Material Selection & Hydraulics & Types | DAIER Chemical Separation Standard

Engineering Reference for Process Engineers

 

Section 4: Material Selection – Detailed Engineering Criteria

18. Q: What is the difference between SS304 and SS316L for tower packing?A: SS304 (18Cr-8Ni) is for general non-corrosive or mildly corrosive service. SS316L adds 2-3% molybdenum (Mo).  (ASTM A240)provides resistance to chlorides, pitting, and crevice corrosion. For seawater, brackish water, or chloride-containing process streams, SS316L with  (ASTM A240) is the minimum specification.

19. Q: What are the chloride limits for SS316L at elevated temperatures?A: At 500 ppm chloride concentration, the critical pitting temperature for SS316L is ~70°C, and the threshold for stress corrosion cracking is ~55°C. For higher temperatures or higher chloride levels, a higher alloy (e.g., duplex, Hastelloy) is required.

20. Q: What is PREN and why is it used?A: PREN (Pitting Resistance Equivalent Number) = %Cr + 3.3×%Mo + 16×%N. SS304 has PREN ~18; SS316L has PREN ~24; Hastelloy C-276 has PREN ~45. Higher PREN indicates better resistance to pitting in chloride environments.

21. Q: What are the temperature limits for plastic packings?A: PP (polypropylene): 80-100°C; PVDF (polyvinylidene fluoride): up to 150°C; PTFE (polytetrafluoroethylene): up to 260°C; PVC (polyvinyl chloride): 60°C. Above these limits, the material softens, deforms, or loses mechanical strength.

22. Q: When is ceramic packing specified?A: Ceramic packing is used for service temperatures above 300°C or in strong mineral acids (sulfuric, nitric, hydrochloric) where metals corrode and plastics soften. Typical applications: sulfuric acid drying towers, chlorine drying, hot gas scrubbing. Porosity and acid resistance must be verified against the specific concentration.

23. Q: Can packing be made from exotic alloys like Hastelloy or Monel?A: Yes. Hastelloy C-276, Inconel 625, Monel 400, and titanium are available for extreme corrosive service (e.g., wet chlorine, hydrochloric acid at high temperature). Cost is 5-20× higher than SS316L.

Section 5: Hydraulics and Performance Prediction

24. Q: What is pressure drop in a packed column and how is it calculated?A: Pressure drop is the gas-phase resistance across the packed bed, expressed as mm H₂O or Pa per meter of packing height. It is calculated using correlations such as the generalized pressure drop correlation (GPDC) or the Stichlmair model. Pressure drop increases with gas velocity and liquid load.

25. Q: What are the loading point and flooding point?A: Loading point: gas velocity where liquid begins to accumulate in the packing, causing a slope change in the pressure drop curve. Flooding point: gas velocity where liquid is prevented from flowing downward, causing loss of separation. Normal operation is at 50-80% of flooding velocity.

26. Q: How is flooding velocity predicted?A: Flooding velocity is predicted using the Sherwood-Eckert flooding correlation or the generalized pressure drop correlation (GPDC). The calculation requires flow rates, densities, viscosity, and packing factor F. Flooding velocity u_flood is proportional to √(ρ_L/ρ_V).

27. Q: What is the F-factor and its typical range?A: F-factor = u × √ρ_V, where u is superficial gas velocity (m/s) and ρ_V is vapor density (kg/m³). Units: Pa⁰·⁵. Typical operating F-factors: random packing 0.5–2.5; structured packing 1.0–3.5; wire mesh packing 0.3–1.5.

28. Q: What is liquid holdup and why does it matter?A: Liquid holdup is the fraction of liquid retained in the packing bed. It affects residence time, pressure drop, and column dynamics. Operating holdup is typically 0.05–0.20 m³/m³ for random packing and 0.05–0.10 for structured packing.

29. Q: What is the minimum wetting rate and why is it important?A: Minimum wetting rate is the lowest liquid flow rate required to completely wet the packing surface. Below this rate, dry patches appear, reducing efficiency. Typical values: 0.1–0.5 m³/(m·h) for random packing, lower for structured packing.

30. Q: How does pressure drop scale with packing size?A: For the same packing type, pressure drop is inversely proportional to nominal size. Doubling the size reduces pressure drop by approximately 50-70%, but also reduces efficiency by 20-40%.

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