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The process of refining of crude oil is used to separate crude into fractions of various derived products.

For example, crude contains fractions of gasses such as methane, ethane, propane, butane, liquids such as gasoline and naptha, middle-distillates such as jet fuel and diesel, and heavier molecules. These are separated in a first stage fractioning column.

The heaver molecules are separated in a vacuum fractioning process, which follows the first stage.

If I understand correctly, this is used to separate various grades of fuel oil from the heaviest of compounds.

Some of these fuel oil components can be cracked into lighter molecules such as gasoline. (Usually a higher octane than the straight run gasoline from the first process.)

The remaining fuel oil components, of which there are typically three grades, are used to power large combustion engines, such as those found on ships. The heaviest of these grades is so viscous that it requires heating before it can be pumped through pipes or burned in a combustion engine.

If I understand correctly, what remains after this is bitumen, which is used in asphalt.

In terms of economics, it is not totally obvious what is categorized as "waste product". Some of these products (presumably the bitumen) have a lower economic value than the raw crude.

I don't know what the typical value of the heavy fuel oils is compared to the crude. Of course, the values will fluctuate as a result of economic conditions in supply and demand chains.

My question is, what happens to the waste products which cannot be converted into some saleable product? Are these products discarded? Are they buried in landfill?

I am assuming the following (I may be wrong):

  • All the methane, ethane, propane and butane is either used to power the refinery itself (in the heating processes), or sold in gas cylinders, or sold to power gas fired power plants
  • All the gasoline and naphtha produced from the various processes is sold, mostly as a blended commercial gasoline product
  • All the diesel and kero products are blended and sold as grades of diesel or Jet Fuel
  • The heavier fuel oil products are either sold and used to power large industrial equipment such as commercial and naval ships, power plants, or these products are used in the cracking process to produce lighter chain molecules
  • What remains is bitumen and is considered to be waste product

Does anything else remain and is considered "waste"?

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    $\begingroup$ Wikipedia compiles some fields of application of bitumen. $\endgroup$
    – Buttonwood
    Commented May 2 at 10:23
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    $\begingroup$ Bitumen is hardly a waste product given how many road surfaces are created from it. $\endgroup$
    – matt_black
    Commented May 2 at 10:37
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    $\begingroup$ There is no fraction that would be wasted. They need to be purified, though. $\endgroup$
    – Mithoron
    Commented May 2 at 13:48
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    $\begingroup$ Don't forget coke, which is the "final product" in my opinion. $\endgroup$
    – Stian
    Commented May 3 at 5:42
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    $\begingroup$ A big part of modern engineering is making sure that nothing is really a waste product - partly for simple reasons of economics (someone who can make use of more of the products will likely be more profitable) and in a much bigger part, environmental legislation. When crude oil first started being used in the US, it was used to produce kerosene for lighting (seen as a "cleaner" alternative to coal gas); what happened to the waste products (making up the vast majority of the input)? Mostly the highly toxic waste was just dumped into rivers or burnt off. Gasoline was a waste byproduct once. $\endgroup$
    – Luaan
    Commented May 3 at 11:15

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There is also petroleum coke, produced by spraying the very heaviest fractions at high temperature and pressure into a drum to try to eek out any remaining volatile components. The remaining product is almost all fixed carbon and sulfur.

The highest grades of coke can be used to produce graphite. The lower grades make an excellent replacement for coal if you can handle the high sulfur (2 - 3 times higher than typical coal), and the hardness.

The Portland cement industry makes abundant use of pet coke as modern preheater kilns completely absorb the sulfur from fuels and it becomes part of the cement (substituting for gypsum that would otherwise need to be added at a later step).

Petroleum coke is generally sold at a price which makes sure all of it will be sold (usually about half the price of coal), so it is probably the closest to a true "waste" product from a refinery.

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A big part of modern engineering is making sure that nothing is really a waste product - partly for simple reasons of economics (someone who can make use of more of the products will likely be more profitable) and in a much bigger part, environmental legislation. When crude oil first started being used in the US, it was used to produce kerosene for lighting (seen as a "cleaner" alternative to coal gas); what happened to the waste products (making up the vast majority of the input)? Mostly the highly toxic waste was just dumped into rivers or burnt off. Gasoline was a waste byproduct once. Heck, the very idea of using a chemical resource to produce energy reeks of (irresponsible) "waste disposal" nowadays - we can't use it productively? Burn it to release energy!

Your list of possible uses of all those products is already quite large, and is not exhaustive by any means. Remember, when you can't just dump or burn the waste (as was long possible, and is still being done when you can get away with it), it accumulates - that's expensive. And given the huge volumes of crude oil processed, even a small percentage amount of waste would accumulate very quickly indeed. And we're talking about stuff that's very toxic, flammable and hazardous, and generally quite hard to store safely.

What exactly each refinery does is very specific, depending on the demands they can hope to satisfy. As weird as it sounds, even today, many oil plants and refineries just burn off excess methane, for example - it's a very valuable byproduct... but only if you can reasonably get it to consumers. There's also a lot of variability with regards to the input feed - crude oil is ridiculously far from being a homogenous substance. Every oil field has a very different combination of hydrocarbons (and other stuff, like various sulfuric compounds) that heavily affects the outputs.

Of course, while petroleum distillation is a very visible and known part of the refinery, refineries don't just separate what's already there. There's a lot of processing steps that change the kinds of outputs you get; there's certain flexibility to allow for changing product demands. And while we talk about things like "gasoline", that's not really a homogeneous chemical product either - it's generally a mix of hydrocarbons (and additives) that satisfies conditions that fit within the requirements specified for particular ICEs. "Gasoline" from two different refineries (even from the same company) can be a very different material indeed. This is in contrast to competing products like ethanol, which are actually just that - ethanol, water, some impurities. This of course requires engines that burn gasoline to be a lot more lenient towards fuel used than engines that only need to burn ethanol or methane (and indeed, gasoline engines usually didn't need much beyond tweaking the ignition to use ethanol or methane instead of gasoline).

So, primarily, you want to sell as much product as possible, and if it makes sense, use the less wanted products for running the refinery. Indeed, a lot of the heating and mechanical elements use fuels or electricity generated on-site. It's a lot easier to power your own refinery with excess methane than building and maintaining a gas pipeline to the nearest gas power station.

If you can't sell the products you get from separation and distillation directly, you have two main options - change it to something you can sell, or dump it. Dumping is severely limited by environmental legislation (and to a lesser extent, practicality); for example, you'll be dumping a whole lot of waste water, but you need to do a lot of processing to make it reasonably safe to be released into the environment.

Worldwide, the main use of petroleum products is in chemical engineering - producing things like fertilizers, plastics etc.; in the US, the majority is used for transportation. According to US EIA, staggering 43% of all petroleum consumed is used just for the production of gasoline, some 20% for diesel fuel and 8% for jet fuel - and that's just transportation. So yes, in the US, energy production is a massive part of what petroleum is used for; it's a great waste of a valuable chemical resource if you ask me :D

It also drives the need to use the waste from the main product somehow - hence all the push to use bitumen for asphalt production (and the associated "we need more roads!", both helping sell the waste and further encourage fuel consumption), or petroleum coke as an industrial fuel source (fairly poor coke, mind - you wouldn't use it for steel production for example). The price of such products is generally very low (petroleum coke tends to be about 1/3rd to 1/4th the price of coal coke, just to get rid of it). The waste sulphur has also historically been a big problem in the US due to the massive volume of petroleum used for fuel purposes. Sulphur is the start of a lot of other chemical processes... but if you can't use it, it's yet another bulky, hazardous chemical. Again, it's in the interests of the oil companies to boost demand, for example by encouraging the use of artificial fertilizers (and indeed, the US oil industry managed to get people quite addicted to "cheap" fertilizers that make it even harder to de-oilify the economy).

There's also the very heavy fuel oils. Noone would ever use them as fuel if they had a choice - they're even more toxic, hard to work with and extremely dirty. But the oil companies still found ways to work around the environmental legislation to sell that product, as horrible as it is - used mainly by seafaring ships that burn it where it's not covered by the environmental legislation. Though it seems there's finally some coordinated push to stop that nowadays, with stricter world-wide regulation coming.

So yeah, what with the waste? If you can, sell it. If you can't, use it. If you can't, convince people how useful and important it is. If you can't, turn it into something more useful. If you can't, dump it. If you can't, dump it even further away. The oil industry is very good at pushing tons of horrible consequences on others :D And because the main product is still fuel in the US, the other products can be sold pretty cheaply and still remain profitable (indeed, paying people for taking the "waste" off their hands would often be cheaper than handling the waste "properly" or turning it into other, more in-demand products).

The big challenge is that by far the main reason we use petroleum at all is how cheap it is; it doesn't take long for additional processing to eliminate that advantage. In fact, there was a time when bioethanol was cheaper than gasoline in the US after the government eliminated alcohol taxes - which completely coincidentally resulted in the Prohibition (Rockefeller funding and owning several of the "grass roots" organizations pushing for alcohol prohibition is completely coincidental, of course :P ).

Engineering alone only explains a small part of the mess that is oil production and refinement - but as it advances, it has the potential to make more valuable products, rather than just finding a spin on selling the less valuable products. For example, finding buyers for bitumen/asphalt used to be very important in the 50s-70s in the US - modern refineries tend to crack it instead, to get shorter chains. But even today, burning off the unwanted products is still fairly significant, and the amount of e.g. methane just leaking into the atmosphere is scary. You can change pretty much everything into more useful products, but that makes the processes more expensive. The oil and gas industry is very protective of their massive profit margins.

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Most of the excess products are reused, whether thats Bitumen(check this out https://infinitygalaxy.org/bitumen-applications-and-uses/).

However there's always a small amount of waste generated during refining that can't be used or processed further.

These are typically not found in the crude oil and are:

Water: Wastewater from various processes needs to be treated before disposal or reuse.

Solid Waste: Spent catalysts and other solid materials are disposed of according to strict environmental regulations. Landfills are usually a last resort.

Also as you mentioned, some heavier fractions can be broken down (cracked) into lighter, more valuable products like gasoline or diesel through processes like fluid catalytic cracking (FCC). (check this out: https://www.eia.gov/todayinenergy/detail.php?id=9150)

I hope it helps.

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  • $\begingroup$ If possible check this out as well: utpedia.utp.edu.my/id/eprint/6722/1/… $\endgroup$
    – Ronith
    Commented May 2 at 11:14
  • $\begingroup$ FCC is probably not the best example process here (in the context of OP talking about vacuum distillation products), since it is typically fed with light vacuum and/or heavy atmospheric gas oil. Heavy vacuum gas oil is typically sent to hydrocracking instead, its chains are too long to be efficiently processed by FCC. And the very heaviest non-bitumenous fraction (vacuum residue) is typically cracked in a (delayed) coker. $\endgroup$
    – TooTea
    Commented May 3 at 9:09
  • $\begingroup$ Yeah that’s right. However the question was about the other waste products(solid waste and water) $\endgroup$
    – Ronith
    Commented May 3 at 10:18
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The vast majority of what goes into an oil refinery is sold in some form or other. Not everything can be used straight from the fractionation column. In particular, straight run gasoline has a rather low octane number and may be upgraded by reforming. Usually there is high demand for additional material in this molecular weight range. This can be made either by "cracking" heavier molecules or by combining lighter ones - known as "alkylation." These processes are optimised in various ways including to improve octane rating of the product and to ensure nothing is wasted.

Waste hydrocarbons that cannot be converted into useful products are used as fuel in the refinery.

One particular example is in hydrogen production as explained below.

Many processes on modern refineries (hydrocracking, hydrodesulfurization) require hydrogen gas. This is on the increase due to the fact that much of the light, sulphur free crude in the world has already been extracted. Therefore a refinery will have a steam methane reformer converting steam and hydrocarbon into hydrogen and CO2:

CH4 + 3H2O -> CO  + 3H2 (steam methane reformer, endothermic)
CO  +  H2O -> CO2 +  H2 (shift reaction, exothermic)
-----------------------
CH4 + 4H2O -> CO2 + 4H2

The raw hydrogen contains CO2 together with traces of reactants. These impurities are absorbed on molecular sieves in a process called pressure swing absorption [1]. The off gas released when these molecular sieves are regenerated constitutes a particular disposal problem as it is mostly CO2 mixed with a small amount of combustibles. Special burners are used on the steam methane reformer to burn both a hydrocarbon makeup gas and also the off gas from the pressure swing absorption. Typically the burners have 2 separate gas inlets.

[1] Pressure swing absorption is now the preferred method of separating hydrogen and CO2 from steam methane reformers. Previously a temperature swing process dissolving the CO2 in alkaline solutions (typically of ethanolamines) was used. This process still finds many applications in removing CO2 but has fallen out of favour in hydrogen plants.

Another major waste product is hydrogen sulfide gas from sweetening operations such as hydrodesulfurization. This is converted to saleable sulfur by the Claus process; adoption of these processes (resulting from the desire to reduce acid rain and avoid poisoning vehicle catalyic convertors) has caused decline in the sulfur mining industry. In this, exactly one third of the H2S is burned to make a mixture of H2S and SO2 which then reacts to form sulfur and water:

 H2S + 1.5 O2 ->  H2O + SO2
2H2S + SO2    -> 2H2O + 3S
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3H2S + 1.5 O2 -> 3H2O + 3S

So basically a refinery will crack or burn heavy material, or sell it as coke or bitumen, and burn any light gas waste on site. Refineries are net consumers of energy so there is usually plenty of energy demand. Oil fields, on the other hand, typically produce some gas together with the oil, and it may not be economic to transport it to a user (an issue known as "stranded gas.") In places where it is allowed, stranded gas is still flared (or even simply vented) continuously (something you wouldn't see on a refinery except in an emergency.) Where environmental law does not allow gas flaring and cryogenic natural gas liquefaction is not economic, gas is most simply converted to methanol, although it can be converted to liquid hydrocarbon if there is enough of it.

Oil refineries and oil fields also generate oily water waste. There will be separate drainage systems for clean and oily water to try to keep the oily waste concentrated, but the issue is the same as for the hydrogen reformer off gas mentioned above - the amount of useful combustible material in the water is small. This water has high "chemical oxygen demand" (if it is released bugs will feast on it and use all available oxygen, which kills the fish.) Therefore it is often sent to a water treatment plant where this can occur in a controlled manner, similar to what happens in a sewage farm.

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In refining crude oil, almost every component is transformed into a valuable product, minimizing waste. Methane through butane gases often power refinery operations or are sold for energy. Gasoline, naphtha, diesel, and jet fuel are processed and sold for their respective uses. Heavier fuel oils power ships or are further cracked into lighter products. Bitumen, far from waste, is vital for asphalt in road construction. The true "waste" is minimal, consisting mainly of refinery residues like sludge, which undergoes treatment or is used in low-grade fuel blending, adhering to environmental standards. Essentially, refineries aim for maximal conversion of crude into saleable products, reflecting the industry's efficiency and environmental responsibility.

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