(soko Banja 2007) Pinch Analysis Of Yeast Fermentation Plant- Tmf Skopje - Anastasovski, Meshko, Markovska, Raskovic

SOKO BANJA 2007

PINCH ANALIZA POSTROJENJU ZA FERMENTACIJE KVASCEM PINCH ANALYSIS OF YEAST FERMENTATION PLANT A. Anastasovski1∗, L. Markovska2, V. Meško2, P.Rašković3 1 2

Factory of Yeast and Alcohol, MK-7000 Bitola, Republic of Macedonia

Faculty of Technology and Metallurgy, Ss Cyril and Methodius University, P. O. Box 580, MK-1001 Skopje, Republic of Macedonia 3

Faculty of Technology, University of Nis, Serbia,

Abstract: Rapid increasing of energy prices all over the world and concern for the environmental impacts are the reasons which stimulate developement of new methods for energy conservation measures in chemical industry. This paper presents a research on the case study of plant for ethanol and yeast production. For the purpose of research, physical model of the plant is recognized as steady state and divided in energy subsystems, which are separately optimized by the use of heuristic rules. Data extraction phase is limited only on the streams which connect the subsystems. The use of Pinch analysis enabled the synthesis of new heat exchanger network, which improve the energy and economic parameters of the plant. Key words: process integration, heat exchanger network synthesis, pinch technology, chemical industry, yeast fermentation plant. 1. INTRODUCTION In response to the staggering environmental and energy problems associated with manufacturing facilities, the process industry has recently dedicated much attention and resources to mitigating the detrimental impact on the environment, conserving resources, and reducing the intensity of energy usage. In the past decades has seen significant industrial and academic efforts [1] devoted to the development of holistic process design methodologies that target energy conservation and waste reduction from a systems perspective. Process integration is a holistic approach to process design and operation that emphasizes the unity of the process. Process integration can be described as “a holistic approach to process design, retrofitting, and operation which emphasizes the unity of the process”, differently to a design approach that optimizes at the unit operation level. Process integration enables the designer to see “the big picture first, and the details later”. Based on this approach, it is not only possible to identify the optimal process development strategy for a given task, but also to uniquely identify the most cost-effective way to accomplish that task.

∗

Coresponding author: A. Anastasovski, E-mail: *[email protected]

Process integration can be broadly categorized into mass [2] and energy(heat) integration [3, 4]. The energy integration deals with the global allocation, generation, and exchange of energy throughout the process [3, 5]. On the other side, mass integration is a part of process integration which has aim to minimize outgoing material compounds through material streams via change of its concentrations. This kind of integration uses mass transfer phenomena for compound concentration changes with mass exchangers using processes such as absorption, adsorption, extraction, ion exchange, leaching and striping. The implementation of PI methods can lead to significant energy savings and waste reduction (primary wastewater minimization). Some of research centers [6] reported that “PI is probably the best approach that can be used to obtain significant energy and water savings as well as pollution reductions for different kind of industries”. 2. PINCH ANALYSIS One of the most extensively studied and single most important industrial application area for process integration is Heat Exchanger Network Synthesis (HENS). Principal aspect of HENS can be found in the fact that most industrial processes involve transfer of heat, either from one process stream to another process stream (interchanging) or from a utility stream to a process stream. Consequently, target in any industrial process design is to maximize the process-toprocess heat recovery and to minimize the utility requirements. To meet this goal, industrial cost-effective HEN (consisting of one or more heat exchangers that collectively satisfy the energy conservation task), is of particular importance. Pinch technology presents a simple HENS method based on the I Law and some constrains originate from II Law of Thermodynamic. Pinch Technology is originally developed as a tool for the design of energy-efficient heat-exchange networks during the late 1970s and early 1980s, in response to the sharp increase in the price of energy. Since then, Pinch technology based techniques have found application in a wide range of fields, including distillation column profiling, low-temperature process design, batch process integration, emissions targeting and water and wastewater minimization. As an introduction to the concepts of Pinch technology, it is instructive to consider a simple heat exchanger presented in Fig. 1a. Temperature-enthalpy (T-H) diagrams shown in the Fig. 1b and Fig. 1c, are used to present the change of thermodynamic parameters of hot and cold streams, which passes through exchanger. For heat-exchange between two fluids, thermodynamic equilibrium is established when a zero temperature difference (temperature driving-force) exists between the fluids exchanging energy. This situation (Fig. 1b) corresponds to the maximum possible level of heat-recovery (which minimizes the amount of external utilities) but requires a heatexchanger of infinite heat-exchange area, and therefore capital cost. To make the design feasible and to have the heat exchanger of a finite size, there has to be some positive temperature difference between the hot and the cold stream at each point along the exchanger. Graphically, in T-H diagram, the ΔTmin > 0 can be obtained by horizontal moving the cold curve in H positive direction, until the temperature difference at the cold end of the exchanger reaches some minimum desired value. By this operation, the size of the overlapping zone is reduced and the amount of utility duties is increased. So, this is the price to be paid for having a feasible process.

Figure 1. Thermodynamic analyze of single heat exchanger

In the case of HENS task (energy system involve more than a two streams) the essential task is to represent the all individual heat sources (hot streams) and all heat sinks (cold streams) on a single diagram, and on that way determine the minimum utility duties for the entire system. Such a diagram, called Pinch diagram, state as the most important tool in Pinch technology, which provides the framework for creation a feasible heat exchange network. The initial step, in constructing the pinch diagram∗, is the creation of Composite Curves (CC), which represents all available (Hot Composite Curve, HCC) or required (Cold Composite Curve, CCC) heat inside the energy system. The construction of CC (either HCC or CCC not both) is presented on Fig. 2. and it consists of four steps: 1) Plotting of all process streams, on a single T-H diagram, with notation that T-axis is absolute and the H-axis is relative. In the case of HCC the streams should be plotted in T positive direction, since in the case of CCC the direction is opposite. Streams are shifted (along H-axis) in the position which does not enable the overlapping of its enthalpy changes. ∗ In this paper we present only graphical tools for Pinch diagram. In the case of making software this operation can be performed by the use of Tabular Approach and Temperature-Interval Diagram (TID)

Figure 2. Construction of composite curve

2) Definition the temperature interval boundaries (denoted as Tk , k = 1,2 ,..N ), by drawing horizontal lines at the supply and target temperatures for each stream. By this step problem get the sequential form. 3) Drawing a new line across the every interval, by applying the linear superposition of enthalpy changes. New lines are connecting and formed one mutual line. 4) Eliminating the original process stream lines from the diagram and leaving only the new mutual - CC line. Final Pinch diagram is made by plotting the CC curves together, on the same T-H axes (Fig. 3). To represent feasible heat exchange from hot to cold streams, the hot composite curve should lie completely above the cold composite curve.

Figure 3. Pinch diagram and grand composite curve

The minimum vertical distance between the CC represents the overall minimum approach temperature for heat exchange†, ΔTmin . For proposed value of ΔTmin , starting and ending point of CCs define the minimum utility duties QCUmin, QHUmin. By moving the HCC in negative and CCC in positive T direction, the composite curve will come in the, so-called, shifted position (presented by dash line in Fig. 3a). Modified position corresponded to the modified interval temperatures, which are obtained by increasing the CCC temperature by ΔTmin / 2 and decreasing the HCC temperature by ΔTmin / 2 . Such temperature shifting ensures the existence of the ΔTmin between the utilities and the process streams. In shifted position composite curves touch each other in the point-named Pinch Point, or simply Pinch. Which temperature is marked as T pinch . The other two important points, for further analysis, are placed on the same ordinate Pinch point of HCC curve which temperature is marked as T pinch ,Hs and Pinch point of CCC curve which temperature is marked as T pinch ,Cs . The temperature difference between these points and pinch point is ± ΔTmin / 2 . By the use of enthalpy (horizontal) differences between the shifted composite curves, it is possible to create another graphical approach, known as the Grand Composite Curve-GCC (Linnhoff et al., 1982). The GCC provides the same information as Pinch diagram, but in a slightly different fashion which enable the selection of appropriate levels of utilities to meet over all energy requirements.

Figure 4. Thermodynamic analyze of HEN and grid diagram †

In the terminology of Pinch technology exist two different approach temperatures: Heat Recovery Approach Temperature (HRAT), which is defined as the smallest vertical distance between the CC, and Exchanger Minimum Approach Temperature (EMAT) which is defined as the minimum allowable temperature difference for the individual heat exchangers. In this paper minimum approach temperature, ΔTmin is related to the HRAT

Pinch diagram and composite curves enable the similar consideration like in the case of single heat exchanger followed by presentation on Fig. 4. The maximum theoretical heat exchange in the network is obtained when the curves (in this case they are not in shifted position) touch each other at the pinch point (Fig. 4a), meaning that somewhere in the network there is one or more exchangers of infinite size ( ΔTmin = 0 ). The maximum practical heat recovery, corresponding to the lowest practical energy consumption can obtained for a specified value of ΔTmin > 0 (Fig. 4b). As before, the price to be paid for having finite temperature differences in the network is the additional energy requirement, i.e. the consumption of both hot and cold utilities is increased (for ΔQHU ,add = ΔQCU ,add ). Again, the optimum value of ΔTmin is to be determined so that the total cost of constructing and operating of network will be at minimum. Finally, the design of fully integrated heat exchanger network (which allow the maximum energy recovery) is developed on the grid diagram followed by pinch design rules (Fig. 4c). The majority of processes exhibit a pinch at some intermediate temperature between the hottest and the coldest process stream. In some cases, known as Threshold Problems, pinch diagram indicate the need for only single utility, either hot or cold but not both (Fig 5a) This situation occurs when the specified ΔTmin equals the threshold value ΔTThreshould . When ΔTmin < ΔTThreshould the result is no pinch and only one utility is required (with the same value as in the case of ΔTmin = ΔTThreshould ).The utility usage only increases when ΔTmin > ΔTThreshould ; Both hot and cold utilities are then required and the problem is pinched.

Figure 5. Threshold problem

Figure 6. Performance targeting

For heat recovery systems with a specified value for the minimum allowable approach temperature ( ΔTmin ), important feature of Pinch diagram is the ability to identify the

maximum recoverable heat (or minimum consumption of utilities) without designing the HEN. In addition, besides the performance target of energy consumption, it is also possible to set the target for Minimum Total Heat Transfer Area(Amin) and Minimum Number of Units (Nmin) before the design phase is started. Performance targets, together with economical parameters (cost of utilities, cost equations for heat exchangers, payback time, interest rate, operating time per year…), can be combined in order to estimate the Total Annual Cost (TAC) of design. Further, the performance target can be calculate for different values of ΔTmin , in order to identify a good starting value for the level of heat recovery. This exercise of preoptimization (Ahmad and Linnhoff, 1990) [7] has been referred as "Super-Targeting". The pinch divides the process into two separate subsystems, which are in enthalpy balance with the corresponding utility. Above the pinch, only the hot utility is required and below the pinch, only the cold utility is required. Hence, for an optimum design, no heat should be transferred across the pinch. This is known as the key concept in Pinch Technology which is expressed through the rules that form the basis for practical network design: 1. Don't transfer heat across the pinch (Fig. 7a); 2. Don't use a cold utility above the pinch (Fig. 7b). 3. Don't use a hot utility below the pinch (Fig. 7c); Violation of any of the above rules results in higher utility requirements, presented as Qtr on the Fig 7.

Figure 7. Graphical presentation of pinch rules The complete HENS design by Pinch Technology can be illustrate by graphical flowchart presented in Fig. 8. Flowchart summarized the four phases [8,9]: 1. Data Extraction phase, the main goal of this phase is to identify the process streams inside the flowsheet of plant and potential utilities, which could be used for building the new, or retrofitting the existing, heat exchanger network. This is a primal and maybe the most important step in pinch design and it can be described more like art than science. Data that has been inappropriately extracted for a physical model of the plant usually leads to results which can disturb overall mass and energy balance of the final solution or might suggest that there is no scope for energy savings.

Figure 8. Flowchart of Pinch technology

2. Targeting phase, where is possible to quantify targets for minimum utility consumption, minimum number of units and minimum area ahead of the actual design stage. This phase is used to determine the optimum level of heat recovery or the optimum ΔTmin value, by balancing energy and capital costs. 3. Design phase, where an initial heat exchanger network, that satisfies the previously defined performance target, is established. The design starts where the process is most constrained, at the pinch, and is carried out separately above and below pinch. ΔTmin is the agreed minimum approach temperature for each heat exchanger, and exchangers are placed between streams such that this requirement is fulfilled. To achieve this, stream splitting may be necessary. The duty of each heat exchanger is made as large as possible in order to minimize the number of units in the design. Utility exchangers are placed on streams which do not meet the target temperatures, when using only process exchangers. 4. Optimization, the maximum energy recovery HEN from the initial design is simplified and improved economically. The strict decomposition at the Pinch normally results in networks with at least one unit more than the minimum number, as well as a few units of inappropriate small area∗. By manipulating with Heat Load Loops, Heat Load Paths, stream splitting and restoring ΔTmin the final solution is is improved in order to achieve a more cost optimal HEN. 3. PHISICAL MODEL OF REFERENCE PLANT In this paper, yeast fermentation plant has been examined (Fig.9). Phisical model of the plant is presented on Fig. 10.

Figure 9. Phisical model of the plant (presented as “black boxes” units) ∗

Even though extensions such as the Driving Force Plot and the Remaining Problem Analysis help the engineer to also minimize total heat transfer area, Total Annual Cost is not necessarily at its minimum, and some final optimization is required.

For the purpose of energy integration plant is divided to A, B, C, D, E and F subsystems. Subsystems represent : • “A”-preparing of raw materials, • “B”-drying stage of fresh yeast. • “C”-distillery part of the plant, • “D”-process of separation of yeast by filtration • “E” and “F” fermentation units, centrifugation and yeast cream storage tanks. In the first part of research, every subsystem has been optimized separately and after that, possible heat integration of the plant has been performed taking into consideration the outlet and inlet streams of each subsystems. More detailed flowsheets of sybsystems are presented on Fig 10.

Figure 10. Flowsheet with selected subsystems of process plant Shwds – hot water distribution system, Shwhs – hot water heating system, Shwt – to hot water tank, SS – steam, Scw – cooling water, Sprm – prepared row material, Sw – waste, C – column, H – heat exchanger, Dr – dryer, Ffiltration, R – reservoir, PS – phase separator, CE – centrifuge

In the subsystem “A” there is the need of hot water (Shw) for fermentation feed solutions preparing such as molasses solution and feeding salt`s solutions. In the subsystem “B” there is the need for hot air for drying (S26) and also this subsystems have outlet waste stream of hot air (S22) which can be used for air heating of production hall. Subsystem with waste high energy content streams (S14, S15) is subsystem “C”. Subsystem which needs cooling utility is subsystem “D”. There is the need for cold washing water (S21 - S20). Subsystems “E” and “F” have no interesting streams for heat integration. Thermodynamic parameters of streams which conect the subsistems are presented in Table 1. For the purpose of creating the pinch design task, data exstraction phase is based on the selection of streams with higher and lower temperatures then the average ambient temperature. The streams are bold in Table 1. The quantity of energy which is consisted in outlet waste energy streams, can be compared as money value and is 1074.00 MJ/h (low pressure steam has price of 14.66 EUR/GJ – at the moment of calculation), so it is about 136000.00 EUR/year. Cost of cooling in the process, where S19 is waste cold stream, is more then about 1100.00 EUR/year. So the total cost is approximately 137100.00 EUR/year.

Table 1. Characteristics of process streams Stream

S5 S6 S7 S8 S9 S10

S14 S15 S18 S19 S20 S21 S22 S23 S23` S24 S25

S26 SSP Sp1 Sp2 Sproduct

Composition Hot water Hot water NaOH solute Evaporative components Evaporative components Sludge, row material Organic components, water Organic acids, water Yeast cream Water Cold water Water Air with dust Saturated Steam, 3 bar Condensate mixed with steam, 3 bar Cooling water (outlet) Cooling water inlet Air Semiconducted product Product 1 Product 2 biomass

Flowrate (m3/h)

Supply temperature(oC)

Target temperature (oC)

Heat capacity (kJ/kg oC)

5

5

90

4.187

30 30 variable variable 6

5 40 105 – 120 60 80

90 90 -

4.187 5.5 -

3.94

106

25

4.2

1.3

105

25

4.204

3.35 4.9 4 4 25000 1.1 1.1

6 7-10 (6*) 15 (7*) 18 35 80

15 15 (7*) -

3.56 4.187 4.187 4.187 1 -

1.5 1.5

30 18

-

4.187 4.187

11000

35

90

0.99

5 0.6 0.2 2

60 20 25 12

6

3.2

4. TARGETING PHASE OF PINCH ANALYSIS Targeting of previously defined HENS tasks is based on the stream data presented in Table 2.

Table 2. Stream data for targeting phase Stream S5 SHWHS S14 S15 S26

Flow (m3/h) 5 8 3.94 1.3 11000

Tinlet (oC) 5 20 106 105 35

Ttarget (oC) 90 60 25 25 90

Heat capacity (kJ/kgK) 4.187 4.187 4.2 4.204 1

Conductivity (W/mK) 0.6 0.6 0.8 0.8 2.9 10-2

Density (kg/m3) 1000 1000 625.15 584 4.1 10-3

Viscosity (cP) 1.2 1.2 1.5 1.5 2 10-2

Pressure (bar) 6.00 3.00 1.25 1.21 2.00

Total heat exchange area calculation is based on Eq. (1) as sum of heat exchange area for enthalpy intervals determined on composite curves [5, 10]. n intervals 1 ⎛ hot streams qi cold streams qi ⎞ (1) + ∑ Anetwork = ∑ ⎜ ∑ ⎟ ΔTLM ,k ⎝ i hi hi ⎠ k =1 i Number of heat exchange units is determined by Eq (2). N u ,min = ( N A − 1) + ( N B − 1) (2)

The economical estimation is based on Eq.(3) and Eq.(4). Operating cost represents cost for utility using and they are calculated with Eq.(5), as well as total annual costs, Eq.(6).

c

⎛A ⎞ Capital Costs Index = a + b ⎜ exchange ⎟ ⋅ Shell ⎝ Shell ⎠

(3)

PL

⎛ ROR ⎞ ⎜1 + ⎟ 100 ⎠ ⎝ Anualized Factor = PL OC = UChu .Qhu ,min + UCcu .Qcu ,min

(4)

TAC = Annualized factor ⋅ CC+OC

(6)

(5)

For the project it is proposed its life to be 5 years with 10% pay back. The utilities present in the system are low pressure steam with cost of 14.66 EUR/GJ and cold water (cold utility). The initial ΔTmin for starting with pinch analysis is adopted to be 10oC. That value is random and designer chooses it. With that value start all calculations of pinch analysis algorithm. Targeting results are obtained by the use of HX-NET software [10]. HX-NET software generates and draws composite curves which are presented in APENDIX 1, capital costs, number of heat exchanger units and heat exchange area for different values of ΔTmin. Heat exchange area is determined as constant value for ∆Tmin range of 1- 200C. Calculations for ∆Tmin higher then 200C are not determined by software. Operating costs have similar function with the same ∆Tmin constant range. This happened because increasing of ∆Tmin, increase hot and cold utility. Increasing come under 200C and that is the reason for increasing of operational costs to very high values. Optimal value of ∆Tmin is every value in range between 1- 200C, because that range has constant value which represents minimum costs, minimum heat exchange area and minimum number of heat exchangers. Value of ∆Tmin is used the initial value of 100C. Minimum number of heat exchangers is 5. 5. DESIGN OF HEAT EXCHANGER NETWORK

After performing pinch analysis using HX-NET, the HEN could be designed. Using pinch technology rules for HEN design, the designer can make many alternatives for different combinations of connecting heat exchanger units. This software warns the user when some of pinch technology rules are broken.

Figure 11. Designed of HEN – alternative-1 / variation -2 (Shwhs – hot water heating system)

Figure 12. Designed of HEN – alternative-2

In this work two alternatives of possible HEN are made (Figs.11 and Fig.12). Alternative-1 (Fig. 11), also presented on HX-NET designed grid diagram (Fig.23) has two variations. These variations are made to use some of the already installed equipment. There are a few questions on how to use some streams, such as separated steam from condensate at different pressures (SS-outlet, S23, S24,SW), and what could be made with outlet hot

air stream (S22). It is proposed to use injector to take separated steam (phase separation) back to the main plant inlet utility stream, as well as condensate to bring back to steam boilers with pumps. Hot air stream S22 could be used for air conditioning of plant hall, before filtrating by air filters. A great part of the energy is used for heating cold water (hot water for process needs), also heating rooms and production halls (hot water heating system, Shwhs), so the way of solving this case has to be focused on minimizing costs for preparing hot water for this purposes.

Figure 13. Grid diagram for new HEN designed with HX-NET (alternative -1/ variation -1) The final results obtained for the both alternatives are shown in Tables 3 and 4. The unit E-111 has the same performances in both alternatives. In the alternative-2 heat exchanger E103 enables using the heat of the waste streams S14, S15, to S5 more efficient, because inlet temperature of S5 into E-110 is higher (47.3 oC by First Law of Thermodynamics). In this alternative, with installation of E-103, increase number of heat exchangers, so it indicates that could be increase capital costs. The sum of energy which will be transferred in both alternatives is the same quantity. New HEN designed for alternative No. 2 consist closed loop of heat exchangers, which is not permitted by pinch technology rules. Heating streams are different (steam and waste stream) and couldn’t be mixed, also stream Shwhs is closed circular stream with different energy needs and couldn’t be mixed with S5. This alternative shows some imperfections of Pinch technology. Table 3. Results for alternative-1/variation -2) Unit 2

Heat exchange area (m ) LMTD (oC) Ft – factor Heat transfer (MJ/h)

E – 101

E -108

E - 109

E - 110

E – 111

71.3 36.76 0.9984 837.4

28 26.42 0.9585 255

62 57.52 0.9984 2.482

24.9 54.34 0.9985 942.08

19.5 79.78 0.998 1085

Table 4. Results for alternative-2

Unit 2

Heat exchange area (m ) LMTD (oC) Ft - factor Heat transfer (MJ/h)

E - 101

E - 103

E – 108

E - 109

E - 110

E – 111

71.3 36.76 0.9984 837.4

2.61 51.07 0.9978 47.9

25.87 23.09 0.9648 207.67

62 57.52 0.9984 2.482

24 53.46 0.9995 893.9

19.5 79.78 0.998 1132.2

One of the most important calculations for optimization i.e. determination which alternative is better for its realization as a project, is economical calculations. For that purpose the CAPCOST software has been used [12]. To estimate equipment cost the bar module method determined using data for heat exchange area, operating pressure and construction material, has been used. CEPCI index for 2006 is 516.8 [13]. CAPCOST software use module costing technique, which is common technique to estimate the cost of a new chemical plant. Such cost estimation is accepted as the best for making preliminary cost estimation. With this estimation, sum of direct and indirect costs is given as multiplication of purchased cost of equipment for base conditions (using the most common material, and operating near ambient pressures) and multiplication factor (for specific conditions) representing Bar Module Cost. The results of equipment costs estimated by CAPCOST software are given in the Table 5. Table 5. Estimated equipment costs Heat exchangers Air filter unit Air Fan Sum

114900.00 $ 1000.00 $ 1500.00 $ 117400.00 $

Production costs presented by the energy, which is saved with this integration, are given in the Table 6. Table 6. Calculated saved energy by process heat integration Stream S14 (12 months using) Stream S15 (6 months using) Cooling integration Heating of rooms (6 months) Sum of cost saving/year approx.

105000.00 EUR 16200.00 EUR 1000.00 EUR 47570.00 EUR 170000.00 EUR

The sum of equipment cost is 117 400.00 $ and the bar module cost is 566300.00 $, which means that the sum of direct and indirect costs for new plant is 566 300.00 $. For economical calculation taxes for profit are assumed to be 42%. In the alternative -1/variation -1, heat exchanger network uses three already installed heat exchangers and two new heat exchangers: E-109, E-110 and E-111 are not installed, and instead of them use already installed R-1, hot water tank with direct injection of steam on it, and existing heat exchangers H-2 and H-3 (Fig.10). The alternative -1/ variation- 2, uses two already installed heat exchangers H-2 and H-3. The alternative-2 is similar to alternative-1/ variation - 1 upgraded with another new heat exchanger E-103 (Fig. 12). No discount and discount cash flow for plant 5 years life time are given in Fig. 14. During the plant life time the amount of no discount cash flow is the same for every year except for the last, where the end value of the plant is included. Depreciation of the plant is calculated with Straight Line Depreciation Method [14], which means equal depreciation amount per year. Using interest rate for depreciation of money discount cash flow plot can be

calculated, and it is presented in Figure 9a, also. This calculation can be used for obtaining cumulative no discount and discount cash flow (Fig. 15). In the year of investment cash flows are negative, because there is no income profit. In the first year of plant life, there is incoming profit which leads to positive value of cash flow. At the end of plant life, the value is positive since it contents salvage and working capital. Using the cumulative cash flow (Fig 15) the economical parameters can be determined. These parameters are based for decision making which project is better. Using these plots, it could be determined the values of ROROI (Rate Of Return Of Interest) which is 30.31%, as well as CCP=1.515 (Cumulative Cash Position). The value of NPV (Net Present Value) at the end of plant life with 8 % interest is 115155.00 $ and PVR (Present Value Rate) is 1.2 (Table 7).

Figure 14. No discount cash flow for alternative-1/variation-1

Figure 15. Cumulative discount and no discount cash flow for alternative-1/variation-1 Table 7. The calculated economical parameters for investigated alternatives of HEN Investigated alternatives of HEN Alternative-1/ Variation-1 Alternative-1/ Variation-2 Alternative-2

ROROI (%) 30.31 26.99 26.33

CCP 1.515 1.349 1.316

NVP ($) 115155.00 47312.00 31403.00

PVR 1.200 1.070 1.044

The economical parameters for the investigated alternatives shown on Table 7 could be compared to determine the most profitable project. These economical parameters are better if their values are higher. Alternative -1/variation -1 has the highest rate of return of investment and net present value at the end of plant life. That means alternative 1/ variation- 1 is the best

case, and then alternative -1/ variation -2 is following. The economical parameters, ROROI, CCP, NVP and PVR for the alternative-2 are the lowest which means that this alternative is not acceptable for additional detailed investigation. The values of rate of return, plant life, depreciation method, CEPCI index, rate of interest, and taxes for all investigated cases are the same. This helps in decision making in a right way. 6. CONCLUSON

The three investigated cases done by HX-NET software using pinch technology are all profitable. That confirms calculated economic parameters for each alternative. Comparing the two alternatives it is obtained that alternative-1/variation-1 is better than alternative1/variation-2, while alternative-2 is less profitable than alternative-1. Also is calculated possibility of using the prepared hot air outlet for hall air conditioning excludes present heating with hot utility and optimized subsystem “D” with minimum cold utility using for both alternatives. This work is proof for saving energy with small investment in production plant. The pinch technology method gives a clear overall view respect to energy consumption efficiency in process plants and should be implemented regularly for designing new and investigating existing plants in order to choose the optimal alternative. NOMENCLATURE A - area (m2) Aexchange –heat exchange area, m2 c - velocity (m/s) CC – installed capital cost, $ CCP – Cumulative Cost Position CEPCI – Chemical Equipment Plant Cost Index cp - specific heat capacity (J/kg K) Cs - cold process streams CU - cold utility TLM –LMTD - logarithmic temperature difference, oC ΔTmin – minimum temperature difference, oC Ft – factor – LMTD correction factor h - specific enthalpy (J/kg) HE – Heat Exchanger

UChu – Hot utility cost, $/kW year Nu, min – Unit target OC – Operating costs, $/year p - pressure (bar) PL – Plant Life Psys - piping system PVR – Present Value Rate Qcu,min – Energy target of cold utility, kW Qhu, min – Energy target of hot utility, kW ROR – Rate Of Return ROROI – Rate Of Return Of Investment t - temperature (0C)

HEN – Heat Exchange Network Hs - hot process streams HU - hot utility

α - heat transfer coeff. (W/m2 K) Subscripts

HWHS – Hot Water Heating System K - thermal conductivity (W/mK) LP - linear programming mu - monetary unit (€) NA – number of process and utility streams above pinch NB – Number of process and utility streams below pinch NLP - nonlinear programming NPV – Net Present Value $, MKD, EUR etc T - temperature (K) UCcu – Cold utility cost, $/kW year

add - added in - inlet out - outlet min - minimum max - maximum RT - retrofit GS - grassroot sp - supply tg - target tr - transfer

Greek symbols

REFERENCES

[1] Lj. Markovska, V. Meshko, R. Kiprijanova, A. Grizo, : Optimal design of shell and tube heat exchangers. Bulletin of Chemists and Technologists of Macedonia, Vol. 15, No.1 , pp. 39-44, 1996. [2] M.M.El-Halwagi, Pollution Prevention Through Process Integration: Systematic Design Tools, Academic Press, San Diego (1997) [3] B.Linnhoff, User guide on process integration for the efficient use of energy, The Institution of Chemical Engineers, UK, 1994 [4] R.Smith, Chemical Process Design, McGraw Hill, New York, 1995 [5] Introduction to Pinch, available from URL (2007) http://www.envormntalexpert.com/software/linnhoff/Pinch%20Intro.pdf [6] Process integration, CANMET Energy Technology Centre (CETC) – Varennes, Canada. See also http://cetc-varennes.nrcan.gc.ca/en/eb_o.html [7] Ahmad, S. and Linnhoff, B., SUPERTARGETING: Different Process Structures for Different Economics, J. of Energy Resourses Technology, 11(3):131-136, 1989 [8] Rašković, P. Industrial energy system optimization based on heat exchanger network synthesis. Ph.D.Thesis: Mechanical Engineering, University of Niš, 2002 [9] Rašković, P., Ni Pinch-software tool for heat exchanger network synthesis, Computational engineering in fluid dynamic and energy technology, I professional seminar, Niš, Serbia, 2004. [10] J. G Mann, Jr , Process Integration: Unifying Concepts, Industrial Applications and Software Implementation, Ph Dissertation, (1999) available from URL http://scolar.lib.vt.edu/theses/available/etd-102199-101855/ [11] HX-NET Manual, Ver. 5.0 (2001) HYPROTECH CO. [12] CAPCOST Manual, Ver. 2 (2002) [13] CEPCI index data available on URL: http://ca.geocities.com/[email protected]/ [14] Richard Turton, Richard C. Bailie, Wallace B. Whiting, Joseph A. Shaeiwitz, Analysis, Synthesis and Design of Chemical Processes, Prentice Hall (2002)

APENDIX 1

Figure A1. Hot (red) and cold (blue) composite curves for analyzed system (alternative -1/ variation -1)

Figure A2. Balanced hot (red) and cold (blue) composite curves with demand of utilities (alternative -1/ variation -1)

Figure A3. Shifted hot (red) and cold (blue) composite curves (alternative -1/ variation -1)

Figure A4. Plot of temperature against enthalpy - grand composite curve (alternative1/ variation -1)

Figure A5. Plot of cold driving force in HEN (alternative -1/ variation -1)

Figure A6. Plot of hot driving force in HEN (alternative -1/ variation -1)

Figure A7. Plot of range targets for hot utility (alternative -1/ variation -1)

Figure A8. Plot of range targets for cold utility (alternative -1/ variation -1)

Figure A9. Plot of total heat exchange area for different values of Dtmin; (alternative -1/ variation -1)

Figure A10. Plot of operating cost index for different values of Dtmin; (alternative -1/ variation -1)