SABP-A-012.pdf

Previous Issue: 16 March 2011 Next Planned Update: TBD

Revised paragraphs are indicated in the right margin Page 1 of 92

Primary contact: Soliman Nour Eldin, Mahmoud Bahy Mahmoud on +966-3-8809449

CopyrightSaudi Aramco 2013. All rights reserved.

Best Practice SABP-A-012 21 July 2013

New Projects Energy Efficiency Optimization Review Methodology

Document Responsibility: P&CSD/Energy Systems Division

New Projects Energy Efficiency Optimization Review Methodology

Document Responsibility: P&CSD/Energy Systems Division SABP-A-012

Issue Date: 21 July 2013

Next Planned Update: TBD New Projects Energy Efficiency Optimization Review Methodology

Page 2 of 92

Table of Contents

Page

1 Introduction 3

1.1 Definition 4

1.2 Purpose and Scope 4

1.3 Intended Users 4

2 New Projects Energy Assessment 4

2.1 Project Phases 4

2.2 Energy Efficiency Optimization Tasks Description 5

2.3 Solution Approach 5

3 Energy Assessment Methodology 6

3.1 Energy Assessment Procedures during Project Study Phase 6

3.2 Energy Assessment Procedures during Project Proposal Phase 24

3.3 Quick Guidelines for Efficient Energy System Design 31

4A Appendices for Short-cut Assessment Tools 35

4A.1 Steam and Power Model 35

4A.2 Pinch Method for utilities Targeting and Selection 36

4A.3 Cogeneration Targeting and Drivers Selection 54

4A.4 Cooling Water and Refrigeration System Targeting 72

4A.5 Tri-generation 90




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1 Introduction

Energy conservation in Saudi Aramco became everyones business. It is mandatory for each existing process facility to find cost effective solutions to save energy and achieve

more with less in their facilities. It is also equally important for each new project to be

designed and operated in an energy-conscious manner.

A vital contribution towards the success of the company wide energy conservation

policy comes through documenting the company best practices in methodology; tools

and applications in the field of energy efficiency optimization. Besides, capturing the

knowledge of the in-house expertise in such field and distributing such knowledge

among our facilities and engineering services departments. Hence, a consistent effort

has been exerted in Saudi Aramco to produce Best Practices to help our engineers

achieve their energy efficiency optimization mission through the design and building of

energy conscious facilities following the same new paradigm implemented in the

existing facilities.

This particular Best Practice document introduces a brief methodology for grassroots

projects energy assessment, associated with short-cut tools that can help satisfy the

above mission.

The first and most important thing to learn and apply from this quick review

methodology for energy efficiency optimization in grassroots project is that;

Our Big Picture Includes Process and Utility Plants

It is important during the early phase of any project that we see its big picture.

In this document when we talk about project phases we mean only the following three

phases; project studies phase, design basis scoping paper preparation, and project

proposal phases.

We need to make sure that the system-approach that take into consideration the

process(es), hot and electricity utilities, and the cooling and refrigeration utilities needs

is utilized. This approach has to prevail on the current state-of-art sequential sub-

system by sub-system approach during the project study phase.

Removing some degrees of freedom from our options subjectively shall be avoided as

much as possible. During feasibility study phase, it is absolutely necessary to

investigate different combined process and utilities system schemes.




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1.1 Definition

The term Energy Assessment refers to the methodology of collecting and analyzing available energy utilities related process data without losing the

context of the whole process needs in order to establish the big picture of the energy requirements for a particular facility and identify component-based-

energy efficiency optimization opportunities from the operating cost point of

view and capital cost of energy and process sub-systems point of view too.

Striking the right balance between such costs will define the close-to-optimum

solution of the energy problem in the design of any new plant. In grassroots

projects available data are mostly uncertain, time is critical and there are infinite

combinations of options. Therefore, the energy assessment process of any new

project has to be conceptual, fast but rigorous-oriented with the right level of

details at each phase of the project.

1.2 Purpose and Scope

The purpose of this best practice document is to describe a methodology for the

quick review of new projects from energy efficiency optimization point of view.

Besides, introducing short cut tools by which quick assessment for energy

efficiency improvement can be conducted. Its scope include quick energy

assessment methodology in a step-by-step manner, simple models for data

representation, and short cut tools for evaluating process schemes for energy

efficiency optimization.

1.3 Intended Users

This Best Practice manual is intended for use by project and process engineers

in Saudi Aramco, who are responsible for process &facilities planning, process

engineering and energy systems engineering. This particular document will

enable them to conduct quick review of new projects from energy efficiency

optimization point of view to make sure that they are planning for and designing

of new energy-conscious facilities in Saudi Aramco.

2 New Projects Energy Assessment

2.1 Project Phases

In Saudi Aramco our projects have four main phases. These phases are the

project study phase, design basis scoping paper phase, project proposal phase

and finally expenditure request approval and completion phase.




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During each of these phases it is important to develop the appropriate level of

details in our modeling and assessment techniques to be able to render at the end

of all project phases a facility which is going to be optimal. This process shall

proceeds in a way that does not hinder next phase decisions from being optimal

too. It is the same philosophy used in dynamic programming approach where

while the flow of information details goes from left to right on the time and

information maturity scales of new projects, the optimization process starts from

right to left.

2.2 Energy Efficiency Optimization Tasks Description

Energy Efficiency Optimization objective aims to specifying the near-optimal

design that minimizes the new plants energy consumption at minimum deficiency in energy supply of the utility systems to the plants process at minimum capital cost. Following that, the task will be to list all possible design

options/actions/modifications necessary to achieve the specified/desired process

target(s). This includes identification of all related engineering activities in a

minimum possible time using uncertain plant data and without any interruption

to the overall project schedule. Currently, the scope of the energy efficiency

optimization of new projects assessment include the power, heating and cooling

systems that are mandatory to satisfy certain process demands along the life of

the project.

2.3 Solution Approach

Nowadays in Aramco for the sake of simplicity and timely results,

decomposition and heuristic techniques are adapted in lieu of the time-

consuming but more beneficial Mathematical Programming/Optimization

Techniques.

The evolutionary approach can be adapted versus the more time consuming

revolutionary approach. The old projects data base shall be fully utilized to

facilitate the energy review process and result in merits.

The plants energy utility needs shall be defined with reasonable level of flexibility and the energy utility system; electricity, fuel, steam and other

energy-related utilities shall be defined one by one to find the near- optimal

consumption of such utilities that guarantee minimum deficiency in the utility

supply to plant processes subject to controlled minimum capital cost.

The company reliability figures shall prevail at least for the time being.




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On the macro level the energy system components are generation, distribution

and utilization. The objective will be to minimize waste in energy fresh

resources and capital in these three components. This can be done via the

continuous upgrade of the efficiency of energy system components in

generation, distribution and utilization. However, the utilization component has

a unique feature, where its boundaries are not completely dictated by the

process. Therefore, the room of improvement in this component can have

tangible impact on the process capital cost in addition to energy utility system

cost.

3 Energy Assessment Methodology

3.1 Energy Assessment Procedures during Project Study Phase

Preliminary review of similar old process designs, system drawings and data analysis

Understand the Big Picture of the old plant and the new plant-wide operations

Understand process energy needs and utility systems preference of both the old and the new plants

Understand the interaction between the process and hot utility system

Understand the interaction between the process and the cold utility system

Establish your desired objectives Targeting for the new project (power, steam, fuel, water)

Identify All Opportunities for energy savings in the old project/existing facility

Define Obvious Quick-hit savings (e.g. better plot plan, considering cogeneration scheme,etc.)

Prepare do and do not do list for the new project during the study phase

Challenge every process step in the old design to generate new process alternatives for the sake of a lower energy systems capital and operating costs

From the available data, establish at least two or more process design schemes

Propose scope for the second level of the review process that includes more definitive assessment with some economic analysis including the simulation

of the defined process schemes.

Propose plant-wide energy-utility strategy




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Discuss your findings with the project study team

There are three essential tasks that need to be conducted during the review of the

old project schemes in order to draw useful conclusions for the new project

process and utility design

(A) Data analysis, Models building and establishing Targets

(B) Insights, Opportunities and Estimated savings potential

(C) Screen and Formulate Improvement Strategy

These tasks can be explained in details as follows:

1. Site survey through templates, checklists and interviewing of process owners/proponents to gather the right amount of data that enable the energy

team build the plants big picture and understand the goals and the constraints of the facility

2. Define the criteria for focusing on potential areas of interest (when to be rigorous and get to the second level of details)

3. Develop site energy/utility nominal design/normal operation models with the appropriate level of details in a high level generic path diagrams for, power, fuel, H2, steam, water, nitrogen and air. Preliminary purpose of

these models will be to understand what is going on in the energy utility

system, locate the energy consumption elephants (ECEs) in both process and utility plants and generate insights for energy saving opportunities

4. Add more depth in the level of details of the energy utility model for each ECE and/or other criterion of focus

5. Define the effect of disturbances and uncertainty on the energy utility system models

a. Sources of disturbances

b. Site energy utility balance under disturbances

c. Nominal and dynamic targeting of energy utility systems

d. Check that the big picture depicted for the process and the utility plants is correct with enough degree of confidence before you proceed

6. Target (order of magnitude targeting)

a. _ Identify main processing issues that affect utility utilization

b. _ Link utility-utility interactions

c. _ Integrate and qualitatively optimize site utilities




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7. Integrate core processes among themselves and with utilities

8. Develop a comprehensive initiatives list via identifying and estimating energy utility savings opportunities

9. Develop word strategies for realizing savings for the new facility goals, analysis of the results and the mapping of the opportunities onto the new

facility strategy

Power and Heat Supply Decision

The following example addresses the problem of using cogeneration or not using

cogeneration to satisfy the process heat and power supply to the process. The example

below is an actual study conducted by one of our external consults. It shows that for

process heating and power supply requirements, it is important to consider as much as

possible number of options and economically screen them before you decide where to

go for this issue.

Option 1 Base Case

Steam raised in boilers at 150 psig, power purchased from SEC and all equipment on

electric drives.

Initial steam demand estimate is given below.

Summer

Year Water

Cut

Desalter

Heater

Stabiliser

Reboiler

Stripping

Steam

Other

Users

% MMBTU/h MMBTU/h MMBTU/h Mlb/h Mlb/h Mlb/h Mlb/h MMBTU/h

2011 1 14 167 181 198 30 95 323 295.2

2015 11.1 39 169 208 227 30 95 352 321.7

2022 30 126 173 299 327 30 95 452 413.1

2030 51 153 173 326 357 30 95 482 440.5

Total Steam DemandSummer Duty

Winter

Year Water

Cut

Desalter

Heater

Stabiliser

Reboiler

Stripping

Steam

Other

Users

% MMBTU/h MMBTU/h MMBTU/h Mlb/h Mlb/h Mlb/h Mlb/h MMBTU/h

2011 1 113 178 291 318 30 95 443 404.9

2015 11.1 210 185 395 432 30 95 557 509.1

2022 30 531 197 728 796 30 95 921 841.8

2030 51 600 201 801 876 30 95 1001 914.9

Winter Duty Total Steam Demand




Page 9 of 92

Estimated Power Demands

Wat

er C

ut1%

1%6%

9%11

%14

%17

%19

%22

%24

%27

%30

%32

%35

%38

%40

%43

%46

%48

%51

%

Year

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2021

2022

2023

2024

2025

2026

2027

2028

2029

2030

Powe

r req

uire

men

tAq

uife

r WIP

sM

W75

7575

7575

7575

7593

9393

9393

9393

9393

9393

93

Form

atio

n W

IPs

MW

99

99

933

3333

3333

3333

3333

3333

5050

5050

ESPs

MW

67

912

1417

2125

2933

3844

4955

6269

7683

9199

Oth

er G

OSP

MW

4646

4646

4646

4646

4646

4646

4646

4646

4646

4646

Utili

ties

MW

55

55

55

55

55

55

55

55

55

55

Tota

l Pow

erM

W13

914

014

314

514

717

517

818

220

521

021

522

022

623

223

824

526

927

628

429

2




Page 10 of 92

The steam demand ranges from 323 Mlb/h for summer 2011 to 1001 Mlb/h for Winter

2030.

Minimum criteria to be used for phased installation of equipment is 7-10 years, however

from the above table it can be seen that >50% of final capacity is required by 2015.

Therefore, 100% capacity installation is required from 2010.

4 x 50% units will be installed each with capacity of 500 Mlb/h, giving N+2 intallation

in year 2030. It is assumed that one boiler will be down for maintenance at any one

time and that the steam load will be shared equally between the remaining boilers.

Refer to tables below showing steam demand and boiler turndown.

During summers the required steam demand can be met by a single boiler. However it is

assumed that the load is shared by two boilers to allow speedy ramp-up should one

boiler trip. It is possible to share this load over 3 boilers but the boilers would be

operating at close to 20% turndown.

Summer

2011 2015 2022 2030

Total Steam demand Mlb/h 323 352 452 482

Running boilers (N+1) 2 2 2 2

Production per boiler Mlb/h 161.5 176.0 226.0 241.0

Turndown % 32% 35% 45% 48%

Winter

2011 2015 2022 2030

Total Steam demand Mlb/h 443 557 921 1001

Production per boiler 2 3 3 3

Production per boiler Mlb/h 221.5 185.7 307.0 333.7

Turndown % 44% 37% 61% 67%




Page 11 of 92

Operational cost factors will be based on power import and fuel consumption as shown

in the following table:

Operation Cost Factors 2011 2015 2022 2030

Power Import, Summer MW 139 147 220 292

Power Import, Winter MW 139 147 220 292

Fuel, Summer MMBTU/h 410 447 574 613

Fuel, Winter MMBTU/h 563 708 1,170 1,272

Option 1a Steam Generation at 750psig

This option looks at raising steam at 750 psig (52 bara) and letting down through steam

turbine drivers for the compressors.

The GOSP gas compressors power demands are constant throughout the life of the plant; therefore 100% steam capacity would be required from 2010.

Compressor Description Power per item MW

Total operating power

K-100A/B Atmospheric Compressor 12,500 12,500

K-101/2 A-C HP compressor 9,960 19,920

K-103 A/B Propane Compressor 3,159 3,159

It is estimated that 35 MW of power is available from 454 t/h (1001 Mlb/h) steam

through 750 100 psig pass-out turbines. This matches the operating duty for all the running gas compressors.




Page 12 of 92

Stand-by machines will be electric motors.

750 psig

1001000 lb/h

4 ST Drivers for Compressors

2 x 10 MW

35 MW 1 x 12.5 MW

1 x 3.2 MW

100 psig

60 psig

Condensate

BOILERS

Condensing

turbine

Excess steam in the summers and during the early years can be used to generate

electricity via a condensing steam turbine generator and hence reduce the amount of

purchased power required further. Refer to tables below:

Summer

2011 2015 2022 2030

Steam Produced Mlb/h 1001 1001 1001 1001

Steam for Process Heating Mlb/h 323 352 452 482

Excess Steam Mlb/h 678 649 549 519

Power produced hp 24726 23668 20021 18927




Page 13 of 92

Winter

2011 2015 2022 2030

Steam Produced Mlb/h 1001 1001 1001 1001

Steam for Process Heating Mlb/h 443 557 921 1001

Excess Steam Mlb/h 558 444 80 0

Power produced hp 20350 16192 2918 0


in the following table.




Fuel, Summer MMBTU/h 1,279 1,279 1,279 1,279

Fuel, Winter MMBTU/h 1,279 1,279 1,279 1,279




Page 14 of 92

Option 2 PWIPs with individual CGTs & WHRUs

Base Case :

Power Water Injection Pumps (PWIP) are electric motor driven and installed in 2

phases. Each PWIP is limited to 400MBPOD or a motor size of approx 25,000 hp with

4 PWIPs installed in Phase 1 and a 5th

installed in 2019.

This option considers same capacity and phasing of PWIPs as base case, however each

PWIP is connected to a combustion gas turbine (CGT) with a waste heat recovery unit

generating low-pressure steam. Refer to sketch below:

CGT

GE Frame 5 WIP

Fuel & Air

25,000 hp

WHRU

From other

WHRUs

138.9 Mlb/hTo Users

Base load

from Boilers

300 Mlb/h

All other drivers are electric motor with power purchased from SEC.

Back-up steam production by 3 x 50 % boilers is required in case WHRUs fail, i.e., 3 x 500.5 Mlb/h boilers (3 x 227 t/h) (number of back-up boiler tbc)

Assume that back-up boilers are operating at 30% turndown. In the summers & early

years it is assumed only one back-up boiler is running at turndown, in order to minimize

heat bypassed to GT/WHRU exhaust.




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Each PWIP is coupled to a GE Frame 5 CGT complete with a WHRU, which can

produce up to 139 Mlb/h steam.

In 2018 at end of Phase 1 total steam that can be generated by WHRUs is 556 Mlb/h.

With back-up boilers operating at 30% turndown there is excess heat from the WHRUs

which is discharged to the GT exhaust.

In 2030 when 5 PWIPs are installed total steam production from WHRUs will be 695

Mlb/h. 306 Mlb/h steam made up from boilers.

Summer Winter

2015 2030 2015 2030

No PWIPs 4 5 No PWIPs 4 5

Power Produced hp 100000 125000 Power Produced hp 100000 125000

Steam from WHRUs Mlb/h 556 695 Steam from WHRUs Mlb/h 556 695

Steam from Boilers Mlb/h 150 150 Steam from Boilers Mlb/h 150 305

Process Steam req'd Mlb/h 395 482 Process Steam req'd Mlb/h 705 1001

Installation requirements:

2010-2018 4 x Frame 5 GE CGTs direct drivers for PWIPs complete with WHRU

[555.6 Mlb/h steam + 100,000 hp (74MW) Power]

3 x 500 Mlb/h back-up boilers

2018-2030 1 additional Frame 5 GE CGT complete with WHRU

[694.5 Mlb/h steam + 125,000 hp (93MW) Power]

Other considerations:

In years 2010 to 2015 there will be excess heat available from WHRU. This can be

used to raise excess LP steam that can be used for BFW preheat or Crude preheating or

by-passing the WHRU to stack.


in the following table (excess heat loss via GT Stack is also included):




Page 16 of 92






Excess Heat from GT Exhaust, Summer MMBTU/h 389 360 399 369

Excess Heat from GT Exhaust, Winter MMBTU/h 267 151 -1 -1

Option 2a PWIPs with individual CGTs & WHRUs

This option is same as Option 2 except 4 off larger PWIPs (and associated GT Direct

Drives) are installed in year 2010. The best GT match is Siemens SGT-700 for the new PWIPs duty. The Siemens GT operates at higher power output and lower steam production.

Refer to sketch below:

CGT

SGT-700 WIP

Fuel & Air

30,345 hp

WHRU

From other

WHRUs

115 Mlb/hTo Users

Base load

from Boilers

541 Mlb/h




Page 17 of 92

All other drivers are electric motor with power purchased from SEC.

Back-up steam production by 3 x 50% boilers is required in case WHRUs fail, i.e., 3 x 500.5 Mlb/h boilers (3 x 227 t/h) (number of back-up boiler tbc)

Assume that back-up boilers are operating at 30% turndown. In the Summers & early

years it is assumed only one back-up boiler is running at turndown, in order to minimise

heat bypassed to GT/WHRU exhaust.

Each PWIP is coupled to a Siemens GT-700 complete with a WHRU, which can produce up to 115 Mlb/h steam.

In 2018 at end of Phase 1 total steam that can be generated by WHRUs is 460 Mlb/h.

With back-up boilers operating at 30% turndown there is excess steam or heat lost with

by-pass of WRHU to the stack.

Summer Winter

2018 2030 2018 2030

No PWIPs 4 4 No PWIPs 4 4

Power Produced hp 121380 121380 Power Produced hp 121380 121380

Steam from WHRUs Mlb/h 460 460 Steam from WHRUs Mlb/h 460 460

Steam from Boilers Mlb/h 150 150 Steam from Boilers Mlb/h 250 541

Process Steam req'd Mlb/h 395 482 Process Steam req'd Mlb/h 705 1001

excess/ (make-up) Mlb/h 215 128 excess/ (make-up) Mlb/h 5 0

Installation requirements:

2010-2030 4 x Siemens GT-700 complete with WHRU

[460 Mlb/h steam + 123,000 hp (92 MW) Power]

3 x 500.5 Mlb/h back-up boilers

Other considerations:

In years 2010 to 2015 there will be excess heat available from WHRU. This can be

used to raise excess LP steam that can be used for BFW preheat or Crude preheating or

wasted via GT stacks.

Towards 2030 and in winter, two back-up boilers are required to operate @ 54%. This

is due to the lower steam production from the Siemens GT-700, the closest GT size to the PWIPs power rating.


in the following table (excess heat loss via GT Stack is also included).




Page 18 of 92






Excess Heat from GT Exhaust, Summer MMBTU/h 293 264 162 132

Excess Heat from GT Exhaust, Winter MMBTU/h 171 55 1 1

Option 3 Cogeneration sized for heat match

One back-up boiler will be running at 30% turndown. Base heat load provided by

WHRUs of the GTGs and all the drivers including PWIPs are electric motor.

Central Cogen sized for process heat match remaining power purchased from SEC.

2 x 50% back-up boilers.

2030 steam demand requires 3 x GE Frame 6 CGT.

350 Mlb/h

CGT

GE Frame 6 GEN

Fuel & Air

72,415 hp

WHRU

From other

WHRUs

335.1 Mlb/h To Users

From Boilers




Page 19 of 92

Total Power generated in 2030 (2 GTGs) is 108 MW.

Therefore, purchase power required is 183MW.

Installation.

2010- 2018 2 x GE Frame 6 CGTs complete with WHRU (only 1 operating to

reduce heat waste via GT stacks) with 2 x 500.5 Mlb/h back-up boilers

[670 Mlb/h Steam + (54MW) Power]

~93 MW Purchase Power required in 2018

1 more GE Frame 6 CGT complete with WHRU (with 2 operating)

[670 Mlb/h Steam + (108MW) Power]

~183 MW purchase Power required


in the following table (excess heat loss via GT Stack is also included):




Fuel, Summer MMBTU/h 789 789 1,388 1,388

Fuel, Winter MMBTU/h 789 885 1,515 1,642

Excess Heat from GT Exhaust, Sum MMBTU/h 330 271 561 516

Excess Heat from GT Exhaust, Winter MMBTU/h 86 7 -1 30




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Option 4 Cogen sized for PWIP Steam Turbine Drivers

PWIPs driven by steam turbine, all other drivers electric motor.

Central Cogen raising enough steam to drive PWIPs

Back-up by 2 x 100% boilers & SEC

Assume PWIP size & phasing as per option 2.

125,000 hp

177,415 hp

CGT

GE Frame 7 GEN

Fuel & Air

171,250 hp

WHRU

2336.8 Mlb/h

From other

WHRUs

584.2 Mlb/h

750 psig

WIPs

To process

heating

1001 Mlb/h

150 psig

Condensing

turbine

2921 Mlb/h

150 psig

1920 Mlb/h




Page 21 of 92

In 2018, PWIP power requirement is 100,000 hp. This requires approx 2337 Mlb/h

steam from 750 175 psig pass out turbine. Power required for remaining drives is approx 155,560 hp (116 MW).

In 2030 PWIP power requirement is 125,000 hp. This requires approx 2921 Mlb/h

steam from 750 175 psig pass out turbine. Power required for remaining drives is approx 274,910 hp (205 MW).

However, process heat requirement in 2030 is only 1001 Mlb/h (60 psig), therefore,

excess steam is routed to condensing turbine to generate more power.

In 2030 5 x GE Frame 7 CGTs are required to raise steam for PWIP steam turbine

drives, which will generate 856,250 hp. An additional 177,415 hp is generated by the

condensing steam turbine giving total available power generated = 1,033,665 hp.

This is well in excess of the required 274,910 hp.

Installation:

2010- 2018 4 x GE Frame F CGTs complete with WHRU

[2337 Mlb/h Steam + 685,000 hp (511 MW) Power]

2 x 1001 Mlb/h back-up boilers

1 x 174,333 hp (130 MW) condensing turbine

1 additional GE Frame 7 CGT complete with WHRU

[2921 Mlb/h Steam + 1,033,665 hp (769 MW) Power]




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Option 5 Cogeneration sized to match total power demand

No base load boilers required, as the GT system has a N+2 supply arrangement.

In this option all drivers are electric motor and all power is produced by central CGTs

with no back up from SEC. Therefore, N+2 CGTs are required.

Steam is raised at 750 psig in WHRU and passed through a steam turbine. 150 psig

steam is extracted for process heating demand.

Total power requirement in 2030 is ~300 MW or 402,310 hp

CGT

GE Frame 7 GEN

Fuel & Air

88,370 hp

WHRU

From other

WHRUs

302.5 Mlb/h

750 psig

To process

heating

1001 Mlb/h

150 psig

87,165 hp

To

condenser

907.5 Mlb/h

1210.0 Mlb/h

209.0 Mlb/h

GTs running at 100% rate, the actual maximum power or heat demand is supplied with

around 90% turndown on the GT and associated steam turbine generator.




Page 23 of 92

Installation:

2010 - 2018 5 x GE Frame 7 CGTs complete with WHRU (3 operating, two standby),

[907 Mlb/h Steam + 265,000 hp (198 MW) Power]

1 x 87,165 hp (65MW) condensing turbine

(based on maximum of 1210 Mlb/h of steam)

2018 - 2030 1 additional GE Frame 7 CGT complete with WHRU

[1210 Mlb/h Steam + 440,645 hp (328MW) Power]

In summer, the plant power demand will dictate the turndown ratio of the operating GT

machines, and in all cases, there will be excess heat. In winter 2015 & 2022, the heating

requirement will dictate the GT turndown rates.


in the following table (excess heat loss via GT Stack is also included).



Power Import, Winter MW 0 -21 -31 0



Excess Heat from STG condenser, Summer MMBTU/h 455 427 410 142

Excess Heat from STG condenser, Winter MMBTU/h 455 356 294 142

Excess power in 2015 & 2022, can be reduced by the installation of after burner to

divert energy from power to heat.

The heat loss is via condenser not GTG stacks.




Page 24 of 92

3.2 Energy Assessment Procedures during Project Proposal Phase

The following are the procedures which normally render an acceptable energy

efficient process design in a project proposal phase:

1- Data extraction for the study to be done for each stream that needs to be

heated or vaporized and any stream that needs to be cooled or condensed

in the base design case. As if each stream will be handled through utilities. (No integration in the base case design).

2- Targets for energy utility to be calculated for the process with integration

and without integration.

3- The grand composite curve for the base case design shall be utilized to

help show the right/optimal level of utility mix. for heating and cooling

utilities.

4- The same graph (GCC) needs also to be utilized to show the potential

cogeneration opportunities and best drivers for the process, if any.

5- List of possible design and operational modifications to be investigated to

explore its impact on the utility consumption and other process units.

6- These steps should be done for at least 6 DTmin., before selecting the right

one. Of-course, in such cases a preliminary evaluation of the HENs capital

cost will be needed, or whatever targeting method you use, to reach the

close-to-optimum DTmin. (These calculations can be done easily using

state-of-the art software(s) like SPRINT, currently available at Saudi

Aramco ESU)

7- Preliminary HEN synthesis developed will render several process

initiatives for improving the design from energy efficiency point of view

compared with the base case design.

8- The process scheme produced may have some environmental, safety and

control/operability constraints that may justify forbidding streams

matching and warrant the removal of some streams from the heat

integration schemes or even removing all of them from integration

scheme; it does not matter as long as the design is pursued systematically

and the techno-economical justifications are detailed and documented.




Page 25 of 92

9- One-by-one, of problem stream(s) shall be taken out from the integrated process scheme and its energy impact in Dollars is defined. In the same time an engineered solution for solving this problem/ constraint, safety;

control/operability or other problem, shall be suggested and its impact

shall also be roughly quantified/estimated in Dollars if possible and

documented.

10- Trade-off between the energy saving impact $ and for instance the control/operability impact $ shall be calculated, documented and shown in

the energy assessment study.

11-Other subjective decisions need to be mentioned and documented clearly

with enough techno-economical support as much as possible to support the

decisions of accepting or rejecting process initiatives for the sake of energy

efficiency optimization.

In general, there are very important constraints in form of early decisions taken

at early stages of the project life that confine the scope of work in any energy

efficiency optimization study. It will not be practical, logical and even

beneficial to continue arguing about the logic or correctness of past decisions

because the review process shall move on fast but with enough rigors and

without losing the essence of why we are doing energy studies for new designs.

In order to get the best out of any energy study, we suggest that you explore few

important modifications that would have the most impact on the base case

design from energy efficiency point of view and also help save significant

capital cost.

The following example is an actual one about an oil and gas separation project

where the base case design has been studied from energy efficiency optimization

point of view by an outside consultant/engineering company and has been

reviewed with the comments below.

The proposed comments are a result of small effort spent on an energy study

review with the available information at that stage bearing in mind that only

major things shall be reported back for consideration. Changes have to be

practical and do not have any major change on the project schedule. However, it

may help correct some of the quit clear points in the base case design.




Page 26 of 92

The first most important item which is fundamental and does not even need

investigation is the unnecessary recycle of the NGL stabilizer over head gas

stream back to the process. This recycle in base case design is not technically

useful. Such type of recycles has to be eliminated as long as these recycle

streams have no separation sink. These recycle streams normally, do not only

affects the size of all equipment, piping,etc., down the stream it joins resulting in huge capital waste but also has no production benefit from NGL separation

point of view. It also affects energy utilities such as the refrigeration package

capital and operating cost. In any case recycle streams without separation or

conversion sink should not be recycled back to the process.

Deleting NGL Stabilizer OVHD Recycle Example:

The two graphs below show the place of the recycle that need to be demolished

and an idea that need to be investigated with others by the process designers to

explore the extra capital cost used due to the recycle and to enhance if possible

the amount of NGL that can be recovered. Here below some ideas that can be

explored along the major change of using de-ethanizer instead of NGL stripper,

for instance.

260 Psig445 Psig

425 Psig

TEG unit

NGL Stripper

HP gas from inlet manifold

Dried HP gas to export

Condensate Feed Drum

GOSP condensate from

condensate inlet manifold

440 Psig

Should not be recycled




Page 27 of 92

260 Psig445 Psig

425 Psig

TEG unit

NGL Stripper






440 Psig

Should not be recycled

Sales Gas

NGL

Condenser

This condenser could use the process stream that have a temperature of

50 F and the rest can come from the refrigeration package

260 Psig

445 Psig

425 Psig

TEG unit

NGL StripperAn idea to avoid recycle and possible

Increase in NGL recovery






440 Psig

330 Psig

NGL

sales Gas

sales Gas

-New HP flash drum or small stripper with 20% of the feed load to

recover more NGL

-Smaller existing Stripper using less steam &redesigned to allow more NGL recovery instead of the heavy components loss in the top




Page 28 of 92

Heat Integration between Compressors and Crude Stabilization process

Example:

The graph below suggests that integration between the discharge of compressors

and the crude stabilization process can be done through several options upon the

implementation of pinch techniques. One option is possible through a hot water

system. This integration option can result in huge steam saving and savings in

the fin-fans electric power loads as well as a reduction in capital. The scheme

below can have different options based upon the location of the pump-

around/inter-heater and the water return temperature. Note that we are only

giving here a configuration while several configurations can also be produced

and explored. The savings here in capital and operating cost is quite clear and

there is no operability problem but simulation and more in-depth review of the

process will be warranted.




Page 29 of 92

Steam System Optimization Example:

The graph below shows that electricity can be generated from the proposed

utility system design to minimize the power purchased from the grid.

In the current base case design, only 4 MW could be generated from the current

situation using BPST generator.

Khurais Project

Combined Heat & Power System

*- One working to

support the 573 Klb/hr

process by 168 Klb/hr 133 psig

*- One on standby 428 Deg. F

*- One shutoff 573 Klb/hr

168 Klb/hr 4 MW

95 psig

365 Deg. F

716.17 Klb/hr 24.83 Klb/hr

1.65 Klb/hr

40 psig

320 Deg. F

24.5 Klb/hr

High Pressure

Low Pressure

Mid Pressure

HRSG

4GT

3 Boilers

at 50% load

Process LP Steam

Demand

Process MP Steam

Demand

BPST




Page 30 of 92

The proposed scheme below shows that via increasing the HRSG pressure and

temperature, it is possible to produce about 20 MW power of electricity.

Khurais Project

Combined Heat & Power System

*- One working to

support the 573 Klb/hr

process by 168 Klb/hr 625 psig

*- One on standby 700 Deg. F

*- One shutoff 573 Klb/hr

168 Klb/hr 20 MW

95 psig

365 Deg. F

716.17 Klb/hr 24.83 Klb/hr

1.65 Klb/hr

40 psig

320 Deg. F

24.5 Klb/hr

High Pressure

Low Pressure

Mid Pressure

HRSG

4GT

3 Boilers

at 50% load

Process LP Steam

Demand

Process MP Steam

Demand

BPST

The HRSG HP Steam can be utilized to drive a steam turbine generator for

power recovery. The steam balance and the steam property will not be affected.

In general it is recommended to produce the steam at the highest possible

pressure to generate more power. The optimum steam pressure can be decided

by the designer.

Heat Integration of NGL Separation Section Example:

The graph below suggests that simple pinch calculation might also be useful in

exploring the best way to match the shown hot and cold streams in order to

further minimize the utility consumption. The result may exhibit no need to

modify the existing design especially after the consideration of modifying the

NGL recovery and stopping the recycle, however it may worth its exploration.

It is important also to consider both the NGL cold section and the refrigeration

system simultaneously to minimize capital and compressor work-shaft.




Page 31 of 92

260 Psig445 Psig

425 Psig

TEG unit

NGL Stabilizer






440 Psig

Cold streams to be heated

Hot streams to be cooled

It is important to note that the above mentioned suggestions and others in line

with it can saves energy utility in form of steam consumption, electricity

consumption and increase the in-situ generation of electricity to reduce the

purchased power.

It may also result in an increases the NGL recovery and reduces the overall

process plant and utility plant capital cost due to the elimination of boilers, fin

fan coolers and the reduction of the capital cost. These benefits need to be

verified by process designers via simulation and economic analysis.

3.3 Quick Guidelines for Efficient Energy System Design

Consider the process and hot utility system simultaneously and optimize the

CHP system

Strongly consider the use of Cogeneration if your power-to-heat ratio is

rendering high cogeneration efficiency with respect to central power

generation plants efficiency

Do not allow the carrying through of the undesired species with main

streams, (gas, water or other species) especially if heating or cooling is

required along its path




Page 32 of 92

Later in the project phase watch for robust condensate recovery system

Do not forget Piping insulation for long distance pipelines

Consider having flexible operation of main equipment to allow for its load

management

Optimize air compressors design

Consider the use of Economizers and Pre-heater in the boilers

Consider the use of turbo-expander instead of JT valves and to drive gas

compressors

Watch for power generation from high pressure liquids

Re-consider the use of gas turbines versus the more efficient steam turbines

Increase boiler steam pressure and temperature to the extent that matches

process needs unless electricity generation is the controlling factor

Use auxiliary turbines to minimize steam let downs

Use steam in the process optimally to save capital cost

Consider using air pre-heaters for combustion air

Use ASD on BFW pumps

Integrate the flue gases in with the rest of the process using grand composite

curve developed by pinch technology (see later section)

Recover valuable gases from fuel gases and fully utilize the streams pressure

Minimize the H2 wheel in your plant

Cool down the inlet temperature to compressors

Reduce cooling medium return temperature in refrigeration cycles

Consider heat rejection of the refrigeration system in process cold or even

hot section, to the ambient and to another refrigerant

Use highest efficiency turbines in your CHP system (thermo-flow software

can help in such selection)

Utilize motors instead of turbine drivers if it is more economical since they

are more efficient

Optimize steam use in strippers

Minimize live steam utilization

Consider Mechanical energy integration




Page 33 of 92

Reduce natural gas consumption by understanding fuel gas sinks and

constraints

Reduce fuel gas use via considering energy integration

Keep H2 separate from fuel gas system, also measure the composition of

off-gas streams and recover C2 and C3+

Avoid unnecessary processing of off-gas

Avoid unnecessary processing of wastes and inert

Minimize the unnecessary production of off-gas

Avoid unnecessary recycles

Adjust operating pressures and optimize process interaction

Optimize your piping system to minimize excessive pressure drops

Re-use lowest quality water

Maximize use of stripped sour water and Minimize generation of wastewater

Eliminate direct water injection for cooling purposes

Eliminate live steam used for re-boiling and stripping where it is only used

for BTU value

Minimize or eliminate live steam consumption in sour water strippers by

replacing it with re-boilers

Boiler blow-down could be considered for cooling tower make-up

Extract the low pressure steam from the boiler blow-down

Use process water effluent as a source on the next lower water quality level

In general eliminate live steam usage since it becomes water and follows an

energy path through the plant consuming more energy to process it

Should live steam becomes necessary optimize the amount used through

optimal pressure conditions

Use lowest quality water possible for desalter operation

Minimize water used in desalting and/or carried through to desalting

Automate desalter operation, avoid water slipping through with crude during

desalting/maximize the separation of free water upstream of the crude

desalting (each Ib of water will require roughly Ib steam for processing)

Minimize the water-wheel in the plant




Page 34 of 92

Maximize utilization of treated oily-water from the waste-water treatment

plant

Consider adjustable speed motors/devices for pumps, compressors, .etc

Increase waste heat steam generation

Insulate condensate return lines, valves, flanges,etc.

Cooling- tower blow-down should not be treated but segregated to sewer

Boiler blow-down should not be sent to wastewater treatment but segregate

to sewer

In your plot-plan make sure that energy exporters are close to energy

importers

Avoid non-isothermal mixing of streams

Use cooling water instead of air, if possible, to cool down compressors

discharge

Illustrative Examples for Quick Energy Efficiency Optimization in New Design

Compression Energy % Savings Due to Decrease in compressors Inlet temperature

% Energy saving in a compressor energy consumption = {1- (Tnew/Told)} * 100

Tnew is the new inlet temperature

Told is the old inlet temperature

Back pressure turbines energy available for integration

Thermal energy available for Integration (Q) = Outlet steam flow* (Vapor enthalpy-

liquid enthalpy)

Outlet steam flow= Inlet steam flow (1- actual wetting factor)

Actual wetting factor can be assumed between (8 to 15) %

% Energy saving in heat pumps/refrigeration cycles due to decrease in reject

temperature

W2/W1 = (T reject 2 Tc)/ (T reject 1 Tc)

Treject is the temperature at which heat is rejected to the cooling medium (water)

Tc is the temperature at which heat is taken into the refrigeration) cycle




Page 35 of 92

4A Appendices for Short-Cut Assessment Tools

4A.1 Steam and Power Model

The basic Steam Mass Balance does not require high accuracy as long as the

developed model still makes sound engineering sense. (i.e., output is much

higher than input)

Common engineering sense shall be used to estimate what the unknowns. For

example condensate return, blow-down and flares can be defined after getting

good idea about main consumers.

chemicals

Proc. #1

Proc. #1

Proc. #1

Proc. #2

Proc. #3

Proc. #4

BFW

Raw water

Make-up Treatment Plant

MP Process

Condensate

LP Process

Condensate

HP Process

Condensate

Process Condensate

Est. 50 % Returned

HP Boiler

MP Boiler

HP

MP

LP

Vent

Effluent

Deaerator

98 t/h

68 t/h

0.0 t/h

21 t/h 0.0 t/h

68 t/h

30 t/h

0.0 t/h

8 t/h

30 t/h

0.0 t/h

Vent

1 t/h

2 t/h

1 t/h

18 t/h

7 t/h 4 t/h

27 t/h 9 t/h

38 t/h

(42+5) t/h

98+5 t/h

5 t/h

0.0 t/h

0.0 t/h

0.0 t/h

6.28 MW




Page 36 of 92

4A.2 Pinch Technology for Utilities Targeting and Selection

The purpose of this section is not to conduct a pinch study but to get some

energy targets regarding the utilities consumption for a desired plant area.

This can be done essentially via three methods, graphical, algebraic and using

mathematical programming/optimization. In this document the only one method

is going to be explained. In Saudi Aramco we have some software(s) that can be

used to conduct in depth analysis.

Targeting Using Graphical Method:

Any heat exchanger can be represented as a hot stream that is cooled down by

another cold stream and/or cold utility and a cold stream that is heated up by a

hot stream and/or hot utility with a specified minimum temperature approach

between the hot and the cold called Tmin.




Page 37 of 92

The process exhibited below in the graph shows the situation when the two

streams do not have a chance of overlap that produce heat integration between

the hot and the cold.

PROCESSH CFeed Product

0

20

40

60

80

100

120

0 10 20 30 40 50 60 H

T HOT UTILITY

COLD UTILITY




Page 38 of 92

Moving the cold stream to the left on the enthalpy axis without changing its

supply and target temperatures till we have small vertical distance between the

hot stream and the cold stream we obtain some overlap between the two streams

that result in heat integration between the hot and the cold and less hot and cold

utilities. As been depicted in the graph below with shrinkage in the hot and cold

lines span.


0

20

40

60

80

100

120

0 10 20 30 40 50 60 H

T HOT UTILITY

COLD UTILITY

HEAT

RECOVERY

Pinch(MAT)


0

20

40

60

80

100

120

0 10 20 30 40 50 60 H

T HOT UTILITY

COLD UTILITY

HEAT

RECOVERY

Pinch(MAT)




Page 39 of 92

For demonstration, all hot streams will be represented in the process by one long

hot stream to be called the hot composite curve. Same thing be done for all cold streams in the process.

The next step will be drawing the two composite curves/lines on the same page

in Temperature (T)-Enthalpy diagram with two conditions:

1- The cold composite curve should be completely below the hot composite

curve, and

2- The vertical distance between the two lines/curves in terms of temperature

should be greater than or equal to a selected minimum approach

temperature called global Tmin

The resulting graph is depicted below and known as thermal pinch diagram.

Net Heat Sink

Above the Pinch

Net Heat Source

Below the Pinch

Opportunity for

heat recovery

Net Heat Sink

Above the Pinch

Net Heat Source

Below the Pinch

Opportunity for

heat recovery




Page 40 of 92

Grand Composite Curve (G.C.C)Should Be Drawn To Scale

T* (K)

Enthalpy ( kW)

700 1400 2100 2800200

300

400

500

600

Total hot utility required is equal to 2620 kW

Hu1

Hu2

Hu3

Multiple utility targeting/selection using Grand Composite Curve (GCC)

Upon maximizing heat recovery in the heat exchanger network, those heating

duties and cooling duties not serviced by heat recovery must be provided by

external utilities.

The most common utility is steam. It is usually available at several levels.

High temperature heating duties require furnace flue gas or a hot oil circuit.

Cold utilities might be refrigeration, cooling water, air cooling, furnace air

preheating, boiler feed water preheating, or even steam generation at higher

temperatures.




Page 41 of 92

Although the composite curves can be used to set energy targets, they are not a

suitable tool for the selection of utilities. The grand composite curve drawn

above is a more appropriate tool for understanding the interface between the

process and the utility system. It is also as will be shown in later chapters a very

useful tool in studying of the interaction between heat-integrated reactors,

separators and the rest of the process.

The GCC is obtained via drawing the problem table cascade as we shown

earlier.

The graph shown above is a typical GCC. It shows the heat flow through the

process against temperature. It should be noted that the temperature plotted here

is the shifted temperature T* and not the actual temperature. Hot streams are

represented by Tmin/2 colder and the cold streams Tmin/2 hotter tan they are in the streams problem definition. This method means that an allowance of

Tmin is already built into the graph between the hot and the cold for both process and utility streams. The point of zero heat flow in the GCC is the pinch point. The open jaws at the top and the bottom represent QHmin and QCmin respectively.

The grand composite curve (GCC) provides convenient tool for setting the

targets for the multiple utility levels of heating utilities as illustrated above.

The graphs below further illustrate such capability for both heating and cooling

utilities.




Page 42 of 92

The above figure (a) shows a situation where HP steam is used for heating and

refrigeration is used for cooling the process. In order to reduce utilities cost,

intermediate utilities MP steam and cooling water (CW) are introduced.

The second graph (b) shows the targets for all the utilities. The target for the

MP steam is set via simply drawing a horizontal line at the MP steam

temperature level starting from the vertical axis until it touches the GCC.

The remaining heat duty required is then satisfied by the HP steam. This

maximizes the MP steam consumption prior to the remaining heating duty be

fulfilled by the HP steam and therefore minimizes the total utilities cost.

Similar logic is followed below the pinch to maximize the use of the cooling

water prior the use of the refrigeration.




Page 43 of 92

The points where the MP steam and CW levels touch the GCC are called utility

pinches since these are caused by utility levels. The graph (C) below shows a

different possibility of utility levels where furnace heating is used instead of HP

steam. Considering that furnace heating is more expensive than MP steam, the

use of the MP steam is first maximized. In the temperature range above the MP

steam level, the heating duty has to be supplied by the furnace flue gas. The flue

gas flowrate is set as shown in graph via drawing a sloping line starting from the

MP steam to theoretical flame temperature Ttft.

If the process pinch temperature is above the flue gas corrosion temperature, the

heat available from the flue gas between the MP steam and pinch temperature

can be used for process heating. This will reduce the MP steam consumption.

In summary the GCC is one of the basic tools used in pinch technology for the

selection of appropriate utility levels and for targeting for a given set of multiple

utility levels. The targeting involves setting appropriate loads for the various

utility levels by maximizing cheaper utility loads and minimizing the loads on

expensive utilities.




Page 44 of 92

MP

CW

Refrigeration

T*

H

T-tft(C)




Page 45 of 92

Normally, Plants Operations have choices of many hot and cold utilities and the graph below shows some of available options. Generally, it is recommended to

use hot utilities at the lowest possible temperature while generating it at the

highest possible temperature. And for the cold utilities it is recommended to use

it at the highest possible temperature and generate at the lowest possible

temperature. These recommendations are best addressed systematically using

the grand composite curve.

ProcessHeat

Pump

Boiler House

And Power Plant

Hot Oil

Circuit

Furnace

Fuel

Cooling

Towers

Refrigeration

Steam

Turbines

Gas

Turbines

Air preheat

W

W

W

W

BFW

preheat

Hot and cold utilities




Page 46 of 92

The graph below shows that utility pinches are formed according to the number

of utilities used. Each time a utility is used a utility pinch is created. It also shows that the GCC right noses sometimes known as pockets are areas of heat integration/energy recovery. In other words it does not need any external

utilities. These right noses/pockets are caused by;

- Region of net heat availability above the pinch

- Region of net heat requirement below the pinch




Page 47 of 92

GCC curve can be used by engineers to select the best match between utility

profile and process needs profile. For instance, the steam system shown below

needs to be integrated with the process demands profile to minimize low

pressure steam flaring and high or medium pressures steam let downs. Besides

it helps selecting steam header pressure levels and loads.

chemicals

Proc. #1

Proc. #1

Proc. #1

Proc. #2

Proc. #3

Proc. #4

BFW

Raw water

Make-up Treatment Plant

MP Process

Condensate

LP Process

Condensate

HP Process

Condensate

Process Condensate

HP Boiler

MP Boiler

HP

MP

LP

Vent

Effluent

Deaerator

Vent




Page 48 of 92

T

H

CW

BFW

LP

HP

MP

Superimposed Utility Profile with Process Profile

Nominal Case Supply-Demand Matching Problem

Process GCC




Page 49 of 92

The superimposed steam system on the process grand composite curve shows

that while process heating needs can be achieved electricity can also be

generated to satisfy process demands and/or export the surplus to the grid.

The graph below shows how we can use the GCC not only to select utility type,

load but also to define the steam headers minimum pressure/temperature to

minimize driving force and save energy.

T

H

CW

BFW

LP

MP

HP

Qh

Qc




Page 50 of 92

Grand Composite Curve can also be utilized to select the load and return

temperature of hot oil circuits. The graph below shows that while in many cases

the process pinch can be our limiting point in defining the load (slop of the hot

oil line) and the return temperature of the heating oil. In some other cases the

topology of the GCC is the limiting point not the process pinch. This is also

shown in the second graph below. This practical guide to select the load and the

target temperature of the hot oil circuits is also applicable to furnaces as will be

shown later in this chapter.

CW

Refrigeration

T*

H

Process

Pinch

Hot Oil

T return

T supply

Process Pinch temperature is the Limiting temperature for the Hot oil return temperature




Page 51 of 92

CW

Refrigeration

T*

H

Process

Pinch

Hot Oil

T return

T supply

Process Pinch temperature is not the Limiting temperature for the Hot oil return temperature

But the topology of the GCC curve

CP-min




Page 52 of 92

Grand composite curve (GCC) can also be used to select the process

refrigeration levels and the synthesis of the multiple-cycles refrigeration systems

as we did in the steam system. The schematic graph below shows a simplified

refrigeration system.

Schematic Diagram for multi-level Refrigeration System

Work

Process

0C

CW

Process

-35C

Process

-65C

25C

-5C

-40C

-70C

Compressor

Condenser




Page 53 of 92

The GCC as we mentioned before can be used to place the refrigeration levels as

we did with steam levels. The graph below shows how we can do that.

T

H

Tcw

We can place the refrigeration levels like steam levels.

Maximizing the highest temperature load to minimize the lower temperature loads

- 5 C

- 40 C

- 70 C

When a hot utility needs to be at a high temperature and/or provide high heat

fluxes, radiant heat transfer is used from combustion of fuel in furnace. Furnace

designs vary according to the function of the furnace, heating duty and type of

fuel, and method of introducing combustion air.




Page 54 of 92

4A.3 Cogeneration Targeting and Drivers Selection

Steam and power balances provide the link between the process utility

requirements and the utility supply. They determine the basis for cogen or no cogen decision, import power requirements or power export potential, boiler sizes, fuel consumption, steam-turbines flows, boiler feed-water requirements,

steam flows in various parts of the process,etc.

An easy way to explore the site power and steam optimal generation and

utilization is through what is called site hot and cold composite curve. It is

important to emphasize on that we recommend, on the contrary of most

literatures, that you include other process steam demands in the balance

calculation in order to depict more accurate picture.

Constructing the site- source and sink composite curves

The first step in constructing the site source-sink composite diagram is to draw

the site-source composite curve and the site-sink composite curve via looking at

each process grand composite curve and extract the source(s) and sink(s)

streams while ignoring the pockets, areas of process heat integration, as shown

in graphs below. Source streams are the ones that have negative slopes, while

the sink streams are the streams that are having positive slopes.




Page 55 of 92

Process

(A) GCC

Process

(B) GCC

T*

H

T*

H

Process A heat

sink profile

Process A

heat source profile

Process B heat

sink profile

Process B heat

source profile




Page 56 of 92

Now let us use a simple example to show that site composite curves can be

drawn the same we do for drawing single process composite curves.

Data for Constructing Composite Curves

Stream Type Supply Temp

(C) Target Temp.

(C) FCp (kW/ C)

Source/Hot 170 70 10

Source/Hot 120 30 20

Sink/Cold 50 90 40

Sink/Cold 20 110 18

For the simple example shown in the table above, first step will be tabulating the

site sources and sinks as shown. The second step in developing the site-

composite curves now is the development of the two tables below. These two

tables, list all the source and sink streams temperatures of each process (A,

B,.N), extracted from its grand composite Curves like the ones shown above, in an ascending order with the cumulative enthalpy (result of adding the

enthalpy of all source streams or sink streams laying together in a certain

temperature interval) corresponding to the lowest hot temperature and lowest

cold temperature respectively equal to zero.

In every temperature interval the cumulative source/hot load is calculated using

the following formula:

H= FCp * (Tsupply Ttarget)

In every temperature interval the cumulative sink/cold load is calculated using

the following formula:

H= FCp * (Ttarget Tsupply)




Page 57 of 92

Source streams temperature list Cumulative Enthalpy (H)

T0=30 H0=0.0

T1=70 H1=800

T2=120 H2=2300

T3=170 H3=2800

Sink streams temperature list Cumulative Enthalpy (H)

T0=20 H0=0.0

T1=50 H1=540

T2=90 H2=2860

T3=110 H3=3220

Temperature (T) - Enthalpy (H) Diagram

T

H

30

20

Site-source composite curve

Site-sink composite curve




Page 58 of 92

The site-sink/cold composite curve shall lie completely below or to the left of

the site-source/hot composite curve and this can be done via dragging the site-

sink/cold composite curve to the right on the enthalpy axis (H). This process

shall stop at a vertical distance between the cold and the hot composite curve for

a temperature equal to reasonable minimum temperature approach.

Temperature (T) - Enthalpy (H) Diagram

T

H

30

20

Site-source composite curve

Site-sink composite curve

Site-Minimum Heating Utility

Site-Minimum Cooling Utility

Qh =480 kW

Qc=60 kW

Minimum Temperature Approach




Page 59 of 92

It is important to note that the construction of the grand composite curve of each

process relies on a built-in Tmin between the hot composite and the cold composite curves. It is a Tmin/2 (half Tmin) lower shift in the actual hot streams temperatures and Tmin/2 upper shift in the actual cold streams temperatures. Since the heating and/or cooling utilities are going to be used as

buffer for the purpose of integration among different processes it is important to

have another shift in hot and cold streams temperatures, which is complete

Tmin instead of half Tmin. If these curves are drawn without considering hot utility/steam as a buffer the graphs will look like the composite curves shown

above. However, in order to better show site-steam generation capability from

the site-source composite curve and its demand based upon the site-sink

composite curve we need to plot the two composites curves as shown below.

Total Site Profiles

T

H

Site Source Profile

Site Sink Profile

Hot streams to be cooled/

steam generation/supply

Cold streams to be heated/

steam Demand




Page 60 of 92

The site composite curves drawn this way can be utilized to select the required

utility mix and its temperature range. The site composite curve shown above to

the left defines for the site, the overall cooling requirements from both enthalpy

and temperatures points of view. The utility selection shall start from the top-to-bottom with the intention of maximizing steam generation. As depicted in the graph, the highest temperature cooling utility in the shown case is medium

pressure steam generation using process high temperature source stream(s).

Again, it is beneficiary for the site to maximize the use of such cooling utility

(high pressure steam generation). The second highest temperature cooling

utility is the generation of low pressure steam. This cooling utility has to be

maximized too. The residual heat that needs to be rejected to the environment

can be then handled using air or water cooling systems. The site sink composite

curve to the right shows the site needs for heating utilities. The process of

selecting the heating utilities on that side is a bottom-to-top marsh. We start at the lowest possible temperature heating utility and we maximize it. In our

case here it is a low pressure steam utility. The next lowest-heating utility is a

medium pressure steam, and also it has to be maximized. The rest of the heating

utility demands can now be handled using high pressure steam.

Targeting for steam Generation/Supply and Demand

T

H

Site Source Profile

Site Sink Profile

LP

MP

HP

CW

LP

MP




Page 61 of 92

Studying the process heating and cooling demands should not be done in

isolation of the process needs for electricity. The interaction between the

process units, hot utility and cold utility systems is extremely important.

Sometimes it is not very clear to the straight forward old perceived intuitions.

Accurate process steam demands and generation capabilities are essential for

proper targeting of the site cogeneration design.

After recovering heat between process steam generation and process steam

usage, the balance of the heating demand and other process steam users will be

satisfied by fuel fired in the utility boilers to generate the required steam

demands. Normally, very high pressure steam will be produced to produce

power and use the exhausted steam in satisfying the process demand.

The shaded area, in the left graph below, is a region where higher pressure steam

is expanded through steam turbine to lower pressure steam to produce power.

This shaded region can be used roughly to compare between the amounts of

power that can be produced from a site at different scenarios. The site steam

headers might also have a pinch where above it there is a steam supply

deficiency and below it there is a surplus of heat/steam supply and the site needs

to reject it to the environment. This is normally rejected to water or air coolers.

In order to maximize the true cogeneration of power and steam from the site,

low pressure steam generated is expanded to vacuum pressure steam, which is

ultimately condensed using cooling water.




Page 62 of 92

While this graphical procedure can render some insights we recommend that

you use algebraic method to with simple equations for steam turbine to estimate

the exact amount of power that can be co-generated with steam need to satisfy

the process demand. Schematic representation of the method is shown to the

right of the graph below.

Steam generated

by the process

Steam Consumed

by the processLP

MP

HP

Fuel

VHP

VP

CW

E1

E2

VPE3




Page 63 of 92

Back to the graphical method that can give very useful insights, the graph below

can be used to as we said before in getting an idea about amounts of power that

can be produced from a site in different scenarios.

Fuel

VHP

VP

H

T




Page 64 of 92

In steam turbine situation, the larger the flow of steam through the turbine, the

greater is the amount of power that will be produced and the larger the pressure

difference and hence the larger the saturation temperature difference across the

turbine, the greater the potential for power generation. Such po

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