Indian Journal of Fibre & Textile Research Vol. 29. June 2004. pp. 233-238

Techno-economic control systems for polyester manufacturing

A A Itagi'. Mandar Shete. Kapil Asher & Nitin Kanade

D.K.T.E Society's Textile and Engineering Institute. Ichalkaranji 416 liS. India

Received 2 July 2002; revised received 28 November 2002; accepted 24 February 2003

A new approach for process control in polyester manufacturing plants is presented. which is based on the di stributed control system and simulation. enabling a high degree of automation. The technology seems to be stable process with operational ease providing protection against misguidance and wastage. Although a high initial investment is called for nonetheless high investments can be recovered in just three years of installing this technologically superior and economically viable control system.

Keywords: Distributed control system. Polyester. Programmable logic controller IPC Code: Int. CI.7 G06F 9/00

1 Introduction Through the decades, the process system for

polyester manufacturing control architecture has witnessed technology advances. In the industrial sectors, it has moved from pneumatic controls (some still around) to electronics (vacuum tubes) to transistors to integrated circuits. Direct digital controls (DOC) led to distributed control system (DCS). Automation has undergone a paradigm shift with innovations in electronics and control to achieve its current form where complex processes take place merely with a push of a button or even click of a mouse. The new age smart instruments, namely digital communications, sophisticated control systems and the concept of completely integrated factory, are all set to change manufacturing forever. With robotic­controlled plants, networking with TCP/IP, optic fibre and satellite communication, the automation indeed has come a long way. Today, sitting in the comforts of the room or even while globetrotting, a CEO can visualize and control operations and manufacturing process on the plant floor.

Polyester manufacturing is a very sensitive and delicate process which requires a high degree of engineering knowledge and excellent equipment and more so precise equipment control to guarantee desired quality. Although the textile is under a sustained recession, the technology solutions are available to the manufacturer to reduce manufacturing

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costs by better machine control. The process control in polyester manufacturing needs multi-attention as there are many a parameters to take stock of, necessi tating a limited degree of automation and, in the present work, this is precisely the reason to undertake a training and study project to validate the new approach which is based on the distributed control system (DCS) for process control in polyester manufacturing right at the place of its usage (industry). Study has been carried out at Indorama Synthetics [India] Ltd and Sanghi Polyester Ltd, to observe the technical and economical feasibility of DCS technique in polyester manufacturing plants.

2 Digital Control Mechanism Digital systems are any computerized systems or

microprocessor-based devices used for the control or data acquisition.

2.1 Distributed Control System

The distributed control system is a comprehensive hardware and software package that encompasses all the functionality required to implement control and data acquisition functions. This includes operator graphic interface (VDUs), alarm package, historical data collection package, continuous and discontinuous control, and standard hardware communication capability to the other digital systems. This system along with programmable logic controller (PLC) and supervisory control and data acquisition system (SCADA) offer wide ranging applications for control and data acquisition. A typical DCS system is shown in Fig. I

234 INDIAN J. FIBRE TEXT. RES., JUNE 2004

OPERATOR CONSOLES

D D REDUNDANT

DATA

FIELD INSTRUMENTS FIELD INSTRUMENTS

Fig. I-Typical distributed control system

2.2 Programmable Logic Controller It is a hardware and software combination that

encompasses both control and data acquisition functions. A typical PLC system consists of a processor and an VO subsystem consisting of groups of rack-mounted cards. While the control and data acquisition functions are executed in the processor, the signal conversion for field devices is implemented in the VO subsystem. Many PLC manufacturers also offer hardware redundancy (usually for the processor only). The PLC is designed as a stand-alone device that makes it an attractive choice for dedicated control functions.

2.3 Supervisory Control and Data Acquisition System It is a system that gathers and sends information to

remote locations. It simply monitors and logs activity; it also interfaces with the primary controllers by sending set points or calculated values. A primary feature of the SCADA system is its abil ity to communicate using a variety of different methods . This makes it especially useful in getti ng information to and from remote locations or other digital systems using a variety of different protocols. The SCADA systems are capable of communicating over telephone lines, UHF/VHF radios, microwave systems and fibre optic cable.

3 Polyester Product and Process in Nutshell The automation and control can be well appreciated

if the salient features of the so-called sensitive process of polyester manufacturing are considered.

3.1 Continuous Polycondensation Two main raw materials PTA (terephthalic acid)

and MEG (mono ethylene glycol) are pumped through

metering system to slurry mix tank at desired mole ratio. The mixing of PTA and MEG is a continuous process and the slurry thus formed is supplied to the esterifier reactor through injection nozzles. The reaction of PTA and MEG mixture is accomplished in esterifier reactor under required heating conditions by an external heat exchanger and a vapour separator. Excess glycol and water in the form of vapour are taken at the top of the vessel to a column where these are condensed and separated, glycol is pumped back for making the slurry. The oligomers formed in the esterification reactor are filtered and the product is fed to a continuous polymerization reaction vessel where ol igomer further reacts to form polyester polymer of the desired viscosity under specified and controlled temperature and vacuum conditions. The reaction links polymer chains with evolution of glycol. Excess glycol vapour is removed, condensed and taken to slurry preparation system. The polymer from the final reactor is then pumped through gear pumps to staple fi bre spinning and chips route continuously.

3.2 Filament Yarn (POY) Molten polymer from poly condensation is passed

through polymer cooker and after filtration and pressure boosting it is distributed to the spinning manifolds and then to spinning positions. The polymer melt from the spinning position is extruded through the spinnerets by the variable speed drive operated spinning pumps. The extruded filaments are cooled by precisely controlled conditioned air in quench chamber and after passing through the finish appl ication system the filaments are taken on take-up winders and finally wound on bobbins.

3.3 Polyester Staple Fibre (PSF) The second stream of the polymer melt from poly

condensation plant is fed to the spinning manifolds after filtration and is extruded through specially built spinnerets. The molten polymer is driven through the spinneret by variable speed driven metering pumps. The extruded filaments are cooled down by quench air at regulated velocity, temperature and humidification. The filaments from all the spinning positions are then combined in the form of ' tow' and collected in continuous driven traversing cans. The tow is cut and doffed in the cans automatically. The tow from number of cans depending upon the denier of final product is processed through the fibre draw line. The staple fibre so produced is then cut and bales are produced.

IT AGI el al.: TECHNO-ECONOMIC CONTROL SYSTEMS FOR POLYESTER MANUFACTURING 235

3.4 Chips Section Third stream of the polymer melt from the

polycondensation is discharged through the metering pumps to the casting heads. The molten ribbons coming out of the casting head are taken in the cooling chambers of chips cooler where the molten ribbons are cooled by cooling water stream. The solidified polymer ribbons are then cut by the chips cutter and dried in the drier. The dried chips are then collected in different classified storage silos.

4 Scenarios in Polyester Manufacturing Units The polyester manufacturing plants of India

ventured into the field of digital control around 1995-96. Almost six plants came up with DCS technology for PET manufacturing. These are Reliance Industries

Ltd (Hazira) with 1200 tones/ day capacity, Indorama Synthetics (Nagpur) with 650 tones/day and Sanghi Polyesters (Hyderabad) with 150 tones/day .

This project was taken to study the DCS technology and non-DCS controls right at the place of usage (Industry) and to bring forth the advantages of new technology . For this, Indorama Synthetics and Sanghi Polyesters were chosen for the project. Bird 's eye view of some non-DCS and DCS features are shown in Tables 1 and 2 respectively.

5 Results and Discussion During the past decade, the capabilities of PLC and

DCS have changed to the extent that today many applications that used to be the exclusive province of

Table I --Summary of different non-DCS system features

Feature Relay Solid state control Micro processor Mini computer PLC

Hardware cost Low Equal Low High Low to hi gh Versatility Low Low High High High Usability Yes Yes No No Yes Trouble shooting Yes No No No Yes Computer compatibility No No Yes Yes Yes Arithmetic capability No No Yes Yes Yes Information gathering No No Yes Yes Yes Programming cost High Hi gh Very high High Low Reusabil ity No No Yes Yes Yes Space considerations Largest Large Small Average Small

Table 2-Summary of different DCS systems specifications

Specification Tala Honeywell Foxboro ABB-Advant

Name of system TDC-3000 FX8-5.5 Advant 5 Capacity of the system Small/medium Medium/large SmalllMedium

industries industries industries Color hard copier Possible 8 Pin dot printer Inkjet pri nter Alarm and logging Possible Possible Possible Language Special Basic Software Software Non open S/W Open S/W Open S/W Style of operation Touch screen and fl at Trackball+ mouse Trackball+ mouse

keyboard No. of stations on one network 64 992 Nodes 99 CRT resolution 640 x 448 dots 1024 x 768 1024 x 768 No. of trend page 8 Pens per page 8 Pens per page 8 Pens per page Time of rccording 96 h No limit No limi t Use of graphics 336 Pages 3-D graphic No. of real time trends 80 No. of historical trends 64 Instrument diagram window Not possible Yes Yes Alarm window No Yes Yes Trend window No Yes Yes Operator guide window No Yes Yes Dynamic window No Yes Yes Help window No Yes Yes Special display function No Yes Yes User window No Yes Yes Engineering keyboard Yes Yes Yes Voice alarm Yes Yes

236 INDIAN J. FIBRE TEXT. RES., JUNE 2004

one or the other can be handled by both. Manufacturers who had been making relay for logic and interlock applications developed PLC. Therefore, in the past it made good sense to use each type of controller in the area of superior experience. If the bulk of the I/O is digital, the logical choice is to use a DCS whereas if the I/O is mostly analog, a PLC system is selected. This logic while still valid to some extent is no longer universally true, and personal preference and end user familiarity have become decisive factors in system selection. In terms of pros and cons between the two designs, PLC I/O is likely to be more rugged and DCS system is likely to handle discrete logic faster than a PLC. DCS and PLC historically have functional differences although a DCS is meant for analogue loop control and PLC is meant for digital I/O or relay logic, today the lines between the various systems have blurred. A DeS can replace relay logic (digital). A PLC will implement analogue loop control. Some digital control systems do not fall into any existing specific category but functionally compete with the DCS or PLC, i.e. the pc-based control system. Features of PLC and DCS as per the study are given in Table 3.

Operations in DCS are quite handy. The alarm system relieves the attendant from scrupulous observation. The alarm also indicates the type and location of fault, which further eases the operations. In a PLC system, alarms are in the form of hooters and depending on the hooter alarm signal (noise) the operator does the corrective action. The store requirements, viz inventories, for DCS system are less than that for PLC system. In a DCS system, if fault occurs at a particular location it can be easily identified, whereas for a PLC the whole cabling has to be changed and controller setting has to be redone. Repair and maintenance in DCS has less failure risks while PLC demands more components during maintenance thus adding to the cost.

Table 3--Comparison between DCS and PLC Systems

Feature DCS PLC

Investment A T Cost of production T A Labor T A Operational ability Good Poor Stores and spares T A Down time T A Repair and maintenance T A Quality of product A T Accuracy A T Trouble shooting and startup Better Poor Space T A Historical logging and trending Good Poor Cabling T A

The DCS system gives more accurate and effective control over process parameters and/or variables especially at polymerization stage than that is possible wi th the PLC system. However, there is no direct effect on quality of POY yarn in case of both these systems. Thus, there is no apparent difference in quality of POY yarns. Loop control accuracy in DCS system is 0.8 whereas in PLC it is 1.2. Econo!TIic considerations of DCS and PLC are shown in Tables 4-6. Total cost for DCS and PLC systems using Foxboro 4.1 version for 150 tones/day (in Indian rupees) production is estimated to be: for DCS, 40 crores and for PLC, 30 crores; liable to change depending upon user specifications.

Table 4--Summary of cost estimate for DCS system (Foxboro 4.1 version) prepared for 150 tones/day of plant production

Parameter Quantity

Hardware Control unit 4 Data acquisition unit 2 Operator console 2 Hard copy device 4 Interface unit 2 Arching unit 2 Communications I 110 Card file 17 1I0'S (FBM) I Engineering station I Total hardware cost

Software Configuration Batch Total software cost

Other expenses Space Technical support Factory acceptance test Field installation and checkout Site acceptance test Documentation Project management Training (10 people)" Maintenance and repair (12 months) Total other expenses

Total DCS cost b

"Excludes travel and boarding.

Unit price Total (US$)

50,000 2,00,000 15,000 30,000 70,000 I AO,DOO 3,000 12,000 9,000 18,000 30,000 60,000 8,000 8,000

2470 42,000 30,000 30,000 91,000 91,000

6,31,000

1,65,DOO 2,55,000 4,20,000

10,000 12,000 18,000 9,600

12,000 6,000 31,200 6,000 60,000

1,64,800

12,15,800

bExcludes field instruments and cabling, sensors and activators, controller basically flapper nozzle assemblies and card control, cabling and wiring in the field bus, building and furniture, and pre-commissioning activities.

ITAGI el al.: TECHNO-ECONOMIC CONTROL SYSTEMS FOR POLYESTER MANUFACfURING 237

The difference in profit between DCS and non -DCS system is Rs 620/metric tones [Rs 6230(DCS)­Rs 561O(PLC)] of polyester. Thus, in case of DCS system, Rs 620 extra can be generated by saving in manufacturing. Profit difference per day will be Rs 620 x150=Rs 93,000/day. Considering 100 % capacity utilization of the polyester plant, profit difference/day is Rs 93,000 x 350 workdays, i.e. Rs 3,25,50,000/annum. Hence, Rs 3,25,50,000 is generated as saving through DCS system utilization for one year and for 3 years, it is Rs 3,25,50,000 x 3,

Table 5-Summary of cost estimate prepared for PLC system (Foxboro 4.1 version) for 150 tones/day of the plant production

Parameter

Hardware Control unit (2400) Frequency invertors Master control panel,(2) Printers 1400 AID and DI A Converters 80 Control panels Total hardware cost

Software PLC configuration Total software cost

Other expenses Integration space Technical support Factory acceptance test Labour Field installation and check out Site acceptance (SAT) Documentation and support systems Project management Training (20 people)" Maintenance Total other expenses

Total PLC cost b

"Excludes travel and boarding.

Price (US$)

74,000 1,20,000 20,000 30,000 80,000 3,24,000

2,40,000 2,40,000

23,000 50,000 18,500 30,000 20,000 8,000 16,000 35,000 10,000 70,000

2,80,000

8,44,000

~xcludes field instruments and cabling, sensors and activators, controller basically flapper nozzle assemblies and card control, cabling and wiring in the field bus, building and furniture, and pre-commissioning activities.

i.e. Rs. 9,76,50,000. Hence, Rs 10 crore extra invested in the installation cost can be recovered within a span of 3 years and a month. Each supplier of the DCS or PLC normally provides fully configured software after ascertaining various requirements of the user and hence these costs can be treated as plant specific. Allocation of labour for the two systems is given in Table 7.

For control room, polymerization and spinning, the DCS stations require -14 m2 while PLC stations require 32 m2 and the personnel required are plant and requirement specific as shown above.

Thus, there is considerable saving in labour cost with the DCS system when compared with the PLC system by reduction in the labour required for plant operation.

Table 6----Cost of production for metric tones of polyester (POY) [Based on 150 tones/day production, Prices in Indian rupees.l

Parameter DCS PLC

Raw material PTA 22,000 22,000 MEG 17,000 17,000 Additives 600 600 Spin finish oil 50 50 Processing charges Power and fuel 2340 2360 Electricity 3700 3800 Furnace oil 1330 1330 Miscellaneous 1800 1640 Stores and repairs 300 460 Packaging 700 700 Administration. selling and distribution Salaries, PF, Welf~ 3000 3400 Transport and handling 800 800 Taxes and rent 150 150

Cost of production 53770 53290 Selling price/tones 60000 60000 Profit/tones' 6230 5610

• Assuming that an increase in selling price and cost price wiD be proportional. The difference between the DCS and PLC profit would remain the same.

Table 7---Labour organization in polymerization, spinning and heat treatment media (HTM)

Parameter First shift

Control room 6E+ 120 Maintenance 4E+ 180 General 4E+400

E-Engineer, O-Operator and helper. Total Engineers: 32 (DCS) and 37 (PLC).

DCS Second shift

3E+80 3E+ 100 3E + 180

Total operators and helpers: 142 (DCS) and 184 (PLC).

PLC Third shift First shift Second shift Third shift

3E+80 8E+ 160 3E + 120 3E+ 120 3E + 100 5E + 200 3E+ 120 3E+ 120 3E + 180 6E+500 3E + 250 3E+250

238 INDIAN J. FIBRE TEXT. RES., JUNE 2004

6 Futuristic The future belongs to the fieldbus, ethernet, internet

and the very latest 'wireless' networking. Unfortunately, in developing countries like India where most of the modern plants are more than 20 years old, the absorption of new technology is not easy with the high initial investment. But the stakes are high too! The dividend in terms of energy savings, better producti vi ty, optimum use of human resources and profitability is too high to ignore. By adapting newer technologies, it could be possible to withstand

the international competition and move onto the new horizons of iudustrial prosperity.

Acknowledgement The authors are thankful to the management of

Indorama Synthetics (India) Ltd and Sanghi Polyester Ltd for their kind permission and active co-operation extended to the entire project team during their stay and study at their firms. They are also thartkful to Dr C D Kane, Principal, D.K.T.E'S Textile and Engineering Institute, for encouragement and support.