WirelessHART Successfully Handles Control

MAKING IT WORK

WirelessHART Successfully Handles ControlTests on stripper and absorber show results rival those with wired devices

By Frank Seibert, University of Texas at Austin, and Terry Blevins, Emerson Process Management

IN RECENT years the Separations Research Program (SRP)at the University o exas at Austin has been involved intesting pilot plant processes that require signicant equipmentmodications and additions. In these activities, process setupand decommissioning take about three times longer than theactual runs. So, decreasing setup time and pilot plant down-time are a high priority. Use o wireless transmitters provides a new degree o exibility in reconguring the process withoutinstalling or relocating transmitter wiring.

Te exibility o being able to work on the absorber andstripper without disconnecting cables oers ongoing benetby reducing downtime and maintenance. However, many measurements that are candidates or switching to wirelesstransmitters serve or control. Tus, measurement reliability and control perormance achievable using wireless transmit-ters were o primary concern. o allow direct comparison o the perormance o wireless versus wired installations, wirelesstransmitters were installed in parallel with wired transmittersor a ew critical loops on an absorber and stripper process.Tis article presents the results o these tests.

PROCESS EQUIPMENTTe equipment setup used to evaluate application o wire-less transmitters in control currently serves or testing new energy-efcient stripping processes associated with carbondioxide capture rom coal-red power plants. From a process standpoint, this involves an absorber to remove CO

2 

rom a simulated stack-gas stream using an amine-basedsolvent and then a stripper to recover the CO

2and recycle

the regenerated amine solution to the absorber. Te photoshows the equipment.

Te absorber generally tests proprietary solvents andpacking or tray internals. Te absorption column providestwo 10-t beds o packing. A liquid collector and liquid re-distributor are located in the middle o the column. In the SRPCO

2-capture experiments, ambient air is blended with recycled

stripped CO2

and make-up CO2

and ed to the bottom o thecolumn. Carbon dioxide is absorbed into the downowing amine solvent as the gas ows up through the packing. Teexiting solvent, reerred to as the rich amine, goes to the strip-ping section to remove the absorbed CO

2.

Te stripper tests packings and determines the energy efciency associated with a proprietary solvent. Tis columnalso has two 10-t beds o packing. A liquid collector andliquid re-distributor are located in the middle o the column.Te solvent leaving the absorber, reerred to as the rich amine,is heated with the stripper bottoms liquid using a cross-plateexchanger and then is ed to the top o the stripper. A kettle-type reboiler generates vapor to strip o some o the carbondioxide. Te overhead rom the stripper goes to a shell-and-tube condenser where the vapor is condensed and lean amine isreturned to the absorber eed tank.

MAKING IT WORK

WIRELESS INSTALLATIONOver the last ew years development and design o wireless elddevices or the process industries has rapidly advanced. Ater a review o available wireless technology, WirelessHAR devices were selected or installation at the SRP. (For details about thetechnology, see “Make the Most o WirelessHAR,” www.ChemicalProcessing.com/articles/2008/071.html.) Pressureand dierential pressure transmitters were put in to supporttesting o control o column pressure and steam ow.

Te WirelessHAR gateway was centrally located in theprocess area and connected to the control system via Ethernet. A DeltaV control system installed in the SRP provides standardsupport or the WirelessHAR gateway as well as congura-tion, setup and diagnostics o WirelessHAR devices. Tus,the eort to integrate these devices into the control systemsotware conguration matched that associated with tradi-tional eld devices. Because these devices are battery powered,no power wiring was required to install them in the process.

Te existing control modules or column pressure andsteam ow control were modied to a llow the operator toswitch between wireless and wired inputs. When control with wireless input was selected, the PID modicationsor wireless control were automatically used. A lso, the new measurements and a switch to select the source o controlinput were added to the operator displays. Tus, using theirnormal displays the operators can visually compare wiredand wireless measurements at all times and select the mea-surement to be used in control.

Metrics were added to the control module to calculate theintegral o absolute error (IAE) while on automatic control. Also, the number o new measurement values used in controland the duration o control were automatically captured toallow comparison o wired versus wireless control perormance.

THE TRIALSPreliminary tests o a wireless pressure transmitter and a steam-ow dierential pressure transmitter took place during a recent packing characterization test to determine distillation

efciency and hydraulic perormance o Raschig-JaegerRSP-250 structured packing. (Te stripper also may be usedas a distillation column.) Te column was operated at totalreux using the cyclohexane/n-heptane binary system. Teeect o pressure on distillation perormance also was studied.Te standard test pressures were 2.4, 4.83, 24 and 60 psia. Inthe wireless testing, the base pressure was 24 psia. Te projectsponsor, Raschig-Jaeger, had agreed to allow the wirelesstesting during the runs.

Figure 1 shows pressure control achieved on the column with a wired pressure transmitter and a wireless one. For thesetests, the proportional-integral-derivative (PID) tuning, lter-ing and control conguration were exactly the same. As thisdemonstrates, there are no visible dierences in perormance.

Similar results were observed using wired versus wirelesstransmitters or steam ow control. Filtering was 10 sec.or the wired measurement and zero or the WirelessHARone; the original plant PID tuning was used or both wiredand wireless control. Te same dynamic control was seen orset-point changes.

 As the table details, comparable control perormance versus wired, as measured by IAE, was achieved or both pressurecontrol and steam ow control using WirelessHAR measure-ments with the PID modied or wireless communication.However, WirelessHAR used one-tenth the number o mea-surement samples or ow control and one-sixth the numberor pressure control as wired control to reduce battery drain.

Tese test results indicate the combination o  WirelessHAR transmitters with PID modications or wire-less communication perormed as reliably as the standard SRPhard-wired transmitters using traditional PID. Additional tests were carried out where compositional and packing efciency HEP (height equivalent to a theoretical plate) data wereobtained using the SRP hard-wired transmitters and then theoperation was transitioned to the wireless transmitters and

REFERENCES1. Broadley, S., Karschnia, R., Nixon, M. and Blevins, T., “Us-

ing Wireless Transmitters in Single-Use Disposable Biore-

actor Control,” IFPAC Conf., Baltimore, Md. (Jan. 2009).

2. Nixon, M., Chen, D., Blevins, T. and Mok, A., “Meeting

Control Performance over a Wireless Mesh Network,” 4th

Annual IEEE Conf. on Automation Science and Engineer-

ing (CASE 2008), Washington D.C. (Aug. 2008).

3. Johnston, G. and Munns, A., “Make the Most of Wire-

lessHART,” Chemical Processing, May 2008, www.Chemi-

calProcessing.com/articles/2008/071.html.

4. Chen, D., Nixon, M., Blevins, T., Wojsznis, W., Song, J.

and Mok A., “Improving PID Control under Wireless Envi-

ronments,” ISA Expo Houston (Oct. 2006).

Table 1. Wired and wireless control provided comparable results.

Using WiredMeasurements

Using WirelessHARTMeasurements

Loop FIC202Column

Steam Flow

PC215ColumnPressure

FIC202Column

Steam Flow

PC215ColumnPressure

Setpoint Average 511.32 24.01 509.66 24.01

IAE on Control 9,134.33 145.46 10,645.15 198.60

Number of Measure-ment Updates Whileon Control

13,655.00 6,649.00 1,184.00 912.00

Time on Control, sec. 6,830.00 6,829.00 5,926.50 5,925.00

Control Performance

MAKING IT WORK

control. No dierences in the relative bottoms and distillatecompositions or HEP were observed in the wireless mode.

A POSITIVE STEPInstallation o WirelessHAR transmitters or stripper steamow and column pressure has eliminated the cost to relocate wiring. Column-pressure and heater steam-ow control using  WirelessHAR transmitters and PID modied or wire-less communication provide the same dynamic response andperormance as that achieved using wired transmitters andtraditional PID.

Te application o WirelessHAR is being extended toother areas at SRP. For example, three wireless temperature

transmitters (4 RDinputs per device)are being put in tomonitor temperaturedistribution across

the absorber. Also, two wireless pH transmitters arebeing installed in the lean liquid input and rich liquidoutlet streams o the absorber to indirectly monitorCO

2concentration in the amine solution. Using 

 wireless t ransmitters should improve the accuracy andreliability o the temperature and pH measurements— and hence enhance stripper and absorber operation— because the battery-powered transmitters aren’t assusceptible to the ground loops that oten plague a  wired instal lation.

FRANK SEIBERT, P.E. is technical managerof the Separations Research Program at theUniversity of Texas at Austin. TERRY BLEVINSis a principal technologist for Emerson ProcessManagement, Austin, Texas. E-mail them [email protected] and [email protected].  

RELATED CONTENT ONCHEMICALPROCESSING.COM

“Wireless Proves its Worth,” www.ChemicalProcessing.

com/articles/2009/066.html

“Make the Most of WirelessHART,” www.ChemicalProcess-

ing.com/articles/2008/071.html

“Wireless Star ts to Mesh,” www.ChemicalProcessing.com/

articles/2008/208.html

“Wireless Proponents Take HART,” www.ChemicalPro-

cessing.com/articles/2007/155.html

Performance Comparison

Figure 1. Similar control results wereachieved with wired (top) and wireless(bottom) measurements.

Reprinted with permission from Chemical Processing, January 2011. On the Web at www.chemicalprocessing.com.© PUTMAN. All Rights Reserved. Foster Printing Service: 866-879-9144, www.marketingreprints.com.

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