sabp-a-012.pdf
TRANSCRIPT
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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
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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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Document Responsibility: P&CSD/Energy Systems Division SABP-A-012
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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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Document Responsibility: P&CSD/Energy Systems Division SABP-A-012
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Next Planned Update: TBD New Projects Energy Efficiency Optimization Review Methodology
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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
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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
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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%
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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.
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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
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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
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 85 94 170 243
Power Import, Winter MW 88 100 183 257
Fuel, Summer MMBTU/h 1,279 1,279 1,279 1,279
Fuel, Winter MMBTU/h 1,279 1,279 1,279 1,279
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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.
Operational cost factors will be based on power import and fuel consumption as shown
in the following table (excess heat loss via GT Stack is also included):
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Operation Cost Factors 2011 2015 2022 2030
Power Import, Summer MW 64 72 126 198
Power Import, Winter MW 64 72 126 198
Fuel, Summer MMBTU/h 1,222 1,222 1,480 1,480
Fuel, Winter MMBTU/h 1,222 1,222 1,575 1,677
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
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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.
Operational cost factors will be based on power import and fuel consumption as shown
in the following table (excess heat loss via GT Stack is also included).
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Operation Cost Factors 2011 2015 2022 2030
Power Import, Summer MW 45 53 126 198
Power Import, Winter MW 45 53 126 198
Fuel, Summer MMBTU/h 1,135 1,135 1,135 1,135
Fuel, Winter MMBTU/h 1,135 1,135 1,530 1,632
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
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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
Operational cost factors will be based on power import and fuel consumption as shown
in the following table (excess heat loss via GT Stack is also included):
Operation Cost Factors 2011 2015 2022 2030
Power Import, Summer MW 84 93 111 183
Power Import, Winter MW 84 93 111 183
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
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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.
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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.
Operational cost factors will be based on power import and fuel consumption as shown
in the following table (excess heat loss via GT Stack is also included).
Operation Cost Factors 2011 2015 2022 2030
Power Import, Summer MW 0 0 0 0
Power Import, Winter MW 0 -21 -31 0
Fuel, Summer MMBTU/h 1,118 1,186 1,962 2,603
Fuel, Winter MMBTU/h 1,118 1,355 2,240 2,603
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.
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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.
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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.
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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
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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
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
HP gas from inlet manifold
Dried HP gas to export
Condensate Feed Drum
GOSP condensate from
condensate inlet manifold
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
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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.
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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
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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.
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260 Psig445 Psig
425 Psig
TEG unit
NGL Stabilizer
HP gas from inlet manifold
Dried HP gas to export
Condensate Feed Drum
GOSP condensate from
condensate inlet manifold
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
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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
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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
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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
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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
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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.
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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
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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.
PROCESSH CFeed Product
0
20
40
60
80
100
120
0 10 20 30 40 50 60 H
T HOT UTILITY
COLD UTILITY
HEAT
RECOVERY
Pinch(MAT)
PROCESSH CFeed Product
0
20
40
60
80
100
120
0 10 20 30 40 50 60 H
T HOT UTILITY
COLD UTILITY
HEAT
RECOVERY
Pinch(MAT)
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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
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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.
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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.
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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.
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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.
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MP
CW
Refrigeration
T*
H
T-tft(C)
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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
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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
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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
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T
H
CW
BFW
LP
HP
MP
Superimposed Utility Profile with Process Profile
Nominal Case Supply-Demand Matching Problem
Process GCC
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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
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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
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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
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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
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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.
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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.
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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
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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)
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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
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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
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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
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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
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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.
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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
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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
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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