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BTEX Prediction & Removal in Amine Units

BTEX Prediction & Removal in Amine Units. Luke Burton Chad Duncan Armando Diaz Miguel Bagajewicz. Project Objective. To study means of reducing incineration expenditures associated to BTEX capture in Amine units, through Process parameter optimization

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BTEX Prediction & Removal in Amine Units

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  1. BTEX Prediction & Removal in Amine Units Luke Burton Chad Duncan Armando Diaz Miguel Bagajewicz

  2. Project Objective • To study means of reducing incineration expenditures associated to BTEX capture in Amine units, through • Process parameter optimization • Alternative/Additional Technologies to capture BTEX • Specifics: • BTEX content of needs to be kept under the EPA emission limit of 25 Ton/year . • If this is achieved, a reduction in incineration temperature from 1500 oF to 1350 oF can be accomplished with an associated savings of $303 Thousand. • Alternative Technologies, if they exist, ought to have a lower cost.

  3. Project Methodology • Discuss Existing Simulators and compare their capabilities of predicting • Acid Gas flowrate and composition • BTEX capture • Determine ways of using these simulators to make approximate predictions • Assess the ability of process parameter manipulation to achieve the reduction of BTEX capture goal. • Study Alternative Technologies • Adsorbents • Ionic Liquids

  4. Modeling Objective • Commercial Simulators seem not to reproduce reliable results in the case of Amine units, especially when BTEX capture is of interest. • Ideal Objective: Have a simulator that will use the right thermodynamic equation of state and liquid activity coefficients • Achievable Objectives: Use existing simulators and supersede them with additional data and make conclusions.

  5. Amine Plant at Glance

  6. BTEX Problems in Amine Unit • Flash Drum • BTEX is emitted to the atmosphere, possible violating EPA guidelines. • Acid gas stream • BTEX present has to be incinerated at high temperatures, therefore incurring a high fuel cost. • Sweet gas stream • Some BTEX will be present, so it is removed in glycol unit.

  7. PRO II Amine Unit Simulation Same inlet conditions were used: Feed gas (575MMSCF), T (85°F), P (500psia), same compositions. Results were compared to 92 wt% of CO2 usually found in acid gas stream.

  8. AmineCalc Amine Unit Simulation Same inlet conditions were used: Feed gas (575MMSCF), T (85°F), P (500psia), same compositions. Results were compared to 92 wt% of CO2 usually found in acid gas stream.

  9. CO2 Results from Pro II and AmineCalc

  10. Credibility • Which simulator is correct? • AmineCalc renders 99 wt% of CO2 in the acid gas • Pro II renders 94 wt% of CO2 in the acid gas, closer to the 92 wt % reported from field data. • Thermodynamic packages in AmineCalc and Pro II might explain why.

  11. EOS in AmineCalc • Uses Peng-Robinson equation of state. • Not as thorough as Pro II as far as the thermodynamics. Binary interaction coefficient calculated by using simple cubic mixing rule. • Mixing rule have been shown to be incapable of modeling real systems.

  12. EOS in Pro II • Pro II uses SRKM equation of state to calculate the vapor phase enthalpy and density, and liquid and vapor phase entropy. • ω, cij, kij, b, are parameters that are easily obtained. • Binary interaction coefficients mixing rule developed by Prausnitz, and shown to perform better than simple cubic mixing rule.

  13. BTEX Predictions

  14. USE OF EXTERNAL DATA • We used the solubility data found in Developments and Applications in Solubility. (Coquelet et. al. 2007) • In this book, the activity coefficients of benzene, toluene, ethylbenzene, and xylene are calculated experimentally for different mixtures of MDEA/DGA and Water.

  15. Contactor Tower Results Contactor Tower • Experimental results can be used to calculate how accurate are the simulator results; more specifically the molar composition in the liquid stream. Sweet Gas Amine Liquid tray 5 Feed Gas Vapor tray 6 Bottoms liquid

  16. Contactor Tower Results V Flash G L2 L2 G V L1 L1

  17. Contactor Tower Results V Flash G L2 L1

  18. Regenerator Tower Results Regenerator Tower • Experimental results can be used to calculate how accurate are the simulator results; more specifically the molar composition in the acid gas stream. Acid Gas Vapor tray 3 Liquid tray 2 Rich Amine Lean Amine

  19. RESULTS

  20. CONCLUSION • It is our belief that Pro II produces good answers for flows and CO2 concentrations in the amine unit. • Pro II and AmineCalc overestimates the solubility of BTEX in the contactor. • We do not have the right thermodynamics in Pro II or AmineCalc, or any simulator. • Despite the above, we have a credible way of estimating solubilities based on experimental data.

  21. Glycol Dehydration Units • Unit removes water from sweetened natural gas. • Glycols such as DEG or TEG usually used for these tasks. • Two commercially available simulators: GlyCalc and Pro II. • Interfaces for Glycalc and Pro II are shown.

  22. Glycol Dehydration Units Milagro Data: 49 MMSCFD 104 °F 887 psig 10gal/min glycol 382 °F • Unit removes water from sweetened natural gas. • Glycols such as DEG or TEG usually used for these tasks. • Two commercially available simulators: GlyCalc and Pro II. • Interfaces for Glycalc and Pro II are shown.

  23. GlyCalc Contactor Tower • In contactor tower, VLE calculations using Kremser-Brown approximation. • Approximation used to calculate K-values. • Contactor tower not rigorously modeled by using stage by stage flash calculation. • L and V is assumed to be average in every stage.

  24. GlyCalc Regenerator • For regenerator, manual notes: • “to avoid complex heat and material balances that would be needed if the regenerator were rigorously modeled, a simple empirical calculation is used”

  25. Results

  26. Conclusions • GlyCalc produces better results for BTEX in dehydration unit. • We believe Glycalc would be able to predict the amount of BTEX present in dehydration unit. • GlyCalc would not be able to accurately predict duty in regenerator due to its simple correlation used for energy balance.

  27. BTEX Solutions

  28. Reduction Possibilities • Two different ways to remove amine exist • Reduce absorption in amines • Certain parameters can obtain this • Remove BTEX prior/post amine unit treating • Solvent • Alternative Technologies

  29. First Solution • Changes in parameters such as amine flow rate, temperature and pressure of towers, etc. may reduce BTEX capture. • We performed a few simulations in Pro II to get a preliminary sensitivity analysis for the affect of temperature.

  30. Parameter Adjustments • It is our belief that this route will not solve the emission problems.

  31. Second Solution • Solvents can be used: • Water • Alternative Technology • Adsorbents • Activated Carbon • Silica Aerogels • Macroreticular Resins • Ionic Liquids

  32. Removal by Solvent • Removal By Water

  33. CONCLUSION • Manipulating the amine unit parameters (T, P, and flow rates) will not lead to the order of magnitude changes needed to reduce the emission. • This conclusion is based both on considering results of Pro II directly and calculations based on experimental results. • Water is also not a good solvent to remove BTEX due to separation complications. • This leads to the investigation of other alternative technologies

  34. Activate Carbon • Activated Carbon has a density of about 350 kg/m3 and surface area of 500 m2/g • Can only be used 2 cycles before 50% adsorption reduction occurs

  35. Macroreticular Resins • Macroreticular resins have an adsorption of BTEX of about: • 350 mg BTEX/1000 mg of adsorbent *(Lin (1999)) • Can adsorb and desorb BTEX for 42 cycles before a 10% reduction in adsorption

  36. Silica Aerogels (SAG) • Hydrophobic material that has low density (0.3-0.05 g/cm3), high porosity, and high surface area (500- 1000 m2/g). SA can be used up to 14 cycles!

  37. Incineration Results • From Pro/II, it was calculated how much fuel gas (methane) will be needed to fully incinerate the acid gas stream at 1500°F by using a Gibbs reactor. • These calculations were based from on the following field data:

  38. Flame Temperature Verification • We took initial and final moles from Pro II. Reaction was carried while keeping the vent gas temperature at 1500°F. • Pro II results agreed with field data within 1.3% margin of error. • Pro II results agreed with hand calculations within .64% margin of error.

  39. Excess Air Limits • The Limit of excess is such that the mole percent of oxygen released to the atmosphere must be between 1-3% (Lewandowski, 2000). • Lower limit due to formation of CO below 1% O2 • Upper limit exist to reduce formation of NOx which occur above 3% oxygen • This data is backed by Ignacio plant data with O2 level of 2% in outlet stream

  40. Flame Temperature VOC • The flame of incinerator must be risen to a temperature, Auto Ignition Temperature, high enough to combust VOC’s: • In order to incinerate at this temperature, long residence times in incinerator must be used • A common rule of thumb for 99% incineration efficiency at .5 seconds is to add 400°F onto AIT. * (Lewandowski, 2000).

  41. Fuel Cost **Cost of Methane at $5/MMBtu**

  42. SAG Adsorption Process Column 1 To Amine Unit/ Oxidizer Acid Gas/Raw Gas Column 2 To Design One tank opened while the other is closed, and they will run for 12 hr periods. From the columns, the BTEX can be removed by using three different designs. These columns could be used up front of amine unit or in Acid Gas.

  43. Comparison of Two Designs Removing the BTEX present in the columns by blowing air through the columns. Instead of burning the air/BTEX stream, run the stream through a condenser, and then pass it through a flash.

  44. Activated Carbon Acid Gas • Activated Carbon cost $4 per kg. • Used Pro-II Results from Milagro Type Plant • This design would have an additional cost of $191,000 • In order for a saving of $100,000 to be reached price would have to be reduced to $1.15 per kg • 71% discount needed

  45. Silica AerogelsAcid Gas • Silica Aerogels cost $37 per kg from Cabot. • Used Pro-II Results from Milagro Type Plant • This design would produce a savings of $76,000 • In order for a saving of $100,000 to be reached price would have to be reduced to $34 per kg • 8% discount needed

  46. Macroreticular ResinsAcid Gas • Macroreticular resins cost $43 per kg from Dow Chemical. • Used Pro-II Results from Milagro Type Plant • This design would produce a savings of $61,000 • In order for a saving of $100,000 to be reached price would have to be reduced to $35 per kg • 19% discount needed

  47. Conclusions from Adsorption • There exist a saving of $303,000 in reducing the flame temperature from 1500°F to 1350°F. • This savings can then be used to design adsorption columns to remove BTEX. • Out of all the adsorbents studied silica aerogels proved to be the best adsorbent on the basis of savings and reduced cost.

  48. Ionic Liquid Background • Ionic liquids can be used to remove carbon dioxide. • The expense of using these liquids will be examined in comparison with that of the amine unit.

  49. Amine Unit Cost

  50. Ionic Liquid Conclusion

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