Process Instruments Inc.

Process Instruments Inc.

Raman Fiber Multiplexer

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The Fiber Optic System for Raman Spectroscopy (U.S. patent No. 6,859,581B1) can be used to expand the use of our Raman Spectrometer. Through switching the excitation and collection fibers. Our raman spectrometer can monitor up to 18 probes. This can be done with minimal signal loss, and very high repeatability. The Fiber Optic Multiplexer had become popular with feedforward applications for refineries, where our PI-200 and multiplexer control their blending header while looking ant the incoming component streams. The Fiber Optic Multiplex system can come in a variety of options. We offer a 5, 9 or 18 Max channel configurations.  

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Continuous in situ monitoring of all the critical amine parameters: amine concentration, H2S, and CO2 levels can provide almost instantaneous feedback on important changes in the amine process which can lead to real time process control, improved finished product quality, reduced overall cost, and minimized waste.

A schematic representation of a typical amine process with the locations of the Raman probes used to monitor the rich and lean amines shown in red is displayed to the right.  The Raman instrument is designed to be located in a centralized control room and is connected to the different fiber optic cables associated with each Raman sample probe.  Raman has an advantage over infrared in that there is no interference from water so the probe can be directly inserted into a process stream.  Infrared requires a sample conditioner.  A single Raman system can be multiplexed up to 18 channels/streams.

Acid Gases in Amines

The Raman spectrum of a typical rich amine s ample containing CO2 and H2S is displayed to the right.  The Raman peak located at 2570 cm-1 is from the H2S and the 1387 cm-1 peak is from CO2.  The fact that intensity from each acid gas peak is linearly proportional to the gas’s concentration allows chemometric techniques such as Partial Least Squares (PLS) to model all of the spectral variations and correlate them to concentrations.  Current, standard laboratory practice requires laborious titration of grab samples to determine amine, H2S and CO2 levels in both the rich and lean amine streams.  Many laboratory measurements have to be performed to properly control and monitor the amine streams.  Consequently the amine streams are often not efficiently used, at great expense to the refinery, natural gas provider, and power plant.  Real time monitoring would also eliminate the formation of unknown excessive acid gas concentrations that form acid solutions in the aqueous stream and damage critical plant infrastructure.

CO2 Monitoring

Raman spectra from a pure DGA amine sample with no CO2 adsorbed (black circles) and on-line grab samples containing different CO2 concentrations (colored lines).  The CO2 1387 cm-1 Raman band has a signal intensity that is c.a. 4.5 times less then the H2S band.  The nine DGA amine samples we analyzed did not have laboratory CO2 concentrations.  The figure to the right illustrates how the CO2 peak is not observed in the pure DGA sample and that the peak intensity varies within the nine DGA samples.  The varying intensity indicates that Raman analysis can easily distinguish different CO2 concentrations.

CO2 Sequestration

Raman spectra from mixtures of different amine streams (MEA, MDEA, DEA) and CO2 all at different concentrations are displayed to the right.  Each subplot shows a varying concentration of a single component while the other component's concentrations remain constant.  Distinct spectral features for each of the different components can be observed, and indicate that Raman spectroscopy can be used for on-line monitoring of streams containing multiple amine species.

Typical online results for the combined amines (MEA, MDEA, DEA) streams with various CO2 loading.  The plot shows Raman predicted concentrations vs. the measured laboratory concentrations.  Even though these models were generated with a limited number of samples (50 total), their standard errors of predictions (SEP) are comparable to most conventional laboratory techniques.

DGA Monitoring

Raman spectrum of a rich Diglycolamine (DGA) sample that contained 35.1 % DGA, and 21,290 ppm H2S is displayed to the right.  The Raman peak located at 2570 cm-1 is from the H2S and the 1387 cm-1 peak is from CO2.

Typical online partial least squares (PLS) analysis of the DGA concentrations. The plot shows Raman predicted values vs. the measured laboratory values.  The standard error of prediction for the PLS model with outliers removed was 0.56 %.

H2S Monitoring

Typical online partial least squares (PLS) analysis of the H2S (mole/mole) concentrations. The plot shows Raman predicted values vs. the measured laboratory values.  The standard error of prediction for the PLS model with outliers removed was 0.0057.

Typical online partial least squares (PLS) analysis of the H2S (ppm) concentrations. The plot shows Raman predicted values vs. the measured laboratory values.  The standard error of prediction for the PLS model with outliers removed was 745 ppm.

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