Moisture Measurement in the Recycle Gas of a Reformer

Moisture Measurement in the Recycle Gas of a Reformer

Description
In reforming units platinum catalysts are used to convert low-octane naphtha into a high-octane product called “reformate”. Monitoring the moisture content of the reformer gas (also called recycle gas) play a crucial role in this process:

  • On the one hand it is an indicator for the efficiency of the entire process
  • On the other hand a moisture value above 30 ppm is hazardous for the catalysts in the reformer. It reduces their service life considerably and leads to high regeneration costs.

Therefore it is important to determine the moisture content of recycle gas (measuring range 0 – 30 ppm) in a way that is precise, reproducible and long-term stable.

Solution
HYGROPHIL F 5673 reliably and precisely measures the moisture in gases and liquids inline in the range of the dew point temperatures of -80 °C to +20 °C. A very high accuracy in the desired measuring range 0 – 30 ppm can be achieved with a customer-specific special calibration in the medium itself (at an accredited institute). HYGROPHIL F 5673 exhibits no cross-sensitivity towards the substances in the process. The sensor can be installed directly in the pressure line up to 200 bar (and removed under pressure at any time using the BARTEC BENKE sensor retraction tool) or in a BARTEC BENKE Sample Conditioning System. Due to the special calibration the moisture sensor is very precise, maintenance-free and long-term stable.

Measuring principle
The sensor element consists of a robust multi-layer with optically high and low refractive layers. The moisture content equilibrium then causes water molecules to settle in the layer. The different refractive indices (air: 1.00 / water 1.33) cause a wavelength shift in the incoming light which is proportional to the prevalent moisture in the medium. This shift (i.e. no intensity measurement => path-neutral) is evaluated by a polychromator and allocated to a dew point.

The sensor can be used in zone 0 (cl. 1 / div. 1). The measuring system has ATEX, CSA/UL and GOST approval.

The working conditions for trace moisture measurement in catalytic reformers are between 30° to 50°C gas temperature, a pressure from about 30 to 50 bar and a moisture content between 0 and 30 ppm. The main elements of cycle gas are hydrogen H2 (80%), hydrocarbons C1 to C4 and traces of HCl, which is purged out of the catalyst with water.

The moisture in the process is often measured by aluminum oxide sensors, which are installed in an analytic/ sample system. Nevertheless the residual amount of HCl effects the lifecycle and the drift behavior of sensors, which leads to higher maintenance efforts and reduces the process stability. The aluminum oxide sensors must be recalibrated in intervals of up to 3 months. It is not exactly identifiable, if the moisture in the process reaches a critical value, which would require a flow-rate reduction of the reactor, or if the sensor just shows drifting values.

Experience has shown that annual checks of the fiber optical measuring principle of the moisture sensor L1661 are sufficient, because the sensor is handled without electrochemical implementation.

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