ERT Applications in Gas-Liquid Two-Phase Flow Measurement

TL;DR · One-Sentence Summary

ERT for gas-liquid two-phase flow works best when the continuous phase is electrically conductive (water-based solutions, electrolytes). It measures cross-sectional conductivity distribution to infer bubble location and gas holdup. If your continuous phase is gas or oil, use ECT instead.

Why ERT is Suitable for Gas-Liquid Two-Phase Flow

The electrical contrast between phases in gas-liquid flows is stark:

Phase	Conductivity	Permittivity	Implication for ERT
Gas	≈ 0	≈ 1	Very low conductivity → signal “void”
Water-based liquid	High	≈ 80	High conductivity → strong background signal

This large conductivity difference (often orders of magnitude) makes bubbles appear as clear “low-conductivity regions” in ERT reconstructed images, ideal for:

• Gas holdup measurement (cross-sectional and local gas fractions)
• Bubble dynamics tracking (rise velocity, coalescence and breakup)
• Flow regime identification (bubble flow, slug flow, annular flow, etc.)

Typical Applications of ERT in Gas-Liquid Two-Phase Flow

1. Airlift Bioreactors

Airlift reactors use bottom gas injection for agitation and oxygenation, common in biopharma and wastewater treatment:

• Monitoring objectives: Bubble distribution uniformity, dead zone detection, correlation between gas holdup and aeration rate
• ERT advantage: Real-time cross-sectional gas-liquid distribution observation without process disturbance
• Engineering considerations: Electrode material must resist corrosion (316L stainless steel, titanium), good biocompatibility required

2. Gas-Liquid Dispersion in Stirred Tanks

In chemical stirred tanks, gas is injected from bottom or sidewall and dispersed by impeller:

• Monitoring objectives: Impeller dispersion efficiency, bubble size distribution, gas-liquid mixing uniformity
• ERT advantage: Multi-plane ERT can observe dispersion effects at different heights above the impeller
• Engineering considerations: Electrode placement must avoid impeller blade disturbance zones

3. Pipeline Gas-Liquid Two-Phase Flow

Common in petrochemical gas-liquid multiphase transport and wet natural gas pipelines:

• Monitoring objectives: Flow regime identification (stratified, slug, annular), slug prediction, gas-liquid interface position
• ERT advantage: Provides cross-sectional information, complementing differential pressure and single-point probes
• Engineering considerations: High-pressure applications require special electrode packaging, reliable waterproof sealing

4. Gas-Liquid Separators

Vessels for gas-liquid separation and phase disengagement:

• Monitoring objectives: Separation efficiency, liquid level and interface position, foam layer thickness
• ERT advantage: Can observe phase distribution evolution during separation
• Engineering considerations: Large-diameter vessels require consideration of electrode array coverage

Measurement Principle: From Conductivity to Gas Holdup

Calibration and Normalization

Typical ERT measurement workflow in gas-liquid two-phase flow:

1. Full-liquid calibration: Pipe filled with liquid phase, measure reference conductivity σ_ref
2. Empty-pipe calibration (optional): Pipe filled with gas phase, obtain lower reference
3. Online measurement: Measure σ_meas under actual operating conditions
4. Normalization: Calculate σ_norm = (σ_meas − σ_air) / (σ_ref − σ_air)
5. Gas holdup inversion: Convert σ_norm to gas holdup α_gas using EMA model

From Image to Gas Holdup

Path A: Threshold-based on image

• Reconstruct normalized conductivity distribution map
• Set threshold to binarize image (low conductivity = gas phase)
• Count low-conductivity pixel proportion → cross-sectional gas holdup

Path B: EMA model-based

• Use raw conductivity measurements directly
• Apply EMA model (e.g., Maxwell, Bruggeman)
• Weighted averaging of multiple electrode pair results → mean gas holdup

Flow Regime Identification

ERT images themselves can be used for flow regime identification:

Flow Regime	Image Features	Gas Holdup Range
Bubble flow	Dispersed small bubbles, uniform distribution	< 25%
Slug flow	Large gas bubbles alternating with liquid slugs, significant cross-sectional occupation	25% - 60%
Annular flow	Liquid film at wall + gas core at center, annular high-conductivity region in cross-section	> 60%

Combined with time-series analysis, ERT can also track slug frequency and bubble rise velocity.

Engineering Practice Considerations

1. Electrode Selection and Installation

• Material: 316L stainless steel (standard), titanium alloy (high corrosion), Hastelloy (strong corrosion)
• Shape: Rectangular or circular point electrodes, typically 10×10 mm to 20×20 mm
• Installation: Welded or flange-embedded, ensure flush with pipe inner wall to avoid flow disturbance

2. Frequency Selection

Common ERT excitation frequencies for gas-liquid two-phase flow:

• Low frequency (1 kHz - 10 kHz): Suitable for high-conductivity media, but polarization effects are significant
• Medium frequency (10 kHz - 100 kHz): Compromise, suitable for most gas-liquid two-phase flows
• High frequency (100 kHz - 1 MHz): Reduces polarization, but capacitive coupling begins to intervene

Frequency selection requires trade-offs between polarization, capacitive coupling, and equipment complexity.

3. Temperature and Conductivity Compensation

Water conductivity varies significantly with temperature (≈ 2%/°C), industrial sites require:

• Online temperature measurement: Install PT100 / thermocouple near ERT plane
• Conductivity-temperature calibration: Pre-characterize medium conductivity vs. temperature curve
• Real-time compensation: Correct measurements to reference temperature based on temperature

4. Avoid Boundary Effects

• When bubbles cling to pipe wall, very low conductivity near electrodes amplifies measurement error
• Can be suppressed by adjusting regularization parameters in image reconstruction algorithms
• Or use “guard electrode” structures to improve edge sensitivity

5. Safety Current Limits

ERT injection current must ensure safety:

• Human safety: Typically < 10 mA RMS (must comply with GB/IEC standards)
• Process safety: Avoid electrochemical products, electrode heating
• Intrinsically safe applications: Requires isolation barriers, explosion-proof packaging

Boundaries and Limitations

Situation	Why ERT Doesn’t Work	Alternative Solution
Gas as continuous phase	No conductive path for electrodes	Switch to ECT
High-viscosity media	Bubbles difficult to disperse, images blurred	Ultrasound, Process Tomography
High-pressure supercritical gas	Gas-liquid conductivity difference reduced	ECT, radiography
Internal pipe coating	Electrodes isolated from medium	Non-contact ECT
Bubble size < electrode	Single bubble has negligible effect on electrode measurements	Higher electrode count, higher frequency

Want to Learn More?

For detailed theoretical derivation and industrial application cases of ERT gas-liquid two-phase flow measurement, refer to the review paper:

Z. Cui, Q. Zhang, K. Gao, Z. Xia, H. Wang, “Electrical Impedance Sensors for Multi-Phase Flow Measurement: A Review”, IEEE Sensors Journal, Vol. 21, No. 24, Dec. 2021, pp. 27252–27267. DOI: 10.1109/JSEN.2021.3124625

Next Steps

• Selection phase: Read ERT vs ECT: How to Choose to confirm technical approach
• Deepen principles: Read Phase Fraction Measurement by Tomography for EMA model details
• Project consultation: Contact us, provide medium information, pipe dimensions, and measurement objectives—engineers will provide specific sensor solutions and quotations