Design finalreport2

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Design finalreport2

  1. 1. Chemistry Ammonia act as a weak base in water where it dissociates to form ionized ammonium as shown in Equation (1). In order to remove ammonia we need to shift the reaction left to yield more unionized ammonia, which can be easily liberated out of water. Consequently we can increase the pH to make this shift occur and as shown on Figure 995, that for a pH above 10 almost 90% of the ammonia exists as the desired unionized form and at pH 11 almost 99% of the ammonia exist as the desired form. At pH less than 10, unionized ammonia content decreased significantly, thus for a robust operation it is desirable to keep the pH above 11. (1) Figure 995. Ammonia Dissociation at Different pH (Gay, 2009) Pilot Plant Study A pilot stripping column is constructed to simulate the ammonia stripping process. The data obtained showed significant ammonia removal at pH 10 and greater and at pH 10 and greater less than 20 ppm of ammonia is left in the treated stream.
  2. 2. Mass and Energy Balance Mass Balance The backwash system contain complex system that allows different modes of operation to regenerate the backwash sand filter without causing any down time. For different modes of operation, flow path and flow rate are altered resulting difficulty to perform mass balance. However the backwash system is designed to meet the objective of delivering 10 m3 /hr of feed to the stripping column and thus simplified process shown in Figure 996 is used to perform mass balance. Table 994. Mass Balance Table Mass Balance (kg/hr) Storm Water Heated Feed Alkaline Stream Air Air + NH3 Water Sulfuric Acid Product NH3 10 10 0 0 10 0 0 0 H2O 9964 9964 0.25 0 0 9964 0.25 9965 NaOH 0 0 0.25 0 0 0.25 0 0.25 H2SO4 0 0 0 4268 4268 0 0.25 0.25 Air 0 0 0 0 0 0 0 0 Sum 9974 9974 0.5 4268 4278 9964.25 0.5 9965.5 The hand calculated mass balance is shown in the Table xxx. Storm water contains ammonia and trace amount of other impurities which may yield unexpected reaction and affect the result of this study. However, a pilot plant study was conducted by Yara, and no unexpected reaction was observed and the design objective of less than 20 ppm of ammonia in the product stream was achieved. Consequently, it is safe to assume impurities pose negligible effect on the removal of ammonia. Under the condition that the storm water is assumed to contain only ammonia and no unexpected reactions would occur in the process, the mass balance is significantly simplified and the hand calculated data Table xxx replicated the data calculated using HYSYS, Figure 998. Energy Balance Energy balance was performed on the over all system and the system has a net of 540 kW consumption. This system was assumed to operate under adiabatic, 100% efficient with no unexpected reactions. In order to consider these factors we will have to consult distributors to find the type of insulation, the brand of pumps and perform a more detailed reaction analysis. These details are recommended for future work but are not included in the scope of this design.
  3. 3. There are three types of energy conversion considered and they are shown in Table xxx below. The three types of energy include chemical, electrical, and thermal energy conversion. The chemical energy is associated with enthalpy of dissociation, via when solid NaOH dissociates in water heat is produced. Enthalpy of dissociation calculation was performed and a sample calculation is shown in Appendix A. The electrical energy are associated with running the pump. The thermal energy is required to bring up the temperature of storm water and air feed. When comparing chemical, electrical and thermal energy transfer, heating yield the most significance and enthalpy of formation is almost negligible. The total energy balance yield a deficit of 540 kW for an adiabatic, 100% efficient system with no unexpected reactions. Table xxx. Energy Balance Table Energy Balance ΔHf (kJ/mol) Amount (mol/hr) Energy Input (kW) -44.5 6.25 -0.077 -206.96 2.54 -0.15 3.62 10 0.010 Pumps 40 horsepower 29.8 29.8 Cp (kJ/kg*K) ∆T Air 4268 kg/hr 1.005 40 47.66 Storm Water 9974 kg/hr 4.179 40 463.13 Sum 540.40 Sample Calculation: Enthalpy of Ammonia Dissociation at Standard Condition Table xxx. Enthalpy of Formation (Politechnika Gdanska) Species ΔHo f (kJ/mol) NH3(aq) -80.29 H2O(l) -285.83 NH4 + (aq) -132.51 OH- (aq) -229.99
  4. 4. HYSYS Simulation Figure 996 HYSYS Simulation The process simulation was performed using AspenTech’s HYSYS 2006 version and the pfd is shown in Figure 996. The purpose of this simulation is to assess the feasibility of the chemistry of removing Ammonia to the desire level of less than 10 ppm. Control and backwash sand filtration systems are not included because HYSYS lacked the ability to perform these simulations. For the complete over view and discussion of the process please see the PFD section below. A decision tree was used to find a suitable simulation thermodynamic model is shown in the Apendix Figure 999. The decision tree has suggested Henry’s Law to be sufficient, however there is not such model in the ASPEN HYSYS 2006 version. Consequently we have chosen General NRTL model which complies with Henry’s law at low pressure and temperature, and thus was an ideal selection. As shown in Figure 996, the Storm Water feed is preheated to 60 degree Celsius and alkalized with NaOH to PH 11 and sent to the randomly packed stripping column. According to our chemistry, more than 99% of the ammonia will be in the unionized form at this temperature and pH and can be easily removed. The column packing and counter current air provides the disturbance required to break the suspension of the ammonia and water and thus liberating ammonia out of water. The resulting product stream contains almost no ammonia as shown in the simulation report, Figure 998.
  5. 5. PFD The complete process Flow diagram is shown below. Figure 997. Process Flow Diagram NaOH/H2SO4 Addition T-100 NaOH Mixing tank and T-300 H2SO4 are units used to add NaOH and H2SO4 respectively. Sodium Hydroxide is required to raise the pH of the storm water to shift the ammonia from ionized form to unionized form for ease of separation as discussed in the chemistry section above. The amount of NaOH required to alkalize 10 m3 of storm water to PH 11 is determined to be 0.4 kg/hr and amount of H2SO4 required to neutralize the same amount of sodium hydroxide is determined to be 0.5kg/hr. These results are based on following equations (2) and (3) and the sample calculation for the amount of NaOH required is shown in Appendix A. (2) (3) From our research that PH 10 is sufficient to achieve our design objective, however at pH 10 the process are not robust for fluctuation and may result ineffective removal of ammonia when pH is below 10. Since these
  6. 6. calculations show that the amount of acid and base required is small and thus it is economically sustainable to bring pH to 11 and make the process more robust to fluctuations. In reality, because the predominant importance of maintaining desired pH, pH controls are implemented to prevent undesired pH change and the details will be discussed in the process control section.
  7. 7. Figure 998. HYSYS Simulation Report
  8. 8. Figure 999. EOS Flow Table (Hamid, 2007, p. 8)
  9. 9. Hazard and Operability Study The relative risk of a deviation was defined as the product of the frequency and consequence. Both frequency and consequence were ranked relatively on a scale of 1-4, where 4 represent a frequent deviation or severe safety concerns. Frequency rating is based on Ulrich, where likeliness of incident for common plant units is listed. Consequence rating is based on human judgment with reference to Material Safety Data Sheets listed in Appendix D. The maximum relative risk achievable on this scale is 16. The 3D diagram of the relative risk graph is shown below in Figure 995. Figure 995. HAZOP Heat Exchanges Stripping Column NaOH Mixing Tank Sulfuric Acid Mixing Tank Backwash Sand Filter Pumps Piping 0 2 4 6 8 10 12 RelativeRisk HAZOP
  10. 10. HAZOP analysis identified some severe problems such as heat exchanger subject to no flow, backwash sand filter subject to high flow or high pressure, stripping column subject to high temperature etc. Most these problems were identified as the result of pressure build up from abnormal flow condition. Thus flow meter and pressure relief should be installed to provide methods of controlling flow and mitigate the risk of pressure build up. For detailed HAZOP and list of recommendations please refer to Table 996. HAZOP Decision Table in Appendix E. Table 996. HAZOP Decision Table Company: Yara HAZOP Team: Ahmed Agina, Hoss Agina, Jian Gu, Jingjin Chen Process: Ammonia Absorbtion Drawing: Ammonia absorbtion column Date: March 10, 2014 Item: Heat Exchanger Process Parameter Guide word Possible Causes Possible consequences Freque ncy Consequ ence Ri sk Recommendati on Storm water flow into heat exchanger No Frozen Pipe Pressurized pipe (steam generation) 4 3 12 Implement flow control Plugged Pipe Pressure relief valve Pump malfunctioned Higher Improper flow control Insufficient removal of ammonia 3 2 6 Lower Improper flow control Pressurized pipe (steam generation) 3 3 9 Implement flow control Cavitation in pump Pressure relief valve Heated oil flow into heat exchanger No Pump malfunctioned Pressure build up 1 3 3 Pressure relief valve Pressure Higher Storm water flow below normal Pipe rapture hazard 3 3 9 Flow rate meter for monitoring Storm water flow Pump Caviation Lower Leakage None 1 0 0 None Pump malfunctioned Temperature Higher Storm water flow below normal Pressure build up 3 3 9 Flow rate meter for monitoring Storm water flow Lower Oil flow rate below normal Poor Ammonia removal 1 2 2
  11. 11. Item:Absorptio n Column Storm water flow into column No Frozen Pipe None 4 0 0 Flow rate meter for monitoring Storm water flow Plugged Pipe Pump malfunctioned Higher Improper flow control Insufficient removal of ammonia 3 2 6 Flow rate meter for monitoring Storm water flow Flooding/Leaking Storm water solution Lower Improper flow control None 3 0 0 Flow rate meter for monitoring Storm water flow Air rate into column No Pump malfunctioned Poor ammonia removal 1 2 2 Preform regular lab test for the product Pressure Higher Improper flow control Flooding/Leaking Storm water solution 3 1 3 Flow rate meter for monitoring Storm water flow Lower Leakage None 1 0 0 Pump malfunctioned Temperature Higher Storm water flow below normal Incrustation 3 4 12 Flow rate meter for monitoring Storm water flow Lower Heat exchanger malfunctioned Poor ammonia removal 1 2 2 Preform regular lab test for the product Item:NaOH mixing tank Storm water flow into column No Frozen Pipe None 4 0 0 Plugged Pipe Pump malfunctioned Higher Improper flow control Overflow 3 2 6 Flow rate meter for monitoring Storm water flow Improper NaOH mixing Lower Improper flow control None 3 0 0 NaOH Flow No PH control malfunctioned Poor ammonia removal 3 2 6 Preform regular lab test for the product Pressure Higher Improper flow control Flooding/Leaking NaOH solution 3 3 9 Flow rate meter for monitoring Storm water flow
  12. 12. Lower Leakage None 1 0 0 Pump malfunctioned Temperature Higher Heat exchanger malfunctioned Incrustation 1 4 4 Clean the tank peroidically Lower Heat exchanger malfunctioned None 1 0 0 Item:H2SO4 mixing tank Storm water flow into column No Frozen Pipe None 4 0 0 none Plugged Pipe Pump malfunctioned Higher Improper flow control Overflow 3 2 6 Flow rate meter for monitoring Storm water flow Improper H2SO4 mixing Lower Improper flow control None 3 0 0 H2SO4 Flow No PH control malfunctioned Basic solution/bad product 3 3 9 Preform regular lab test for the product Higher PH control malfunctioned Acidic solution/bad product, PH induced corrosion 3 3 9 Preform regular lab test for the product Lower PH control malfunctioned Basic solution/bad product 3 3 9 Preform regular lab test for the product Item: Back wash sand filter Storm water flow into filter No Frozen Pipe None 4 0 0 None Plugged Pipe Pump malfunctioned Higher Improper flow control Pressure build up 3 4 12 Flow rate meter for monitoring Storm water flow Lower Improper flow control None 3 0 0 None Percipate removal Lower Sand was not regenerated in time crustation/ Pump damages 2 4 8 Strict maintenance scheduel Pressure Higher Storm water flow above normal Equipment rapture hazard 3 4 12 Flow rate meter for monitoring Storm water flow Lower Leakage None 1 0 0 None Pump malfunctioned Temperature Higher Heat exchange malfunctioned None 1 0 0 None
  13. 13. Lower Heat exchange malfunctioned None 1 0 0 None Item: Pumps Storm water flow into pumps No Frozen Pipe Cavitation in pump 4 2 8 Implement flow control Plugged Pipe Pump malfunctioned Pressure Loss Pump was not primed Pump Caviation 1 2 2 Flow rate meter for monitoring Storm water flow High Excess friction from blocked pipe Pressure build up 4 3 12 Pressure relief valve Control valve malfunctioned Temperature Higher Heat exchanger malfunctioned Cavitation in pump 1 2 2 Flow rate meter for monitoring Storm water flow Item: Pipe Blockage No Frozen Pipe Pressure build up 4 3 12 Implement flow control Plugged Pipe Pressure relief valve Pump malfunctioned Precipatation on the pipe wall Signific ant Incrustation Pressure build up 2 3 6 Replace pipe if necessary Corrosion Signific ant failed PH control Leakage 3 3 9 Use Polyethylene or Fiber glass pipe Steam generation Signific ant Flow varation Pressure build up 3 3 9 Implement flow control Heat exchanger malfunction Pressure relief valve
  14. 14. Sulfuric Acid S2SO4
  15. 15. Sodium Hydroxide (NaOH)
  16. 16. Ammonium Hydroxide (NH4OH)
  17. 17. Appendices Sample Calculation: amount of NaOH required to alkailize 10 m3 /h storm water to pH 11 M http://www.engr.usask.ca/classes/CHE/422/notes/ClassOrientation-Final%20Report%20Jan%202014.pptx http://www.engr.usask.ca/classes/CHE/422/notes/2011/guideforreport.doc

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