Gas Compressors – Interpreting MPC Patch Colors with Chemically Sound Accuracy

This refined version keeps your powerful field insight—but aligns it with rigorous lubricant chemistry, so both engineers and chemists will agree with it.

---

1. What the MPC Test Actually Measures

The Membrane Patch Colorimetry (MPC) test:

• Passes a diluted oil sample through a membrane
• Captures:
  • insoluble particles
  • borderline soluble (near-saturation) polar degradation products
• Produces:
  • ΔE value → quantity
  • patch color → qualitative indication of deposit nature

Critical clarification:

MPC does not measure fully dissolved degradation products
and does not separate vapor / oil / sludge phases physically

---

2. Correct Interpretation of “Top – Middle – Bottom” in Your Image

Your visual model is useful—but must be framed correctly:

Visual Layer	Chemically Accurate Meaning
TOP	Low molecular weight, soluble oxidation products (pre-varnish stage)
MIDDLE	Near-solubility-limit species captured by MPC
BOTTOM	Insoluble, high molecular weight deposits (sludge/varnish)

Important:

These are conceptual stages of degradation, not physical layers inside the MPC test.

---

3. How Gas Composition Influences Oil Degradation (Chemically Correct View)

In gas compressors, gas ingress affects oil through:

• dissolution under pressure
• chemical interaction with base oil and additives
• catalytic effects (via water, acids, or contaminants)

This modifies:

• oxidation rate
• degradation pathways
• polarity and solubility of by-products

---

4. Gas Type vs Degradation Pathway (Refined and Chemically Accurate)

---

A. Sweet Natural Gas

Dominant Effects:

• Limited oxygen → mild oxidation
• low acid formation

Chemistry:

• formation of oxygenated hydrocarbons (low MW)
• early antioxidant depletion

MPC Appearance:

• light amber / golden

Interpretation:

Early-stage oxidation products approaching solubility limit

---

B. Sour Gas (H₂S / CO₂)

Dominant Effects:

• acid formation (carbonic + weak sulfur acids)
• accelerated oxidation and additive depletion

Chemistry:

• formation of:
  • polar oxidation products
  • sulfur-containing compounds
• increased reaction rates via acid catalysis

MPC Appearance:

• dark amber → brown → black

Chemically Accurate Note:

Darkening is primarily due to high molecular weight oxidation/polymer products,
not sulfur alone

---

C. Wet Gas (CO₂ + H₂O)

Dominant Effects:

• hydrolysis + oxidation coupling
• additive degradation (especially R&O systems)

Chemistry:

• organic acid formation
• oxygenated polar species (e.g., ketones, aldehydes, carboxylic acids)

MPC Appearance:

• reddish brown → dark brown

Important Clarification:

Reddish tones are commonly observed with certain oxidation intermediates,
but are not uniquely diagnostic of CO₂ + water

---

D. Hydrogen-Rich Gas

Dominant Effects:

• limited oxygen availability
• slower oxidation kinetics

Chemistry:

• oxidation still occurs via:
  • residual oxygen
  • thermal degradation
  • catalytic metals

MPC Appearance:

• light amber → brown

Correction to common assumption:

Hydrogen does not prevent oxidation—it may only reduce its rate under certain conditions

---

E. Inert Gas (N₂ / He)

Dominant Effects:

• minimal chemical reactivity

Chemistry:

• degradation dominated by:
  • thermal oxidation
  • system contaminants

MPC Appearance:

• very light amber → straw

Interpretation:

Low varnish tendency unless driven by non-gas factors

---

F. Flare / Associated Gas

Dominant Effects:

• highly variable composition
• mixed oxidation, sulfidation, and hydrocarbon reactions

Chemistry:

• multi-path degradation:
  • oxidation
  • polymerization
  • additive depletion

MPC Appearance:

• dark brown → black

---

G. Process Gas (Refinery / Petrochemical)

Dominant Effects:

• aromatics + unsaturated hydrocarbons
• high temperature operation

Chemistry:

• polymerization and condensation reactions
• formation of:
  • lacquer
  • coke-like material

MPC Appearance:

• very dark brown → black

Important:

At this stage, deposits transition from varnish to carbonaceous residues

---

5. What MPC Color CAN and CANNOT Tell You

✅ What It CAN Indicate:

• Relative severity of deposit formation
• General nature of degradation (mild vs severe)
• Presence of high molecular weight oxidation products

❌ What It CANNOT Reliably Identify:

• Exact chemical species
• Specific gas contamination source
• Precise degradation mechanism

---

6. Correct Way to Use MPC Color in Gas Compressors

Always combine:

Parameter	Why
MPC ΔE	Quantifies varnish potential
MPC Color	Indicates qualitative nature
TAN	Detects acid formation
RULER	Tracks antioxidant depletion
Water	Identifies hydrolysis risk
Operating conditions	Validates mechanism

---

7. Critical Limitation Section

MPC interpretation is semi-quantitative and qualitative.

Limitations include:

• Color depends on:
  • base oil type (Group I vs II vs III)
  • additive chemistry
  • contamination (metals, soot, process ingress)
• Different chemistries can produce similar colors
• Fully dissolved degradation products are not measured

---

8. Final Chemical Insight

Gas composition does not directly dictate varnish color—
it influences degradation pathways that alter the distribution, polarity, and molecular weight of oxidation products, which are then partially reflected in MPC patch appearance.

---

9. Final Engineering Takeaway

• Use MPC color as a trend and pattern recognition tool
• Not as a standalone diagnostic
• Always interpret within:
  • gas environment
  • oil chemistry
  • operating conditions

---

---

In these images, the “Top (vapor)” region should be understood as a conceptual representation of early-stage, low molecular weight oxidation products that remain fully dissolved in the oil, rather than an actual vapor phase or a physically separate layer. Chemically, it reflects the initial degradation products formed through gas–oil interactions (e.g., mild oxidation, acid formation) before they grow in molecular weight and polarity to approach their solubility limit. These species are typically not captured effectively by the Membrane Patch Colorimetry (MPC) membrane and therefore appear lighter in color, indicating that the oil is in a pre-varnish or early varnish formation stage, where degradation is ongoing but deposits have not yet precipitated.

Question

Can we relate this article with Delta a and Delta b in the MPC test , can those help in knowing the gas influence on the oil ?

§∆E = √( L^2+ a^2+ b^(2 ) )

§L = black/white; a = red/green; b = blue/yellow

Can Δa and Δb in MPC Help Identify Gas Influence in Gas Compressors? — Chemically Accurate Answer

Short answer:

They can provide useful directional insight, but they cannot uniquely identify the gas type or contamination source.

Let’s go deeper—this is where your concept becomes powerful if framed correctly.

---

1. What Δa and Δb Actually Represent in MPC

In the Membrane Patch Colorimetry (MPC), color is measured in CIELAB space:

• ΔL → lightness (brightness / darkness)
• Δa → red ↔ green axis
• Δb → yellow ↔ blue axis

Interpretation:

• +Δa → more red tones
• +Δb → more yellow tones
• Low ΔL → darker patch (heavier deposits)

---

2. Linking Δa / Δb to Oil Degradation Chemistry

From a chemistry standpoint:

Δb (Yellow Component)

Typically associated with:

• early oxidation products
• low to medium molecular weight compounds
• oxygenated species (e.g., aldehydes, ketones)

➡️ Represents:

Initial oxidation stage / soluble varnish precursors

---

Δa (Red Component)

Often linked to:

• more complex oxidation products
• conjugated structures (e.g., quinone-like species)
• some polymerized degradation compounds

➡️ Represents:

More advanced oxidation chemistry

---

Darkening (Low ΔL + mixed Δa/Δb)

Indicates:

• high molecular weight compounds
• polymerization
• carbonaceous material

---

3. Now the Critical Question: Can This Be Linked to Gas Type?

Yes — but only indirectly and probabilistically

Gas composition influences:

• oxidation pathways
• reaction rates
• types of degradation products

This can shift Δa / Δb trends, but:

❗ There is NO one-to-one mapping (e.g., “CO₂ = red” is not scientifically valid)

---

4. Practical Interpretation in Gas Compressors

Now we translate your concept into a chemically defensible diagnostic model:

---

Case 1: Sweet Gas Systems

Expected trend:

• High Δb (yellow)
• Low Δa

Meaning:

• early oxidation
• mild degradation

---

Case 2: Wet Gas (CO₂ + Water)

Possible trend:

• Increasing Δa (reddish tones)
• Moderate Δb

Meaning:

• oxygenated degradation products
• acid-driven chemistry

⚠️ Important:

Red tones are suggestive, not definitive proof of water/CO₂

---

Case 3: Sour Gas (H₂S + CO₂)

Expected:

• Lower ΔL (darker patch)
• Mixed Δa and Δb

Meaning:

• accelerated degradation
• formation of heavier molecules

---

Case 4: Process / Flare Gas

Expected:

• Very low ΔL (dark)
• Δa and Δb less meaningful individually

Meaning:

• polymerization dominates
• carbonaceous deposits

---

5. What Δa and Δb Can Actually Help You Do

✅ They CAN:

• Track evolution of degradation chemistry
• Differentiate:
  • early oxidation vs advanced degradation
• Detect shifts in degradation pathway over time

---

❌ They CANNOT:

• Identify exact gas composition
• Replace gas analysis or process data
• Provide root cause alone

---

6. Most Powerful Use (Your Concept — Now Refined)

Instead of saying:
❌ “Δa and Δb tell you which gas is present”

Say:

“Δa and Δb trends, when correlated with operating conditions and gas environment, can provide insight into changes in degradation pathways influenced by gas contamination.”

---

7. Advanced Insight (This is Where You Add Real Value)

In gas compressors, combine:

Parameter	Insight
Δb ↑ first	early oxidation
Δa ↑ later	advanced oxidation
ΔL ↓	heavy deposits forming
TAN ↑	acid-driven degradation (CO₂ / H₂S influence)
Water ↑	hydrolysis contribution

---

8. Final Chemically Correct Conclusion

Δa and Δb do not directly identify gas contamination, but they reflect the evolving chemical nature of oxidation by-products, which can be influenced by gas composition in compressor systems.

---

Final Engineering Insight

If you trend:

• Δa, Δb, ΔL + TAN + water + operating conditions

You move from:
👉 “Oil looks bad”

to

👉 “I understand how the degradation pathway is evolving—and why.”

---