Introduction
Chlorine is one of the key raw materials in the chlor-alkali and PVC industries. From chlorine production and liquefaction to hydrochloric acid synthesis and downstream PVC manufacturing, accurate chlorine concentration measurement is essential for process optimization, product quality, and operational stability.
In these processes, chlorine online analysis is required at different concentration levels:
percentage-level chlorine measurement for process control and chlorine utilization optimization;
ppm-level chlorine measurement for residual chlorine control and emission monitoring.
The main applications include:
Chlorine liquefaction tail gas analysis (10–30% Cl2)
HCl synthesis chlorine control (1–5% Cl2)
Residual chlorine monitoring in HCl gas (<200 ppm Cl2)
Chlorine emission monitoring (<10 or 100 ppmppm level)
For a broader overview of chlorine (Cl₂) online analysis applications from ppm-level residual chlorine measurement to percentage-level process control, see our guide: Online Chlorine (Cl2) Gas Analysis: Industrial Applications from ppm to Percentage Levels.
1. Chlorine Measurement Points in Chlor-Alkali and PVC Production
A simplified process chain is:
Brine Electrolysis → Chlorine Gas Production → Chlorine Liquefaction → HCl Synthesis → PVC Production
Typical chlorine analysis points include:
| Process Section | Measurement Point | Typical Cl2 Range | Main Purpose |
|---|---|---|---|
| Chlorine liquefaction | Tail gas | 10–30% Cl2 | Liquefaction efficiency and chlorine recovery |
| HCl synthesis | Furnace outlet / dry HCl gas | 1–5% Cl2 | Reaction control |
| HCl gas quality control | HCl gas main line | <200 ppm Cl2 | Residual chlorine monitoring |
| Exhaust treatment | Treated exhaust / stack | <10 or 100 ppmppm level | Environmental compliance |
2. Chlorine Liquefaction Tail Gas Analysis
Process Background
In chlor-alkali plants, chlorine produced by electrolysis is normally compressed and liquefied for storage and transportation.
The liquefaction process does not convert all chlorine gas into liquid chlorine. A residual gas stream remains after condensation.
Monitoring chlorine concentration in this tail gas provides information about process efficiency.
Typical Measurement Point
Chlorine liquefaction unit tail gas
Process:
Chlorine gas → Compression → Liquefaction → Liquid chlorine product → Tail gas
Typical Measurement Range
Approximately 10–30% Cl2, (depending on liquefaction conditions and recovery system design)
Why Online Chlorine Analysis Is Required
Continuous chlorine measurement helps:
evaluate liquefaction performance;
reduce chlorine losses;
optimize recovery operation;
improve material balance calculations.
3. HCl Synthesis Furnace Chlorine Control
Process Background
Hydrogen chloride is produced by reacting hydrogen and chlorine:
H2+Cl2→2HCl
In industrial HCl synthesis furnaces, the hydrogen/chlorine ratio must be carefully controlled.
A controlled chlorine excess may be used to ensure complete hydrogen conversion.
Typical Measurement Point
HCl synthesis furnace outlet / dry HCl gas stream
Typical Measurement Range
Approximately 1–5% Cl2, (depending on operating conditions)
Why Online Chlorine Analysis Is Required
Continuous chlorine measurement supports:
H2/Cl2 ratio control;
reaction optimization;
stable HCl production;
reduced raw material consumption.
4. Residual Chlorine Analysis in HCl Gas
Process Background
After HCl synthesis, small amounts of unreacted chlorine may remain in the HCl gas stream.
Residual chlorine concentration is an important quality parameter because excessive free chlorine may affect downstream PVC production.
Typical Measurement Point
HCl gas main line
Typical Measurement Range
Typically below 200 ppm Cl2
Why Online Chlorine Analysis Is Required
Online residual chlorine measurement helps:
control HCl product quality;
detect abnormal synthesis conditions;
reduce corrosion risks;
improve downstream process stability.
5. Chlorine Emission Monitoring in PVC and Chlorine Plants
Process Background
Chlor-alkali and PVC production facilities may generate chlorine-containing exhaust gases from process vents, treatment systems, and auxiliary operations.
After gas treatment systems such as scrubbers, chlorine concentration may be continuously monitored before discharge.
Typical Measurement Point
treated exhaust gas line;
stack emission monitoring system.
Typical Measurement Range
<10 or 100 ppmppm level (typically configured according to emission regulations and plant requirements)
Why Online Chlorine Analysis Is Required
Continuous emission monitoring helps:
verify scrubber performance;
maintain environmental compliance;
provide real-time emission data.
6. Chlorine Analyzer Technology for Chlor-Alkali and PVC Processes
UV-based chlorine analyzers are widely used for continuous chlorine measurement because chlorine has strong absorption characteristics in the ultraviolet range.
Advantages include:
direct gas-phase measurement;
fast response;
continuous operation;
no chemical reagents;
suitability for corrosive chlorine-containing streams.
7. Key Considerations for Chlorine Analyzer Selection
7.1 Measurement Range
Different process points require different analyzer ranges:
| Application | Required Range |
|---|---|
| Chlorine liquefaction tail gas | 10–30% Cl2 |
| HCl synthesis control | 1–5% Cl2 |
| Residual chlorine monitoring | <200 ppm Cl2 |
| Emission monitoring | <10 or 100 ppmppm level |
A single analyzer configuration may not cover all applications; measurement range selection should match the process objective.
7.2 Sampling System Design
Because chlorine is highly reactive and corrosive, analyzer systems require:
compatible materials;
reliable sample conditioning;
proper moisture control;
stable gas handling.
7.3 Process Objective
The purpose of chlorine measurement varies by location:
| Measurement Purpose | Example |
|---|---|
| Process optimization | HCl synthesis chlorine control |
| Recovery optimization | Chlorine liquefaction tail gas |
| Quality control | Residual chlorine in HCl |
| Environmental compliance | Exhaust monitoring |
Conclusion
Chlorine (Cl2) online analysis plays an important role throughout the chlor-alkali and PVC production chain. These applications are part of a wider range of industrial chlorine analysis solutions covered in our guide to Online Chlorine (Cl2) Gas Analysis: Industrial Applications from ppm to Percentage Levels. By applying suitable online chlorine analyzers at different process points, chlor-alkali and PVC plants can optimize process control, improve chlorine utilization, maintain product quality, and achieve more stable operation.
For product information, please visit:
Ultraviolet photometry analyzer.
For further technical consultation or application support, please contact us at sales@mzdd.de .