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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 utilities very important.

    However, the other side of the coin is reliability. The utilities systems form the backbone of any industrial site. 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

    ESL-IE-04-04-17

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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 be optimized 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 utility and 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-if capability to deal with the unexpected.

    •  On-line real-time capability, with connections to ‘live’ prices and process information management systems, 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 utility demands. The corrective action after an unexpected boiler failure depends very much on the duration for which the boiler will be unavailable.

    Aspen Utilities™ provides such a tool.

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

    ESL-IE-04-04-17

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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  R A  T  E   G I   C 

    T   O  O L 

    DCS

    Data Improvement Data Validation & Reconcil iation

    Real Time Data Base

    Off-line Decision Making

    Off-line Decision Making

    Lowest Cost Scheme

    Lowest Cost Scheme

    On-line Decision Making

    On-line Decision Making

    Lowest Cost Operation

    Lowest Cost Operation

     

    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 between energy efficiency and reliability, such as:

    •  Hot standby

    •  Load shedding

    •  Drive switching

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

    ESL-IE-04-04-17

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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 process involving 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 standby capacity 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.

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

    ESL-IE-04-04-17

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    Minimum Hot Standby, Mlb/hr 

       C   o