum heat capacity rate of 0.514 MW/K with two oil circuits, as reported by Rodera and Bagajewicz28 using MILP formulation. Simplification of an oil network, as illustrated through example 3, provides additional flexibility to the plant designer. For a complete HEN with an oil network, a rigorous economic model may be developed for technoeconomic evaluation of optimal HEN. Energy economic analysis with detailed cost benefits for complete integrated multiple plants is to be carried out for the final design of optimal HEN. In addition to these, issues such as restricted matches and flexibility in interplant heat transfer may also be addressed. Future research is directed toward these issues.

■

■

REFERENCES

(1) Linnhoff, B.; Flower, J. R. Synthesis of Heat Exchanger Networks. I. Systematic Generation of Energy Optimal Networks. AIChE J. 1978, 24, 633. (2) Bandyopadhyay, S.; Sahu, G. C. Modified Problem Table Algorithm for Energy Targeting. Ind. Eng. Chem. Res. 2010, 49, 11557. (3) Bandyopadhyay, S.; Varghese, J.; Bansal, V. Targeting for Cogeneration Potential through Total Site Integration. Appl. Therm. Eng. 2010, 30, 6. (4) Morton, R.; Linnhoff, B. Individual Process Improvements in the Context of Site-Wide Interactions. Inst. Chem. Eng., Annu. Res. Meet. Bath, UK 1984. (5) Ahmad, S.; Hui, D. C. W. Heat Recovery between Areas of Integrity. Comput. Chem. Eng. 1991, 15, 809. (6) Klemes, J.; Dhole, V. R.; Raissi, K.; Perry, S. J. Targeting and Design Methodology for Reduction of Fuel, Power and CO2, on Total Site. Appl. Therm. Eng. 1997, 17, 993. (7) Sadhukhan, J.; Zhu, X. X. Integration Strategy of Gasification Technology: A Gateway to Future Refining. Ind. Eng. Chem. Res. 2002, 41, 1528. (8) Varghese, J.; Bandyopadhyay, S. Targeting for Energy Integration of Multiple Fired Heaters. Ind. Eng. Chem. Res. 2007, 46, 5631. (9) Zhang, B. J.; Luo, X. L.; Chen, Q. L.; Hui, C.-W. Heat Integration by Multiple Hot Discharges/Feeds between Plants. Ind. Eng. Chem. Res. 2011, 50, 10744. (10) Kovac Kralj, A. A Simple Graphical Utilities Targeting Method for Heat Integration between Processes. Ind. Eng. Chem. Res. 2012, 51, 14171. (11) Laukkanen, T.; Tveit, T.-M.; Fogelholm, C.-J. Simultaneous Heat Exchanger Network Synthesis for Direct and Indirect Heat Transfer inside and between Processes. Chem. Eng. Res. Des. 2012, 90, 1129. (12) Varbanov, P. S.; Fodor, Z.; Klemes, J. J. Total Site Targeting with Process Specific Minimum Temperature Difference (ΔTmin). Energy 2012, 44, 20. (13) Varghese, J.; Bandyopadhyay, S. Improved Areaenergy Targeting for Fired Heater Integrated Heat Exchanger Networks. Chem. Eng. Res. Des. 2012, 90, 213. (14) Sieniutycz, S.; Jeżowski, J. Maximum Heat Recovery and Its Consequences for Process System Design; Elsevier: Amsterdam, 2013; pp 475−497. (15) Dhole, V. R.; Linnhoff, B. Total Site Targets for Fuel, CoGeneration, Emissions, and Cooling. Comput. Chem. Eng. 1993, 17, S101. (16) Hui, C. W.; Ahmad, S. Minimum Cost Heat Recovery between Separate Plant Regions. Comput. Chem. Eng. 1994, 18, 711. (17) Kralj, A. K.; Glavic, P.; Krajnc, M. Waste Heat Integration between Processes. Appl. Therm. Eng. 2002, 22, 1259. (18) Gorsek, A.; Glavic, P.; Bogataj, M. Design of the Optimal Total Site Heat Recovery System Using SSSP Approach. Chem. Eng. Process. Process Intensif. 2006, 45, 372. (19) Coetzee, S. W. A.; Majozi, T. Steam System Network Synthesis Using Process Integration. Ind. Eng. Chem. Res. 2008, 47, 4405. (20) Matsuda, K.; Hirochi, Y.; Tatsumi, H.; Shire, T. Applying Heat Integration Total Site Based Pinch Technology to a Large Industrial Area in Japan to Further Improve Performance of Highly Efficient Process Plants. Energy 2009, 34, 1687. (21) Hackl, R.; Andersson, E.; Harvey, S. Targeting for Energy Efficiency and Improved Energy Collaboration between Different Companies Using Total Site Analysis (TSA). Energy 2011, 36, 4609. (22) Chen, C.-L.; Lin, C.-Y. Design of Entire Energy System for Chemical Plants. Ind. Eng. Chem. Res. 2012, 51, 9980.

ASSOCIATED CONTENT

S Supporting Information *

Tables S1 and S2 and Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

■

i, j, k, l = index variables P = plant

AUTHOR INFORMATION

Corresponding Author

*Phone: +91-22-25767894. Fax: +91-22-25726875. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

NOMENCLATURE CPDjl = heat capacity rate required by the jth demand of the lth plant (kW/K or MW/K) CPikjl = heat capacity rate transferred from the ith source of the kth plant to the jth demand of the lth plant (kW/K or MW/K) CPSik = heat capacity rate produced by the ith source of the kth plant (kW/K or MW/K) NDk = number of demands in the kth plant NSk = number of sources in the kth plant Q = heat duty (kW or MW) TDjl = temperature of the jth demand of the lth plant (K) TSik = temperature of the ith source of the kth plant (K) ΔTmin = minimum approach temperature (K)

Abbreviations

GCC = Grand Composite Curve HEN = heat exchanger network LP = linear programming MGCC = modified Grand Composite Curve MILP = mixed integer linear program MINLP = mixed integer non linear program MPTA = modified problem table algorithm PTA = problem table algorithm RMGCC = reflected modified Grand Composite Curve FCCU = fluidized catalytic cracking unit CDU/VDU = crude/vacuum distillation unit VBU = visbreaker (thermal cracking) PLAT = platformer (reformer) NHT = naphtha hydrotreater DHT = diesel hydrotreater KHT = kerosene hydrotreater P = Plants Superscripts

CU = cold utility HU = hot utility 13155



Article

(23) Chew, K. H.; Klemes, J. J.; Wan Alwi, S. R.; Abdul Manan, Z. Industrial Implementation Issues of Total Site Heat Integration. Appl. Therm. Eng. 2013, 61, 17. (24) Zhang, B. J.; Luo, X. L.; Chen, X. Z.; Chen, Q. L. Coupling Process Plants and Utility Systems for Site Scale Steam Integration. Ind. Eng. Chem. Res. 2013, 52, 14627. (25) Liew, P. Y.; Lim, J. S.; Wan Alwi, S. R.; Abdul Manan, Z.; Varbanov, P. S.; Klemeš, J. J. A Retrofit Framework for Total Site Heat Recovery Systems. Appl. Energy 2014, DOI: 10.1016/j.apenergy.2014.03.090. (26) Rodera, H.; Bagajewicz, M. J. Targeting Procedures for Energy Savings by Heat Integration across Plants. AIChE J. 1999, 45, 1721. (27) Bagajewicz, M.; Rodera, H. Energy Savings in the Total Site Heat Integration across Many Plants. Comput. Chem. Eng. 2000, 24, 1237. (28) Rodera, H.; Bagajewicz, M. J. Multipurpose Heat-Exchanger Networks for Heat Integration Across Plants. Ind. Eng. Chem. Res. 2001, 40, 5585. (29) Bagajewicz, M.; Rodera, H. Multiple Plant Heat Integration in a Total Site. AIChE J. 2002, 48, 2255. (30) Bade, M. H.; Bandyopadhyay, S. Thermal Integration of Heat Transfer Fluid Systems. Asia-Pac. J. Chem. Eng. 2014, 9, 1. (31) Dowling, A. W.; Biegler, L. T. Optimization-Based Process Synthesis for Sustainable Power Generation. Chem. Eng. Trans. 2013, 35, 1. (32) Smith, R.; Ochoa-Estopier, L. M.; Jobson, M. The Use of Reduced Models in the Optimisation of Energy Integrated Processes. Chem. Eng. Trans. 2013, 35, 139. (33) Hipólito-Valencia, B. J.; Rubio-Castro, E.; Ponce-Ortega, J. M.; Serna-González, M.; Nápoles-Rivera, F.; El-Halwagi, M. M. Optimal Design of Inter-Plant Waste Energy Integration. Appl. Therm. Eng. 2014, 62, 633. (34) Sahu, G. C.; Bandyopadhyay, S. Mathematically Rigorous Algebraic and Graphical Techniques for Targeting Minimum Resource Requirement and Interplant Flow Rate for Total Site Involving Two Plants. Ind. Eng. Chem. Res. 2012, 51, 3401. (35) Linnhoff, B.; Hindmarsh, E. The Pinch Design Method for Heat Exchanger Networks. Chem. Eng. Sci. 1983, 38, 745. (36) Papoulias, S. A.; Grossmann, I. E. A Structural Optimization Approach in Process synthesisII: Heat Recovery Networks. Comput. Chem. Eng. 1983, 7, 707. (37) Rosenthal, R. E. GAMSA User’s Guide 2014; GAMS Development Corporation: Washington, DC, USA, 2014. (38) Fraser, D. M.; Gillespie, N. E. Application of Pinch Technology to Retrofit Energy Integration of an Entire Oil Refinery. Chem. Eng. Res. Des. 1992, 70, 395.

13156


Minimization of Thermal Oil Flow Rate for Indirect Integration of

Recommend Documents