Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
Подождите немного. Документ загружается.
THE REFINERY ENVIRONMENTAL ISSUES 613
and 7 ppm in summer. Aquatic life (fish and tadpoles) continually consume oxygen
from the water, but aquatic plants produce oxygen naturally maintaining the balance
required to sustain fish life. Any oxidizable contaminant introduced into the natural
waterways consumes the dissolved oxygen to be oxidized. The dissolved oxygen will
be depleted below saturation point and will be replenished only by re-aeration. The
relative rate of replenishment to its saturation level will depend on a time factor
equivalent to the stream distance or the stream flow rate.
Oxygen depletion occurs by the introduction of one or more of the following contam-
inants entering the waterway:
r
Natural pollution by surface run-off rainwater, or melting snow, in the form of
soluble salts leached from the earth
r
Natural pollution caused by decay of organic plants from swamps or other sources
r
Human and animal life excretion
r
Chemical pollution from reducing agents in industrial plant wastes. Such as sulfides,
nitrites, ferrous salts, etc.
r
Biochemical pollution from such industrial wastes as phenols, hydrocarbons, car-
bohydrates, and the like
The degree of oxygen depletion from the pollution sources described above may be
catalogued by the following terms:
BOD—Biological Oxygen Demand.
COD—Chemical Oxygen Demand.
IOD—Immediate Oxygen Demand.
BOD. Since all natural waterways will contain bacteria and nutrient, almost any waste
compound introduced into the waterway will initiate biochemical reactions. These
reactions will consume some of the dissolved oxygen in the water. This is illustrated
in Figure 14.1.
The depletion of oxygen due to biological pollution is not very rapid. It follows the
laws of first order reaction. Because of this the effect of BOD is measured in the
laboratory on a five day basis, and has been universally adopted as the measure of
pollution effect. The “Ultimate”BOD is a measure of the total oxygen consumed when
the biological reaction proceeds to almost completion. The “5 day”BOD is believed
to be approximately the Ultimate. Figure 14.2 shows the “5 day”BOD versus the
“Ultimate”.
In summary BOD measures organic wastes that are biologically oxidizable.
COD. The COD is a measure of the oxygen depletion due to organic and inor-
ganic wastes which are chemically oxidizable. There are several laboratory methods
614 CHAPTER 14
Bacteria
Oxygen
Protein Biochemical Oxidation.
Bacteria
Nutrient
Nutrient
Oxygen
Carbohydrate Biochemical Oxidation.
Proteins
SO
4
+ H
2
O
NH
3
+ H
2
O
Amino Acids
CO
2
+ H
2
O
+
Fatty Acids
Alcohols
Carbohydrates
+
CO
2
+ H
2
O
NH
3
+ H
2
S
Figure 14.1. Typical BOD chemistry.
Ultimate BOD
BOD Curve
BOD
Inactive
5 day BOD.
100
0
Time Days
Figure 14.2. Lab determination of 5-day BOD versus ultimate BOD.
THE REFINERY ENVIRONMENTAL ISSUES 615
Table 14.2. Relationship between BOD and COD
Source Pollutants BOD “5 day” COD “2 hr dichromate”
Brewery Carbohydrates,
Proteins
550 –
Coal gas Phenols, Cyanides,
Thiocyanates,
Thio sulphates
6,500 16,400
Laundry Carbohydrates, Soaps 1600 2700
Pulp mill Carbohydrates,
Lignins, Sulfates
25,000 76,000
Domestic sewage Solids, Oil and Grease,
Proteins, Carbohydrates
350 300
Petroleum refinery
(Sour water)
Phenols, Hydrocarbons,
Sulfides
850 1,500
Petroleum refinery Phenols, Sulfides,
Hydrocarbons,
Mercaptans, Chlorides
125 2,600
accepted to measure the oxygen depletion effect of these pollution. The two most
widely accepted are the “4 hour permanganate”method or the “2 hour dichromate”
method. Although there is no generalized correlation between BOD and COD, usually
the COD will be larger than the BOD. The following table illustrates how different
wastes exhibit a different relationship between COD and BOD (Table 14.2).
IOD. Oxygen consumption by reducing chemicals such as sulfides and nitrates is
typified as follows:
S
2−
+ 2O
2
−→ SO
2−
4
NO
−
2
+
1
/
2
O
2
−→ NO
−
3
These types of inorganic oxidation reactions are very rapid and create what is measured
in the laboratory as immediate oxygen demand. If waste contaminants contain these
inorganic oxidizers the “5 day”BOD test will include the consumption of the oxygen
due to IOD also. A separate test to determine IOD must be made and this result
subtracted from the “5 day”BOD to arrive at the true BOD result.
Toxic pollutants common to oil refining
Toxic pollutants that are most commonly contained in untreated refinery aqueous
wastes are:
Oil. Heavy oil and other hydrocarbons are the most problematic pollution to be found
in refinery water effluent. All refineries exercise the most stringent methods to control
and remove these undesirable pollutants. Indeed in many cases the treated effluent
616 CHAPTER 14
streams leaving the refinery may well be purer than incoming portable water used in
the processes.
Phenols. These chemicals often formed in refinery processes such as catalytic and
thermal crackers, are highly toxic to aquatic life in concentrations of 1 to 10 ppm.
Apart from their toxicity phenols also unpleasant taste and odor to drinking water in
the range of 50 to 100 ppb. In concentrations of 200 ppm and more, these chemicals
can also deactivate water treatment plants such as trickle filters and activated sludge
units.
Caustic soda and derivatives. Solutions containing sodium hydroxide are used in a
number of refinery processes. Inevitably some of this chemical enters the refinery’s
waste water system. This contaminant is toxic to humans and marine life in even low
concentrations. The spent caustic (compounds leaving the process) such as sodium
sulfide is even more injurious.
Aqueous solutions of ammonium salts. The most common of these are ammonium
sulfide NH
4
SH, and ammonium chloride. Both these salts are present in effluent
water from the crude distillation unit overhead accumulator, however the sulfide
salt is present in all aqueous effluents from the cracking processes, and the hydro
treaters. Other ammonium salts are also present in hydro cracking and deep oil hydro
treating.
Acids in aqueous effluents. The most common of these are from the alkylation pro-
cesses which use either hydrofluoric acid or sulfuric acid. In some isomerization
processes hydrochloric acid is used to promote the aluminum chloride catalyst. In
some older processes sulfur dioxide is used to remove aromatics. This effluent usu-
ally leaves these plants as dilute sulfuric acid.
Ketones, furfural, and urea. These compounds are used in the refining of lube
oils. MEK and Urea are used in the dewaxing processes while furfural is used in
the extraction processes for finished lube oil stock. All of these compounds are
toxic.
Treating refinery aqueous wastes
The treatment of aqueous wastes from oil refineries fall into three categories. These
are:
r
In-plant treatment, these are onsite processes usually sour water strippers, spent
caustic oxidizers, and spent caustic neutralizers
THE REFINERY ENVIRONMENTAL ISSUES 617
To firebox
Top Tray
Bottom Tray
Sour Water
Stripped Water
Figure 14.3. Sour water stripper with no reflux.
r
The API separator, or similar oil/water separating device
r
Secondary treatment, which includes chemical coagulation, activated sludge pro-
cesses, trickle filters, air flotation and aerators
Most energy refineries contain only the first two of the above categories.
Sour water strippers. This is one of the most common treating process. Its purpose is
to remove the pollutant gases included in process plant effluents. The more common
pollutants are ammonia, ammonium salts, and hydrogen sulfide. The sour water strip-
per is almost always located in the process area of the refinery. The feed to the stripper
is the effluent water from the crude unit overhead condenser, the water phase from
the desalter, the condensed water from the vacuum unit’s hot well, and all the water
condensate from the hydro-treater product steam strippers. The sour water stripper is
usually a single trayed tower with no reflux or a similar tower with an overhead reflux
stream. Figure 14.3 shows a simple stripper tower with no reflux and Figure 14.4
shows a stripper with an overhead condenser and reflux.
618 CHAPTER 14
Reflux Drum
Top Tray
Reflux
Bottom Tray
Sour Water
Stripped water
Figure 14.4. Sour water stripper with reflux.
Both these type of sour water stripping lend themselves to tray by tray mass and heat
transfer. The sour water feed is introduced on the top tray of the tower while steam
usually at a rates of 0.5–1.5 lbs/gallon of feed is introduced below the bottom tray. In
the case of a tower with reflux the reflux enters the tower with the fresh feed. The design
of both towers utilize the partial pressure relationship of NH
3
and H
2
S in aqueous
solution. A series of graphs giving these relationships are given in Appendix 14.1
of this chapter as Figures 14.A.1–14.A.10. An example of the design for a sour water
stripper with no reflux is given in Appendix 14.2.
Spent caustic disposal. The other major effluent from oil refining is the spent caustic
streams from hydrogen sulfide removal and also the removal of phenols. Refiners usu-
ally have the following options in the disposal of these streams. In order of preference
these can be listed as follows:
THE REFINERY ENVIRONMENTAL ISSUES 619
Phenolic spent caustic.
r
Disposal by sales
r
Disposal by dumping at sea
r
Neutralizing with acid
r
Neutralizing with flue gas
Sulfidic spent caustic.
r
Disposal by sales
r
Oxidation with air and steam
r
Neutralization with acid and stripping
r
Neutralization with flue gas and stripping
Both the phenolic and sulfidic spent caustic have a commercial value in industrial
areas and where transportation costs make sales an attractive economic option. In the
case of the phenolic spent caustic, processors recover, separate, and purify various
cresylic acid fractions for commercial use and sales. In the case of the sulfidic spent
caustic this can be sold as sodium sulfide with some additional processing.
Neutralizing phenolic spent caustic. As listed above phenolic spent caustic can be
neutralized using acid or flue gas. When neutralized the mixture separates into two
liquid phases. The upper phase contains the acid oils while the lower phase is an
aqueous solution of sodium sulfate or sodium carbonate. The neutralization using
either acid or flue gas can be accomplished in a batch or continuous operation. Batch
neutralization using acid is considered to be the preferable route in this case. Flue
gas neutralization, although practical and commercially used in many installations is
more complex in design than the acid application and does not lend itself readily to
batch operation. It may require in some instances large pipe-work, switch valves, and
ablower.
A diagram of a phenolic caustic batch neutralizer is shown in Figure 14.5.
The process is fairly simple involving the following steps:
1. Charging the spent caustic batch into the neutralizer vessel.
2. The addition and thorough mixing of the acid neutralizer into the batch. This has
to be accomplished slowly and carefully to avoid lowering the pH of the batch too
far. A pH of between 4 and 5 will be required to free the phenols completely.
3. A settling period is necessary to allow the sprung acids to separate from the sprung
water.
4. This step is the pumping out and disposal of the sprung water.
Figure 14.5. Batch neutralization of spent caustic with acid.
THE REFINERY ENVIRONMENTAL ISSUES 621
5. The sprung acid is steam stripped to remove any residual H
2
S and light mercaptans.
6. Pump out the interface material which will be included in the next spent caustic
batch.
7. Finally pumping out of the sprung acid.
The neutralization step is exothermic giving out around 125 Btu/lb of sprung acid.
As the objective of the process is to produce a phenol-free sprung water for disposal
the system temperature should be kept as low as possible until the sprung water is
removed. For example the solubility of phenol in pure water at 120
◦
F is about 11%wt
while its solubility at 100
◦
F is 8.5 wt%. For this reason therefore steam stripping
should not begin until the sprung water has been removed. Routing the sprung water
to the sour water stripper ensures the removal of any entrained H
2
S in that stream.
Mixing of the acid/spent caustic is usually achieved using a mixing valve at the inlet
of the neutralizer. An educator or a jet nozzle inlet to the vessel itself would ensure
complete mixing if this is required.
Spent caustic oxidation
Spent caustic cannot be steam stripped to remove the sulfides contained in it due
to the H
2
S removal process in which the caustic was used. This is because sodium
sulfide does not hydrolyze even when heated. Acids could be used, of course, to
neutralize the spent caustic which would release gaseous H
2
S. This would however
be a costly procedure and causes a potential air pollution problem. The alkaline sulfide
can be economically oxidized to form thiosulfates and sulfates. This is the process
most commonly used in refineries where only sulfides are the pollutants in the spent
caustic and the release of gaseous H
2
S is a problem. The oxidation process also applies
to the presence of ammonia sulfide in the stream. The process has not been applied to
wastes containing a higher percentage of phenols, because the phenols interfere with
the oxidation process.
Oxidation of sulfides to thiosulfates
The oxidation of Na
2
S or (NH
4
)
2
S to the corresponding thiosulfates may be expressed
in ionic terms as follows:
2S
2−
+ 2O
2
+ H
2
O → S
2
O
2−
3
+ 2OH
−
lb 2(32) 2(32) 18 112 2(17)
H
f
2(−10) 2(0) +68 +154 2(+55)
where H
f
is the theoretical heat of formation in kcals/gm-mole in dilute solution. The
plus sign denotes heat evolved while the minus sign denotes heat absorbed.
622 CHAPTER 14
Vent gases
Oil
Separator
Feed Water Containing
water
Sulfides Oil to Waste
Oxidized
Effluent.
steam
Oxidizer Tower
Air
Figure 14.6. Schematic of an oxidizer plant.
Similarly the oxidation of NaSH or NH
4
SH to the corresponding thiosulfate can be
expressed as follows:
2SH
−
+ 2O
2
→ S
2
O
2−
3
+ H
2
O
lb 2(33) 2(32) 112 18
H
f
2(+3) 2(0) +154 +68
In both cases the theoretical oxygen required is 1 lb/lb of S, and the theoretical air
required would be 4.33 lbs/lb of S. The theoretical heat of reaction in both cases is
216 kcals/gm-mole of S
2
O
3
or 6,100 Btu evolved per lb of sulfur.
Figure 14.6 shows a typical oxidizing unit. Briefly the unit consists of an oxidizing
tower, and an overhead separator. The sour water feed enters the oxidizing tower after
preheating by exchange with the hot oxidizer overhead effluent. Heating steam and air
is injected into the sour water feed stream before entering the oxidizer tower. The steam
flow is controlled by the temperature of the inlet mixture while the air is flow controlled
to meet the reactor condition premise. The oxidizing tower itself is divided into three
sections with mixing nozzles connecting the inlet to each section. The reactants (air
and the sour water feed ) enter the base of the oxidizer also through a mixing nozzle
and flow upwards through the sections of the tower. The oxidized effluent leaves the
top of the tower to be cooled and partially condensed by exchange with the sour water
feed. The cooled effluent enters a separator operating under pressure control. The vent