1 July 2005

MEMS advance analysis

Gas chromatographs get technology boost; refineries gain an edge

By Harald Mahler

The petroleum refining industry is one of the key market segments for process GCs. Approximately 20% of process GCs sell into this market.

There is a worldwide trend to mega-refineries in green field projects such as Jamnagar refinery in India or Ras Laffan in Qatar.

Refinery applications for on-line chromatographs apply to a significant number of applications simply from the analytical point of view.

To measure lower hydrocarbons up to C4 and inert gases like hydrogen, CO, or CO₂ in various refining processes are typical measuring tasks.  Microelectro mechanical systems (MEMS), in a recent technological advance, are bringing the advantages of their science to chromatography and process analysis. Here is how.

MEMS solid position exists

Products based on microsystem technology have structures in the micrometer dimension and acquire their functionality through the design of the microstructures.

Microsystems combine several optimized micro components into one overall system, which fulfills one or several functions.

Microsystems with integrated microelectronic components are MEMS products and provide extended functionalities.

People in their everyday lives encounter MEMS, but most do not know it.

MEMS products first installed in vehicle airbag systems, video projectors, and ink jet printers. A MEMS accelerometer sensor informs the airbag when to trigger; an optical MEMS sensor is utilized for the phenomenal clearness of modern projectors; and a MEMS nozzle allows ink jet printers to produce high-resolution printouts.

The motivation for using MEMS for the development of new and innovative products is a combination of lower manufacturing costs, compact size, low weight, and low power consumption, as well as increased intelligence and multi-functionality.

These benefits establish the opportunity to improve existing solutions or to provide new products with special functionalities for additional fields of application. The future for the MEMS market is promising. The prediction is it will grow at an average yearly rate of 16% over the next five years.

MEMS have consolidated their position in established markets and are continuously finding new applications. This trend in favor of MEMS technology has also characterized new developments in the process industry, e.g. in process analytics. Consider the following product technologies:

• Micro process technology with micro-structured components works for manufacturing in the process industry and delivers particular benefits for chemical synthesis. The small dimensions of the critical components, like micro reactors, mean significant heat generation and flammable reactants and solvents are easier to control. In addition, the product yield is better.
• Micro spectrometers allow the spectroscopist to acquire spectra of extremely small samples and with little sample preparation. Measurements can be made while light is transmitted through the sample, reflected from it, or even when the sample is made to emit light, e.g. due to fluorescence.
• Micro process chromatographs open a new dimension to simplify the analytical system and its installation capabilities outside of analyzer shelters. There is a high potential that micro process GCs will also participate in this trend.

Injection and detection

The process GC here uses MEMS technology in microchip scale.

The miniaturization of all essential components using this innovative technology allows an extremely compact design of the whole device in the size of a football. This also results in high robustness against ambient influences (degree of protection is IP65 and NEMA4X, ambient temperature range -20°C to +55°C) and simplified design engineering for the pressure resistant casing to guarantee explosion protection.

This design offers the possibility to install this analyzer directly at the sample point even in remote locations.

The analyzer is modular way and comes as three main units: the analytical module, the electronic section, and the pneumatic interface. All three parts integrated into housing similar to that of a transmitter.

Even as features based on software functionalities are more and more important, the heart of each gas chromatograph is still the analytical hardware around the chromatographic column. These provide separation of the gas mixture into individual components and allow detection according to the measuring task.

Column switching systems are in process gas chromatography as a standard tool for cleaning the system, coupled with short cycle times, which results in optimum repeatability. The switching of the internal gas streams typically uses pneumatically driven valves.

The most important column switching configurations are straight-on, back flush, back flush sum, heart-cut, and distribution. Depending on the valve type, the practicability for column switching systems can have limits depending on the occurrence of pressure pulses when switching the valves, by diffusion of control medium into the sample path, by chemical reactions, or adsorption of sample constituents with the valve material.

Most all valve types have a significant dead volume. This can lead to peak broadening and to a loss of separation capacity, especially with capillary columns.

These disadvantages disappear when using valveless column switching systems. This analyzer uses MEMS-based technology for key components like injection and detection. The MEMS-based valveless column-switching unit eliminates dead volume. This guarantees the best coupling to the applied high-speed narrow-bore capillary columns.

Typically, the adjustment of gas flow in a chromatographic system happens manually by an empirical optimization method of restrictors and electronic pressure controllers. Especially for valveless column-switching this adjustment presupposes experience and time to balance the various internal gas flows. 

No significant degradation

We evaluate process analyzers, such as process GCs, according to their performance in the field, where rough climate conditions frequently exist. 

Besides the basic requirement that all measuring components be separate from other components, typically parameters like repeatability, detection limit, linearity, and analyzing time specify a process chromatographic solution. Additionally, basic conditions like ambient temperature range on site or sample conditions, such as temperature or pressure variations of the process sample, are also important.

Typically, conventional process GCs mount inside air-conditioned (HVAC) analyzer shelters. HVAC units guarantee very low temperature variations in the range of ±3°C. The failure of the HVAC could affect the reproducibility of the measuring system.

Ambient temperature influences are much more important if the process GC is at the sampling point, protected in a simple transmitter box, or just by a roof. The installation costs of such simple installations are significantly less than installation in analyzer shelters, but the demand on the performance of the analyzer is higher.

The analyzer must be able to compensate for climatic influences. It also must provide representative and repeatable result at changing ambient temperatures from the day-night shift or seasonal thermal fluctuations. 

As a general rule, a sample conditioning system mounts at some location prior to the analyzer. One of its tasks is to control the pressure in a range, which the analyzer can handle. If slight sample pressure changes appear at the injection device, the injection volume of a vapor sample is also varying because of the compressibility of vapor samples. This has an influence on the accuracy of the measuring results.

Process GCs achieve reliabilities in the range of 95 to 99%. It is also necessary the mean-time-to-repair be as short as possible, in case of failures.

Practical tests have demonstrated the replacement of the analysis module is possible in less than 30 minutes. No significant degradation of the data or results quality took place as a consequence of the removal and reinstallation of the module. 

Modern communication concepts

Process analyzers, such as process GCs, usually get some kind of protection against climatic influences. The GC’s mount together with other analyzers in centralized walk-in type air-conditioned shelters.

Sometimes the GCs are in open, more cost-effective, three-sided stands, or non walk-in type cabinets. The preferred option depends on local (e.g. hazardous area, space requirements) climatic pre-conditions or on the user’s philosophy. In most cases, cost is important.

Micro process GCs open new opportunities regarding the system integration installation in the process environment. The size of the Micro GC (12”H x 14”W x 9”D) versus the size of a conventional GC (39”H x 26 1/116”W x 16 3/16”D) enables a smaller capital investment without any degradation of analyzer performance.

Nevertheless, there are process requirements as a result of which the plant operator might prefer an analyzer installation as close as possible to the process sampling point.

For example, in an acetylene plant, the fast analysis of nitrogen is important to control the process. Avoiding the longer distances to transport the sample from the extraction point to the analyzer shelter is important. A micro GC can go directly inside the plant even above the ground at higher platforms.

Some process samples tend to change during transportation from the sampling point to the remote analyzer shelter where the GC is. When upgrading process plants, space restraints could occur, which prevent traditional analyzer concepts using shelters. Both are further aspects for installation of process GCs at the sampling point.

The primary advantage of such solutions is significantly lower capital costs compared to installations in shelters.

The micro GC is best qualified for in-stallations directly at the sampling point:

• Minor requirements regarding consumables: no instrument air necessary, low gas and power consumption
• Reduced space: Installation in small transmitter box or under a rain roof is sufficient 
• Practical service concept with easy monitoring of the analyzer status from remote location and short time to repair

Process GCs are applicable within a plant infrastructure on various occasions. Therefore, in recent years, many different concepts have been developed and used to integrate the analyzers in the plant-specific communication environment. In the past, analogue links to dedicated communication systems (DCS) have been dominant.

The information content for this type of data link is limited to the measured value. Another disadvantage is the high hardware expense when wiring all individual measuring components, especially for multi-component analysis. Digital com- munication techniques can provide additional information such as status messages, sample stream name or number, or date and time stamp and are predominantly the preferred solutions today.

Optimization in process plants is necessary in order to increase their efficiency and for competitive reasons. 

This also influences modern communication concepts in terms of data safety. DCS links are increasingly redundant.

The schematic shows a redundant industrial Ethernet network with process GCs (micro and conventional GCs), which are in an analyzer shelter or sit directly at the sampling point. The network compo-nents are standard switches with an extended lifetime (MTBF of 52 years). A promising alternative, especially for communication from remote locations, is the application of wireless network compo-nents, which we see here in the upper right.

Industrial applications

Applications are required in saturates gas plants, summarization plants, visbreakers or reformers, and platformers, for example. Depending on the sampling points, the measured components and its concentration range are varying.

Another market segment where process GCs based on MEMS technology exceed the requirements is the natural gas industry.

One of the key applications using process GCs is the measurement of lower hydrocarbons and inert gases (N₂, CO₂, C1 to C5, sum C6+, or C6, C7, C8, C9 individually). Based on these components, the GC calculates the calorific value, density, or WOBBE index.

The components or fractions of these have to see analysis in various plant locations within the huge transportation grids worldwide. Additional applications are required in natural gas processing plants for sulfur and carbon dioxide removal or for the separation of natural gas liquids.

Liquefied natural gas (LNG) terminals (in liquefaction as well as in LNG vaporization plants) and floating production storage off-loading ships in offshore applications are examples where Process GCs utilize state of the art technology to determine the CV value.

The limited measuring task, especially for CV determination, allows the standardization of the application and the whole analyzer to a high degree.

The use of micro process GCs opens new opportunities of cost-effective field installations.

The Micro GC is able to measure pipeline quality natural gas with repeatability for CV and density of better than 0.01 %. Based on the MEMS concept with narrow-bore capillary columns and multi-line and in-line detection, the analyzer provides high separation capacity for all measuring components within a short analysis time of 180 seconds.

An accuracy for CV and density of <0.1 % can be achieved. Special CV control operation software is available to meet the requirements in terms of verification and access to a fiscal metering mode.

Micro process GCs provide multiple application possibilities in the field of process gas chromatography. The analytical concept presented is MEMS-based technology.

An analyzer is available that works for various analytical tasks in the oil and gas industry. Features like narrow-bore capillary columns and multi and in-line detection guarantee high separation power and repeatability of measuring results within a short analysis time.

The compact and modular analyzer concept allows flexibility in field installations such as decentralized locations close to the sampling point or inside of shelters. Therefore, micro Process GCs are a promising tool for the planner and user to reduce their capital investment.

The simplification of analytical systems using a novel mathematical network model will support the trend of standardization of process GC solutions.

Its communication structure allows the integration into modern communication networks such as redundant solutions via well-proven industrial Ethernet. Decentralized solutions using compact field mountable process GCs will probably expand.

ABOUT THE AUTHOR

Harald Mahler (harald.mahler@siemens.com) is an ISA presenter and an analysis expert. He lectured at the ISA Analysis Division Symposium in Houston in April. For introduction to the mathematical network model behind this MEMS GC: http://www.isa.org/intech/july07/GCcoverstory.