ON-LINE ANALYSIS OF AROMATIC

HYDROCARBONS IN PROCESS WATER

INTRODUCTION

Aromatic Hydrocarbons such as benzene and its deriva-

tives, toluene and xylene often referred as BTX

are used as starting materials in a multitude of

products from pharmaceuticals, dyes, polymers to

agrochemicals.

These aromatics are known to cause health effects

from chronic exposure and even benzene is listed as a cancer causing substance. In production processes cooling water and condensed water can become contaminated   with these compounds. It is therefore necessary to have reliable and fast methods to determine and control the amount of these compounds as well as effective methods for treatment and rem-oval.

                           Figure 1: BTX solvents

Gas chromatography is the most commonly used

method for measurement. However, there is demand

for alternative methods utilising fast on-line analysis

with lower running costs.                                                                                                                                                                                                                                                                                               

THE METHOD

Aromatic hydrocarbons show good absorption

characteristics in UV-VIS region due to the high level

of conjugated bonds present. Figure 1 gives an example of different concentrations of a benzene, toluene,xylene mixture in water.

A method using absorbance characteristic of aromatics in the region 190–300 nm was developed to measure these compounds in the ppm range.

A PLS algorithm is used to determine the concentration of individual and total aromatics.

                                                                                                                          Figure 2: LabView screenshot

THE   SYSTEM

The industrial diode array system MCS 500 UV based on the established ZEISS MCS polychromator technology

provides the spectral information in the UV range required for the developed method.

High sensitivity, high wavelength stability and low straylight guarantee excellent reliability of the determined values.

Pre-aligned flow cell, 2-channel plug-in multiplexer to measure two streams or sample/reference, industrial standard

outputs (RS 422, 4-20 mA) and an easy-to-use on-line software package complete the system.

Systems can be cascaded and controlled via one Host PC to measure compounds in different streams.

THE   SOFTWARE

The DM1 software package is designed to run under WINDOWS®NT and includes the PLS/IQ module from

GALACTIC to work with any PLS model designed under GRAMS 5.

The programm allows to display the results of up to 4 channels including the measured absorbance spectra.

Concentrations of the compounds can be displayed, saved and/or sent via analog output.

Digital controlled devices like valves, shutter etc. to switch streams or references can be included in the automated

signal processing.

THE   BENEFITS

♦   Analysis and response time greatly reduced

♦   Fast and continuous monitoring

♦   Established and proved system and method

♦   High reliability, low maintanance

♦   Easy to use software

♦   Speed and full spectrum advantage of photodiode array detection

♦   Possibility to combine systems to monitor various streams

♦   Fiber optics allow flexibility in installation and location of the system

SYSTEM   SPECIFICATIONS

Wavelength range                                              200 – 620 nm (190–1020 nm optional)

Wavelength resolution                                       0,8 nm/diode (2,4 nm Rayleigh)

Wavelength   reproducibility                               < 0,05 nm

Measurement time                                             < 2s

PC Interface                                                       RS422

Distance between PC and System                     Up to 80 m with standard cable

   Up to 2,5 km with waveguide transmission

Analog output (4 – 20 mA)                                  4 channels (standard)

   NI PC ISA card

Optical Fiber (Probes-System)                           Quartz, 600 µm core, SMA

   Maximum distance 10 m

Housing                                                             19” slide-in rack

19” protective housing