Docshare

Published on April 2017 | Categories: Documents | Downloads: 62 | Comments: 0 | Views: 314
of 6
Download PDF   Embed   Report

Comments

Content

International Journal of Modeling and Optimization, Vol. 1, No. 1, April 2011

Improving of Refinery Furnaces Efficiency Using
Mathematical Modeling
Mir Esmaeil Masoumi and Zahra Izakmehri


process is generally done by furnaces. Furnaces, in essence,
are a kind of heat exchanger that transfer the thermal energy
obtained from burning fossil fuels in a closed space to a
process liquid which in coils or locked up pipe flows.
Heaters are usually designed for uniform heat
distribution .the average radiant heat flux specified is defined
as the quotient of total heat absorbed by the radiant tubes
divided by the total outside circumferential tube area inside
the firebox, including any fitting inside the firebox. The rows
of convection tubes exposed to direct radiant shall be
considered as being in the radiant section and the maximum
radiant heat absorption rate shall apply to these tubes,
irrespective of whether extended surface elements are used or
not. The maximum radiant heat flux density is defined as the
maximum heart rate to any portion of any radiant tube [1].
One of the most common furnaces in industry is the draft
type which operates by high temperature difference between
burner and stack. This means gases density inside furnace will
be less the density of the air of surrounding area. This
difference in the density causes that pressure inside furnace to
be less than pressure of the air at each point of the same height
outside furnace. Therefore, all points inside furnace have
lower pressure relative to the pressure of surrounding area.
This results in a relative negative pressure. This phenomenon
is termed „chimney effect‟ or „natural draft‟. Under influence
of this phenomenon, the air required for burning is naturally
sucked in and after mixing with fuel and burning, resulted
gasses from burning transfer their heat to process liquid and
exit stack [2].
Furnace designs vary as to its function, heating duty, type
of fuel and method of introducing combustion air. Different
typical furnace configurations for petroleum applications are
shown in Fig. 1. The preferred design of furnaces is mostly of
the radiation–convection type, since it uses the flue gas heat
more effectively getting higher thermal efficiency and lower
fuel consumption (lower operating costs) than the stand alone
convection or radiation types. Some types of process fired
heaters presented in Fig. 1 are: (a) radiant, shield, and
convection sections of a box-type heater; (b) heater with a
split convection section for preheating before and soaking
after the radiant section as can be seen in Fig. 1, furnaces have
some common features, however. The main parts of a furnace
are the radiation chamber, convection section, burners, tubes,
and stack. The heat input is provided by burning fuel, usually
oil or gas, in the combustion chamber. Fuel flows into the
burner and is burnt with air provided from an air blower [3].
Increase in thermal performance of furnaces, given
increase of fuel price in recent years, is a very important issue.
Correct design and optimally setting operational condition
has impact enhancing performance of furnace. Thermal

Abstract—Approximately 75% of energy consumption in
petrochemical and refining industries is used by furnaces and
heaters. Ambient air conditions (pressure, temperature and
relative humidity) and operational conditions such as
combustion air preheating and using excess air for combustion,
can affect the furnace efficiency. If the furnaces are operated at
optimized conditions, the huge amounts of savings in energy
consumptions would be achieved. By modeling and optimizing of
a furnace the optimal operation conditions can be obtained. The
aim of this paper is providing a mathematical model which is
able to calculate furnace efficiency with change in operating and
combustion air conditions. In this paper the furnace of
atmospheric distillation unit of a refinery in Iran was considered
as a case study. Presented model, first examines changes in
ambient air conditions and then presented optimized design of
the furnace including excess air reduction and preheating of
burning air methods. The furnace is modeled mathematically
and simulated by software. Verification of the developed model
against the design data highlighted the reliability of the model
predictions. The optimal operation conditions to get the
maximum efficiency are introduced. The most commonly used
optimization methods (excess air reduction and air preheating)
are applied to the furnace. The results shows that the preheating
of air up to 485.6 and reducing of the excess air until 15%,
reduces the exhaust gas temperature from 1000 to 402 and
increases the furnace efficiency from 63% to 89% . This is a
significant saving in energy. Also by increasing the heat transfer
area, the furnace capacity could be increases up to 30% without
any change in furnace efficiency. The results show that by
investment of 5.23 M$, could be earn 5.81 M$/Yr saving in
energy costs, then the payback period was 0.9 year. Economical
results also show that the investment purchases and saving
benefits cover each other with acceptable payback period in all
cases of optimization methods.
Index Terms—Eefficiency, Furnace, Modeling, Optimization

I. INTRODUCTION
Approximately 65-90% of total refineries energy for
heating is provided by furnaces. Chemical industries such as
oil, gas and petrochemical comprise a set of diverse heating
and cooling processes in many of them it is necessary that
some of liquids to be heated to a certain temperature. This

Manuscript received March 15, 2011.
M. E. Masoumi, Islamic Azad University, Tehran North Branch, Faculty
of Engineering, Chemical Engineering Department, P.O.Box:19585/936,
Tehran, IRAN (correspondence author, phone: +98 21 88 77 07 42; fax: +98
21 88 78 72 04; e-mail: [email protected])
Z. Izakmehri, Islamic Azad University, Tehran North Branch, Faculty of
Engineering, Chemical Engineering Department, P.O.Box:19585/936,
Tehran, IRAN (e-mail: [email protected])

74

International Journal of Modeling and Optimization, Vol. 1, No. 1, April 2011

efficiency usually is defined as ratio of absorbed heat to total

incoming energy [4].

Fig. 1. Different box furnace configurations.

Galitsky & Worrel by controlling variables such as
percentage of excess air and amount of oxygen in outgoing
gasses showed and assessed importance of performance
increase of used energy in furnaces and its relation to decrease
of operational cost and amount of pollution. By these methods,
up to 18% saving in furnace energy consumption could be
achieved [5].
Jegla, using optimization of stack temperature and air
heating system, registered a new method for furnace operation.
This method is based on process integration using pinch
technology and is for saving in energy consumption. This
paper shows that using of gasses exiting stack energies for
heating the air by a little change in operational parameters,
could reduces annual energy costs of a refinery up to 20% [6].
Also in recent years, jegla presented a method for design of
furnaces burner which was based on models developed by
Lobo-Evans, Bloken and by defining a target function based
on minimizing annual costs of furnace, presented its optimum
design [7].
There are different methods for increasing a draft type
furnace performance. The most common and effective ways
are:
1) Reduction of thermal wasting in walls using insulation
2) Improvement of temperature condition in burner
3) Improvement in energy recycling in transport sector
4) Reduction of unburned carbon on internal and external
surface of the furnaces.
5) Installation of pre-heater
6) Control of excess air
Amongst the above mentioned methods, pre-heating of air
is normally applied possible for large furnaces and method of
excess air control is one of the most common methods which
are recommended for furnaces with low thermal performance.
However its effectiveness is and depended on operational
conditions and can be studied [8].
Burning process requires a certain amount of air that for its
accurate calculation, fuel combination should be determined.
If in burning process of hydrocarbons there is
not enough oxygen available, compounds like carbon
monoxide are created which have undesirable effects on bioenvironment. Therefore for obtaining full burning and
ensuring polluting substances are not formed, a percentage of
excess air is normally considered for the combustion process.
Despite the fact that use of excess air, prevents production
of compounds like carbon monoxide, but in practice, its
amount can not be more than an optimum level. Since any
75

excess air which does not react with air will escape from the
exits stack, the more its amount is, the more thermal energy is
wasted. Hence as a principle for design and operation of
furnaces the amount of excess air is regulated in an optimal
condition which prevents both incomplete burning of the fuel
and thermal energy losses. Two main parameters used in
examining a furnace performance are temperature of exiting
gasses from stack and amount of excess air (or oxygen), in
stack gasses. As a rule of thumb, reduction of excess air of
stack gasses to the amount of 10% or reduction of temperature
stack gasses for 20ºC by pre-heater of the air, will cause 1%
increase in furnace performance.
In order to enhance furnace or boiler‟s efficiency and
improvement of its functioning condition, the first and most
effective action is regulation of excess air. At the moment, in
most furnaces and boilers, amount of excess oxygen and draft
of stack gasses are measured which are proportional to excess
air. Desirable amount of excess oxygen in furnaces and
boilers gas fuel shown by analyzer in exiting gasses is 3% and
suitable amount of draft is about - 0.3 [9].
If there is no sufficient air for burning of fuel, then diffusion
of unburned hydrocarbons and monoxide will increase.
However a high level of excessive air in combustion process
will produce NOx Fig.2 shows amount of diffusion of CO and
NO with excess air in burning stoichiometric methane with
the air in ambient temperature. Increasing the excess air, the
amount of CO decreases but that of NOX decreases sharply
before declining. Therefore it is crucial to have an optimum
amount of excess air in the combustion process in order to
control both CO and NOX.
Burning efficiency depends
on ratio of fuel to the air. In practice, use of 2 to 3% excess
oxygen (about 15% excess air) indicates most suitable
performance [10].

Fig. 2. Emissions of pollution for stoichiometric burning of methane with air

In this paper, an optimal mathematical model for designing
of industrial furnaces is developed. The presented model first
examines changes in ambient air conditions such as
temperature, pressure, and relative humidity and then design
the furnace using best optimize methods including reduction
of excess air and pre-heating of burning air. The

International Journal of Modeling and Optimization, Vol. 1, No. 1, April 2011

aim of this paper is to provide a mathematical model which is
able to calculate furnace performance at various conditions,
and then optimize it. In addition, study of economic costs is
amongst goals of this research.

Mean Beam Length is defined in terms of the ratios of length,
height and in terms of diameter and height [14].
(13)
The exchange factor (F) is a function of gas emissivity (PF)
and the ratio of refractory area (AR) to the equivalent cold
plane area (

II. FURNACE MODELING
The combustion equation of hydrocarbon in air can be
represented as in Eq. (1).
a CXHY+ b O2 +c N2 +d H2O→e CO2 +f H2O+ g O2 +c N2
(1)
Using mass balance the percent of O2, on a dry volume
basis can obtained by Eq. (2).

The wall area in terms of length (L), center-to-center
spacing (C), and number of tubes per row (N) is defined as:

Ambient air temperature, pressure, and humidity affect
the air flow rate [9]. If the air temperature increases, the air
flow rate decreases. As definite temperature, pressure and
humidity of air increases, water vapor in ambient air is
decreases and result to decrease the percent of O2. So the air
flow rate through the burner at definite pressure drop by using
corresponding-states defines as Eq. (3) [11,12].

The furnace design requires the computation of the heat
transfer areas in each furnace section. In particular, the heat
transfer area for the convection section (ACon) is computed as
follows:
Where LM refer to the log mean base temperature difference
which is defined as follows:

The absorbed heat in convection and radiation section of
furnace by crude oil can be represented by Eq. (4).
QA= QRad + Qcon = (Hout- Hin) = Moil Cp (Tout –Tin)
(4)
Furnace thermal efficiency is defined as the percent ratio of
the total heat absorbed in a furnace to the net heat-released.
Then considering radiant heat loss and heat losses by hot flue
gases discharged through the stack, the net heat released
represented as Eq. (5).
(5)
The gas heat content entering the stack (

Heat transfer coefficient could estimate from trial term:
The wall effect usually ranges in magnitude between 6% and
15% of the sum of the pure convection and radiation
coefficients [13].
(19)
The gas radiation coefficient is defined as:

depends on

gas temperature at that point (TS) and excess air (x) used in the
gas combustion according to a given functionality. The heat
loss (Q1) is considered in the range from 1% to 3% of the
net-heat release (Qf). From Eq. (4) and (5), the fired box
efficiency (
, which depends on gas temperature at stack
inlet and excess air, is computed according to the following
equation [3] :

Where:

Refractory walls radiation coefficient is defined as:
where:

The convection gas film coefficient is defined as:
The heat absorbed in radiation chamber
to lobo-Evans method is defined as Eq. (8) [13].

(24)

according
where:

The absorptive ( depends on tube spacing (C) and the outer
diameters of tubes (D0) and defined by Eq. (9).
III. CASE STUDY
The emissivity (PF) can be correlated as a function of the gas
average temperature and PL factor like Eq. (10).

For verification of the proposed model with literature, the
distillation unit furnace of a refinery in Iran was considered as
a case study. The operational data and furnace parameters are
listed in table 1.
All data needed for furnace design calculations was not
accessible, so for calculating
that present in
Eq. (8) until Eq. (23) Petrosim simulating software was used.
Also by using from Eviews software, furnace process data
was fitted and constant parameters in mentioned Eq's was

Partial pressure of CO2 and H2O (PL), is a function of carbon
– hydrogen ratio of the fuel and percentage of excess air can
be defined as Eq. (11).
where:
76

International Journal of Modeling and Optimization, Vol. 1, No. 1, April 2011

calculated and presented in table 2.
A computer program was prepared using MATLAB
software for designing the furnace and calculating the
parameters. It has also been used for computing economical
aspects of the furnace.
A comparison between basic designed and calculated
parameters of studied furnace by prepared software was
achieved and presented in table 3.
The results show that the modeling results and design data
are very consistent (Table 2). This indicates that the
developed model was reliable and it could be used for
studying the effect of ambient conditions on furnace design
ambient temperature and relative humidity at atmospheric
pressure on oxygen demand of furnace. As can be seen by
increasing the ambient air temperature, the oxygen demand of
furnace is reduced. Fig. 4 shows the effect of excess air and
stack flue gas temperature on heat lost from furnace. As can
be seen by increasing the excess air, the heat loses from
furnace is increased.

each other from economical point of view. Also presented
results compared with design and in the case evaluated by
energy saving. This studied furnace used 100% of excess air
in operational condition, have 1000 stack temperature and
63% thermal efficiency.

Fig. 3. %O2 vs. ambient air temperature as a function of relative humidity

TABLE1: THE PARAMETERS OF STUDIED REFINERY FURNACE

Furnace type: box type
Fuel combinations

Ambient temperature ( )
Relative humidity
Firebox temperature ( )
Fuel temperature ( )

Co (0.25%), N2(4.95%)
H2(23.79%), H4(23.5%),
C2H6(23.61%),C3H6(2.92%)
C3H8 (6.53%), C2H4 (7.471%)
41-113
70-75%
1550-1650
60
Fig. 4. %Heat loss vs. %O2 as a function of stack flue gas temperature

TABLE2: THE CONSTANT PARAMETERS OF DIFFERENT ESQ.‟S WHICH WAS
CALCULATED BY SOFTWARE FOR THE STUDIED CASE

Eq.

C1

C2

C3

C4

C5

6
9
10
12
14
20
22
24

3.12 E-5
0.0016
-9.517 E-9
0.0007
-0.0156
-0.0086
8.91 E-16
1.389 E-8

1.3
-0.0907
0.32
-0.02
-1.2698
31.2
0.0025
4.286 E-6

0.51
1.1554
-0.091
0.278
0.069
-35.21
-7.55 E-14
0.011

------0.008
---1.88
6.7 E-6
-0.83
-0.143

-------0.341
---0.0703
----------

TABLE3: COMPARISON OF BASIC DESIGN AND CALCULATED PARAMETERS OF
STUDIED

FURNACE

parameters

model

design

Tg (
TW ( )
Tair )
Qrad (Mbtu/hr)
Qconv (Mbtu/hr)
Acon (ft2)

1700
734.6
485.6
168.47
59.3
18779

1660
798
509
180.13
60.04
18579

Fig. 5. Results of the proposed Eq. (7) to calculate efficiency as function of
stack gas temperature and excess air.

The studies shows that by repairing heater wall and adjust
damper the excess air and stack flue gas temperature could be
reduced by 40% and 800
respectively. Then the furnace
efficiency increases up to 76%. If the investment expenses of
heater wall maintenance and damper adjustment control
system are calculated and compared with saving due to
increased furnace efficiency, the payback period was
calculated 0.76 year which was acceptable from economical
point of view.
The preheating of combustion air is the relevant method
for more reducing on excess air, stack exit flue gas
temperature and increasing furnace efficiency. So if a
preheater set in the incoming combustion air line, by using
energy balance calculations and dew point restrictions,
preheated air temperature calculated 485.6
In result, exit
flue gas of stack temperature reduced to 402 and excess air
consumptions will reduce to 15%. Also the furnace efficiency

Eq. (7) shows that the thermal efficiency percent (
) for
a gas is a function of stack gas temperature (Ts) and excess air
percent (x), so the thermal efficiency could be calculated
using this equations. Fig. 5 presents the results of this
calculation.
IV. OPTIMIZATION
For furnace optimization and increasing of efficiency,
reduction of excess air, air preheating and increasing of heat
transfer area was considered in this work and compared with
77

International Journal of Modeling and Optimization, Vol. 1, No. 1, April 2011

At= Area of tubes in convection (ft2)
Acon= Convection heat transfer area (ft2)
ACP= Cold plane area (ft2)
AR= Refractory area (ft2)
Ab= Area of burner throat (ft2)
a=Volume fraction of hydrocarbon in the ambient air-fuel
mixture
b= Volume fraction of o2 in the ambient air-fuel mixture
c=Volume fraction of N2 in the ambient air-fuel mixture
c1, c2, c3= Constant of coefficient
C= Distance between tube centers (ft)
d=Volume fraction of H2O in the product mixture
e= Volume fraction of CO2 in the product mixture
f= Volume fraction of o2 in the product mixture
F= Correction factor
G= Mass velocity at minimum cross-section (Btu/ft2.hr)
hcr= Gas-radiation coefficient (Btu/ft2.hr)
hcc= Convection gas film coefficient (Btu/ft2.hr)
hr= Total apparent gas film coefficient (Btu/ft2.hr)
H= Height of firebox (ft)
kb= Pressure loss coefficient through burner
LM= logarithm means temperture difference from flue gas
to fluid (
MLB= Mean beam length (ft)
MOil= Oil mass fow-rate (MBtu/hr)
moair= Mass flow rate (Ib/hr)
N= Total number of radiant
∆pb= Airside pressure drop across burner (psig)
PF= Gas emissivity
Qcon= Heat transfer rate absorbed in the convection section
(Mbtu/hr)
QA= Heat absorbed by the oil (Mbtu/hr)
Qrad= Heat transfer rate absorbed in the radiant section
(Mbtu/hr)
Qgs= Heat content of gas leaving the convection section
(Mbtu/hr)
Qn= Net heat-released from the fuel combustion (Mbtu/hr)
Tcw= Average tube wall temperature (convection section)
(
Tgc= Avarage gas temperature(convection section) (
Ti= Inlet temperature of oil
Tc= Cross-over oil tempreture
Tg= Exit gas tempreture
Ts= Inlet stack tempreture
Tw= Average tube wall (
U= Over-all transfer cofficient (btu/ft2.hr)
W= Wide of firebox (ft)
Greek symbols
= Absorptivity of a tube surface

increase up to 89%. If the capital investment expenses of air
preheater apparatus are calculated and compared with saving
due to increased furnace efficiency, the payback period was
calculated 1 year which was acceptable from economical
point of view.
The details of results in different cases are summarized in
table 4. Case 1 refere to the existing furnace, case 2 refere to
reducing of excess air strategy and case 3 for combustion air
preheating plus reducing of excess air. As can be seen the
results of case 3 are more economic than case 2. Also in case 3
furnace is operated at normal design conditions.
In the revamping project where the increasing of the
process throughput are considered, the main goal of the
furnace optimization is the incresing of the furnace capacity
without any changing in efficiency of the furnace. So the
studies of this work show that the increasing of the furnace
capacity up to 30% without any compromising in furnace
efficiency are possible. Increasing of the furnace capacity
requires more heat transfer area in furnace. If the investment
expenses of added area are calculated and compared with
saving due to increased furnace capacity, the payback period
was calculated 0.9 year which was acceptable from
economical point of view. The details of results are
summarized as case 4 in table 4.
TABLE 4: OPTIMIZATION RESULTS OF STUDIED FURNACE IN DIFFERENT
STRATEGIES

Parameter

Case 1

Case 2

Case 3

Case 4

Excees air %

100%

40%

15%

15%

Tg (

1000

800

402

402

Efficiency ( )

63%

76%

89%

89%

0.9964

0.9964

0.9964

0.9964

Arad (ft2)

6535.4

6535.4

6535.4

9219.6

PF

0.1795

0.3559

0.38

0.37

F

0.3643

0.5139

0.5304

0.4

Qrad (Mbtu/hr)

63.5

89.67

92.5

95.7

QF (Mbtu/hr)

203.63

231.3

228.9

297.6

hcc (btu/ft2. .hr)

4.47

4.255

4.11

4.11

hcr (btu/ft2. .hr)

2.42

2.0522

1.73

1.73

hcw (btu/ft2. .hr)

9.95

9.95

9.95

9.95

U (btu/ft2. .hr)

7.2328

6.6393

6.183

6.183

LM (

709.267

562.7

436.08

436.08

ACon (ft2)

10993

15094

20920

23889

E. saving (Mbtu/hr)

3.52

3.94

4.401

5.81

Payback time (yr)

-

0.76

1.06

0.9

V. CONCLUSIONS
In this paper different method of energy saving in the
refinery furnaces was evaluated. The results show that the
control of excess air has significant effect on increasing
furnace efficiency but did not adequate in furnace energy
saving projects. So to increase the efficiency, combustion air
preheating in line with beside of excess air reduction should
be considered. Also by optimizing the furnace conditions,
incresing of furnace capacity without changing of furnace
efficiency would be possible in refinery proposed optimizing
strategies are promising.
Nomenclatures:
Aw= Area of walls in convection (ft2)

= Stefan bolltzman constant
= Fired heater efficiency
Subscripts:
ATAP= Actual temperature and actual pressure
CXHY= Hydrocarbon
REFRENCES:
[1]

78

Alireza bahadori, Hari B.Vuthaluru. "Novel predictive tools for design
of radiant and convection sections of direct fired heaters", Applied
Energy, Vol. 87, pp.2194-2202,2009.

International Journal of Modeling and Optimization, Vol. 1, No. 1, April 2011
[2]
[3]

[4]
[5]

[6]

[7]

Francis Wildy, "Fired Heater Optimization", AMETEK
Process
Instruments, 2000.
S. Mussati, Juan I."Manassaldi,Mixed Integer Non Linear
Programming Model For The Optimal Design Of Fired
Heaters",Applied Thermal Engineering, Vol. 29, pp.2194-2204, 2009.
Hassan Al-Haj Ibrahim."Thermal efficiency of fired heater," 2008.
Worrell E,Galitsky C. "Energy efficiency improvement and cost saving
opportunities for petroleum refineries", Lawrence Berkeley National
Laboratory report LBNL-56183. Berkeley, CA.
Z .Jegla, P. Stehlik, J. Kohoutek, "Plant energy saving through efficient
retrofit of furnaces", Applied Thermal Engineering Vol.20, pp.
1545-1560, 2000.
Z Jegla,"The Conceptual Design of a Radiant Chamber and
Preliminary Optimization of a Process Tubular Furnace ", Heat
Transfer Engineering, Vol.27, pp.50-57, 2006.

[8]

[9]

[10]
[11]
[12]

[13]
[14]

79

Jegla,Z., Kohoutek, J., And Stehlik, P.," Global Algorithm For
Systematic Retrofit Of Tubular Process Furnaces", Applied Thermal
Engineering, Vol. 23, pp.1797-1805, 2003.
Taal, M., Bulatov, I., Klemes, J. And Stehlik, P., "Cost Estimation And
Energy Price Forecasts For Economic Evaluation Of Retrofit
Projects,Applied Thermal Engineering", Vol.23, pp. 1819-1835, 2003.
A.Garg.”Revamp fired heater rating “.hydrocarbon processing,
pp.67-80, 1998.
Bussman W, Baukal C. "Ambient condition effects on process heater
emissions", Energy, 2008.
W.R. Bussman,C.E. Baukal,"Ambient condition effects on process
heater efficiency", John Zink Co. LLC, 11920 East Apache, Tulsa,
Energy,2009.
W.E. Lobo, J.E. Evans, "Heat transfer in the radiant section of
petroleum heaters",Trans AIChE , 743–778. 1939
Nelson, W.L. "Petroleeum Refinery Engineering", 4th ed, Mc
Graw-Hill.

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close