Post your response to the blog discussing why refinery wastewater cannot be treated in municipal wastewater treatment plants.

Among the various supporting processes comes the treatment of wastewater. Refineries use a great amount of water in many different processes including desalting, distillation, cracking, and coking. As stated in Lesson 10, there are a few types of wastewater, including cooling water, process water and steam, storm water, and sanitary sewage water. Process water and steam is usually heavily polluted since it comes in direct contact with petroleum distillates. Several pollutants can be found in wastewater, such as aromatic compounds, heteroatoms like amines, phenols, and cyanides, and acids which all have the potential to harm humans or other wildlife.

It is very important that wastewater be treated prior to being sent off to a municipal wastewater treatment plant. Sour water is contaminated with solid particles that must be stripped of like sulfur in a stripping unit, and oils that must be separated by skimming the oil that floats on top of the denser water. A secondary treatment process utilizes microorganisms as biological contactors to help separate the pollutants from the wastewater. Much of this necessity stems from the implementation of the Clean Water Act of 1972 and the incorporation of National Pollutant Discharge Elimination permits (NPDE). There are multiple stages the wastewater must go through before it is suitable enough to be treated at public facilities.

Write a blog post discussing the objectives of catalytic reforming and limits on catalytic reforming capacity in the U.S. refineries.

Catalytic reforming is another catalytic conversion process utilized by many petroleum refineries. This was introduced in a similar time frame as catalytic cracking – during World War II. Of course, there was a huge increase in the demand for high-octane gasoline during this time frame due to the necessity of fuel demanded from US aircraft.

There are four different reactions of catalytic reforming which all achieve the same objective of increasing the octane number. Dehydrogenation, dehydroisomerization, and dehydrocyclization are all highly endothermic reactions which produce aromatic compounds and hydrogen gas in large yields. These reactions require high temperatures and relatively low pressures, however the hydrogen pressure must be significantly high enough in order to avoid deactivating the catalyst surfaces due to coke deposition.

Hydrogen is the most valuable byproduct obtained from catalytic reforming. This is because this element can be used to essentially ‘clean up’ fuels further through processes of hydrotreating and hydrocracking. There are a couple of limits posed on catalytic reforming. Usually the feedstock must be hydrotreated before reforming can take place since the platinum catalyst used in reforming can be hindered by exposure to sulfur, nitrogen, or other heteroatom contaminants. Also, the United States and Europe hold a limit on the levels of benzene and the total aromatics for gasoline, therefore placing a limit on the amount of reformate able to be used in the blending of gasoline. The overall goal of catalytic reforming is to obtain high-octane-number gasoline while also acting as the sole internal source of the byproduct hydrogen.

Another limit pertaining to catalytic reforming is the occurrence of the undesirable side reaction of hydrocracking. This process consumes the valuable hydrogen byproduct while forming gaseous hydrocarbons which in turn decrease the yield of reformate. The principal approach to achieve high yields and high quality of reformate is increasing the selectivity of desirable reactions by means of finding the proper balance between acidic and metallic sites.

Write a blog post comparing catalytic cracking and catalytic hydrocracking processes with respect to feedstocks, process objectives, and products.

Catalytic cracking is a process utilized by petroleum refineries which has been around for nearly a century. The reason this process became so popular is mainly due to its increased yield of gasoline with a higher octane rating, compared to a yield that would be achieved from thermal cracking. This process differs in a few ways from thermal cracking, as catalytic cracking incorporates the use of a catalyst, is more flexible in its feedstock, and does not require as high of temperatures and pressures as thermal cracking would.

Amongst the different types of catalytic cracking processes is Fluid Catalytic Cracking (FCC), introduced in 1942. This process in particular has seen a large use in the refining industry as it has a very flexible feedstock, which is usually straight-run atmospheric gas oil (AGO) and light vacuum gas oil (LVGO). Utilizing an acidic catalyst, long chains of n-alkanes are broken into shorter branched chains of isoalkanes, as well as cycloalkanes and aromatics. Different methods of catalytic cracking are used in refineries to produce LPG, cycle oils, and light hydrocarbons such as propane and butane, in addition to high-octane gasoline. Through alkylation and polymerization, these light hydrocarbons are used as feedstock to produce higher molecular weight isoalkanes and olefins which ultimately end up in the high-octane gasoline pool. Coking occurs during the cracking reactions, which works to deactivate the catalyst. When the coke is burned off with air, the temperature of the catalyst particles increases from the heat released by this, which provides the required energy for cracking to occur with minimal loss. This is what ultimately makes the cracking process so thermally efficient.

In 1958, the first commercial use of a process known as catalytic hydrocracking occurred. While catalytic cracking can support a wide range of feedstocks, hydrocracking is even more flexible and selective. Its main use in the refining industry is for its ability to produce light and middle distillates including large amounts of hydrogen, as well as its respect for environmental regulations that seek to limit the quantities of sulfur and aromatic hydrocarbon emissions. While these are all useful items, the overall goal of this process is to produce diesel and jet fuel from highly aromatic feedstocks such as residue and LGO from FCC. This is achieved by causing a decrease in the molecular weight and boiling point of heavy oils. The incorporation of bi-functional catalysts systems helps to keep coking under control. Hydrocracking involves a hydrogen-addition process, providing high yields of the desired distillates while avoiding the production of low-grade byproducts such as heavy oils, gas, and coke.

Thermal cracking is a process that manipulates long straight chains found in gas oils and other crude fractions, into shorter straight chain alkanes. The chemistry involves free radical reactions which are the key factor concerning the relatively low octane numbers of gas oils that undergo thermal cracking. Without this process, vacuum distillation residue (VDR) would essentially be a useless byproduct. Thermal cracking allows this residue to be converted into distillate fuels along with its primary goal – coke.

Thermal cracking was initially introduced in the early 1900s in order to produce more motor gasoline and high-octane gasoline for aircrafts. It wasn’t until the 1930s and 1940s, when catalytic cracking was introduced, that the petroleum industry seemed to lose its interest in thermal cracking. Today, there is still a desire for such a process, mainly in countries where the chief petroleum fuel in high demand is diesel fuel. Thermal cracking is also used for VDR with visbreaking and coking processes.

There are two (but technically three) main types of thermal cracking during coking, which include delayed coking and fluid coking. The third is known as flexi-coking, a derivative of fluid coking utilized to maximize the yield of distillate products. There is a notable market for this rejected carbon since coke has economic value as it can be used as fuel or as filler when producing anodes for the electrolysis of alumina.

Write a post comparing the solvent dewaxing and catalytic dewaxing processes.

Wax is made up of long-chain paraffins and it is a desirable by-product, particularly lube oil base stock. Dewaxing is the process of removing wax from feedstocks that would otherwise readily solidify, such as DAO from deasphalting and HVGO from vacuum distillation. There are two commercial methods of dewaxing. On utilizes a physical process known as solvent dewaxing, while the other method of catalytic dewaxing involves a chemical process.

Solvent dewaxing is a physical process which separates the wax with respect to freezing and solvent transport. This method uses stage-wise refrigeration of the feedstock after being mixed with the solvent. Wax crystals are then carried to a rotary filter via the solvent to be separated on a filter cloth. This layer of wax is collected and taken to a steam-stripping unit to recycle the solvent separated from the wax product, known as slack wax. This product has several marketable uses, such as paraffin wax for candles, microwax for cosmetics, and for petroleum jelly. The refrigerator’s temperature can be manipulated to control the desired pour point of the resulting lube oil base stock product.

Catalytic dewaxing is a chemical process, which removes the wax by means of selective reactions of long chain n-alkanes. This method is technically a low-severity conversion process, which involves a selective catalytic cracking of n-paraffins. Molecular sieve catalysts, known as zeolites, host selective cracking of n-alkanes while simultaneously keeping out bulky i-paraffins. Therefore, this process increases the ratio of i-paraffins to n-paraffins in the product, in turn lowering its pour point.

In comparison, catalytic dewaxing has an advantage over solvent dewaxing in the fact that it yields a lube oil base stock with a lower pour point as well as a higher yield of this product. Catalytic dewaxing also poses the flexibility to produce both lube oil base stock along with light distillates such as gasoline.

Post a blog to comment on how solvent fractionation works and review the parameters to describe the solvent power for non-polar solvents.

Solvent extraction is a process in which compounds may be separated based on their relative solubilities. It is basically the extraction of a substance from one liquid into another liquid phase. As opposed to distillation which exploits the different boiling points of the feedstock to achieve fractionation, deasphalting utilizes a solvent extraction process that factors in solubility or insolubility of compounds in a certain solvent. Vacuum distillation residue (VDR) is completely dissolved in aromatic solvents, like benzene and toluene. A paraffin solvent (n-heptane) is mixed with VDR in toluene, and the soluble portion of VDR in the n-heptane is called maltenes. The n-heptane solubles can then be further separated using a lighter and weaker solvent, such as n-pentane. These solubles can be separated even further utilizing a lighter solvent like propane. Figure 5 of Lesson 5 shows the overall process of solvent fractionation of VDR.

Rather than being considered a suspension of discrete asphaltene particles in VDR, this residue is actually considered a solution (one-phase material). The gradient solubility model is a widely acknowledged hypothesis which explains this observation. This model declares that asphaltene molecules can dissolve in resins which can then be dissolved in oil, ultimately yielding a single phase solution. The asphaltene is able to be forced out of solution in VDR by solvent extraction. The degree of solubility of a compound in a solvent is dependent upon the dissolving power of the solvent which is measured for non-polar solvents by Hildebrand Solubility Parameters (HSP).

There are two different Hildebrand Solubility Parameters. The first relates solubility to the ratio of surface tension to the cubic root of molar volume, in which solubility increases as surface tension increases and as molar volume decreases. The second parameter equates solubility to the square root of the ratio of latent heat of vaporization to molar volume, where solubility increases with an increasing heat of vaporization. It is clear why aromatic solvents are stronger solvents than aliphatic hydrocarbons.

Solvent Extraction: http://en.wikipedia.org/wiki/Liquid%E2%80%93liquid_extraction

Before the crude can enter the vacuum distillation unit (VDU), it must first be introduced into the Crude Distillation Unit (CDU) flash zone in order to be separated into vapor and liquid streams. Separation of light and heavy crudes is achieved through Atmospheric Distillation (CDU). Vapor fractions rise to the top of the column while the liquid fraction can be sent to VDU after it is introduced to stripping by steam, in order to recover the components dissolved in the heavier liquid that has settled at the bottom of the CDU. This vacuum chamber is vital since there must be a pressure drop which ultimately results in a decrease in temperature. High temperatures are unwanted since this would bring about cracking and inevitably coking on the column’s metal surfaces, interfering with fractionation.

To control the risk of coking, the ideal vacuum distillation temperature in the vacuum distillation column can be found using the Watson Characterization Factor (Kw). This will calculate the upper bound temperature limit for vacuum distillation to avoid coking. Below this range of temperatures, coking risk would be considered negligible while the upper bound represents uncertainty of the probability of coking. To be sure, the distillation temperature would be set lower than this temperature band. One should also take into account the hydrocarbon composition of the crude as paraffinic crudes are more susceptible to coking.

There are three different distillation methods that are used in crude oil refineries, and those are True Boiling Point Distillation (TBP), ASTM Distillation (ASTM), and Equilibrium Flash Vaporization (EFV). Each of these processes vary in the way distillation is achieved, so some methods are more effective than others.

True Boiling Point Distillation (TBP) is the most ideal process. It is a batch distillation operation which utilizes over 100 theoretical plates and a high reflux ratio of 100. This high number of plates along with a high reflux ratio allows for the best possible separation in distillation processes. These parameters also yield a pure compound since the temperature remains constant during evaporation of the unwanted compound.

The next method is ASTM Distillation. This process also uses a batch operation, but it does not include contact plates and it therefore has a reflux ratio of 0. Reflux inside the fractionation column flows downwards and cools vapors flowing upwards. The more reflux provided for a given number of plates results in more effective distillation, which is why this process is not as good as TBP.

The third distillation method is Equilibrium Flash Vaporization (EFV). This method is known yield the lowest degree of separation out of the three. EFV involves separation of the liquid and vapor in a flash drum after heating the flowing feedstock.