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Water Management in Process Plants

Water Management in Process Plants. David Puckett Débora Campos de Faria Miguel J. Bagajewicz. Sources of Refinery Wastewater. Caustic Treating. NH 3 and H 2 S Water Contamination. Distillation. Water Contamination with Organics. Amine Sweetening. NH 3 and H 2 S Water Contamination.

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Water Management in Process Plants

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  1. Water Management in Process Plants David Puckett Débora Campos de Faria Miguel J. Bagajewicz

  2. Sources of Refinery Wastewater Caustic Treating NH3 and H2S Water Contamination Distillation Water Contamination with Organics Amine Sweetening NH3 and H2S Water Contamination Merox Sweetening NH3 and H2S Water Contamination Water Contamination with NH3, H2S, and Organics Hydrotreating Desalting Saline Water Contamination

  3. Water Management Methods • Wastewater produced in industrial processes can be handled in three fashions. • End-of-Pipe Cleanup • Reuse • Regeneration

  4. Regeneration Methods • API separator and activated carbon to remove organics from distillation and hydrotreating wastewater. • Reverse osmosis to removesaline contamination from desalting wastewater. • Chevron wastewater treatment to remove acid gas contamination from caustic treating, sweetening, and hydrotreating wastewater.

  5. Wastewater Optimization • Current methods of optimizing water reuse and regeneration rely on several assumptions. • Operating and capital costs are functions solely of treated water flow rate. • Fixed process outlet concentrations.

  6. Wastewater Optimization A FCI $458000 22% of the amount of contaminant removed, 89% of the FCI B FCI $408000 51% of the amount of contaminant removed, 78% of the FCI C FCI $317000

  7. Wastewater Optimization • Depending on the contaminants present and the treatment processes used, the assumption that regeneration costs are dependent solely on flow rate may not be valid. • The optimum solution for a water allocation problem must take into account factors other than flow rate.

  8. Removal of Organics Distillation Wastewater Contaminated with Organics Hydrotreating API Separator and Activated Carbon Adsorber Wastewater Free of Organics

  9. API Separator • Removes multi-phase contamination through differences in specific gravity.

  10. API Separator • Appropriate for use with any contaminant that forms a distinct phase in the process water. • Oil and Light Organics • Organic and Inorganic Sediment

  11. API Separator Simulation • The basis of the separation is Stokes’ Law. • For a given contaminant in water, rate of settling is determined solely by contaminant particle size. • Quality of separation can be improved through flocculation and coagulation. Buoyant Force Drag Force Gravitational Force

  12. API Separator Simulation • Process water contaminant concentration does not change quality of separation. • Percentage of contaminants removed on a volume basis determined based on a normal distribution of particle radii.

  13. Quality of Separation vs. Length Separator Depth = 1 m Separator Width = 2 m Entrance to Separator at 0.5 m Process Water Flow Rate = 1 m3 / s Contaminant SG = 0.95 Mean Contaminant Diameter = 0.5 mm Quality of separation improves with increasing length, but with diminishing returns.

  14. Quality of Separation vs. Specific Gravity Separator Depth = 1 m Separator Width = 2 m Separator Length = 25 m Entrance to Separator at 0.5 m Process Water Flow Rate = 1 m3 / s Mean Contaminant Diameter = 0.5 mm Separation quality is poor for contaminants similar in density to water.

  15. Quality of Separation vs. Particle Diameter Separator Depth = 1 m Separator Width = 2 m Separator Length = 25 m Entrance to Separator at 0.5 m Process Water Flow Rate = 1 m3 / s Contaminant SG = 0.95 Quality of separation improves with increasing particle diameter, but with diminishing returns.

  16. Quality of Separation vs. Wastewater Velocity Separator Depth = 1 m Separator Width = 2 m Separator Length = 25 m Entrance to Separator at 0.5 m Contaminant SG = 0.95 Mean Contaminant Diameter = 0.5mm Quality of separation improves with decreasing velocity. A velocity of zero would give perfect separation.

  17. Quality of Separation vs. Settling Distance Separator Depth = 1 m Separator Width = 2 m Separator Length = 25 m Contaminant SG = 0.95 Process Water Flow Rate = 1 m3 / s Quality of separation improves with decreasing settling distance. Separators that handle oil will have entrance as close to water surface as possible.

  18. Equipment Cost vs. Flow rate Separator Depth = 1 m Separator Width = 2 m Mean Particle Diameter = 1 mm Process Water Flow Rate = 0.5 m3 / s Equipment cost is sensitive to both flow rate and separation quality.

  19. Operating Cost vs. Flow rate Separator Depth = 1 m Separator Width = 2 m Mean Particle Diameter = 1 mm Process Water Flow Rate = 0.5 m3 / s Operating cost is sensitive only to flow rate.

  20. API Separator Performance • With a normal distribution of particle diameters, quality of separation can be solved analytically. • A bit impractical to implement.

  21. API Separator Performance Varying SG Varying h Varying L Varying Dp Varying F/A The approximation is quite close for all variables. The worst fit is for changes in settling height.

  22. API Separator Equipment Cost Varying ΔSG, Dp, h Varying F and %QS Equipment cost is dependent on flow rate, quality of separation, specific gravity of contaminant, contaminant particle size, settling distance, and the price of steel.

  23. API Separator Operating Cost Operating cost is dependent solely on flow rate.

  24. Activated Carbon • Removes soluble contaminants through adsorption onto the activated carbon surface.

  25. Activated Carbon • Appropriate for use with liquid or gaseous contaminants that are water soluble or form emulsions. • Dissolved Organics • Insoluble Organics of < 150 microns Droplet Size • Dissolved Gases

  26. Activated Carbon Simulation • Separation will follow the Langmuir isotherm. • For the Langmuir isotherm, the rate of adsorption, assuming negligible pore holdup and spherical adsorbate particles, is as follows.

  27. Activated Carbon Simulation • For a fixed bed adsorber, process water should reach equilibrium with activated carbon prior to end of bed. • A constant length of bed is required for adsorption.

  28. Water Treatment Rate vs. Adsorbant Surface Area Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Concentration = 0.25 kg / m3 Greater adsorbant surface area results in faster adsorption and a faster rate of treatment.

  29. Time in Service vs. Adsorber Diameter Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Concentration = 0.25 kg / m3 A greater diameter has more adsorbant per unit length and thus will take longer to saturate.

  30. Time in Service vs. Adsorber Height Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Concentration = 0.25 kg / m3 The saturation wave travels through the adsorber at a constant speed.

  31. Outlet Concentration vs. Inlet Concentration Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Outlet concentration from adsorber is dictated by adsorption thermodynamics.

  32. Equipment Cost vs. Flow Rate Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Concentration = 0.25 kg / m3 Wastewater pump and column diameter must scale for flowrate.

  33. Operating Costs vs. Flowrate Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Concentration = 0.25 kg / m3 A greater flow rate means more regenerations per year in addition to increased pumping work.

  34. Equipment Cost vs. Inlet Concentration Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Flow Rate = 7.035 m3 / hr No changes in the adsorber need to be made to accommodate a greater inlet concentration.

  35. Activated Carbon Aerogel Contaminant Diffusivity = 5 * 10 ^ -11 m2 / s Langmuir Coefficient = 0.06 m3 / kg Saturation Adsorption = 0.4 kg / kg adsorbant Inlet Contaminant Flow Rate = 7.035 m3 / hr A greater inlet concentration has the same effect as a greater flow rate. More contaminant must be adsorbed necessitating more regenerations.

  36. Activated Carbon Performance • Outlet concentration can be calculated analytically.

  37. Activated Carbon Equipment Cost Varying F and CIN Varying H Varying D Equipment cost is dependent on flow rate, inlet concentration, column measurements, and the prices of steel and activated carbon.

  38. Activated Carbon Operating Cost Varying CIN Varying F Operating cost is dependent only on flow rate and concentration.

  39. Removal of Salts Wastewater Contaminated with Salts Desalting Reverse Osmosis Separation Wastewater Free of Salts

  40. Reverse Osmosis • Removes salts from process water by forcing water against the salt concentration gradient.

  41. Reverse Osmosis • Suitable for the removal of any soluble contamination. • Soluble Salts • Soluble Organics • Microorganisms

  42. Reverse Osmosis Simulation • Separation proceeds based on Fick’s First Law. • For reasonably dilute solutions, the van’t Hoff approximation of osmotic pressure can be used. • Quality of separation is fixed by type of membrane used.

  43. Flow Rate vs. Membrane Area Membrane Thickness = 0.002 m Membrane Permeability = 1.92 * 10-9 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Brine Ion Concentration = 100 mol / m3 Ion Rejection Percentage = 0.99 Flow rate and membrane area are linearly related, as would be expected from Fick’s Law.

  44. Flow Rate vs. Membrane Thickness Membrane Area = 100 m2 Membrane Permeability = 1.92 * 10-9 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Brine Ion Concentration = 100 mol / m3 Ion Rejection Percentage = 0.99 Flow rate and membrane thickness are inversely related, as would be expected from Fick’s Law.

  45. Flow Rate vs. Brine Pressure Membrane Area = 100 m2 Membrane Thickness = 0.002 m Membrane Permeability = 1.92 * 10-9 ((m3 m) / (m2 sec atm)) Brine Ion Concentration = 100 mol / m3 Ion Rejection Percentage = 0.99 Flow rate is zero when the pressure gradient is equal and opposite to the osmotic pressure gradient.

  46. Flow Rate vs. Rejection Percentage Membrane Area = 100 m2 Membrane Thickness = 0.002 m Membrane Permeability = 1.92 * 10-9 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Brine Ion Concentration = 100 mol / m3 A higher rejection percentage results in a larger osmotic pressure gradient.

  47. Flow Rate vs. Brine Concentration Membrane Area = 100 m2 Membrane Thickness = 0.002 m Membrane Permeability = 1.92 * 10-9 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Ion Rejection Percentage = 0.99 Again, flow rate is zero when the pressure gradient is equal and opposite to the osmotic pressure gradient.

  48. Flow Rate vs. Temperature Membrane Area = 100 m2 Membrane Thickness = 0.002 m Membrane Permeability = 1.92 * 10-9 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Brine Concentration = 100 mol/m3 Ion Rejection Percentage = 0.99 The van’t Hoff approximation introduces a dependence of osmotic pressure on temperature.

  49. Equipment Cost vs. Flow Rate at 1463 ppm Inlet Membrane Thickness = 0.0001 m Membrane Permeability = 9.17 * 10-10 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Base Ion Rejection Percentage = 0.8 Equipment cost increases exponentially with process water purity as membrane rejection is fixed so membranes must be worked in series to achieve higher purity.

  50. Equipment Cost vs. Flow Rate at 59 ppm Inlet Membrane Thickness = 0.0001 m Membrane Permeability = 9.17 * 10-10 ((m3 m) / (m2 sec atm)) Brine Pressure = 10 atm Base Ion Rejection Percentage = 0.8 Equipment costs are dependent on the relative inlet/outlet concentrations, not the absolute concentrations.

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