INSTITUTE OF TECHNOLOGY İZMİR

FIRE HEATER ENGINEERING ECONOMICS & DESIGN

Ş. Selcen KARAKOÇ Instıtute of Technology İzmir B.Dilhan CAM Instıtute of Technology İzmir

1

ABSTRACT A fired heater is a direct-fired heat exchanger which uses the hot gases of combustion to raise the temperature of a feed flowing through tubes. The process fluid is first heated in the convection section preheat coil which is followed by further heating in the radiant section. In both sections heat is transferred by both mechanisms of heat transfer, radiation and convection. In this project a fired heater design, the natural gas and the 25% excess air were reacted in order to obtain efficient combustion heat for the system. The heat duty was found as 11.61 106kj/h . The temperature was 623 K and using 64 tubes.

2

1. INTRODUCTION 1.1 Fired Heater A heat exchanger is a device that is used for transferring thermal energy betweentwo or more fluids, between a solid surface and a fluid, or between solid particulates anda fluid, at diff erent temperatures and in thermal contact. Typical applications imply heating or cooling of a fluid stream of concern and evaporation or condensation of single- or multicomponent fluid streams.[1] A fired heater is a direct-fired heat exchanger that uses the hot gases of combustion to increase the temperature of a feed flowing through coils of tubes aligned throughout the heater.Fired heaters are used throughout hydrocarbon and chemical processing industries such as refineries, gas plants, petrochemicals, chemicals and synthetics, olefins, ammonia and fertilizer plants. Most of the unit operations require one or more fired heaters as start-up heater, fired reboiler, cracking furnace, process heater, process heater vaporizer, crude oil heater or reformer furnace. The functions of fired heaters in chemical plants are many ranging from simpleheating or providing sensible heat and raising the temperature of the charge to heating andpartial evaporation of the charge, where equilibrium is established between the unvaporisedliquid and the vapour. The mixture leaves the furnace in the form of a partially evaporatedliquid in equilibrium.Fired heaters are usually classified as vertical cylindrical or box-type heaters depending onthe geometrical configuration of the radiant section.[2] In the cylindrical-type furnace, the radiation section is in the shape of a cylinder with a vertical axis, and the burners are located on the floor at the base of the cylinder.Heat transfer to the tubes on the furnace walls is predominantly by radiation. The heatexchange area covers the vertical walls and therefore exhibits circular symmetry withrespect to the heating assembly. In the radiant section, the tubes may be in a circular patternaround the walls of the fire box or they may be in a cross or octagonal design which willexpose them to firing from both sides. The shield andconvection tubes are normally horizontal. Cylindrical heaters with vertical tubes (Fig. 1) are commonly used in hot oil services andother processes where the duties are usually small, but larger units, 100 million kJ/hr andhigher, are not uncommon.Heat transfer in the shieldsectionwill be by bothradiation and convection. The tubesizesusedwillnormally be between 75 and 150mm diameter.The tube size and number of passesuseddepend on the application and the processfluidflow rate.[3][4]

3

Cylindrical heaters are often preferred to box-type heaters. This is mainly due to the moreuniform heating rate in cylindrical heaters and higher thermal efficiency. Furthermore,cylindrical heaters require smaller foundations and construction areas and theirconstruction cost is less.

Figure 1.1.1: Illustration of fired heater with side view of top section Radiant Section: The radiant tubes, either horizontal or vertical, are located along the walls in the radiant section of the heater and receive radiant heat directly from the burners or target wall. The radiant zone with its refractory lining is the costliest part of the heater and most of the heat is gained there. This is also called the firebox. Convection Section: The feed charge enters the coil inlet in the convection section where it is preheated before transferring to the radiant tubes. The convection section removes heat from the flue gas to preheat the contents of the tubes and significantly reduces the temperature of the flue gas exiting the stack. Too much heat picked up in the convection section is a sign of too much draft. Tube temperature is taken in both convection and radiant sections. Shield Section: Just below the convection section is the shield (or shocktube) section, containing rows of tubing which shield the convection tubes from the direct radiant heat. Several important measurements are normally made just below the shield section.

4

Stack and Breeching: The transition from the convection section to the stack is called the breeching. By the time the flue gas exits the stack, most of the heat should be recovered and the temperature is much less. From a measurement point of view, this location places fewer demands on the analyzer but is much less desirable for the ability to control the process. Burner: Traditional premix burners on a process heater premix the fuel with the primary air which is inspired to the burner by the fuel gas flow. The pressure of the fuel gas supply is important since low gas pressure degrades performance. The primary air flow should be maximized without lifting the flame off the burner. Most of the air (as primary air) is delivered to the burner along with the fuel. Secondary air is introduced and adjusted with the registers.

Figure 1.1.2:Premix burner with products of combustion A correctly set burner, with good air-fuel mixing, produces the maximum flame temperature in a compact conical flame. The flue gas contains a minimum of oxygen together with levels of combustibles (CO and H2) in the 100 to 200 ppm range and a minimum of NOx. CH4 + 2O2 + 2.72N2 = CO2 + 2H2O + 2.72N2 + ppm CO + ppm H2 + ppm NOx Too much or too little secondary air gives poor combustion. A minimum excess air level is required for complete combustion but too much excess air reduces flame temperature and drops efficiency.[2],[3] Flame and Effective gas temperature Flame

and

effectivegas

accuratelydeterminedbeforeanalysis

temperatures of

the heat

5

are

keyvariablesthatneed

transfer in

the

to

be

radiantsection of

firedheaters.Flametemperature is the temperatureprovided by the combustion of a fuel. This temperaturedependsespicially on the calorificvalue of the fuel. A theoretical or ideal flametemperaturemay perfectmixing.

be

But

calculatedassumingcompletecombustion evenwhencompletecombustion

is

of

the

fuel

assumed,

and the

actualflametemperaturewouldalways be lowerthan the theoreticaltemperature. 1.2 Flash DrumPart In an equilibriumflash process, a feedstream is separatedintoliquid and vapor streams at equilibrium. The composition of the streamswilldepend on the quantity of the feedvaporized. The groupsincorporating the liquid and vaporflowrates and the equilibriumconstantshave a general significance in separation process calculations

Figure1.1.3: Flash drumseperation process In this project the vertical fired heater and flash drum design were done. For fired heater the specifications were identified for each segments and design parameters were calculated in terms of achieving desired process fluid outlet temperature.[3]After heated fluid exit from the heat exchanger it is feeded to flash drum for separation and then the vapor part was sent to distillation column.

Figure1.1.3: Layout of the system 6

2.CALCULATION Modeling of fired heaters is generally based on the two section, radiant and convection sections. When the fired heater was designed, we calculated within each section and four heat transfer elements. There are the process fluid, the flue gas, fluel gas and air. When fired heater was designed some important key variables serve help us determination of heater performance ; -

Heat exchange areas

-

Heat transfer rates to the process fluid

-

Process fluid and flue gas temperatures

-

Flame and tube skin or tube wall temperatures

-

Process fluid flow rate

-

Fuel flow rate and combustion

-

Process fluid pressure drop and the pressure profile in the heater and stack

2.1 Mass Balances

Figure 2.1: Boiling point diagram for methanol/Methyl oleate at 101325 Pa(chemcad result) The diagram used for to determine the temperature of the entrance and exit liquid and vapor stream. First of all the exit temperature was determined based on the chemcad result as 564.89 K and this is proved by checking the figure 2.1. Since the reboiler act like a tray therefore the leaving liquid composition would be in equilibrium with the leaving vapor composition. The methanol mole fraction in liquid was set as 0.003 and the vapor methanol mole fraction in equilibrium with liquid phase was found as approximately 0.24. 7

Figure 2.2: Equilibrium mole fractions for methanol/methyl oleate at 101325 Pa(chemcad result) In order to calculate the inlet liquid and outlet vapor, amount, temperature and composition, the mass and energy balance were done around the reboiler. The exit amount, temperature and composition for the reboiler was calculated for previous project by using chemcad program. V= The total amount at the exit gas phase from reboiler (kmol/h) L= The total amount at the entrance liquid phase to reboiler (kmol/h) W= The total amount at the exit liquid phase from reboiler (kmol/h) xM= Fraction of methanol at the exit liquid phase from reboiler. xB = Fraction of methyl oleate at the exit liquid phase from reboiler. yM= Fraction of methanol at the exit vapor phase from reboiler. yB= Fraction of methyl oleate at the exit vapor phase from reboiler. xLM= Fraction of methanol at the entrance liquid phase to reboiler. xLB = Fraction of methyl oleate at the entrance liquid phase to reboiler.

8

Material balance for methanol; L=V+W

(1)

L* xLM= V* yM+ W* xM(2) W= 37.962 kmol/h xM= 0.003 yM= 0.24

Energy balance for methanol; For the energy balance the sensible heat of vaporization and latent heat of the methanol and biodiesel were calculated. hL= Enthalpy of liquid entering the reboiler (j/kmol) hW=Enthalpy of liquid leaving the reboiler (j/kmol) HV= Enthalpy of the vapor leaving the reboiler (j/kmol) CpyM= Vapor specific heat capacity of methanol (j/kmol K) CpyB= Vapor specific heat capacity of methyl oleate (j/kmol K) CpM= Liquid specific heat capacity of methanol (j/kmol K) CpB= Liquid specific heat capacity of methyl oleate (j/kmol K) λM= Latent heat of methanol (j/kmol) λB= Latent heat of methyl oleate (j/kmol) qR=Reboiler duty (j/h) hW= xM(CpM (T-Tref))+ (1-xM ) (CpB (T-Tref))

(3)

Tref was set by considering the low boiling point component. The lowest boiling point compound is methanol as 337.6 K.

9

HV= yM(λM + CpyM (T-Tref))+ (1-xM ) (λB + CpyB (T-Tref))

(4)

At 564.89 K CpyM= 64501.38 j/kmolK CpB= 848026.59 j/kmolK λM= 3.802* 102 j/kmol λB= 7.18 * 107 j/kmol Since at 564.89 K methyl oleate was about to changing phase. Therefore while the leaving vapor enthalpy was calculated the sensible heat of vaporization of methyl oleate was neglected just latent heat of vaporization of methyl oleate was considered. In order to calculate leaving liquid enthalpy , just the methyl oleate sensible liquid heat was considered because at that temperature , the amount of liquid methanol could be neglected. hW= (1-0.003) (848026.59 j/kmolK (564.89 K-337.6K)) hW= 190597303.4 j/kmol HV= 0.24(3.802* 102 j/kmol + 64501.38 j/kmolK (564.89 K-337.6K))+ (1-0.24) *(7.18 * 107 j/kmol) HV= 71823997.17k/kmol qR= 11612.969*106 j/h L*hL+ qR= W*hW+ V*HV

(5)

If the equation (1) , (2) and (5) were calculated simultaneously and by usind the trial and error method by xLM assumed as 0.17; L = 120 kmol/h

10

While the xLMwas 0.17 by using fig 2.1 temperature was found as 365 K and h Lwas found as 1509532.8 j/kmol . In order to satisfy this result enthalpy of liquid entering the reboiler is calculated by using equation (3) . Again the liquid enthalpy of methanol was negligible because of the amount of methanol in liquid phase is too small at that temperature. At 365 K CpB=662563.7045 j/kmolK hL= (1-0.17) (662563.7045 j/kmolK (365 K-337.6K)) hL= 15078391.55 j/kmol

When this two results were compared , they were really closed to each other then the fallowing results can be considered as correct results. L= 120 kmol/h V=82.038 kmol/h XLM=0.17 2.2. The Energy Balance We consider at the above parameter but stack was negligible. Energy balance was calculated in the heater. Qin =Qout

(6)

Qin = Qrls + Qair + Qfuel + Qflu+Qfluid

(7)

Qout = Qu + Qlosses

(8)

There are source of heat input; the combustion heat fuel, Qrls and the sensible heat of the combustion air, Qair, natural gas ,Qfueland natural gas when applicable fluid,Qfluid ,heat flue gas from combustion Qflu. This heat input equal in the radiant section is absorbed by in the radiant QR , shield Qshld section and heat loss Qloss. Heat loss is by radiation through the furnace walls. The heat of radiant, QR includes radiation, Qr and convection, Qc.

11

Firstly Qin calculated, we set 25% air combustion with fuel gas. The composition of fuel gas were obtained from the literature and it is stayed in Table 2.1 after that by using these value the specific heat and ratio of fuel gas and air calculated as 12.9

by using

software program [5] . Different flow rate of air and fuel gas flow rate were tried to determine how much air and fuel gas need to heat process fluid. Methane and air combustion equation determine how much flue gas occurs from combustion. CH4 + 2O2 +2.72N2--- > CO2 + 2H2O+2.72N2

Table 2.2.1 The composition of natural gas and 25% excess air Air%

0,25

Fuel gas in

%

CO2

0,08234

CH4

0,8043

N2

0,7179

C2H6

0,0902

O2

0,0382

C3H8

0,0454

SO2

0,00188

N-C4H10

0,0032

H2O

0,15966

I-C4H10

0,002

N-C5H12

0,0002

I-C5H10

0,0004

N2

0,01735

CO2

0,0352

H2S

0,0009

12

Table 2.2.2 Flow rate of natural gas, air and flue gas from combustion

mfuel kmol /hr

mair kmol /hr

mflue kmol/hr

30,91003274

400

711,4235166

46,3650491

600

1067,135275

54,09255729

700

1244,991154

61,82006547

800

1422,847033

69,54757365

900

1600,702912

77,27508184

1000

1778,558791

85,00259002

1100

1956,414671

92,73009821

1200

2134,27055

100,4576064

1300

2312,126429

108,1851146

1400

2489,982308

115,9126228

1500

2667,838187

For the combustion process, flame temperature had to be determined. Flame temperature is the temperature attained by the combustion of a fuel. According to methane flame temperature is define theoretical flame temperature 2000K but actual flame temperature, 1855. A theoretical or ideal flame temperature may be calculated assuming complete combustion of the fuel and perfect mixing. But even when complete combustion is assumed, the actual flame temperature would always be lower than the theoretical temperature.[5] Qcombustion=ΣWi*ʃ Cp * dt

(9)

Qcombustion = Heat of combustion of fuel based on the gross calorific value. Wi = Mass of a flue gas component. The five components considered are CO2, N2, O2, SO2 and water vapour. Cpi = Molar heat of a flue gas component t1 and t2 = Initial and final temperatures. Qcombustion=711.42kmol/hr 27.95 kg/kmol 1.40 kj/kg K (1855K-298K) =43.6 10 6kj/hr Qlosses=5% Qcombustion [3]

(10)

13

Qlosses=2.18 106kj/hr Inlet temperature of process fluid fed at 365 K and boiling point of process fluid is 565 K so that out fluid temperature must be reach 623 K. The outlet fluid temperature is higher than boiling point of the process fluid temperature because of there can be some heat losses along the transportation between the fire heater to flash distillation unit. The reference temperature Tris set 288.5 K. the specific heat of process fluid was taken by ChemCad Qfluid= Qoutfluid-Qinfluid

(11)

Qin,fluid= N Cpfluid,in (Tin-Tr)

(12)

Qin,fluid=120 kmol/hr 577.1 kj/kmol K (365K-288.5K)=5.3 106 kj/hr Qout,fluid= N Cpfluid,out (Tout-Tr)

(13)

Qout,fluid=120 kmol/hr 853.8 kj/kmol K (565K-288.5K) =34.2 106kj/hr Qfluid =34.2 106kj/hr-5.3 106 kj/hr=28.9*106kj/hr Qair= Wair*Cpair*(Tin,air –Tr)

(14)

Qair=400 kmol/hr*28.97 kg/kmol*(298-288.5)*1.005 kj/kg K=11.06*104kj/hr Qfuel=Wfuel*Cpfuel*(Tin,fluel-Tr)

(15)

Qfuel=30.9 kmol/hr*19.9 kg/kmol*2.34 kj/kgK*(298-288.5)= 13.73*103kj/hr Qflue=711.42kmol/hr*1.4 kj/kmolK*(1278K-288.5K)= 9.91*105kj/hr Qin = Qrls + Qair + Qfuel - Qflu+Qfluid

(16)

Qin =43.6 10 6kj/hr-9.91*105kj/hr+13.73*103kj/hr+11.06*104kj/hr-28.9*106kj/hr =11.6*106kj/hr Heat transfer is affected by radiation in theory by the Stefan-Boltzman law for Qr=σT4

[6]

In design of the fired heater, the mathematical solution of radiative heat transfer is more complicated however as it involves the calculation of heat exchange factors as a function of the furnace geometry and the calculation of the absorptivity and emissivity of the combustion gases. There are also other factors to be considered such as the emissivities of the

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surface and the effects of re-radiation of the tubes. The construction materials were selected from Table 2.2.3. Temperature of flue gas is 1005 K so 60 Cr and 40 Ni stainless steel was chosen. Table 2.2.3 According to flame gas, tube material [7]

Qout = Qu + Qlosses

(17)

Qu=Qradiation+Qconvecrion+Qshld

(18)

The tube wall temperature is calculated by the average tube wall temperature [3] Tw=100 0.5 Tw=100 0.5 Qradiation=σ

α Ac

(19) = 347 K F (Tg4-Tw4)

(20)

α: the relative absorption effectiveness factor, 0.835 F: radiation exchange factor, 0.97 [5] σ: Stefan-Boltzman constant, 2.041*10-7 ( kj/hr.m2K4) [5] Ac: Cold plane area of the tube bank (m2) [5] Ac=Ntube*tube space*length (m2) Ntube= number of tube in radiation section Qradiaition=2.041*10-7*0.835*0.97(52*2.2*0.1)*(12784-3474)=50.9*105kj/hr

15

Qconvection= hconvection*At*(Tg-Tw) hconvection: film convective heat tranfer coefficient, 30.66k (j/hr.m2.C) [5] At=Area of the tubes in bank (m2) At=п*Dtube*L*Ntube Qconvection=30.66*(3.14*0.99*2.2)*52*(1005-74) =10.15*105kj/hr for radiation section Qconvection=30.66*(3.14*0.99*0.37)*12*(1005-74) =39.4*103 kj/hr for convective section Qshld= σ

α Ac

F (Tg4-Tw4)shld

(21)

Ac= (Ntube-1)*tube space*length (m2)

(22)

Qshld=2.041*10-7*0.835*0.97((52-1)*2.2*0.1)*(12784-3474) =49.9*105kj/hr for radiation section Qshld=2.041*10-7*0.835*0.97((12-1)*0.37*0.1)*(12784-3474) =18.1*104kj/hr for convective section Qout = Qu + Qlosses Qout=50.9*105+18.1*104+49.9*105+39.4*103+10.15*105+2.18 106 Qout=11.603*106kj/hr Qout was calculated for each different diameter. According to the general energy balance Qin is equal Qout. Qoutwhich was calculated for 0.099m diameter is equal Qin, 11.68*106kj/hr.

16

Table 2.2.4 Calculation parameter mfuel kmol /hr mair kmol /hr

Q in(kj/hr)

Di(m)

Space(m)

Qout(kj/hr)

30,91003274

400

11681791,88 0,025400051 0,031750064 3482683,905

46,3650491

600

35160980,6

54,09255729

700

45850574,97 0,088900178 0,093980188 10454841,75

61,82006547

800

56540169,33 0,099060198 0,101600203 11681794,01

69,54757365

900

67229763,69 0,127000254 0,132080264 14715227,66

77,27508184

1000

77919358,06 0,152400305 0,157480315 17555484,93

85,00259002

1100

88608952,42 0,177800356 0,182880366 20395742,2

92,73009821

1200

99298546,79 0,203200406 0,208280417 23235999,47

100,4576064

1300

109988141,1 0,228600457 0,233680467 26076256,74

108,1851146

1400

120677735,5 0,254000508 0,259080518 28916514,01

115,9126228

1500

131367329,9 0,279400559 0,284480569 31756771,28

0,050800102 0,055880112 6194455,848

The heat exchanger lenght,diameter,tube diameter and other parameters obey and satify the standarts. The heat exchanger lenght and diameter were set as 2.7m and 1m. They were set by looking at the standarts. For different tube diameters the heat duties were calculated after that when the chemcad heat duty result fit with the calculated result ,the tube diameter was found. The results were tabulated in Table 3.2 The thermal efficiency of the fired heater can then be written as: N=(Qu/Qin)*100 N=(9.4*106/11.6*106)*100=80.6 % 2.3 Tube inside parameter calculations 2.3.1 Density of the Mixture The diameter of the tube was calculated in the previous calculations as 0.09m. The tube lengths were calculated as 0.37m for the convectional part and 2.2 m for the radiation part.

17

Liquid density of liquid leaving the reboiler at 564.89 K ; ρM= Pure methanol density kg/h m3 ρB= Pure methyl oleate density kg/h m3 ρmix= xM * ρM +xB * ρB ρM= 788.39 kg/h m3 ρB= 656.94 kg/h m3 ρmix= 0.03*788.39 kg/h m3+ 0.997*656.94 kg/h m3 ρmix= 678.6209 kg/h m3 Molecular weight of the methanol= 32.4 kg/kmol Molecular weight of the methyl oleate = 296 kg/kmol Relative mass of the entry liquid= xLM* MWM + xLB* MWB Relative mass of the entry liquid= 0.17*32.4 kg/kmol + 0.83*296 kg/kmol Relative mass of the entry liquid=251.1268 kg/kmol

Relative mass of the exit vapor= yM* MWM + yB* MWB Relative mass of the exit vapor = 0.24*32.4 kg/kmol + 0.76*296 kg/kmol Relative mass of the exit vapor =232.736 kg/kmol

Relative mass of the exit liquid = yM* MWM + yB* MWB Relative mass of the exit liquid = 0.003*32.4 kg/kmol + 0.996*296 kg/kmol Relative mass of the exit liquid =296.084 kg/kmol The volume of the vapor is calculated from fallowing equation; Vv=zc*V*T*

18

zc= compressibility factor P=101325 Pa (operating pressure) T= 564.89 K R= 8314.34 Pa K/ kmol zc= 0.9848 Vv= 0.9848*82.038 kmol/h * 564.89 K/ *8314.34 PaKm3/kmol /101325 Pa Vv= 3745.545 m3 The volume of liquid= W* Relative mass of the exit liquid /ρL VL= 37.962 kmol/h*296.084 kg/kmol/678.6209 kg/h m3 VL= 16.56 m3 The exit density of the reboiler was calculated as fallows ; ρexit=W* Relative mass of the entry liquid / Total volume ρexit ρexit= 8.01 kg/m3 2.3.2 Friction Loss Cross sectional area and total cross section area of the tubes the tube numbers were found in the previous section as 12 for convection part and 52 as radiation part. All the calculations were done for the optimum values as diameter, lengths of the tubes and the number of the tubes. Table 2.3.2.1 Specifications of the tubes Diameter

Length

Area

Total area

0.09m

0.37 m

0.0785m2

0.0942m2

0.09m

2.2 m

0.0785m2

0.4082m2

19

Mass flux and homogenous velocity were calculated as fallows; G=L* Relative mass of the entry liquid/total area of the tubes ν= G/ ρexit Table 2.3.2.2. Mass flux and homogenous velocity of fluid Total area

Mass flux 319906.7516kg/m2h

0.0942

2

0.4082

73824.635kg/m h

Homogenous

Homogenous

velocity m/h

Velocity m/s

39937.45261 m/h

11.09m/s

9216.335 m/h

2.6m/s

Calculation of reynold and fanning factor; Viscosity of the fluid = 4.356 m Pa/h Re=

Fig 2.3.2.1. Fanning factor(taken from Geankoplis C.) ε was found from Geankoplis for commercial steel Table 2.3.2.3. Reynold number and fanning factors ε

ε/D

Fanning factor

7344

0.000046

0.00046

0.01

1694

0.000046

0.00046

0.015

Homogenous

Reynold

velocity

number

39937.45m/h 9216.33 m/h

20

At tube exit, pressure drop per unit length; ΔPf= 4*ƒ*ΔL/D*ν2/2 Table 2.3.2.4. Pressure drop per unit length ΔL

ƒ

ΔPf

0.37m

0.01

72.95 Pa

2.2m

0.015

34.65 Pa

At tube entry, liquid only, pressure drop per unit length; Velocity= G/ ρmix Velocity for 0.37m length tube = 471.40m/h Velocity for 2.2 m length tube = 108.78m/h

Table 2.3.2.5. At tube entry, liquid only, pressure drop per unit length; Velocity

Reynold

ƒ

Pf

Average Pf

Pressure drop over the tubes

471.40m/h

7344

0.01

4.65Pa

38.08Pa

14.35 Pa

108.78m/h

1694

0.015

0.37Pa

17.5Pa

38.5Pa

Average Pf was calculated as ; (At tube exit, pressure drop per unit length+ At tube entry, liquid only, pressure drop per unit length)/2

Pressure drop over the tubes was calculated as; Average pressure drop * Length of the tubes

21

Static pressure of the tubes; νi= 1/ρL νo=1/ρexit Ps= g= 9.8 m/s2 Table 2.3.2.6. Total pressure drop over the tubes and P head Ps

Total pressure drop over the tubes

P head

130.48 Pa

144.84Pa

2460.68Pa

775.83Pa

814.36Pa

14631.07 Pa

Total pressure drop over the tubes was calculated ; (Pressure drop over the tubes+ Static pressure of the tubes)/2 Phead = ρL*L*g As it can be seen from the table available head is larger than the total pressure drop over the tubes. It shows that this pressure drop is available for this project.

22

2.4 Flash Distillation calculation

Figure 2.4.1 Flash distillation Calculate maximum permissible vapor velocity, uperm (m/s) Uperm=Kdrum ρl: liquid density, 649.06 kg/m3 ρv: vapor density, 2.64 kg/m3 Kdrum: K factor (m/sec)

23

Table 2.4.1. Separator types and K factors[8]

Table 2.4.2.Upermfor different Kdrum K

Uperm=K*((ρl-ρv)/ρv)^0,5

0,05

0,78239386

0,06

0,938872632

0,07

1,095351403

0,08

1,251830175

0,09

1,408308947

0,1

1,564787719

Calculate cross-sectional area, Ac (m) Ac= V: vapor flow rate, 0.0225 kmol/h MW: molecular weight vapor, 246.46 kg/kmol

24

Table 2.4.3. Cross sectional area Ac=V*MW/Uperm*ρv 2,793655152 2,32804596 1,995467965 1,74603447 1,55203064 1,396827576

Calculate drum diameter, D, and height, h

Figure 2.4.2: Flash distillation minimum size

25

The height of the vessel is composed of a number of terms. Droplet settling length is the length from the center line of the inlet nozzle to the bottom of the mist eliminator. There is height equal0.75 D [8]. Then height from the bottom of the inlet nozzle to liquid surface is required to prevent nozzle flooding. The feed in the flash is minimum 0.5D. Another important term liquid height is minimum 0.25 D.

feed D( diameter)

Table 2.4.4. Process of flash distillation size parameter height

bottom(0.75*D)

liquidhight(0.25*D) above(D+0.5*D)

total height

1,886477239 1,414857929

0,47161931

2,829715858

3,301335167

1,72211023

1,291582672

0,430527557

2,583165344

3,013692902

1,594364265 1,195773198

0,398591066

2,391546397

2,790137463

1,491391207 1,118543405

0,372847802

2,23708681

2,609934612

1,406097114 1,054572836

0,351524279

2,109145672

2,46066995

1,333940848 1,000455636

0,333485212

2,000911272

2,334396484

All of values provide the desired vapor amount and by looking at result the minimum value height and diameter were selected for flash drum design because it gives the minimum fix cost.

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2.5.Cost Analysis For combustion process the natural gas and %25 excess air were reacted in vertical fired heater. According to excel calculation the required amounts were found as 30.9 kmol/h for natural gas and 400 kmol/h excess air The plant operation time was set as 8500h and in this time period natural gas consumption rate was calculated as follows. Utility cost Density of natural gas: 110kg/m3 Moleculer weight=19.99kg/kmol 30.9kmol /h*19.99kg/kmol*8000 = 4941528kg/year 4941528kg/110kg/m3=44922m3/h Naturel gas cost(tl/m3)=0.89tl [9] 44922m3*0.89tl /m3= 39981tl=50609$/year Fixed costs The equipments are the vertical fired heater and drum. For the vertical fired heater the cost was calculated by looking at the heat duty.By using equipment cost program was found as 531000$ (2007) [10] 531000*590/500=626580$ For the drum cost analysis; Cp=20000 Bare model cost (CBM)=Cp*(B1+B2*Fp*Fm) Fp=1 Fm=1.5 B1=1.74

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B2=1.55 Cbm=20000*(1.74+1.55*1.5*1)=81300$ The total fixed costs=626580$+81300$=707880$ Cost of manufacturing(COM)=0.280 Fixed Costs+ 1.23*(Cost of raw material+cost of utilities) COM=0.280*707880+1.23*50609$=260456 $

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3. RESULTS AND DISCUSSIONS Table 3.1. Process data sheet for box-type fired heater Unit process conditions Process fluid

Methanol+methyloleate

Fluid flow rate (kmol/h)

120

Inlet temperature

365

outlet temperature

623

ınlet pressure(atm)

1

outlet pressure(atm)

0,9733

efficiency

0,80

Fuel characteristics Type of fuel

Natural gas

Nett calorific value (kJ/kmol)

927844.41

Molar heat (kJ/kmol.K)

2.34

Temperature (◦C)

25

Flow of fuel (kmol/h)

30.9

Molecular weight (kg/kmol)

19.99 CH4 (80.43), C2H6 (9.02), C3H8

Composition (% mol)

(4.54), iso-C4H10 (0.20), n-C4H10 (0.32), isoC5H12 (0.04), n-C5H12 (0.02), CO2 (3.52), H2S (0.09), N2 (1.735).

Air characteristics Molar heat (kJ/kmol.K)

1.005

Flow of air (kmol/h)

400

Air temperature (◦C)

25

Percentage of excess air

25%

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The chemcad results gives the heat duty as a 11200000kj/h, the heater length was fixed as 2.5 m and with different tube diameter the heat duty was calculated. Radiation heat duty was added to convection heat duty. In Table 3.2 QR+Qshield* parts shows the our term by term calculation. However QR+Qshield part was calculated by making over all energy balances around the heater and then these terms were taken from the equality . The very close values were obtained for tube diameter 0.09m. The optimization was done by considering the heat duty since costs were not change in this process .There was an desired heat duty and we had to provide this value by using combustion heat duty.

Table 3.2. The optimization of the process with various tube diameter

Tube

Tube

#

of pressure

diameter(m)

space(m)

Tubes

drop(atm)

lenght(m) QR+Qshield* QR+Qshield

0,025400051

0,031750064

112

0,0014

2,5

3488712,289

11651594,8

0,050800102

0,055880112

95

0,0082

2,5

6205937,611

37012276,5

0,088900178

0,093980188

76

0,0134

2,5

10474647,34

47156549,1

0,099060198

0,104140208

64

0,0267

2,5

11612969,93

57300821,8

0,127000254

0,132080264

57

0,056

2,5

14743357,06

67445094,5

0,152400305

0,157480315

39

0,067

2,5

17589163,54

77589367,2

0,177800356

0,182880366

25

0,07453

2,5

20434970,03

87733639,8

0,203200406

0,208280417

19

0,08965

2,5

23280776,51

97877912,5

0,228600457

0,233680467

14

0,0965

2,5

26126582,99

1080221853.

The number of total tubes was 64, pressure drop was also found as 0.027atm.it is very small value which means there wont be any problem in distillation column because columnt is working at 1atm.The QR+Qshield values are so close and also these values provides the chemcad reboiler heat duty which was 11200000kj/h.

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Table 3.3.Final design parameters Fired heater Inside diameter of the tube, (m)

0,09

Tube spacing(m)

0.104

Tube Length L (m)

2.2

Number of tubes Nt

64

Heater dimeter(m)

1

heaterlenght(m)

2.5

pressure drop(atm)

0.026

construction material

stainless steel

Flash Drum height(m)

2.33

diameter(m)

1.33

.

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4.CONCLUSION

A fired heater is a direct-fired heat exchanger which uses the hot gases of combustion to raise the temperature of a feed flowing through tubes. The process fluid is first heated in the convection section preheat coil which is followed by further heating in the radiant section. In both sections heat is transferred by both mechanisms of heat transfer, radiation and convection, where radiation is the dominant mode of heat transfer at the high temperatures in the radiant section and convection predominates in the convection section. In this project a fired heater and the flash drum design were done. For fired heater design, the natural gas and the 25%excess air were reacted in order to obtain efficient combustion heat for the system. The heat duty was found as 11612969,93 kj/h. The heat which was calculated for the providing enough heat in order to raise the outlet process fluid temperature. The temperature was found as 623K. The tube diameter is 0.09m, number of tubes for radiant part is 52, for convective part is 12 were found. The heat exchanger diameter is 1m and height is for radiant part 2.2m, for convective part is 0.5m.The pressure drop during the tube was calculated as 0.027 atm. After fired heater design the flash drum design was done. The height of drum was obtained as 2.33 m, the diameter of the drum was calculated as 1.33m. The total cost was calculated as 260456 $ in terms of operating costs and fixed costs.

32

5.REFERENCES [1]Online

Resources,

web

link,

[Available

2012]

http://media.wiley.com/product_data/excerpt/10/04713217/0471321710.pdf [2]WildyF.,Fired Heater Optimization, AMATEK process Instruments,p:1-6 [3] Al H.,IbrahimH.,Fired process heaters,Al-Baath University, p:327-329 [4]Sınnott R.,TowlerG.,Chemical engineering design,Elsevier, p:934 [5] Online Resources, web link, [Available 2012], http://heaterdesign.com/design0.htm [6]IncroperaF.,DewittD.,Bergman

T.,

LavineA.,Fundamental

of

heat

and

mass

transfer,Wiley,sixth edition. [7]OnlineResources,weblink,[Available2012] http://kolmetz.com/pdf/ess/PROJECT_STANDARDS_AND_SPECIFICATIONS_fired_heat ers_Rev01.pdf [8]BubbicoR.,Gas liquid separators,“Sapienza” University of Rome [9] ]

faturalandırmaveenerjipiyasasıdüzenlemekurumu

[10]OnlineResources,weblink,[Available2012] http://www.matche.com/EquipCost/Crystallizer.htm

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