Worked Example: Gas Scrubber Debottlenecking & Internals Rating: Wire Mesh vs High-Capacity Axial Cyclones

1. Problem Statement & Vessel Geometry

An offshore gas production platform is planning an infill drilling campaign that will increase natural gas production from the current $55\text{ MMSCFD}$ up to $115\text{ MMSCFD}$.

The main high-pressure suction scrubber (V-101) protecting the export gas compressors is currently equipped with a standard knitted wire mesh demister pad. Replacing the entire pressure vessel is prohibited due to offshore crane load limits, deck space constraints, and high vessel fabrication lead times ($>14\text{ months}$).

Vessel & Operating Data

• Vessel Type: Vertical Gas Scrubber (Knockout Drum)
• Inside Shell Diameter ($D_i$): $1,800\text{ mm}$ ($1.80\text{ m} \approx 70.87\text{ inches}$)
• Operating Pressure ($P$): $60.0\text{ barg}$ ($61.01\text{ bar abs} \approx 885\text{ psia}$)
• Operating Temperature ($T$): $38.0\text{ }^\circ\text{C}$ ($311.15\text{ K}$)
• Gas Density ($\rho_g$): $48.50\text{ kg/m}^3$ (evaluated via PR-EOS at $60\text{ barg}$, $38\text{ }^\circ\text{C}$, $SG = 0.65$)
• Condensate Liquid Density ($\rho_L$): $740.0\text{ kg/m}^3$
• Liquid Entrainment Loading: $0.08\text{ m}^3\text{ liquid per } 100,000\text{ Sm}^3\text{ gas}$
• Existing Internals: Standard 304SS Knitted Wire Mesh Pad ($150\text{ mm}$ thick, $K = 0.107\text{ m/s}$)

Engineering Objectives

1. Determine the maximum gas capacity ($Q_{max}$) of the existing wire mesh pad and evaluate whether it can handle the expanded $115\text{ MMSCFD}$ rate.
2. Evaluate the Gas $F$-factor and assess liquid re-entrainment and flooding risk.
3. Size a retrofit with an Axial Cyclone Deck ($K = 0.280\text{ m/s}$) and calculate the debottlenecking capacity multiplier.
4. Verify downcomer liquid drainage capacity to ensure internal drainage does not back up onto the mist eliminator face.

2. Step-by-Step Engineering Calculations

Step 1: Vessel Cross-Sectional Area

The internal cross-sectional area of the $1,800\text{ mm}$ vessel is: $$A_{vessel} = \frac{\pi \cdot D_i^2}{4} = \frac{\pi \cdot (1.80\text{ m})^2}{4} = \mathbf{2.5447\text{ m}^2}$$

Step 2: Souders-Brown Maximum Velocity for Wire Mesh Pad

The Souders-Brown terminal gas velocity is: $$v_{max} = K \cdot \sqrt{\frac{\rho_L - \rho_g}{\rho_g}}$$

For a standard knitted wire mesh pad, $K_{mesh} = 0.107\text{ m/s}$ ($0.35\text{ ft/s}$). Applying a high-pressure derating factor ($C_{derating} \approx 0.90$ for $P > 40\text{ bar}$ per GPSA Section 7): $$K_{mesh,derated} = 0.107 \times 0.90 = 0.0963\text{ m/s}$$

Evaluating the density ratio: $$\sqrt{\frac{\rho_L - \rho_g}{\rho_g}} = \sqrt{\frac{740.0 - 48.50}{48.50}} = \sqrt{\frac{691.50}{48.50}} = \sqrt{14.258} = 3.776$$

The maximum allowable gas velocity across the wire mesh pad is: $$v_{max,mesh} = 0.0963\text{ m/s} \times 3.776 = \mathbf{0.3636\text{ m/s}}$$

The maximum allowable actual volumetric flow rate: $$Q_{act,max,mesh} = v_{max,mesh} \cdot A_{vessel} = 0.3636\text{ m/s} \times 2.5447\text{ m}^2 = 0.9253\text{ m}^3/\text{s} = 3,331\text{ m}^3/\text{h}$$

Converting to standard gas flow rate ($Sm^3/h$ and $MMSCFD$): Gas expansion ratio from actual to standard conditions: $$\frac{\rho_g}{\rho_{sc}} = \frac{48.50\text{ kg/m}^3}{0.795\text{ kg/Sm}^3} = 61.0$$ $$Q_{std,max,mesh} = 3,331\text{ m}^3/\text{h} \times 61.0 = 203,190\text{ Sm}^3/\text{h} = \mathbf{59.4\text{ MMSCFD}}$$

The existing mesh pad is rated for a maximum of $59.4\text{ MMSCFD}$.

Step 3: Performance Assessment at Expanded 115 MMSCFD Rate

At the target expanded throughput of $Q = 115.0\text{ MMSCFD}$ ($393,300\text{ Sm}^3/\text{h}$): $$\dot{m}_g = 393,300\text{ Sm}^3/\text{h} \times 0.795\text{ kg/Sm}^3 = 312,670\text{ kg/h} = 86.85\text{ kg/s}$$

Actual volumetric gas flow rate through the vessel: $$Q_{act} = \frac{\dot{m}_g}{\rho_g} = \frac{86.85\text{ kg/s}}{48.50\text{ kg/m}^3} = \mathbf{1.7908\text{ m}^3/\text{s}} \quad (6,447\text{ m}^3/\text{h})$$

Actual superficial gas velocity ($v_g$): $$v_g = \frac{Q_{act}}{A_{vessel}} = \frac{1.7908\text{ m}^3/\text{s}}{2.5447\text{ m}^2} = \mathbf{0.7037\text{ m/s}}$$

Velocity Utilization vs Mesh Pad: $$\text{Utilization} = \frac{v_g}{v_{max,mesh}} = \frac{0.7037\text{ m/s}}{0.3636\text{ m/s}} \times 100\% = \mathbf{193.5\%} \quad (\approx 94\% \text{ OVER CAPACITY!})$$

Evaluating the Gas $F$-factor: $$F = v_g \cdot \sqrt{\rho_g} = 0.7037\text{ m/s} \times \sqrt{48.50\text{ kg/m}^3} = 0.7037 \times 6.964 = \mathbf{4.90\text{ Pa}^{0.5}}$$

• Catastrophic Failure Diagnosis: For wire mesh demister pads, the absolute flooding and re-entrainment limit occurs at $F = 2.50\text{ Pa}^{0.5}$. At $F = 4.90\text{ Pa}^{0.5}$, the gas velocity is twice the flooding threshold.
• The upward aerodynamic shear force will prevent any liquid from draining. Liquid will rapidly bridge across the knitted wires, forming a liquid head that violently re-atomizes into large droplets, causing catastrophic liquid carryover into the compressor suction!

Step 4: Rating the Axial Cyclone Deck Retrofit

To achieve $115\text{ MMSCFD}$ within the existing $1,800\text{ mm}$ shell, an Axial Cyclone Deck is selected. Axial cyclones use fixed swirl vanes to generate centrifugal accelerations exceeding $1,000\text{ G}$, flinging droplets against cyclone tube walls where liquid drains into an isolated lower collection chamber.

• Design $K$-Factor: $K_{cyclone} = 0.280\text{ m/s}$ ($0.92\text{ ft/s}$).
• Applying high-pressure derating ($0.92$ for $60\text{ bar}$): $$K_{cyclone,derated} = 0.280 \times 0.92 = 0.2576\text{ m/s}$$

Calculating allowable gas velocity for the cyclone deck: $$v_{max,cyclone} = K_{cyclone,derated} \cdot \sqrt{\frac{\rho_L - \rho_g}{\rho_g}} = 0.2576\text{ m/s} \times 3.776 = \mathbf{0.9727\text{ m/s}}$$

Allowable volumetric flow rate: $$Q_{act,max,cyclone} = v_{max,cyclone} \cdot A_{vessel} = 0.9727\text{ m/s} \times 2.5447\text{ m}^2 = 2.4752\text{ m}^3/\text{s} \quad (8,910\text{ m}^3/\text{h})$$

Allowable standard gas capacity: $$Q_{std,max,cyclone} = 8,910\text{ m}^3/\text{h} \times 61.0 = 543,500\text{ Sm}^3/\text{h} = \mathbf{159.0\text{ MMSCFD}}$$

Evaluating the Debottlenecking Capacity Multiplier: $$\text{Capacity Multiplier} = \frac{Q_{max,cyclone}}{Q_{max,mesh}} = \frac{159.0\text{ MMSCFD}}{59.4\text{ MMSCFD}} = \mathbf{2.68\times} \quad (+168\% \text{ Capacity Gain})$$

At the required $115\text{ MMSCFD}$ operating rate: $$\text{Cyclone Utilization} = \frac{115.0\text{ MMSCFD}}{159.0\text{ MMSCFD}} \times 100\% = \mathbf{72.3\%}$$ The cyclone deck operates comfortably within its high-efficiency sweet spot ($70\%\text{--}80\%$ loading), guaranteeing $>99.9\%$ droplet removal down to $5\text{ }\mu\text{m}$.

Step 5: Liquid Downcomer Drainage Verification

At $115\text{ MMSCFD}$ with liquid loading of $0.08\text{ m}^3 / 100,000\text{ Sm}^3$: $$\dot{V}_L = 393,300\text{ Sm}^3/\text{h} \times \frac{0.08\text{ m}^3}{100,000\text{ Sm}^3} = 0.315\text{ m}^3/\text{h} = 8.75 \times 10^{-5}\text{ m}^3/\text{s}$$

For cyclone deck liquid downcomers, maximum liquid drainage velocity must be $\le 0.50\text{ m/s}$ to prevent gas pull-through. Required downcomer cross-sectional area: $$A_{downcomer} \ge \frac{8.75 \times 10^{-5}\text{ m}^3/\text{s}}{0.50\text{ m/s}} = 1.75 \times 10^{-4}\text{ m}^2 \quad \implies \quad D_{pipe} \ge 15\text{ mm} \quad (0.6\text{ in})$$

Installing standard $2\text{ inch}$ ($50\text{ mm}$) Sch 80 downcomer pipes with a sealed submerged dipleg provides an immense safety factor, ensuring drainage is completely unchoked.

3. Debottlenecking Summary & Comparison Table