Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
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THE REFINERY ENVIRONMENTAL ISSUES 643
Controlling emission pollution from the refining processes
Most oil refineries do not have the very hazardous emissions usually met in petro-
chemical or chemical plants. In most refineries the pollution that is met with is in the
burning of fuels that may contain high levels of sulfur or nitrogen compounds. The
emission of VOCs is usually restricted to storage tanks and in some cases relief valves
open to atmosphere. The release of particulates from certain process plants such as
the FCCU and the Coking units is an emission that refiners can handle quite well.
In all cases the pollution control in a refinery starts with the design of the processes
and systems themselves, and extends to their proper operation. Some of the measures
undertaken to meet the Clean Air Act in the refining industry are as follows:
Fired heaters
Almost all refinery heaters have double service burners. That is the heater burners
can fire fuel gas, or fuel oil. The fuel gas stream will usually have been treated for
the removal of sulfur, in the gas treating plant. So normally this fuel source is not a
major pollutant problem. However all refineries have a ‘waste liquid’pool, and this
is used as the fuel oil stream to the heaters. Unless properly treated either in terms of
the streams that are routed to this pool or indeed the fuel oil pool itself, it becomes
the source for SO
2
pollution. The treatment must reduce the fuel oils sulfur content to
acceptable levels that, when burnt in the fire box, the flue gases satisfy the SO
2
levels
called for by the Act. Almost all refineries today hydrotreat most product streams
for the removal of sulfur and nitrogen except perhaps the very heavy residue stream.
Where these residue streams are used as part of the fuel oil pool they are usually
blended with hydrotreated middle distillate streams (gasoil, or even kero) to reduce
the total stream sulfur. The individual refinery planning schedules will be tailored to
meet this refinery fuel criteria.
The emission of NO
x
is quite another problem and this is usually controlled by the
minimum amount of excess air that is necessary for proper heater operation. The
solution to this problem often starts in the fired heater manufacturer’s design shop
where the best heater fire box geometry can assist in lowering or at least maintaining
the required excess air criteria and in the use of low NO
x
burners. Modern refinery
design engineering takes note of the stack height requirements so that the emission
fall out avoids populated areas as much as possible. It is now common refining
engineering practice among major contractors and heater manufacturers to utilize
developed computer programs which define the fallout contours at ground level
distances from fired heater stacks. These contour map programs are based on fuel
type and burning data, the proposed stack height, the prevailing wind direction,
and speed. The maps are used in the refinery layout studies (for new refineries
being engineered) in terms of locating processes and their fired heaters on the plant
644 CHAPTER 14
site. This assists in avoiding wherever possible particulate and other pollutants
contaminating high populated and ecologically sensitive areas.
Particulate emission from the fluid catalytic cracking unit
The process most vulnerable to this type of pollutant emission is the Fluid Catalytic
Unit (FCCU). In this case the emission of the particulates is either a problem in
operation or less frequently a break down of protection equipment—in particular the
cyclone separators located in the critical areas of the process (for example, in the re-
actor outlet and the regenerator flue gas outlet). The most common problem in fluidized
bed processes are air surges that cause a disruption of the fluidized bed. In the case of
the FCCU this fluidized bed is the catalyst bed being regenerated in the regenerator.
This disruption results in loss of the catalyst to atmosphere from the regenerator exit
stack. The incorporation of a CO boiler however does minimize this emission to
some extent. Usually these air surges are minimized by good flow control systems
with some anti surge system. The malfunction of the cyclones is a much more serious
problem which results in a plant shut down for repair.
VOC emission from refinery facilities
The major sources of atmospheric pollution by VOC emission in refinery are as
follows:
r
Relief and vent valves open to atmosphere
r
Leaks from poorly maintained control valves and flanged joints
r
Storage tanks
r
Rail and road tanker filling facilities
r
Ship loading and unloading facilities (jetty area).
Relief and vent valves normally discharge to a closed relief header which is routed to
flare system. However in some cases, because of the location of the relief and/or vent
valve and the quantity of the discharge material the valve(s) are open to atmosphere.
A typical example is the crude distillation unit relief valves. These are located at the
top of the distillation towers at about 200 ft above grade. Steam injection facilities
are installed at the valve discharge to facilitated atomization and dispersion. Never-
theless when these valves do open there will be some considerable VOC emission to
atmosphere. Any unscheduled event that would cause these valves to discharge are
extremely rare and do not warrant the extensive costs of increased flare header and
flare design that would be necessary to cater for these valve discharges.
Leaks from control (and other valves), pump seals, packing, and flanged joints are
related entirely to the refinery’s maintenance policy and program.
THE REFINERY ENVIRONMENTAL ISSUES 645
Perhaps the biggest source of VOC from a refinery and its operation is from the
storage tanks. This emission occurs during the tank filling, particularly in the case of
fixed roof tanks and LPG bullets and spheres. The emissions from floating roof tanks
are generally quite low and the function of a properly installed roof should suffice in
minimizing the emission from this source. Many of today’s refineries have installed
a closed inert gas circulating system. This system circulates an inert gas (mostly
nitrogen) throughout the refinery for many purposes which requires the absence of
oxygen in the air. One of these is the use of inert blanketing of fixed roof tanks.
Blanketing is often required in intermediate tanks whose contents are feed to processes
which need to eliminate oxygen. Extension of this inert gas to all fixed roof tanks as a
blanket prevents the emission of VOC during their filling. The storage of LPG is also
a potential for emitting VOC to the atmosphere. One method used to combat this is to
float the make/break valves of the spheres and bullets on the refinery fuel gas main.
Spillages from truck and railcar loading facilities have always been problematic.
Most spillages occur when filling the vessels. Most loading facilities nowadays have
automatic shut off on the loading arm nozzles, very similar to those commonly used
in public filling stations. These operate using a level sensitive device which closes the
filling valve at a prescribed filling level in the tank. The other source of VOC emission
is a poorly designed slop system that allows the draining of bottoms from road tankers
and railcars. Such a system should allow for steam out facilities on vessels carrying
heavy petroleum cuts. Vents on this system should be routed to the flare header.
The jetty and ship loading and unloading is also a potential source of VOC emission.
Again as in the case of the tank farm and product loading facilities the most vulnerable
product in this respect is the handling of the light hydrocarbon streams. In the loading
of LPG a flash recycle system has been found to be effective. In this system the flashed
LPG which occurs as the LPG first enters the ships tank which when empty would be
close to atmospheric pressure and temperature. This flashed vapor is then routed
under pressure control to a compressor and cooler assembly where it is liquefied and
returned to the LPG feed stream. The system continues to operate throughout the
loading activity acting then as the loading relief system.
Other sources of emissions of atmospheric pollutants
The most notable of these is the refinery sulfur recovery plant, and in particular the
sulfur storage pit. Most refineries have a sub-surface tiled pit for storing the liquid
sulfur product from the sulfur plant. This pit is covered by concrete or other suitable
slabs. These are sealed to prevent the emission of the SO
2
that is always present above
the liquid sulfur. The SO
2
that does accumulate is normally vented off to a small
bullet partially filled with water. The ensuing sulfuric acid formed is either used in
the refinery for caustic neutralization or sold to other users.
646 CHAPTER 14
In bitumen production the heavy vacuum residue, which is the basis for most bitumen
grades, may be air blown. This becomes necessary to meet certain bitumen grades.
The air used in this air blowing procedure must be perfectly dry to avoid an explosion
in the reactor vessel. (Refer to Chapter 3.9 for details on the air blowing process.)
Under normal operating conditions the vapors exiting the reactor vessel are routed to
the flare header. Occasionally however an explosion does occur in the reactor. When
this does happen the explosion doors (located at the top of the vessel) blow open.
This emits the reactor vapors which are high in SO
2
and VOCs. The plant usually
returns to normal pretty quickly, and the explosion doors close as the pressure inside
the vessel returns to normal. There is no way to eliminate the effect of this occurrence
except, perhaps, to ensure that the air used be dry.
14.3 Noise pollution
Noise problems and typical in-plant/community noise standards
Noise has been widely organized as a major industrial/environmental problem in
most processing plants because of the risk of hearing loss involved when workers are
exposed to high noise levels.
The high noise levels in process plants can be attributed to a great number of sources.
Major noise sources are compressors, fans, pumps, motors, furnaces, control valves,
steam and gas turbines, and piping systems. The noise generating mechanism for each
piece of equipment is complex but, in most cases, the noise levels can be reduced to
desired limits through the implementation of proper noise control measures.
With increasing awareness of the noise problem and its effect on the general public,
regulations on noise standards have been adapted in many countries throughout the
world. In the United States, the Occupational Safety and Health Act (OSHA) of 1970
(29 CFR 1910.95) and the Noise Control Act of 1972 and later amendments have
served as basic guidelines for noise control requirements. OSHA contains maximum
permissible sound pressure levels for each daily time of exposure. These guidelines
are presented in Table 14.6 and serve as a basis for in-plant noise criteria for any
process plant constructed in the United States.
Conversely, community noise criteria are more variable and depend on a number
of factors including local ordinances, existing noise levels and the site of the plant
with respect to the community. Some typical community noise limits are shown in
Table 14.7.
The art of acoustics and noise control is beyond the scope of this paper. Only the most
important concepts necessary for an analysis of process plant noise will be considered.
THE REFINERY ENVIRONMENTAL ISSUES 647
Table 14.6. OSHA noise exposure
limits
Duration per Sound level,
day, hr DBA
890
692
495
397
2 100
1.5 102
1 105
0.5 110
0.5 or less 115
The classification of different areas of the community in terms of environmental noise
zones shall be determined by the Noise Control Office, based upon assessment of
community noise survey data.
The Noise Control Act of 1972, administered by the Environmental Protection
Agency, was intended to establish federal noise emission standards. This act served a
broader scope of coordinating all noise control efforts and places the primary respon-
sibility for noise control on the states. Please refer to current and local regulations.
Fundamentals of acoustics and noise control
Several factors contribute to this problem.
Table 14.7. Typical community noise limits
Noise levels, DBA
Noise Zone Classification
Receiving land
use category Time period Rural Suburban Suburban Urban
One and two
family residential
10 pm–7 am
7 am–10 pm
40
50
45
55
50
60
Multiple dwelling
residential
10 pm–7 am
7 am–10 pm
45
50
50
55
55
60
Limited commercial
some dwellings
10 pm–7 am
7 am–10 pm
55
60
Commercial 10 pm–7 am
7 am–10 pm
60
65
Light industrial,
heavy industrial
Any time 70
75
648 CHAPTER 14
(1) Sound pressure level
Sound is a fluctuation in the pressure of the atmosphere at a given point. Sound
pressure level is expressed as a ratio of the particular sound pressure and a refer-
ence sound pressure:
SPL = 10 log
p
2
P
2
ref
where
SPL = sound pressure level in dB (decibel)
P
2
= mean square amplitude of the pressure variation
P
ref
= 2 × 10
−5
N/m
2
(2) Sound power level
Sound power level is defined as the ratio of the particular sound power and the
reference power:
PWL = 10 log
W
W
ref
where
W = sound power (rate of acoustic energy flow) in acoustic watt
W
ref
= 10
−12
Watt.
The relationship between SPL and PWL is:
SPL = PWL + K
where
K = a constant dependent upon geometry and other aspects of the situation.
(3) Wavelength
Consideration of the wavelength is important to noise control.
It is defined as:
λ =
C
f
where
λ = wavelength in feet
C = speed of sound in feet per second
f = frequency, cycles per second (Hz)
(4) Octave band
An Octave refers to a doubling of frequency. Generally, the audible frequency
range consists of ten preferred octave bands with following center frequencies:
31.5, 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000, and 16,000.
THE REFINERY ENVIRONMENTAL ISSUES 649
Table 14.8. A-weighting network band
correction
Octave band (Hz) Band correction (dB)
63 −25
125 −16
250 −9
500 −3
1,000 −0
2,000 +1
4,000 +1
8,000 −1
(5) A-weighted sound pressure level (dBA)
Most noise regulations set the maximum allowable noise limits based on the use
of the “A”weighting network which provides a popular means of rating noise.
This network is designed to account for the response of the human ear. The other
“B”and “C”weighting networks are no longer in common use. Table 14.8 shows
A-weighting network band correction for each octave band to convert the sound
pressure level (dB) to an A-weighted sound pressure level (dBA).
(6) Adding decibels
The noise levels expressed in decibels cannot be added arithmetically but the
addition should be performed on the basis of energy addition. Therefore, the
combined dB level is determined by:
dB
total
= 10 log
N
i
10 dB
i
/10
where
dB
total
= the combined dB level
dB
i
= the individual dB level
N = the total number of dB levels
(7) Sound fields
A sound field is a description of the relationship between the PWL of the source
and the SPL at different points in the surrounding space.
(a) Idealized sound field:
The sound field for an idealized sound sources which can be considered as a
very small, uniformly pulsating sphere is given by the following equation:
SPL = PWL − 20 log r + K dB
where
r = distance from source to measurement point
K = a constant
650 CHAPTER 14
(b) Non-idealized sound fields:
(i) Outdoor:
The sound field of a directional source radiating over a plane (hemispher-
ical radiation) is given by:
SPL = PWL + 10 log
Q
r
2
+ 2.5dB
where
Q = directivity factor in the direction of interest
r = distance from source in feet.
(ii) Enclosed space:
If source is radiating in an enclosed space, the field equation becomes:
SPL = PWL + 10 log
Q
4r
2
+
4
R
+ 10.5dB
where
R = room constant in sqft.
(8) Directivity
The directivity factor may be defined as the ratio of the mean square sound
pressure at a given distance in a particular direction to the value which would
exist if the source were non-directional. The directivity is defined by:
DI = 10 log Q
where
Q = directivity factor in the direction of interest
Some typical directivity indices are shown in Table 14.9.
(9) Sound propagation
The propagation of sound waves can be affected by a number of factors. The
factors important to noise control consists of sound absorption, transmission
loss, barriers, atmospheric, and terrain effects.
Table 14.9. Some typical directivity indices
Location QDI
Near a single plane surface 2 3 dB
Near the intersection of
2 plane surfaces
46dB
Near a corner formed by
3 plane surfaces
89dB
THE REFINERY ENVIRONMENTAL ISSUES 651
(a) Sound absorption:
Sound waves traveling in an enclosed space are affected by the absorptive
quality of the incident surface. The amount of absorption is expressed as:
R =
S · ab
1 − ab
where
R = room constant in sqft.
S = total interior surface area in sqft.
ab = average absorption coefficient
(b) Transmission loss:
The sound isolating capability of a wall is defined as:
TL = 10 log
1
τ
dB
where
τ = transmission coefficient (ratio of transmitted sound intensity to
incident sound intensity)
(c) Barriers:
Appreciable sound attenuation can often be obtained by interposing a barrier
or acoustical shield between the source and receiver. The sound attenuation
of a barrier is given approximately by:
B = 10 log
20H
2
λr
where
B = reduction of the sound pressure level at a given frequency
H = effective barrier height
r = distance from source to barrier
λ = wavelength of sound at the frequency being considered
(d) Atmospheric and terrain effects:
The propagation of sound outdoors at long distances may be significantly
influenced by atmospheric and terrain effects. Sound propagated through the
atmosphere is subject to small energy losses due to molecular effects. This
loss is dependent upon air temperature and relative humidity. The molecular
effects for 72
◦
F and 50% RH> are shown in Table 14.10.
The other effects resulting in noise reduction include attenuation due to sub-
stantial vegetation, the effects of uneven terrain and tall buildings, and the
effects of wind and temperature gradients. These effects are more complex
and, in most cases, can be neglected in a process plant noise analysis.
652 CHAPTER 14
Table 14.10. Molecular effects T =
72
◦
F; RH = 50%
Attenuation
Frequency (Hz) (dB per 1,000 ft)
500 1
1,000 2
2,000 3
4,000 8
8,000 15
Coping with noise in the design phase
Because of local, state and/or federal noise regulations which establish the maximum
noise reception limits, permission to build or expand any significant industrial facility
may be dependent on predicting that the reception limits set by the controlling agency
will not be exceeded.
Consequently, noise control engineering must be commenced early in the design stage.
A typical noise control program adopted by some major engineering companies is
shown as Figure 14.9:
Plant Noise Design Targets
Mechanical Equipment Noise
Criteria
Preliminary Noise Contour
Map
Noise Control Budget
Pilot Plan Assistance
Noise Control
recommendations
Final Noise Contour Map
Figure 14.9. A typical noise control program flowchart.