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Environmental aspects of Petroleum refining

Briefly, petroleum production and petroleum refining produce chemicalwaste. If this chemical waste is not processed in a timely manner, it can

become a pollutant. A pollutant is a substance present in a particular location (ecosystem)when it is not indigenous to the location or is present in a greater-than-natural concentration. The substance is often the product of humanactivity. The pollutant, by virtue of its name, has a detrimental effect onthe environment, in part or in toto. Pollutants can also be subdividedinto two classes: primary and secondary.

Source Primary pollutant Secondary pollutantA primary pollutant is a pollutant that is emitted directly from thesource. In terms ofatmospheric pollutants, examples are carbon oxides, sulfur dioxide, andnitrogen oxidesfrom fuel combustion operations:

2(C) Petroleum + O22CO2(C) Petroleum + O2 CO 2

2(N) Petroleum + O2 2NO

(N) Petroleum + O2 NO

2

(S) Petroleum + O2 SO 2

2SO2 + O2 2SO 3

Hydrogen sulfide and ammonia are produced from processing sulfur-containing and nitrogen-containing feedstocks:

(S)Petroleum + H2 H 2S + Hydrocarbons

(N)Petroleum + 3H2 2 NH

3 + Hydrocarbons

A secondary pollutant is a pollutant that is produced by interaction of aprimary pollutantwith another chemical. A secondary pollutant may also be produced bydissociation of a

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primary pollutant, or other effects within a particular ecosystem. Again,using atmosphere as an example, the formation of the constituents ofacid rain is an example of the formation of secondary pollutants:

SO2 + H2O H 2 SO3 (Sulphurous Acid)

SO3 + H2O H

2 SO4 (Sulphuric Acid)NO + H2O HNO 2 (Nitrous Acid)2NO2+2H2O 2HNO 3 (Nitric Acid)

In addition, hazardous waste is any gaseous, liquid, or solid wastematerial that, if improperly managed or disposed of, may posesubstantial hazards to human health and the environment. In manycases, the term chemical waste is often used interchangeably with theterm hazardous waste.

Gaseous Emissions:

Gaseous emissions from petroleum refining create a number ofenvironmental problems. During combustion, the combination ofhydrocarbons, nitrogen oxide, and sunlight results in localized low-levelsof ozone, or smog. This is particularly evident in large urban areas andespecially when air does not circulate well. Petroleum use inautomobiles also contributes to the problem in many areas. The primaryeffects are on the health of those exposed to the ozone, but plant lifehas been observed to suffer as well.

Acid gases corrode refining equipment, harm catalysts, pollute the

atmosphere, and prevent the use of hydrocarbon components inpetrochemical manufacture. When the amount of hydrogen sulfide islarge, it may be removed from a gas stream and converted to sulfur orsulfuric acid. Some natural gases contain sufficient carbon dioxide towarrant recovery as dry ice, that is, solid carbon dioxide.

Petroleum refining produces gas streams that often contain substantialamounts of acid gases such as hydrogen sulfide and carbon dioxide.More particularly hydrogen sulfide arises from the hydrodesulfurizationof feedstocks that contain organic sulfur:

(S) Feedstock + H2 H 2S + Hydrocarbons

In summary, refinery process gas, in addition to hydrocarbons, maycontain other contaminants, such as carbon oxides (COx, where x=1 and2), sulfur oxides (SOx, where x=2 and 3), as well as ammonia (NH3),mercaptans (RSH), and carbonyl sulfide (COS).

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From an environmental viewpoint, it is not the means by which thesegases can be utilized that is of concern, but it is the effects of thesegases on the environment when they are introduced into theatmosphere. In addition to the corrosion of equipment of acid gases, theescape into the atmosphere of sulfur-containing gases can eventually

lead to the formation of the constituents of acid rain, that is, the oxidesof sulfur (SO2 and SO3). Similarly, the nitrogen-containing gases can alsolead to nitrous and nitric acids (through the formation of the oxides NOx,where x=1 or 2) which are the other major contributors to acid rain. Therelease of carbon dioxide and hydrocarbons as constituents of refineryeffluents can also influence the behavior and integrity of the ozonelayer.

Hydrogen chloride, if produced during refining, quickly picks up moisturein the atmosphere to form droplets of hydrochloric acid and, like sulfurdioxide, is a contributor to acid rain. However, hydrogen chloride may

exert severe local effects because, unlike sulfur dioxide, it does not needto participate in any further chemical reaction to become an acid andunder atmospheric conditions that favor a buildup of stack emissions inthe area of a large industrial complex or power plant, the amount ofhydrochloric acid in rainwater could be quite high.

Natural gas is also capable of producing emissions that are detrimentalto the environment. While the major constituent of natural gas ismethane, there are components such as carbon dioxide (CO), hydrogensulfide (H2S), and mercaptans (thiols; RSH), as well as trace amounts ofsundry other emissions. The fact that methane has a foreseen and

valuable end-use makes it a desirable product, but in several othersituations it is considered a pollutant, having been identified agreenhouse gas.

Acid rain has a pH less than 5.0 and predominantly consists of sulfuricacid (H2SO4) andNitric acid (HNO3). As a point of reference, in the absence ofanthropogenic pollution sources the average pH of rain is _6.0 (slightlyacidic; neutral pH 7.0). In summary, the sulfur dioxide that is producedduring a variety of processes will react with oxygen and water in theatmosphere to yield environmentally detrimental sulfuric acid. Similarly,

nitrogen oxides will also react to produce nitric acid.

Another acid gas, hydrogen chloride (HCl), although not usuallyconsidered to be a majoremission, is produced from mineral matter and the brines that oftenaccompany petroleum during production and is gaining increasingrecognition as a contributor to acid rain. However, hydrogen chloridemay exert severe local effects because it does not need to participate in

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any further chemical reaction to become an acid. Under atmosphericconditions that favor a buildup of stack emissions in the areas wherehydrogen chloride is produced, the amount of hydrochloric acid inrainwater could be quite high.

In summary, and from an environmental view point, petroleum andnatural gas processing petroleum but the constituent s are, in themajority of cases can result in similar, if not the same, gaseousemissions as coal. It is a question of degree insofar as the compositionof the gaseous emissions may vary from coal to, the same. There are avariety of processes that are designed for sulfur dioxide removal fromgas streams: but scrubbing processes utilizing limestone (CaCO3) or lime[Ca(OH)2] slurries have received more attention than other gasscrubbing processes. The majority of the gas scrubbing processes isdesigned to remove sulfur dioxide from the gas streams; someprocesses show the potential for removal of nitrogen oxide(s) too.

LIQUID EFFLUENTS:

Crude oil, as a mixture of hydrocarbons, is (theoretically) abiodegradable material. However, in very general term s (and asobserved from elemental analyses), petroleum is a mixture of:

(a) Hydrocarbons,(b) Nitrogen compounds,(c) Oxygen compounds,(d) Sulfur compounds, and

(e) Metallic constituents.

However, this general definition is not adequate to describe thecomposition of petroleum as it relates to the behavior of thesefeedstocks.

Crude oil also contains appreciable amounts of organic nonhydrocarbonconstituents, mainly sulfur-, nitrogen-, and oxygen-containingcompounds and, in smaller amounts, organometallic compounds insolution and inorganic salts in colloidal suspension. Theseconstituents appear throughout the entire boiling range of the crude oil

but tend to concentrate mainly in the heavier fractions and in thenonvolatile residues.

Although their concentration in certain fractions may be quite small,their influence is important. For example, the thermal decomposition ofdeposited inorganic chlorides withevolution of free hydrochloric acid can give rise to serious corrosionproblems in the distillation equipment. The presence of organic acid

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components, such as mercaptans (RSH) and acids (RCO2H), can alsopromote environmental damage. In catalytic operations, passivation andpoisoning of the catalyst can be caused by deposition of traces of metals(vanadium and nickel) or by chemisorption of nitrogen-containingcompound s on the catalyst, thus necessitating the frequent

regeneration of the catalyst or its expensive replacement. This carrieswith it the issues related to catalyst disposal.

In all cases careful separation of reaction products is important to therecovery of well refined materials. This may not be easy if thetemperature has risen as a consequence of chemical reaction. This willresult in a persistent dark color traceable to reaction products that areredistributed as colloids. Separation may also be difficult at lowtemperature because of high viscosity of the stock, but this problem canbe overcome by dilution with light naphtha or with propane.

SOLID EFFLUENTS:

Catalyst disposal is therefore a major concern in all refineries. In manycases the catalysts are regenerated at the refinery for repeated use.Disposal of spent catalysts is usually part of an agreement with thecatalysts manufacturer whereby the spent catalyst is returned fortreatment and re-manufacture.

The formation of consider able quantities of coke in the cokingprocesses is a cause for concern as it not only reduces the yield of liquidproducts but also initiates the necessity for disposal of the coke. Stock-

piling to coke may be a partial answer unless the coke containsleachable material s that will endanger the ecosystem as a result of rainor snowmelt.

In addition, emission of sulfur oxides (particularly sulfur dioxide) isgenerated from combustion of sulfur-containing coke as plant fuel.Sulfur dioxide (SO2) ha s a wide range of effects on health and on theenvironment. These effects vary from bronchial irritation upon short-term exposure to contributing to the acidification of lakes. Emissions ofsulfur dioxide therefore, are regulated in many countries.