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01 February 2004

From lab to real world

New age thinking utilizes digital communications, OPC.

By Ted Henry and Brian Smith

Process analyzers at the NOVA Chemicals (Canada) Ltd. Styrene II facility had insufficient sensitivity to measure product contaminants reliably. After the company realized the situation, they decided to kick off a project to replace the eight online gas chromatographs (GCs) in an effort to increase instrument sensitivity and to improve accuracy/repeatability and on-stream time to meet the ever increasing demands from operations and process control engineers. The team also received funding to install a zero-grade air and hydrogen generator in the process analyzer shelter.

At this NOVA site, the laboratory team has responsibility for the online and laboratory-based gas chromatograph instrumentation. The main lab is an ISO 9000 accredited facility, and the technicians utilize the lab-based instrumentation and "grab samples" to calibrate most of the online instrumentation.   The team felt that lab-based equipment would provide superior performance compared to the capabilities of current process analyzers available. To employ lab-based gas chromatographs online required solutions to three technical barriers.   The first was identifying a lab-style gas chromatograph with supporting software that would be capable of meeting the requirements of process applications, while being small enough to fit into the footprint of the existing process analyzers. Second was finding a means of complying with regulatory standards when locating general purpose instruments in an electrically classified area. The last barrier was developing a means of communicating the instruments' output to the distributed control system (DCS).

SELECTION PROCESS

NOVA's two petrochemical operations' laboratories in the Sarnia area employ primarily Agilent Technologies brand gas chromatographs. A product bulletin announced an addition to their line up, a small, single detector instrument—the 6850 GC. In addition, the company's Cerity software could network these instruments, and it included an automation table capable of providing a "repeat sequence" functionality. The automation table also provided the capability of sequencing multiple methods and "multistreaming" each analyzer.   After reviewing the specs, the team decided to travel to Agilent's Marshallton, Delaware production facility to go over the details and to get assurances from Agilent that they would support the team's efforts to employ the instrument and the software in the online applications.  The team already had experience with Parker-Hannifin's gas generators and selected them to provide utility gases to the analyzers. NOVA received technical support from Parker-Hannifin for the enclosure design used to house these devices.

REGULATORY APPROVAL

A meeting was held with the Technical Standards & Safety Authority (TSSA) authorities in Toronto, the division dealing with "hazardous locations certification," to discuss NOVA's preliminary GC and gas generator enclosure designs. This regulatory organization has responsibility for Canadian Standards Association (CSA) approvals. The meeting was on design considerations needed to satisfy the NFPA 496 with a Z-purge panel. Given the shelter's existing Class 1, Division 2, Groups C&D rating, TSSA agreed that using a nitrogen purge media would satisfy the requirements of NFPA provided NOVA's contractor conducted a twenty-four-hour test of each enclosure for oxygen ingress. The eight GC and two gas generator enclosures went to CSA inspectors in three groups for "special acceptance." The testing satisfied TSSA, and the enclosures received CSA approval.

From a project hazard and operability study, the asphyxiation risk associated with the use of nitrogen as a purge media had to go under the microscope. The shelter in question has a heating, ventilation, and air conditioning (HVAC) system that provides a once through, positive pressure, conditioned air supply that alarms remotely upon extended loss of building pressurization. The safety review showed that this risk was adequately addressed given the HVAC flow rate combined with an oxygen sensor/alarm system and the fact that each enclosure only used ~20-scfh (standard cubic feet per hour) of nitrogen, routed to the exterior of the building, to maintain the Z-purge. The NOVA Styrene II site has a pipeline nitrogen supply, so the projected cost of this utility was minor. The design of the enclosure had to take into consideration the ergonomics of servicing the instrument, heat dissipation, the effects of static electricity, along with the possibility of catastrophic failure of the pressurized gas/liquid sample or hydrogen supply tubing.

The laboratory team developed operating and training procedures to eliminate explosion/fire and personnel exposure hazards associated with housing the GCs and gas generators inside the enclosures with the pressurized supply lines described above.

COMMUNICATION TO THE DCS

An investigation into communication alternatives led to a move away from the analog 4–20 mA signals traditionally associated with the analyzer to a digital link to the DCS via OPC.

Using the existing fiber-optic cable, the team extended the plant process control local area network to the analyzer shelter. The team designed a network for the online GCs to operate by either of two available servers—one located in a purged enclosure in the analyzer shelter or the primary server in the lab. Custom software executes at the end of each analysis, utilizing a post-run macro option. This post-run program writes analysis data, along with the "time of analysis" (TOA) to the DCS OPC server. OPC connectivity is via commercially available software. The software is currently capable of supporting eight GCs, with two streams per GC and a maximum of five peaks per stream.

To address a possible communication failure that would result in the DCS displaying "stale" data (last value), NOVA created a program to monitor the last TOA from each instrument—a watch dog program. If forty-five minutes elapse without an update, a DCS alarm triggers and data exits the DCS display and control applications.

GAS GENERATORS

The team designed a single Z-purged enclosure to accommodate either a zero-air or hydrogen gas generator. Both installations required CSA special acceptance from the TSSA.

A bank of cylinders provides backup to the zero-air generator that takes its primary feed from the plant's instrument air system. The generator model selected has a capability of providing up to 18 liters per minute.

The hydrogen generator selected is capable of providing up to 1,200 cubic centimeters per minute at 100 pounds per square inch (psi). The generator typically operates at 45% of this capacity. The original bank of hydrogen cylinders that supplied the analyzer shelter now backs up the generator.

The hydrogen produced supplies the analyzer's flame ionization detector (FID) and the analytical columns/split vents. Using hydrogen as the column carrier gas typically halves the time required to complete an analysis. All the process analyzers now employ a hydrogen carrier with the exception of the Styrene analyzer, where hydrogenation interfered with component measurements.

This generator utilizes a palladium membrane for purification of the hydrogen gas produced, which virtually eliminates the need for any maintenance. Providing utility gas to the analyzers simply involves a delivery of  15 imperial gallons (IG) of water to the shelter once every 45 days.

Careful consideration must be given to the purging of the hydrogen generator enclosure. NOVA installed modified gas separators inside the generator that facilitated external venting of oxygen and hydrogen normally released within the generator's shell. A 1.0-standard  cubic-foot-per-minute nitrogen purge ensures the NFPA standard of less than 60% limiting oxidant concentration(LOC) would be adhered to in the event of a catastrophic failure of the generator's solid polymer electrolyte membrane.

LAB GC SYSTEM

After close to one year, the GCs have been very reliable and stable instruments. Of the seven instruments online, only one has experienced mechanical problems due to water contamination of the EPC. Analyzers with elevated solids loadings have had several sample valve related problems but the use of finer filtration media seems to have corrected this problem. Most analyzers have been able to run upwards of four to six months without technician intervention.

Based on performance, grab-sample calibration checks for final product analyzers have been reduced to once every two weeks while in-process analyzers are now done monthly. Following the initial method development, technicians have only reported component retention time (Rt) adjustment in 2 of the 7 operations methods.

The new installations have delivered a number of enhancements to NOVA Styrene II's process analyzer operations:

1. Economical installation. When the team installed the full complement of eight GCs the project cost was very competitive with traditional process analyzers while future upgrades can be completed at minimal cost. The process analyzer installation is now broken down into its component parts; GC plus purged enclosure plus computer. Replacement of the GC should be limited to the instrument itself while the other components would be reused. 

2. The elimination of a limited reporting range encountered with 4–20 mA communications. The GC's auto-ranging FID has provided accurate reporting throughout the FID's entire dynamic linear range (107 ± 10%). This translates to a range extending from the <100-parts-per-million (ppm) level up to 60-80% w/w. During start-ups and process upsets, operators now have real-time data to help accelerate the recovery to on-specification product.

3. A much tighter analytical column installation. The use of routine automated bakes combined with electronic split vent control has eliminated the need for back-flush and heart-cut valves. This minimizes dead volume and the number of points where leaks might occur, thereby improving the peak sharpness and eliminating Rt shifting. The bakes can provide cleaning of the GC's four heated zones—injector valve plus injector inlet plus analytical column plus FID—and are normally scheduled on a three-day frequency for a thirty duration. 

4. Control features normally reserved to lab-based GCs. Temperature ramped oven to 350°C, inlet and FID with electronic pressure control ranged from 0 to 100 psi, programmable for constant pressure or constant flow and three available pressure ramps, Gas Saver mode and electronic split ratio adjustment without affecting column flow or head pressure.  An automated shutdown of column carrier on loss of inlet set point. All NOVA's installations use isothermal programming to provide maximum analysis throughput.

5. The GC is equipped with VICI’s new, durable mini-diaphragm valve. VICI claims this valve is capable of 1 million injections between rebuilds which equates to over ten years of operations in most applications. To obtain this performance VICI stipulates all solids greater than 2 micrometers be removed from the sample stream. NOVA designed and constructed a sample conditioning system that utilizes a three-tier circulation loop employing self-cleaning, bypass filtration to minimize technician intervention. An armoured, flow-indicating switch is located on the return from the sample valve providing confirmation that the GC is measuring a representative sample. Should the flow be interrupted for more that five minutes, an alarm annunciates in the central control room and data displayed on the DCS disappears.

6. Portability of the GC. The GCs are no longer hardwired. The only thing linking a GC to its location is the instrument's serial number, so they are interchangeable in the different applications. During the upgrade, downtime of the analyzers while converting while converting to 6850’s was normally limited to less than eight hours by simply extending the sample loop tubing to a temporary location while the original GC was replaced.

7. Very fast start-up time. The GC oven measures approximately 8-inches high by 8-inches wide by 4-inches deep. In most applications, the GC will come to ready from a cold start-up within five minutes. Previously, analyzers could take up to forty-five minutes to reach steady isothermal operating conditions.

8. The network of process analyzers can be controlled remotely. With a telephone line and the required network access, full control of the GCs and their methods is possible from any location.

9. Very high quality air for the GC FID. The zero-grade air generator guarantees production of <0.05-ppm total hydrocarbon. In comparison, cylinder supply of zero-air has a specification of <1.0-ppm total hydrocarbon. This has made a significant improvement to GC baseline noise, thereby increasing the sensitivity to a sub-ppm level.

10. Very high purity hydrogen gas for the GCs’ carrier and FID. The hydrogen generator product has a purity of 99.99999%. Elimination of oxygen from carrier gas to the analytical column would extend the life of the stationary phase and help reduce retention time shifting.

11. Reduced hazards associated with cylinder handling and leaking connections. The installation of a hydrogen generator almost eliminates the need to change out cylinders. In addition, it provides improved reliability for the process analyzers themselves, as now there is reduced likelihood of running out of a utility gas supply.

THE OTHER SIDE

Installation of the GC into online applications and gas generators has several limitations.

1. The facility must have an economical nitrogen supply to provide purge media.

2. You will likely have to install fiber-optic cable to service the analyzer shelter, depending on its proximity to a network switch.

3. You may have to upgrade the analyzer shelter's HVAC and safety systems to address the increased use of nitrogen as a purge media.

4. After a power failure, in NOVA's case the GCs do not automatically resume operation. The vendor is working on that problem.

5. A network of analyzers using Cerity software is limited to 32 GCs and a maximum of eight GCs per server.

6. The current custom OPC interface communicating with the DCS limits each GC to two streams per analyzer and five components per stream.

7. Data saved by the Cerity software goes to an Access database.

8. The Windows 2000 platform is subject to memory loss.

9. The hydrogen generator is limited to the operation of 14 GCs before cylinder consumption will begin.

10. The zero-grade air generator is limited to the operation of 60 GC FIDs.

When compared to the original installations, the use of laboratory quality  GCs in the role of process analyzers has demonstrated improvements in the areas of: ease of use, robustness/reliability, sensitivity and repeatability.

In addition, using gas generators to support process analyzer operations have enhanced GC performance and reliability while improving safety by reducing the risk associated with cylinder handling.

The Ethernet connectivity and transmission via fiber-optic cable has proven to be a very reliable means of communicating process analyzer data.. When coupled with a watch dog program and flow switches in line with the sample streams, a very high confidence level can be assigned to the data being delivered to the DCS.

Reduced GC cycle time and improved overall performance should enhance benefits delivered from advanced process control (APC) applications. IT

Behind the byline

Ted Henry is a process analyzer/quality control lab coordinator. He holds a B.Sc. degree in Chemistry from University of Western Ontario.  Brian Smith is a senior instrument/electrical designer at Sarnia, Ontario–based NOVA Chemicals (Canada) Ltd.