Advanced Control of Multiple Sulfur Units

Dustin Beebe

Prosys, Inc.

• Brief Introduction of ProSys

• Wild Loads Overview

• Control Issues

• Control Solutions

• Benefits

Presentation Agenda

• ProSys is a leading global provider of process control engineering services and solutions.

• Founded in 1989.

• Offices in:

Baton Rouge

Houston

Hurth, Germany

About ProSys

Engineering Services

• Basic Control

• Advanced Control

• Alarm Management

• Operator Interface

• Dynamic Simulation

• Many more

Engineering Services

Software Solutions

• Basic Control

• Advanced Control

• Alarm Management

• Operator Interface

Software Solutions

• A control scheme to control multiple units receiving wild feeds from multiple sources or providing a utility or product to multiple users.

• Developed using standard Proportional Integral (PID) Derivative controllers and custom programming within the control system.

• This is an example of Advanced Regulatory Control (ARC) –no predictive process model.

• This control scheme is installed in multiple industrial locations.

Presentation OverviewPresentation Overview

What is a Wild Load?

• A wild load is a supply or demand to/from multiple locations external to a group of units that must deal with it.

• May be controlled by user or producer

• The wild load is not controllable by the unit which must receive or supply the stream.

• Must respond to unpredictable and sometimes large, rapid changes in demand

• Usually little or no surge capacity

Wild Load Handler Examples

Producers

• Boilers/Steam distribution

• Hydrogen plants

• Syngas plants

• Air compressors

Acceptors

• Acid gas handling

• Waste gas incinerators

• Wastewater treatment

We will call these Wild Load Handlers (WLH).

When multiple WLH in parallel are used to handle feed from or supply to multiple sources, the control system must:

• Respond to uncontrolled load changes;

• Ensure that the wild load is properly balanced so that one WLH is not overloaded while another runs well below its capacity;

• React rapidly to one WLH trip, raising the loads on the others to maintain stability.

Control Issues

Consequences of Control Failure

• Plant upsets in the units that use or generate the load

• May need to initiate steam shedding

• May need to reduce production

• May need to flare sour gas

• May lose control with inadequate instrument air

• Lost production/environmental exceedances

Typical Approach

Typical approach to the control problem is to fix the load on most WLHs, and use only one as the swing unit.

Disadvantages

• Lack of capacity to react to load changes

• WLH loads not balanced

• If one WHL trips, taking control of others is difficult, and requires substantial operator intervention.

Control System Overview

Load Balancing Advanced Regulatory Control (ARC)

• Monitors header pressure (or total flow) and loads on each WLH

• Adjusts target feed or product rates for each WLH

• Supplemental ARCs adjust other key variables within each WLH in response to load and composition changes

Control System Overview

Load Balancing ARC

• Master controller maintains gas header pressure

• Pressure cascades to a total flow controller

• Slave controllers maintain feeds to each WLH

• All flows are P/T compensated

Control System Overview

Load Balancing ARC

Constraints included for each WLH as necessary

• Temperature/pressure limits within each WLH

• Equipment capacity

• Valve saturation

Sulfur Rec

Unit #1

Sulfur Rec

Unit #2

Sulfur Rec

Unit #3

Sour Gas Source

Sour Gas Source

Sour Gas Source

Sour Gas Source

FY

Sum

FC

Master

FC

PC

FC

FC

PV

PV

PVSP

SP

SP

SP

PV

Constraints

XY

Constraints

XY

Constraints

XY

Gain =

1.5x with 1 unit out

3x with 2 units out

Load Balancing: Sulfur Recovery Units Example

Control System Overview

Adaptive Gains

• Adaptive gains employed in the master flow controller to improve header pressure control

• Based on number of WLHs available (not at constraint), gains on setpoints of flow controllers in other WLHs adjusted

• Examples

• If 3 WLHs installed in parallel, only 2 are available, gains to the available WLHs must be 3/2 x the base value

• If 4 installed, 3 available, gains are 4/3 x

Boiler #1

Boiler #2

Boiler #3

Steam User

Steam User

Steam User

Steam User

FY

Sum

FC

Master

FC

PC

FC

FC

PV

PV

PVSP

SP

SP

SP

PV

Constraints

XY

Constraints

XY

Constraints

XY

Fuel

Fuel

Fuel

Gain =

1.5x with 1 unit out

3x with 2 units out

Load Balancing: Boilers Example

Boiler

FC

Master

FC

SP

Constraints

XY

From Fuel Gas Header

Desired fuel gas flow from master

FC

CC

Air flow (cross-limiting)

Max capacity

PC Steam drum pressure

Constraint Block

With

Auto-bias

Constraint Control: Boiler Example

Supplemental ARCs

• Within each WHL, individual controls must also deal with changing loads.

• Must be considered with load-balancing design

• Details of ARCs that accomplish this are beyond the scope of this presentation.

• Examples of supplemental ARCs• Air/O2 balancing to SRU burners

• Fuel/Air cross-limiting controls

• Feeds ratio control

• Feed forward furnace controls

• BFW controls (3 element)

• Respond to uncontrolled load changes.

• Ensure that the wild load is properly balanced so that one WLH is not overloaded while another runs well below its capacity.

• React rapidly to a WLH trip, raising the loads on the others to maintain refinery stability.

Load Balancing ARC with Adaptive Gain

Handling Control Issues

• Improved control – fewer impacts to users or upstream units

• Fewer issues when a trip occurs• No need to adjust production/operation in user plants• Keep production units on line

• Better control within WLH

Benefits

• Little or no operator intervention unless all units are at constraint limit

• Handled by ARC, without model-predictive control (MPC)

• No external MPC application• No process model required• Process need not be linear

Benefits

Refinery fighting control issues in sour gas header

• Before ARC installed, multiple operating incidents and several permit violations over year’s time due to ineffective controls.

• After ARC, no violations in a full year

• Project cost recovered within a few months

• Can now withstand sudden loss of a sulfur unit without incident

Before/After: Case Study

Connect with ProSys

For more information on ProSys, click here.