A MINIATURIZED PROCESS GAS CHROMATOGRAPH - A SINGLE ANALYZER MODULE

Transcription

1 A MINIATURIZED PROCESS GAS CHROMATOGRAPH - A SINGLE ANALYZER MODULE Ulrich Gokeler Friedhelm Mueller, Udo Gellert Siemens Applied Automation Siemens AG 7101 Hollister Road A&D PI Houston, TX 77040, USA Karlsruhe, Germany KEYWORDS Process Gas Chromatographs, u-pgc, MicroSAM, MEMS, Decentralized, Transmitter GC ABSTRACT On-line Process Gas Chromatographs are versatile analytical tools used to measure qualitatively and quantitatively one or several specific components in various matrixes typically in multiple sample streams automatic and unattended. To address the required flexibility and versatility concerning analytical and environmental requirements and due to economical issues, typical on-line instruments use a fraction of their hardware and analytical capabilities. Due to tremendous progress in miniaturization technology it is not only possible to integrate analytical hardware and functionality but also electronic and computing capabilities in a fraction of the space compared to traditional analyzers. Certain loss of analytical flexibility is offset by the economical advantage of the manufacturing technique. A micro single analyzer module Process Gas Chromatograph has been developed. Utilizing silicon wafer technology and compact electronic modules, the analyzer has been specifically designed for on-line process applications utilizing narrow bore capillary columns, valveless column switching and semi valveless injection techniques. Cost of ownership aspects have specifically been addressed utilizing a modular design, where traditional maintenance is substituted with replacements of entire standardized analytical or electronics modules and by minimizing environmental influences, consequently enabling the device to be used in locations without climate control requirements. The presentation describes the design of this miniaturized Process GC, specifically focusing on installation and application issues as well as addressing maintenance concerns. 1

2 INTRODUCTION Analytical instruments in general and Process Gas Chromatographs (PGC) in specific are indispensable tools for quality and process control in a wide variety of applications and locations. The historical incremental improvements of hardware and software as well as versatility and reliability lead to a more wide spread utilization and acceptance. Driven by the desire to increase reliability and measurement precision and simultaneously reduce cost of ownership the utilization of Process Gas Chromatographs has undergone significant changes over time. Because environmental changes can and typically adversely impact analyzer precision, analyzer enclosure or shelters are used. Air-conditioned analyzer shelters also improve the working conditions and performance of analyzer technicians that contributes to the improvement of on-line availability. However, the practice of locating analyzers in their individual enclosures close to a specific or few sampling points is rather expensive. By concentrating analyzers in a few locations within a plant, piping the samples to these locations, providing utilities from power supply, auxiliary and calibration gases, sample recovery systems as well as communication hubs, the infra structure cost is significantly reduced. Furthermore, by sacrificing sample frequency by analyzing similar sample streams with the same analyzer, the number of analyzers can be reduced. Therefore the philosophy of centralized analyzer shelters, utilizing a high density of multi stream analytical instruments and also providing a comfortable human environment, has increased reliability and reduced overall costs and is applied widespread today. Depending on the individual situation, sampling points, sample composition, number of streams and frequency required, this concept can also have some significant disadvantages. From an analytical perspective, the piping of the individual sample to a central location utilizing fast and analytical loops not only may require heat tracing, but also promotes compositional changes due to stratification, absorption or chemical reactions during transport. Additionally high sample flows may be required to accommodate acceptable sample lag time with all the associated sample vent or recovery issues. Another issue may be the combination of several samples on a common analyzer that may increase analyzer complexity and memory aspects. It certainly involves a reduction of data frequency and consequently impacts on process control issues not to mention the possible loss of measurements of several sampling points in the case of an off line analyzer. Because the wide variety of possible applications Process Gas Chromatographs have to perform and due to the limited market size that does not permit to provide several more dedicated but simpler analytical hardware, traditional Process Gas Chromatographs typically have a wide range of analytical capabilities. Consequently, even though a specific application is only utilizing a fraction of the analyzer s capabilities, the instrument size, complexity and costs are driven by a major part by the overall capabilities. Despite centralization of analyzers, due to the associated minimization of initial hardware acquisition costs, installation costs and cost of operation, the overall costs of ownership is still significant [1]. Thus permanent installations of on-line Process Gas Chromatographs are frequently not the first choice. Either frequent or infrequent manually sampling and laboratory analysis is performed or no analysis is performed at all. Either way it will impact process control and product quality. 2

3 The desire to overcome these issues by augmenting existing analyzers with simple, cost effective and dedicated Process Gas Chromatograph hardware for more decentralizing installations is obvious. Such an analytical design would complement the presently wellestablished traditional Process Gas Chromatograph for installations and measurements at individual sampling points and simultaneously expand the range of applications, justifying the economical viability. The wide range concerning issues of centralized and decentralized installations have been discussed in detail before [2] with the following conclusions: Decentralized Process Gas Chromatographs can be beneficial. But when, how and where is project, situation and application dependent and have to be evaluated case by case. To achieve theses benefits decentralized Process Gas Chromatographs could provide, they have to have certain hardware characteristics. Some of the benefits decentralized PGC s could provide are also available today by proper application of existing Process Gas Chromatography techniques [3]. It may be necessary for a number of more complex applications to utilize several dedicated analyzers. Thus the benefit of initial cost savings may be smaller than desired but the benefits in system simplicity, reliability and especially sample information gain will more than compensate for it. INSTALLATION REQUIREMENTS A simple, cost effective and more application dedicated Process Gas Chromatograph is able to provide analytical information for some sampling points where traditional PGC s are utilized in a simpler and more cost efficient manner. More significantly are the benefits for sampling points that have not been considered so far utilizing on-line automatic Process Gas Chromatography such as: - Remote sampling points where traditional PGC s have been cost prohibitive. - Sampling points that have sample integrity issues. - Additional sampling points that would benefit from high data frequency during process changes for fast process control recovery. Such points have not been considered because sample lack time and installation costs. They could benefit from the multiple analytical detector capabilities and associated Chemometrics as described below. - Sampling points with critical space restrains that could not accommodate shelters. - Sampling points that utilize other types of analyzers subject to interference such as TCD s or IR s and where a fast and cost efficient PGC could provide sufficient fast data frequency and better interference free accuracy. - Sampling points served by manual sampling, laboratory analysis or at-line analyzers. - Multiple similar applications not requiring the universal flexibility of traditional PGC s. Because of installation costs, performance and maintenance issues, this type of analyzer has to have different capabilities than traditional PGC s as discussed previously [4, 5]. Of course, keep in mind that the analyzer is only one part of an analytical system. Present sample conditioning systems require the majority of the system maintenance necessary. Sample preparation or stream selection systems must have enhanced capabilities to be able to utilize the described benefits because the possible close coupling of sampling point and analyzer, flow capacity, sample system size, performance and maintainability has to be 3

4 addressed. This involves design changes of present sample system modules as well as communication capabilities to the relevant modules for observation, adjustment and controlling purpose. The present discussion, development and availability of new, smart and standardized sample system devices fit very appropriately [6-7]. FIG.1 MicroSAM - SINGLE ANALYZER MODULE ON-LINE PROCESS GAS CHROMATOGRAPH MOUNTED ON A POST ANALYZER DESIGN It has been determined that by just miniaturizing existing traditional analytical techniques and hardware, the items subject to maintenance and failures would remain. In opposite, the mounting environment would amplify these issues detrimentally. Due to tremendous progress in miniaturization technology using Miniaturized Electro Mechanical Systems (MEMS) it is not only possible to densify analytical hardware and functionality but also the electronic and computing capabilities in a fraction of the space than traditional analyzers [8-11]. Certain loss of analytical flexibility is offset by the economical advantage of the manufacturing technique. The Micro Single Analyzer Module (MicroSAM ) Process Gas Chromatograph [12] has been developed. Utilizing silicon wafer technology and compact electronic modules, the analyzer has been specifically designed for on-line process applications. Costs of ownership aspects have specifically been addressed utilizing a modular design. Traditional maintenance has either been eliminated or substituted with replacements of entire standardized analytical or electronics modules and by minimizing environmental influences the device can be used in many locations without climate control. Consequently in 4

5 moderate climates, the analyzer requirements are satisfied by the utilization of a sun/rain roof or a simple weather protective enclosure. The objective of the new design is to provide process gas chromatography capabilities not only in traditional analyzer shelter installations but also at specific sampling points. Therefor, the basketball size analyzer can be close coupled via the sample preparation system directly to the sampling point. The miniaturized Process Gas Chromatograph consists of 3 modules in a cast alloy enclosure suitable for installations in electrical hazardous areas without purge requirements, whether on a wall, post or pipe. The three modules, the analytical, pneumatic and electronic modules are standardized and interchangeable in any respect. The only differentiation is the analytical module separation columns. In order to utilize MEMS technology and in order to have the widest possible separation flexibility, narrow bore capillary columns, valveless column switching, semi valveless injection techniques and micro detection devices are utilized. Because of the tremendous separation power of narrow bore capillary columns, even if utilized with operating parameter outside the columns optimum range, the availability of a wide variety of columns from many sources and the ability to be accommodated in a small confined space, capillary columns are the ideal choice. Furthermore, the tremendous separation power also permits the majority of applications to be performed at isothermal temperature that in turn permits analyzer hardware simplification [13-16]. The widespread successful application of valveless column switching according the LIVE principle over the last twenty years has proven that maintenance free column switching is very beneficial for Backflush, Heart Cutting and Distribution to two main columns [17-19]. About half of all detectors used in Process Gas Chromatography are Thermal Conductivity Detectors (TCD). By improving the possible detection levels, TCD utilization can be even further expanded. MEMS technology permits the relative simple production of multiple Thermal Conductivity Detectors that are as small as the separation column s cross section. Such detectors can be used anywhere in the separation system, not only at the end of the columns but anywhere as inert inter column detectors without any loss of resolution. This in turn opens significantly new analytical and validation possibilities. For example, a µ- TCD column in-line detector right after the injection valve can be used to monitor injection quantity. By means of this value, sample presence, sample pressure and injection valve functionality can be monitored continuously and sudden loss or slow degradation can be detected so failures are predictable. An in-line column detector located in the middle of the pre column may already provide enough analytical information [20]. Consequently during process upsets it may not be necessary to proceed with the entire separation. Therefore the cycle time can be shortened significantly and a higher data frequency can be obtained, especially during the time when a high data frequency is more beneficial than accuracy [21]. The standardized separation system consists of pre column and one or two main columns. The valveless LIVE columns switching is applied exclusively either as Backflush only, as Heart Cut or as Distribution from per column to two main column. Detectors are located on every column inlet and vent. Therefore utilizing six detectors simultaneously permits the monitoring of every fraction of the total sample and therefore provides additional validation parameters. Also inter-column detectors, before and after the cut or distribution device permits the monitoring of complete column switching transfers and provides a very 5

6 easy way to recognize and adjust correct switching times. This arrangement significant simplifies the understanding of the analytical separation for the users. 150 µm Filament Wire FIG.2 µ-thermal CONDUCTIVITY DETECTOR WITH FILAMENT WIRE AND, A 150 µm DIAMETER AND AN INTERNAL VOLUME OF ABOUT 0.1 µ l Contrary to traditional sample injection, the MicroSam injection valve consists of two sections. The initial primary micro-machined injection valve injects a certain volume of sample into the carrier gas stream. The sample pressure is equilibrated to the carrier gas pressure. The following MEMS designed valveless injection device utilizes pressure differential to cut a certain slice of the initially injected homogenous sample plug. The left over sample plug is vented. The time width of the cut determines the actual injection volume into the separation column. For example, within certain limits that can permit a sample stream dependent injection volume. It also permits an injection volume reduction if the concentration of certain constituents measured exceeds certain preset limits and permits measuring range or dynamic range extension. Because the actual volume size or even variations of the volume initially injected is not significant, the importance of proper function of the initial injection valve is of reduced importance. Even if the valve ages and sample leaks continuously into the carrier gas flow, it is just vented without reaching the separation column. Consequently, by monitoring the injection TCD after switching Backflush off and prior actual sample injection, not only can any leakage be monitored periodically which makes injection valve maintenance predictable, but also the mean time between valve failures can be extended. 6

7 Signal Signal Time Zeit FIG.3 DETECTOR RESPONSE OF INJECTION QUANTITY INTRODUCED UTILIZING THE VALVELESS INJECTION METHOD WITH VARIOUS INJECTION DURATION (in 1/100 sec). Sample µ Membrane valve Carrier gas valve µ-live - Injection µ-live - Switching Column 1 Column 2 FIG.4 STANDARD ANALYTICAL MODULE CONFIGURATION WITH MAIN INJECTION VALVE, VALVELESS INJECTION AND COLUMN SWITCHING SYSTEM, PRE AND MAIN SEPARATION COLUMN AND 6 INTER COLUMN AND VENT DETECTORS All the analytical devices are integrated in an exchangeable module that is insulated towards the analyzer enclosure. Standardized pneumatic, feed and vent lines establish the connection to the pneumatic module and the outside. Electrical connections provide power and signal from the electronic to the analytical module. The module housing itself contains the heating source that permits column temperatures of up to 200 o C. 7

8 The pneumatic module includes the supply gas distribution and vents as well as the electronic pressure regulators (EPC) and solenoid Valves (SV). EPC s are used for carrier gas supply for the valveless column and injection techniques. Due to the design of the EPC s pressure changes in the millisecond range can be executed which permits precise and reproducible switching activities. FIG.5 STANDARDIZED ANALYZER MODULE INCLUDING INJECTORS, COLUMNS, VALVELESS COLUMN SWITCHING AND DETECTORS The solenoid valves integrated into the pneumatic modules are for the primary injection valve as well as for external actuation of calibration or validation streams. The pneumatic module is a subassembled module and can either be exchanged as it is or the individual solenoid valves and EPC can be exchanged individually. In order to provide reliability, simplicity and size, all electrical devices not necessary for proper functionality of the analyzer have been removed. For example, integrated power supply only requires a bigger analyzer housing and produces heat without adding any benefits. If a 24 V DC power source is not available, a universal power supply can be mounted easily and cost efficient externally, even in a redundant manner in a junction box close by. The same philosophy applies to analog and digital in- and outputs. Furthermore, the importance of analog and digital devices is greatly minimized with the utilization of digital communication. Standardized Ethernet TCP/IP communication is already common today to network a great number of analyzers for observation, maintenance and reporting purpose. Additionally using MODBUS or OPC reporting, data from any device, even from different manufacturers can be queried, exchanged or linked together, having a common and standardized link to process control systems. With the expected upcoming of individual digitally addressable sample preparation devices, sample preparation can not only be 8

9 monitored and controlled but there are virtually no limitations concerning digital or analog in- and outputs. 1,500,000 Injection Peak C 2 1,000,000 N 2, C 1 C 3 500,000 CO 2 Signal 0 n-c 4 i-c 4 neo-c 5 i-c 5 n-c 5 Main Column Outlet TCD TCD1 TCD2 TCD3-500,000 i-c 5 n-c 5 n-c 6 Pre Column Outlet TCD Main Column Inlet TCD -1,000, Time / s Time / sec. FIG.6 MULTI CHANNEL CHROMATOGRAM OF C1-C5 SEPARATION IN ABOUT 60 SECONDS UTILIZING ISOTHERMAL MICROBORE CAPILLARY COLUMNS CONCLUSION Complementary to traditional on-line Process Gas Chromatographs, the new analyzer design permits field installations in a simple and more cost efficient way. Utilizing MEMS technology it is possible to simplify, miniaturize and modularize the analytical system to a very beneficial degree. It is expected that for selected installations and applications, cost of ownership improvements compared to traditional process gas chromatographs are beneficial. Optimizing the analytical system concerning simplicity and maintenance issues was possible by applying valveless switching techniques and multiple detectors. These techniques also permit additional analysis validation and leads to an increase in confidence with regard to the analytical results generated. The objective of the new device is to have the opportunity and the benefits of simple installations without extensive shelter 9

10 requirements, at remote sampling points or where universally the need for traditional analyzers is not required or can be complemented. REFERENCES 1. Gunnell,J.J. et al, Process Analytical Systems: A Vision for the Future, 14 th IFPAC, January Gokeler, Ulrich et al, Decentralizing Process Gas Chromatographs Would there be any benefits?, ISA AD 2001, Houston, TX, April Bade, Robert et al, Applications in Parallel Chromatography, IFPAC Gokeler, Ulrich et al, Single Analyzer Module Process Gas Chromatograph, ISA 2001, Houston, TX, September Gokeler, Ulrich et al, Application Benefits of a Single Versatile Compact Process Gas Chromatograph, ISA 2001, Houston, TX, September Doe, S. Surface Mount Technology for Sample Conditioning Systems, Proceedings of the 46 th Annual ISA Analysis Division Symposium, Vol. 34, Houston,TX, Hughes, R., An Introduction to the Advanced Modular Sample System and a Concept for Cost reduction, Proceedings of the 46 th Annual ISA Analysis Division Symposium, Vol. 34, Houston, TX, Rubrecht,R et al, Abformverfahren fuer mikrostrukturierte Bauteile aus Kunststoff und Metall, 4.Statuskolloquim des Programms Mikrosystemtechnik, Forschungszentrum Karlsruhe, March 30/31, Annual ISA Analysis Instrumentation Symposium, ISA Proceedings, May Reston, R., Design and Performance Evaluation of a Gas Chromatograph Micromachined in a Single Crystal Silicon Substrate, Air Force Institute of Technology, AAI , Hidgon, W.R., Microfluid applications of MEMS in analytical instruments, Proceedings Sensors Expo, Detroit, IL, MicroSAM: Multiple Patents Pending for various hardware, functionality or applications. 13. Mahler, Harald et al, Multi Column System in Gas Chromatography, Chapter 9 of Adlard, E.R.; Chromatography in the Petroleum Industry, Elsevier, Gokeler, Ulrich et al, 8 th International Symposium Capillary Chromatography, Italy, Sandra, R., Vol. 1, 518 (1987) 15. Clauss, Fernand et al, 15 Jahre Kapillarsaeulen in der Prozess Gas Chromatographie, Labor Praxis, 2-4, 1993, Vogel Verlag 16. Mueller, Friedhelm et al, Experience and Problems with Capillary Columns in Process Chromatography, Chromatographia, 10 (1977) 17. Application Note 220 LIVE Injection, Siemens Applied Automation, Application Note 95, Valveless LIVE Column Switching, Siemens Applied Automation, Gokeler, Ulrich et al, Column Switching Alternatives for simplifications in Process Gas Chromatography, ISA AD 2000, South Charleston, WV, April In-Line Detection, Patent Pending 21. Fast/Precise Method, Siemens Patent Pending 10