1. Economizer Overview
An economizer in a cryogenic NGL recovery plant is a heat exchanger that recovers cold energy from the process effluent streams to pre-cool the incoming feed gas. This internal heat recovery reduces the external refrigeration or expansion work required to achieve the target cold temperatures, dramatically improving the energy efficiency and economics of the plant.
Why Economizers Are Essential
Without economizers, all the cold energy in the residue gas and NGL product streams would be wasted as these streams are warmed to pipeline or storage temperatures. In a typical cryogenic plant processing 200 MMscfd, the economizer system recovers the equivalent of 2,000–5,000 HP of refrigeration, which would otherwise need to be supplied by external refrigeration compressors or additional turboexpander capacity. The economizer is therefore the single most impactful piece of equipment for plant energy efficiency.
Role in the Process
| Function | Description | Benefit |
|---|---|---|
| Feed gas pre-cooling | Cools incoming feed gas against cold residue gas | Reduces external refrigeration requirement |
| Residue gas warming | Warms cold residue gas to pipeline temperature | Meets pipeline delivery temperature specification |
| NGL product cooling | Sub-cools NGL product for storage stability | Reduces flash losses in downstream storage |
| Reflux generation | Provides cold reflux to demethanizer column | Improves C2+ or C3+ recovery |
Types of Economizer Heat Exchangers
| Type | Application | Temperature Range |
|---|---|---|
| Brazed aluminum (BAHX) | Primary gas-gas and gas-liquid exchangers in cold box | −269°C to +65°C (−452°F to +150°F) |
| Printed circuit (PCHE) | High-pressure gas-gas service; compact installations | −200°C to +900°C (−328°F to +1650°F) |
| Shell and tube | Warm end exchangers; kettle reboilers for demethanizer | −100°C to +300°C (−148°F to +572°F) |
| Plate-fin (wound coil) | Large LNG and deep NGL recovery plants | −269°C to +65°C |
2. Thermodynamic Basis
The economizer operates on the fundamental principle that heat flows from hot to cold. The feed gas (warm) transfers heat to the process effluent streams (cold), simultaneously cooling the feed and warming the effluent. The exchanger is constrained by the second law of thermodynamics: the cold stream outlet temperature can never reach the hot stream inlet temperature.
Energy Balance
Q = mhot × Cphot × (Thot,in − Thot,out)
Q = mcold × Cpcold × (Tcold,out − Tcold,in)
Where Q = heat duty (BTU/hr), m = mass flow rate (lb/hr), Cp = heat capacity (BTU/lb·°F)
Effectiveness
ε = Qactual / Qmax
Qmax = (mCp)min × (Thot,in − Tcold,in)
Typical economizer effectiveness: 0.85–0.95
Phase Change Effects
In cryogenic NGL processing, the feed gas undergoes partial condensation as it cools through the economizer. This phase change releases latent heat, which significantly increases the total heat duty compared to a purely sensible heat exchange. The heat capacity of the condensing feed gas stream is not constant and must be evaluated using process simulation (HYSYS, ProMax) at multiple temperature intervals to accurately size the exchanger. Simple LMTD calculations assuming constant Cp will significantly undersize the economizer.
Temperature-Heat Duty Diagram
The composite heating and cooling curves plotted on a temperature vs. cumulative heat duty diagram reveal the minimum temperature approach (pinch point) and the thermodynamic feasibility of the heat exchange. The pinch point constrains the maximum heat recovery achievable.
| Diagram Feature | Significance |
|---|---|
| Pinch point | Location of minimum approach temperature; limits heat recovery |
| Curve crossing | Indicates thermodynamic infeasibility; design must be modified |
| Wide approach (warm end) | Heat available but not utilized; opportunity for additional recovery |
| Phase change plateau | Flat region on cooling curve where condensation occurs |
3. Feed-Effluent Heat Exchange
The primary economizer function is feed-effluent exchange, where the cold residue gas leaving the demethanizer column (or cold separator) cools the incoming treated feed gas. This is the largest single heat exchange service in the plant.
Typical Stream Conditions
| Stream | Inlet Temp (°F) | Outlet Temp (°F) | Pressure (psig) |
|---|---|---|---|
| Feed gas (hot side) | 80 to 100 | −30 to −80 | 800–1,100 |
| Residue gas (cold side) | −100 to −150 | 50 to 80 | 250–450 |
| NGL product (cold side) | −50 to −100 | 60 to 90 | 300–600 |
CO2 Freeze-Out Risk
Carbon dioxide in the feed gas can freeze (solidify) at temperatures below approximately −109°F (−78.5°C) at typical processing pressures. CO2 freeze-out in economizer passages causes blockage, reduced heat transfer, flow maldistribution, and potential mechanical damage to brazed aluminum exchangers. Feed gas CO2 content must be reduced to 1–2 mol% (or lower for deep ethane recovery) before entering the cryogenic section. Process simulation must verify that no point in the economizer reaches the CO2 solid formation temperature at the local pressure and composition.
Multi-Stream Exchangers
Brazed aluminum heat exchangers (BAHX) can accommodate multiple streams simultaneously in separate passages within a single core. A typical cryogenic economizer may have 3–6 streams exchanging heat within one cold box assembly.
| Stream | Direction | Phase |
|---|---|---|
| Inlet feed gas | Hot → Cold | Gas, then partial condensation |
| Residue gas from demethanizer | Cold → Hot | Gas |
| NGL product from demethanizer | Cold → Hot | Liquid |
| Demethanizer reflux | Hot → Cold | Liquid sub-cooling |
| Side reboiler duty | Hot → Cold | Boiling liquid return to column |
4. Joule-Thomson Valve Positioning
The Joule-Thomson (JT) valve creates the cold temperatures needed for NGL condensation by expanding high-pressure gas through an isenthalpic throttling process. The position of the JT valve relative to the economizer determines the process configuration and the NGL recovery efficiency.
JT Before Economizer
| Aspect | Description |
|---|---|
| Process | Feed gas expands through JT valve first, then enters economizer cold side |
| Temperature | Maximum cooling achieved immediately; cold two-phase flow enters exchanger |
| Advantage | Simple configuration; good for moderate NGL recovery |
| Disadvantage | Two-phase flow in exchanger reduces heat transfer; poor distribution |
JT After Economizer (Pre-Cooled Expansion)
| Aspect | Description |
|---|---|
| Process | Feed gas pre-cooled in economizer, then expanded through JT valve |
| Temperature | Pre-cooling shifts JT expansion to lower starting temperature |
| Advantage | Deeper cooling; higher NGL recovery; better exchanger performance |
| Disadvantage | Two exchangers or two zones needed; more complex piping |
Turboexpander vs. JT Valve
Modern cryogenic NGL recovery plants predominantly use turboexpanders rather than JT valves for the primary expansion. Turboexpanders achieve isentropic (rather than isenthalpic) expansion, producing colder outlet temperatures for the same pressure ratio and recovering useful work to drive a compressor. A turboexpander plant typically achieves 10–15% higher ethane recovery than an equivalent JT plant at the same inlet conditions. The economizer design principles remain the same regardless of expansion device type, but the stream temperatures and duties differ.
5. Cold Box Design
The cold box is an insulated enclosure containing the brazed aluminum heat exchangers and associated cryogenic piping. The cold box protects the exchangers from ambient heat gain and moisture ingress, which could cause ice formation and blockage.
Cold Box Components
| Component | Purpose | Design Notes |
|---|---|---|
| BAHX cores | Multi-stream heat exchange | Aluminum alloy 3003; brazed construction; rated to −452°F |
| Insulation | Minimize heat leak from ambient | Perlite fill (expanded volcanic glass); inert gas purge |
| Steel enclosure | Contain insulation; support exchangers | Carbon steel shell; weather-tight; access panels |
| Nozzle transitions | Connect aluminum exchanger to steel piping | Bi-metallic (aluminum-to-stainless) transition joints |
| Nitrogen purge | Keep insulation dry; inert atmosphere | Continuous or intermittent N2 supply; moisture monitoring |
Mercury in Feed Gas
Trace mercury in natural gas is extremely damaging to brazed aluminum heat exchangers. Mercury amalgamates with aluminum, causing liquid metal embrittlement and catastrophic structural failure. Mercury removal to below 0.01 micrograms per normal cubic meter is required upstream of the cold box. Mercury removal units (MRU) using sulfur-impregnated activated carbon are standard upstream of all cryogenic plants processing mercury-containing gas.
6. Approach Temperature Selection
The approach temperature (the minimum temperature difference between the hot and cold streams in the exchanger) is a critical design parameter that balances capital cost (exchanger size) against operating cost (energy efficiency).
Approach Temperature Guidelines
| Service | Typical Approach (°F) | Typical Approach (°C) | Notes |
|---|---|---|---|
| Gas-gas (cryogenic cold end) | 3–10 | 2–6 | Tight approach justified by high cold energy value |
| Gas-gas (warm end) | 10–25 | 6–14 | Wider approach acceptable; lower temperature driving force value |
| Gas-boiling liquid (reboiler) | 5–15 | 3–8 | High heat transfer coefficient on boiling side |
| Condensing-gas | 5–15 | 3–8 | Phase change enhances heat transfer |
Economic Optimization
Reducing the approach temperature increases the exchanger surface area (and cost) but reduces the external energy requirement. The optimal approach temperature minimizes the sum of annualized capital cost and annual operating cost. At current compression costs, the economic optimum for cryogenic gas-gas exchangers is typically 3–8°F. Below 3°F, the exchanger cost increases exponentially with diminishing energy savings. Above 10°F, the increased compression cost dominates.
7. Process Configurations
The economizer configuration varies depending on the NGL recovery target (ethane rejection vs. recovery), plant capacity, inlet gas composition, and the expansion device (JT valve or turboexpander).
Common Cryogenic Plant Configurations
| Configuration | C2 Recovery | C3 Recovery | Key Economizer Feature |
|---|---|---|---|
| Simple JT | 15–30% | 60–75% | Single feed-residue exchanger |
| JT with external refrigeration | 30–50% | 80–90% | Two-stage cooling: propane chiller + JT |
| Turboexpander (GSP) | 80–90% | 98%+ | Multi-stream cold box with side reboilers |
| Enhanced turboexpander (RSV, OHR) | 90–97% | 99%+ | Subcooled reflux generation; additional cold recovery |
| Ethane rejection mode | 1–5% | 95–99% | Warmer cold box temperatures; modified stream routing |
GSP (Gas Subcooled Process) Economizer
The GSP process, widely used for ethane recovery, uses the economizer to generate subcooled liquid reflux for the demethanizer column. A portion of the feed gas is condensed and subcooled in the cold box, then flashed to low pressure as reflux to the top of the demethanizer. The cold box must handle three or more streams: feed gas cooling, residue gas warming, and reflux sub-cooling.
Dual-Mode Operation
Many modern plants are designed to switch between ethane recovery and ethane rejection modes in response to NGL market prices. The economizer must accommodate both operating modes with different flow distributions, temperatures, and duties. This flexibility requires careful design of the cold box with appropriate stream bypasses and control valves to manage the transition between modes without shutting down the plant.
8. Operational Considerations
Economizer performance directly impacts plant NGL recovery, residue gas delivery temperature, and overall energy efficiency. Monitoring and maintaining economizer performance is essential for optimal plant operations.
Performance Monitoring
| Parameter | Monitoring Method | Warning Sign |
|---|---|---|
| Approach temperature | Inlet/outlet temperature measurements | Increasing approach indicates fouling or flow maldistribution |
| Pressure drop | Differential pressure transmitters | Rising ΔP suggests fouling, hydrate, or CO2 freeze |
| UA product | Calculated from duty and LMTD | Declining UA indicates reduced heat transfer effectiveness |
| Cold box insulation | External surface temperature survey | Hot spots indicate insulation settling or moisture ingress |
Common Operational Issues
| Issue | Cause | Mitigation |
|---|---|---|
| CO2 freeze-out | Excessive CO2 in feed; operation below freeze temperature | Upstream amine treating; temperature monitoring; operating limits |
| Hydrate formation | Water in feed gas above hydrate formation conditions | Upstream dehydration to <1 lb/MMscf; glycol injection |
| Mercury damage | Trace mercury amalgamation with aluminum | Mercury removal unit upstream; periodic monitoring |
| Thermal stress | Rapid temperature changes during startup/shutdown | Controlled warm-up/cool-down rates per manufacturer limits |
| Flow maldistribution | Fouling in individual passages; two-phase flow | Proper header design; periodic cleaning; inlet separation |
Startup and Shutdown Procedures
Brazed aluminum exchangers are sensitive to thermal shock. Temperature change rates must be limited to the manufacturer's specified maximum (typically 1–3°F per minute) during startup, shutdown, and mode transitions. Exceeding these rates can cause differential thermal expansion between the aluminum fins and the parting sheets, leading to fin debonding, internal leaks, and potential catastrophic failure. Automated startup sequences with temperature-controlled ramp rates are standard practice.