Wastewater Treatment
Wastewater Treatment szw5009Considering the vast amounts of water used in a refinery, wastewater treatment constitutes a very significant supporting process for safe operation. Figure 10.9 lists the different types of wastewater, pollutants involved in wastewater streams, and the major refinery units that generate significant amounts of wastewater. The four types of refinery wastewater include cooling water, process water and steam, storm water, and sanitary sewage water. Among these, the most heavily polluted wastewater stream that requires serious treatment is the process water and steam that come into direct contact with petroleum fractions. Storm water may be contaminated because of incidental exposure to pollutant sources on refinery surfaces and accidental spills. Cooling water and sanitary sewage water may not require much treatment before they are sent to public water treatment facilities. One rule of thumb is to avoid mixing different types of wastewater streams to reduce the load on the treatment units.
Pollutants found in the wastewater streams include hydrocarbons with particular concern for toxic aromatic compounds, such as benzene; heteroatom compounds, such as mercaptans, amines, phenols, and cyanides; dissolved gases such as H2S and NH3, and acids, such as H2SO4 and HF; and suspended and dissolved solids. The refinery units that generate the most significant amount of wastewater are desalting, distillation, thermal and catalytic cracking, coking, as well as heat exchangers and storage tanks [5].
Figure 10.9. Refinery wastewater, water pollutants, and refinery units.
Image reads:
Types of wastewater
a. Cooling water
b. Process water and steam
-heavily polluted
c. Stormwater
d. Sanitary sewage water
Pollutants involved
a. Liquid hydrocarbons
b. Suspended solids, dissolved solids
c. Mercaptans (RSH)
d. Phenols (RC6H5O), amines (-NH2)
e. H2S, NH3, cyanides (-CN)
f. H2SO4, HF
Refinery units involved
Desalter, distillation, thermal cracking, visbreaking, coking, catalytic cracking, heat exchangers, storage tank drainage
[5] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 290-293.
Wastewater characterization
Wastewater characterization azs2Standard measurements used for wastewater characterization are listed in Figure 10.10. Biochemical Oxygen Demand (BOD) measures the amount of oxygen consumed by microorganisms in decomposing organic matter, whereas Chemical Oxygen Demand (COD) measures the total oxygen consumption by organic and inorganic chemicals present in water. Both measurements relate to the level of contamination in wastewater, and they are used to gauge the effectiveness of the wastewater treatment processes. Other water quality parameters include the amount of suspended solids, hydrocarbon content, nitrogen content, phenols content, and acidity.
Primary and secondary treatment processes
Primary and secondary treatment processes azs2Figures 10.11 and 10.12 illustrate the primary (physical) and secondary (biological) treatment processes, respectively. The primary treatment of sour water contaminated with oils and solid particles involve the stripping of dissolved H2S using steam, float/sink density separation for skimming the floating oil, and the settling tanks to separate heavier oil and solids, usually in multiple stages, before the treated water can be directed to public treatment facilities. The secondary treatment uses micro-organisms to further remove organic contaminants.
Figure 10.11. Primary Treatment of Wastewater - Physical
Simplified primary treatment diagram. Sour water and steam undergo stripping. H2S is removed and sent to a Claus Unit. Everything else is sent, with wastewater to the separator (float, sink). Oil floats and is removed, the rest goes to two successive settling tanks (which sometimes include flocculation agents) where oils and solids are removed. Finally, the remaining water is sent to a public treatment facility.
Figure 10.12. Secondary treatment of wastewater - Biological
Simplified secondary treatment diagram. National pollutant discharge elimination (NPDS) permits are needed according to the clean water act of 1972 and amendments of 1977. The materials collected are considered toxic and are regulated by the resource conservation and recovery act of 1976.
Wastewater and air enter biological contactors. From there some products go to surface disposal and some of the products go to biomass waste, then dewatering and is finally turned into biocoke.