Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
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THE REFINERY ENVIRONMENTAL ISSUES 623
Table 14.3. Typical design criteria for sulfide oxidizers
Temperature at bottom,
◦
F 165–225
Pressure at bottom, psia 75–100
Inlet air, (lbs/gal)/(1,000 ppm·S) 0.05–0.075
Inlet air, lb/lbs 1.4–2.1
Superficial air, ft/sec 0.08–0.1
Sulfide oxidation rate, (lbs/hr)/cuft 0.35
gases leave through this controller to flare or other suitable disposal equipment (such
as a heaters firebox). The oil phase of the effluent is skimmed off as a side stream
while the water phase after settling leaves the bottom of the separator as the oxidized
effluent.
High temperature and low pressures tend to vaporize some of the water in the top
section of the oxidizer. This results in more sulfides being stripped out into the vapor
phase. Thus it is recommended that the oxidizer top pressure be at least 25–40 psi
above the vapor pressure of the water at the oxidizer top temperature. Typical design
criteria for these oxidizing units are given in Table 14.3.
Oxidation of mercaptans
If caustic soda has been used to remove H
2
S there will almost certainly be mercaptans
present in the sour stream. Caustic soda will also react with and remove these from
the process stream. Certainly in treating these light streams from sour crudes the
mercaptan content of the spent caustic stream could be as high as 3,000 ppm of
sodium mercaptide sulfide. The greater part of the mercaptides will be oxidized to
hydrocarbon disulfides according to the equation:
2RSNa +
1
/
2
O
2
+ H
2
O −→ RSSR + 2NaOH.
Almost all the mercaptides will be oxidized in a concurrent flow reactor. However
some of the mercaptides will be partially hydrolyzed and it can be assumed that
there will be a certain amount of free mercaptans present in accordance with the
equilibrium:
RSNa + H
2
O ←→ RSH + NaOH.
The free mercaptan formed by this reaction will be stripped out of solution and leave
the reactor in the overhead vapor phase. Now if the separator is operating at 62 psia and
125
◦
F, the vapor pressure of water at 125
◦
F is 2 psia, then the partial pressure acting
to condense the overhead vapor is 60 psia. Methyl mercaptan may condense under
these conditions but the heavier mercapans and the disulfide certainly will condense.
The disulfides are immiscible in aqueous alkaline effluent and will form the upper oil
layer to be skimmed off for disposal. The mercaptans that are condensed will form
624 CHAPTER 14
mercaptides with the NaOH present. Since this will really occur in the feed/effluent
exchanger when the reformed mercaptides are still in contact with the reactor air
supply and most if not all will be oxidized to disulfides again.
The theoretical oxygen requirement for oxidizing mercaptides to disulfides is 0.25
pounds per pound of mercaptide sulfur, and the theoretical air requirement is 1.09
pounds per pound of sulfur. For convenience, the following value of the theory in this
case can be taken as:
1 Theory = 0.0091 (lb air/gal)/(1,000 ppm RSH-S)
Oxidation of sulfide to sulfate
The oxidation of sulfides to thiosulfides only reduces the ultimate oxygen demand of
the sulfides from two pounds of oxygen per pound of sulfides to one pond per pound of
sulfide. The oxidation to thiosulfate will remove the short term or immediate oxygen
demand. Thiosulfate limits are not usually stipulated in water pollution regulations
while sulfides are always stipulated. In those cases where thiosulfates are a regulated
limit it will be necessary to convert the thiosulfate to the sulfate. The chemistry
relating to this may be written as follows:
S
2−
+ 2O
2
−→ SO
2−
4
The above equation applies equally to aqueous ammonium sulfide and to sodium
sulfides.
The theoretical oxygen requirement is 2 pounds per pound of S, and the theoretical air
requirement is 8.66 ponds per pound of S. The theoretical heat of reaction is 12,700
Btu evolved/lb of S. That is one theory of air is:
0.072 (lb air/gal)/(1,000 ppm S)
The following table compares those units oxidizing to thiosulfates to those oxidizing
to sulfates (Table 14.4). Although many units processing the sulfides to sulfates do
not use a catalyst but convert the sulfide to a mixture of thiosulfates and the sulfate.
To convert the sulfide completely to the sulfate a copper chloride catalyst is used to
accelerate the reaction rate. The reaction of this conversion is quite slow without the
catalyst.
Oil–water separation
Most aqueous effluents from a refinery will contain oil. This oil content has to be
reduced to at least 10 ppm before it can be deposited into a river, lake or ocean. The
oil contamination sources are from process water run down, paved area drainage,
THE REFINERY ENVIRONMENTAL ISSUES 625
Table 14.4. Comparison of sulfide oxidation processes
Oxidizing to Partial oxidizing Total oxidizing
thiosulfate only to sulfate to sulfate
Anions produced
SO
4
% Nil 46 100
S
2
O
3
% 100 54 Nil
% Sulfide oxidized to:
SO
4
Nil 34 100
S
2
O
3
100 66 Nil
Sulfide oxidation rate,
(lbs/hr)/cuft. 0.33 0.033 0.035
Air rate in theories
Oxidation to S
2
O
3
2.05 1.55 6.53
Oxidation to actual anions 2.05 1.17 3.27
Air–water flow Concurrent Countercurrent Countercurrent
Catalyst used None None CuCl
2
storm catch-pots, tanker ballast pump out, and tank farm diked areas. All the water
from these sources is treated in oil separation processes. The most important of these
oil–water separation process is the API Separator.
The API oil–water separator
The design of an API separator is based on the difference in gravity of oil and water
in accordance with the general laws of settling. These laws are given by the following
equations:
V = 8.3 × 10
5
×
d
2
S
η
(Stokes Law)
V = 1.04 × 10
4
×
d
1.14
× S
0.71
S
0.29
c
× µ
0.43
(Intermediate Law)
V = 2.05 × 10
3
d × S
S
c
1/2
(Newton’s Law)
where
V = settling rate in ins per min.
d = droplet diameter in ins
S = specific gravity differential between the two phases.
η = viscosity of the continuous phase (water in this case) in centipoises
S
c
= continuous phase specific gravity.
626 CHAPTER 14
When the Re number is < 2.0 use Stokes Law. When Re number is between 2 and
500 use the Intermediate Law. When the Re number is >500 use Newton’sLaw.
The Re number in this case is given as
10.7 × dVS
c
η
As a guide the minimum droplet size of the oil in water can be taken (as a minimum)
0.008 ins when the oil gravity is 0.850 or lighter and 0.005 ins when the gravity of
oil is greater than 0.850.
The above laws of settling in oil refining processes usually apply to the hydrocarbon
being the continuous phase with water (the heavier phase) being the one that settles
out. In the case of the API separator the reverse is true. That is the continuous phase
is water and the lighter oil phase is the one that is separated to be disposed as the
product skimmed from the surface. In the design of this system Stokes Law is used to
reflect the rate at which the oil rises to the surface through the water. This modified
Stokes law may be written as follows:
V
r
= 6.69 × 10
4
d
2
S
η
where
V
r
= rising rate of the oil phase in ft/min
d = droplet diameter in ins.
S = difference in Specific gravity of the phases.
η = viscosity of the continuous phase (water) in centipoises.
An example of the application of the equation in the design of an API separator is
given in Appendix 14.3.
The oil phase from the separator is removed using specially designed skim pipes and
an oil sump. A simplified diagram of a typical API separator is given in Figure 14.7.
Ancillary equipment used in the design and operation of the API separator are de-
scribed in the following items.
Oil skim pipes. As a good part of the oil will separate from the water on leaving the
inlet flume and two oil retention baffles with skim pipes are provided. One is located
ahead of the flow baffle and the other ahead of the overflow weir. The distance between
these baffles will be the function ‘L’as calculated in Appendix 14.3. The API manual
recommends 10 ins diameter skim pipes where the horizontal run is less than 40 ft.
Larger diameter skim pipes will be installed in greater horizontal runs.
The oil sump. The oil recovery sump is located at one edge mid way along the
horizontal run. The size of the sump is based on the oil content of the inlet stream to
THE REFINERY ENVIRONMENTAL ISSUES 627
Waste Water Feed
Oil Sump
Trash
Screen
Retention Baffles
A A
Oil Skimmer
Sludge Sumps
Plan View,
Distribution
Baffle
Figure 14.7. Schematic diagram of a typical api separator.
the separator plus four times that amount as water delivered by the skim pipes. For
example if the inlet stream contains 200 ppm by volume of oil and the water outlet
contains 50 ppm Then the oil removal from the separator will be 150 ppm or 0.015%
vol which in the case of the specification given in Appendix 5.3 will equate to:
0.00015 × 600 × 1,440 = 130 gals/day.
Use the API recommendation to add to this figure four times this figure for the amount
of water contained with the oil delivered by the skim pipes. Then the total liquid to
the sump will be 650 gals/day. For ease of operation a sump size is often based on
a predicted weekly accumulation which amounts to 4,550 gals. Assume the sump
cross sectional area is 144 sqft (12 ft × 12 ft) then the liquid depth to NLL should
be 4,550/(7.48 × 144) = 4.2 ft and the HLL can be set at 5 ft. To ensure adequate
gravity head for the oil removed by the skim pipes the bottom of the sump will be
8 ft below the NLL of the separator.
The sump pump. The sump pump will be a standard vertical pump with a capacity to
pump out the entire sump in about 1 hr or less. Normally refineries pump the sump
oil to the ‘Bottom Settling Tanks’of the refinery. These are tanks used to store the
bottoms of crude oil tanks which usually contain high water and sediment content not
suitable for the normal process plants. The contents are allowed to sit for some time
628 CHAPTER 14
and then returned as feed to the normal refinery processes. The bottom settling from
these tanks are routed to the API separator.
Oil retention baffles. API recommends that oil retention baffles have a depth below
the level in the separator of 0.6–0.8 of the liquid level. Further the baffle should extend
1 ft above the normal liquid level of the separator.
Distribution baffles. An important feature in the design of the API separator is a
means of distributing the influent evenly. API recommends either ‘V’shaped baffles
or pipes installed at the inlet of the separator to be used for this purpose. It is also
recommended that the the open area between slots be 3–7% of the chamber cross
sectional area.
Sludge sumps. Sludge sumps are located immediately downstream of the distribution
baffles. API recommends that these sumps should be at least 30 ins deep. That is 30
ins below the floor of the separator. The sludge pump suction is to extend to 6 ins
of the sump bottom and the suction line will be 3 ins diameter. Normally the sludge
bottom will be 4 sqins in cross sectional area or 2 ins × 2 ins. API recommends that
these sumps slope a minimum of 1.7 ins vertical to 1.0 ins horizontal. In the case of
the example design in Appendix 5.3 an 18 ins × 18 ins sumps sloping to the bottom
of 2 ins ×2 ins would give a slope of 1.9 which would meet the API recommendation.
Such a layout could accommodate 6 sumps.
Trash rack. API recommends that a trash rack made up of series of 3/8 ins rods on
1.5 ins centers be installed as a trash trap.
Storm surge ponds
These ponds are installed to provide storage for maximum rainfall conditions. There
are several forms of these surge ponds, some requiring pumps, some located upstream
of the API separator, and some downstream of the separator. In most cases the storm
drain system is directed to the storm surge pond. Thus in a storm the excess rainfall is
held in this pond and fed to the API separator, over a period of time and at a rate that
will not exceed the separator’s capability to handle the water effectively. In this way
the refinery ensures that any oily water will not by-pass the separator under the worst
condition. The size of the surge pond must be able to handle the maximum rainfall
and the flow from all catch basins and open culverts that form part of the refinery
drainage system.
Surge ponds are constructed with a shallow depth over a large surface area. Its location
is usually only slightly elevated above the API separator but near the low point of
the refinery. Thus, even when the pond is full the water, it will not ‘back-up’to the
process area drains.
THE REFINERY ENVIRONMENTAL ISSUES 629
Other refinery water effluent treatment processes
Oxidation ponds
Oxidation ponds are usually used as a secondary effluent cleanup after the API sepa-
rator. There are three types of these ponds which are:
Aerobic. Where the oxidation of the water utilizes oxygen from the atmosphere plus
oxygen produced by photosynthesis.
Anaerobic. Where oxidation of the wastes does not utilize oxygen.
Aerated. Where oxidation of the wastes utilizes oxygen introduced from the atmo-
sphere by mechanical aeration.
Aerobic oxidation. Consider a shallow pond containing bacteria and algae. The bac-
teria will utilize oxygen to oxidize bio-chemically the incoming water. In so doing
it will produce H
2
O, CO
2
, and perhaps NH
3
. The algae will utilize sunlight plus the
H
2
O and CO
2
to produce oxygen. Oxygen in turn produces additional bacteria and
algea growth. This cycle is shown below:
Sunlight
Photosynthesis
New algae growth
(algae)
O
2
H
2
O + CO
2
Waste water
Oxidation
New bacteria growth
(bacteria)
Waste water out at reduced BOD
Air flotation
The purpose of the air flotation process is the clarification of waste water by the
removal of suspended solids and oil. This is achieved by dissolving air in waste water
under pressure, and then releasing it at atmospheric pressure. The released air forms
bubbles which adhere to the solid matter and oil in the waste water. The bubbles
cause the adhered matter to float in the froth on the surface of the water bulk. The
dissolved air in the water also achieves a reduction in the BOD of the effluent stream.
Figure 14.8 shows the principle of a typical air flotation process.
630 CHAPTER 14
Mixing chamber Flotation Vessel
Waste water
CleanWater
Sludge
Air Drum
Aerated Water
Compressed Air
Recycle Water
Figure 14.8. A typical air flotation process.
The process shown is the preferred recycle type. The process can also be designed
for a once through operation. Typical design criteria for the recycle process is given
in the Table 14.5.
Referring to Figure 14.8 the flocculating chemical and pH control chemical are mixed
in the mixing tank with the raw water feed before the feed enters the flotation vessel.
The processes described above are those found in oil refining most often. Indeed of
the processes described above most refineries only use the API separator and the surge
ponds to meet the oil/water separation required.
Table 14.5. Design criteria for a typical recycle air flotation unit
Recycle rate 50% of raw feed rate
Air drum pressure 35–55 psig
Air drum retention time 2 min of recycle flow (from Liquid level to drum bottom)
Flotation tank retention 15–20 min of total flow (raw feed plus recycle)
Flotation tank rise rate 3.0 gal/min of total flow per sqft of liquid surface
pH 7.5–8.5
Flocculating chemical 25 ppm of alum in total flow.
Air rate 0.25 to 0.5 Scft/100gals of total flow
Flotation tank liquid depth 6–8 ft (to meet requirements above)
Reference
M.R. Beychok. Aqueous Wastes from Petroleum and Petrochemical Plants, John Wiley &
Sons, New York, 1967.
THE REFINERY ENVIRONMENTAL ISSUES 631
14.2 Emission to the atmosphere
Oil refiners have two responsibilities with regard to atmospheric emissions. The first
is to produce products that minimize the release of toxic or unacceptable emissions
when used or stored. Much of this subject has been dealt with in Chapter 2 of this
volume. The other responsibility, of course is to identify and control those unsavory
emissions from the refining operation and processes. This is in keeping with all other
industries that contain processes that have atmospheric emissions. It is this second
responsibility that is addressed in this part of the chapter. It begins with a synopsis
of the Clean Air Act. This Act is the basis of all standards and limits of control in
the atmospheric emission from industrial and other sources. The sections of the Act
given in the following sections refer to the USA although most other countries follow
these items quite closely.
Features of the Clean Air Act
The Clean Air Act reflects a numbers of revisions of a law first passed in 1970. This
summary covers some of the important provisions of the Clean Air Act.
This summary is only a brief introduction to the Clean Air Act. The document itself
is much more extensive and provides details of all the most complicated aspects of air
pollution. The reader should consult the latest versions and amendments to the Clean
Air Act.
The role of the federal government and the role of the states
Although the Clean Air Act is a federal law covering the entire country, the states do
much of the work to implement the Act. For example, a state air pollution agency holds
a hearing on a permit application by a power or chemical plant or fines a company
for violating air pollution limits.
Under this law, EPA sets limits on how much of a pollutant can be in the air anywhere
in the United States. This ensures that all Americans have the same basic health and
environmental protections. The law allows individual states to have stronger pollution
controls, but states are not allowed to have weaker pollution controls than those set
for the whole country.
The law recognizes that it makes sense for states to take the lead in carrying out the
Clean Air Act, because pollution control problems often require special understanding
of local industries, geography, housing patterns, etc.
States have to develop state implementation plans (SIPs) that explain how each state
will do its job under the Clean Air Act. A SIP is a collection of the regulations a
632 CHAPTER 14
state will use to clean up polluted areas. The states must involve the public, through
hearings and opportunities to comment, in the development of each SIP.
EPA must approve each SIP, and if a SIP isn’t acceptable, EPA can take over enforcing
the Clean Air Act in that state.
The United States government, through EPA, assists the states by providing scientific
research, expert studies, engineering designs and money to support clean air programs.
Interstate air pollution
Air pollution often travels from its source in one state to another state. In many
metropolitan areas, people live in one state and work or shop in an-other; air pollution
from cars and trucks may spread throughout the interstate area. The Clean Air Act
provides for interstate commissions on air pollution control, which are to develop
regional strategies for cleaning up air pollution. The Clean Air Act includes other
provisions to reduce interstate air pollution.
International air pollution
Air pollution moves across national borders. The law covers pollution that originates
in Mexico and Canada and drifts into the United States and pollution from the United
States that reaches Canada and Mexico.
Permits
One of the major breakthroughs in the Clean Air Act is a permit program for larger
sources that release pollutants into the air [2].
[2] A source can be a power plant, factory or anything that releases pollutants into
the air. Cars, trucks and other motor vehicles are sources, and consumer products and
machines used in industry can be sources too. Sources that stay in one place are referred
to as stationary sources; sources that move around, like cars or planes, are called mobile
sources.
Requiring polluters to apply for a permit is not a new idea. Approximately 35 states
have had state-wide permit programs for air pollution. The Clean Water Act requires
permits to release pollutants into lakes, rivers or other waterways. Now air pollution
is also going to be managed by a national permit system. Under the new program,
permits are issued by states or, when a state fails to carry out the Clean Air Act
satisfactorily, by EPA. The permit includes information on which pollutants are being
released, how much may be released, and what kinds of steps the source’s owner
or operator is taking to reduce pollution, including plans to monitor (measure) the
pollution. The permit system is especially useful for businesses covered by more than