article hydrocarbon april 2007

5/25/2018 Article Hydrocarbon April 2007

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Rob Howard, Sebastien Osta, Francois van Niekerk and Jos Mara Ferrer, AspenTech

Europe, discuss how the use of best practices combined with the latest technologies

can significantly extend the benefits delivered by advanced process control (APC).

EXTENDINGAPCBENEFITS

www.hydrocarbonengineering.com Reprinted fromApril2007 HYDROCARBONENGINEERING

APC is now a mature and well proven technology in

the process industries with demonstrated benefits

that typically offer paybacks in months. Ensuring

a proper ROI requires a concerted corporate widestrategy and utilisation of the latest innovations,

supported by considerable inhouse expertise.

The good news is that APC technologies are

continually evolving, with enhancements to the

basic software and integration with a series of

additional tools that deliver incremental benefits.

These advancements make APC easier to implement,

operate and maintain so that users can generate

value more quickly and sustain that value over alonger period1. More advanced software platforms

have also been developed to address particular

control challenges.

This article reviews these advancements,

describing some of the key new tools and exploring


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Reprinted from HYDROCARBONENGINEERING April2007 www.hydrocarbonengineering.com

some of the methodologies and best practices available

for the future APC implementations.

Process analysis and benefitsestimatesThe benefits of APC applications have been so well

documented that many companies have adopted a

strategy of standardising the technology and rolling itout company wide. While it was possible in the past

to get projects approved in this manner for the larger

process units, today more operators are implementing the

technology on the second tier or smaller process units in

refineries and chemical complexes. Although the software

and implementation costs have come down over the

years, there is an increasing emphasis on identifying the

benefits from the proposed applications to ensure project

approval. Conducting thorough yet rapid assessments of

the benefits from APC applications that capture technical

and operations management attention speeds up decision

making, ensures that the benefits are actively pursued and

allows them to be thoroughly post audited.

Benefits of improved control translate into economic

benefits through one or more of the following areas:

Increased production of more valuable products via

increased throughput/capacity and improved product

recovery/yield.

Improved quality control.

Improved reliability.

Reduced energy consumption.

Changing the average operating point on the plant

is the key to realising benefits, as shown in Figure 1; sounderstanding current operations is critical to estimating

the possible improvement.

Best practice guidelines

Process analysisAt the heart of any benefits analysis is a detailed

process operations analysis and review to understand

the prevailing economics of the unit, operating

objectives, different operating modes (eg. seasonal and

market driven), equipment limitations and constraints,

instrumentation and quality information. This knowledge is

best gathered in a workshop environment where the key

operating, technical and planning/commercial personnel

are led through a structured discussion on the process,

beginning with the big picture before diving into details.

The key question to be answered is why is the plant

operated in this way/to this constraint, and how would

changing this make more money? A relentless focus on

exposing the real (as opposed to the inferred, assumed or

historical) constraints is required.

Understanding constraintsThe all too frequent answer to a question relating to

an operating parameter setpoint or limit (eg. furnace

throughput or reflux rate) is that is the design value or

we are already operating 10% above the design. The

consultants response should be what measurement on

what piece of equipment is the real limiting constraint,

and who do we need to speak to in order to understand

the constraint in more detail and what changes might be

possible?

In Europe today many of the major constraints are

not even process ones but quoted as limits by the

authorities. Not only are there operating permits that

define, for example, maximum FCC feedrates, but also

an ever increasing number of environmental emissions

constraints. Understanding the nature of these regulations

is important as some apply not only in bulk across a site

(total sulfur emissions) but also per furnace or stack (totalduty or NOx and SOx concentrations). In many sites these

higher level constraints are mapped onto lower level local

constraints as absolutes, when in fact they are normal

variables to be controlled like any other, where APC can

make a significant difference.

Data collectionHaving understood all the major operating issues and

constraints, the historical process performance of the

plant should be analysed in this context. How much

data of what frequency is required for this analysis?

The simple answer is to collect as much as is needed

to capture the full operating constraint environment ofthe plant. As a minimum it is recommended that several

months of data, from a winter and summer operation, is

collected, and preferably a full year of data. Since different

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Figure 1. APC profit.

Figure 2. Prototyping APC at design phase.

Figure 3. Aspen DMCplus controller integrated in Aspen HYSYS

Dynamics.


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benefits are normally achievable across the seasons it is

good practise to be able to validate with actual data the

basis for the amount of time in the year that a particular

benefit is claimed. This data can be an hourly snapshot

or even daily average data. Additionally, a period of high

frequency data for one to two weeks should be collected

for the period when onsite. This data can be analysed to

produce simplified models of the plant and determine keygain relationships between manipulated and controlled

variables.

During the onsite time limited process testing should

be used where necessary to understand important

relationships, particularly in areas where operations

personnel have not typically made moves in the plant.

This also helps to validate the proposed operating limit

adjustments or to determine the true operating limits.

The techniques for processing the data and

quantifying the benefits achievable from APC have been

covered well by Canney2, and include the following:

Evaluating the differences between best, worst

and typical operations. For looking at data overa long period of time, the gap between average

performance and the 90% percentile gives a

reasonable indication of the improvement possible.

Reduced standard deviation in key process variables.

This is the traditional statistical approach, which

assumes a certain reduction possible (typically 50%

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is used, but post audits frequently show 70 - 80%

reduction), and then ties this back to an increase in

a production rate or yield. Care should be taken to

ensure that this approach takes account of all other

constraint impacts.

Finally, having translated the process constraint and

economic knowledge into a set of key plant performance

parameters or KPIs whose current performance is definedtogether with expected improvements from the APC, a

full benefits analysis summary can be prepared. With a

benefits estimate firmly grounded in data from the current

plant operation the next phases of the project should

proceed smoothly.

APC prototyping in theengineering design phaseDuring the engineering design of a new plant, engineering

firms typically take overall responsibility for the design,

construction and startup as part of a turnkey contract;

the plant is usually accepted by the client after 6 - 12

months of stable operation. In many cases, the first

thing the client does after accepting the plant is to

start improvement programmes aimed at optimising

current operations. These programmes often identify

APC as a first logical step, which can then require new

instrumentation, changes in the basic controller layout,

plant tests, and new hardware and software installations,

which is not desirable for a newly commissioned plant.

Therefore it clearly makes better sense to include APC

implementation within the scope of the engineering firm.

Since they design the processes using rigorous simulation

models, including detailed dynamic simulations to verify

control layouts, safety scenarios and startup/shutdown

procedures, they already possess the deep understanding

of process dynamics required to implement APC

successfully. The existing models can be used to design

and prototype the APC system, so that the most suitable

instrumentation and control layout can be selected from

the outset3. This approach, as shown in Figure 2, has the

advantage of shortening the time to optimum operation,

minimising the impact of plant tests, and reducing the

overall cost.

Logically, this approach is more suitable for processes

where the dynamic model provides enough rigour and

fidelity to the real process, for example gas plants, NGL

recovery, LNG liquefaction and general distillation.

To support this methodology, engineering simulation

tools such as Aspen HYSYS Dynamics have beenintegrated with APC software such as Aspen DMCplus

(Figure 3) to efficiently design and verify the optimum APC

structures4, 5.

Before performing step tests in the real plant, the

dynamic model serves as a virtual plant where the virtual

step tests can be performed (Figure 4). With the results,

a prototype or draft APC model can be obtained, which

makes it possible to analyse the dynamic responses of

the plant and calculate the optimum move size for the real

step tests. If Aspen SmartStep is used to automate the

real plant tests, the prototyped Aspen DMCplus model

can be used as the good seed model for the automated

testing tool, minimising the impact on plant operationsand personnel.

For new or existing plants, the advantages of this

approach are:Figure 5. Remote visualisation using production control web server.

Figure 4. Virtual tests performed in Aspen HYSYS dynamic model.


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Reprinted from HYDROCARBONENGINEERING April2007 www.hydrocarbonengineering.com

Minimises or eliminates step testing:

Generates seed model for Aspen SmartStep.

Calculates optimum amplitude of moves.

Provides confidence with the plant models.

Improves and verifies APC models:

Not contaminated by unmeasured disturbances.Verifies linearity range of the process.

Hidden problems arise when models do not match.

FeedForward moves can be imposed.

Better testing and training:

Controller is tested in a wider range of operation.

Basic control layout improvements can be quickly

studied.

Enables models to be reused for retesting (revamps

or operation mode changes).

Simulation models available during plant

shutdowns.

Rich, risk free training of operators and engineers.

Successfully maintaining APCapplications

Value delivered by APC can be sustained by adopting

best practice technologies to improve and optimise

performance.

Remote visualisationThe APC engineer cannot always be onsite to monitor the

way the controller is being used. In a plant environment,

with operators changing shifts every eight hours or so,

the APC engineer has to be able to react quickly when a

possible incorrect use of the tool has been identified, andto deliver training in real time to the operator.

In this situation, technologies are available to provide

remote visualisation of what the multivariable controller

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is doing through a standard web interface (Figure 5). This

provides:

Remote navigation and trending.

Role based assess to tuning parameters.

Visualisation of the custom calculations and current

model matrix.

Management functions of the controller.

Performance analysisA critical part of being able to maintain an APC application

is the ability to perform a quick analysis of what the

multivariable controller did in the past. If the APC engineer

is not able to understand the reason why a controller

did not behave properly or why an operator turned the

application off under particular circumstances, then the

situation will repeat in the future.

To address this situation, tools exist that allow users

to analyse controller performance using a series of user

friendly features:

Automatic configuration of each controller in thetoolkit.

Trending of any external/internal parameters present

in the database over several months using industry

standard historian package.

Drill down graphical web based analysis to detect

underperforming applications.

Detailed analysis and understanding of the

performance compared to baseline or benchmark key

performance indicators.

Adapting the control applicationAnother important maintenance function is the abil ity to

adapt the control application to a changing operating

environment. In a typical plant situation, the process is

changing continuously as a result of operational issues

such as fouling or instrument failures, while operational

objectives are revised in order to adapt to changes

such as increased market demand or varying feed

composition.

New technologies now exist that automate the

process of updating the control application. These tools

provide a fast cycle upgrade of the control application that

consist of:

Automatic step testing with reduced supervision.

Daily reinjection of improved models.Web based analysis to check new results in terms of

controller performance.

Sustained value programmeAnother fundamental requirement for maximising returns

from an APC application is the creation of a dedicated

maintenance organisation as part of a sustained value

programme. This will help management to retain their

focus on the importance of this function after the APC

project is completed, and will also help ensure that APC

maintenance retains an identified budget.

A sustained value programme can be based on

internal or external resources, depending on the existinglocal support and the complexity of the application.

Agnes Devichi, project manager at Naphtachimie (France),

which implemented a complex APC and real time

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Figure 7. Aspen State Space Controller Desktop.

Figure 6. Block structure of State Space Controller.


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optimisation system on its ethylene plant in partnership

with AspenTech, stated that a sustained value programme

is absolutely necessary to cope with the complexity and

the size of the installed applications. The programme gave

full attention to the continuous training of operators and

engineers, and was structured to give flexible support

to the plant accommodating changes in the production

schedule and demand across the different maintenanceareas.

APC using state spacecontrollers

An important growth area for the application of APC

systems is in the use of state space models. The

AspenTech state space controller (SSC) advanced

process technology (Figure 6) is a fifth generation MPC

algorithm6that has been designed from the start to

provide a platform for future control applications. It

was built using the newest software architecture, which

will enable extension and enhancement without the

limitations associated with older software generations.

The SSC controller is best suited for small to medium

parts of a unit and is initially targeted for non-olefins

and speciality chemicals manufacturing, which has

traditionally been underserved due to project complexity

and implementation costs.

SSC has been designed with the non-expert user in

mind and this improvement in the tools for implementation

will provide faster time to benefits as required by the latest

ARC advice1. The user can maintain the SSC application

through a web browser interface (Figure 7) which allows

for global collaboration and remote monitoring. The

supporting infrastructure and tools allow for long term

support and sustaining the controller benefits through the

entire lifecycle of the application.

Conclusions

While the use of APC is well established, the introductionof the latest technologies, methodologies and best

practices can provide process manufacturers with

significant opportunities to increase the benefits they

realise from APC implementations. Recent advancements

make APC easier to implement, operate and maintain, so

that users can generate value more quickly and sustain

that value over a longer period.

ReferencesFISKE, Tom, Advanced Process Control: the Right Stuff, ARC

Insights, November 2005.CANNEY, William M., The future of Advanced Process Control

promises more benefits and sustained value, Oil & Gas Journal,

April 2003.

VALAPPIL, Jaleel, MESSERSMITH, David, WALE, Sanjay andMEHROTRA, Vibhor, Lifecycle Dynamic Modelling in the design

and Testing of Advanced Process Control, IEEE AdvancedProcess Control Applications for Industry Workshop, May 2006.

TRIVELLA, F. and MARCHETTI, G., Integration for innovation,Hydrocarbon Engineering, November 2004.

ALSOP, N. and FERRER, J.M., What dynamic simulation

brings to a Process Control Engineer - Applied Case Study toa Propylene/Propane Splitter, ERTC Computing, London May

2004.

FROISY, Brian, Top Model Hydrocarbon Engineering, January2006.

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article hydrocarbon april 2007

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