SUPERIOR PLANT EFFICIENCY THROUGH SEAMLESS SYSTEM


INTEGRATION OF PROCESS GAS CHROMATOGRAPHS


Typical distillation towers - A signifi cant application fi eld for process gas chromatographs © Siemens AG 2022, All rights reserved


Process analytics, in conjunction with sophisticated system integration, plays an important role in the effi cient operation of chemical production plants. A practical example of a distillation process provides insights into important aspects of planning and implementation.


Process optimization by using process analytics


What is the motivation to implement additional costly equipment in a process plant? Nowadays, the market pressure is enormous – for suppliers of process analyzers, but also for operators of chemical production plants. Energy is increasingly expensive and need to be saved whenever possible. An excellent tool is advance process control (APC) and optimization. To implement APC, data must also be provided by the analyzer in such a way that the operator can react to process changes in time to prevent plant degradation. Sometimes even small traces of a process poison are suffi cient to destroy the catalyst material. The analysis of trace components is becoming increasingly important to meet quality standards. A good example is the measurement of ammonia traces in C2 products or arsine traces in C3 products. The producers want to avoid out-of-specifi cation product batches, which means that in worst case a signifi cant amount of sample has to be reprocessed. The solution is an effi cient process optimization through the use of online analytics.


An excellent application for the analyzer is the fractionating tower, which helps customers to better control their plant. Various control elements such as fl ow or temperature are needed for plant optimization. In addition also analytical control is important to measure the purifi cation that’s occurring in a distillation tower. Are the chemicals leaving the top or the bottom the correct purity? A signifi cant value for the user when implementing process gaschromatographs (process GC) is specifi cally getting closer to maximum allowed purity levels of samples and therefore closer to any product specifi cation. Without process GCs operators have to use a much more conservative set-point for the refl ux rate with much smaller fl uctuation. Unfortunately, by refl uxing or recycling so much, energy costs increase because more heating and cooling is required than needed. Additionally, the high recycling rate reduces also the through-put. Ultimately, the GC allows the operator to know exactly what the refl ux rate would be, even as the different conditions change over time. Energy costs are minimized with maximum production rate. A process GC integrated into a distillation tower can provide tremendous user benefi ts. It could increase through-put of the distillation tower and energy consumption in the range of 5 to 15%. It is crucial that process control is supported by the use of process analytics in such a way that it operates the process at an economic optimum. Therefore, the payback time is often less than one year and one analysis per day can already be profi table! Strong arguments for process analytics!


Process analyzer technology


Process gas chromatographs has been established in the process industry since decades specifi cally for applications to optimize distillation towers. Users appreciate this technology, even if it sometimes seems complex, because


PIN April / May 2022


• Dual oven confi guration • Capability of 2 trains per oven


* Detection of non-hydrocarbons using TCD Detection of sulfur components by FPD


Picture 2a and 2b:


Typical GC confi guration for liquid samples with key analytical componets (above) and standard GC design (right), options to integrate multiple analytical trains. © Siemens AG 2022, All rights reserved


• the measurement is proven in use and the analyzer system can be easily automated.


• plenty of components can be simultaneously measured and


• the analysis is typically interference free because the principle of chromatography is based on a physical separation of any substance - either a gas or a liquid, as long as it can be evaporated without leaving any residue.


There are multiple options to implement the required measurements for a specifi c application. For complex liquid sample streams, analytical separation trains are often combined with a liquid injection valve, capillary separation columns preferably with valveless column switching and, depending on the measuring task, a fl ame ionization (FID), thermal conductivity (TCD) or fl ame photometer detector (FPD). Due to the high analytical fl exibility, several separation trains can even be integrated in one GC (in the use case below using 4 analytical trains). For the injection valve also inert versions are available. The combination of capillary columns with a valveless switching device provides a superior separation power compared to packed columns and valves. The Siemens FID offers the advantage that the detector is placed outside the analytical oven and is heated separately. This supports the high availability of the GC by avoiding detector corrosion due to condensation effects.


The following arguments speak for this technology:


• Process chromatography is a powerful analytical technique to measure beside vapor samples also liquid matrices over a wide range of boiling points.


• Method development and application know-how is crucial for demanding applications especially for liquid process streams.


• MAXUM Ed. II gas chromatograph offers fl exible analysis tools, such as various injection valves or separation techniques for the analysis of simple to complex sample mixtures.


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