Improving Energy Efficiency of a Dyes Intermediates Synthesis Plant. A Developing Country Specific Case Study

21st European Symposium on Computer Aided Process Engineering – ESCAPE 21 E.N. Pistikopoulos, M.C. Georgiadis and A.C. Kokossis (Editors) © 2011 Elsevier B.V. All rights reserved.

Improving Energy Efficiency of a Dyes Intermediates Synthesis Plant. A Developing Country Specific Case Study Zsófia Fodora, Paul Krajnikb, Petar Sabev Varbanova, JiĜí Jaromír Klemeša a

Centre for Process Integration and Intensification - CPI2, Research Institute of Chemical Technology and Process Engineering, FIT, University of Pannonia, Egyetem utca 10, 8200, Veszprém, Hungary b HUNTSMAN Global Head / Processing & Engineering, Textile Effects, Basel, Switzerland 4057

Abstract In this contribution an energy efficiency analysis of a dyes intermediates synthesis plant in India has been performed to identify the potential for energy savings. The paper presents a developing country specific case study identifying the energy sources and sinks as well as the scope for energy savings of a chemical site. It aims at improving the energy efficiency and reducing pollution. The analysed plant has low energy efficiency for heating and electricity generation. The first part of a site process has been analysed to identify the potential for utility saving and the extent of the possible improvements to the process operation. Keywords: Energy Efficiency, Developing Country. Heat Integration, Industrial Case Study

1. Introduction Heat Integration (HI) is a resource-driven, structured approach used to tackle a wide range of improvements related to the resource efficiency of process and site utility systems. Using Pinch Analysis (Linnhoff et al., 1982) allows quickly identifying the scope for energy savings at an early stage of the analysis or synthesis of a process. HI has been applied mostly in the developed countries for several decades. HI implementation reduced energy consumption and CO2 emissions in various processes and Total Sites. Companies regularly practising PI include Shell, BP, PETROBRAS, MOL, Mitsubishi Chemicals, Exxon, Total, Monsanto and others (PIRC, 2010) achieving serious savings in energy consumption up to 20-30% (Klemeš et al., 2010). India has the world's second largest labour force of 467 million workers. India's external trade has reached a relatively moderate share of 24% of GDP in 2006, up from 6% in 1985 (Policy Brief, 2007). India's share of world trade was about 1.68% in 2008. The last year it was the world's eighteenth largest exporter (International trade statistics, 2010). Major exports include textile goods next to petroleum products. India has been fast developing country; however improving energy efficiency via HI has not been widely applied yet. It has been hindered by a number of factors, lack of maintenance, as well as reliable and consistent process data. This paper is an attempt to demonstrate the opportunities offered by HI in the developing world.

Improving Energy Efficiency of a Dyes Intermediates Synthesis Plant. A Developing Country Specific Case Study

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2. Methodology The presented analysis follows the procedure of HI developed by (Linnhoff et al., 1982) and gradually extended by numerous contributions (Friedler 2009, 2010). The lower bounds on the use of utilities for heating and cooling need to be identified in the first step. To accomplish this, a set of data suitable for evaluating the energy needs is extracted. This includes process stream temperatures and loads from representative heat and mass balances for a process. The data have been analysed by Pinch Analysis using the Composite Curves (CCs) plot. The options for improving the heat recovery by adjusting process design parameters have also been evaluated. The parameters varied include soft outlet temperatures of the hot streams and the pressure in process evaporators (Klemeš et al., 2010). Process flowsheets usually contain data, which are not all relevant to the energy analysis. It is necessary to extract the information that captures the heat sources and sinks, and their interactions with the process. The required data include stream properties as mass flowrate, specific heat capacity, supply and target temperatures, and heat of evaporation for streams with a phase change. A key objective of data extraction is to recognise, which parts of the flowsheet can be subject to change during the analysis. There are possibilities to make modifications such as re-piping, adding heat exchangers, temperature changes in the process or changing the utility (e.g. MP steam instead of HP steam). Certain rules should be followed to obtain credible results (Klemeš et al., 2010). To Vacuum System Na2SO4*10H2O

Ice water

3

Pusher Centrifuge 1 Water

4 Melt 3

1 Steam

4 Step Evaporator Pusher Centrifuge

Black Anhydrid

Vacuum Evaporator

Water 4

4

2 5

Water

Water 2

5

3

Pusher Centrifuge

2

MLAnh. Na2SO4 Tank Anhydrid

To Incinerator

Fig. 1: Process flowsheet

3. Industrial problem typical for developing countries 3.1. Process description The plant has been commissioned 12 years ago and recently acquired by a western investor who is making an attempt to improve the energy efficiency and to reduce the carbon footprint. Sodium sulphate (Na2SO4) is important for the manufacture of textiles. It helps reducing negative charges on fibres and dyes can penetrate evenly. It is produced from natural minerals as Mirabilite, Glauberite and Thernadite. Secondary

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sources of Na2SO4 are by-products of chemical processes where the neutralization of acid effect (e.g. H2SO4) or alkaline (e.g. NaOH) tail solutions produces a Na2SO4 solution. After the consultation with the plant management it has been decided to examine a part of the flowsheet (Fig. 1), which consumes 45 % of the heating utility in the dyes synthesis plant producing approx. 300 t Na2SO4 per month. The total steam consumption for the flowsheet is 13.3 t/h if the cycle time is 48 h. From this amount 2.2 t/h is used directly to heat up the process and 11.1 t/h steam condensate is not utilised. The plant has been partly retrofitted and improved to make the cycle time more effective and improve the productivity and competitiveness of the plant. The cycle time was reduced from 48 h down to 24-30 h. The total steam consumption changed to 10 t/h and the direct steam to heat up the process to 1.1 t/h. The unutilised condensate has been reduced to 9 t/h. 3.2. Data extraction Following the data extraction rules (Klemeš et al., 2010), five streams were extracted from the flowsheet (Fig. 1). The stream properties are shown in Tab. 1 and 2. Tab. 1: Extracted cold streams Cold Stream No. 1 1 2 3

Segment

1 2 -

Ts °C

Tt °C

5.00 100.00 49.42 36.00

100.00 101.00 51.42 40.00

CP kW/°C 5.82 626.82 500.00 65.00

Q kW 552.19 626.80 1000.00 260.00

Tab. 2: Extracted hot streams Hot No. 4 5

Stream

Ts °C 66 66

Tt °C 42 60

CP kW/°C

Q kW

19.14 166.66

459.5 1000.0

There is a heating loop for the Melt Operation (Fig.1), where Stream 3 is circulating the melting mass at high flow rate, which needs heating. Currently this heat is provided by Stream 4. The temperature of this Stream was labelled in the flowsheet provided by the plant as 42 °C. After analysing the configuration of the heat exchangers and the streams, it was concluded that this was unlikely because the temperature differences would be too small. Consequently it was assumed that the 42 °C was the final temperature of the hot water stream (Tab. 2). Fig. 2 shows the current heat exchanger network (HEN) for the process streams in Tab. 1 and 2. The topology includes three recovery heat exchangers and a heater. The lower input/output temperature differences for the heat exchangers are ǻT E1 = 26.6 °C, ǻTE2 = 11 °C, and ǻTE3 = 6 °C. The heater duty on Process Stream 1 (Fig. 2) is partly spent on preheating Stream 1 to saturation temperature (352 kW) and the remaining duty powers the evaporation in the first effect.

Improving Energy Efficiency of a Dyes Intermediates Synthesis Plant. A Developing Country Specific Case Study

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3.3. Heat Integration analysis The CCs (Fig. 3) had been constructed. From the heat and mass balance the temperatures and heating or cooling requirements are identified. The temperature differences in the existing HEN were analysed to select a ǻTmin value for the Pinch Analysis. In Fig. 2 the smallest temperature difference is 6 °C (on E3). However, this temperature difference is the rather uncertain piece of information from the plant data. Therefore the Pinch Analysis has been carried out using sensitivity analysis by varying ǻTmin from 6 °C up to 12 °C. E3 42 °C

1

5 °C

4 CP = 8.31 kW/°C 66 °C

E2

3

49 °C

199.5 kW E2 1000 kW

36 °C

19.1

166.6

5

100°C H 979.5 kW * 51 °C

E1 2

CP [kW/°C]

66 °C

E1

60 °C

CP = 10.83 kW/°C

E3

40 °C

5.8 500 65

260 kW

*Two segments: Preheat and Evaporation

Fig. 2: Process Grid Diagram T°(C) 120 100 80 60 Hot CC Cold CC 36 deg Cold CC 38 deg Cold CC 40 deg

40 20 0 0

1000

2000

3000 Q (kW)

Fig. 3: Problem Composite Curves

The results identified the data from Tab. 1 and 2 as a threshold problem with ǻTmin,threshold = 11.5 °C. Below this value the plant does not need utility cooling. A higher ǻTmin value is desirable since the lower values in the sensitivity range are likely to require larger heat exchangers. ǻT min = 10.2 °C was chosen for the remaining analysis – a value lower than the threshold producing the diagram in Fig. 3. The outlet (target) temperature of Process Stream 3 does not need to be fixed. This is a typical case of a “soft” temperature (Klemeš et al., 2010).

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To improve the heat recovery and the utility targets, process modifications could be applied. One option is the soft target temperature of Process Stream No. 5, which provides heating for Vacuum Evaporator Stream No. 2. To be able to use its potential, an additional degree of freedom to reduce the temperature of the potential process Pinch Point is needed. It is controlled by the pressure in the Vacuum Evaporator. The degree of freedom is the evaporation pressure reduction from 0.12 bar down to 0.1 bar or even lower. After making the improvements, the heating utility target is reduced by 96.7 kW from 979.5 kW to 882.8 kW while external cooling is not required.

4. Conclusions and future work The HI analysis allows setting targets for minimum energy consumption prior to the HEN design. It identifies the scope for energy saving both grassroots and for existing processes, which is the analysed case. The data relevant for utility targeting have been extracted using the flowsheet and plant measurements where available. The targets have been obtained for a range of ǻTmin values clearly identifying a threshold problem requiring only utility heating. The target for the current operating conditions and ǻT min = 10.2 °C is identical with the heating utility of the current HEN. However, by exploiting soft data and a slight evaporator pressure reduction, it has been possible to save more than 10% hot utility. However, the magnitudes of the recommended changes for the running plant had to be doublechecked against the margins of measurement error and after this verification evaluated the plant management. The steam saving of approximately 100 kW is relatively small compared with the overall steam consumption of the Total Site, indicating that there is further potential for higher savings. However in this case the scope of analysis needs to be extended to include other parts of the process or rather the whole site. The analysis shows the steps in the right direction provided an evidence for the plant management to extend the analysis into the Total Site. Further improvements could involve analysing the batch processes on the site using a tailored method – e.g. as the one developed by Foo et al. (2008), as well as evaluation of the plant environmental impact (Pavlas et al., 2010).

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