Cross-unit Thermal Integration With Hot-oil Loops - Polley 2013

UKHTC2013/Please paste your Paper No here

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13th UK Heat Transfer Conference, UKHTC2013

2 - 3 September 2013 Imperial College London

THERMAL FLUIDS: THE KEY TO IMPROVING THE

ENERGY EFFICIENCY OF LARGE CHEMICAL SITES

G.T.Polley1 , M. Picon Nunez and E.Tamakloe

1 Department of Chemical Engineering, University of Guanajuato, Mexico

Abstract

Large factories often contain a mix of processing plants that are in energy surplus (so only consume cold utility) and

plants that have an energy deficit (and demand hot utility). For instance, a factory that produces soaps and

detergents may produce its own sulphuric acid in a highly exothermic reaction. Heat generated in this process could

be used to drive distillation columns used in plants producing surfactants.

Process integration technology has developed a tool, given the name site composites, for the analysis of such

factories. However, in applying this tool it is often assumed that subsequent energy recovery will be undertaken

using steam-mains.

The capital cost of a site energy recovery system is dominated by piping cost with installed cost of piping being

proportional to the diameter raised to a power 3. Since the economic diameter of a pipe used to transport steam is

very much larger than that required to transport a liquid, the use of thermal fluids (e.g. Dowtherm, or pressurised

water) could lead to substantial capital cost savings.

Furthermore, where a factory has heat demands that are at significantly different temperature levels the use of a

steam main can prove impractical. This is not the case with a thermal liquid system.

This paper addresses the issues involved in cross-site integration and presents a case study using a description of a

plant taken from the literature.

Keywords: process design, capital cost saving, energy efficiency, process integration, heat recovery

1 Introduction

The purpose of this paper is to look at the considerations that need to be made when evaluating how

the energy efficiency of factories can be improved through cross-site thermal integration and to

demonstrate how thermal fluids could provide a cost effective means of achieving this. A thermal

fluid may take the form of a specially prepared hydrocarbon (e.g. Dowtherm) or a simply be water at

high pressure.

Investment capital is a limited resource. Piping costs form a major component of investment needs for

cross-site integration projects. Here, the use of thermal fluids has a major advantage.

Obviously the cost of piping increases with increasing size. The size of pipe required to transport a

liquid is generally much smaller than that required to transport a gas. So, investment costs could be

reduced substantially if thermal fluids are used for cross-site heat transmission.

Our consideration of cross-site integration starts with a consideration of the design of a new factory.

This is followed by a consideration of how cross-site integration energy can be used to save energy in


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an existing factory. Finally, we consider the case of energy saving in a Fluidised Bed Catalytic

Conversion that has been the subject of previous discussion [1,2].

2 Utility Profiles

The tool developed for cross-site integration has been given the name Site Composite Curves.

These are derived from the analysis of the Grand Composite Curves of the individual processes

operating within the factory.

The Grand Composite Curves indicate the temperature levels at which heat is in deficit (Figure 1) or

at a surplus (Figure 2). Where a surplus covers a deficit the deficit can be satisfied using a heat

recovery network.

Figure 1. Demand for Heat Figure 2. Surplus Heat

Figure 3. Utility Demand Profiles

Removing the pockets from the analysis (as proposed by Klemes et al [3]) we have two individual

utility profiles. Above the pinch we observe how hot utility is demanded at specific temperature

levels. Below the pinch we see how cold utility is demanded at specific temperature levels. If the

pinch point for one part is higher than that of another the cold utility transferred from that plant can

be used to satisfy part or all of the hot utility demand in the other.

Site Composites are built up by combining the profiles of the individual plants. The temperature axes

used in the Grand Composite Curve are so called interval temperatures where the temperature of the

hot streams have been reduced by half of the minimum temperature approach assumed for the heat


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recovery and the cold streams have been increased by half of the minimum temperature approach

assumed for the heat recovery. For cross-site integration the use of real (rather than interval

temperatures) is more logical. So, the utility profiles are best converted to real temperatures by

increasing the temperatures on the hot utility line by half of the assumed temperature approach abd

decreasing those on the cold utility line by this quantity.

Figure 3. Site Composites Figure 4. Integration Using Steam Mains

If steam generation is the chosen means of transferring heat across the factory we observe that for this

set of site composites no fewer than five individual mains would be required.

In Figure 5 we show how a thermal fluid could be used. The full load could be transferred using a

single recirculation loop. However, temperature approaches would be less than the specified minimum

at several locations within the system. This can be resolved by using two individual recycle loops

(Figure 6) or by using a single oil loop and a single steam main (Figure 7).

Figure 5. Single Loop: Thermal Fluid Figure 6. Two Loop System

Figure 7. Thermal Fluid plus Steam Main

3 Piping Costs

The quantity of metal used in a straight length of pipe can be shown to vary with the square of the pipe

diameter. However, bends, tee junctions, valves and most pipe fittings vary with the cube of pipe


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diameter. For many projects the relationship between pipe size and cost will be closer to cubic than

square. So, a 200mm pipe used to transport steam will be over fifty times more expensive than a

50mm pipe used to transport a liquid.

In 2006 the supply cost of 200mm pipe components were around the values listed in Table 1.

Installation costs (fitting to existing pipe-bridge) are listed in Table 2. The capital cost index for 2006

is 500. That for 2011 is 586.

Component Cost

Straight Pipe 37.5 GBP/meter

90 degree bend 50 GBP

Gate Valve 850 GBP

Bolted Connection 60 GBP

Welded Flange 20 GBP

Table 1. Supply Costs

Component Cost

Straight Pipe 46.3 GBP/metere

90 degree bend 180 GBP

Gate Valve 210 GBP

Bolted Connection 90 GBP

Welded Flange 95 GBP

Table 1. Installation Costs

The amount of metal used in a straight pipe is varies (approximately) with the square of the pipe

diameter. That used in piping components varies with the cube of the pipe diameter. These

relationships were found to correlate cost data (both supply and installation) for other pipe sizes.

Using the shell-and-tube heat exchanger purchase cost algorithm published by Saunders [4] a

comparison can be made between heat exchanger and piping costs. A typical 100m run of 200mm pipe

was equivalent to an additional one square metre of surface in a typical industrial heat exchanger.

Figure 8. Mapping of Pipe Bridges

X

Y

1

7

2

8

3

11

9

5

10

6

412

13


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Use of a 50mm pipe, rather than a 200mm pipe, would be equivalent to an additional 50 square metres

of surface in a typical industrial heat exchanger.

The first step in the consideration of piping needs is the mapping of the position of the existing pipe

bridges using a set of three dimensional co-ordinates (Figure 8). Pipe lengths and needs for fittings

such as bends and tees can be determined from this map.

4 Examination of Differing Plant Combinations

The length of piping required to transfer heat between individual plants is obviously a function of

geographical location. Therefore, the closer the plants the higher the justified heat transmission. Rather

than solely look at the whole factory the analysis should also be applied to differing combinations of

plant.

This analysis does not require specialist software. A spread-sheet can be developed for this purpose.

This could contain the following:

1. Temperature-enthalpy information for all of the process streams present in a given zone

2. Information relating the distances between individual zone limits

3. A variable that can be used to specify if a given zone is to be included in the analysis

4. Pipe cost equations

5. Size of pipe to be used to link the individual zones

5 Use of Existing Process Flows

Steam pipes are not the only pipes carry fluids between individual plants. Products and recovered feed

components are also transferred between geographical zones. In some situations there will be benefit

in adjusting the temperature at which the transfer is made in order to reduce the energy consumption in

one of the zones. Tamakloe & Polley [5] presented that allows the engineer to quickly determine how

transfers between zones are best undertaken.

6 Utility System Stability

Plant throughput can vary on an hourly basis The variation of feed to two individual crude distillation

units encountered in one large refinery is displayed in Figure 9. This resulted in changes in steam

generation (Figure 10).

0

100

200

300

400

500

600

700

5/1/01 6/8/01 7/16/01 8/23/01 9/30/01 11/7/01 12/15/01 1/22/02 3/1/02 4/8/02

Flow,

MBD

C2+ Feed

C3+ Feed

Combined Feeds

Figure 9. Throughput Changes


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Boiler Steam Gen

0

500

1000

1500

2000

2500

4/1/2001 5/21/2001 7/10/2001 8/29/2001 10/18/2001 12/7/2001 1/26/2002 3/17/2002

Total

HP

Stm

Gen,

Klb/h

Figure 10. Changes in Steam Demand

This situation leads to individual steam generators being regularly taken on and off stream. This is

expensive and inefficient. It also leads to variations in fired heater feed temperatures. When these

temperatures fall the rate of fouling within the fired heaters increases with further losses in site energy

efficiency and increased generation of both carbon dioxide and NOx.

Site stability and fired heater inlet temperatures can be maintained if heat storage is used to cope with

demands resulting from throughput variation. Steam cannot be stored. However, it can be used to heat

thermal fluid which is easily stored.

7 Case Study

We now consider the case of energy saving in a Fluidised Bed Catalytic Conversion that has been the

subject of previous discussion [1,2]. The Process Flow Diagram for the plant is shown in Figure 9.

The plant is large and can be considered to be in two separate geographic zones: fractionation zone

and gas processing zone. The fractionation zone contains the main distillation column and the

catalysed bed reactor. The gas processing zone contains the sponge absorber and all of the other

distillation columns.

The energy consumed in the fractionation zone is just 3.2 MW. That consumed in the gas processing

zone is 74.1 MW. When energy analysis of the full system is undertaken (assuming a minimum

practical temperature approach for heat recovery of 10 C) it is found that the plant requires 36 MW of

heat. Clearly there is significant opportunity to utilise heat currently rejected to the environment in the

fractionation zone to drive some of the distillation columns situated in the gas processing zone. The

scope for additional heat recovery is 74.1+3.2-36 = 41.3 MW.

(Since, flue gas is the utility being used in the fractionation area another possible energy saving

opportunity arises. The flue gas leaves the heater at high temperature (over 360 C). This could be

matched against thermal fluid in a new heat recovery unit and transported to the gas processing zone.)

The Grand Composite Curve for the Fractionation Zone is shown in Figure 12. That for the Gas

Processing Zone is presented in Figure 13.

We observe that 40 MW of heat can be transferred by raising steam in the fractionation zone and using

this to drive the high temperature re-boiler in the gas processing zone. The existing re-boiler is already

driven using steam, so the required modification (which would be checked) is likely to be the

installation of the new steam pipe into the existing steam supply.

An alternative would be to use a thermal fluid. There would be significant savings in terms of piping

cost. However, the practicality of using the existing re-boiler would need to be checked. If the re-

boiler was a horizontal unit (steam being condensed within the tubes) then it may be possible to use

thermal fluid on the tube-side (with heat transfer being enhanced through the installation of tube

inserts).


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Figure 11. Process Flow Diagram of FCC Plant

Figure 12. GCC: Fractionation Zone Figure 13. GCC: Gas Processing Zone

Finally, we observe that there are additional re-boilers operating in the gas processing zone. These

operate at the following interval temperatures:

Re-boiler B 130 C Re-boiler C 110 C Re-boiler D 90 C

These units have a combined load of 25 MW.

Assume that this heat can be provided by cross-site integration.

Factors that need to be considered when integrating re-boilers have been discussed by Polley et al [5].

One important factor is the temperature at which heat is provided to the re-boiler. If this is too high

both the thermal performance and operational stability can be adversely affected. One means of

controlling the local temperature within a supply loop is to recycle cooled fluid.


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5 Conclusions

In many factories there is significant scope for improving energy efficiency by transporting heat across

the site. There are several ways in which this can be achieved and the designer should be aware of all

of them.

Capital cost considerations are often an over-riding factor. Suitable cost equations exist for the supply

and installation of pipework are available.

Previous approaches have appeared to favour the use of steam mains to cross-integrate plants.

However, greater transfer of heat could be achieved through the use of a thermal fluid. This results

from a better temperature profile (a sloped profile replaces a number of isothermal steam levels) and

capital cost savings associated with smaller pipe diameters.

Capital cost considerations favour integration of neighbouring plants. Again, in some cases a steam

main can be used and this is particularly cost effective if equipment does not need to be modified.

However, even in this situation use of a thermal fluid should be considered for it is often possible to

modify existing exchangers at relatively low cost.

The conditions of the utility (steam or thermal fluid) and the resulting behaviour of heat exchangers

should be considered when evaluating integration opportunities. Super-heated steam should not be

used. The temperature at which thermal liquid enters a re-boiler can have adverse effect on re-boiler

performance.

References

[1] Al-Riyami B.A., Klemes J & Perry S., Heat integration retrofit analysis of a heat exchanger

network of a fluid catalytic cracking plant, Appl.Thermal Engng. 2001, vol 21, pp1449-1487

[2] Polley G.T. & Kumana J.D. Energy saving retrofit of an FCC Plant, paper presented at NPRA

Annual Meeting, 2007, San Antonio, Texas

[3] Klemes J., Dhole V.R., Raissi K., Perry S.J. & Puigjaner L, Targeting and design methodology

for reduction of fuel, power and CO2 on total sites, Applied Thermal Engineering, 17,993f,1997

[4] Saunders E.A.D., Heat Exchangers, Longman Scientific & Technical, Harlow, Essex, UK,

1988

[5] Tamakloe E.K. Simple Procedure for Determining the Scope and Means of Transferring

Heat Between Zones of Integrity, paper presented at AIChE Spring Meeting, 2011

[5] Polley G.T., Vazquez Ramirez E.E., Riesco Avila M., & Jantes Jaramillo D. Thermal Integration

of Re-boilers, Chemical Engineering, April 2012

Acknowledgement

We would like to acknowledge the large contribution made by J.D.Kumana to this work. The insight

he provided to utility system behaviour and industrial needs was invaluable.

Documents

Cross-unit Thermal Integration With Hot-oil Loops - Polley 2013