Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

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applying the following equation to estimate emissions from all sources in

a stream:

EVOC ¼ FA  WFVOC  N ð17:9Þ

where EVOC is the emission rate of VOC from all sources in kg/year, FA is the

applicable average emission factor, WFVOC is the average weight fraction of

VOC in the stream, and N is the number of pieces of equipment grouped in the

relevant category according to we ight fraction of VOC.

The following steps for fugitive emission calculation are:

1. Develop an inventory of the nu mber and type of fugitive sources:The

number and service type of each equipment in the refinery must be

determined.

2. Group the inventory into streams: To simplify calculations, it is recom-

mended that the equipment/service mode combinations identified in

Step 1 (i.e. valves in gas service) be grouped into streams according to the

approximate weight fraction of VOC in each stream.

3. Note operational hours: The number of operational hours needs to be

estimated.

4. Use emission factors to estimate emission rates: Use the relevant emission

factors and the equation given above to calculate the emissions from each

equipment type. These emissions should then be added to derive a total

emission rate for all equipment pieces quantified using this methodology.

Table 17.8 presents the emission factors required to estimate emissions

using the steps discussed above.

Example E17.6 illustrates the application of the average emission factor

approach.

Example E17.6

In the calculations of fugitive emissions from a reﬁnery site, the following

inventory steps are carried out:

1. A particular section of a refinery has a count of 300 valves (Step 1)

2. It is ascertained that 200 of these are in gas service (Step 1)

3. Within these valves in gas service, it is ascertained that 100 (N ¼ 100) valves

are with 80 wt% (non-methane hydrocarbons) (WFVOC ¼ 0.8) and 20 wt%

methane. Therefore, the rest of the valves do not emit any VOCs and will not

be included (Step 2)

4. It is estimated that this group of valves operates for 5500 h/year (Step 3)

5. The appropriate emission factor for valves in gas service is FA ¼ 0.0268 kg/

h/source (from Table 17.8) (Step 4).

Calculate the emission rate of VOC.

436 Chapter 17

Solution:

The ﬁnal emission estimate for the group of 100 valves speciﬁed above is

calculated from equation (17.9)

EVOC ¼ (0.0268) (0.8) (100) (5500) ¼ 11,790 kg VOC/year.

17.3.1.1.4. Gas Dispersion Air pollution models constitute a set of

formulas that take into account:



Sources of pollution in a given area



The amounts of pollutants emitted by each source



Chemical reaction transformations



Different meteorological conditions and topographical features



Other factors that affect dispersion of pollutants

The short term concentration model for pollutant sources uses the

steady-state Gaussian plume equation for a continuous elevated source.

For each source and each hour, the origin of the source’s coordinate system

is placed at the ground surface at the base of the source. The x-axis is

positive in the downwind direction, which the y-axis is crosswind (normal)

to the x-axis, and the z-axis extends vertically. The fixed receptor locations

are related to each source’s coordinate system. The hourly concentrations

Table 17.8 Average fugitive emission factors (API, 1991)

Equipment Service Emission factor (kg/h/source)

Connectors Gas 2.50  10

4

Light liquid 2.50  10

4

Heavy liquid 4.34  10

3

Flanges Gas 2.5  10

4

Light liquid 2.5  10

4

Heavy liquid 4.68  10

3

Compressor seals Gas 0.636

Pump seals Light liquid 0.114

Heavy liquid 3.49  10

3

Valves Gas 0.0268

Light liquid 0.0109

Heavy liquid 0.87  10

3

Open-ended lines All 2.30  10

3

Pressure relief valves Gas 0.16

Sampling connections All 0.015

Drains All 0.032

Environmental Aspects in Refining 437

calculated for each source at each receptor are summed to obtain the total

concentration for all sources.

The hourly concentration at downwind distance x (meters) and cross-

wind distance y (meters) is given by (Wark and Warner, 1981)

Cðx; y; zÞ¼

ð2pÞs

exp 



exp

 z  HðÞ

þ exp

 z þ HðÞ

"#()

ð17:10Þ

where Q is the pollutant emission rate (mass per unit time), s

, s

are the

standard deviation of lateral and vertical concentration distribution (m),

¼104 X

0.894

(at neutral stability class), s

¼61 X

0.911

(at neutral stability

class), X is the downwind distance (km), u is the mean wind speed (m/s) at

release height, and

H is the source height (m).

Equation (17.10) includes a vertical term, a decay term, and dispersion

parameters. It should be noted that the vertical term includes the effects of

source elevation, receptor elevation, plume rise, limited mixing in the

vertical, and the gravitational settling and dry deposition of larger

particulates.

Example E17.7

1000 kg/h fuel is burned in a furnace. The fuel contains 4 wt% sulphur. The

furnace stack is 35 m high. A residential area is about 1 km downwind from

the stack. On a day of 5 m/s wind, calculate the amount of SO

that reaches the

residential area.

Solution:

Emission rate of SO

from the furnace stack ¼ (1000 kg/h) (0.04 S) (64/32) ¼

80 kg/h SO

Q ¼ 22.2 g/s

Y ¼ 0 since the residential area is downw ind

Z ¼ 0 since residential area is at ground level

X ¼ 1.0 Km, then

¼ 104 X

0.894

¼ 104 m

¼ 61 X

0.911

¼ 61 m

ðpÞs

exp

ðHÞ

"#()

22:2ð10

pð104Þð61Þð5Þ

exp

 35ðÞ

2ð61Þ

"#()

¼ 189

This 189 mg/m

is equivalent to 72.3 ppb which is greater than the standard

daily average of SO

concentration of 60 ppb.

438 Chapter 17

17.3.1.2. Treatment and Control

Pollution con trol equipment sh ould be desig ned and ope rated in s uch a

way that the substances released from the equipment have the m inimum

influence on the environment. Table 17 .9 liststypicalairemissionsand

suggested reduct ion methods for each pollutant. In g eneral, pollution

control equipment should be kept running during start-up and shutdown

for as long as is necessary to ensure compliance with release limits.

Table 17.9 Potential BAT

air emission reduction requirements for reﬁneries

(CONCAWE, 1999)

Process Air em issions Emission reduction method

Incinerator SO

,CO

and

particulates

Use reﬁnery or natural gas fuel

Incinerator NO

Use low NO

burner (<100 mg/Nm

)

HDS unit H

S Use alkanolamine adsorption

Gas processing VOCs, H

S Maintenance

Sulphur recovery

plant

S, SO

Maintenance

Catalytic

reformer

Use low NO

burner and ﬂue gas

recirculation (<100 mg/Nm

)

Alkylation unit HF Maintenance and continuous

monitoring of emissions

Fluid catalytic

cracking

Hydrogenate the vacuum distillate and

install FGD

(<100 mg/Nm

)

Fluid catalytic

cracking

Use SCR system (<100 mg/Nm

)

Fluid catalytic

cracking

CO Install CO boiler (<100 mg/Nm

)

Thermal

cracking

(visbreaking)

Use low NO

burner (<100 mg/Nm

)

Storage tanks VOCs Install double seals on ﬂoating roofs

Steam boilers (gas

fuel)

Use reﬁnery or natural gas fuel

Steam boilers (gas

fuel)

Use low NO

burner (<100 mg/Nm

)

Steam boilers

(residual fuel)

Install FGD system

Steam boilers

(residual fuel)

Use SCR

system (<100 mg/Nm

)

best available techniques;

flue gas desulphurization;

selective catalytic reduction.

Environmental Aspects in Refining 439

To ensure continued optimu m perf ormance , all equipment should ha ve

regular preventative maintenance according to operational requirements.

Since the main air pollutants in a refinery are the sulphur compounds, such

as sulphur oxides and hydrogen sulphide, Chapter 15 discusses acid gas

processing and treatment technology in detail.

17.3.1.2.1. NO

Emission Treatment The major NO

emission sources in

a petroleum refinery are combustion processes. NO

emissions are usually

the result of the rea ction between oxygen and nitrogen at temperatures

above 1093



C (2000



F) in the air/fuel combustion flame. NO

emission

control technologi es a pplicable to these sou rces incl ude co mbustion

controls, S elective Catalytic Reduction (SCR) systems, Selective Non

Catalytic Reduction (SNCR) systems and a combination of these

techniques.

Most SCR systems use anhydrous ammonia as a reducing agent to convert

any NO

in a flue gas stream into nitrogen over a catalyst. The reactions are :

4NO þ 4NH

þ O

! 4N

þ 6H

O ð17:11Þ

2NO

þ 4NH

þ O

! 3N

þ 6H

O ð17:12Þ

Catalysts are usually oxides o f titanium, tungsten, vanadium and

molybdenum or in combination. The c atalyst o perates at tempe ratures

in the range of 127–232



C (260–450



F). Zeolite catalysts have been

applied successfully in SCR systems at 232



C(450



F). Over 95% NO

emission reduction ca n be achieved. In SCR operation care shou ld

be taken to reduce fouling and corrosion that poison the catalyst

(Ja afar et al., 2003).

Example E17.8

Flue gas at a ﬂow rate of 200 kmol/h from the FCC catalytic regenerator

contains 35 mol% NO

, 12 mol% NO and 47 mol% CO

. The ﬂue gas enters

SCR with 95% conversion of nitrogen oxides. The following two reactions take

place at 232



C and atmospheric pressure:

4NO þ 4NH

þ O

! 4N

þ 6H

2NO

þ 4NH

þ O

! 3N

þ 6H

Ammonia and oxygen are added to reactor. The reactor products are then cooled

using a ﬂash drum to 50



C and separated to collect water from the bottoms and

goes to the stack. Use UNISIM simulator, perform the material balance on

the process.

Solution:

The process ﬂow chart is shown in Figure E17.8, and a summary of results is

shown in Table E17.8.

440 Chapter 17

17.3.1.2.2. VOC Emission Treatment VOC emissions from refinery units

are controlled by incineration in a CO boiler, carbon adsorption, flares,

catalytic oxidation and condensation. Adsorption systems have been applied

to the control of VOC emissions from catalytic reformer purge gas streams.

All of these VOC emission control systems are capable of controlling

95–99% of the emissions from oil refinery sources.

17.3.1.2.3. CO Emission Treatment CO incinerators or boilers generally

oxidize 95% or more of the CO emissions to carbon dioxide. Wet gas

scrubbing (WGS) technology allows refiners to meet FCC emission regula-

tions. WGS advantages are:

Reactor

SCR

Flue Gas

Oxygen

Ammonia

Cooler

Flash

drum

Top

Bottoms

Figure E17.8 Process flow chart

Table E17.8 Summary of results

Feed to reactor

To p f r o m f l a s h

drum

Bottoms from

flash drum

(kmol/h) 70 3.4 0.095

NO (kmol/h) 24 1.2 0

(kmol/h) 106 103.5 3.05

(kmol/h) 50 11 0

(kmol/h) 160 1.146 2.5

O (kmol/h) 0 34.73 198.96

(kmol/h) 0 122.55 0

T (



C) 232 50 50

P (kPa) 101 96 96

Environmental Aspects in Refining 441



Replaces costly CO boiler



Reduces particulate and SO

emission



Produces environmentally safe wastewater discharge



Collects catalyst

After effective separation from the scrubbing liquid, the clean flue gas is

vented to the atmosphere and the liquid containing the particulates. SO

and NO

are disposed in an environmentally acceptable manner.

Figure 17.1 shows the WGS unit in the FCC process.

Seawater scrubbing uses the natural alkalinity of the seawater to remove

. The achieved environmental benefits of SO

recovery can be as high as

99%. The sulphur content of the effluent seawater is increased by approxi-

mately 3%. Special maintenance should be carried out to minimize corrosion.

17.3.1.2.4. Particulate Emission Control WGS is very efficient (>90%)

for removal of particulates (4–10 m m) from the FCC regenerator exit

(Figure 17.1). Cyclones could be the first choice clean-up device for

particulates. As a particulate emission control, cyclones are situated inside

Expander

Cyclone

Regenerator

Air

Feed

Catalyst

Reactor

Hot flue gas

Superheated

Steam

Water

Flue Gas

Clean gas to

stack

Caustic and

makeup

water

WGS

Figure 17.1 Treatment of FCC flue gases byWGS

442 Chapter 17

the FCC reactor and regenerator. They are high-velocity devices which

return recovered catalyst to a dust hopper. Cyclones are efficient at remov-

ing particles in the range of 10–40 mm and above. Efficiencies can range

from 30 to 90%. Cyclones are more effective for coarser particles.

On the other hand, electrostatic precipitators (ESP) employ an electro-

static field to apply a charge to particulate emissions and then collect them

on grounded metal plates. ESP units are very efficient (99.8%) for removing

finer (4–10 mm) particulates from FCC regenerator gas. Typical particulate

emission levels achieved with ESP is <50 mg/Nm

. As a consequence of

the particulate reduction, the metals in the flue gas can be reduced to less

than 1 mg/Nm

. Particulate emissions from the FCC can thus be reduced to

1.1–2.3 kg/h. In the regenerator, the spent catalyst is regenerated by burn-

ing off the coke by utilizing air for combustion (Figure 17.2). The flue gas

from the regenerator is usually routed through multistage cyclone separa-

tors, power and heat recovery equipment, particulate emission control

equipment and then released to the atmosphere via the stack. The flue gas

from flue gas cooler passes through the ESP unit and the particulates flowing

through an electrostatic charged field are reduced to acceptable limits. Clean

gas from the ESP is vented to the atmosphere via the stack. The catalyst fines

are collected in hoppers below the ESP. Periodically, the hoppers are drained

and the catalyst fines are removed for further disposal (CONCAWE, 2006).

Expander

Cyclone

Regenerator

Air

Feed

Catalyst

Reactor

Hot flue gas

Superheated

Steam

Water

++++++

ESP

Flue Gas

withdust

Clean Gas

from dust

Dust

Collection

Figure 17.2 Treatment of FCC flue gases by ESP

Environmental Aspects in Refining 443

The ESP design basically depends on the gas flow velocity V

, the

electric field and the drift velocity o. The very high voltage differential

between electrodes causes electrons to pass at the high rate from the centre

wire into the passing gas stream. The electrons attach themselves to the gas

particulates. The particulates with negative ions migrate to the collecting

plates, which have either a positive charge or grounded. The migration or

drift velocity o is calculated (Wark and Warner, 1981) as:

o ¼

2:2  10

14

ð17:13Þ

where E is the electric voltage per distance (V/m), d

is the particulate

diameter (mm) and m

is the gas viscosity (kg/m h). The ESP efficiency  is

calculated based on plate spacing S, gas and drift velocities:

 ¼ 1  exp 



ð17:14Þ

Example E17.9

ESP with plates spacing of 23 cm is attached to 50 kV of electric power. The

ﬂue gas ﬂows at 1.5 m/s and carries particulates with 0.5 mm at 100



Calculate the plate length to capture 95% of the particul ates.

Solution:

Since the electric power is applied midway between plates, the distance from

wire to plate is 0.23/2 ¼ 0.115 m.

o ¼

2:2  10

14

2:2  10

14

ð50; 000=0:115Þ

ð0:5Þ

0:0863

¼ 0:024 m=s

0:95 ¼ 1  exp 

0:024L

1:5ð0:115Þ



gives L ¼ 21:5m

If we increase the efﬁciency to 99%, the length will increase to 33 m.

17.3.1.2.5. Emission Monitoring Continuous emission monitoring is

recommended for petroleum refinery air emission point sources for gas

temperature and gas flow rate as well as for NO

,SO

, CO, VOC and

total reduced sulphur concentration. In refinery steam boilers, process

heaters and process furnaces, it is required to monitor CO, NO

and SO

emissions. Catalytic cracking units have to monitor both NO

and SO

certain locations. The refinery must show evidence that the released emis-

sions are prevented or minimized through continuous monitoring.

444 Chapter 17

17.3.1.3. Waste Minimization and Prevention

The recent trend on this subject is to investigate pollution prevention

opportunities other than end-of-pipe treatment. Pollution prevention pro-

vides a wide range of options for reducing emissions to levels well below

those provided by treatment methods. The highest priority in pollution

prevention is given to source reduction, while the end-of-pipe treatment is

usually kept as the last and unfavourable option. The main advantage of the

pollution prevention approach is that it improves production and reduces

the adverse impacts of pollution before they happen.

Pollution prevention options are source reduction and recycle/reuse.

Source reduction includes actions to reduce the quantity or the toxicity of

the waste at the source. Examples include equipment modification, design and

operational changes of the process, redesign of products, substitution of raw

materials and use of environmental friendly chemicals. On the other hand,

recycle/reuse involves the use of pollutant-laden streams within the process.

Typically, separation technologies are key elements in recycle/reuse systems to

recover valuable materials such as solvents, metals, inorganic species and water.

17.3.2. Wastewater

Wastewater includes condensed steam, stripping water, spent caustic solu-

tions, a cooling tower and boiler blowdown, wash water, alkaline and acid

waste neutralization water, and other process-associated water. Wastewater

typically contains hydrocarbons, dissolved materials, suspended solids,

phenols, ammonia, sulphides, and other compounds. Water usage in the

refinery generates large volumes of water contaminated with oil and other

chemical impurities. Part of this water can be reused while the remainder is

discharged to water bodies. The discharged part must meet the environ-

mental regulations in its contents of metals and pollutants.

17.3.2.1. Emission Estimation

The first refining step is desalting in which a hot water wash extracts the

salts. If the feedstock contains aromatics with good solubility such as

benzene or toluene then some will be in the desalted effluent.

Process wastewater arises from desalting crude oil, steam stripping opera-

tions, pump cooling, product fractionator reflux drum drains and boiler blow

down. Table 17.10 shows typical wastewater coming from refinery units.

Recirculated cooling water must be treated to remove dissolved hydro-

carbons. Because the water is saturated with oxygen from being cooled with

air, the chances for corrosion are increased. One means of corrosion

prevention is the addition of a material to the cooling water that forms a

protective film on pipes and other metal surfaces. Therefore, the effluent

water from at the cooling tower contains dissolved hydrocarbons, antic-

orrosive chemicals and corrosion material.

Environmental Aspects in Refining 445