WO2006037368A1

WO2006037368A1 - Process for reducing the organic acid content of hydrocarbon feedstocks

Info

Publication number: WO2006037368A1
Application number: PCT/EP2004/011361
Authority: WO
Inventors: Elizabeth Marques Moreira; Claudia Maria De Lacerda Alvarenga Baptista; Paul O'connor; Henrique Soares Cerqueira
Original assignee: Petroleo Brasileiro SA Petrobras; Albemarle Netherlands BV
Current assignee: Petroleo Brasileiro SA Petrobras; Albemarle Netherlands BV
Priority date: 2004-10-04
Filing date: 2004-10-04
Publication date: 2006-04-13
Anticipated expiration: 2007-04-04

Abstract

A process for reducing the organic acid content of a crude oil feed by contacting the oil feed with an adsorbent comprising anionic clay or a heat-treated form thereof in a reactor vessel under agitation and under a flow of inert gas at a temperature in the range of 200 to 500°C.

Description

PROCESS FOR REDUCING THE ORGANIC ACID CONTENT OF HYDROCARBON FEEDSTOCKS

The present invention relates to a process for reducing the organic acid content of crude oil feeds.

The organic acids present in crude oils and petroleum fractions are mainly naphthenic acids. Naphthenic acids are predominantly monocarboxylic acids having one or more cycloaliphatic groups alkylated in various positions with short chain aliphatic groups and containing a polyalkylate chain terminating in the carboxylic acid function. They are represented by a general formula C_nFWzOa, where n indicates the carbon number and z specifies a homologous series. The z is equal to 0 for saturated acyclic acids and increases to 2 in monocyclic naphthenic acids, to 4 in bicyclic naphthenic acids, to 6 in tricyclic acids, and to 8 in tetracyclic acids. Naphthenic acids in the range of C₇ to C12 consist mainly of monocyclic acids. The more complex acids contain larger proportions of multicyclic condensed compounds.

The molecular weight of the naphthenic acids present in crude oils, as determined by mass spectrometry, generally varies between 200 and 700.

A significant quantity of crude oils has recently been found worldwide, particularly the recent offshore findings in Brazil. These crude oils, which exhibit a relatively high density and a high content of naphthenic acids, challenge downstream operations due to high corrosion rates. As a result, the price of the oil decreases sharply with the increase of the total acidity number (TAN).

The straightforward solution is to blend the high-acidity crude with low-acidic ones. Another option is the addition of a basic solution of corrosion inhibitors (US 5,182,013 and 1)8 4,647,366;, the drawback being the formation of a stable emulsion, which is difficult to separate from the treated crude. The pre-treatment of crudes by making use of many different basic compounds to neutralize the acids present in the oil has also been investigated. Compounds like monoethanolamine (US 4,589,979), ethoxylated amine (US 5,961 ,821), ammonia (US 6,258,258), polymeric amines (US 6,281 ,328), aqueous solutions of hydroxides or ammonium hydroxide (WO 01/79386), or a mixture of alcohol with metal carbonates, hydroxides and/or phosphates (US 6,190,541) are disclosed as being effective. Nevertheless, the consumption of such compounds is a major inconvenience.

US 6,086,751 discloses reduction of the acidity by means of a moderate thermal treatment, at temperatures in the range of 320 to 42O⁰C and reaction times of

2 hours. Under those conditions thermal cracking is minimized and most of the acids are converted to water and CO₂. The presence of water inhibits the decarboxylation reaction. It is therefore important to remove the water prior to the thermal treatment and to keep removing it (and the CO₂) as it is formed during the treatment.

Another possibility is pre-treatment of the crude oil under fixed bed hydrotreating conditions - temperature: 200-370⁰C, H₂ partial pressure: 50-500psi - for selectively converting low-molecular weight acids, which are thought to be the main cause of corrosion (US 5,871 ,636 and US 5,910,242). A Group Vl metal or a Group VIlI noble metal may be used as catalyst. The drawback to such a process is the investment (and catalyst) cost and the hydrogen consumption. According to US 5,928,501, a slurry reactor may also be used for the hydrogenation of naphthenic acids, achieving complete elimination of the acids using a similar catalyst and pressure, but at higher (380-450⁰C) temperatures.

Another approach is the use of adsorbents to adsorb the acids. WO 04/005434 discloses a reduction of the naphthenic acidity of desalted and dewatered petroleum oils by using an adsorbent. Disclosed adsorbents include carbon black and spent or coked FCC catalysts.

US 5,389,240 discloses a fixed bed process for sweetening liquid hydrocarbon feedstocks. The first step in this process involves the removal of naphthenic acids; the second step is the removal of mercaptans from an alkaline environment. It is mentioned that petroleum fractions having an end boiling point up to about 600⁰C and TAN values of 0.003 to 4 mg KOH/g can be treated by this process. Disclosed petroleum feedstocks are kerosine, middle distillates, light gas oil, heavy gas oil, jet fuel, diesel fuel, heavy naphtha, lube oil, stove oil, and heating oil; with kerosine with a TAN of 0.01-0.06 mg KOH/g being preferred. The sulfur levels of these petroleum fractions is about 0.05-0.8 wt% (as S).

The first step involves flowing the feedstock in the presence of oxygen or air and at a preferred temperature of 30-80⁰C through a fixed bed of an adsorbent containing a metal oxide solid solution. The metal oxide solid solution contains a divalent and a trivalent metal oxide and is prepared by calcining a hydrotalcite-like material at a temperature between 400-750⁰C.

Although this document mentions that the reaction temperature during naphthenic acid removal may be as high as 400°C, the use of such a high temperature in said process will never be considered by those skilled in the art. First of all, such temperatures would result in thermal cracking of the petroleum fractions, which is in general undesired. Second, the use of such high temperatures in this oxygen- requiring process will result in the formation of (a) explosive mixtures of light cracking products and oxygen and (b) undesired O-containing compounds like phenols.

Crude oils contain far more heavy metal and hetero atom (S, N) contaminants than the petroleum fractions of US 5,389,240 do. Crude oil feeds usually have a sulfur content of at least 1.5 wt% (as S), typically 2-3 wt%, although sulfur levels above 5 wt% are possible.

If the process of US 5,389,240 should be used for organic acid removal from crude oils, it is to be expected - because metal oxide solid solutions are good adsorbents for sulfur and nitrogen-containing materials - that sulfur and nitrogen-containing materials preferentially adsorb on the solid solutions before giving organic acids a chance to adsorb. Further, the heavy metals present in the crude oils are expected to poison the solid solution's adsorption sites. The same would be expected for anionic clays, the precursors of said solid solutions. Therefore, the process of US 5,389,240 cannot be used for the removal of organic acids from crude and heavy oils.

Unexpectedly, it has now been found that, under specific conditions, anionic clays and their heat-treated forms can be successfully used for the removal of organic acids from crude and heavy oils.

The present invention therefore relates to a process for reducing the organic acid content of crude oil feeds by contacting a crude oil feed with an adsorbent comprising an anionic clay or a heat-treated form thereof in a reactor vessel under agitation and under a flow of inert gas at a temperature in the range 200 to 500⁰C.

Without wishing to be bound to any theory, it is thought that the success of the process is due to in situ regeneration of the adsorption sites. This in situ regeneration is probably due to decomposition of the organic - e.g. naphthenic - acids, thereby regenerating the adsorption sites. Further, in this temperature range, acid adsorption seems to be favoured over adsorption of the sulfur contaminants.

Using the process of the invention, crude oils with a TAN up to 10 can be treated. Further, this process at the same time reduces the viscosity of the crude oil. Crude oil feed

Crude oil feeds that may be used in the process of the invention include any organic acid-containing crude oil that is liquid or liquefiable at the temperature applied during the process. Both whole crudes, i.e. unrefined, undistilled crudes, and topped crudes may be used.

The crude oil feed preferably has a Conradson Carbon content of at least 3, more preferably of at least 5. Their sulfur content preferably is at least 1.5 wt% (as S), more preferably 2-3 wt%. The Total Acid Number (TAN) of the crude oil feed preferably is up to 10, more preferably 2 to 5. This Total Acid Number refers to the amount of milligrams of KOH required for neutralizing one gram of oil and is determined using ASTM D- 664-04.

The adsorbent

The adsorbent to be used in the process of the present invention comprises an anionic clay or a heat-treated form thereof. In addition, the adsorbent may contain a matrix material, such as alumina, silica, silica-alumina and/or magnesia. Other materials that may be present in the adsorbent include compounds containing metals selected from the group consisting of Ca, Ba, K, and Na.

The adsorbent preferably contains 50-100 wt%, more preferably 80 to 100 wt% of (heat-treated) anionic clay.

The adsorbent is preferably used in the form of shaped particles, such as microspheres, extrudates, beads, or pellets. These particles have a diameter of preferably 0.1-100 mm, more preferably 0.1-10 mm, and most preferably 0.1-3 mm.

The BET surface area of the (heat-treated) anionic clay preferably ranges from 60 to 300 m²/g, more preferably from 150 to 250 m²/g. Anionic clays are layered structures corresponding to the general formula

[M_m ²⁺ M_n ³⁺ (OH)_2m+2n.K Xn/z^Z"). bH₂O wherein M²⁺ is a divalent metal, M³⁺ is a trivalent metal, m and n have a value such that m/n = 1 to 10, preferably 1 to 6, and b has a value in the range of from 0 to 10, generally a value of 2 to 6, and often a value of about 4. X is an anion with valance z, such as CO₃ ^2", OH^", or any other anion normally present in the interlayers of anionic clays. It is more preferred that m/n should have a value of 2 to 4, more particularly a value close to 3. In the prior art, anionic clays are also referred to as layered double hydroxides and hydrotalcite-like materials.

Anionic clays have a crystal structure consisting of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay in which Al is the trivalent metal, Mg is the divalent metal, and carbonate is the predominant anion present. Meixnerite is an anionic clay in which Al is the trivalent metal, Mg is the divalent metal, and hydroxyl is the predominant anion present.

In hydrotalcite-like anionic clays, the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers may contain anions such as NO₃ ^", OH, Cl^", Br^", r, SO₄ ^2", SiO₃ ^2", CrO₄ ^2", BO₃ ^2", MnO₄ ^', HGaO₃ ^2', HVO₄ ^2",

ClO₄ ^", BO₃ ^2", pillaring anions such as V10O28^6" and Mo₇O₂/^", monocarboxylates such as acetate, dicarboxylates such as oxalate, alkyl sulfonates such as lauryl sulfonate.

Upon thermal treatment at a temperature above about 20O⁰C, anionic clays are transformed into so-called solid solutions, i.e. mixed oxides that are re-hydratable to anionic clays. At higher temperatures, above about 800⁰C, spinel-type structures are formed. These are not re-hyd ratable to anionic clays.

The thermally treated anionic clays that can be used in the process of the present invention include solid solutions and spinel-type materials, with solid solutions being preferred.

For the purpose of the present invention various types of (thermally treated) anionic clays are suitable. Examples of suitable trivalent metals (M³⁺) present in the (thermally treated) anionic clay include Al³⁺, Ga³⁺, In³⁺, Bi³⁺, Fe³⁺, Cr³⁺, Co³⁺, Sc³⁺, La³⁺, Ce³⁺, and combinations thereof. Suitable divalent metals (M²⁺) include Mg²⁺, Ca²⁺, Ba²⁺, Zn²⁺, Mn²⁺, Co²⁺, Mo²⁺, Ni²⁺, Fe²⁺, Sr²⁺, Cu²⁺, and combinations thereof. Especially preferred anionic clays are Mg-Al and Ca-Al anionic clays. Anionic clays with any type of stacking can be used, e.g. conventional 3Ri stacking or the 3R₂ stacking described in WO 01/012550.

Suitable anionic clays can be prepared by any known process. Examples are co- precipitation of soluble divalent and trivalent metal salts and slurry reactions between water-insoluble divalent and trivalent metal compounds, e.g. oxides, hydroxides, carbonates, and hydroxycarbonates. The latter method provides an inexpensive route to anionic clays.

Process conditions

The process according to the present invention is conducted in a slurry phase reactor, by mixing the adsorbent with the crude oil feed. The adsorbent/oil weight ratio in the slurry preferably is in the range of 0.01 to 1 , more preferably 0.02 to 0.08.

The process is conducted under an inert gas flow. Suitable inert gases include nitrogen and argon. During the process, light gases may be produced, such as butanes and lighter, H₂O, CO, and CO₂. This gas formation may be due to decomposition of the organic acids and/or some cracking of the crude oil feed.

The temperature in the reactor ranges from 200⁰C to 500⁰C, preferably from 250 to 400°C, and more preferably from 300 to 350⁰C. The latter range is preferred, because it results in minimized thermal cracking of the crude oil feed. Above this temperature, gas production and coke deposition will increase. On the other hand, some thermal cracking of the crude oil may be desired and higher temperatures are then preferred.

The pressure in the reactor is preferably atmospheric, more preferably in the range of 0.1 to 1.0 MPa.

The crude oil feed is preferably contacted with the adsorbent for a period of 15 minutes to 5 hours, more preferably 30 to 120 minutes.

After this period, the adsorbent may be removed from the oil feed using a conventional solid/liquid separation technique, such as decantation or centrifugation. The oil fraction can then be further refined, whereas the adsorbent may be regenerated by calcination, i.e. controlled burning in the presence of air, at a temperature of 200-40Q°C.

Alternatively, the adsorbent and oil feed are not separated, and the slurry obtained from the process of the invention can be submitted to further refining during which the adsorbent may be present as a useful additive.

FIGURE

Figure 1 illustrates a laboratory scale installation suitable for performing the process of the invention. EXAMPLES

General test procedure

The adsorbent was loaded in the reactor (2), which was then closed and purged with a stream of nitrogen at a flow rate of 100 mL/min for nearly 30 minutes. Cooling water was turned on and the adsorbent was heated up to 80⁰C. A Campos Basin oil, Rio de Janeiro, Brazil, having TAN of 3.2 mg KOH/g (1 kg), - pre-heated at 80°C in a reservoir (1) - was drained towards the interior of reactor (2) with the aid of nitrogen. The flow rate of nitrogen was then reduced to 5 mL/min and reactor (2) was heated to the contacting temperature, under constant agitation with an impeller (3) at about 300 rpm.

The condenser (5) and the cold finger (6) were used to condense the light gases formed. The volume of gas evolving during the process, minus the volume of purge gas, was measured by a humid gas meter (7) and collected in sample bags (8) for chromatographic analysis.

The process was conducted for 60 minutes, after which the TAN of the treated oil was measured. To this end, a sample of the treated oil feed was centrifuged at ambient temperature for 5 minutes at 2,000 rpm and the TAN of the liquid phase was determined using a Metrohm™ titration processor employing ASTM Method D- 664.

EXAMPLES 1-5 AND COMPARATIVE EXAMPLES 6-9

The general test procedure described above was applied in order to test the suitability of (thermally treated) anionic clays for the removal of naphthenic acids from crude oils. Different contacting temperatures and adsorbent/oil weight ratios were used: see Table 1.

The adsorbent used was a powder containing 100 wt% of either Mg/AI anionic clay or a solid solution prepared by calcining this anionic clay at 350⁰C (Example 4) or 400⁰C (Example 5). The anionic clay had a specific surface area of about 200 m²/g.

In Table 1 , the results are compared with those of kaolin (i.e. a cationic clay), coke fines, and two blank tests, Comparative Examples 6-9, respectively. The blank tests only involved the thermal treatment, without any adsorbent.

The results clearly indicate that anionic clays can be used to reduce the acid content of crude oils. Solid solutions obtained by calcination of anionic clay are even more suitable.

The Table further shows that a contacting temperature of 350⁰C is preferred above one of 250°C.

Table 1

Example T (⁰C) Adsorbent Adsorbent/oil TAN (wt/wt) (mg KOH/g oil)

1 250 anionic clay 0.1 1.35

2 350 anionic clay 0.1 0.32

3 350 anionic clay 0.02 0.53

4 350 anionic clay calcined 0. 1 0.13 at 35O⁰C

5 350 anionic clay calcined 0.1 0.12 at 400⁰C

6 350 kaolin 5 1.5

7 350 coke fines 0.02 1.29

8 250 blank 0.00 3.20

9 350 blank 0.00 1.52

Claims

1. A process for reducing the organic acid content of a crude oil feed by contacting the oil feed with an adsorbent comprising an anionic clay or a heat-treated form thereof in a reactor vessel under agitation and under a flow of inert gas at a temperature in the range of 200 to 500⁰C.

2. A process according to claim 1 wherein the crude oil feed and the adsorbent are used in an adsorbent/oil weight ratio of from 0.01 to 1.

3. A process according to claim 2 wherein the adsorbent/oil weight ratio ranges from 0.02 to 0.08.

4. A process according to any one the preceding claims wherein the crude oil feed is contacted with the adsorbent for a time period of from 15 minutes to 5 hours.

5. A process according to claim 4 wherein the time period is from 30 to 120 minutes.

6. A process according to any one of the preceding claims wherein the pressure in the reaction vessel is from 0.1 to 1.0 MPa.

7. A process according to any one of the preceding claims wherein the adsorbent comprises 50 to 100 wt% of (heat-treated) anionic clay.

8. A process according to any one of the preceding claims wherein the adsorbent has a BET surface area of between 60 and 300 m²/g.

9. A process according to any one of the preceding claims, followed by separating the adsorbent from the oil feed and regenerating the adsorbent by calcination.