Flow Assurance

Hydrate Inhibitor Injection Rate: MEG & Methanol Engineering Guide

Calculate hydrate inhibitor dosing using Hammerschmidt equation for methanol and glycol systems.

Launch Calculator Hammerschmidt equation

Methanol

K = 2,335

MW = 32.04, high losses

MEG

K = 2,335

MW = 62.07, regenerable

Corrosion

10-50 ppm

Continuous injection

Contents

Use this guide to:

  • Calculate inhibitor concentration for hydrate suppression.
  • Size methanol or glycol injection systems.
  • Determine corrosion inhibitor dosing.

1. Hammerschmidt Equation

Predicts hydrate temperature depression based on inhibitor concentration in the aqueous phase.

Hammerschmidt Equation: ΔT = K × W / (M × (100 - W)) Solved for concentration: W = 100 × M × ΔT / (K + M × ΔT) Where: ΔT = Hydrate temperature depression (°F) K = 2,335 (universal Hammerschmidt constant, °F form) W = Weight % inhibitor in aqueous phase (0-100) M = Molecular weight of inhibitor (g/mol)

Inhibitor Constants

Inhibitor MW K Max W ΔT @ 25 wt%
Methanol 32.04 2,335 ~80% 24.3°F
Ethanol 46.07 2,335 ~70% 16.9°F
MEG 62.07 2,335 ~70% 12.5°F
DEG 106.12 2,335 ~65% 7.3°F
TEG 150.17 2,335 ~60% 5.2°F

Example: Methanol Concentration

Given: Need 30°F hydrate suppression using methanol K = 2,335, M = 32.04 W = 100 × 32.04 × 30 / (2,335 + 32.04 × 30) = 96,120 / 3,296 = 29.2 wt% methanol in aqueous phase

2. Methanol Injection

Methanol is effective but lost to both gas and liquid hydrocarbon phases. Total requirement = aqueous + gas losses + HC losses.

Total methanol requirement: MeOH_total = MeOH_water + MeOH_gas + MeOH_HC In aqueous phase (Hammerschmidt): MeOH_water = W × W_rate / (100 - W) [lb/day] Lost to gas phase: MeOH_gas = Kᵥ × P × Q_gas [lb/day] Where Kᵥ = vapor distribution factor (see table) Lost to hydrocarbon liquid: MeOH_HC ≈ 0.5-2% of condensate volume

Methanol Vapor Loss Factor (Kᵥ)

T (°F) 30 40 50 60 70
Kᵥ (lb/MMSCF/psi) 0.0015 0.0022 0.0032 0.0045 0.0062
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Example: Methanol Injection Rate

Given: 10 MMSCFD, 50 bbl/day water, 1000 psia, 40°F Need 25 wt% MeOH in water Water mass: 50 bbl × 350 lb/bbl = 17,500 lb/day MeOH in water: = 0.25 × 17,500 / (1 - 0.25) = 5,833 lb/day MeOH to gas (Kᵥ = 0.0022 at 40°F): = 0.0022 × 1000 × 10 = 22 lb/day Total: 5,855 lb/day ÷ 6.6 lb/gal = 887 gal/day (21 bbl/day)

⚠ Safety: Methanol is toxic and flammable. Follow applicable handling codes.

3. Glycol (MEG) Injection

Glycols are preferred for pipelines because they're regenerable with minimal vapor losses.

MEG vs Methanol

Factor Methanol MEG
Effectiveness (ΔT per wt%) Higher Lower
Recovery Usually lost Regenerated (80-90%)
Vapor loss 2-10% <0.1%
HC solubility loss 1-2% <0.5%
Cost driver Operating (makeup) Capital (regen unit)
Best application Short-term, remote Long pipelines, offshore

Glycol Injection Calculation

Lean glycol injection rate: G_lean = W_water × C_rich / (C_lean - C_rich) Rich glycol concentration: C_rich = 100 × M × ΔT / (2,335 + M × ΔT) [from Hammerschmidt] Where: G_lean = Lean glycol rate (lb/day) W_water = Water production (lb/day) C_lean = Lean glycol concentration (typically 80-90 wt%) C_rich = Rich glycol concentration (from required ΔT) K = 2,335 (universal constant, °F)

Example: MEG Injection Rate

Given: 100 bbl/day water, need 25°F suppression Lean MEG = 85 wt%, M = 62.07, K = 2,335 Rich MEG concentration (Hammerschmidt): C_rich = 100 × 62.07 × 25 / (2,335 + 62.07 × 25) = 155,175 / 3,887 = 39.9 wt% Water mass: 100 bbl × 350 lb/bbl = 35,000 lb/day Lean MEG rate: G_lean = 35,000 × 39.9 / (85 - 39.9) = 1,396,500 / 45.1 = 30,965 lb/day Volume: 30,965 lb ÷ 9.3 lb/gal = 3,330 gal/day (79 bbl/day)

4. Corrosion Inhibitors

Film-forming corrosion inhibitors protect against CO₂ and H₂S attack. Dosing is typically ppm-based on produced water volume.

Dosing Rates

Application Typical Rate (ppm)
Sweet gas (CO₂ only) 10-25
Sour gas (H₂S present) 25-50
High CO₂ (>5 mol%) 50-100
Produced water systems 25-75

Injection Rate Calculation

Continuous injection: Rate (gal/day) = Q_water (bbl/day) × ppm × 42 / (ρ × 10⁶) Where: Q_water = Water production rate (bbl/day) ppm = Target concentration ρ = Inhibitor density (lb/gal), typically 8-9 42 = gal/bbl Example: 500 bbl/day water, 50 ppm, ρ = 8.5 lb/gal Rate = 500 × 50 × 42 / (8.5 × 10⁶) = 0.12 gal/day

References

  • GPSA, Section 20
  • NACE SP0106 – Internal Corrosion Control
  • API RP 14E – Offshore Platform Piping

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Frequently Asked Questions

How is the Hammerschmidt equation used to calculate inhibitor concentration?

The Hammerschmidt equation relates the hydrate temperature depression to the weight percent of inhibitor in the aqueous phase. It uses inhibitor-specific constants for methanol, MEG, and other glycols to determine the minimum concentration required for a target temperature depression.

What is the methanol vapor loss factor and why does it matter?

The methanol vapor loss factor (Kᵥ) accounts for methanol that partitions into the gas phase rather than remaining in the water phase where it provides hydrate inhibition. This loss must be added to the calculated injection rate to ensure adequate protection.

How does MEG compare to methanol for hydrate inhibition?

MEG has negligible vapor losses and can be regenerated and recycled, making it preferred for continuous operations. Methanol is cheaper per unit but has significant vapor-phase losses and is typically used for intermittent or batch treatments.

How are corrosion inhibitor injection rates determined?

Corrosion inhibitor dosing rates are based on the volume of produced water and are typically specified in parts per million (ppm). The injection rate calculation multiplies the target concentration by the water production rate.