Sustainable Project 1 Report Final2

LAMAR UNIVERSITY

DAN. F. SMITH DEPARTMENT OF CHEMICAL ENGINEERING

SPRING 2016

Sustainable Process Engineering – Project 1

Date: 02/16/2016

SUBMITTED BY:

Dan Philip Fernandes L20368579

Mozammel Mazumder L20261213

Tejus Mane L20359647

Sustainable Process Engineering pg. 1

TABLE OF CONTENTSProject Statement........................................................................................................................................31) Introduction and Background.........................................................................................................4

1-1) Benefit of choosing a quencher over a heat exchanger...............................................41-2) Types of Quencher.......................................................................................................................4

1-2-1) Spray Towers........................................................................................................................51-3) Types of Quenchers.....................................................................................................................71-4) Spray Nozzles.................................................................................................................................71-5) Process Description.................................................................................................................... 8

2) Mass and Energy Balance of Quench Tower.............................................................................82-1) Corresponding changes in nozzle pressure...................................................................102-2) Residence/Quench Time and Evaporation Time........................................................112-3) Quench water requirements when flue gas conditions change............................12

A) Flue gas temperature rises to 570 degrees celsius...................................................12B) Flue gas temperature flowrate decreases to 10,000 Nm3........................................13C) Flue gas temperature temperature decreases by 50 degrees celsius...............13D) Flue gas moisture increases by 10%...............................................................................14

The final scenario is to see the changes in the system which is an increase in 10% of moisture content in the flue gas. Increasing the moisture content will increase the mass flowrate as there is more water vapor flowing through the flue gas, therefore the new flue gas flowrate becomes 2612.2 kg/hr.....................................................................143) Quench Tower Design.......................................................................................................................15

3-1) Heat Transfer Calculations....................................................................................................153-2) Mass Transfer Calculations...................................................................................................163-3) Quench Tower Design specifications................................................................................173-3) Quench Tower Contact Length............................................................................................18

4) Controller Design of Quench Tower...........................................................................................194-1) Simulating the quench tower...............................................................................................204-2) Dynamic Simulation of quench tower..............................................................................224-3) Testing the controllers............................................................................................................254-4) P&ID Design and Control Strategy.....................................................................................27

4-4-1) Process Control and Normal Operations...............................................................294-4-2) Emergency Plant Trip System and Safety Features..........................................304-4-3) Startup of the Quench Tower......................................................................................304-4-4) Shutdown of the Quench Tower................................................................................30

5)Nozzle Design........................................................................................................................................ 315) Conclusion............................................................................................................................................. 326) References............................................................................................................................................. 33


Project Statement

A spray quenching tower need to quench flue gas (inlet condition: flowrate= 20000 Nm3, 16.1 m/s, p=1 atm) from 550 oC to 200 oC within 1 second. The water particle should be no more than 75 µm under normal condition. The composition of the flue gas (weight %) is given below

H2O 9.39N2 65.81O2 10.72SO2 0.01CO2 12.97AR 1.11

1) Conduct mass and energy balance of the flue gas and spray water system. If the inlet temperature is increased by 20 oC, what kind of change be the spray system make? If the inlet flue gas flow rate is decreased to 10000 Nm3, how should the spray system respond? Any other scenarios?

2) The flowrate, flue gas temperature, local moisture level, inlet water temperature, air temperature all changes. The control strategy should be able to respond fast, and control the temperature accurately. Develop a good P&ID of the aforementioned system (use Microsoft visio).

3) Provide the key control parameters.4) The calculation should be documented in EXCEL


1) Introduction and Background

Quencher is the sudden cooling of a process fluid like gas. It can also be used for removing impurities and avoiding side reactions. This rapid cooling is used by many essential applications in the petrochemical industry. Cooling by liquid quenching is essentially accomplished by introducing hot gases in a liquid contacting device.

The principal of operation is simple but difficult to design as it incorporates heat and mass balances to be taken for a liquid drop and the entire column. The rate of evaporation depends on the heat and mass transfer, the drop surface area and the relative motion of drops in the gas. The mechanism of operation is that when a liquid evaporates the energy necessary to vaporize the liquid comes at the expense of the hot combustion gases, which results in the reduction of the gas temperature. If the operation is adiabatic, the gas leaves the quencher saturated with water vapor. The following background information will go over the design that is designated by the project statement.

1-1) Benefit of choosing a quencher over a heat exchanger

For the same amount of cooling a larger heat exchanger is required. Direct contact is more efficient than indirect contact as in the case of a heat exchanger.

1-2) Types of Quencher

There are three types of quenchers:

1. Spray Tower2. Venture scrubbers3. Packed Towers

1-2-1) Spray Towers

Spray towers are gas liquid contacting empty cylindrical vessels made of steel or plastic and nozzles that spray liquid in the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed downward from one or more levels. This flow of inlet gas and liquid in the opposite direction is called countercurrent flow.

To provide a large surface area for the gas liquid contact and to maximize the number of fine droplets, many nozzles are provided in the contact area of the tower.


Figure 1 - Example of spray quench tower


LIQUID INLET

GAS OUTLET

In a spray tower design, a smaller droplet size or a larger liquid to gas ratio (L/G). For this project, a droplet size of 75 microns is considered.

1-3) Types of Quenchers

1. WET WALL QUENCHER: It uses a large amount of excess liquid which results in a wetted vessel wall. It provides a robust system for the process of quench liquid recovery from the gas stream and the optimization of the recirculation. Excess water (10% or more) is used to assure gas cooling and all internal walls are wetted.

2. DRY WALL QUENCHER: It requires all spray drops to be completely evaporated without any liquid touching the wall. Dry wall quencher is mostly used for very large flows. The spray nozzles are usually directed downstream

The process design concept is very important factor in choosing a wet wall or dry wall system. Wet wall system must include hardware to remove and manage the large excess of quench liquid. Smaller diameter systems favor wet wall configuration. A dry wall system requires an extremely small drop size to assure all drops evaporate before they reach the wall.

This project requires the use of a dry wall quencher on account of a larger gas flow and smaller droplet diameter of 75 µm, as well as due to the corrosive nature of the flue gas.

1-4) Spray Nozzles

It is a device which facilitates the formation of spray. The dispersion of a liquid as fine droplets of spray is called atomization. Spray nozzles are used to increase the liquid surface area so as to enhance the evaporation. It is a device which uses the pressure energy of the liquid to increase its velocity through an orifice and break it into drops.

Following are the types of spray nozzles:

1. Hydraulic Spray2. Gas(air) atomized spray3. Rotary4. Ultrasonic


The flue gas velocity for this project is at 16.1 m/sec, which is relatively fast, therefore the cone angle of the nozzle will be greatly reduced.

1-5) Process Description

Flue gas with an inlet flowrate of 20000 Nm3/h and pressure of 1 atm and temperature of 550 deg C enters the quench tower from the bottom. There is a flow transmitter and temperature transmitter to measure the flow and temperature of the flue gas respectively. The tower is operated at one atmosphere.

Quenching liquid is water, which is fed through pumps via a cooler through a nozzle arrangement at the top of the tower. Instrument air at a pressure of 45 psi is mixed with quench water in a two fluid nozzle. There is a pressure transmitter to measure the instrument air pressure. The temperature and flowrate of the water are measured by their respective transmitters. The flow of quench water is counter current to the gas flow. The contact area is 0.7 m and is located at the top of the tower. The flue gas is cooled as soon as it comes into contact with the water droplets and it is cooled in one second. The cooled flue gas exits from the top of the tower at 200 deg C. Since the tower is of dry design, the water is also evaporated and exits with the flue gas.

2) Mass and Energy Balance of Quench Tower

The flue gas specs that are provided in the project statement as used as the basis of our calculations for the mass and energy balance. The initial energy balance done is a simplified version, which takes into account some assumptions: no surrounding heat loss is taken into considerations, complete energy transfer is occurring from the flue gas to the quenching water which results into the water getting completely evaporated, and other thermodynamic calculations are simplified.

The energy balance is done before the mass balance in this case to find out the quantity of water that would be required to cool the flue gas in one second. The energy balance is done time independent at this point to just get a water flowrate after which the tower can be designed that can take into account the droplet diameter and quenching time.

To calculate the energy lost from the flue gas, the simple relation can be used:


˙Qfluegas=mc p ∆ t

The c p value at T = 550 degrees Celsius can be achieved by HYSYS and ∆ t=(550−200 )=350

The following table summarizes the inlet flue gas conditions as well as the thermodynamic values calculated by HYSYS under the Antoine fluid package as that is the closest to ideal, and the liquid flows are calculated as ideal.

Inlet Flue Gas ConditionsUnits

Pressure atm 1Temperature degrees Celsius 550Molar Flowrate Nm^3/hr 20000Mass Flowrate kg/hr 2.53E+04Superficial Velocity m/s 16.1Density kg/m^3 0.4196Mass Enthalpy kJ/kg -1813Cp kJ/Kg C 1.22

Table 1 - Inlet Flue gas conditions and thermodynamic properties

Thus we get ˙Qfluegas=¿ 10798830 kJ/hr

Before going further, an average temperature of water is chosen as 27 degrees Celsius as it is taken as the average temperature of water in Texas, hence it would require minimal cooling/heating in summer/winter.

In order to find the mass of water required to cool down the flue gas, the relationship can be set up as follows since the heat required to cool down the gas needs to be provided by the water. The water is required to be heated from 27 degrees to 200 degrees Celsius. The specific heat of water changes from liquid to gas form as well as the heat of vaporization needs to be taken into consideration.

Qfluegas=−Qwater=m cpliq (100−27 )+m c pvapor (200−100 )+m H vap

The specific heats were also calculated by HYSYS which is given in the table below:

Quench Water ConditionsTemperature degrees celsius 27Density kg/m^3 997.7Mass Enthalpy kJ/kg -1.58E+04Cp kJ/Kg C 75.75Hvap kj/kg 2257Cp H2oVap kJ/Kg C 1.879mHvap kJ/ hr 3,056,304.579


Table 2 - Quench Water Thermodynamic conditions

In order to simplify calculations, a starting value for water mass flowrate was defined in excel. The solver function is used to change the water flowrate until it matched heat required to cool the flue gas. The water mass flowrate is calculated as:

Water Mass Flowrate kg/hr 1,354.14

The water flowrate calculated here is under ideal conditions without accounting the mass transfer.

With a water flowrate, the outgoing mass flow of the gas out of the tower can be calculated:

Outlet Flue Gas Mass Flowrate kg/hr 26,644

A low water flowrate can be expected also for the quench water in the simulation due to water’s high capacity to absorb heat and smaller water droplet diameter.

2-1) Corresponding changes in nozzle pressure

In order to effectively deliver the changes in flowrate of water to the quench tower, the nozzle pressure is required to change appropriately. The given equations by spray systems company describes the relationship between desired flowrate and required nozzle pressure:

Q1

Q2=

( P1 )n

( P2 )n

where P1 and P2 refer to current nozzle spray pressure and new nozzle spray pressure in bars while Q1 and Q2 refer to current nozzle flowrate and desired nozzle flowrate in l/min. The relationship between the two can be seen in the following graph:


0 5 10 15 20 25 30 35 400123456789

f(x) = 0.00734096975420575 x² − 0.0386286101623002 x + 0.1382375209478

Required Flowrate vs. Required Nozzle Pressure

Series2Polynomial (Series2)

Desired Flowrate (l/min)

Noz

zle

Pre

ssu

re (

bar

)

Figure 2 - Required Flowrate vs. Required Nozzle Pressure graph

The control action will also require the nozzle pressure to change but the relationship is not completely linear, but rather second order polynomial.

2-2) Residence/Quench Time and Evaporation Time

In order to effectively control the outlet temperature to 200 degrees Celsius, the control spray system will require to change the max diameter of the drop to keep the effective quench time at 1 second.

The time to evaporate one droplet of water can be given by the following expression from BETE literature:

θ=EvaporationTime=ρl λ Do

2

8 k g ∆Twhich can be calculated using the following data:

Density kg/m^3 996.1Conductivity J/m s K 0.614Hvap J/kg 2257

Table 3 - Evaporation Time data


Do and delta T refer to the diameter of the water droplet in m and LMTD respectively. This will change based on water flowrate and flue gas conditions which will be calculated in the case studies.

The quench time which can be referred to the residence time is calculated by the following formula developed by Hardison of UOP Air Correction Division

t=quench time= droplet radius0.123∗(Gas temperature−Droplet Temperature )

The project requires quench time to be no more than 1 s, therefore the required droplet diameter can be back-calculated (using goal seek)

For the base case where water flowrate required is 1,354 kg/hr the following quench and evaporation time are found:

Residence Time required to cool sec 1Diameter of water droplet microns 32.16482165Evaporation time of 1 drop (thetha) sec 2.66314E-09

2-3) Quench water requirements when flue gas conditions change

The project requires seeing the changes in the quench water requirements when the inlet flue gas temperature increases to 570 degrees Celsius, when the flue gas flowrate decreases to 10,000 Nm3 , when the flue gas temperature falls to 500 degrees Celsius and if the flue gas moisture level increased by 10%. The changes can be seen in excel document with the changes modified to give the corresponding water flowrate .

A) Flue gas temperature rises to 570 degrees celsius

With the temperature of the flue gas rate increased, a corresponding increase in water flowrate is observed as expected, as well as a small droplet size is required for rapid evaporation.

New Flowrate when flue gas increases by 20 degrees celsiusMass Flowrate kg/hr 1431.524393

Required Nozzle Pressure atm 3.34


Residence Time required to cool sec 1

Diameter of water droplet microns 33.4

Evaporation time (thetha) sec 2.68E-09

B) Flue gas temperature flowrate decreases to 10,000 Nm3

Similarly the flowrate is changed is 10,000 Nm3 and goal seek is used to find the new quench water flowrate and water droplet sizesince our equations do not change.

New Flowrate when flue gas flowrate decreases to 10,000 Nm^3 (446.1 kmol/hr)

Mass Flowrate Kg/hr 653.57Required Nozzle Pressure atm 0.61


Diameter of water droplet microns 33.4Evaporation time (thetha) sec 2.68E-09

Notice that the droplet size is not required to be changed since we are not dealing with a change in temperature but rather a change in flowrate.

C) Flue gas temperature temperature decreases by 50 degrees celsius

Another scenario labeled as case 3 requires the changes in spray system required when the flue gas temperature drops 50 degrees Celsius.

Spray system changes when flue gas temperature is 500 degrees celsius




Diameter of water droplet microns 29.1Evaporation time (thetha) sec 2.62e-9

D) Flue gas moisture increases by 10%

The final scenario is to see the changes in the system which is an increase in 10% of moisture content in the flue gas. Increasing the moisture content will increase the mass flowrate as there is more water vapor flowing through the flue gas, therefore the new flue gas flowrate becomes 2612.2 kg/hr.

Spray system changes when flue gas moisture increased by 10%



Diameter of water droplet microns 32.1Evaporation time (thetha) sec 2.66e-9

This method does not take into account changes in mass and heat transfer due to the increase in moisture. A rudimentary way to calculate the required residence time is

t=residence time=50∗√ X – from Mass Transfer Operations

X refers to the mass moisture content of the medium required to be dried, but this is usually used in calculating drying time for solids. For this case, it can be attempted by assuming our X to be the moisture content of the flue gas:

X= mass of watermas sof dry gas

= 2612.222915.269

=0.11

t=50∗(0.11)0.5=16 sec

A residence time of 16 seconds far exceeds the project requirements.

3) Quench Tower Design


3-1) Heat Transfer Calculations

In order to find the heat transfer coefficient between the quench water and surrounding gas that can then be used to calculate the amount of energy transferred per second that will influence the physical design of the quench tower.

To describe the mass transfer operations occurring, the Prandlt number can be calculated with fluid properties of the quench water (calculated by HYSYS)

Pr ¿c p μk

Similarly Reynolds is calculated by:

ℜ¿ Dvν

With the two properties, the Nusselt number can be calculated using the following correlation:

Nu=2+0.6 R e0.5 Pr0.33

From Nusselt’s number, the heat correlation and total energy transferred by a 75 micron droplet can be calculated:

Nu=hDk

˙Q=Nu∗(−π D2 kc)∗( ∆ T

D)

The following table summarizes the results of the calculations:

Heat Transfer CalculationsArea of droplet 1.77E-08 m^2Prandlt Number 105.0136808Cp 75.75 kJ/Kg CCp 75750 j/kg KViscosity 0.8512 cPViscosity 0.0008512 Pa S


Reynolds Number 175.5345395Nusselts 39.4461714h (heat transfer coeffecient) 1.17E-06 W/(m^2 K)Reynolds Number 175.5345395

Table 4 - Summary of heat transfer calculations

Since the amount of energy required to cool the flue gas is known from the energy balance, therefore the number of droplets required to be sprayed within one second is known by dividing the heat transfer of droplet by total heat transfer required within one second.

Total number of drops required to cool down gas 1.30255E+13Total area of drops in m^2 2.30E+05

Table 5 - Area of water droplets required

3-2) Mass Transfer Calculations

For calculating the mass transfer coefficient between the droplet and surrounding gas, the diffusion coefficient needs to be calculated which is calculated by the following formula derived by Analysis of vapor droplet paper from University of Shangai:

DiffusionCoeffecient=D=435.7 T23

p (V a

13+V b

13)2 √ 1

M A+

1M B

x 10^-4

This allows us to find the Schmidt number from:

Sc= μρD

and using the similar correlation from heat transfer operations, we can calculate the Sherwood number by the Froessling equation:

Sh=2+0.552 R e12 S c

13

Using the definition of Sherwood number, the mass transfer coefficient is calculated:


K=Sh x DL

The following tables summarizes the mass transfer calculations:

Temperature of mass transfer (K) 300.15 KPressure (atm) 1 atmMolar Volume of Water (cm^3/mol) 18 cm^3/molMolar Volume of flue gas (cm^3/mol) 66491897.04 cm^3/molMolecular Weight of Water (g/mol) 18 g/molMolecular Weight of Gas (g/mol) 27.9 g/molD (Diffusivity Coeffecient) 3.55169E-06 cm^2/sD (Diffusivity Coeffecient) 3.55169E-10 m^2/sViscosity 0.8512 cPDynamic Viscosity 0.0008512 Pa SSchmidt Number 2396.60847Sherwood Number 108.3812051K (Mass Transfer Rate) 5.13E-04 m/s

Table 6 - Summary of Mass Transfer Operations

3-3) Quench Tower Design specifications

Quench tower design calculations are used to get the physical parameters of the tower. The following equations are from Quencher Design book.

Before calculating the diameter of the tower, the tower area needs to be calculated using:

A=W G

3600∗V s ρg

Where W G is the mass flowrate of the flue gas, Vs is the velocity of the gas and ρg is the density of the gas. From the area, the diameter can be calculated as:

D=(√ 4 πA )

In order to calculate the height of the tower, the superficial mass velocities of the liquid and gas streams need to be calculated as follows:


G=Gas Mass FlowArea

L= Liquid Mass FlowArea

The following relationship gives the length of the quench tower as described below:

Z0.5= q0.43

∗G0.8∗L0.4∗A

The tower design specs are calculated and summarized in the table below:

Quencher DesignArea 47.46 m^2D (Diameter of Tower) 6.1 mFlue Gas Mass Flowrate 2.53E+04 kg/hrWater Mass Flowrate 3757 kg/hrG 5.33E+02 kg/m^2 hrL 79.17 kg/m^2 hrZ^0.5 31.15Z (length of tower) 5.58 mV (Volume of tower) 264.89 m^3

3-3) Quench Tower Contact Length

From the previous section, the quench tower diameter was determined to be 6.1 meter. The diameter can be used to determine if it will be sufficient to cool down the flue gas within one second by finding the contact length required between the quenching water and flue gas.

The number of drop occupying a radial surface of tower diameter D and length of 75 microns (maximum droplet size) is

No . of droplets= Tower DiameterDroplet Diameter


The total number of droplets required for cool down of flue gas has been calculated in section 2-2 as 4.7e4 drops. This can determine the effective contact length required by following formula:

Contact Length= Number ofdrops perradial sectionof diameter DMax Droplet Diameter

The following table displays the results for effective contact length of the quench tower:

Number of drops per radial section of tower D 8.14E+04length per 75 micrometer (how many radial D can stack up per droplet diameter) 1.60E+08Length per micrometer 2.13E+06Contact length (meter) 2.13

The effective contact length is determined to be 2.13 m which is much less than the actual tower length calculated by the design equations therefore, the tower will be adequate for the cooling purposes expected from the project statement.

4) Controller Design of Quench Tower

To design the P&ID of the quench tower, it is required to determine the control strategy which will take into account disturbances such as flue gas temperature, flue gas flowrate, water temperature, local moisture level and outside air temperature. This would also require simulating the controllers in order to get the key control parameters of the controllers.

4-1) Simulating the quench tower

Before designing the control strategy of the quench tower, a steady state simulation is run before converting into dynamics mode. In this case, Aspen HYSYS is used to simulate the quench tower with the Antoine fluid package as it is closest to ideal gas model.


In order to simulate the quench tower, a simple flash tank is used to model the quench tower since only the outgoing gas temperature needs to be controlled against disturbances. The figure shows the PFD of the steady state simulation:

Figure 3 - PFD of steady state simulation

The flash tank has a liquid output labeled as purge but there is no flow through it since all our water will be evaporated in order to cool down the gas. The adjust function is used to determine the flowrate of quench water to make sure the outlet gas is 200 degrees Celsius. The following table summarizes the results of the steady simulation.


The results of the water flowrate from the simulation and the calculation done in the mass and energy balance are different. This difference can be accounted due to some of the non-idealities that are being taken into account by HYSYS as well as the oversimplification of the mass and energy balance in the excel calculations.


Figure 4 - Stream Summary

4-2) Dynamic Simulation of quench tower

After setting up the steady state simulation, the controllers are added to the simulation. In this case, our primary controller is a temperature controller that manipulates the flow of quench water to keep the outgoing gas temperature constant. In order to take account for some of the disturbances and for efficient controlling of the system, a feedforward controller is added to the temperature controller with the flue gas flowrate acting as the measured disturbance. The control strategy is explained further in the P&ID design.

Figure 4-PFD of dynamic simulation

The temperature controller is set as direct acting since outgoing gas temperature and quench water flowrate have a direct relationship i.e. as temperature of outgoing gas increases, flowrate of water increased by moving the valve actuator of VLV-100 upwards. TIC-100 is a PI controller rather than a PID since time is sensitive for this operation therefore adding a derivative term could cause the valve to chatter.

In order to get effective control of the tower, the controllers have to be tuned against the disturbance such that the response is quick and steady. First, the temperature controller is tuned, while the feedforward controller is set on manual. The temperature controller is tuned by manipulating the Kc (gain) and the Ti (integral time). Figure 3 represents the final tuning parameters of the temperature controller TIC-100.


Figure 5- Control Parameters for TIC-100

Similarly, the feedforward controller is tuned to give the following control parameters.


Figure 6- Feedforward controller parameters

The following table summarizes the key control parameters:

TIC-100Kc 10.9Ti 0.137

Feedforward Controller

Kc 0.8Tp1 18Tp2 1

Table 7 - Summary of key control parameters


4-3) Testing the controllers

The controllers are tested against the disturbances listed in the project statement. First the flue gas inlet temperature is increased to 570 degrees Celsius and the controller responds as shown in figure A. Table A shows the legend for the strip chart.

Strip Chart LegendProcess Variable ColorInlet Flue Gas Temperature

Orange

Inlet Flue Gas Molar Flowrate

Pink

TIC-100 Controller Output BlueQuench water mass flowrate

Purple

Outlet Gas temperature PV Green


Figure 7 - Controller Action against increasing the temperature of inlet flue gasFigure 8 Controller Action against increasing the temperature of inlet flue gas

Outlet Gas temperature Setpoint

Red

Quench Water Temperature

Cyan

Table 8 - Legend for strip chart in figure 8,9 and 10

It can be seen that the controller effectively counters this increase in flue gas temperature by quickly increasing quench water flowrate from 3757 kg/hr to 3981 kg/hr. All other parameters are unchanged while the outlet temperature barely increases.

Figure B below shows the controller response to a decrease in the molar flow rate of flue gas to 10000 Nm^3/hr.


Figure 8- Controller action for decreasing in flue gas flowrate Figure 9- Controller action for decreasing in flue gas flowrate

As the feedforward measures the disturbance before affecting the outlet flue gas temperature, it is able to effectively counter this decrease in flue gas flowrate by decreasing the water flowrate to 1878 kg/hr. The temperature of output gas is unchanged due to the feedforward controller counteracting this disturbance before it affected the process.

Finally figure 7 displays the controller action if the quench water temperature suddenly decreased to 22 degrees Celsius. From figure 7, it can be observed that changes in water temperature don’t affect the outlet flue gas as much and the controller is quick enough to respond to its fluctuation without having to add another controller to account for it.

4-4) P&ID Design and Control Strategy

Figure A displays the P&ID designed for the quench tower operation using Microsoft Visio. The following section describes the control strategy of the whole unit (not just the simplified simulation). The difference between the simulation and the P&ID is that an air cooler is added to cool the water in order to maintain its temperature at 27 degrees Celsius during hot summer months. This way, another temperature controller can account for the changes in air temperature and local moisture level by changing the fan speed. Emergency and safety feature have also been added to the P&ID.


Figure 10- Controller action against decreasing the water temperature to 22 degrees Celsius


Figure 71- P&ID of the quench tower

4-4-1) Process Control and Normal Operations

There are three controllers:1. TIC 101: It controls the outlet flue gas temperature by regulating the quench water

flow by sending a signal to the control valve FPV1012. FIC 101: It controls the flue gas flow rate disturbance by regulating the amount of

quench water flow by sending a signal to the control valve FPV101. This is a Feedforward controller.

3. SIC 101: It controls the temperature of the quench water by controlling the speed of the fan blades of the air cooler.

4. PIC101: Controls the instrument air pressure in the instrument airline.

Once the quench tower is started up and stabilized, monitor the outlet temperature of the flue gas at 200 deg C, by looking at TT 101 at the flue gas outlet. The controller TIC 101 is to be set in Auto mode with the SP set at 200 deg C. If the outlet temperature of the flue gas increases the controller TIC 101 will take corrective action by opening the quench water flow control valve which is an air-to-open (air failure-close). It is a direct acting controller.

If there is a disturbance in the flowrate of the flue gas inlet, the FIC 101 which is a feedforward controller will reject the disturbance by regulating the flow of the quench water flow control valve FPV101. Both output signals from the TIC 101 and FIC 101 are added in a logic controller which is an adder block (a part of the feedforward system), whose electrical output of 4 to 20 milliamps signal goes to the input/output convertor which converts it to a pneumatic pressure signal of 0.1 kg/cm2 to 1.2 kg/cm2 and controls the actuator of the control valve FPV101.The FF and FB control works on the same valve. It is given as the + symbol on the P&ID.

In order to control the inlet water temperature of the quench, the temperature transmitter on the water line sends the PV signal to the SIC controller whose set point is inputted. If the inlet water temperature rises, the SIC 101 takes action by increasing the speed of the fan which sucks in more ambient air for the cooler fins which helps cool the water.

Instrument air controller PIC 101 which belongs to the valve PPV101 should be put in auto with the set point of 45 psi.

The valves HPV102 is air to close (air fail open) and Valve HPV 103 is air to open (air failure to close). If, by any chance there is a failure of these valves, there is a relief valve to handle the load of the flue gas if the pressure rise to 2 atmospheres (set point of the relief valve).


4-4-2) Emergency Plant Trip System and Safety Features

In the case of an instrument air supply failure, quench pump failure or total power failure to the plant. The vent valve HPV102 (solenoid valve) will open and the flue gas outlet valve HPV103 will close. This is to safe guard the downstream equipment from overloading and damage due to the hot temperature of the flue gas.

4-4-3) Startup of the Quench Tower

After a turnaround, before starting the plant, ensure there is power supply and instrument air, then stroke check the control valve. Keep the vent valve open, and close the flue gas outlet valve HPV103.Check that all the manholes are closed and all drains are closed. Check whether all transmitters are in working condition.Ensure that there is sufficient water in the tank. Then open the suction valve of the centrifugal pump. Check the lube oil level of the pump, and the shaft for free rotation. Then start the pump, and observe the pump amps. Slowly open the discharge valve of the pump till the pump pressure reads 3 bar. Then start the air cooler fan.

Put TIC controller in auto and to set its respective set point at the corresponding load of the flue gas. Once the flue gas enters, the control valve FPV101 will open correspondingly to control the temperature of the flue gas at 200 deg C. Once you get 200 deg C, open the flue gas outlet valve HPV103 and close the vent gas valve HP102. The plant has now reached stabilization.

4-4-4) Shutdown of the Quench Tower

To shut down the quencher, first open the Vent valve HPV102. Then close the flue gas valve HPV103. Next open the spillback line of the pump and close the pump discharge valve. Then stop the pump. If the shutdown is for a turnaround, once the flue gas has stopped flowing open the drain valve of the quench tower to drain any liquid water accumulated during upset conditions or abnormal operation of the cooling tower during shutdown.

CAUTION: Care should be taken and ensured that the drain valve of the quench tower should be closed at all times during normal operations to prevent the flue gas from bypassing through it. Failure to comply would cause a serious injury or death to the plant operations personal.


5)Nozzle Design

An ideal atomizer should possess the following characteristics:1) Ability to provide good atomization over a wide range of liquid flow rates.2) Rapid response to changes in liquid flow rate.3) Freedom from flow instabilities.4) Low power requirements5) Capability for scaling, to provide design flexibility.6) Low cost, light weight, ease of maintenance, and ease of removal for servicing.7) Low susceptibility to damage during manufacture and installation.

We have chosen the plain orifice for our nozzle design. Here the atomization of a low viscosity liquid is most easily accomplished by passing it through a small circular hole. If the liquid pressure exceeds the ambient gas pressure by about 150 kPa, a high velocity liquid jet is formed that rapidly disintegrates into a well atomized spray. Since our liquid pressure exceeds the ambient pressure by 200 kPa, it is well suited.

The sprays produced by plain orifice atomizers have a cone angle that usually lies between 50 to 150. This cone angle is only slightly affected by the diameter and length/diameter ratio of the orifice and is mainly dependent on the viscosity and surface tension of the liquid and the turbulence of the issuing jet.

Spray nozzle design for Quench Tower TK101:

Type of nozzle selected: Plain-Orifice atomizers

Flow Number (FN): Flow rate,kg/sec (Pressure differential, Pa) ^0.5*(liquid density,kg/m3)

PLAIN ORFICE ATOMIZE DESIGN

Flow rate of water to nozzle

62.61667 kg/sec

Inlet pressure to nozzle 3 baroutlet pressure from nozzle

1 bar

liquid density 996.1 kg/m3

Flow Number 4.419025 m2

Table 9 - Flow Number Calculation


The spray angle from jet mixing theory of Abramovich is

= tanθ -10.13(1+ density of flue gas/density of liquid)

Flue gas density = 0.4196 kg/m3

Spray Angle θ = 7.40

Nozzle diameter = Reynolds number*viscosity of liquid/density*sqrt(2*(outlet press-inlet press)/density))

Reynolds Number 1413

Viscosity 0.8512 cp

Inlet pressure 3 bar

Outlet pressure 1 bar

Density of liquid 996.1 kg/m3

Diameter 0.060255 m6.5 cm

Table 10 - Nozzle Diameter Calculation

Nozzle diameter = 6.5 cm

5) Conclusion

With the given project statement, there has been various methods implemented in designing the column using simple mass and energy balances using excel as well going into some fluid dynamic calculations as well as mass transfer operations. The control scheme of the quench tower is designed using fundamental control operations such as PID controller and feedforward controllers. The quench tower is simulated to determine the controller parameters as well as come with a P&ID that takes into account all emergency systems as well start-up and shutdown operations.


6) References

-BETE Fog Nozzle, Inc. Spray Gas Quench Design Considerations. Greensfield: BETE Fog

Nozzle, n.d. Print.

-Grosshans, Holger. Evaporation of a Droplet. Rep. Lund: Lund U, 2012. Print.

-Lipp, Charles W., Thomas J. King, and C. P. Christenson. "A Fresh Look at Design of Spray

-Gas Quench Systems Using the Theory of Inventive Problem Solving (TIPS)." 21st Annual

-Conference on Liquid Atomization and Spray Systems (2008): n. pag. Web.

- Jiang, Lu, Yu Min, Tao Le-Ren, and Zhao Ji-Zhe. "Analysis of Vaporization of a Single

Water Drop and Study on Its Vaporization Time." 25.1 (2003): n. pag. Web.

-Niessen, Walter R. "Incineration Systems: Flue Gas Conditioning." Combustion and

Incineration Processes: Applications in Environmental Engineering. New York: M.

Dekker, 1978. 333-34. Print.

-PNR. Spray Engineering Handbook. N.p.: PNR, n.d. Print.

-"Quencher Design." Academia. N.p., n.d. Web. 5 Feb. 2016.

http://www.academia.edu/4216703/7_Quencher_Design

- Atomization and sprays (pg 106 and pg 296) -: Arthur H.Lefebvre

- Fluent 6.3 User guide Plain Orfice Nozzle:

http://aerojet.engr.ucdavis.edu/fluenthelp/html/ug/node824.htm


http://www.academia.edu/4216703/7_Quencher_Design

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