1. Cyclosep Overview
Cyclonic separators, often referred to as Cyclosep or inline separators, use centrifugal force rather than gravity to separate phases. By imparting a spinning motion to the incoming multiphase fluid, denser liquid droplets and solid particles are driven outward to the vessel wall while lighter gas exits through a central vortex finder. This principle allows separation in much smaller vessels than conventional gravity-based designs.
Compact size
Reduced weight and footprint
Centrifugal forces 500–3,000 times gravity allow vessel diameters 50–80% smaller than gravity separators for the same throughput.
Fast response
Short residence time
Separation occurs in seconds rather than minutes, making cyclonic units ideal for slug handling and rapid process changes.
No moving parts
High reliability
Static swirl elements generate centrifugal force with no rotating equipment, reducing maintenance and increasing uptime.
How Cyclonic Separation Works
A cyclonic separator works by converting the linear momentum of the inlet stream into rotational motion. The key stages of separation are:
- Swirl generation: The multiphase fluid enters through tangential inlets or passes through stationary swirl vanes (axial cyclones) that impart a high-velocity spin.
- Centrifugal separation: The spinning flow creates a strong centrifugal force field. Denser phases (liquids, solids) migrate radially outward to the wall while lighter gas migrates inward toward the axis.
- Liquid film drainage: Separated liquid forms a film on the inner wall and drains downward by gravity into a liquid collection section or sump.
- Gas exit: Clean gas exits through a central vortex finder tube, which prevents re-entrainment of separated liquid.
- Liquid exit: Collected liquid exits through a bottom drain or is routed to a downstream liquid handling vessel.
Cyclonic vs. Gravity Separation
| Feature | Cyclonic Separator | Gravity Separator |
|---|---|---|
| Separation force | 500–3,000 × g | 1 × g |
| Vessel diameter (same duty) | Much smaller | Larger |
| Residence time | 1–5 seconds | 1–5 minutes |
| Liquid surge capacity | Minimal | Significant |
| Droplet removal | > 10–15 μm | > 100–300 μm |
| Pressure drop | 1–5 psi | 0.5–1 psi |
| Weight | 30–50% of gravity unit | Baseline |
| Cost | Lower capital, lower installation | Higher capital for large vessels |
| Best application | Bulk separation, scrubbing, debottlenecking | Primary separation with liquid storage |
2. Separation Principles
The physics of cyclonic separation is governed by the balance between centrifugal force driving droplets outward and drag force resisting their radial motion. Understanding these fundamentals is essential for proper sizing and performance prediction.
Centrifugal Force
Droplet Separation Efficiency
A droplet is separated when the centrifugal force exceeds the gas drag force. The critical (cut) droplet diameter defines the smallest droplet that can be separated with a given efficiency:
Factors Affecting Separation Efficiency
| Factor | Effect on Efficiency | Design Implication |
|---|---|---|
| Tangential velocity | Higher velocity = better separation | Optimal range exists; too high causes re-entrainment |
| Cyclone diameter | Smaller diameter = higher G-force | Use multiple small cyclones in parallel for high flow |
| Gas density | Higher pressure = higher drag | Performance decreases at very high pressures |
| Liquid loading | High liquid = wall film thickens | Excessive liquid causes re-entrainment from film |
| Droplet size distribution | Larger droplets = easier separation | Inlet conditioning can improve droplet size |
| Swirl vane angle | Steeper angle = more spin | Balanced against pressure drop penalty |
Pressure Drop
Cyclonic separators inherently have higher pressure drop than gravity vessels because energy is required to spin the fluid. The pressure drop depends on the swirl intensity and gas velocity:
3. Design Configurations
Cyclonic separators come in several configurations, each suited to different applications, flow rates, and space constraints. The choice depends on the required separation performance, allowable pressure drop, and physical installation requirements.
Axial Flow Cyclone (Inline)
The most common configuration for midstream applications. Flow enters one end and exits the other, with swirl vanes mounted inside a cylindrical body. This design installs directly in the pipeline.
- Swirl element: Stationary vanes at the inlet impart spin to the gas-liquid mixture
- Separation zone: Liquid migrates to the wall in the cylindrical body section
- Vortex finder: Central tube at the gas outlet prevents liquid re-entrainment
- Liquid drain: Annular slot or drain holes at the wall collect separated liquid
- Best for: Compressor inlet scrubbing, pipeline pigging receivers, wellhead separation
Tangential Inlet Cyclone
Similar to traditional cyclone separators used in solids removal. The inlet enters tangentially to the cylindrical body, creating natural spin without internal vanes.
- Higher G-forces: Can achieve higher centrifugal forces than axial designs
- Longer body: Requires more vertical height for effective separation
- Conical section: Tapered bottom section accelerates the vortex for finer separation
- Best for: Sand and solids removal, high-efficiency liquid knockout
Multicyclone Bundle
Multiple small-diameter cyclone tubes arranged in parallel inside a pressure vessel. Each tube handles a fraction of the total flow, and the small diameter produces very high G-forces.
- High efficiency: Small tube diameters (2–6 inches) produce G-forces of 1,000–3,000 g
- Scalable: Add or remove tubes to match flow rate requirements
- Pressure vessel: Tubes are contained within a standard ASME vessel
- Best for: Gas scrubbing where very low liquid carryover is required
Configuration Comparison
| Configuration | G-Force | ΔP (psi) | Cut Size (μm) | Liquid Handling |
|---|---|---|---|---|
| Single axial cyclone | 200–1,000 | 1–3 | 10–20 | Moderate |
| Tangential inlet | 500–2,000 | 2–5 | 8–15 | Good |
| Multicyclone bundle | 1,000–3,000 | 3–8 | 3–10 | Limited per tube |
| Hybrid (cyclone + gravity) | 200–1,000 | 1–3 | 10–20 | Excellent |
Hybrid Cyclone-Gravity Systems
Many modern separator designs combine cyclonic inlet devices with a conventional gravity separation vessel. The cyclone performs bulk liquid removal at the inlet, reducing the required gravity settling area and improving overall separation performance.
4. Sizing Methodology
Sizing a cyclonic separator involves determining the required swirl intensity, body dimensions, and liquid handling capacity. The methodology differs from gravity separator sizing because the driving force is centrifugal acceleration rather than gravitational settling.
Step 1: Define Process Conditions
Step 2: Determine Cyclone Diameter
The cyclone diameter is selected to achieve the target tangential velocity and G-force. Smaller diameters produce higher G-forces but handle less flow per unit.
Step 3: Determine Body Length
The cyclone body length must provide sufficient residence time for the droplets to travel radially from the center to the wall:
Step 4: Liquid Handling
The liquid drainage system must handle both steady-state liquid load and slug volumes. Key considerations:
- Liquid film thickness: Must be thin enough to avoid re-entrainment (typically < 0.1 inch)
- Drain slot sizing: Annular slots or drain holes must pass the liquid without restricting flow
- Liquid sump: A small collection vessel below the cyclone provides buffer volume
- Level control: Required to prevent liquid from backing up into the cyclone body
Step 5: Number of Parallel Elements
For multicyclone bundles, the number of parallel elements is determined by the total flow rate and the capacity of each individual cyclone tube:
5. Applications
Cyclonic separators are used across the midstream and upstream sectors wherever compact, high-efficiency separation is needed. The following are the most common applications in pipeline and gas processing operations.
Compressor Inlet Scrubbing
Protecting compressors from liquid carryover is the most common midstream application for cyclonic separators. Liquid droplets in the compressor suction gas cause valve damage, cylinder erosion, and efficiency loss.
- Inline cyclone scrubbers reduce footprint vs. conventional vertical scrubbers
- Target liquid carryover: < 0.1 gal/MMSCF for reciprocating compressors
- Must handle liquid slugs from upstream pigging or process upsets
- Often combined with a small downstream knockout drum for slug handling
Subsea Separation
Subsea cyclonic separators are used for seabed gas-liquid separation in deepwater production systems. The compact size and lack of moving parts make them ideal for subsea deployment.
- Subsea boosting and pumping systems require separation before liquid pumps
- Compact inline cyclones fit within subsea manifold structures
- Reliability is critical since intervention is extremely expensive
- Design must account for hydrate formation and wax deposition
Offshore Platform Debottlenecking
Cyclonic separators are frequently used to debottleneck existing offshore platforms where adding conventional vessels is impractical due to weight and space limitations.
Pipeline Liquid Removal
Inline cyclonic separators installed at pipeline receipt points or meter stations remove condensed liquids before custody transfer metering or downstream processing.
Application Selection Guide
| Application | Preferred Configuration | Key Design Criteria |
|---|---|---|
| Compressor inlet scrubber | Axial inline | Low ΔP, slug handling, < 0.1 gal/MMSCF |
| Subsea separation | Axial inline or tangential | Reliability, compact, hydrate resistance |
| Platform debottleneck | Multicyclone bundle | High efficiency, fit within existing vessel |
| Pipeline receipt point | Axial inline | Low ΔP, moderate liquid handling |
| Sand removal | Tangential inlet (desander) | Solids handling, erosion resistance |
| Fuel gas conditioning | Multicyclone or inline | Very low carryover for gas turbines/engines |
| Wellhead separation | Tangential or axial | High GOR, slug tolerance, sand handling |
6. Worked Example
Size an axial inline cyclonic separator for a compressor inlet scrubber application.
Step 1: Calculate Actual Gas Flow
Step 2: Select Cyclone Diameter
Step 3: Verify G-Force
Step 4: Estimate Body Length
Step 5: Liquid Handling
Step 6: Pressure Drop Estimate
Summary
| Parameter | Value |
|---|---|
| Cyclone body diameter | 14 inches |
| Body length | ~6 ft (10 ft overall) |
| G-force | ~130 g (single body); 465 g with 4-inch tubes |
| Pressure drop | ~0.4 psi |
| Liquid sump volume | ~40 gallons |
| Weight (estimated) | ~2,000 lb (vs. ~6,000 lb for conventional scrubber) |
7. Operations & Troubleshooting
Performance Monitoring
Cyclonic separators require regular monitoring to ensure separation efficiency is maintained:
- Differential pressure: Monitor ΔP across the cyclone. Increasing ΔP may indicate fouling, scale buildup, or liquid overload.
- Gas outlet liquid content: Sample or use inline analyzers to verify liquid carryover remains below specification.
- Liquid drain rate: Verify liquid is draining properly. Slugging or erratic drain flow may indicate level control issues.
- Pressure drop trending: Plot ΔP vs. flow rate over time. Deviation from the baseline curve indicates internal changes.
Common Problems and Solutions
| Problem | Likely Cause | Solution |
|---|---|---|
| High liquid carryover | Flow rate above design, liquid overload | Reduce flow, check upstream liquid sources, add parallel cyclone |
| Increasing ΔP | Fouling, scale, wax deposition | Clean internals, add inhibitor injection upstream |
| Poor turndown performance | Low velocity reduces centrifugal force | Consider variable geometry or partial bypass |
| Liquid re-entrainment | Liquid film too thick, drain blockage | Clear drains, increase drain capacity, reduce liquid load |
| Erosion damage | Sand or solids in process stream | Install upstream desander, use erosion-resistant materials |
| Vibration | Flow instability, slug flow | Install flow conditioning upstream, add slug catcher |
Maintenance Considerations
- Inspection interval: Internal inspection every 2–5 years depending on service severity and erosion/corrosion rates
- Swirl vane condition: Inspect for erosion, corrosion, and fouling. Replace if vane geometry has degraded significantly.
- Vortex finder: Check for erosion at the leading edge and seal integrity. A damaged vortex finder dramatically increases liquid carryover.
- Drain system: Verify drain slots and piping are clear. Blockage causes rapid performance degradation.
- Wall thickness: Measure wall thickness at areas of high velocity and potential erosion, particularly opposite the inlet and at the cone section.
Operational Limits
| Parameter | Normal Range | Action Level |
|---|---|---|
| Flow rate (% of design) | 50–110% | < 40% or > 120%: evaluate performance |
| Pressure drop | Within ±20% of baseline | > 50% increase: inspect internals |
| Liquid carryover | < 0.1 gal/MMSCF | > 0.5 gal/MMSCF: investigate and correct |
| Liquid level in sump | 25–75% of range | High-high level: trip or alarm |
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