Retrofit Design oon Synthesis and P

* Siti Rafidah Ab. Rash* Faculty of Chemical Engineerin

Abstract — Due to the current worldwide enecrisis, all possible options of reducing the enare explored. This drives chemical processing energy-intensive consumers to strive for projects such as retrofitting the heat exchangeThe retrofit project is focusing on the rconsumption in the plant to its minimum en(MER). As a result of this, the plant enerincrease tremendously. In this work, pinexploited in analysing the potential energy sagain. As a case study of the work, a typical considered. The pinch calculation shows thsaving is 48,417 kW. This amount of energy 57% of the current hot and cold utility cindicates the high potential of attaining the eviolation of one of the pinch technology rule wabove the pinch is the reason of the excessive usthe analysis, there are two coolers situated abfour heat exchangers have low area efficiencplant heat exchanger network is retrofittedminimum number of network structures anmodifications. Minimal modifications are eeconomic feasibility. This is showed in the which it gives the payback period of the investm

Keywords - retrofit, heat exchanger networkmethanol production

I. INTRODUCTION One of the major challenges in produc

minimising the capital and operating costs. an issue especially for a large scale plants. of energy consumption also causes the increcost.

In order to reduce energy and thus opeproducers focus on efforts to increase efficiency. The precariousness of fuel penvironmental regulations are two majorealising the minimisation of production cost

Heat integration using the Pinch Tecrepresents a very important tool for oconsumption in existing industrial plantsthereby reducing the consumption of boutilities. Reducing utilities consumption not burning of fossil fuels, but also cuts carbon which are a primary cause of greenhouse

f Heat Exchanger NetwPurification Unit of Me

hid, *Ummi Kalthum Ibrahim and *Sakinah Mohg, Universiti Teknologi MARA, 40450 Shah Alam

ergy and economic nergy consumption plant as one of the energy efficiency

er network (HEN). reduction of heat nergy requirement rgy efficiency will nch technology is ving and economic methanol plant is

e potential energy accounts 74% and onsumptions. This

economic gain. The which is no cooling se of the utilities. In bove the pinch and cies. Therefore, the d by modifying a nd heat exchanger nsured to achieve economic analysis

ment by 0.4 years.

k, pinch technology,

ction industries is This has become The huge amount

ement of operating

erating cost many the plant energy price and tighter or constraints in t.

chnology concept optimizing energy s via retrofitting, oth hot and cold only decreases the dioxide emissions effect. Therefore

energy saving in process planwarming control [1].

Pinch Technology is a systand retrofit or improvement optimisation. In this work, thito achieve its objectives. Itechnology was developed to imin grassroots HEN design. Tecon pinch analysis was first int[2].

In principle, retrofit describto allow modifications to takactivity, most of the time, is toor parts. In this work, the propassembly of HEN on Synthetypical methanol plant and it isconstraints.

The objective of this worenergy saving via establishmetargets of the existing unit effectively.

II. APPLICATION OF PINCHMETHAN

A typical methanol plant units namely as feed purificatioreforming, synthesis gas compcrude methanol distillation.

Natural gas, the feedstocksteam and converted to synthnickel catalysts. The reformedmixture of hydrogen, carbon oxthen cooled to ambient temperasynthesis gas is recovered by feed water) preheating, heatidistillation section and by demi

Figure 1. Methanol pro

work (HEN) ethanol Plant

hd Alauddin m, Selangor, Malaysia

nts can be related to the global

tematic technique for the design of process systems for energy is methodology is implemented In the beginning, the pinch mprove the utilization of energy chnique for HEN retrofits based troduced by Tjoe and Linnhoff

bes an activity taken in industry ke place in process plant. The o install or uninstall assemblies posed retrofit will focus on the

esis and Purification Unit of a s subject to the existing process

rk is to establish the potential ent of maximum heat recovery and to retrofit the HEN cost

H ANALYSIS FOR RETROFIT OF A NOL PLANT

consists of several processing

on, syngas synthesis unit, steam ression, methanol synthesis and

k is desulphurised, mixed with hesis gas in the reformer over d gas at the reformer outlet is a xides and residual methane. It is ature. Most of the heat from the steam generation, BFW (boiler

ing of the crude methanol at ineralised water preheating.

oduction block diagram

2011 IEEE Colloquium on Humanities, Science and Engineering Research (CHUSER 2011), Dec 5-6 2011, Penang

978-1-4673-0020-9/11/$26.00 ©2011 IEEE 33

Also, heat from the flue gas is recovered by feed-steam preheating, steam generation and superheating as well as combustion air preheating. After final cooling, the synthesis gas is compressed to synthesis pressure before entering the synthesis loop. The synthesis loop consists of a recycle compressor, feed/effluent exchanger, methanol reactor, final cooler and crude methanol separator.

Crude methanol, which is condensed downstream of the methanol reactor, is separated from unreacted gas in the separator and routed to the crude methanol distillation. Water and minor quantities of by-products formed in the synthesis and contained in the crude methanol are removed by distillation system [3].

Among all the unit, it is known that the methanol synthesis and crude methanol distillation units are very crucial and are therefore become the focus of this study.

III. DATA COLLECTION

A. Thermal Data Data extraction is the most crucial part of a process

integration study. It relates to the extraction of information required for Pinch Analysis from a given process. The minimum data to be collected are material and energy balances data, physical and chemical properties of the materials and costing data that is used to estimate the capital investment and payback period. Most of the data are obtained from the process flow diagram (PFD) available. The plant’s capacity is 1.7 metric tonnes methanol per annum. Tables I and II below show the extracted data of hot and cold streams.

All heat exchangers with continuous flow are then selected and their operational and geometrical features should be known. The geometric data will be necessary when calculating heat transfer coefficients used during the synthesis and validation of the heat exchanger network.

These data should be validated either by calibrated instrumentation in plant or using reconciliation tools available in the market. Data reconciliation is a technique used to improve the quality and accuracy of measurements by manipulating the uses process model constraints ie. mass and energy conservation laws and obtains estimates of process variables by adjusting the process measurements to satisfy the constraints [4].

TABLE I. HOT STREAM DATA

Stream No.

Tsupply (K)

Ttarget (K)

Heat Capacity Flow-rate (kW.K)

Heat Duty (kW)

H1 493 430 473.94 29858 H2 430 408 797.41 17543 H3 493 430 172.03 10838 H4 343 313 952.37 28571 H5 420 388 1046.88 33500 H6 447 420 2122.07 57296 H7 401 400 112456.00 112456 H8 342 314 272.07 7618 H9 401 313 84.07 7398

TABLE II. COLD STREAM DATA

Stream No.

Tsupply (K)

Ttarget (K)

Heat Capacity Flow-rate (kW.K)

Heat Duty (kW)

C1 330 367 474.14 17543 C2 C3

366 366

367 367

0 33500.00

0 33500

C4 356 393 2.70 100 C5 413 414 57296.00 57296 C6 413 414 65384.00 65384 C7 391 392 112456.00 112456

Stream C2 is referring to a stream which has no duty. This is because the stream is a stream which only required or functioning during the start-up period or abnormal cases. Thia is a Pre-run Column Steam Reboiler and being heated by LP (low pressure) steam. In normal operation, the reboiler is bypassed.

B. Cost Data The cost data is obtained from the PFD and plant cost data

of the designed process. The hot utilities used in this methanol production plant is LP steam at 16 barg and the cold utility is supplied by water recirculated from sea water, cooling tower and boiler feed water. As for the initial work, the cost data shown in Table III obtained from Al-Riyami et al. [5] is used.

TABLE III. UTILITY COST DATA Utilities Medium Heat Duty

(kW) Cost

(RM/kJ) Hot Utility LP Steam 65,484 1.197E-5

Cold Utility Seawater 28,571 1.397E-5

Cooling water 15,016 5.134E-6 Boiler Feed Water

(BFW) 40,696 1.313E-5

IV. ANALYSIS OF THE EXISTING HEAT EXCHANGER NETWORK

From Tables I and II, the existing heat exchanger network

(HEN) is found to have nine hot and seven cold streams. Figure 2 shows the grid diagram for the existing HENs. There are a total of 12 heat exchangers in the existing process. This includes four process-to-process heat exchangers, five coolers (C) and three heaters heated by LP steam. The existing hot and cold utility requirements are 65,484 kW and –84,283 kW respectively, with a total heat recovery of 220,795 kW by process-to-process heat exchangers.

The minimum temperature difference (ΔTmin) of the existing HEN is 7°C. The types of heat exchangers used in the synthesis and purification units are floating head heat exchangers. This is due to its versatility and the ease of cleaning and inspection.

2011 IEEE Colloquium on Humanities, Science and Engineering Research (CHUSER 2011), Dec 5-6 2011, Penang

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Figure 2. Existing heat exchanger network (HEN)

Figure 2 shows that two coolers exist above the pinch in the current synthesis and purification unit layout, indicating that the fundamental rules for maximum heat recovery have been violated (refer to streams H1 and H3). Looking at the difference between the targeted and the actual utility usages, clearly, there are opportunities for further heat recoveries. However, for a retrofit problem, it may not be possible to achieve the minimum utility targets without incurring significant modifications on the existing plant [6].

V. HEAT EXCHANGER NETWORK IMPROVEMENT

A. Retrofitting The Problem Table, developed by Linnhoff and Flower in

the year of 1978, shows the relevant process streams data to the energy analysis in an organized table format. These data are used to generate the energy balance to apply Pinch Technology [7].

Composite curves of hot and cold streams were constructed from the stream data from Tables I and II as shown in Figure 3. The figure shows that the system has a minimum hot utility (QHmin) target is 17,067 kW, while 35,866 kW for the minimum cold utility (QCmin). For ΔTmin of 7°C, the pinch point temperatures were located at 63°C or 336K (cold) to 70°C or 343K (hot).

Figure 3. Composite curves for synthesis and purification unit of methanol plant

From the analysis, the amount of hot utility saving is up to 74% while the cold utility is 57%. The large opportunities of saving justify the retrofitting work for the synthesis and purification unit of methanol plant.

Additional heat may be recovered through integration of streams H3 and C6, H3 and C6, and also H9 and C6. The matches fulfilled the matching rule in pinch technology. These new matches reduced the current utility consumptions from 65,484 kW and 84,283 kW to 17,067 kW and 35,866 kW for hot and cold utility respectively. These reductions are as per the target values in the composite curve as shown in Figure 3.

Figure 4. New Retrofitted Heat Exchanger Network

Figure 4 is the new retrofitted heat exchanger network which shows three new process-to-process heat exchangers need to be installed to achieve the minimum utility targets. As mentioned earlier, stream C2 is a stream that is required during start-up period. Therefore the load or duty is zero during normal operation. In short, the physical heat exchanger for stream C2 is not available for heat integration or retrofitting. The existing heater is remained unaffected.

In Figure 4, there are three new process-to-process heat exchangers. The corresponding heat exchange areas are calculated for the use of capital cost investment calculation [8]. The estimated value of the overall heat transfer coefficient obtained from literature is about 1500 W/m2/K [9].

TABLE IV. NEW HEAT EXCHANGERS DESIGN DATA Heat

Exchanger No.

Temperature (K) Heat Duty

(kW)

Area (m2)

Thi Tho Tci Tco ΔTlm

E1 493 430 413.6 413.7 10.6 10,838 680.4

E2 493 430 412.1 413.6 16.3 29,858 1221.9

E3 401 343 413.0 413.1 9.2 7,398 533.7

Table IV shows the design data for the new heat exchangers in the network. These heat exchangers are the results of the new retrofitted HEN.

C

C

C

C

C

H

H

H

473.9

CP (Kw/K)

797.4

172.0

952.4

1046.9

2122.1

112456

272.07

84.1474.1

0

33500

2.7

57296

65384112456

H1

H2

H3

H4

H5

H6

H7

H9

H8

C2

C3

C4

C5

C6

C7

493 430

430 408

493 430

420 388

447 420

401 400

367 366

367 366

393 356

414 413

414 413

392 391

367

401

342

343

343 336

29858

10838

33500

57296

112456

0

100

65384

28571

7618

7398

17543

Legend:

H

C

Integrated HE

Heater

Cooler

C

C

H

H

H

1

1

473.9

CP (Kw/K)

797.4

172.0

952.4

1046.9

2122.1

112456

272.07

84.1474.1

0

33500

2.7

57296

65384112456

H1

H2

H3

H4

H5

H6

H7

H9

H8

C2

C3

C4

C5

C6

C7

493 430

430 408

493 430

420 388

447 420

401 400

367 366

367 366

393 356

414 413

414 413

392 391

367

401

342

343

343 336

7398

10838 17543

112456

0

100

413.7

28571

7618

29858

2

2

33500

3

3

57296

Legend:

H

C

New HE

Existing HE

Heater

Cooler

2011 IEEE Colloquium on Humanities, Science and Engineering Research (CHUSER 2011), Dec 5-6 2011, Penang

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B. Capital Cost The total heat transfer area calculated is 2,435 m2. By using (1) [10], the estimated purchased heat exchanger cost is RM 4,294,690. Using a module factor of 3.29, the total installed heat exchanger cost calculated using (2) is RM14.13 million.

HE cost (RM) = (33,422 + 1784 × Area(m2)0.81).(CF) (1)

Total cost = Module factor x Equipment Cost (2)

C. Comparison Study Equation (3) is used to calculate the cost savings of

utilities based on the cost data in Table III. From Tables V and VI, the total estimated cost savings is RM 38.9 million per year.

For cold utilities, the total cooling water and boiler feed water savings is about 57% or RM17.9 million per year. The total hot utility savings is about 74% or RM 21.0 million per year. This gives a payback period of 0.4 year calculated using (4). Thus, it can be concluded that the HEN retrofit based on pinch design approach is an effective means for a process plant to further reduce energy consumption.

Cost saving of utilities (RM/year) = cost of utilities (RM/kJ) × 360 days × 86400 s/day × Heat load reduction (kW) (3)

PBP = (4)

TABLE V. SAVINGS IN COLD UTILITY

Utilities Medium Utility consumption before retrofit (kW)

Utility consumption after retrofit (kW)

Percentage of

savings (%)

Cost Savings

per year (RM)

Cold Utility

Seawater 28,571 28,571 0% 0

Cooling water

15,016 7,618 51% 1.23 mil

Boiler Feed

Water

40,696 0 100% 16,62 mil

Total cold

utilities

84,283 64,437 57% 17.85 mil

TABLE VI. SAVINGS IN HOT UTILITY

Utilities Medium Utility consumption

before retrofit (kW)

Utility consumption after retrofit

(kW)

Percentage of savings

(%)

Cost Savings

per year (RM)

Hot Utility

LP Steam

65,484 17,067 74% 21.0 mil

VI. CONCLUSION

Pinch retrofit method has been applied to synthesis and purification unit of a methanol plant. The result shows that the retrofitted heat exchanger network is able to recover up to 57% and 74% of cooling and heating respectively. Meanwhile, the capital investment required for the project is about RM 4.3 million. The total annual savings in utility consumption is RM 38.9 million, giving the payback period for the investment of 0.4 years.

NOMENCLATURE ΔTmin Minimum temperature difference C Cooler Ci Cold stream CF Conversion Factor H Heater HE Heat exchanger HEN Heat exchanger network Hi Hot stream LP Steam Low pressure steam PBP Payback period QCmin Minimum cold utility QHmin Minimum hot utility Tci Cold stream temperature inlet Tco Cold stream temperature outlet Thi Hot stream temperature inlet Tho Hot stream temperature outlet Tlm Log mean temperature Tsupply Supply temperature Ttarget Target temperature

REFERENCES

[1] Souza J. N. Et. Al (2005), “Energy Integration – An Example in a Retrofit of a Petrochemical Plant”, 4th Mercosur Congress on Process Systems Engineering.

[2] Tjoe, T. N. and Linnhoff, B. (1986). “Using Pinch Technology for Process Retrofit”. Chemical Engineering Journal. 93: 47–60.

[3] “ Methanol Process Description,” Available: http://www.uhde.eu/competence/technologies/gas/techprofile. [Accessed: Jul, 2011]

[4] Siti Rafidah A.R., et.al (2009), “Data Monitoring and Reconciliation for Refinery Hydrogen Network”, International Seminar on Advances in Renewable Energy Teknology, 24-32.

[5] Al-Riyami, B. A., Klemeš, J. and Perry, S. (2001) “Heat Integration Retrofit Analysis of a Heat Exchanger Network of a Fluid Catalytic Cracking Plant”, Applied Thermal Engineering. 21: 1449–1487.

[6] Sharifah Rafidah W. A. et.al, (2009), “Cost-Effective Retrofit of a Palm Oil Refinery Using Pinch Analysis”, Jurnal Teknologi. 29-40

[7] Linnhoff, B., J.R. Flower (1978). “Synthesis of Heat Exchanger Networks. Part I: Systematic Generation of Energy Optimal Network”s. AIChE, J., 24 (4), 633.

[8] Cheresources.com. Chemical Engineering Tools and Information. Assessed on July 2011. http://www.cheresources.com.

[9] Incropera, F. & Dewitt, D. (2003). “Fundamental of Heat and Mass Transfer”, Ed. LTC.

[10] Guthrie, K. M. (1996). “Capital Cost Estimating”. Chemical Engineering. 76: 114–142.

2011 IEEE Colloquium on Humanities, Science and Engineering Research (CHUSER 2011), Dec 5-6 2011, Penang

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