CHAPTER 5

PROCESS INTEGRATION

5.1ABSTRACTThis plant is designed to produce product of 45,000 metric tonne per year of polytetrafluoroethylene. The production rate of the product was determined by break even analysis. Generally, PTFE has many unique properties that made their chemical is in highest ranking of applications. It involves in material coating, use in laboratory equipment that can resist corrosive and can be an insulator. PTFE also can used to insulate much data communication cable, and also can act in manufacture of semi-conductors, as their properties that low electrical conductivity. In this plant, there are two processes in which preparing the monomer which is tetrafluoroethylene and the polymerisation process. For, monomer preparation, pyrolysis reaction took place while suspension polymerisation type was chosen. As for the market demand and supply, polytetrafluoroethylene is monopoly by Asian Countries. By comparing the demand in Asia, Japan has largest demand of PTFE compared to other Asian Country. However, the main producer and manufacturer of PTFE is comes from China. Although the demand for PTFE is high at Japan, the new plant is decided to build in China because the availability of raw material supply. Therefore, the plant is to be built in Suzhou National New & Hi-Tech Industrial Development Zone because the area provide low land cost and many facilities. In process flow diagram, there are seven major equipments which have been detailed design to ensure it suitable for the process. Besides, the process control systems have been designed in all equipments in PTFE plant to make sure that the flow in each process runs in desired condition. In economic analysis, all costs involved in the plant had been calculated and profitability analysis had been done. The payback period for this PTFE plant is about 5 years which is in the range of industrial payback period. In environmental aspect, waste water treatment and waste gas treatment involved in the process. For gas treatment, incineration is used. Last but not least, some hazards analysis had been done and plant layout had been sketched to ensure the safety of the process condition, people, and environment.

5.2 INTRODUCTION OF HEAT EXCHANGER NETWORKSEnergy conservation is very important in process design. Energy can be conserved by using process integration method. In recent years, much work has been done in developing a perfect method for energy integration and the efficient design of heat exchanger networks (Gundersen and Naess, 1988). However, Bodo Linnhoff had successfully developed a method that had been usually used until now: pinch technology. The term was derived from the fact that in a plot of the system temperature versus the heat transferred. A pinch usually occur between the hot stream and cold stream curve. It has been shown that the pinch represent a distinct thermodynamic break in the system and that, for minimum energy requirements, heat should not be transfer across the pinch. (Linnhoff and Townsend,1982)

Figure 5.2 Hot and cold stream composite curve

5.3 PINCH TECHNOLOGYPinch Technology provides a systematic methodology for energy saving in processes and total sites. The methodology is based on first and second law thermodynamic principles. Pinch Technology gives three rules that form the basis for practical process design:a) No external (utility) cooling above the Pinch Temperature.b) No external (utility) heating below the Pinch Temperature.c) No heat transfer between process streams across the Pinch Temperature.By using Pinch Technology, appropriate changes in the core process conditions that can have an impact on energy savings can be identified. After the heat and material balance is established, targets for energy saving can be set prior to the design of the heat exchanger network. The Pinch Design Method ensures that these targets are achieved during the network design. In PTFE production plant, there are nine streams with five hot streams and 4 cold streams that need to be considered for the problem of integrating the utilization of energy. The heat capacity flow rate of each stream is shown as CP. For streams where the specific heat capacity can be taken as constant, and there is no phase change, CP will be given by:

Wherem = mass flow rate, kg/sCp = average specific heat capacity between

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PropertiesStream 1Stream 2Stream 3Stream 4Stream 5Stream 6Stream 7Stream 8Stream 9

Temperature, T (oC)2577102308850-357529350

Temperature difference, T ( oC )131312214.5890162675301330

Mass flow, m (kg/h)1.45x1048.66x1048.66x1042.58x1043.52x1042.58x1049.41x1038.71x1046.39x105

Mole Heat capacity, Cp (J/kg oC)2.87x103786.49x1028.47x1022.49x1038.74x1023.29x1031.89x1023.41x102

Heat capacity, mCP (kW/oC) 11.61.8815.65.8211.96.268.594.5760.3

Table 5.3.1 Aspen Data Properties

5.3.1 ENERGY TARGETING PROCEDUREPinch Analysis provides a target for the minimum energy consumption. There are two procedures to achieve the energy targets for the process, which are: a) Problem Table AlgorithmCalculates the process heat surpluses and deficits within some specified temperature intervals.

b) Composite Curves The cumulative process heat availability (surplus) and heat requirement (deficit)

5.3.2 PROBLEM TABLE METHOD Problem table method (PTA) was developed by Linnhoff & Flower (1978) as a mean of determining the utility needs of a process and the location of the process pinch. The procedure for PTA is as follows: 1. Convert the actual stream temperatures, Tact, into interval temperatures, Tint. The use of the interval temperature rather than the actual temperatures allows the minimum temperatures difference to be taken into account (Tmin = 10 K).

2. Note that any duplicated interval temperatures.

3. Rank the interval temperatures in order of magnitude, showing the duplicated temperatures only once in the order.

4. Carry out a heat balance for the streams falling within each temperature interval.

For mixture: (Where (Cp) mix = heat capacity mixture yi = mass/mole fraction of i-component Cpi = heat capacity of i-component

For the nth interval:

Where Hn = net heat required in the nth intervalCPc = sum of the heat capacities of all the cold streams in the interval CPh = sum of the heat capacities of all the hot streams in the interval Tn= interval temperature difference = (Tn-1 Tn)

5. Cascade the heat surplus from one interval to the next down the column of interval temperatures.

6. Introduce just enough heat to the top of the cascade to eliminate all the negative values.

Construct table by calculate:

Since, Tmin = 10 oC

Therefore:

Hot streams,

Cold streams, Stream no.Stream typeTactual ( oC )Tinterval( oC )Tinterval( oC )mCP (kW/ oC )

TinitialTfinal

1-2Hot251230-171311.6

5-6Cold779072-85131.88

7-8Hot10290107-951215.6

13-14Cold35522.530-7457155.82

16-17Hot850-40905- -3594011.9

19-23Cold-35127-40-1221626.26

20-21Cold7575070-7456758.59

27-28Cold2933024-3253014.57

25-29Hot 35020355-2533060.3

Table 5.3.2.1 Summary Process Data for Each Stream

Then, draw temperature interval to shows the series of temperature intervals for hot and cold stream.

Figure 5.3.2.1 Temperature Interval1 3 5 9 2 4 6 7 8 11.6 15.6 11.9 60.3 1.88 5.82 6.26 8.59 4.57 845 755 1.99x104 527.5 1.06x104 345 1.87x102 335 1.50x102 132 1.37x103 97 95 5.80x103 85 82 80 40 34 20 1.01x103 15 2.84x103 7 -30 2.44x101 -45 Hot Hot Hot Hot ColdCold Cold Cold Cold

Cp (kW/k)StreamQ=CpT (W)

Tranked (oC)Tint (oC)CP(kW/K)=CPC-CPH

H(kW)=CP x Tint

845---

7559011.91.071x103

527.5227.53.317.53x102

345182.5-2.51-4.581x102

3351057.795.779x102

13220353.221.08x104

973546.961.644x103

95262.561.251x102

851060.686.068x102

82345.081.352x102

80246.9693.9

404055.552.222x103

34661.373.682x102

201465.949.232x102

15577.543.877x102

7817.241.379x102

-30375.642.087x102

-451511.91.785x102

Table 5.3.2.2 Problem Table Method

Interval T (oC)Grand Composite net heat, H (kW)Hot and Cold Cascaded heat (kW)

845-0

7551.071x1031.071x103

527.57.53x1021.824x103

345-4.581x1021.366x103

3355.779x1021.944x103

1321.08x1041.275x104

971.644x1031.439x104

951.251x1021.452x104

856.068x1021.512x104

821.352x1021.526x104

8093.91.535x104

402.222x1031.757x104

343.682x1021.794x104

209.232x1021.887x104

153.877x1021.925x104

71.379x1021.939x104

-302.087x1021.960x104

-451.785x1021.978x104

Table 5.3.2.3 Cascading Heat Table AnalysisCascading the heat from one interval to the next indicates that the temperature difference is such that the heat can be transferred between the hot and the cold streams. As the value is all positive, there is no pinch occur as it is thermodynamically feasible. From Table 5.4: Hot and Cold Cascaded heat (kW) 1. At interval temperature, T = 750 oC

2. At interval temperature, T = 350 oC

Cascade Diagram Pinch temperature

Heat Load (W)

Temp (oC)HOT UTILITYCOLD

UTILITY0

1.071x103

1.071x103

1.824x1037.53x102

-4.581x102

1.366x103

1.944x1035.779x102

1.08x104

1.275x1041.644x103

1.439x104

1.452x104441.251x102

6.068x102

1.512x104

1.526x1041.352x102

9.392x101

1.535x104

2.222x103

3.682x1021.757x104

1.939x1041.925x1041.887x1041.794x1041.379x1023.877x1029.232x102

2.088x102

QC=1.978 x104

1.960x104

1.785x102

1.978x104

Figure 5.3.2.2 Cascaded DiagramTherefore, 1. Minimum amount of heat must be Removed by Cold Utilities is 1.978x104 kW.

2. Pinch Point at interval, T = 845oC.

a) Cold Stream, T = 850 oC.

b) Hot Stream, T= 840 oC.

5.3.3 GRAND COMPOSITE GRAPH A graph of net heat flow against shifted temperature can be plotted from the data of the Problem Table (heat cascade). This graph is known as the Grand Composite Curve (GCC). GCC is done to highlight the difference between heat available from the process hot streams and the heat required by the process cold streams, relative to the pinch, at a given shifted temperature Temperature (oC)Grand Composite net heat, H (kW)

8450

7551.071x103

527.51.824x103

3451.366x103

3351.944x103

1321.275x104

971.439x104

951.452x104

851.512x104

821.526x104

801.535x104

401.757x104

341.794x104

201.887x104

151.925x104

71.939x104

-301.960x104

-451.978x104

Table 5.3.3.1 Data for plotting the grand composite curve

Figure 5.3.3.1 Grand Composite Curve5.3.4 Design for Heat Exchanger Network (HEN)5.2.4.1 Grid RepresentationGrid representation is a method to present a heat exchanger as a grid. Heat exchangers are drawn as circles connected by a vertical line. The circles connect the two streams that are in between heat being exchanged. Thus, the streams would flow through the actual exchanger. Heaters or coolers will be drawn as a single circle, connected to the appropriated utility (Sinnot et. al, 2013).In PTFE production process, only below pinch grid diagram is needed. The network design below the pinch is as follows:

CPHOT CPCOLD

Figure 5.3.4.1 Grid diagram of PTFE production processFCPH

H12511.6150.8 H210215.6 187.2 H385011.9 10591 H435060.3 19899 C1901.88 24.44 C2522.55.822837.25 C31276.26 1014.12 C47508.595798.25 C53304.571375.5719778.372837.25150.8187.2676.125798.2524.441254.94120.63

5.2.5 Minimum Number of Heat Exchangers For simple networks, Holmann (1971) has shown that the minimum number of exchangers is given by;

Where; Zmin= NHE= minimum number of exchangers needed, including heaters and coolers N= Ns+Nu-1= the number of streams, including the utilities NHE= Number of Heat Exchanger NS= Number of stream NU= Number of Utilities

Therefore, Above Pinch:

Below Pinch:

Figure 5.3.4.2 Process flow diagram of PTFE production before heat integration

Figure 5.3.4.3 Process flow diagram of PTFE production before heat integration

5.3.6 Comparison Before and After Heat Integration5.3.6.1 Total Energy Can Be Saved

By implementing the heat integration, the heat removal from condenser can be utilized to heat up the reactant mixture.UtilityBefore integrationAfter integration

Hot (kW)3.142x1040

Cold (kW)1.237x1041.978x104

Total (kW)4.379x1041.978x104

Table 5.3.6.1 Comparison Before and After IntegrationTherefore, percentage (%) total energy can be saved:

5.4 ConclusionAs a conclusion, by doing heat integration, the energy requirement can greatly be reduced as shown at the step above. By reducing the energy requirement it will help reduced the cost and the payback period of the plant.

5.5 References

Gary Smith, Ajit Patel. (1987, November). Step by step through the pinch. Energy Integration, pp. 26-31.Richard Turton. (2012). Analysis, Synthesis, and Design of Chemical Processes. Pearson.