Retrofit of the Heat Recovery System of a Petroleum Refinery Using Pinch Analysis

doi:10.4236/jpee.2013.15006

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Journal of Power and Energy Engineering, 2013, 1, 47-52

http://dx.doi.org/10.4236/jpee.2013.15007 Published Online October 2013 (http://www.scirp.org/journal/jpee)

Retrofit of the Heat Recovery System of a Petroleum

Refinery Using Pinch Analysis

John M. Joe, Ademola M. Rabiu

Department of Chemical Engineering, Cape Peninsula University of Technology, Cape Town 8001, South Africa.

Email: [email protected]

Received September 2013

ABSTRACT

Energy efficiency has become an important feature in the design of process plants with the rising cost of energy and the

more stringent environmental regulations being implemented worldwide. In South Africa, as elsewhere, most process

plants built during the era of cheap energy place little emphasis on the need for energy recovery due to the abundance of

cheap utilities sources such as co al. In most of these plants, there exist s ignificant potential for substantial process heat

recovery by conceptual design of the heat recovery system. By maximizing heat recovery from the processes, there will

be a reduction in the process utilities requirement and the associated environmental effects. Pinch analysis has been

demonstrated to be a simple but very effective tool for heat integration and optimization of chemical plants. This study

uses the pinch principle to retrofit the heat exchanger networks (HEN) of the crude distillation unit of an integrated pe-

troleum refinery to evolve a HEN that features optimum energy recovery. The network was further relaxed by trading

off energy cost with capital cost to obtain an optimal HEN topology not too different from the existing network. The

simulation works were implemented in AspenPlus v8.0 environment. Analysis revealed that 34 per cent saving on

energy usage per annum is realizable. This significant saving in energy also results in diminished gaseous pollutants

associated with energy usage.

Keywords: Plant Retrofit; Heat Integration; Crude Distillation Unit; Heat Exchanger Network; Pinch Analysis;

Optimization; Remaining Problem Analysis

1. Introduction

The issues of energy sustainability and security [1] and

the increasingly stringent environmental regulations have

combined to elevate the challenge of energy efficiency to

a high-priority issue [2] for energy intensive industries. It

was reported that the Chinese petroleum refining industry

consumes approximately 15% of industrial fuel oil and

10% of industrial coal [3]. Plants retrofitting towards

cost-effective energy reco ver y and h ence improv ed energy

and utilities usage become attractive and are being im-

plemented worldwide. It has become imperative to im-

prove on the overall economic and environmental foot-

prints of existing plants to enhance their competitiveness

[3] as well as sustainability. The ultimate aim is to syn-

thesize a process that is cost-effective and environmen-

tally benign [4]. It is expected that many process plants

in South Africa have inefficient heat recovery system.

Before the energy crisis, the chemical industries saw lit-

tle use of heat integration to reduce energy usage, since

energy (particularly from fossil fuels) was relatively in-

expensive and abundant [5]. Little emphasis was placed

on optimal process heat recovery because of the cheap

availability of utilities sources at the time of plant con-

struction. The sharp rise in fuel price, dwindling fossil

fuels reserves, and the growing awareness of the envi-

ronmental problems associated with fossil fuels consump-

tion, have however combined to drive the impetus for

design of energy ef ficient pla nt [6]. Ana lyse s of the energy

recovery system of plants design with rule of thumbs have

revealed potential significa nt saving s in utilities usage [7,

8] by placing better matches between the process streams.

A design that maximizes energy recovery using heat ex-

chan ger n et wor ks t o mat ch t he p roc es s hot and cold streams,

will result in minimal external heating and cooling re-

quirements to meet the energy needs of the plant. For

instance, Kemp [9] reported a potential saving of between

30 to 100 per cent in utilities usage in various units of a

petroleum refinery.

This study is motivated by the need to improve on the

energy efficiency of petroleum refineries designed with

rule of thumbs available at the time, by modifying the

existing heat exchanger networks and hence saving on

utilities usage and the emissio n of gre enhouse ga ses from

fossil fuels consumption. The Crude Distillation Unit (CDU)

Retrofit of the Heat Recovery System of a Petroleum Refinery Using Pinch Analysis

[10] consumed the most energy in a refinery, consuming

as much as 2 per cent of the total crude oil processed [11].

It is therefore expected that the use of PT to analyze the

energy recovery system of the CDU will reveal better

matches for process energy transfer, and hence saves on

utilities usage and the associated environmental benefits,

for instance, the emission of obnoxious pollutants. It in-

vestigated the potential saving in utility usage in the crude

distillation unit of a petroleum refinery. A preliminary

energy audit of the plant was carried out using AspenPlus

v8.0 to locate the unit with highest utility requirement

and offers the most potential energy savings. The Re-

maining Problem Analysis (RPA) variant of the Pinch

Technology was applied to locate and reassigned only

the heat exchangers that are inefficiently placed.

2. Process Heat Int eg ration

A design of an efficient and cost effective process heat

recovery system employing series of heat exchangers will

promote efficient utilities usage and requirement leading

to savings in energy cost [12]. The practical importance

of HENs can be found in the fact that most industrial

processes inv olve transfer of heat, either from one process

stream to another process stream or from a utility stream

to a process stream [13,14]. Consequently, the target in

any industrial process design is to maximize the process -

to-process heat recovery and to minimize the utility re-

quirements. To meet this goal, cost-effective HEN con-

sisting of one or more heat exchangers that collectively

satisfy the energy conservation task, is of particular im-

portance.

The use of Pinch Technology (PT) for retrofitting and

grassroot designs, has been found to result in considera-

ble saving for instance in energy usage. This saving di-

rectly improves the economics of the plant. Pinch tech-

nology is one of the least complicated and most effective

technologies for Heat Exchanger Networks (HEN) de-

within the chemical plant. It is based on sound thermo-

dynamic principles without including heavy mathemati-

cal calculations and interpretations. The use of pinch tech-

nology for plant modification is centered on the trade-off

between the savings in utilities versus the cost of the

prop osed c hanges in the plant . The “pinc h ” point represents

the bottleneck of heat recovery. The key concept of pinch

analysis is the setting of energy targets [9] with the aim

to achieve maximum energy saving by maximising process

to process heat recovery and minimising the use of hot

and cold utilities.

3. Methods

3.1. Data Extraction and Energy Target

The design and operating data of the refinery unit were

obtained. A detailed modelling and simulation of the unit

was carried out in AspenPlus v8.0 environment for con-

vergence test to reconcile the streams enthalpy data. The

enthalpy data required were extracted. This was done in

order to scope the existing network for potential energy

and cost savings. Thermodynamic profiles of the process

streams using the Problem Table algorithms, Composite

Curves and Grand Composite Curves were studied to

determine the targets for the hot and cold utilities and the

position of the pinch. This profile also indicated the

maximum energy recovery possible at the chosen ∆Tmin..

3.2. Maximum Energy Recovery Network Desig n

The existence of a topology trap was first investigated by

studying the influence of ∆Tmin on the utilities require-

ment. This is also used to obtain an optimum ∆Tmin for

the retrofitting study.

To achieve the target obtained above, a new heat ex-

changer network featuring maximum energy recovery

(MER) was obtained using the remaining problem analy-

sis approach as developed by Tjoe and Linnhoff [15].

Retroffiting of HEN is agreed upon (for instance [5]) to

be more tasking than grassroot design. For the results of

the retrofit project to be implementable, the resulting

networks while featuring significant energy savings must

not be markedly different from the existing networks.

Otherwise the modification required will be major and

costly offsetting the targeted saving in energy cost. Re-

main problem analysis as a technique for retrofitting of

the HEN is an essential principle to develop an optimized

HEN close to the existing plant topology. This is neces-

sary because of layout considerations and the cost impli-

cations of matches.

The methodology will keep as much as possible to the

existing topology of the plant, meaning the exchangers

that did not violate the pinch were left untouched. This

entails identifying the heat exchangers working across

the pinch and hence inefficiently placed or streams that

were inefficiently matched while leaving the other heat

exchangers intact. The objective of the evaluation of the

existing network was to use the existing area within this

network more effectively. This was done to identify the

area used due to criss-crossing [15]. The new network

features maximum energy recovery.

3.3. MER Network Relaxation

An optimization of the process was done for possible

network relaxation. A trade-off looking at the existing

process and the MER design was carried out focusing on

the investment and energy costs. The MER network was

examined for possible network simplification, by identi-

fying and removing loops to trade-off capital cost of the

required heat transfer unit and energy cost of the saved

Retrofit of the Heat Recovery System of a Petroleum Refinery Using Pinch Analysis

utilities. This eventually entails reducing the number of

the heat transfer unit thus reducing capital costs while

sacrificing some energy recovery. A comparison of the

cost implications of the various networks was presented.

4. Results

The composite curve from the enthalpy data is presented

in Fig ure 1. From the composite curve, it could be seen

that the overlapping of the cold composite and the hot

composite curve indicates a high possibility of process to

process heat exchange. The minimum utilities require-

ments for the plant are 72 MW and 64 MW for the hot

and cold utility respectively at a ∆Tmin of 15 ˚C. This pre-

sented about 34 per cent potential reduction in utility

requirement. There is only one pinch point, 235˚C and

225˚C for hot and cold streams respectively.

In Figure 2, the plot of the cold and hot utilities at

different ∆T min shows a linear relationship with no dis-

continuity within the range . This signified the absence of

a topology trap. The absence of a topology trap means

that the need for a detailed cost analysis to determine the

optimum ∆Tmin using different algorithms and graphs is

not necessary.

The grid diagram for the existing network is presented

in Figure 3. It shows that seven exchangers are working

across the pinch. Hence, there is no maximum energy

recovery and there is no efficient use of the existing heat

exchangers.

The focus of the study is concentrated only re-assign-

ing these exchangers. Using the remaining problem analy-

sis approach, the network in Figure 4 was obtained. The

complete heat exchanger network design represents max-

imum energy recovery of the process heat utilizing as

many as required heat transfers unit including heat ex-

changers, heaters/furnace and coolers. This network rea l-

ised the utilities targets set abov e.

The relaxation process of the network is an important

part in the optimization of the network. The purpose of

this step is not to eliminate all the lo ops and paths but to

rather reduce the paths and loops. This step involves the

removal of heat exchangers by allowing a small energy

penalties leading to a reduction of units within the net-

work thus reducing capital costs. The ultimate aim is to

find a balanced heat exchanger network, keeping as best

as possible the topology maximising the energy duties

between the streams whilst not incurring heavy capital

costs related to the heat exchanger areas. This network

has minimum impact on the current configuration of the

plant, whist giving a substantial 34 per cent saving on

utilities.

In the MER network, there are a number of loops and

paths, the target of the relaxation step for this design is

not to remove some or all of these loops and paths. The

final relaxed network is presented in Figure 5. This net-

work has six less heat transfer unit compared to the MER

design.

The cost implication of the various networks is as pre-

sented in Table 1.

Figure 1. The composite curve at ∆Tmin of 15˚C.

Retrofit of the Heat Recovery System of a Petroleum Refinery Using Pinch Analysis

Figure 2. Plot of the utility requirements vs ∆Tmin.

Figure 3. The existing heat exchanger network showing across the pinch violation.

Figure 4. The maximum energy recovery network.

Retrofit of the Heat Recovery System of a Petroleum Refinery Using Pinch Analysis

Figure 5. The final relaxed network.

Table 1. Cost comparisons of the heat exchanger networks.

HEN Description Existing HEN MER Network Final Relaxed HEN

Utility Costs ($) 222,536 115,670 149,9 54

Capital Cost (Heat exchangers) ($) 865,045 1,199,490 1,010,176

Capital Cost (Furnace, Coolers) ($) 589,367 542,373 497,866

5. Conclusion

In the current economic climate, with the rising cost of

fuel that provides the industrial plants with their heating

and cooling needs, it has become imperative that the

plant becomes more energy efficient. This study confirmed

that Pinch Techno logy is a very practical, easy a nd intui-

tive method for attaining better process heat integration.

The existing HEN had a number of heat exchangers

working across pinch; these violations were eliminated

during the retrofitting. The final evolved HEN reveal a

potential 34 per cent energy saving in the plant section

studied. A trade-off was explored to find a balance of

utilities usage, number of exchanger units and area. This

was done by using the heat load loops and paths. The

effect of pressure in the evolved HEN network was not

investigated. This must be considered before implement-

ing the recommendations. Alternative methods must also

be used to verify the findings of this study such as math e-

matical optimisation methodologies. In conclusion, pinch

technology is still an important method for evaluating

and scoping for potential energy saving in a chemical

plant. It is particularly relevant today, due to the high

cost of utilities and the adverse environmental impact of

energy usage by industries.

6. Acknowledgements

The authors acknowledge the supports provided by Cape

Peninsula University of Technology, Nigerian National

Petroleum Corporation (NNPC) and the management of

Warri Refinery and Petrochemical Complex, Nigeria.

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