design finalreport2

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.

NH 3+H 2O↔NH 4+¿+OH−¿¿ ¿ (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.

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 0H2O 9964 9964 0.25 0 0 9964 0.25 9965NaOH 0 0 0.25 0 0 0.25 0 0.25H2SO4 0 0 0

4268 4268 0 0.25 0.25

Air 0 0 0 0 0 0 0 0

Sum 9974 9974 0.5426

8 42789964.2

5 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.

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)NaOH (s)+H 2O→Na+¿+OH−¿+H 2O ¿ ¿

-44.5 6.25 -0.077H 2SO4+2OH

−¿→SO42−¿+2H 2O ¿ ¿

-206.96 2.54 -0.15NH 3+H 2O↔NH 4

+¿+OH−¿¿ ¿3.62 10 0.010

Pumps 40 horsepower 29.8 29.8 Cp (kJ/kg*K) ∆T Air 4268 kg/hr 1.005 40 47.66Storm Water 9974 kg/hr 4.179 40 463.13 Sum 540.40

Sample Calculation: Enthalpy of Ammonia Dissociation at Standard Condition

NH 3+H 2O↔NH 4+¿+OH−¿¿ ¿

Table xxx. Enthalpy of Formation (Politechnika Gdanska)

Species ΔHof (kJ/mol)

NH3(aq) -80.29H2O(l) -285.83

NH4+

(aq) -132.51OH-

(aq) -229.99

ΔH=Σ ΔH f , products−Σ ΔH f reactants

ΔH o=(−132.51−229.99 ) kJmol

−(−80.29−285.83) kJmol

ΔH o=3.62 kJmol

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.

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.

NaOH (s)+H 2O→Na+¿+OH−¿+H 2O ¿ ¿ (2)

H 2SO4+2OH−¿→SO4

2−¿+2H 2O ¿ ¿ (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

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.

Figure 998. HYSYS Simulation Report

Figure 999. EOS Flow Table (Hamid, 2007, p. 8)

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.

No flow

Low Flow

Low pressure

Low Temp

0

2

4

6

8

10

12

Heat Exchanges

Stripping Column

NaOH Mixing Tank

Sulfuric Acid Mixing Tank

Backwash Sand Filter

Pumps

Piping

HAZOP

Rela

tive

Risk

Figure 995. HAZOP

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 ChenProcess: Ammonia

Absorbtion Drawing: Ammonia absorbtion column Date: March 10, 2014

Item: Heat Exchanger

Process Parameter

Guide word

Possible Causes Possible consequences

Frequency

Consequence

Risk

Recommendation

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

HigherImproper flow control

Insufficient removal of ammonia 3 2 6

LowerImproper flow control

Pressurized pipe (steam generation) 3 3 9


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


LowerOil flow rate below normal Poor Ammonia removal 1 2 2

Item:Absorption Column

Storm water flow into column No Frozen Pipe None 4 0 0


Plugged Pipe

Pump malfunctioned

HigherImproper flow control

Insufficient removal of ammonia 3 2 6


Flooding/Leaking Storm water solution

LowerImproper flow control None 3 0 0


Air rate into column No

Pump malfunctioned Poor ammonia removal 1 2 2

Preform regular lab test for the product

Pressure HigherImproper flow control

Flooding/Leaking Storm water solution 3 1 3


Lower Leakage None 1 0 0

Pump malfunctioned

Temperature Higher

Storm water flow below normal Incrustation 3 4 12


Lower

Heat exchanger malfunctioned Poor ammonia removal 1 2 2


Item:NaOH mixing tank

Storm water flow into column No Frozen Pipe None 4 0 0 Plugged Pipe

Pump malfunctioned

HigherImproper flow control Overflow 3 2 6


Improper NaOH mixing


NaOH Flow NoPH control malfunctioned Poor ammonia removal 3 2 6


Pressure HigherImproper flow control

Flooding/Leaking NaOH solution 3 3 9


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

HigherImproper flow control Overflow 3 2 6


Improper H2SO4 mixing


H2SO4 Flow NoPH control malfunctioned

Basic solution/bad product 3 3 9


HigherPH control malfunctioned

Acidic solution/bad product, PH induced corrosion 3 3 9


LowerPH control malfunctioned

Basic solution/bad product 3 3 9


Item: Back wash sand filter

Storm water flow into filter No Frozen Pipe None 4 0 0 None Plugged Pipe

Pump malfunctioned

HigherImproper flow control Pressure build up 3 4 12


LowerImproper 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


Lower Leakage None 1 0 0 None

Pump malfunctioned

Temperature HigherHeat exchange None 1 0 0 None

malfunctioned

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


Plugged Pipe

Pump malfunctioned

Pressure LossPump was not primed Pump Caviation 1 2 2


High

Excess friction from blocked pipe Pressure build up 4 3 12


Control valve malfunctioned

Temperature Higher

Heat exchanger malfunctioned Cavitation in pump 1 2 2


Item: Pipe

Blockage No Frozen Pipe Pressure build up 4 3 12Implement flow control

Plugged Pipe Pressure relief valve

Pump malfunctioned

Precipatation on the pipe wall

Significant Incrustation Pressure build up 2 3 6

Replace pipe if necessary

CorrosionSignificant

failed PH control Leakage 3 3 9

Use Polyethylene or Fiber glass pipe

Steam generation

Significant Flow varation Pressure build up 3 3 9


Heat exchanger malfunction


Sulfuric Acid S2SO4

Sodium Hydroxide (NaOH)

Ammonium Hydroxide (NH4OH)

Appendices

Sample Calculation: amount of NaOH required to alkailize 10 m3/h storm water to pH 11

pOH=14−pH

pOH=14−11

pOH=3

¿

¿M

qNaOH=qStormwater×¿

qNaOH=10m3

hr×1000 L1m3

×10−3molL×( 0.040kg

mol)

qNaOH=0.4kghr

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design finalreport2

Documents

energy balance energy

energy balance mass

ppm of ammonia

total energy balance

removal of ammonia

electrical energy

chemical energy

ammonia stripping process