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Optimization of Hot Standby José de Souza and David Holden

Aspen Technology Inc

1293 Eldridge Parkway

Houston TX, 77077

Introduction

Energy Optimization is one of the key issues facing the chemical process industries today. The drivers are

both economic and environmental. Utilities are among the top operating expenses for manufacturers,

reflecting elevated energy prices and the energy intensity of the industry. The difference between the most

energy efficient manufacturers and those with average energy efficiency is approximately 30%, clearly

demonstrating potential for improvement. In addition to capital versus energy considerations,environmental concerns now add new pressures to reduce CO2, NOx and SOx emissions. Deregulation of

the energy supply market in both the USA and Europe has made “intelligent” purchasing and use of utilitiesvery important.

However, the other side of the coin is reliability. The utilities systems form the backbone of any industrialsite. The loss due to an unexpected outage of process units due to lack of steam, fuel or power, usually far

outweighs the energy savings of reducing back-up utility equipment availability. So, how can one reduce

utilities operating costs without compromising the reliability of the site? What is the optimum trade-off

between reliability and cost?

Reliability

Most industrial sites use the so-called “Reliable Capacity” operating philosophy for their utilities systems.Reliable capacity, also referred to as “N-1” Capacity, is defined as the sum of generating capacities for a

given utility minus the capacity of the single largest generator.

For example, if a site has five boilers with the following capacities:

Boiler 1 150 Mlb/hr

Boiler 2 150 Mlb/hr

Boiler 3 150 Mlb/hr

Boiler 4 250 Mlb/hr

Boiler 5 100 Mlb/hr

Total 800 Mlb/hr

Reliable Capacity 550 Mlb/hr (69% of total)

Proceedings from the Twenty-Sixth Industrial Energy Technology Conference, Houston, TX, April 20-23, 2004

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In today’s competitive environment, most industrial sites cannot afford the luxury of an overly conservative

utilities operating philosophy. They need reliability – but not at any cost.

As sites have developed, the above reliability issues have been addressed and redundancy has been built

into the utilities supply systems. As companies focus more and more on cost reduction, this redundancy

can be exploited as flexibility and used to reduce costs. In striving to achieve the best trade-off between

reliability and cost, today’s sophisticated energy management tools can prove invaluable.

Such energy management tools need to provide the following:

• No compromises on utilities reliability. However, the purchase, supply and use of utilities must beoptimized within the constraints of site reliability.

• The ability to quantify the costs of increased reliability.

• A tailored operating philosophy for the site. Each site has different characteristics, both on the utilityand process side, which dictate the best operational philosophy.

• A holistic view of the whole site’s economics, including process and utilities.

• The ability to train operations personnel in optimum economic operations and to provide a what-ifcapability to deal with the unexpected.

• On-line real-time capability, with connections to ‘live’ prices and process information managementsystems, so that decisions are based upon the most up-to-date economic and process information.

• A predictive capability which facilitates decisions based not only on current, but also on future utilitydemands. The corrective action after an unexpected boiler failure depends very much on the durationfor which the boiler will be unavailable.

Aspen Utilities™ provides such a tool.


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Aspen Utilities can be used to identify the most cost-effective utility suppliers and contract parameters for a

given utility system flowsheet. This is applicable for daily/monthly/annual contract nomination, from a

fixed supplier or for contract evaluation studies to determine long-term gas or electricity suppliers. It can

also be used to determine the marginal price of gas and/or electricity on the site, and this can guide

decisions relating to the sale or purchase of electricity and/or gas on the spot market.

Figure 2 provides an overview of Aspen Utilities’ capabilities. Aspen Utilities is aimed at supportingstrategic as well as operational decisions. It provides a common decision-making framework for both types

of decisions. This helps the site management to set one common evaluation tool for all utilities-related

decisions in the site.

12©2000 AspenTech. All Rights Reserved.

System Overview

Tariffs

Operational

Constraints

Plant Demands

Operations Advice

•Turbine/ boiler on/off decisions

•Flow distribution

•Equipment fouling•Multiple objective functions

O P E R A T I O N S

T O O L

Tariff

scenarios

Seasonal

variations

Changes in demand

e.g. site expansion

What-if Analysis•Tariff selection•Optimize Investment options

•Significant changes in demands

•Changes in fuel supply

S T RA T E GI C

T O OL

DCS

Data ImprovementData Validation & Reconcil iation

Real Time Data Base

Off-line DecisionMaking

Off-line DecisionMaking

Lowest CostScheme

Lowest CostScheme

On-line DecisionMaking

On-line DecisionMaking

Lowest CostOperation

Lowest CostOperation

Figure 2:

The basic premise of Aspen Utilities is a common model and optimization basis for strategic and

operations decisions in a site.

A description of the business processes that Aspen Utilities supports is provided in Appendix A.

How does Aspen Utili ties Address Reliability?

There are several ways in which Aspen Utilities helps plant operations to find the correct trade-off betweenenergy efficiency and reliability, such as:

• Hot standby

• Load shedding

• Drive switching


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• Fuel selection

• Electricity supply

• Maintenance planning

• Operator training

Hot Standby

Hot standby is the amount of spare generation capacity which must be available during operations, i.e. it is

the difference between the maximum generation capacity of all the online units and current load. For

example, if a site has 4 boilers, each with a maximum capacity of 200 Mlb/hr, and the current total site

steam demand is 500 Mlb/hr, then if the minimum hot standby is 100Mlb/hr or less, only three of the

boilers need to run, but if the minimum hot standby is greater than 100Mlb/hr, say 150Mlb/hr, then all four boilers need to be running in order to meet this requirement.

The hot standby requirement can be specified in Aspen Utilities. The optimization will then define the

number of the boilers that need to be on line and the optimum setpoint (steam generation) of each boiler to

achieve the lowest cost operation.

If the hot standby requirement is known then this can be specified as a constraint within the optimization.

However, in many cases the definition of hot standby capacity is somewhat of a subjective decision processinvolving the N-1 principle discussed earlier. Increased reliability could be achieved by having the

equivalent of say two boilers on hot standby rather than one. What is the cost of this increased reliability?

If the economics are not clear then Aspen Utilities can be used to develop a more objective decision

process for hot standby capacity. Typically the cost of providing additional hot standby capacity is not

linear with capacity but features steps that correspond to more boilers being brought on line as can be seen

in Figure 3. It can be seen from this plot that at this particular site providing 25% of the hot standbycapacity results in no increase in operating cost, 50% of the total hot standby capacity can be provided at

less than one third of the cost of maximizing hot standby capacity. The appropriate use of this plot can provide useful insights to aid in the decision making process.


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Minimum Hot Standby, Mlb/hr

C o s t o f H o t S t a n d b y , $

Figure 3: Cost of Hot Standby

Load shedding

By providing sufficient hot standby capacity a site may always be able to provide sufficient steam (and

or electricity), there may be times when despite best efforts there is insufficient steam supply. In such

a situation a decision may have to be made and made quickly as to which units to selectively shutdown

in order to prevent the entire site from shutting down due to loss of steam pressure. In this case thedecision as to which units to shutdown should not be made purely on steam usage but more on the

implications of shedding a unit, e.g. lost product, maintenance, renewal of catalyst and so on.

Aspen Utilities can be used to define which units could be shed to meet expected steam raising

capabilities. The value of the steam saved set against the cost of lost production can be used to definethe priority units to shut down. The entire procedure can be automated by defining possible problems

that would lead to reduced steam supply and by providing automatic update of product value etc. The

result, as seen in Figure 4, can be an advisory table, available in real time that advises the appropriateaction for any eventuality. A natural consequence of this is that if in a load shedding run the cost of

load shed is less that the value of the utility going to the unit then the unit will be advised to be shed

even if there is no requirement on the steam demand side.

In the case shown in Figure 4 a trip of any single boiler does not lead to a situation where loadshedding needs to take place. This would be the normal case. However, in situations, for example,

where certain boilers are down for maintenance, a trip of an online boiler may result in the need to

shed load.


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Process A

Section 1

Process A

Section 2Process B Process C Process D

Process D

Mode BProcess E Process F Process G

Co

Load

Load Shed Cost 14400 960 6000 3120 5520 3120 2400 2400 9600 43

Trip Scenario 1 - Boiler A trip 1 1 1 1 1 1 1 1 1

Trip Scenario 2 - Boiler B trip 1 1 1 1 1 1 1 1 1

Trip Scenario 3 - Boiler C trip 1 1 1 1 1 1 1 1 1

Trip Scenario 4 - Gas turbine trip 1 1 1 1 0 0 0 0

0

0

1 20

Trip Scenario 5 - Incinerator trip 1 1 1 1 1 1 1 1 9

Trip Scenario 6 - Exothermic process trip 1 1 1 1 1 1 1 1 14

Figure 4: Load Shedding Advisory System

Drive Switching

Although drive switching options have historically been provided on critical drives in order to improve

reliability, the ability to select either an electric motor or a steam turbine provides some flexibility to reduce

costs by selecting the most cost effective drive. Which drive to use depends on several factors including he

cost of electricity, overall site steam balance and the perceived reliability of turbines versus motors. Theword “perceived” is used here because there is no industry standard as to whether turbines or motors are the

more reliable as issues such overall electricity supply reliability, time to start up steam turbines andoperator preference all come into the equation and deliver different answers for different sites.

Aspen Utilities can make the economic optimum decision as to the lowest cost option for each driveswitching choice within a single optimization run. The cost of “preference” can then be fully quantified.

The cost of switching each drive on or off is specified in the Aspen Utilities model and taken into account

in the economic optimization. Practical constraints like the frequency between switches of a drive can also be imposed on the optimization.

Fuel Selection

A similar situation exists with fuel selection as with drive switching. Many sites have the ability to burndifferent fuels. What may have originally been a fuel supply reliability issue can now be used to reduce

energy costs by selecting the most cost effective fuel mix and distribution. Issues such as complex utilities

contracts (including take or pay), dual fuel capabilities, emissions limits on each fuel and calorific value

can all be taken into account in the optimization.

Electricity supply

When the ability exists to both generate internally and/or to import electricity, Aspen Utilities can be used

to recommend the most cost effective solution. Issues such as complex utilities contracts and electrical

distribution system constraints can all take taken into account. Some of the contracts Aspen Utilities can

handle are:

•

Contracts with peak and off-peak time-windows

• Maximum demand charges

• ‘by-total-usage’ contracts, where the charge is based upon the net usage over an extended period,usually a month, not the current instantaneous value


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Maintenance planning

Correct maintenance plays an important part in maintaining system reliability. However, bringingequipment down for maintenance can also have a big impact of system availability (e.g. less hot standby

capacity when a boiler is brought down) and on operating costs, for example when a gas turbine needs to be

brought down and electricity import increased to compensate.

Aspen Utilities can provide two important capabilities to help with maintenance planning. First, it can

track the performance (efficiency) of utilities equipment and thus highlight when cleaning is appropriate.

(It should be noted that in some cases the tracking of this same information can provide early warning as to possible impending operating problems). Secondly it can be used to advise regarding the optimum

schedule for maintenance when such downtime would have the lowest cost impact.

Operator training

While not a dynamic Operator Training system, Aspen Utilities can be used to train operations personnel as

to the best economic and practical operation of the site utilities system against a complete range of

operating scenarios. Aspen Utilities can provide operator advice as to the best economic operation given

defined or predicted utilities demands from the process units and utilities equipment availabilities. In

addition Aspen Utilities can be used in a “what-if” environment to check specific operating issues. In thisway, operators become more able to deal effectively with abnormal situations and thereby improve both the

overall reliability and cost effectiveness of the utilities systems.

Conclusions

The process industries rely on adequate and reliable supplies of energy. Without the energy system nothing

happens—nothing is heated or cooled, pumped or compressed. Due to the reliance that the industry has onenergy supply, the issue of reliability of the utilities systems has always been of paramount importance.

Energy is also by far the largest operating expense for petroleum refineries and many chemical plants.

Energy accounts for over 40 percent of refinery operating expenses, and over 60 percent of olefin plant

operating expenses.

A common drive in the industry is to reduce costs while simultaneously improving system reliability and

hence availability. While these objectives may not appear to be compatible, it is possible with the help of

today’s advanced information systems to fully address the trade-offs that exist between utilities systems’reliability and cost. Aspen Utilities provides the means to evaluate and achieve the correct balance between

reliability and energy cost reduction and in so doing provide the means to achieve both objectives.


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Appendix A : The business processes approach toEnergy Management supported by Aspen Utili ties

11©2000AspenTech. All Rights Reserved.

Figure A-1: Overview of the business processes supported by Aspen Utilities

Aspen Utilities supports the following business processes

• Demand Forecasting

• Production Planning

• Optimal Plant Operation (On-line optimization)

• Tariff Evaluation and Contract Management

•

Performance Monitoring

• Emissions Monitoring

• Investment Planning

• Cost Accounting


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Aspen Utilities explores the flexibility inherent in the purchase, generation, use and distribution

of utilities on industrial sites and advises on the optimum choice available to the user. Issues such

as different tariffs, alternative fuels, optimum loading of boilers and turbines, choice of

equipment, import, self sufficiency or export of electricity, choice of drives (motor or turbine) etc.

are all taken into account in the optimization.

Benefits of applying Aspen Utilities in the operational management are typically in the range of 2to 5% of site-wide energy costs. These benefits are obtained by:

• Better purchasing – lower contract price

• Better adherence to contract/tariff terms – reduced penalties

• Maximizing use of most efficient equipment

• Correct choice and use of fuels

• Reduced hot standby

• Reduced venting of steam

• Better cost accountancy

• Faster response to problems (and better targeting of problems)

• Optimum scheduling of maintenance

• Reduced manpower for accounting, purchasing etc.

• More profitable trading (import/export).

• Reduced capital investment for improvements in energy efficiency

Aspen Utilities provides a model-centric approach whereby a single rigorous model of the

utilities system is used to address all the important business processes. These are described in

more detail in the following sections.

Demand Forecasting

In order to operate and manage the utilities supply system at lowest cost requires knowledge of the current

demands and likely future demands. This helps to minimize the use of hot standby (e.g. boilers), the

venting of steam due to excess on-line capacity and the loss of supply due to insufficient standby or control.It also ensures that penalties are not incurred due to violation of take-or-pay contracts, maximum demand

charges or load factor clauses in both the electricity and gas contracts.

Production Planning


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This involves taking the demand profiles and, based on availability of utilities generation equipment,

developing an optimized production plan within the constraints of the utilities tariffs. Production planningcan typically be carried out on a tactical basis for the next 24 hours and on a strategic basis to review, for

example, the best configuration when a gas turbine needs to be brought down for inspection or when a

process unit is to be shutdown/started up.

Optimal Plant Operation (On-line optimization)

While a plan may be developed in advance (say every 24 hours), in practice the operations of the plant maychange within that period thereby invalidating the optimum plan. Even if the steam and power demands do

not change within this period, other factors such as electricity price and gas price will vary. This solution

provides on-line advice to operations personnel on how to best configure and operate the system at lowestcost based on current demands and current costs. The operator is also able to use the solution in a “what-if”

environment to evaluate alternative operating modes to accommodate, for example, for a unit shutdown.

Tariff Evaluation and Contract Management

As the markets for both electricity and gas open up to competition the site operator can be faced with a bewildering array of tariff options. Making the right choice of supplier is not simply a case of choosing the

lowest unit cost of utility (e.g., cents per kWh). Most tariff structures include elements of maximum

demand charges and punitive penalties if this maximum demand is exceeded. Without the benefit of

suitable software the task of tariff selection is very labor intensive, with no guarantee that the best optionhas been selected. Companies operating large cogeneration systems (which are increasingly the norm) have

an added complexity in being able to export electricity. Aspen Utilities provides the following capabilities

to assist in contract selection and optimization (see Figure A-2).


Contract optimization - Aspen Utilities

Gas

M3/hr

Jan Dec

Elect

MWe

Jan Dec

SteamT/hr

Jan Dec

GasM3/hr

Jan Dec

Elect.MWe

Jan Dec

One profi le for each header Consumer gas & electricity requirement

Past Profile

Optimized Profile

equipment availability

perational constraints

Al ternati ve fuel

availability

Gas & elect.

bill forselected

contract

Site wide gas and electricity demands

Modeling and Optimization

Gas & Electric ity

relevant tariff structures

multi period

optimization model

Figure A-2: Overview of contract optimization capability in Aspen Utilities


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• Create a demand profile for a long-term period (several months, 1 year, several years) foreach utility.

• Determine minimum, maximum and average demand.

• Define the available utility contract options (costs, structure, limits, etc.)

• Send the demand profile and contract information to an optimizer to determine thecontract mix that would yield the lowest cost solution.

• Achieve streamlined profiles for the overall gas and electricity consumption to minimizethe overall utilities bill for the site.

Once a tariff has been selected and a contract established, there is an on-going requirement to manage the

utilities consumption within the terms of the contract.

The key to driving down the purchase cost of utilities is to reduce both the average demand and themaximum demand and to “exploit” the tariff structures to the advantage of the operating company. In

general, the bigger the gap between the average demand and maximum demand the larger the unit cost ofutility (electricity or gas). Companies can exploit degrees of freedom within the site operations to staywithin the contract constraints. Such flexibility could be to switch off electric motors and switch on steam

turbines, burn fuel oil rather than imported natural gas, etc. This solution provides the operator with an

accurate picture of the current operation, likely future demands and operation, and hence highlights

potential costly problems. The operator is then able to use the solution to identify the best way to avoid the

problem from occurring.

Performance Monitoring

Performance monitoring comprises two aspects:

1. Comparison of the production plan against the actual performance.

2. Tracking of performance of key equipment items

Tracking of plan versus actual is a well-established methodology for continuous improvement. Thesolution allows the operator to develop a daily production plan and to compare the actual daily performance

with the plan. This highlights areas where the plan was not followed and facilitates the understanding of

why the plan was not followed. This, in turn, defines how production plans can be improved (perhaps byincorporating new constraints) and defines areas which could benefit from other improvements such as

equipment modifications, better control, etc.

If the necessary metering is in place then the solution can also track performance of the individual items of

equipment within the utilities system, for example the efficiency of the boilers and gas turbines. This

information can be used to optimize cleaning and maintenance schedules and can also provide earlywarning of operating problems.


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Emissions Monitoring

Increasingly, a permit to operate requires that plants operate within strictly defined environmental

constraints. In such situations, monitoring and regular reporting of emissions is a necessary task. Certain

gaseous emissions such as SOx and CO

2 can be directly related to fuels burnt. Other emissions such as NO

x

require predictive models as the NOx production is not related solely to fuel burnt but is also dependent on

flame temperature, state of the equipment, etc.

When the permit to operate specifies a maximum emissions limit then this can cause a constraint to production and to the operation of the utility system. The solution can provide emissions prediction and

reporting for CO2 and SOx and can be integrated with predictive NOx models.

Investment Planning

While this solution enables the operating company to operate and manage the utility plant to its true potential, there will be limits to optimization imposed by the design of the system. The company can use

Aspen Utilities in an off-line mode to evaluate possible structural changes (e.g. a new steam turbine, neweconomizer, dual fuel firing, etc) and to develop the best investment plan for the utility system.

The product uses a GUI with a drag and drop environment, incorporating a series of standard models,

which the engineer can use to develop a new configuration and evaluate the economic benefits plant-wide

of any change to the current configuration. This provides the operator with the confidence that the bestinformation on demands and current system ability is used when evaluating possible investment options.

This, in turn, ensures limited capital is spent in the best way.

Cost Accounting

In many companies the allocation of costs for utilities can be somewhat arbitrary and hence unreliable.

Aspen Utilities provides the ability to perform accurate cost allocation. Aspen Utilities can also beconfigured to provide utility costs in real time and to also provide true marginal costs, which would apply

to a reduced or increased use of utilities. Real time prices can support decision-making processes such as

the costs of increasing steam use to a particular unit to enhance throughput or the costs of steaming-out ofequipment.

Link with Production Planning and Optimization

The utility system is closely linked with the production processes in the site. The production planning and

the production optimization steps utilize utilities cost as an integral part of the optimization and decision-

making. Aspen Utilities provides a more accurate understanding of the utilities cost for supply andgeneration within the production processes. This integration provides a more consistent basis for site-wide

production management. The impact of utilities equipment scheduling and utilities contract usage can betaken into account in plant production decision-making. Alternatively, the plant production constraints and

opportunities are well accounted for in the utilities optimization.


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Power Import/ExportPower Import/Export

Process

A

Process

B

Process

C

Process

D

FuelFuel

ProductProductSideSide--

ProductProduct

FeedsFeeds

ProductProductFeedFeed

SideSide--ProductProduct

Utility System

Fuel ImportFuel Import

Steam ExportSteam Export

ProductsProducts

• Utility prices

• Utility equipmentschedule

• Optimum contractusage

Plant

Optimization

Production Planning & Scheduling

Figure A-3: Aspen Utilities works with production planning and optimization tools.

Link with Plant Design Improvements

Utilities cost plays an important role in the investment justification for plant revamps. Inaccurate pricing of

steam, gas and electricity can lead to missed design opportunities. Aspen Utilities provides a clearunderstanding of the price of the utility streams but also changes in the marginal prices of these streams in

different operational scenarios. This provides a much more concrete basis for estimating the capital energy

trade-off during the conceptual and front-end design stages during plant revamps. AspenTech’s Aspen

Engineering Suite provides a comprehensive environment to develop the process designs in an integrated

and efficient manner (see Figure A-4).


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Aspen Pinch

Aspen B-JAC

Aspen Plus

Plant simulation

Energy Analysis

Equipment Design

Process D

Utility

streams

Figure A-4: Aspen Utilities provides an accurate understanding of utilities cost impact

during the process design stage.

Multi-site operation

AspenTech helps develop solutions for optimization across multiple sites to provide enterprise-wide

optimization in utility and energy systems (Figure A-5). In such applications, Aspen Utilities is used to

optimize the operation of each individual site. Aspen Utilities then communicates with AspenTech’s MIMIenterprise-wide Supply Chain software to optimize the purchase, sale and operation for the enterprise. This

approach also helps to analyze new power plant investment decisions such as size of the new unit, and to

assess the impact of key design factors on the overall profitability.


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Corporate Purchasing and Trading

Enterprise wide Optimisation

Enterprise Asset Optimisation

Site 1

Industrial

Cogen

Site 2

Industrial

Cogen

Site 3

Power

Station

Local Asset OptimisationLocal Asset Optimisation

Two

Major

SItes

Single

Major

Site

Site 4

Industrial

Cogen

New

Investment

?

Figure A-5: Overview of enterprise wide optimization of utility systems

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