Process Analysis of Refinery Crude Charge Heater

31

Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 ISSN: 2141 – 4068

© Wilolud Journals, 2012 http://www.wiloludjournal.com Printed in Nigeria doi:10.5707/cjengsci.2012.7.2.31.51

PROCESS ANALYSIS OF REFINERY CRUDE CHARGE HEATER

P.U. Uzukwu1 And E.T. Iyagba2

1 African Regional Aquaculture Centre, PMB 5122, Port Harcourt, Nigeria. 2 Department of Chemical Engineering, University of Port Harcourt, Port Harcourt, Nigeria.

ABSTRACT Process analysis of refinery crude charge heater was used to investigate the effect of changes in fuel gas composition on crude charge heater efficiency of Port Harcourt refinery. Material and energy balance around the heater provided the basis for the calculation of heat absorbed by crude charge. The efficiency of the crude charge heater was then calculated as the quantity of heat absorbed by crude charge divided by the quantity of heat generated by combustion of fuel gas and the result multiplied by one hundred. This was done for various forced-draft fan (FDF) damper openings for fuel gas A (from the refinery mixing drum) and B (used to design the heater). Also the percentage oxygen content of the flue gas was calculated for the same FDF damper openings for the two fuel gas samples. For fuel gas A, the crude charge heater efficiency records ranged from 73.34 to 82.62%, with the highest value 0f 82.62% recorded at 45% FDF damper opening. For fuel gas B, the efficiency ranged from 76.51 to 86.59%, with the 45%FDF damper opening also recording the highest value (86.6%). Statistical analysis showed that at various FDF damper openings, the efficiency values of the heater when firing fuel gas A were not significantly (p>0.05) different from the values when firing fuel gas B. Hence, changes in fuel gas composition have no significant (p>0.05) effect on the crude charge heater efficiency. The forced draft fan (FDF) damper opening of 50% recorded the optimal efficiency and optimal oxygen content in the flue gas for both fuel gas A and B. The significance of these findings for turn-around maintenance of other refineries is discussed. KEYWORDS: Crude heater, Process analysis, Heater efficiency

INTRODUCTION Nigeria presently has four federal government owned crude petroleum refineries with total installed capacity of 445,000 barrels per day, with an output which has been sub-optimal. This is due to inefficient performance of their unit operations and processes. As a result Nigeria now imports a substantial quantity of her refined petroleum products needs to satisfy domestic consumption. It has been suggested that one of the sustainable solutions to the problem of inadequate domestic supply of petroleum products is to open up the space of refinery operations in Nigeria to private sector participation. At present this suggestion has not recorded any significant response as only one out of the 18 companies so licensed, Orient Petroleum Limited, has made substantial effort to commence construction. The reasons adduced include: (1) Lack of legal and regulatory framework, (2) Government continued price regulation in a deregulated regime, and (3) Doubt over government’s commitment to incentives offered to investors including tax holidays, federal

government guarantee of foreign loans, guarantee of uninterrupted feedstock supply and allocation of crude to refineries (Abdulahi, 2005).

Cottage refineries of about 5000MT barrel per day should be encouraged to open up frontiers even to indigenous investors as the capital outlay would be more affordable than higher capacity refineries. However, for the scheme to record better participation and consolidation, price regulation should be discouraged. Besides, indigenous capacity building in key refining technologies is also an important critical success factor, of which crude charge heater process analysis is one. A crude charge heater is an enclosed space in which heat is produced from chemical oxidation (combustion) of fuels (Perry, 1963) for crude vaporization at the flash zone of crude column. It is very important in maintaining process temperature needed in the efficient operation of refineries, and requires routine optimization analysis and turn around maintenance for maximum efficiency. The composition of the fuel gas used to design the crude charge heater of Port Harcourt refinery is not exactly the same with that used to operate the heater.

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 The objectives of this study were:

(1) To investigate the effect of changes in fuel gas composition on crude charge heater efficiency of Port Harcourt refinery, and

(2) Demonstrate the procedure for crude charge heater process analysis. It is hoped that this will provide technical knowledge for the turn around maintenance of existing and up coming refinery heaters in Nigeria and else where. MATERIALS AND METHODS Fuel Gas and Crude Sampling The fuel gas (A) that is actually used to fire the heater of Port Harcourt refinery was sampled from the mixing drum (77D01) of the fuel gas unit (Figure 1) where refinery gas streams produced by process units and liquefied petroleum gas (LPG) are received, processed and distributed to heaters and boilers at regulated pressure. The fuel gas which was used to design the Port Harcourt refinery heater is tagged fuel gas (B). Crude Petroleum sample was obtained from tank 50TKO1C of the Port Harcourt refinery. Fuel Gas Analysis and Crude Distillation The fuel gas (A) was analysed in the Port Harcourt refinery laboratory using a gas chromatograph. The operating conditions of the gas chromatograph are: Detector - Thermal conductivity (TCD) Column - 1.5 x 4mm I.D. glass packed with HMPA 30% chromosorb P. Column Temperature - Room (250C) Carrier gas and flow rate - Argon at 60ml/min. Recorder and speed - Chart at 240cm/sec The crude petroleum sample was subjected to true boiling point (TBP) distillation in the Port Harcourt refinery laboratory. The distillation apparatus used was PMA - 3010F model, and ASTM D2892 method was adopted. Four (4) litres charge of the crude oil was used for the distillation in the 30 actual plate vacuum jacketed oldershew column. This corresponded to 15 theoretical plates with a reflux ratio of 4:1 (Imafidon, unpub). The distillation was carried out at atmospheric pressure up to a still-liquid temperature of 313oC. In order to prevent cracking, the distillation was then conducted at 10mm Hg pressures. The distillation was stopped when the still-liquid temperature reached 3500C. Material Balance for Fuel Gas A Fuel gas (A) feed flue gas HEATER 15.29 KNM3/hr CO2 H2O Air (O2, N2) O2 205.8 KNM3/hr N2 System: The heater; Basis: 1 Kg mol/hr fuel gas For steady state operation, the mass balance using the fuel gas (A) analysis (Table 1) is: Input = Output - (1) That is, (0.6335H2 + 0.1886CH4 + 0.1125C2H6+ 0.0016C3H8 + 0.0186C3H6 + 0.0059C4H10 = (bCO2 + cH20 + + 0.0015C5H12 + (3.76a + 0.0384) H2 (2) 0.0005C4 + 0.0009C6H14 + 0.0384N2 +a02 + 3.76aN2)

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 Equating the coefficients of carbon (C), Hydrogen (H), and oxygen (O) in equation 2 we obtain: C: 1(0.1866) + 2(0.1125) + 3(0.0016) + 3(0.0186) + 4(0.0059) + 4(0.0005) + 5(0.0015) + 6(0.0009) = 1(b) ∴ b = 0.5107 H: 2(0.6335) + 4(1866) + 6(0.1125) + 8(0.00016) + 6(0.0186) + 10(0.0059) + 8(0.0005) + 12(0.0015) +

14(0.0009) = 2C = 2.9064 ∴ C = 1. 453 O: 2a = C + 2b = 1.453 + 2(0.5107) ∴ a = 1.237 Substituting for a, b and c in equation 2, we get for 1 Kgmol fuel gas: Fuel gas + 1.237O2 + 4.651N2 = 0.5107CO2 + 1.453 H2O + 4.68 N2 (3) Equation 3 is the theoretical equation for the combustion of 1kgmol fuel gas A. Computation of Mass Flow Rate (kgmol/hr) Fuel Gas to Heater From the heater process control computer print out the flow rate of fuel gas is 15.29 KNM3/hr. ∴ kgmol fuel gas fed to the heater per hour = 15.29/0.0224 = 683 kgmol/hr. Computation of Mass Flow Rate (kgmol/hr) Air fed to Heater. From the heater process control computer printout, the flow rate of air from the 2 forced draft fans (FDF) A and B are 100.9 and 104.9 KNM3/hr respectively(Figure 2). ∴ Total kgmol Air fed to the heater per hour at 100% FDF air damper opening

= 5.91870224.0

9.1049.100 =

+ kgmol/hr

FDF air damper opening was used to regulate the amount of air fed to the heater. The Kgmol Air fed to the heater at various FDF air damper (register) openings were computed as in Table 2. For 683 kg mol fuel gas fed to the heater, the theoretical equation (3) becomes: 683 (fuel gas) + 84502 + 3176.6N2 = 348.8C02 + 992.4H20 + 3203N2 (4)

For 45% FDF damper opening, the excess air equation is: 683 (fuel gas) + 868.202 + 3266.12N2 = 23.202 + 348.8CO2 + 992.4 H20 + 3292.3 N2

% excess 02 = ( )

%75.2845

1008452.868 =−

% excess 02 in flue gas =100(kgmol 02 in flue gas)/kgmol flue gas %5.07.4656

)2.23(100 =

Using the same procedure above the % excess air equations for 50%, 60%, 70%, and 100% FDF air damper openings were also computed: 50%: 683 (fuel gas) + 964.702 + 3629.1 N2 = 348.8 C02 + 992.4 H20 + 3655.3N2 + 119.7 02 60% 683 (fuel gas) + 1157.6 O2 + 4355N2 = 348.8 C02 + 992.4 H20 + 4381.1N2 + 313 O2 70%: 683 (fuel gas) + 1350.6O2+ 5080.7N2 = 348.8 C02 + 992.4 H20 + 5106.9N2 + 5060 100%: 683 (fuel gas) + 1929.4O2 + 7258.13N2 = 348.8 C02 + 992.4 H20 + 7284.4N2 + 1084.4O2

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 The material balance above as well as the corresponding % excess O2 and % excess oxygen in flues gas for 45%, 50%, 60%, 70% and 100% are summarized in table 2. Energy Balance Around the Heater at 100% FDF Air Damper Opening for Fuel Gas Sample A Computation of the Heating Value of Fuel Gas First, the molecular weight and specific gravity, and mean specific heat capacity (CPm) of fuel gas sample A were computed as 10.1kg/kgmol, 0.3498 and 2.5702 respectively as detailed in Tables1 and 2. Heat generated by the combustion of fuel gas is given by the following expression: Q fuel = ∆H298 + ∑[Qm CPM (T2–T1)] p – [Qm CPm (T2–T1)]R Where: T1 = 250C = 298K T2

= Average fire box temperature of heater = 7470C = 1020k. Qm = Mass of material (kg) CPm = Mean CP = 2.5702 P = Products R = Reactants However, Mcker and Fredersdorff (1947) stated that ∆H298 of fuel gas = (155 + 1,425g) 37253 J/m3

Where: g = specific gravity of fuel = 0.3498 ∴ ∆ H298 = 37253 J/m3, = 5815.46 K cal/m3

Mean fire Box Temperature From the heater process control computer print out (fig. 2) the fire box temperatures were 7370C and 7570C. The mean fire box temperature = 7470C ∴ The heating value of fuel gas A (at 298K) of flow rate 15.29KNM3/hr = 5815.45 x 15290 K cal/hr = 88.980 Gcal/hr. Using the same procedure the heating value of fuel gas B was computed to be 83.74 G cal /hr. Computation of Sensible Heat Content of Reactants i. Fuel gas: The sensible heat content of fuel gas can be computed using the expression: Sensible heat content of fuel gas (ShG) = Qm CPm (T2–T1)] Where Qm = Mass flow rate of fuel gas = 6981.1kg/hr CPm = 2.5702 T2 = 7470C T1 = 250C ∴ ShG = 6901.1 x 2.5702 (747 – 25) = 12.806 G cal/hr ii. Oxygen, O2 Air flow rate = 205.8 KNM3/hr

Kgmol 02 in 205 KNM3/hr = 224.0

8.205 (0.21) = 1929.4 kgmol O2

From figure 3 the sensible heat content of 02 (ShO) at average temperature of 3860C (726.80F) is 4900 Btu/lbmol

∴ShO = 4900 Btu

Kcalx

kgmol

bmolx

bmol

Btu 252.0

454.0

1

1

= 2719.5 K cal/Kgmol

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012

But at 100% FDF air damper opening 1929.4 hr

kgmol 20was fed to the heater.

∴ sensible heat content of 1929.4 kgmol O2 = 2719.5 x 1929.4 cal/hr = 5.248 G cal/hr iii. Nitrogen (N2):

kgmol of N2 in 205.8 KNM3/hr air fed to the heater = 13.725879.00224.0

8.205 =x

from figure 3, sensible heat content of kgmol N2 at 3860C is 4900bmol

Btu

1 = 4900 (0.555)

kgmol

Kcal =

2719kgmol

Kcal

for 7258.1 kgmol N2, the sensible heat content = 2719.5 x 7258.1 Kcal = 19.741G cal ∴QReactants = 12.806 + 5. 245 +19.741 = 37.795 G cal/hr Computation of Sensible Heat Content of products for 100% FDF Air Damper Opening The sensible heat content of products, using figure 3, with average temperature of 3860C (726.8 0F) is:

Oxygen (O2): 1084.4 5000Btu

Kcalx

kgmol

bmolx

bmol

Btu

1

252.0

454.0

1

1

= 1084.4 (5000) (0.555) Kgmol

CalK = 3009210 Kcal/hr

Nitrogen (N2): = 7284.4 (5000) x 0.555 = 20214210 Kcal/hr Carbondioxide (CO2): = 348 (7000) x 0.55 = 1351980 Kcal/hr Water (H2Og): = 992.4 (8100) x 0.555 = 4461334 Kcal/hr Qproducts = 3.009 + 2021 + 1.351 + 4.461 = 29.0367G Cal Qfuel = heat generated by combustion of fuel gas in the heater

= ∆ H298 + ∑ {Qm Cpm (T2 – T1)P – (Qm CPm T2 – T1)R}

= 88.981 + (29.036734 – 37.795) G Cal = 88.980 + ( - 9) G Cal = 80.0G Cal COMPUTATION OF HEAT ABSORBED BY CRUDE CHARGE IN THE HEATER FOR FUEL GAS A Construction of TBP and EFV Curves The true boiling point (TBP) curve (figure 4) was first plotted at I atmospheric pressure. Then, the slope of the TBP curve between 10 and 70% points was estimated as follows:

Slope = %)/(%10%%70%

%10%70 0 VolCatVolumeatVolume

TBPatatTBP

−−

= 1070

98368

−−

= 4.50C/ Volume % distilled = 8.10F/Volume % distilled

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 From Figure 6, the slope of the equilibrium flash vaporization (EFV) curve for 8.10F/Vol % distilled was 5.5 0F/Vol % distilled. From Fig 4 the temperature of the TBP curve at 50% volume distilled = 2800C = 5270F From Figure 7, the temperature of the 50% equilibrium flash vaporization (EFV) curve at 5270F = 660F, which is lower than the 50% temperature of the TBP curve. We can see that the 50% EFV temperature = 50% distillation point – 50% flash point = 5270F – 660F = 4610F = 238.30C Now the slope of EFV curve = 5.50F/ % distilled At100% distilled, EFV temperature is given by:

Slope = Vol. F/%5.550100

461 0=−

−= x

Adjacent

Opposite

∴ x = 461 + 5.5 (50) = 7360F (3910C) At O% distilled, EFV temperature = 461 – (50 x 5.3) = 1860F = 860C Using 50% EFV temperature = 238.30C and O% EFV temperature = 860C,the EFV curve at 1 atmospheric pressure was plotted (Fig. 4) Construction of EFV curves at heater inlet and outlet pressures (a) Refinery Heater Inlet Pressure was 16.5 kg/cm (Guarg). = 17.5kg/cm2 (Absolute) At 760mm Hg, 50% EFV temperature = 4610F = 238.30C From Fig. 8 at 13,300 mmHg (760 X 17.5), 50% EFV temp. = 7500F = 3990C ∴ At 100% EFV temperature = 750 + (50 x 5.5) = 10250F = 5520C At O%, EFV temperature = 750 – (50 x 5.5) = 4750F = 2460C Using 50% EFV temperature and O% EFV temperature, the EFV curve at 17.5kg/cm2 pressure (Absolute) was plotted (Fig. 4). (b) Refinery Heater Outlet Pressure was 3.0 kg/cm2 (Guage) = 4.0kg/cm2 (Absolute) Now following the same procedures above the EFV curve at 4 Kg/cm2 pressure (Absolute) was constructed (Fig 4). CRUDE CHARGE PROPERTIES Volumetric flow rate of liquid crude charge to preflash drum (10D02) = 930 m3/hr (Figure 9). Liquid crude flow rate out of preflash drum = 829 m3/hr (figure 9).

Volume % crude vapourized in preflash drum (10D02) = 100 - 930

829 x 100 = 10.86% Vol.

From Figure 5, weight % of crude vaporuized in preflash drum = 8.5 wt %.

Density of crude, cl = 849.7kg/ m3

∴ Mass Flow rate of crude charge to preflash drum

= l c x 930 m3/hr = 849.7kg/m3 x 930 m2/hr = 790221kg/hr.

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 Mass flow rate of preflesh vapour from (10D02) = 0.085 (790221) kg/hr = 67168.8kg/hr. Mass flow rate of crude entering the crude charge heater (10H01) (Figure 6). = (790221 – 67168.8)kg/hr = 723052.8kg/hr. Inlet temperature of crude into Heater (10H01) = 238.5oc Outlet temperature of crude flow out of Heater = (10H01) = 3420C Outlet pressure of crude flow out of Heater = 4kg/cm2(Absolute). From Figure 4, the amount of crude vaporized in the heater at 3420C and 4.0kg/cm2(Absolute).= 60.8 vol. % (= 57wt% in Fig. 5) From Fig 5 the API gravity of crude is 29.2. Percent prefleash liquid crude vaporized in heater = 57 – 8.5 (vaporized in 10D02) = 48.5wt%. Percent preflash liquid crude unvapourized = (100 – 57) = 43wt%. Amount of crude vapourized in the heater = 57 – 8.5 x 790221 100 = 383257.2kg/hr. � Amount of unvapourizes (liquid) crude = 723052.8 – 383257.2 = 339795kg/hr. Mean temperature of crude in heater (Tav) = Tin + Tout = 238.5 + 342.8 = 290.650 2 2

From Figure 10, the quantity of heat absorbed by liquid crude ( Hl) at 555.2 oF and API gravity of 29.2

= 315 Btu x 0.252kcal x 1b = 174.68kcal/kg 1b Btu 0.454kg 339795kg/hr unvapourized crude will absorb 339795 (174.68) kcal/hr = 59.4/Gcal/hr. From figure 10, the quantity of heat absorbed by vapourized crude ( Hv) at 555.20F and API gravity of 29.2 = 405 Btu (0555) kcal x 1b = 224.8kcal/kg 1b kg Btu � 383257.2kg/hr vapourized crude will absorb

224.8k cal x 383257.2kg/hr = 86.16 G cal/hr Kg Total heat absorbed by crude in the heater

= wt fraction of vapourized crude ( Hl) + wt fraction of unvapourized crude ( Hv). = 48.5 (86.15) + 43 (59.41) 48.5743 43 = 48.5 = (45.66 + 27.92) G cal/hr = 73.58G cal/hr. HEAT ABSORBED BY AIR AT THE PRE-HEATER Under steady state conditions, a part of the heat generated by the combustion of fuel gas in the heater is used to preheat the air in the air preheater. Accounting of heat used for air preheating was made as follows: Air inlet temperature in the preheater (Tin) = 250C. Air outlet temperature out of the preheater (Tout) = 2730C. Tav = 25 + 273 = 1490C (3000F). 2 At 100 FDF air damper opening, kgmol of Air fed to the preheater = 205.8 = 9187.5kgmol Air. 0.0224 From Figure 3, the sensible heat content of Air at 3000F = 1900 Btu x 0.252kcal lbmol lbmol Btu 0.454kg = 1054.5k cal/kgmol

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 � 9187.5kgmol Air will contain 1054.5 x 9187.5 = 9.69 G cal/hr Using similar procedures, the sensible heat content of preheated air at 45, 50, 60, and 70 FDF air damper openings were obtained, as: 4.36, 4.84, 5.81 and 6.78 G cal/hr receptivity. The total heat produced in the heater was the sum of the heat produced by the combustion of fuel gas (Q fuel), and the sensible heat content (Qs) of air out of the air preheater. The efficiency of the heater, E was calculated using the expression: E = Heat absorbed by crude (Q crude) x 100 Heat generated by Heater (Q Heater) RESULTS Material Balance FUEL GAS ANALYSIS The results of analysis of fuel gas samples A and B are presented in Table 1 PHYSICAL PROPERTIES OF FUEL GAS The results of the computation of molecular weight and specific gravity of fuel gas samples A and B are also presented in Table 1 while the results of computation of mean specific heat capacity of the fuel gas samples A and B are presented in Table 2. TRUE BOILING POINT DISTILLATION The plots of true boiling point distillation and API gravity versus weight % and volume % distilled are presented in figures 4 and 5 respectively. MATERIAL BALANCE AND THE THEORETICAL EQUATIONS The theoretical equations for the combustion of Ikgmol fuel gas derived from component material balance around the heater for fuel gas samples A and B by equating the coefficients of carbon, hydrogen oxygen, and sulphur were: (Fuel gas A) + 1.23702 + 4.651N2 = 0.5107C02 + 1.453H20 (g) + 4.689N2

(Fuel gas B) + 1.1990702 + 4.5085 N2 = 0.46552C02 + 1.4663 H20(g) + 4.52977N2 + 0.0004S02 . Table 3 shows a summary of the material balance around the heater.

CALCULATION OF KGMOL FUEL GAS AND KGMOL AIR FED TO HEATER. The amount of fuel gas fed to the heater was 683kgmol/hr while the total amount of Air fed to the heater by forced draft fans A and B at 100% FDF damper opening was 9187.5kgmol/hr. Energy balance Mean firebox temperature of heater was 7440C while mean temperature of reactants and products was 3860-C. The molecular weight, specific gravity and mean specific heat capacity (CPm) of fuel gas samples A and B at 3860C are presented in Tables 1 and 2. Sensible heat contents of reactants and products The sensible heat contents of reactants and products for fuel gas A and B at various FDF damper openings are presented in Table 4. The results of the material and energy balance are presented in figures 11 and 12. The excess oxygen values for sample A, ranged from 2.80 to 128 to 128% while oxygen values in the flue gas ranged from 0.5 to 11.2%. For sample B, the % excess oxygen ranged from 6.0 to 136% while oxygen content of the flue gas ganged from 1.06 to 11.40%. The Lowest values were obtained for 45% FDF air damper opening while the highest values were recorded for 100% FDF air damper opening. The value of thermal efficiency of the heater, with fuel gas sample A, ranged from 73.34 to 82.62% with the highest value of 82.62% recorded for 45% FDF air damper opening. For fuel gas B, the values of the thermal efficiency ranged from 76.51 to 86.59% with the highest values of 86.5% also recorded for 45% FDF Air damper

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 opening. In both cases (gas samples) thermal efficiency indicated a strong inverse relationship with the FDF air damper opening. DISCUSSION The results from this study showed generally that the excess oxygen (air) and oxygen content of the flue gas varied directly with FDF air damper opening (Figure 11). This was expected since the amount of air fed to the heater increases with increased damper opening. However, the thermal efficiency records, for both gas samples, indicated strong inverse relationship with the air damper opening (Figure 11). For 40% air damper opening, the system operated with insufficient oxygen. This is uneconomical since insufficient oxygen results to less heat production and smoke. Therefore, the heater should not be operated at 40% FDF air damper opening. From 45% to 100% air damper opening, the heater operated with increasing amount of oxygen in flue gas and decreasing efficiency which is not dramatic (figure 11). It has been observed that the best parameter for assessing the efficiency of a heater is the percentage excess oxygen in the flue gas ; which should not exceed 3%. Based on this recommendation, the 50% FDF damper opening is the best opening (for both gas samples) for the Port Harcourt. Refinery. It has been observed by the operators of the heater that when the heater operates with little oxygen in the flue gas (0.5 – 0.9%) the heater gets overheated and the burner’s efficiency drops. It has also been observed that heaters which are properly run and in good condition usually operate at about 80% thermal efficiency and with air preheating facility, the thermal efficiency could be increased to about 90%. Results showed that with 50% FDF air damper opening and air preheating thermal efficiencies of 82% and 86.6% were obtained with corresponding oxygen content in flue gas of 2.3% and 2.94% respectively for gas sample A and B. Therefore, the 50% FDF damper opening is hereby recommended for Port Harcourt refinery. Comparing the thermal efficiency values for gas samples A and B, it was found that B generally gave relatively higher efficiency than A at all FDF damper openings (Figure 12). However, the differences were not found significant (P>0.05). This is supported by the fact that the correlation of the heater efficiency of both samples A and B showed strong correlation with correlation coefficient r = 0.99 (Figure 12). Furthermore, fuel gas sample B is contaminated with sulphur compound (H2S) and produces S02 in the flue gas which is corrosive, whereas fuel gas sample A is free from sulphur contamination and therefore has no risk of S02 pollution problems. Since the thermal efficiency of the heater when run on fuel gas A is above 60% at 50% FDF air damper opening, one can deduce that the change in fuel gas composition has no significant (P>0.05) effect on the efficiency of the crude charge heater of Port Harcourt Refinery. From the stand point of environmental quality, fuel gas A is even better than fuel gas B which contained Hydrogen sulphide gas. CONCLUSION We therefore concluded that changes in refinery fuel gas composition had no significant (P>0.05) effect on the heating efficiency of the crude charge heater of the Port Harcourt Refinery,and that the FDF air damper opening of 50% produced the best thermal efficiencies of 82% and 86.6% for fuel gas samples A and B respectively with corresponding oxygen contents of 2.3% and 2.94% in the flue gas. REFERENCES Abdulahi,S.: Why private refineries are not coming up in Nigeria. Vanguard Media Limited. p25. (2005). Perry, J.H., Perry. H.H. and Kirkpatrick, S.D.: Chemical Engineering Handbook, McGraw-Hill Book Co .New York. (1963). PHRPM: Process and operating manual of the Port Harcourt refinery project for crude distillation unit(unpublished). Wilson, L. and Hottel, H.C.: Heat transmission in radiant section of tubestills. Industrial engineering chemistry 24:486 (1932) In: Nelson W.L.: Petroleum refining engineering. McGraw-Hill Book Co. (1985). Mcker and Fredersdorff: Refinery gas fuel. Petroleum Refiner, p81. (1947). Nelson, W.L.: Petroleum refining engineering. Fourth edition. McGraw-Hill Book Co. (1985). Imafidon, R.C.: Report on TBP Essay of Crude samples Port Harcourt Refining Company, Eleme (unpublished). Snedecor, G.W. and Cochran, W.C.: Statistical methods. 7th ed. The Iowa University press. 507pp. (1980).

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P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 Table 1: Analysis of fuel gas supply to crude charge heater and computation of molecular weight of fuel gas

Note: Specific gravity (s.g.) according to Mcker and Fredersdorff (1947), of fuel gas = (molecular weight)/28.9

Fuel A, s.g = 3498.09.28

11.10 =

Fuel B, s.g = 9.28

33898.9 = 0.3215 (see Fig. 12)

Table 2: Calculation of mean CP (CPm) of fuel gas A and B at 3860C

Components of fuel gas

Fuel gas Analysis (mole %) Molecular weight

Mole % X molecular weight

Fuel gas A (tank mixing drum 77D01)

Fuel gas B used for heater design

Fuel gas A Fuel gas B

H2 0.6335 0.7225 2 1.2670 1.445 CH4 0.1866 0.11495 16 2.9856 1.839 C2H6 0.1225 0.09467 30 3.375 2.840 C3H8 0.0016 0.03085 44 0.0704 1.354 C3H6 0.0186 - 42 0.7812 - C4H10 0.0059 0.0149 58 0.3422 0.864 C4H8 0.0005 - 56 0.0280 - C5H12 0.0015 0.00232 72 0.1080 0.167 C6H14 0.0009 0.00252 86 0.077 0.21672 H2S - 0.0004 34 - 0.0136 N2 0.0384 0.02127 28 1.0752 0.59556 TOTAL 1.00 1.00 10.11 9.3389

fuel gas Components

Fuel gas Analysis mole % CP of fuel gas components at 3860C (Kcal/kg.k)

Mole % X CP (=CPm fuel gas)

Fuel gas A Fuel gas B Fuel gas A Fuel gas B H2 0.6335 0.7225 3.592 2.2755 2.595 CH4 0.1866 0.11495 0.842 0.15711 0.0968 C2H6 0.1225 0.09467 0.767 0.0863 0.0726 C3H8 0.0016 0.03085 0.746 0.0012 0.0230 C3H6 0.0186 - 0.746 0.0139 C4H10 0.0059 0.0149 0.736 0.0043 0.001017 C4H8 0.0005 - 0.736 0.0004 C5H12 0.0015 0.00232 0.721 0.0011 0.00163 C6H14 0.0009 0.00252 0..715 0.0006 0.00160 H2S - 0.0004 0.623 0.00025 N2 0.0384 0.02127 0.777 0.0298 0.01653 TOTAL 1.00 1.00 2.5702 2.8185

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Table 3: Summary of material balance for the combustion of 683 kgmol fuel gas A and B in the heater at various FDF damper openings

FUEL GAS A FDF Dam-per Ope-ning (%)

Kgmol Air Fed to heater

Kgmol O2

in air Kgmol N2 in Air

kg mol fuel gas fed to he-ater

Kg mol CO2

Kgmol H2O (g)

Kgmol N2

Kgmol O2

% Ex O2

% ExO2 In fuel gas

45 4134.4 868.2 3266.1 683 348.8 992.4 3292.3 23.2 2.80 0.05 50 4593.8 964.7 3629.1 683 348.8 992.4 3655.3 119.7 14.20 2.30 60 5512.5 1157.6 4355 683 348.8 992.4 4381.1 313 36.99 5.20 70 6431.25 1350.6 5080.7 683 348.8 992.4 5106 506 59.80 7.20 100 9187.5 1929.4 7258.13 683 348.8 992.4 7284.4 1084.4 128.30 11.20

FUEL GAS B

FDF Dam-per Ope-ning (%)

Kg mol fuel gas

Kgmol Air Fed to Heater

Kgmol O2 in Air

Kg Mol N2 in Air

Kg mol CO2

Kg mol SO2

Kgmol H2O (g)

Kg mol N2

Kgmol O2

% Ex O2

% Ex O2 In fuel gas

45 683 4134.4 868.2 3266.2 317.9 0.273 1001.5 3280.7 49.2 6 1.06 50 683 4593.8 965.0 3629.06 317.9 0.273 1001.5 3643.6 146.04 17 2.94 60 683 5512.5 1157.63 4354.88 317.9 0.273 1001.5 4369.4 338.67 41 5.62 70 683 6431.3 1350.56 5080.68 317.9 0.273 1001.5 5095.3 531.0 64 7.65 100 683 9187.5 1924.4 7258.13 317.9 0.273 1001.5 1110.04 1110.04 136s 11.4

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Table 4: Sensible heat contents of Reactants and Products of heater combustion reaction for fuel gas samples A and B computed from Figure 3.

Reactants (Sample A) Products (Sample A) FDF Damper Opening (%)

683kgmol fuel gas

O2 N2 Total Gcal/hr

CO2 H2O (g)

N2

O2 Total

45 12.806 2.36 8.88 24.06 1.40 4.50 9.14 0.007 15.04 50 12.806 2.62 9.86 25.29 1.40 4.50 10.14 0.07 16.11 60 12.806 3.15 11.84 27.79 1.40 4.50 12.16 0.10 18.16 70 12.806 3.67 13.82 30.29 1.40 4.50 14.17 0.16 20.23 100 12.806 5.25 19.74 24.06 1.40 4.50 20.21 0.35 26.46

Reactants (Sample B) Product (Sample B) FDF Damper Opening (%)

683 kgmol fuel gas

O2 N2 Total (Gcal/ hr

CO2 SO2 H2O (g)

N2 O2 Total s

45 12.97 2.36 8.88 24.21 1.24 0.0012 4.50 9.10 0.14 14.98 50 12.97 2.62 9.86 25.45 1.24 0.0012 4.50 10.12 0.41 16.27 60 12.97 3.15 11.84 27.96 1.24 0.0012 4.50 12.12 0.94 18.80 70 12.97 3.67 13.82 30.46 1.24 0.0012 4.50 14.14 1.47 21.35 100 12.97 5.25 19.74 37.96 1.24 0.0012 4.50 20.18 3.1 29.02

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Table 5: Summary of energy balance results at various FDF damper openings for fuel gas sample A and B

FUEL GAS A

FDF Air Damper Opening (%)

∆ H298

(Gcal) Q product

(Gcal) Q Reactant

(Gcal) Q fuel gas =z∆ H298+ QP-QR

(Gcal) 45 88.980 15.018 24.050 79.95 50 88.980 16.288 25.288 79.98 60 88.980 18.839 27.796 80.02 70 88.980 21.289 30.300 79.97 100 88.980 29.04 37.795 80.22

FUEL GAS B


∆ H298

Q product (Gcal)

Q Reactant (Gcal)

Q fuel gas = ∆ H298 + QP-QR

(Gcal) 45 83.74 14.98 24.21 74.80 50 83.74 16.27 25.45 74.63 60 83.74 18.80 27.98 74.58 70 83.74 21.35 30.46 74.56 100 83.74 29.02 37.96 74.51

44


Table 6: computation of crude charge heater efficiency (%) for fuel gas samples A and B.

FUEL GAS A


Heat produced by heater, Q heater (Q fuel + Qs)

Heat absorbed by crude (Q crude)

Heater Efficiency (%) E

= heaterQ

xCrudeQ 100

45 84.31 73.58 87.27 50 84.82 73.58 86.74 60 85.83 73.58 85.72 70 86.75 73.58 84.81 100 89.91 73.58 81.84

FUEL GAS B

FDF

Air Damper Opening (%)

Heat produced b4 heater, Q heater (Q fuel + Qs)

Heat absorbed by crude (Q crude)

Heater Efficiency (%) E

= heaterQ

xCrudeQ 100

45 79.16 73.58 92.95 50 79.47 73.58 92.25 60 80.39 73.58 91.53 70 81.34 73.58 90.46 100 84.20 73.58 87.39

45


Table 7: Comparison of physical properties of fuel gas A and B

Physical properties A B Molecular weight (Kg/kmole) 10.11 9.34 Specific gravity o.3498 0.3231 Men specific heat capacity (CPm) Kcal/kg 2.5702 2.8185

77D0

77 E 01

Figure 1: Fuel Gas Unit OF Port Harcourt Refining Company

46


Figure 2: Crude heater (Combustion)

Figure 3: Sensible heat of common gases basis 320F

2 1 4

3

1 = N2, O2, CO, NO, Air

47


Figure 4: TBP and EFV Curves

Figure 5: Graph of weight % vs Volume % and API Gravity

48


Figure 6: Relationship between the slope (%) of various distillation vaporization curves

Figure 7: Relationship between distillation temperatures at 50% vaporized and the flash (E.F.V) temperature at 50%

Fla

sh E

FV

49


Figure 9: Charge Crude (Pretreating) in Heat Exchangers

Figure 8: Vapour pressure and boiling –point corrections for normal paraffin

Above Critical Points Of Pure Compounds

Vap

or

F

50


Figure 10: Heat Content of Petroleum Fractions

BT BT

Figure 11 FDF Damper opening Vs efficiency and oxygen in flue gas

51

P.U. Uzukwu and E.T. Iyagba: Continental J. Engineering Sciences 7 (2): 31 - 51, 2012 Received for Publication: 09/05/12 Accepted for Publication: 11/07/12 Corresponding author P.U. Uzukwu African Regional Aquaculture Centre, PMB 5122, Port Harcourt, Nigeria. E-mail [email protected]

Figure12: Heater efficiencies for fuel, Gas A & B

Figure13: Specific heats of mid Continent oil vapor with a correction factor for ther bases of oil (Helcomb and Brown, Ind. Eng, Chem.).

cp B

TU

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