Measurement of O2 Concentrations in Vacuum Distillation Column in Petroleum Refining

Introduction

O₂ measurement in hydrocarbon vapor is used for safety monitoring in vacuum distillation columns in petroleum refining. With conventional paramagnetic oxygen analyzers, O₂ concentrations are obtained through an extractive sampling system, which conditions the sample prior to being analyzed. Sample systems cause measurement response time delays, which are very critical in safety systems. Moreover there are many problems related to maintenance and operational costs of sample systems. The process gas to be measured contains high concentration of H₂S that leads to quick corrosion of the sample system and analyzer. Another issue when using sampling systems is the sample is not representative of the process after the sample has been conditions and dew point of the sample has been modified, which affects the volume concentration of the sample.

The TDLS8000 Laser Analyzer is the solution to all these problems.

 

Expected Benefits

• High-speed response to meet all requirements of safety control
• Continuous monitoring of O₂ concentration in-situ
• Eliminates the need for sampling and reduces the maintenance and running costs of the sampling system

 

Process Overview

The vacuum distillation column is used to distill the residual oil from the atmospheric distillation column. The distillation must be performed at absolute pressures as low as 2 to 15 kPa so as to limit the operating temperature to less than 370 to 380°C and to avoid hydrocarbons cracking processes. The presence of oxygen in the process can be caused by suction and can lead to explosion. Thereby there are rules to control the O₂ level in vacuum columns. At level 2 to 5 % O₂ the alarm and purging of vapor line by inert gas has to be switched on. That is why high-speed response and high reliability are required for O₂ measurements.

 

Advantage

Solution Details

Measurement System

Measuring points of vacuum distillation column

Pressure Variation Test Data

This graph shows O₂ measurement on 21%, 4%, 0.8% level under condition which gas pressure change from 1 BarA to 0.2 BarA by TDLS, and shows the measurement done stably and correctly without influence of pressure change.

O₂ absorption spectra shape is big changed by pressure change, but the spectrum peak area change to pressure proportionally, so TDLS can compensate pressure influence correctly.

Measurement Value of O₂ Concentration under Pressure Change 1 BarA to 0.2 BarA