sulfuric acid recovery

Acid treating of used lube oils is an obsolete technology to recycle used oil. The acid treating process is not environmentally sound or economically sound when waste by-product management is considered. Only if the hazardous wastes are dumped into the environment does acid treating make a 'profit', and severe environmental damage is a horrible result.

There are several proven technologies for reprocessing used lube oils into high quality lube base stocks which are environmentally friendly and economically sound. They use dehydration, fractionation, and solvent extraction or hydrotreating of the lube stock distillates. The plants are not cheap, but are economically viable.

The sulfuric acid chemically reacts with parts of the used oil. The only way to 'recover' the acid is by burning the sludge in a specially designed sulfuric acid manufacturing plant. The oily sludge has very high hydrocarbon content and burning it makes a lot of water vapor and CO2, which use up to processing capacity of the acid plant. Normal feedstock for acid plants are elemental sulfur and H2S.

The 'sulfur' is burned in air to SO2, which is then catalytically converted to SO3. The SO3 is dissolved in 98% H2SO4 to make 'fuming sulfuric acid (104% H2SO4) , which is diluted back to 98% for product. Oil sludge takes as much as 15 times the acid plant capacity as elemental sulfur fo rthe same 98% acid production.

Hi, oily!

From what I've read on the topic, and I hope to be corrected in here if I'm wrong, there are two issues revolving around using the H₂SO₄ recovery method for sludge.

First, there are several stages of the recovery process that are environmentally harmful and must be undertaken with due care and caution so as not to leave behind harmful residue and by-products from the various stages of recovery. I hope you are taking all precautions necessary in your operations. I have highlighted some information below where environmental concerns seem to arise.

The alkylation of your used-up acid is of extreme importance, so that it is neutralized before being exposed to the earth or water table.

Secondly, the amount of H₂SO₄ used depends upon the amount and type of sludge being treated. Since the viscosity, purity, and amount of sludge being treated determine the treatment, and since the amount of sludge you are refining depends upon the capacity of your equipment, you will only be able to determine an appropriate quantity of H₂SO₄ to add during each batch process.

You probably are already aware of the concentration of acid required. (I have never refined sludge, but I seem to recall that a somewhere around a 4% concentration is used, depending upon the nature of the sludge. Again, I appeal to the chemical engineering guys & gals to supply a correct figure. There is probably a table detailing the type of sludge or its characteristics and the concentration required to refine it.)

The one factor that you can control is the strength of the acid in process, which according to the information I read (below), must be at least at 90%. Therefor, you will have to undertake occasional batch testing to determine the strength of the sulphuric acid during process and add fresh acid as required.

The following information comes from a website

http://www.epa.gov/epaoswer/hazwaste/id/studies/studyp.txt

You can use a tool called "Google" to find more information about your interest. Type "google" into your address line. When the website comes up, type key words like "use sulphuric acid for sludge oil recovery" into the Google Search line. This website will then bring you a large number of topics that match your interests on the internet, along with a 'thumbnail' description (enough to tell you if this is a choice you want to check out). If you like the description, click on the Bold type, and that website will immediately be connected to your computer. This is a very useful tool.

The following description comes from the Environmental Protection Association

3.6.1 Sulfuric Acid Alkylation Process Description

In the sulfuric acid alkylation process, olefin and
isobutane gases are contacted over concentrated sulfuric
acid (H₂SO₄) catalyst to synthesize alkylates for
octane-boosting. The reaction products are separated by
distillation and scrubbed with caustic. Alkylate product
has a Research Octane Number in the range of 92 to 99.
Figure 3.6.1 provides a generic process flow diagram for
H₂SO₄ alkylation.

http://www.cepis.ops-oms.org/bvsars/i/fulltext/petrole/petrole.pdf

http://e-collection.ethbib.ethz.ch/show?type=diss&nr=15447&part=fulltext

The olefin stream is mixed with isobutane and H₂SO₄ in
the reactor. To prevent polymerization and to obtain a
higher quality yield, temperatures for the H₂SO₄ catalyzed
reaction are kept between 40 and 50^oF (McKetta, 1992).
Since the reactions are carried out below atmospheric
temperatures during most of the year, refrigeration is
required. Pressures are maintained so all reaction streams
are in their liquid form. The streams are mixed well during
their long residence time in the reactor to allow optimum
reaction to occur.

The hydrocarbon/acid mixture then moves to the acid
separator, where it is allowed to settle and separate. The
hydrocarbons are drawn off the top and sent to a caustic
wash to neutralize any remaining trace acid. The acid is
drawn from the bottom and recycled back to the reactor. A
portion of the acid catalyst is continuously bled and
replaced with fresh acid to maintain the reactor's acid
concentration at around 90 percent. This spent H2SO4 was a
listing residual of concern.

In the fractionator, the hydrocarbon streams are
separated into the alkylate and saturated gases. The
isobutane is recycled back into the reactor as feed. Light
end products may be filtered with sorbents to remove trace
H₂SO₄ acid, caustic or water. The sorbents (e.g., treating
clays) are study residuals of concern.

Some facilities have neutralization tanks (in and above
ground), referred to as pits, which neutralize spent caustic
and any acid generated from spills prior to discharge to the
WWTP, serving as surge tanks. Neutralizing agents (sodium,
calcium, potassium hydroxides) are selected by the
refineries. If necessary, the influent to the pit is
neutralized and, depending on the neutralizing agent, the
precipitated salts form a sludge. This sludge was also a
listing residual of concern. Sludge may also be generated
in process line junction boxes, in the spent H₂SO₄ holding
tank, and during turnaround. However, due to the aqueous
solubility of sodium, calcium, and potassium sulfates,
sludge generation rates are relatively low and the majority
of neutralization salts (e.g., sodium sulfate) are
solubilized and discharged to the WWTP.

Mark

Oily - Years ago, I performed field service on a plant in Houston, TX that recovered sulfuric acid from sludge. They sprayed the sludge into a furnace and burnt off the oil residue. Pure sulfuric acid remained. My company supplied the variable speed drives which controlled the rate the sludge was spayed into the furnace.