PINCH ANALIZA POSTROJENJU ZA FERMENTACIJE KVASCEM

PINCH ANALYSIS OF YEAST FERMENTATION PLANT

A. Anastasovski1∗, L. Markovska2, V. Meško2, P.Rašković3

1Factory of Yeast and Alcohol, MK-7000 Bitola, Republic of Macedonia

2Faculty 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: *AleksandarA@mt.net.mk

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-to-process 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 heat-exchanger 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 0Tmin >Δ 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 N,..2,1k,Tk = ), 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†, minTΔ . For proposed value of minTΔ , 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 2/TminΔ and decreasing the HCC temperature by 2/TminΔ . Such temperature shifting ensures the existence of the minTΔ 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 pinchT . The other two important points, for further analysis, are placed on the same ordinate Pinch point of HCC curve which temperature is marked as Hs,pinchT and Pinch point of CCC curve which temperature is marked as Cs,pinchT . The temperature difference between these points and pinch point is 2/TminΔ± .

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, minTΔ 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 ( 0Tmin =Δ ).

The maximum practical heat recovery, corresponding to the lowest practical energy consumption can obtained for a specified value of 0Tmin >Δ (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

add,CUadd,HU QQ ΔΔ = ). Again, the optimum value of minTΔ 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 minTΔ equals the threshold value ThreshouldTΔ . When

Threshouldmin TT ΔΔ < the result is no pinch and only one utility is required (with the same value as in the case of Threshouldmin TT ΔΔ = ).The utility usage only increases when Threshouldmin TT ΔΔ > ; 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 ( minTΔ ), 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 minTΔ , in order to identify a good starting value for the level of heat recovery. This exercise of pre-optimization (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 trQ 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 minTΔ 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. minTΔ 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 minTΔ 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, F- filtration, 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 Composition Flowrate (m3/h)

Supply temperature(oC)

Target temperature (oC)

Heat capacity (kJ/kg oC)

S5 Hot water 5 5 90 4.187 S6 Hot water 30 5 90 4.187 S7 NaOH solute 30 40 90 5.5 S8 Evaporative components variable 105 – 120 - - S9 Evaporative components variable 60 - - S10 Sludge, row material 6 80 - - S14 Organic components,

water 3.94 106 25 4.2

S15 Organic acids, water 1.3 105 25 4.204 S18 Yeast cream 3.35 6 - 3.56 S19 Water 4.9 7-10 (6*) 15 4.187 S20 Cold water 4 15 (7*) - 4.187 S21 Water 4 18 15 (7*) 4.187 S22 Air with dust 25000 35 - 1 S23 Saturated Steam, 3 bar 1.1 - - - S23` Condensate mixed with

steam, 3 bar 1.1 80 - -

S24 Cooling water (outlet) 1.5 30 - 4.187 S25 Cooling water inlet 1.5 18 - 4.187 S26 Air 11000 35 90 0.99 SSP Semiconducted product 5 60 - - Sp1 Product 1 0.6 20 - - Sp2 Product 2 0.2 25 - - Sproduct biomass 2 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 Flow (m3/h)

Tinlet (oC)

Ttarget (oC)

Heat capacity (kJ/kgK)

Conductivity (W/mK)

Density (kg/m3)

Viscosity (cP)

Pressure (bar)

S5 5 5 90 4.187 0.6 1000 1.2 6.00 SHWHS 8 20 60 4.187 0.6 1000 1.2 3.00 S14 3.94 106 25 4.2 0.8 625.15 1.5 1.25 S15 1.3 105 25 4.204 0.8 584 1.5 1.21 S26 11000 35 90 1 2.9 10-2

4.1 10-3 2 10-2 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 hot streams cold streams

1 ,

1 i inetwork

k i iLM k i i

q qAT h h=

⎛ ⎞= +⎜ ⎟Δ ⎝ ⎠∑ ∑ ∑ (1)

Number of heat exchange units is determined by Eq (2). ( ) ( ),min 1 1u A BN N N= − + − (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).

ACapital Costs Index

cexchangea b Shell

Shell⎛ ⎞

= + ⋅⎜ ⎟⎝ ⎠

(3)

1100Anualized Factor

PLROR

PL

⎛ ⎞+⎜ ⎟⎝ ⎠= (4)

,min ,min. .hu hu cu cuOC UC Q UC Q= + (5) Annualized factor CC+OCTAC = ⋅ (6)

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 E-103 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 E – 101 E -108 E - 109 E - 110 E – 111

Heat exchange area (m2) 71.3 28 62 24.9 19.5 LMTD (oC) 36.76 26.42 57.52 54.34 79.78 Ft – factor 0.9984 0.9585 0.9984 0.9985 0.998 Heat transfer (MJ/h) 837.4 255 2.482 942.08 1085

Table 4. Results for alternative-2

Unit E - 101 E - 103 E – 108 E - 109 E - 110 E – 111

Heat exchange area (m2) 71.3 2.61 25.87 62 24 19.5 LMTD (oC) 36.76 51.07 23.09 57.52 53.46 79.78 Ft - factor 0.9984 0.9978 0.9648 0.9984 0.9995 0.998 Heat transfer (MJ/h) 837.4 47.9 207.67 2.482 893.9 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 114900.00 $Air filter unit 1000.00 $Air Fan 1500.00 $Sum 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) 105000.00 EUR Stream S15 (6 months using) 16200.00 EUR Cooling integration 1000.00 EUR Heating of rooms (6 months) 47570.00 EUR Sum of cost saving/year approx. 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 ROROI (%) CCP NVP ($) PVR Alternative-1/ Variation-1 30.31 1.515 115155.00 1.200 Alternative-1/ Variation-2 26.99 1.349 47312.00 1.070 Alternative-2 26.33 1.316 31403.00 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 alternative-1/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) UChu – Hot utility cost, $/kW year Aexchange –heat exchange area, m2 Nu, min – Unit target c - velocity (m/s) OC – Operating costs, $/year CC – installed capital cost, $ p - pressure (bar) CCP – Cumulative Cost Position PL – Plant Life CEPCI – Chemical Equipment Plant Cost Index Psys - piping system cp - specific heat capacity (J/kg K) PVR – Present Value Rate Cs - cold process streams Qcu,min – Energy target of cold utility, kW CU - cold utility Qhu, min – Energy target of hot utility, kW �TLM –LMTD - logarithmic temperature difference, oC ROR – Rate Of Return ΔTmin – minimum temperature difference, oC ROROI – Rate Of Return Of Investment Ft – factor – LMTD correction factor t - temperature (0C) h - specific enthalpy (J/kg) HE – Heat Exchanger Greek symbols

HEN – Heat Exchange Network α - heat transfer coeff. (W/m2 K) Hs - hot process streams HU - hot utility Subscripts

HWHS – Hot Water Heating System add - added K - thermal conductivity (W/mK) in - inlet LP - linear programming out - outlet mu - monetary unit (€) min - minimum NA – number of process and utility streams above pinch max - maximum NB – Number of process and utility streams below pinch RT - retrofit NLP - nonlinear programming GS - grassroot NPV – Net Present Value $, MKD, EUR etc sp - supply T - temperature (K) tg - target UCcu – Cold utility cost, $/kW year tr - transfer

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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 (alternative-

1/ 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)