Worked Example: Sizing a Triethylene Glycol (TEG) Dehydration Contactor & Regeneration Skid

Triethylene Glycol (TEG) absorption dehydration is the undisputed industry standard for dehydrating natural gas to prevent pipeline hydrate blockages and internal corrosion. Wet natural gas contacts lean liquid TEG counter-currently in a packed or trayed absorber column (contactor). The rich TEG is subsequently regenerated at atmospheric pressure in an indirect-fired reboiler and stripped of water vapor before being recirculated.

This worked example details the complete process design and equipment sizing for a $10.0\text{ MMSCFD}$ TEG dehydration unit operating at $65.0\text{ barg}$, including contactor column diameter via the Souders-Brown equation, glycol circulation rate, regeneration reboiler thermal duty, and stripping gas requirements.

1. Design Basis & Process Operating Parameters

A front-end engineering design (FEED) study specifies the following inlet conditions for an offshore dehydration module:

Gas Operating Parameters:

• Gas Flow Rate ($Q$): $10.0\text{ MMSCFD}$ ($11,798.7\text{ Sm}^3/\text{h}$)
• Contactor Operating Pressure ($P$): $65.0\text{ barg}$ ($66.01\text{ bar(a)} = 957.4\text{ psia}$)
• Inlet Gas Temperature ($T_{in}$): $35.0^\circ\text{C}$ ($95.0^\circ\text{F} = 308.15\text{ K}$)
• Gas Specific Gravity ($G$): $0.65$ (relative to dry air)
• Gas Compressibility Factor ($Z$): $0.9549$

Target Dehydration Performance:

• Target Sales Gas Moisture Content: $4.0\text{ lb/MMSCF}$ ($64.1\text{ mg/Sm}^3$)
• Lean TEG Concentration: $99.00\text{ wt}\%$ (enhanced to $99.99\text{ wt}\%$ via stripping gas)
• Design TEG Circulation Ratio: $3.0\text{ gal TEG / lb H}_2\text{O removed}$
• Souders-Brown System Factor ($K_{SB}$): $0.16\text{ ft/s}$ ($0.0488\text{ m/s}$) for bubble-cap/structured packing
• Reboiler Operating Temperature: $204.0^\circ\text{C}$ ($400.0^\circ\text{F}$)

2. Step 1: Inlet Water Load & Water Removal Duty

A. Inlet Gas Saturation Moisture Content ($W_{in}$):

Using the Bukacek (1973) correlation at $65.0\text{ barg}$ and $35.0^\circ\text{C}$:

$$W_{in} = \mathbf{80.88\text{ lb/MMSCF}} \quad (1,295.5\text{ mg/Sm}^3)$$

B. Water Removal Duty ($\Delta W$):

$$\Delta W = W_{in} - W_{target} = 80.88 - 4.00 = \mathbf{76.88\text{ lb/MMSCF}}$$

C. Mass Water Removal Rate:

For a $10.0\text{ MMSCFD}$ facility:

$$\dot{m}_{water,day} = 76.88\text{ lb/MMSCF} \times 10.0\text{ MMSCFD} = \mathbf{768.8\text{ lb/day}}$$

Hourly water removal rate:

$$\dot{m}_{water,hr} = \frac{768.8\text{ lb/day}}{24\text{ h/day}} = \mathbf{32.03\text{ lb/h}} \quad (\mathbf{14.53\text{ kg/h}})$$

3. Step 2: TEG Solvent Circulation Rate

The required volumetric circulation rate of lean TEG is determined by the design circulation ratio ($3.0\text{ gal TEG / lb H}_2\text{O}$):

$$L_{TEG}\text{ [GPH]} = \dot{m}_{water,hr} \times \text{Ratio} = 32.03\text{ lb/h} \times 3.0\text{ gal/lb} = \mathbf{96.10\text{ GPH}}$$

Converting to Gallons per Minute (GPM):

$$L_{TEG} = \frac{96.10\text{ GPH}}{60\text{ min/h}} = \mathbf{1.60\text{ GPM}}$$

In standard metric units:

$$L_{TEG,metric} = 1.60\text{ GPM} \times 0.227125 = \mathbf{0.364\text{ m}^3/\text{h}} \quad (\mathbf{363.8\text{ L/h}})$$

Mass Flow of Circulating Glycol:

Pure TEG density at $15^\circ\text{C}$ is $1,125\text{ kg/m}^3$ ($9.39\text{ lb/gal}$):

$$\dot{m}_{TEG} = 96.10\text{ gal/h} \times 9.39\text{ lb/gal} = 902.4\text{ lb/h} = \mathbf{409.2\text{ kg/h}}$$

Rich Glycol Water Pickup:

Water absorbed per pound of TEG: $$\Delta w = \frac{32.03\text{ lb/h}}{902.4\text{ lb/h}} = 3.55\text{ wt}\%$$ Rich TEG concentration exiting contactor: $$w_{rich} = 99.00\% - 3.55\% = \mathbf{95.45\text{ wt}\% \text{ TEG}}$$ Design Rule Check: Rich TEG concentration remains $>95.0\text{ wt}\%$, safely within the GPSA recommended range ($94 - 96\text{ wt}\%$) to prevent thermal degradation and high reboiler loads.

4. Step 3: Contactor Absorber Column Sizing

The contactor diameter is sized to prevent liquid carry-over and entrainment flooding at the top gas outlet.

A. Gas Density at Contactor Conditions ($65\text{ barg}, 35^\circ\text{C}$):

$$\rho_g = \frac{P \cdot M_{mix}}{Z \cdot R \cdot T} = \frac{6,601,250\text{ Pa} \times 0.01883\text{ kg/mol}}{0.9549 \times 8.31446 \times 308.15\text{ K}} = \mathbf{50.80\text{ kg/m}^3} \quad (3.171\text{ lb/ft}^3)$$

B. Liquid Density:

At contactor operating temperature ($35^\circ\text{C}$), TEG density is: $$\rho_L \approx 1,118\text{ kg/m}^3 \quad (69.80\text{ lb/ft}^3)$$

C. Maximum Flooding Velocity via Souders-Brown Equation:

$$v_{max} = K_{SB} \cdot \sqrt{\frac{\rho_L - \rho_g}{\rho_g}}$$

With standard conservative derating for high pressure glycol contactors ($K_{SB} = 0.16\text{ ft/s}$):

$$v_{max} = 0.16 \cdot \sqrt{\frac{69.80 - 3.171}{3.171}} = 0.16 \cdot \sqrt{21.01} = 0.16 \times 4.584 = \mathbf{0.82\text{ ft/s}} \quad (0.250\text{ m/s})$$

D. Design Gas Superficial Velocity:

Sizing at a safe design margin of $70\%$ of flooding:

$$v_{design} = 0.70 \times v_{max} = 0.70 \times 0.82 = \mathbf{0.57\text{ ft/s}} \quad (0.175\text{ m/s})$$

E. Actual In-Situ Gas Volumetric Flow Rate:

$$Q_{actual} = 11,798.7\text{ Sm}^3/\text{h} \times \left( \frac{1.01325}{66.01} \right) \left( \frac{308.15}{288.15} \right) \left( \frac{0.9549}{1.0} \right) = \mathbf{186.2\text{ m}^3/\text{h}} = \mathbf{1.826\text{ ft}^3/\text{s}}$$

F. Required Column Cross-Sectional Area & Diameter:

$$A_{col} = \frac{Q_{actual}}{v_{design}} = \frac{1.826\text{ ft}^3/\text{s}}{0.57\text{ ft/s}} = \mathbf{3.203\text{ ft}^2} \quad (0.2975\text{ m}^2)$$

Solving for column internal diameter:

$$D_{calc} = \sqrt{\frac{4 \cdot A_{col}}{\pi}} = \sqrt{\frac{4 \times 3.203}{\pi}} = 2.019\text{ ft} = \mathbf{24.2\text{ in}} \quad (0.615\text{ m})$$

Selection of Standard Vessel Shell:

In accordance with ASME Section VIII standard pipe cap and shell diameters: * Recommended Inside Diameter: $30.0\text{ in}$ ($762\text{ mm}$ / $0.762\text{ m}$) * Operating Velocity in $30\text{ in}$ column: $0.37\text{ ft/s}$ ($45\%$ of flooding) $\to$ Exceptional safety against foaming and liquid carry-over. * Internals: Structured packing bed height = $4.5\text{ m}$ (equivalent to 8–10 bubble-cap trays), providing total column height $\approx 8.0\text{ m}$.

5. Step 4: Regeneration Skid Reboiler Thermal Duty

The reboiler thermal duty ($Q_{reb}$) supplies sensible heat to raise rich glycol from cross-heat exchanger outlet ($140^\circ\text{C}$) to boiling temperature ($204^\circ\text{C}$), latent heat to vaporize absorbed moisture, and allowance for reflux condenser and shell radiation losses:

$$Q_{reb} = Q_{sensible} + Q_{latent} + Q_{reflux} + Q_{loss}$$

graph LR
    A[Rich TEG at 140°C] --> B[Reboiler at 204°C]
    B --> C[Water Steam to Still Column]
    B --> D[Lean TEG to Surge Tank]
    E[Heating Coil: Sensible 17.5 kW + Latent 9.1 kW + Reflux 2.3 kW + Loss 2.9 kW] --> B

A. Sensible Heat ($Q_{sensible}$):

Mean specific heat capacity of TEG solution $c_p \approx 2.72\text{ kJ/(kg}\cdot^\circ\text{C)}$:

$$\begin{aligned} Q_{sensible} &= \dot{m}_{TEG} \cdot c_p \cdot (T_{reb} - T_{inlet}) \\ &= \frac{409.2\text{ kg/h}}{3,600\text{ s/h}} \times 2.72\text{ kJ/kg}\cdot\text{K} \times (204.0 - 140.0) \\ &= 0.1137\text{ kg/s} \times 2.72 \times 64.0 = \mathbf{17.5\text{ kW}} \end{aligned}$$

B. Latent Heat of Water Vaporization ($Q_{latent}$):

Latent heat of steam at $100^\circ\text{C}$ is $\lambda = 2,257\text{ kJ/kg}$:

$$Q_{latent} = \frac{14.53\text{ kg/h}}{3,600\text{ s/h}} \times 2,257\text{ kJ/kg} = 0.004036\text{ kg/s} \times 2,257 = \mathbf{9.1\text{ kW}}$$

C. Reflux Condenser Duty Allowance ($Q_{reflux}$):

To minimize glycol vaporization losses in the still column overhead, the still reflux coil requires $\approx 25\%$ of latent heat:

$$Q_{reflux} = 0.25 \times 9.1\text{ kW} = \mathbf{2.3\text{ kW}}$$

D. Shell Heat Loss ($Q_{loss}$):

Radiation and convection allowance ($10\%$ of total heat):

$$Q_{loss} = 0.10 \times (17.5 + 9.1 + 2.3) = \mathbf{2.9\text{ kW}}$$

E. Total Required Reboiler Duty:

$$Q_{reb} = 17.5 + 9.1 + 2.3 + 2.9 = \mathbf{31.8\text{ kW}}$$

Converting to field units ($1\text{ kW} = 3,412.14\text{ Btu/h}$):

$$Q_{reb} = 31.8\text{ kW} \times 3,412.14 = \mathbf{108,500\text{ Btu/h}} \quad (0.1085\text{ MMBtu/h})$$

6. Step 5: Stripping Gas Sizing for Ultra-Lean Purity

Atmospheric reboiler boiling at $204^\circ\text{C}$ can only concentrate TEG to $98.7 - 99.0\text{ wt}\%$ due to the thermodynamic equilibrium partial pressure of water vapor. To achieve $99.99\text{ wt}\%$ lean TEG (necessary for extreme dew point depressions $< -40^\circ\text{C}$), dry stripping fuel gas is bubbled through a stripping column beneath the reboiler.

Per GPSA Section 20, stripping gas demand: * Stripping Gas Ratio: $9.1\text{ scf gas / gal TEG circulated}$

$$Q_{strip} = 96.10\text{ gal/h} \times 9.1\text{ scf/gal} = 874.5\text{ SCFH} = \mathbf{14.6\text{ SCFM}}$$

Daily stripping gas consumption:

$$Q_{strip,day} = 874.5 \times 24 = \mathbf{20,988\text{ SCFD}} \quad (594.3\text{ Sm}^3/\text{day})$$

7. Equipment Datasheet & Design Summary

8. Operational Safeguards & Field Design Rules

1. Maximum Temperature Limit ($206^\circ\text{C}$):
2. TEG begins thermal decomposition into organic acids and non-condensables at temperatures above $206.7^\circ\text{C}$ ($404^\circ\text{F}$). The reboiler temperature controller must be hard-interlocked to shut down fuel gas at $205.0^\circ\text{C}$.
3. Hydrocarbon Liquid Carry-In:
4. Liquid hydrocarbons (condensate) entering the contactor dissolve into TEG, resulting in violent foaming in the contactor and flashing in the reboiler. A high-efficiency filter-coalescer separator is mandatory upstream of the contactor.
5. Corrosion and pH Monitoring:
6. Glycol degradation products are acidic. Regular sampling is required to maintain system pH between $7.0$ and $8.5$ using neutralizing amine buffers (such as triethanolamine).