Vacuum Distillation Engineering for Used Lube Oil Re-Refining: High-Vacuum Wiped Film Evaporation, Fractionation & Asphalt Extraction

1. Executive Summary & Process Overview

Used Lubricating Oil (ULO)—collected from automotive crankcases, industrial turbines, hydraulic systems, and transformers—represents a complex hazardous waste stream rich in high-value paraffinic and naphthenic hydrocarbon base stocks. However, virgin lube base stocks in ULO are heavily degraded by operational oxidation, thermal stress, shear breakdown, and contamination.

A typical raw ULO feed matrix contains:

• Water & Emulsions: $3.0 - 8.0 \text{ wt}%$ (free, dissolved, and chemically emulsified water)
• Light Fuel Hydrocarbons: $3.0 - 7.0 \text{ wt}%$ (gasoline, light diesel, cracking fractions, flash-point depressing solvents)
• Degraded Lube Base Stock: $65.0 - 80.0 \text{ wt}%$ (paraffinic, naphthenic, and aromatic hydrocarbons ranging from C_{15} to C_{50})
• Additives & Degradation Byproducts: $10.0 - 18.0 \text{ wt}%$ (viscosity index improvers, dispersants, detergents, zinc dialkyldithiophosphate [ZnDTP], oxidized polymers, polycyclic aromatic hydrocarbons [PAH])
• Particulates & Heavy Metals: Heavy metals (Pb, Cr, Cd, Fe, Cu), soot, carbonaceous sludge, and inorganic ash ($0.5 - 2.5 \text{ wt}%$)

+---------------------------------------------------------------------------------------------------------+
|                                    USED LUBE OIL REREFINING BLOCK FLOW                                  |
+---------------------------------------------------------------------------------------------------------+
                                                     
     Raw ULO Feed 
          │
          ▼
+-------------------+      Water / Light Ends
| 1. Pre-treatment  | ──────────────────────────────► To Wastewater Treatment & Condenser
|  & De-watering    |   (Atmospheric / 100 mbar)
+-------------------+
          │
          ▼  Dry De-watered Oil
+-------------------+      Light Gas Oil (LGO)
| 2. Flash Fuel     | ──────────────────────────────► Flash Point Restoration Fuel Cut
|    Stripping      |   (20 - 50 mbar, 220°C)
+-------------------+
          │
          ▼  Stripped Heavy Feed
+-------------------+
| 3. High Vacuum    |      Asphaltic Residue (Bitumen Modifier)
|   Wiped Film      | ──────────────────────────────► Heavy Bottoms Flux (Residue Sludge)
|   Evaporator (WFE)|   (0.1 - 0.5 mbar, 320°C)
+-------------------+
          │
          ▼  Distilled Lube Vapors (Asphalt-Free)
+-------------------+
| 4. High Vacuum    | ───► Light Base Oil Cut (SN 150 Equivalent)
|   Fractionation   | ───► Medium Base Oil Cut (SN 300 Equivalent)
|     Column        | ───► Heavy Base Oil Cut (SN 500 Equivalent)
+-------------------+   (1 - 3 mbar, Packed Bed Column)
          │
          ▼  Base Oil Cuts
+-------------------+
| 5. Finishing      | ───► Finished Group I / Group II Base Oils
|  (Hydrotreating / |   (ASTM D1500 Color < 1.0, TAN < 0.03 mg KOH/g)
| Solv. Polishing)  |
+-------------------+

The core objective of modern re-refining process plant design is the maximum volumetric recovery of API Group I (SN 150, SN 300, SN 500) and API Group II equivalent base oils, while completely separating water, light-end fuels, sub-micron particulates, organometallic complexes, and asphaltic residues.

Achieving high-yield recovery (> 85% of available base oil fractions) without inducing thermal cracking requires specialized high-vacuum wiped film evaporation (WFE) combined with vacuum fractionation technology.

---

2. Thermal Cracking Kinetics & Operating Window

2.1 Pyrolysis & Thermal Cracking Thresholds

Hydrocarbon base stocks begin thermal degradation (pyrolysis) via homolytic cleavage of carbon-carbon (C-C) bonds when bulk fluid temperatures exceed $340^\circ\text{C} - 360^\circ\text{C}$. The reaction rate constant k_c for thermal cracking follows classical Arrhenius kinetics:

k_c = A  ·  \exp( -(E_a) / (R  ·  T) )

Where:

• A ≈ 1.5 × 10^{14} s^{-1} (pre-exponential factor for heavy alkanes/naphthenes)
• E_a ≈ 230 - 260 kJ/mol (activation energy for thermal cracking)
• R = 8.314 J/mol·K
• T = absolute fluid temperature (K)

If thermal cracking occurs during distillation:

1. Long-chain paraffinic molecules break down into volatile unsaturated light hydrocarbons (alkenes/alkanes), decreasing the flash point of the recovered base oil cuts.
2. Polymerization and condensation of free radicals form heavy coke structures on heat transfer surfaces, causing severe equipment fouling, degradation of overall heat transfer coefficients (U), and rapid equipment shutdown.
3. Total Acid Number (TAN) increases due to thermal breakdown of oxidized sulfur and nitrogen compounds.

2.2 Thermal Stress Mitigation via Short Residence Time

To prevent thermal cracking, the process design must keep bulk liquid temperatures strictly \le 330^\circC and minimize thermal exposure time at elevated temperatures.

                              RESIDENCE TIME COMPARISON
                              
 Conventional Kettle [Reboiler](/process/equipment/reboiler)  [████████████████████████████████████████] 15 - 45 Minutes
 Forced Circulation Reboiler   [████████████████] 3 - 8 Minutes
 Wiped Film Evaporator (WFE)   [█] 1 - 15 Seconds

The Wiped Film Evaporator (WFE) solves thermal stress by spreading the viscous lube feed into an extremely thin turbulent film ($0.1 - 0.5 \text{ mm}$) over a heated cylindrical wall using mechanically driven rotating wipers. This reduces liquid residence time on the hot surface to 1–15 seconds, compared to 15–45 minutes in conventional reboilers.

2.3 Vacuum & Temperature Regimes Across Process Stages

---

3. Mechanical & Process Design Engineering

3.1 Design Codes & Regulatory Compliance

• Pressure Vessel Design: ASME Section VIII Division 1 (Design for Full Vacuum [FV] / External Pressure of $1.013 \text{ bar}$ at design temperature $360^\circ\text{C}$).
• Heat Exchanger Standards: TEMA Class R (Refinery Service) / TEMA Class C.
• Storage & Tanks: API 650 (Atmospheric Lube Storage) & API 2000 (Venting Requirements).
• Rotating Machinery: API 610 (Centrifugal Pumps for Hot Thermal Oil & Residue Sludge) & API 682 (Shaft Sealing Systems).
• Vapor Leak Integrity: EN 13185 / ASME V Article 10 (Helium Mass Spectrometer Leak Detection down to < 1 × 10^{-6} mbar·L/s).

3.2 Wiped Film Evaporator (WFE) Internal Architecture

A Wiped Film Evaporator consists of an outer vertical jacketed cylinder, an internal high-precision rotor equipped with dynamic wiper elements, and an optional internal condenser (Short Path / Molecular Distillation configuration).

                            WIPED FILM EVAPORATOR (WFE) SCHEMATIC
                            
                                 Drive Motor & Gearbox
                                        ││
                                     ┌──┴┴──┐
       Feed Inlet ──────────────────►│      │◄────── Heating Medium Inlet (HTO Hot)
                                     │  │   │
                                     │  ││  │
                                     │  ││  │  <--- Outer Cylindrical Heated Wall
        Internal Condenser ─────────►│  ││  │  <--- Rotating Rotor & Wiper Blades
          (Cooling Medium)           │  ││  │
                                     │  ││  │
                                     │  ││  │
       Distillate Outlet ───────────►│      │◄────── Heating Medium Outlet (HTO Return)
                                     └──┬┬──┘
                                        ││
                                        ▼
                             Asphalt Residue Bottoms

Wiper Mechanism Selection:

1. Hinged Metallic / Carbon-Graphite Vanes:

   • Slotted vanes mounted on rotor arms swing outward under centrifugal force to wipe the wall.
   • Ideal for high viscous residues (> 500 cP at operating temperature) containing particulates up to $50 \mu\text{m}$.
   • Materials: Carbon-filled PTFE (for T < 260^\circC), PEEK, or graphite-impregnated Hastelloy C-276 (for T > 300^\circC).

2. Rigid Roller Rotor System:

   • Precision glass-filled PTFE or metallic rollers held in guide cages roll along the inner vessel surface.
   • Maintains a fixed, uniform film thickness ($0.2 - 0.3 \text{ mm}$). Excellent heat transfer rate (U = 1000 - 1400 W/m²K), but sensitive to large inorganic particulate scoring.

Hydrodynamics of the Wiped Film:

The rotor speed (\omega) generates a bow wave ahead of each wiper blade. The liquid film behind the wiper undergoes rapid surface renewal.

                           WIPER BLADE HYDRODYNAMICS
                           
                 Heated Vessel Wall
  ──────────────────────────────────────────────────────────
     Liquid Thin Film (0.2 mm)   │
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~│  Bow Wave (Turbulent Mixing)
               ◄─────────────────┼─── [ Wiper Blade ] ◄── Rotation Vector
                                 │
  ──────────────────────────────────────────────────────────

The mean film thickness d_f produced by a wiper blade moving at tangential velocity v_t = \omega · r is governed by:

d_f = C  ·  ( (μ  ·  v_t) / (ρ  ·  g) )^{1/2}

Where:

• μ = dynamic viscosity of lube oil film (Pa·s)
• v_t = peripheral rotor velocity ($2.5 - 5.5 \text{ m/s}$)
• ρ = liquid density (kg/m³)
• g = gravitational acceleration ($9.81 \text{ m/s}^2$)
• C = dimensionless geometric wiper constant ($0.45 - 0.65$)

3.3 High Vacuum System Architecture

Achieving and maintaining absolute operating pressures of $0.1 - 0.5 \text{ mbar}$ under high vapor flow rates requires a multi-stage vacuum system:

1. Primary Stage (High Vacuum): Roots Mechanical Booster Pump (positive displacement rotary lobe blower) designed for high volumetric displacement at low suction pressures ($1000 - 5000 \text{ m}^3/\text{h}$).
2. Secondary Stage (Backing Vacuum): Dry Rotary Screw Vacuum Pump or Oil-Sealed Rotary Vane Pump capable of discharging against atmospheric pressure.
3. Cold Vapor Traps: Sub-cooled shell-and-tube condensers operated at -15^\circC to -25^\circC positioned upstream of the vacuum pumps to condense non-condensable light fractions, protecting the vacuum pump fluid.

                             MULTI-STAGE VACUUM ARCHITECTURE
                             
 WFE Vapor Outlet ──► [ Sub-Cooled Cold Trap ] ──► [ Roots Mechanical Booster ] ──► [ Dry Screw Backing Pump ] ──► Vent
                       (-15°C Refrigerated)         (Stage 1 Vacuum: 0.1 mbar)       (Stage 2: Discharge to Atm)

---

4. Sizing Equations & Thermodynamic Mass Balance Logic

4.1 WFE Heat Transfer & Evaporation Surface Area Sizing

The total required thermal duty Q_{total} in the WFE accounts for sensible heating of the feed from inlet temperature T_{in} to bulk boiling temperature T_b, and the latent heat of vaporization Δ H_{vap} of the base oil fractions:

Q_{total} = \dot{m}_{feed}  ·  C_p  ·  (T_b - T_{in}) + Σ ( \dot{m}_{dist,i}  ·  Δ H_{vap,i} ) + Q_{loss}

Where:

• \dot{m}_{feed} = Mass flow rate of stripped feed entering WFE (kg/s)
• C_p = Specific heat capacity of lube oil ($2.2 - 2.5 \text{ kJ/kg}\cdot\text{K}$)
• \dot{m}_{dist,i} = Mass flow rate of base oil fraction i evaporated (kg/s)
• Δ H_{vap,i} = Latent heat of vaporization of base oil cut ($220 - 270 \text{ kJ/kg}$)

The required heat transfer surface area A_{wfe} is calculated via:

A_{wfe} = (Q_{total}) / (U  ·  Δ T_{lm)}

Where the Logarithmic Mean Temperature Difference (Δ T_{lm}) for a thermal oil jacket operating with inlet temperature T_{HTO,in} and outlet temperature T_{HTO,out} is:

Δ T_{lm} = ((T_{HTO,in} - T_b) - (T_{HTO,out} - T_{in})) / (\ln ( \frac{T_{HTO,in) - T_b}{T_{HTO,out} - T_{in}} )}

Overall Heat Transfer Coefficient (U):

For a wiped film evaporator operating on heavy lube oil residues, U is governed by the thermal resistance of the inner liquid film h_i, the shell wall conductive resistance k_w / x_w, and the thermal oil jacket film resistance h_o:

(1) / (U) = (1) / (h_i) + (x_w) / (k_w) + (1) / (h_o) + R_{foul}

• h_i (Wiped film coefficient) = 1200 - 1800 W/m²K
• h_o (HTO jacket side) = 800 - 1200 W/m²K
• x_w / k_w (SS316L / Duplex wall, x_w = 8 mm, k_w = 16.3 W/m·K) = 4.9 × 10^{-4} m²K/W
• Typical overall design coefficient: U_{design} = 650 - 950 W/m²K.

4.2 Molecular Evaporation Dynamics (Langmuir-Knudsen Equation)

At ultra-high vacuum pressures (P < 0.5 mbar), gas dynamics transition from viscous continuum flow to Knudsen molecular flow. The maximum theoretical evaporation rate J_{max} (kg/m²·s) from the liquid film surface is given by the Langmuir-Knudsen relation:

J_{max} = E  ·  P_{sat}(T)  ·  √((M) / (2 π  ·  R  ·  T))

Where:

• E = Evaporation efficiency factor ($0.7 - 0.95$ for high vacuum WFEs)
• P_{sat}(T) = Saturation vapor pressure of base oil component at temperature T (Pa)
• M = Average molecular weight of evaporating base oil cut ($350 - 550 \text{ kg/kmol}$)
• R = Universal gas constant ($8314 \text{ J/kmol}\cdot\text{K}$)
• T = Absolute liquid film temperature (K)

Mean Free Path (\lambda):

To prevent evaporated molecules from colliding with each other and bouncing back into the liquid film, the mean free path \lambda of the vapor molecule must be comparable to or greater than the distance between the evaporation surface and the condenser surface (d_{cond}):

\lambda = (k_B  ·  T) / (√(2)  ·  π  ·  d_m²  ·  P)

Where:

• k_B = Boltzmann constant ($1.3806 \times 10^{-23} \text{ J/K}$)
• d_m = Molecular diameter of base oil hydrocarbon (≈ 0.8 - 1.2 nm)
• P = Absolute system pressure (Pa)

Engineering Calculation Example: At T = 580 K ($307^\circ\text{C}$) and P = 0.1 mbar ($10 \text{ Pa}$), \lambda ≈ 12 - 18 mm. This requires short vapor passage routes and internal condensers (Short Path Evaporator setup) to eliminate vapor line pressure drops.

4.3 Rigorous Plant Mass & Energy Balance (Basis: 10,000 kg/h Raw ULO Feed)

                                  10,000 kg/h RAW ULO FEED
                                              │
                                              ▼
                             +----------------------------------+
                             |      STAGE 1: DE-WATERING        | ───► Water Vapor / Light Ends
                             |    (120°C, 150 mbar, Atmospheric) |      500 kg/h (5.0 wt%)
                             +----------------------------------+
                                              │
                                              ▼ Dry Oil (9,500 kg/h)
                             +----------------------------------+
                             |      STAGE 2: DE-FUELING         | ───► Light Gas Oil (LGO Cut)
                             |       (210°C, 30 mbar)           |      600 kg/h (6.0 wt%)
                             +----------------------------------+
                                              │
                                              ▼ Stripped Lube Feed (8,900 kg/h)
                             +----------------------------------+
                             |     STAGE 3: WFE DE-ASPHALTING   | ───► Heavy Asphalt Residue Bottoms
                             |       (325°C, 0.2 mbar)          |      1,400 kg/h (14.0 wt%)
                             +----------------------------------+
                                              │
                                              ▼ Asphalt-Free Lube Vapors (7,500 kg/h)
                             +----------------------------------+
                             |  STAGE 4: VACUUM FRACTIONATION   |
                             |         (1.5 mbar)               |
                             +----------------------------------+
                                  │           │           │
                                  │           │           └───────────────────────┐
                                  ▼           ▼                                   ▼
                            SN 150 Cut    SN 300 Cut                          SN 500 Cut
                            2,500 kg/h    3,200 kg/h                          1,800 kg/h
                            (25.0 wt%)    (32.0 wt%)                          (18.0 wt%)

Detailed Stream Data Summary Table:

---

5. Metallurgical Specifications & Materials Engineering

The thermal processing of Used Lube Oil exposes equipment to severe corrosive mechanisms:

1. Naphthenic Acid Corrosion (NAC): Severe attack on iron/steel at temperatures between $220^\circ\text{C}$ and $400^\circ\text{C}$ by organic acids (R-COOH) present in degraded oils.
2. Hydrochloric Acid / Chloride Attack: Dissociation of chlorinated additives or salt contamination releasing HCl gas during de-watering/de-fueling stages.
3. Hydrogen Sulfide (H_2S) & Mercaptan Corrosion: Thermal decomposition of sulfur additives (ZnDTP) above $200^\circ\text{C}$.

Material Selection Matrix Across Process Equipment:

+---------------------------------------------------------------------------------------------------------+
|                                    EQUIPMENT METALLURGY MATRIX                                          |
+---------------------------------------------------------------------------------------------------------+

  Component / Vessel          Primary Material          PREN Rating   Corrosion Allowance / Notes
 ─────────────────────────────────────────────────────────────────────────────────────────────────────────
  De-Watering Column          SS316L / Duplex 2205      25 - 35       3.0 mm; Resistant to wet HCl vapor
  De-Fueling Column           SS316L                    25            2.0 mm; Resistant to light acids
  WFE Main Shell Vessel       Duplex 2205 / SS316L Clad 34 - 36       3.0 mm; High yield strength at 340°C
  WFE Rotating Shaft & Rotor  Duplex 2205 / Hastelloy   35 - 68       Zero tolerance; Precision machined
  WFE Wiper Elements          Graphite-PEEK / C-276     N/A           Thermal rating up to 350°C
  WFE Internal Condenser      Hastelloy C-276 / SS316L  68 / 25       Resistant to concentrated acid condensate
  Thermal Oil Jacket Shell    ASTM A516 Grade 70       N/A           Carbon steel suitable for non-corrosive HTO
  Fractionation Column        SS316L / SS304L           25 / 19       Structured packing SS316L
  Hot Product Acid Pumps      Hastelloy C-276           68            Double mechanical seal with Plan 54 barrier

Pitting Resistance Equivalent Number (PREN) Calculation:

For high-temperature acidic environment tolerance, materials are selected based on PREN:

PREN = \% Cr + 3.3  ·  (\% Mo + 0.5  ·  \% W) + 16  ·  \% N

• SS304L: PREN ≈ 19.0 (Unsuitable for WFE main shell due to naphthenic acid pitting).
• SS316L: PREN ≈ 25.0 (Acceptable for fractionation column and low-temp sections).
• Duplex 2205 (UNS S31803): PREN ≈ 34.5 (Optimal for WFE shell; twice the yield strength of 316L, preventing vacuum collapse under heat).
• Hastelloy C-276 (UNS N10276): PREN ≈ 68.0 (Impervious to high-temperature naphthenic acid and chloride corrosion; utilized for WFE internal condensers and dynamic wiper mountings).

---

6. Technology Selection Matrix & Comparative Engineering Analysis

To evaluate separation technologies for lube oil re-refining, process parameters must be analyzed across four distinct evaporator architectures:

---

7. Real-World Commercial Plant Case Study & Operational Data

7.1 Plant Baseline Specification

• Facility Capacity: $150 \text{ Metric Tons/Day}$ ($50,000 \text{ MTPA}$) Raw Used Lube Oil Re-Refining Facility.
• Location / Feed Origin: Mixed Automotive (80%) and Industrial (20%) Hydraulic Oils.
• Technology Installed: Multi-Stage Continuous Vacuum Distillation featuring Pre-Dehydration, Fuel Flash Column, Wiped Film Evaporator (Duplex 2205 construction), and Structured Packing Vacuum Fractionation Tower.

                           PLANT OVERVIEW & MASS YIELD BREAKDOWN
                           
                   ┌──────────────────────────────────────────────┐
                   │  150 TPD Raw Used Lube Oil Feed (100.0%)     │
                   └──────────────────────┬───────────────────────┘
                                          │
             ┌────────────────────────────┼────────────────────────────┐
             ▼                            ▼                            ▼
  ┌──────────────────────┐     ┌──────────────────────┐     ┌──────────────────────┐
  │ Aqueous Waste Stream │     │ Light Gas Oil Fuel   │     │ Asphaltic Bottoms    │
  │    7.5 TPD (5.0%)    │     │    9.0 TPD (6.0%)    │     │   21.0 TPD (14.0%)   │
  └──────────────────────┘     └──────────────────────┘     └──────────────────────┘
                                          │
                                          ▼
                               Distilled Base Oil Vapors
                                   112.5 TPD (75.0%)
                                          │
             ┌────────────────────────────┼────────────────────────────┐
             ▼                            ▼                            ▼
  ┌──────────────────────┐     ┌──────────────────────┐     ┌──────────────────────┐
  │  SN 150 Base Oil Cut │     │  SN 300 Base Oil Cut │     │  SN 500 Base Oil Cut │
  │   37.5 TPD (25.0%)   │     │   48.0 TPD (32.0%)   │     │   27.0 TPD (18.0%)   │
  └──────────────────────┘     └──────────────────────┘     └──────────────────────┘

7.2 Measured Operating Parameters at Steady State

===================================================================================
                  WIPED FILM EVAPORATOR (WFE-301) OPERATING LOG
===================================================================================
Parameter                                Target Unit Value    Measured Value
-----------------------------------------------------------------------------------
Feed Flow Rate                           kg/h                5,562
Feed Temperature (Post De-fueling)       °C                  215
WFE Shell Internal Wall Temperature      °C                  338
Thermal Oil Inlet Temperature (HTO)      °C                  352
Thermal Oil Outlet Temperature (HTO)     °C                  339
Thermal Oil Circulation Velocity         m/s                 2.15
WFE Operating Pressure (Top Head)        mbar (abs)          0.22
WFE Rotor Speed                          RPM                 340
Rotor Motor Current Load                 Amperes             48.2 (55 kW Rated)
Wiper Outer Tip Speed                    m/s                 4.28
Calculated Overall Heat Transfer (U)     W/m²K               1,045
Asphalt Residue Discharge Temperature    °C                  312
Distillate Vapor Temperature             °C                  298
===================================================================================

7.3 Recovered Base Oil Quality Analysis

Following high-vacuum WFE de-asphalting and vacuum fractionation, the base oil fractions were subjected to finishing treatment (solvent clay polishing / hydrotreating). The analytical results confirm complete restoration to API Group I / Group II specifications:

====================================================================================================
                        LABORATORY ANALYTICAL RESULTS (POST FINISHING)
====================================================================================================
Test Property                  Test Method     SN 150 Cut       SN 300 Cut       SN 500 Cut
----------------------------------------------------------------------------------------------------
Kinematic Viscosity @ 40°C     ASTM D445       30.2 cSt         64.8 cSt         102.5 cSt
Kinematic Viscosity @ 100°C    ASTM D445       5.25 cSt         8.45 cSt         11.40 cSt
Viscosity Index (VI)           ASTM D2270      104              101              98
Color                          ASTM D1500      L0.5             L1.0             L1.5
Flash Point (COC)              ASTM D92        204°C            232°C            256°C
Pour Point                     ASTM D97        -12°C            -9°C             -6°C
Total Acid Number (TAN)        ASTM D664       0.015 mg KOH/g   0.020 mg KOH/g   0.028 mg KOH/g
Conradson Carbon Residue (CCR) ASTM D189       0.012 wt%        0.018 wt%        0.035 wt%
Noack Volatility (1 hr @ 250°C)ASTM D5800      13.5 wt%         7.2 wt%          4.1 wt%
Polynuclear Aromatics (PNA)    IP 346          < 0.8 wt%        < 1.0 wt%        < 1.2 wt%
Total Heavy Metals (Pb,Cr,Zn)  ICP-AES         < 1.0 ppm        < 1.0 ppm        < 1.0 ppm
====================================================================================================

---

8. Engineering Best Practices & Operations Manual

8.1 Mechanical Shaft Sealing Systems for Ultra-High Vacuum

The WFE main agitator shaft enters the vessel under extreme vacuum conditions ($0.1 \text{ mbar}$) while surrounded by high-temperature vapors (> 300^\circC).

• Seal Configuration: Dual pressurized mechanical seal arrangement (API Plan 53B or Plan 54).
• Barrier Fluid: Synthetic thermic fluid or high-viscosity silicone fluid compatible with base oil, maintained at a pressure 1.5 to 2.0 bar above atmospheric pressure ($2.5 - 3.0 \text{ bar abs}$).
• Cooling: Integrated water-cooled seal housing to maintain seal face temperatures below $160^\circ\text{C}$, preventing barrier fluid carbonization.

                              MECHANICAL SEAL PLAN 54 INTEGRATION
                              
                                   Barrier Fluid Reservoir
                                   (Pressurized @ 3.0 bar)
                                             │
                                     ┌───────┴───────┐
                                     │  Pumping Unit │
                                     └───────┬───────┘
                                             │
                                             ▼  Cool Barrier Fluid
  Atmosphere (1.0 bar) ───► [ Primary Seal Face ] ───► [ Secondary Seal Face ] ───► Vacuum Space (0.1 mbar)
                                             │
                                             ▼  Hot Return Fluid
                                   [ Heat Exchanger Cooling ]

8.2 Thermal Oil System Velocity & Temperature Control

Local overheating of the heat transfer oil (HTO) inside the WFE outer jacket causes thermal cracking of the synthetic heat transfer fluid, creating gas pockets and carbonaceous insulation on the jacket wall.

• Maintain HTO bulk velocity in the jacket space at > 1.8 m/s using internal directional spiral baffles.
• Limit the maximum differential temperature between HTO supply and return (Δ T_{HTO}) to \le 15^\circC.
• Ensure the HTO heater loop features an automatic high-temperature interlock set at $365^\circ\text{C}$.

8.3 Anti-Coking Startup & Shutdown Protocols

Startup Sequence:

1. Inert the entire process loop using high-purity Nitrogen (N_2, oxygen level < 0.5 vol%) until pressure testing passes.
2. Pull system vacuum down to < 1.0 mbar using the vacuum package.
3. Initiate HTO circulation and heat the WFE vessel to $180^\circ\text{C}$ before introducing liquid feed.
4. Start WFE rotor drive; verify current draw before introducing stripped oil feed.
5. Ramp up HTO temperature gradually ($2^\circ\text{C/min}$) up to target operating temperature ($350^\circ\text{C}$) while establishing full feed rate.

Shutdown Sequence:

1. Cut raw feed supply and transition WFE feed to a flushing cut (clean light distillate or synthetic flushing oil).
2. Maintain rotor rotation and run flushing oil for 30 minutes to clean asphaltic residues off the inner vessel wall and wiper blades.
3. Turn off HTO heating unit; continue rotor operation until vessel wall temperature drops below $120^\circ\text{C}$.
4. Stop WFE rotor; break vacuum using dry Nitrogen gas. Never admit atmospheric air into a hot WFE vessel to prevent catastrophic auto-ignition of residual hot heavy hydrocarbons.

8.4 Vacuum Leak Integrity Maintenance

In-leakage of atmospheric air into high-vacuum WFE vessels operating at > 300^\circC leads to rapid thermal oxidation of base oil vapors, severe color degradation (ASTM color increases from L0.5 to > 4.0), increase in Total Acid Number, and explosive vapor mixture formation.

• Perform quarterly Helium Mass Spectrometer leak detection tests across all flanged joints, sight glasses, instrument ports, and mechanical seal housings.
• Utilize fully welded construction wherever possible, minimizing flanged connections. Where flanged joints are required, specify ASME B16.20 Spiral Wound Gaskets with Flexible Graphite Filler and Inner/Outer Stainless Steel Rings or metal-jacketed O-rings.

---

9. Conclusion & Plant Design Recommendations

Engineering a high-yield, continuous Used Lube Oil re-refining facility requires strict adherence to short residence time high-vacuum distillation principles.

Key design takeaways for process engineers:

1. Prevent Thermal Cracking: Limit bulk oil temperatures to \le 330^\circC and utilize Wiped Film Evaporation to restrict thermal contact time to < 15 seconds.
2. Design for Deep Vacuum: Execute de-asphalting at $0.1 - 0.5 \text{ mbar}$ absolute pressure to maximize base oil evaporation yields (> 85%) without increasing thermal stress.
3. Select High-Grade Alloys: Specify Duplex 2205 for WFE pressure shells and Hastelloy C-276 for internal condensing and wiping components to resist naphthenic acid and chloride corrosion.
4. Enforce Seal & Vacuum Integrity: Implement API Plan 54 double mechanical seals and rigorous helium leak testing (< 10^{-6} mbar·L/s) to guarantee zero air ingress, preserving base oil color, flash point, and chemical purity.