Pickup Trucks and Oil Refinery Discussion - A Jay and Jimmie Production

Good question!

The desalter effluent - also called "undercarry" - is one of the major contributors of effluent at a modern refinery. There are routine operating cycles when the desalter must be rinsed, this is called "blowdown."

Typically, just from the desalter, you can generate 2-3 gallons of contaminated effluent per barrel of crude input. As can be expected, the effluent is contaminated with hydrocarbons, specifically a wide range of polycyclic aromatic hydrocarbons in the form of a coagulated sludge

At one time, the effluent was sent to clarifiers where it was subject to surfactant and emulsifier pretreatment, flocculation, coagulation, with the resulting nasty sludge mess disposed off "somewhere." It's now considered quite hazardous waste. Using the clarifier there was a bit of free product - crude oil that is - skimmed off as well.

More recently, membrane technology has been applied to filter the effluent. Two waste streams are generated from the membrane filters, the permeate which has acceptible levels of TDS to be easily handled by the clarfier, and the concentrate, which tends to be in a slurry form that may be burned

 

That's pretty much the response I was thinking of ....

 

Part II: The Crude Distillation Unit, distillation column

Once the crude leaves the desalter, it is heated in a boiler to about 650 F and fed into a fractionating tower. The fractionator tower is typically 120 ft in height, 15 ft in width. Inside the tower are usually 15 horizontal trays

The trays play an important part in the distillation process. They are either perforated or have “flappers†that allow vapor under pressure to advance up the tower. The trays usually have overflow pipes or scuppers, that allow excess product to drain down to the previous tray, get heated up and vaporize again.

A lot of process water is used in the fractionating tower, in the form of steam. Steam is injected into some trays to help vaporize the product in the tray, which tends to create a small vacuum and encourage the vapor to travel upwards

Some trays are also equipped with pumparounds, or “reflux,†which takes the product from the tray, cools it in a heat exchanger, and sprays it back in to encourage further vaporization.

The crude after preheating is fed near the bottom of the tower. This very hot liquid will almost immediately flash to vapor in the tower. As the vapor rises up the tower, the temperature drops. The lowest level of the tower contains a thick tar like stuff called residuum.

The bottom tray may contain a heavy oil. The middle tray may contain a kerosene type liquid. The top tray may contain a heavy naphtha liquid. The very top of the tower will have gases like butane, and light naphtha.

The fractionating tower is a continuous process. The various tray pumparounds require continuous monitoring so as not to overflow a tray or suck it dry, otherwise an explosion could result

Due to the various amounts of steam and condensed water inside the tower, the residuum at the bottom of the tower will contain a lot of contaminated process water. This water is stripped and separately treated in settling tanks, according to Stoke’s Law, before being sent to the clarifier.

To make it easier to understand, this topic will now have to be split up into subgroups. I’ll next examine what happens to the residuum from the bottom of the fractionating tower

 

1: No, usually 4-6 discrete output trays. Some of the trays are equipped with perforations and no draw-out, along with overflow scuppers, to allow some liquid to condense out, flow back down, and evaporate again. This is used in combination with reflux to encourage more aromatic yield

2. A typical dist tower production cycle will be every 2-3 weeks before acid washing or blowdown is needed. Otherwise the residuum at the bottom will coke up and plug the bottom, and the tray perforations will also coke or gum up. It's a messy job cleaning the trays

3. A lot of the process water consumption in the tower is taken out in various upper tray gas drawdowns, such as mingled with naphtha gases and butanes. The gas is "wet" so has to go through a dryer. Some of this water is also recycled in the tray pumparounds

The water that is stripped from the "wet" gas has to be treated as it contains H2S and other chemicals. The distillation/vaporization in the tower tends to use water. In addition, there is liquid water in the residuum, it is bound due to the surfactants and emulsifiers added as part of the process

 

Crude Part IIa: Vacuum Distillation

We’re still in the fractionating tower process, but will now deal with where all that “stuff†from the tower is sent. Typical of how my mind works, we’ll start at the very bottom: the VDU or Vacuum Distillation Unit

The fractionating tower has at the very bottom the heaviest components of the crude, that did not readily flash evaporate and get condensed out in the various trays. The heavy stuff at the very bottom is called “residuum†and resembles a dark tar-like stuff. At one time, the residuum was turned into asphalt stock and the rest was dumped somewhere

A portion of the residuum is still turned into asphalt stock, but the majority are useful for other things. The next step is to convert this heavy stuff into useful things like gas oils, lube stock, and fuel for the refinery itself. Once every useful component is extracted from the residuum, the remaining coke is sent to a coke furnace to provide steam or other heating duties

Vacuum distillation is still considered a fractionating process, and even the VDU will roughly resemble the fractionating tower. There are key differences though. A vacuum system and water misting is used to create much lower pressure for the hot tar. Like the fractionating tower, a direct-fire boiler is used to heat up the tar again, beyond 600 F.

Once pumped into the VDU, there aren’t trays like the fractionating tower. Instead there are levels filled with “packing.†Packing is any material – plastic, ceramic, metal, etc – that increases the surface area for vapor or liquid to contact. This enhances stripping and recovery quite a bit. Like the fractionator there is also pumparound applied to the different levels, so the cooled product has a chance to be flashed again to recover more useful components

The tar will actually start to vaporize on the packing, and each level produces a certain range of product. The lowest level produces what is known as HVGO or Heavy Vacuum Gas Oil, sometimes called Heavy Virgin Gas Oil. HVGO is typically used as feedstock for another very important refinery process, the Fluidized Catalytic Cracker. I will discuss the FCC later. A lot of lube oil stock is derived from HVGO as well

A middle level produces LVGO or Light Vacuum Gas Oil. LVGO is sent to distillate hydrotreating, to produce valuable gas oils, process oils, and kerosene/diesel. The LVGO – along with the HVGO – are now used to create things like oils and fuels, from what used to be a waste product

The residuum from the VDU can be used to make asphalt, or as coke for fuel within the refinery

 

That's a good question, unfortunately no easy answer

A lot of it depends on the crude source. A "sweet" crude, especially Light Brent Sweet, has the smallest residuum, typically under 5 gals per barrel in a well designed modern refinery process

A very "sour" crude, such as Venezuelan, could produce up to 25 gals of residuum per barrel.

Remember, at one time the residuum was only good for making asphalt and tars. Now with vacuum distillation and other process steps, including catalytic steps, we can derive useful fuels such as diesel and gasoline from it

The waste water from these steps does contribute to total clarifier load at a refinery, which was also a major source of PAH emissions from the waste. With membrane technology, you can filter out the hydrocarbons, leaving behind a concentrated sludge that can be dried and burned as coke fuel

 

I'm still working on trying to draw up some simple schematics. My artsy-fartsy skills are pretty bad even with a computer, I can't draw a straight line to save my life

 

Crude Part IIb: Mid distillate hydrotreating

Continuing further up the fractionating tower, midlevel is where a lot of distillates are recovered after reflux has been applied. These middle-grade distillates are suitable for kerosene, home heating oil, diesel fuel, and jet fuel

The hydrotreating process is *very* important to keep our air clean. One primary use of hydrotreating is to remove most of the sulfur, improving the life and efficiency of catalytic converters and DPF’s. Hydrotreating will also help remove most of the nitrogen and metals that are in suspension from the source crude

It should be noted that crude oil isn’t a uniform substance. Compare Light Brent Sweet to something from Venezuela and you can *smell* the difference. Even crude pumped from the same field isn’t truly homogenous, as the crude can form strata or plumes within the field, of varying quality.

Visually, the blackest looking crude may contain the most sulfur. There actually is crude that is amber in color, this is “sweet†and tends to make high quality product. Unfortunately, most of the crude out there is Sour

“Sweet†crude has the lowest sulfur content and generally requires little initial hydrotreating, unless you need more diesel/jet from the kerosene. A “sour†crude requires a great deal of hydrotreating to clean it up

Even relatively “clean†Sweet crude products require hydrotreating, as just trace amounts of sulfur will damage the delicate catalysts downstream in the Catalytic Reformer. So it isn’t just clean air regs that push the need for hydrotreating

Hydrotreating is actually a combination of hydrogenation and hydrogenolysis. As it implies, hydrogen is a necessary component. Chemically, the hetero-atoms S, N, O are mostly removed, the sulfur and hydrogen turn into hydrogen sulfide. In addition, you can expect some hydrogenation of double bonds and aromatic rings

At this point you may ask “where does the hydrogen come from?†We’ll eventually get to that in another topic, but at this point the answer is: it’s a Reforming byproduct put to good use

Ok, so here is how the hydrotreater works

The mid distillate stock from the fractionating tower is sent to the hydrotreating unit. The hydrotreater requires the use of a catalyst, both CoMo (Cobalt Molybdenum) and NiMo (Nickel Molybdenum) are used as catalysts.

The stock is first mixed with hydrogen gas and recycled hydrogen gas, then heated to about 700 F at up to 120 atmospheres, and sent to the catalytic reactor. It then vaporizes and enters the hydrotreater reactor bed, which has the catalyst within a frame.

Most of the CoMo or NiMo resembles – believe it or not – food pellets you might feed a rodent. Many different shapes are used, depending on the desired surface area and reaction rate

The hot vapor interacts with the catalyst bed and the hydrogen can interact with the catalyst and the sulfur compounds. This is known as hydrodesulfurization, the sulfur has been converted to hydrogen sulfide.

The hot vapor is then drawn out of the reactor and sent through a heat exchanger, which cools it off to about 110 F and 5 atmospheres. This causes condensation and a resulting liquid/gas mixture, which next enters a gas separation vessel.

The gas in the gas separation vessel is mostly hydrogen, with H2S. This is sent to an Amine Contractor , where the hydrogen is recycled back to the reactor to hydrotreat. The H2S is sent to Sour Gas processing, part of the Gas Plant within the refinery complex: more on that later

If anybody wants, I can go into a discussion of how Amines are used to remove the H2S from the Hydrogen gas stream. I have to warn you this becomes very complex and I’m trying to keep this fairly simple

Within the gas separator vessel the liquid is captured and sent to a Stripper, which functionally operates like the fractionating tower. A reboiler is used for the Stripper to further degas the liquid. The residuum from the Stripper is the final product from hydrotreating

So that covers the Hydrotreating operation for mid distillates. I promise to have simple diagrams up in the next 2-3 days, and will add them to the previous posts I have made to try to ease understanding of this very complex chemical process

Ok, any questions? Or, as one of my profs used to jokingly ask after a difficult subject was explained: “any survivors?â€

 

I'm going to now have a few beers and try to work on the artsy-fartsy diagrams. If I don't have any luck, I'll just post a process diagram and hope for the best

 

Good question

Well, actually, *every* part of a refinery is dangerous! Since the fractionating towers, hydrotreating reactor, reformer, etc are completely closed processes, there isn't any open flame or spark that can cause an explosion

However

Say a pipe weld from one of the tower tray pumparounds were to crack, you'd suddenly have hot reflux gushing out, and the tower would suddenly vent to atmospheric pressure. Air, containing oxygen, would enter, and in that environment an explosion is not only possible but very likely.

It has happened before, and is pretty tragic when it does go boom. I guess it's either good luck or good engineering - I'd say both - that keeps it from happening more often

Another danger is improper venting before workers have to enter a closed vessel, say the hydrotreater reactor or gas separator, to clean it out. There are a lot of toxic gases in there, such as H2S, that can kill you pretty quickly

 

Ok I gave up trying to make my own "simple" process diagrams. Even the heavy drinking didn't make me artsy-fartsy.

So I turned to Ronald F. Colwell, PE, of Process Engineering Associates LLC, who has a nice collection of rather easy to understand diagrams. Thank you Mr Colwell!