The fourth type of refining processes constitutes the supporting processes. Figure 10.1 lists the supporting role of these processes as:
Although these processes and units are not involved directly in hydrocarbon fuels production, their roles are essential for the operation of a refinery.
By the end of this lesson, you should be able to:
This lesson will take us one week to complete. Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignment below can be found in this lesson.
Reading: | J. H. Gary and G. E. Handwerk, Chapter 13 (Supporting Processes) |
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Assignments: | Exercise 9 |
If you have any questions, please post them to our Help Discussion (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
The gas streams produced in refinery units such as catalytic crackers, cokers, hydrocrackers, and reformers are sent to the Gas Processing Unit [1] in order to:
In some refineries, Gas Processing Units also function as Light End Units.
[1] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 278-280.
Sour gas separated in the Gas Processing unit is sent to the Amine Unit for acid gas removal using chemical solvents such as monoethanolamine (MEA), or diethanolamine (DEA), as shown in Figure 10.2.
As shown in Figure 10.2, the sour gas is pumped from the bottom of an absorption column to get in contact with the basic solution (typically 15-30wt% diethanolamine) to capture H2S (and other acidic gases such as CO2) in the solution. The rich solution containing the acid gases is sent to a flash drum to recover the C1 and C2 hydrocarbons from the rich solution to be used as fuel gas in the refinery to generate process heat, or steam in fired furnaces. The rich solvent is then sent to a regenerator still to remove the acid gases that are sent to the sulfur recovery unit. The remaining solvent is cooled in a heat exchanger and recycled to the absorption unit to close the loop [2].
[2] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 280-283.
As indicated in Figure 10.3, the objective of the sulfur recovery process is to convert H2S to elemental sulfur. Sulfur recovery takes place in a series of two steps: Claus Process and SCOT Process [3]. In the Modified Claus Process, partial combustion of H2S takes place to generate SO2 that is reacted with the remaining H2S to recover sulfur as elemental sulfur. The Modified Claus Process, once-through burner operation, works only with acid gases that contain more than 50% H2S by volume. In the process, hydrogen in H2S is converted to H2O. The second stage, the SCOT Process, functions as a tail gas clean-up operation to remove the sulfur compounds produced in the side reactions of the Claus Process, i.e., carbonyl sulfide (COS) and carbon disulfide (CS2).
[3] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 283-290.
Figure 10.4 shows the configuration of the multi-step Modified Claus Process that includes two kinds of reactors: a burner reactor and a converter reactor. In the burner reactor, H2S is burned with compressed air to SO2 and H2O. Two critically important variables of the burner reactor are the oxygen to H2S ratio and the reactor temperature. The O2/H2S ratio needs to be one-third of the stoichiometric ratio for complete combustion of H2S. The significance of the O2/H2S will be discussed further in the next section. The temperature in the burner reactor must be maintained typically at 1850°F to make sure that any ammonia present in the feed gas is completely destroyed to protect the catalysts in the converter reactor. The effluent gas from the burner reactor is cooled to 450°F (above the dew point of S) in the waste heat boiler as it enters the converter reactor for catalytic conversion of H2S and SO2 to elemental sulfur and water. The converter effluent is introduced into a condenser unit to obtain elemental sulfur as a liquid product. Small quantities of S produced in the burner reactor may also be recovered after the waste heat boiler. Typically, three sets of converter-condenser units in series are needed to achieve 95% recovery of S in the Modified Claus Process.
Figure 10.5 shows the principal reactions in the Modified Claus Process in the burner and converter-reactor sections. In the burner, H2S is partially oxidized to produce H2O and SO2. In the reactor converter, the burner product SO2 reacts with the remaining H2S to produce elemental sulfur (the intended product in the sulfur recovery process) along with the side product water. Ideally, the final products should consist only of elemental sulfur and water with no H2S or SO2 present. The only way to achieve the intended product mix is to control the O2/H2S ratio in the burner. As can be seen, the stoichiometric ratio of O2/H2S for complete conversion of H2S to SO2 is 3/2 which would effectively convert all H2S to SO2. In order to reserve part of the feed H2S to react with the burner product so that no H2S or SO2 remains in the final product from the converter, the O2/H2S ratio should be controlled at 1/3 of the stoichiometric ratio, that is (1/3)(3/2)=1/2. As a self-check exercise, explain with chemical equations why the desired oxygen/hydrogen sulfide ratio in the feed to the Burner in the Claus Process should be 1/3 of the stoichiometric oxygen/hydrogen sulfide ratio (for complete combustion of hydrogen sulfide). The answer to this exercise is given at the end of the lesson.
As seen in Figure 10.5, the side reactions in the burner produce COS and CS2 which cannot be converted in the catalytic reactions that take place in the converter reactor. Therefore, a tail gas clean-up process, or SCOT Process, is needed to reduce the concentration of these side products to less than 20 ppm by volume in the outlet.
Figure 10.6 illustrates how the SCOT Process is integrated with the Claus Unit to convert COS, CS2 and any remaining SO2 by reacting with H2 in the catalytic reactor back to H2S to be recycled to the Claus Unit to close the loop. The hydrogenating catalysts used in SCOT contain nickel or tungsten on alumina support, and the reaction takes place at 480-570°. By coupling Claus and SCOT processes, more than 99% of sulfur entering the Claus unit can be recovered as elemental sulfur to be sold as a refinery product.
Although refineries produce a significant quantity of hydrogen needed for hydrotreating and hydroconversion processes, in most cases, additional hydrogen is needed particularly for refining the sour crudes. Therefore, a Hydrogen Plant is needed on site to provide the additional hydrogen demand. As seen in Figure 10.7, steam reforming of natural gas is most commonly used in the U.S. to produce hydrogen, whereas partial oxidation of heavy hydrocarbons is preferred in Europe [4].
For the partial oxidation process, a heavy hydrocarbon fraction, typically fuel oil, is reacted at high pressures (1300-1800 psig) with pure oxygen supplied in strictly controlled quantities for partial oxidation of hydrocarbons to carbon monoxide and hydrogen, as shown in Figure 10.7. Carbon monoxide produced in the reaction is converted to hydrogen by catalytic shift reaction with steam. In the purification step, CO2 produced in the shift reaction is removed by absorption in a basic solvent such as potassium carbonate.
Figure 10.8 illustrates the reactions in steam reforming of natural gas (CH4) to produce hydrogen in the U.S. refineries. In the reforming reaction, CH4 is converted to H2 and CO on a NiO/SiO2-Al2O3 catalyst at temperatures of 760-816°C. Reforming is followed by the water gas shift reaction at 343°C to shift CO to H2 and CO2 on a Cr2O3 and Fe2O3 catalyst in multiple catalyst beds with external cooling to control the temperature to achieve high conversion in the exothermic reaction. The product gas is purified by absorption of CO2 in an Amine Unit. In the final step of methanation, residual CO and CO2 is removed by hydrogenation on a Ni/Al2O3 catalyst at 370-427°C.
[4] Petroleum Refining, by J. H. Gary, G. E. Handwerk, M. J. Kaiser, 5th Edition, CRC Press NY, 2007, Chapter 13, Supporting Processes, pp. 273-278.
Considering 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].
[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.
Standard 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.
Figures 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.
Air pollutant emissions from the refinery processes are also controlled. Figure 10.13 lists the major legislations and regulations that affect the environmental impact of refineries in the U.S. [6].
[6] C. S. Khor and A. Elkamel, “Environmental Issues Related to the Petroleum Refining Industry” In Petroleum Refining and Natural Gas Processing, Editors: M. R. Riazi, S. Eser, J. L. Peña, ASTM International, West Conshohocken, PA, 2013, pp. 701-716.
Please take a few minutes to complete the exercise and then answer the questions below.
This week Exercise 9 is due.
Supporting processes are essential to the operation of a refinery. These processes have become more important as the crude oil base has become more sour. The demand for hydrogen has increased to support the required finishing processes for heteroatom removal and recovery of sulfur and metals. Refineries have become major producers of elemental sulfur for the chemical industry.
You should now be able to:
You have reached the end of Lesson 10! Double-check the to-do list below to make sure you have completed all of the activities listed there before you begin Lesson 11.
Please refer to the Course Syllabus for specific time frames and due dates. Specific directions for the assignments below can be found within this lesson.
Reading: | J. H. Gary and G. E. Handwerk, Chapter 13 (Supporting Processes) |
---|---|
Assignments: | Exercise 9 |
If you have any questions, please post them to our Help Discussion Forum (not email), located in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.