DIESEL: MORE EFFICIENT ENERGY SOURCE THAN GASOLINE
SK ANSARI (CHEMICAL ENGINEER)
SEEMA ANSARI (TELECOM ENGINEER)
May 3 - 16, 2010
Growing demand for transportation of people and goods is a challenge facing the world nowadays.
Transportation is almost entirely dependent on petroleum products as its energy sources, currently consuming about 51 per cent of the total world petroleum consumption.
By the year 2020, the Energy Information Administration estimates, that petroleum consumption by transports worldwide would reach as much as 58 per cent of the projected world consumption of 106 million barrels per day. In industrialized nations, the number of vehicles would be two times current by 2050. In developing nations such as China and India, the number of vehicles is expected to increase dramatically. In the next 50 years, the number of vehicles worldwide is projected to rise from 670 million to over 3.5 billion.
As the human population, number of vehicles, and vehicle miles traveled rise urban air pollution in most of the world tends to swell up. Air pollution is known to contribute to cardiovascular and respiratory diseases such as asthma and emphysema.
Worldwide there is evidence of a trend in rising global average temperatures, which many scientists attribute to human activities that are increasing atmospheric concentrations of gases such as carbon dioxide, methane (natural gas), and nitrous oxide, also known as greenhouse gases because they absorb and hold heat within the earth's atmosphere.
Burning of fossil fuels has increased atmospheric concentrations of carbon dioxide (CO2) by about one-third over the past 150 years. Worldwide, an additional 24 billion tons of CO2 per year are now being generated.
In the US alone, transports account for one-third of CO2 emissions. Concern about the perceived threat of global warming has led to an agreement at an international conference in Kyoto, Japan in December 1997 to reduce greenhouse gas emissions to seven per cent below 1990 levels by 2008-2012. The Kyoto Protocol has so far been signed by 55 developed nations. While the US has yet to sign to this Protocol, there is a growing consensus that it is prudent for the US to reduce its emissions of greenhouse gases.
Similarly refiners also faces a significant challenge in the coming years as global transport fuels see a continuing increase in demand for diesel.
There has been a gradual realization that diesel is the most desirable transportation fuel. Processed diesel fuel is essentially sulphur free and is cleaner burning than crude oil derived diesel.
It has been calculated that the production of the diesel rich product spectrum from a GTL (gas-to-liquid) plant has a lower greenhouse gas emission impact than the products from a typical oil refinery using the comparison method recommended by the ISO standard.
GTL diesel is clearly superior when it comes to the local environmental impact at the places where the fuels are used.
Diesel fuel is about 18 per cent heavier than gasoline and consists mainly of hydrocarbons.
Though diesel fuel is heavier than gas, it is lighter than other petroleum-based products such as crankcase oil and lubricating oil. With a flash point of 120-degrees to 160-degrees, depending on the method of distillation, diesel is not as volatile as either kerosene or gasoline. Gas, however, burns considerably cleaner than petroleum-based diesel fuel.
Before shipment from the petroleum distillery, the composition of diesel fuel may be varied by the distilling facility, depending on the latitude of the distillation facility and weather; specifically, the temperature at the time of distillation. The heavier diesel fuel will tend to thicken, or solidify, in cold weather, unlike gasoline which is basically unaffected by colder temperatures.
Because there are no legal standards for premium diesel yet, it is very hard to know if you are buying the good stuff. A task force has drafted standards for premium diesel. When the new specifications are accepted, information will have to be posted on the fuel pump. Retailers will no longer be allowed to label cheap blended diesel as 'premium.' They will have separate pumps with clear labels informing the customer what is being sold. The marketing and labeling will be the same as with regular and premium gasoline. Retailers selling the real thing use this system now. Enforcement of all fuel standards is to be done in all over Pakistan.
Important secondary products are high quality lubricant base oils and olefinic hydrocarbons used in the petrochemical industry. Production of lubricants base oils involves fractionation under vacuum and hydroisomerization. Extractive distillation is used to separate the olefins from each mixed hydrocarbon cut.
The diesel fuel product may be used as a blend material to enhance the properties of crude oil derived diesel.
We will investigate a number of scenarios to demonstrate the potential benefits of available processing schemes to increase the diesel to gasoline ratio. Key consideration areas include crude selection, optimizing cut points, improving diesel recovery from the vacuum distillation unit (VDU), modifying fluid catalytic cracking (FCC) yields, installing residue upgrading, technologies for converting liquefied petroleum gas (LPG) and gasoline to diesel and converting gasoline to petrochemicals.
Several examples will investigate the advantages and disadvantages of each scheme and highlight some potential synergies among available options, as well as include capital and operating cost impacts, impact on overall refinery yield and consideration of carbon emissions.
- Crude and cut point selection.
- Improved diesel recovery.
- Targeting upgrading options.
- Converting LPG and naphtha to diesel.
- Converting gasoline into petrochemicals.
We will consider each option and investigate the impacts that these option can have on the refinery balance and potential to achieve a 100 per cent diesel refinery.
It can be said that 'what is bad for octane is generally good for cetane' so the low aromatics content and low degree of branching are very beneficial for the cetane number. Conversely, the low degree of branching results in poor cold properties and the low aromatics content in a low density. However, hydrotreating, slightly hydroisomerized products in general make a good diesel and an excellent blending component to enhance the properties of crude oil derived diesel blending material. The primary product from HTFT processes is more branched and more olefinic than the LTFT (low temperature Fischer Tropsch) material; moreover, the HTFT product contains some aromatics while aromatics are almost totally absent from LTFT material. As implied above, some aromatic content is desirable to increase the diesel density.
CRUDE AND CUT POINT SELECTION
The crude processed in the refinery and the cut points used to fractionate that crude can have a significant impact on the diesel production from the refinery. Diesel is produced from two main sources in crude straight-run (SR) materials around 150∞C - 360∞C and diesel produced in the upgrading vacuum gasoil (VGO) or residue.
Table 1 also gives an indication of the values of the various fractions relative to the cost of crude oil.
TABLE-1: VALUE OF VARIOUS OIL FRACTIONS
|PRODUCT||TYPICAL RATIO OF VALUE OF PRODUCT TO COST OF CRUDE OIL|
|LPG||1.1 - 1.4|
|Motor Gasoline||1.4 - 1.5|
|Diesel||1.5 - 1.7|
|Naphtha||1.2 - 1.3|
|Gas oils||1.15 - 1.3|
|Jet kerosene||1.3 - 1.4|
|Vacuum gas oil||0.95 - 1.05|
|Heavy Fuel Oil||0.6 - 0.7|
Diesel engines, if properly maintained, are more efficient than gasoline engines in term of both fuel economy and carbon dioxide emission. Emission catalyst, meanwhile, are restricted to platinum. The major problem is emission of sooty particulates, and catalysts are usually used near the exhaust filter to burn them off.
Rhodia Inc. has recently suggested a fuel-borne catalyst to facilitate the burning off. If it is to be added to diesel fuel in a fixed proportion, it will need to be inexpensive, and there is a question as to whether it can be considered a true catalyst.
Rhodia Inc. a global specialty chemicals company set out to develop a fuel additive designed to increase fuel mileage. What they got with a trade name is EOLYS, a fuel born-catalyst (a suspended metallic-oxide) that aids in the near elimination of diesel particulate matter. EOLYS additive in conjunction with a particular filter can reduce particle emission by more than 99.9 per cent.
EOLYS is simply poured into the tank like any other fuel additive. The diesel particulate filters (DPFs) are needed to be replaced every 25,000 to 30,000 miles which become costly. The next generation EOLYS was a Cerium (rare earth) and Iron-based additive intended to increase the longevity of the particulate filter and reduce the need to add EOLYS every time the vehicle was fueled.
A 2.5L bottle is attached alongside the fuel tank fill tube and holds enough EOLYS for 80,000 to 100,000 miles before the bottle needs refilling. This correlates with the replacement of the DPF. The EOLYS metallic particles imbed themselves in the soot particles as they enter the DPF, kind of like packing small stones in a snowball. The EOLYS/soot particles burn off during active regeneration of the particulate filter at 430∞C as opposed to 600∞C for non-EOLYS soot particles, and in one-sixth the time.
The reduction in the amount of the heat needed to generate the filter will allow engineers the option of moving the DPF further down the exhaust trail, opening up the space nearer the exhaust manifold for other catalyst.
The Rhodia system is currently on 14 different platforms in Europe with million of vehicles are installed with EOLYS system. This can work with any type of diesel fuel which does not care what type of fuel it is without a single failure. Further development was carried out in EOLYS system with new iron-oxide based additive which can reduce the particulate up to 7ppm, which can reduce the amount of metal and extend the life of the particulate filter to more than 150,000 miles.
CHARACTERISTICS OF PARTICULATES
Hydrocarbon and oxide of sulphur and nitrogen are passed over the catalytic process and these gases are transformed into carbon dioxide, water, and nitrogen.
MAXIMIZED DIESEL YIELDS
Maximized diesel yields by selecting the right cut points within the refinery are also important. Table-2 shows the yield of gasoline, diesel and FCC diesel wt per cent on feed.
TABLE-2: IMPACT OF GASOLINE, DIESEL, FCC DIESEL
|CASE||MAX GASOLINE WT% YIELDS ON FEED||MAX DIESEL WT% YIELDS ON FEED||FCC DIESEL WT% YIELDS ON FEED|
Of the two types of engines that are currently in production and widely used, the diesel is more efficient than the gasoline engine. Automakers are developing the gasoline direct injection engine, which is expected to be more efficient than the current port fuel injected gasoline spark ignition engine.
Gas turbines provide efficient air transport but attempts to adapt them to ground vehicles have been largely unsuccessful. For the future, if and when the "automotive" fuel cell becomes ready for production, and hydrogen becomes a widely available transportation fuel, there may be a strong competitor to the diesel engine.
In the near- to mid-term, the diesel engine appears to be the most probable engine for improving transportation fuel efficiency.
CONCERN ABOUT DIESEL
The diesel engine has a reputation for emitting large quantities of oxides of nitrogen and soot. To this day, the perception of the "smoky old diesel" persists even with the growing body of scientific and technical knowledge, and research results showing that diesel exhaust can achieve or even improve the standards set for gasoline engines. As "dieselization" becomes more likely because of the growing concern about rising fuel costs and global climate change, and the absence of any other competitive transportation alternative to the less efficient gasoline engine, some environmental groups have raised concerns about increasing usage of diesel engines especially for light duty applications.
TOXIC EMISSION FROM GASOLINE AND DIESEL
Health studies indicate that fine particulates may be highly toxic to the human lung at very low mass concentrations because of:
large numbers per unit mass;
b) high deposition efficiency in the lower respiratory tract;
c) inability of the respiratory tract to clear itself of such particulates; and
d) increased surface area available for interactions with cells.
These tendencies increase as the particles become smaller.
These factors imply that gasoline particulates, because they are smaller than diesel particulates, may be more readily embedded deeper into the lungs. By inference, therefore, gasoline particulates could pose an even greater health risk than diesel particulates since the greater number of our vehicles are gasoline fueled and the greater quantity of fuel consumed is gasoline.
The small size of engine particulates presents difficulty in measuring and characterizing them let alone determining their biological toxicity and human health effects.
Interestingly, aromatics are undesirable in diesel fuel because they lower the cetane number (i.e. reduce "compression ignition" quality of the fuel), but not in gasoline where they enhance octane quality. Hence, enhancing diesel fuel quality tends to remove aromatics, and therefore, lower toxicity.
WHY DIESELIZATION WILL LEAD TO CLEANER AIR?
It is our view that by conducting research to develop emissions control technologies for lean burn (diesel) engines, implementation of these technologies will lead to cleaner air. Diesel engine emissions control technology development has lagged behind gasoline engine emissions control. Technology development has focused on gasoline engine emissions control.
The three-way catalyst system subsequently reduced gasoline emissions to their current very low levels. In the same timeframe that the catalytic converter has reduced gasoline engine emissions, diesel engine manufacturers have focused on reducing diesel engine-out emissions while optimizing engine efficiency.
To say that diesel engine emissions cannot be made as clean as gasoline engine emissions, as some would claim, is therefore premature since research on exhaust emissions after treatment for "lean burn" (diesel) engines is relatively recent.
For many refineries, the back-end boiling point specification limits the amount of diesel that can be produced. Since diesel is the heaviest cut produced in the crude unit, it can be difficult to achieve good separation between the diesel and residue cut. Carryover of residue material into the diesel cut can limit recovery of diesel-range material and reduce diesel yields.
The nature of crude unit design means that it is difficult to get good separation at the bottom of the crude unit. By allowing some diesel to slip into the vacuum unit and recovering light vacuum gas oil (LVGO), good separation can be achieved between the LGVO and VGO as it is at the top of the column and more diesel-range material can be recovered before the boiling point limitation is reached.
This could result in 1 to 2 per cent of crude being recovered as diesel rather being routed to the VGO upgrading unit. The actual impact of this on the gasoline-to-diesel ration will depend on the upgrading units installed and the separation quality achieved in the refinery.
The focus of many European refiners has been on improving the yields of diesel range material from their VGO upgrading unit. Light cycle oil is not a particular good diesel blend stock, often having a low cetane number in the range of 20 to 30 and high density, whereas the minimum cetane number range requires from 40 to 60.
Improved diesel recovery capabilities may allow for downstream optimization. Improved fractionation can selectively yield diesel streams with different desulphurization for the same diesel volume, increase capacity on downstream diesel treaters or create multiple diesel streams; each may optimize desulphurization.
APPROACHES FOR DIESEL RECOVER
Five different approaches can recover more diesel:
- Improving atmospheric tower
- Improving vacuum tower operation.
- Adding diesel recovery between the atmospheric tower and vacuum tower.
- Integrating streams within the unit.
- Refractionating diesel from other product streams.
SK Ansari is chemical engineer and Seema Ansari is telecom engineer.