INTRODUCTION - GAS PROCESS SYSTEM
Natural-gas processing plants purify raw natural gas by removing contaminants such as solids, water, carbon dioxide (CO2), hydrogen sulfide (H2S), mercury and higher molecular mass hydrocarbons. Some of the substances which contaminate natural gas have economic value and are further processed or sold.
Industrial processes are operated by process control systems which is based on the measurement of physical properties and composition of the product. The composition of the product is mainly determined in the laboratory. Laboratory analyses are done within hours and, since such measurement takes a relatively long time it can have a negative impact on the process plant’s throughput and product quality (an upset condition at the front may results in an off-spec condition at the output when not quickly corrected). Therefore fast analyses are required. Some of these analyses are performed on-line in the plant at different critical points in the process. This will speed up the measurement and the operation of the plant, resulting in a better control of product specification. Installing on-line analytical instruments will also minimize the errors that are introduced when taking manual samples.
Micro Process Gas Chromatography (µPGC)
Although the on-line analyses is an improvement for process analyses it can be improved even more. Micro Gas Chromatography (µGC) is an interesting development that could be used for such an improvement. The main reason for this assumption is first speed. µGCs are fast due to the use of narrow and medium-bore columns. As mentioned in (1) it was predicted during the first symposium on vapour-phase chromatography by A.J.P. Martin in 1956 that micro-columns would be needed for GC analyses. These are state of the art technique nowadays for GC and particularly in µGC analyses. Second size and separation power of µGCs allows it to be used for fast analysis and integration in a process plant. Moreover, for a number of practical reasons it is an advantage to miniaturize the equipment for process analysis. Those practical advantages are
- close mounting to the sample take-off point
- lower amounts of sample gas needed
- lower transfer times of the samples
- shorter delay times for analyser results
- low energy and utility consumptions
- easier explosion proof (ATEX, CSA) integration
- increased reliability (24/7) operation
- increased precision and accuracy
process sample to control
Often the focus is only on the analyses itself while 90% of failures with process analyses occurs in the sample handling and treatment. When fast analyses are introduced the speed of sample handling should be adjusted accordingly. On-line process analyses requires a number of essential steps to make it successful. It covers the steps from the process sample take off to the transfer of the results to the process control system, which, in its turn operates the plant to the correct sample quality and yield.
The steps from process sample to process control are
- The sample take-off from the process pipe/vessel
- The sample pre-conditioning
- The sample transport
- The sample conditioning and handling including calibration
- The sample analysis
- The sample data processing, and finally
- The feedback of the results to the process control system
Essential steps in process analyses
The essential steps in process analyses start at the sample take-off point. For this purpose a sample take-off probe is used and mounted to the process pipe to draw a representative sample from the process on 20%-80% in the cross section of the pipe.
Then the pressure and temperature are conditioned in the pre-conditioning system as close as possible to the take off point. This is done at the take-off point in order to get a representative sample, to avoid the use of high pressure sample lines and to make the system faster. Next the sample is transported to the sample handling and conditioning system by heated and/or insulated sample lines. This to avoid condensation and freezing of the sample in the lines. Also heating will minimize wall adsorption effects of low sample component quantities. In some cases the sample line wall is in addition treated and coated for this purpose.
THEORY AND RESEARCH ON ΜGC
In this chapter the theoretical background behind the µGC technology will be discussed, the background of narrow and medium bore columns, micro-injector and micro thermal conductivity detector (µTCD).
Gas processing is necessary to treat natural gas from the production source to pipeline or liquefaction quality for monetization purposes. The natural gas from the source is first treated in a gas processing unit to remove higher molecular weight hydrocarbons (NGL – butane and propane), sulfur compounds, water and other contaminants. Gas not immediately needed in the locality of a production field may be liquidified in order to facilitate transportation to regions of demand. Liquefaction is accomplished by cooling the clean gas in two or three cascade cooling cycles down to the liquefaction temperature of −162°C (−260°F) thereby reducing its volume approximately 600 times. The LNG is then transferred to heavily insulated storage tanks at atmospheric pressure, and from there it is loaded into LNG tankers for shipment.