US3000817A - Method of sweetening petroleum distillate - Google Patents

Description

Sept. 19, 1961 P. E. E. DE RYCKE ET AL 3,000,817

METHOD OF SWEETENING PETROLEUM DISTILLATE Filed March 12, 1959 ALCOHOL OR WATER a FEED 4 I &

TOWER 1 -TOWER SEPARATOR SEPARATOR 9 n A PRODUCT Jacques M L. Rombauf SMALL, THOMAS, DUN/ AM 8 MARX A TTORNE Y 3,000,817 l t/ETHOD F SWEETENING PETROLEUM DHSTILLATE Paul E. E. De Ryeke and Jacques M. L. Rombaut, Antwarp, Belgium, assiguors to Esso Research and Engineering Company, a corporation of Delaware Filed Mar. 12, 1959, Ser. No. 799,013 Claims priority, application Belgium Mar. 26, 1958 13 Claims. Cl. 208-232) The present invention relates to a process of sweetening petroleum distillates and particularly to light petroleum distillates that are refined by distillation at atmospheric pressure.

The sulfur compounds present in crude petroleum are the most objectionable and harmful impurities. These sulfur compounds contribute to the objectionable smell and corrosive properties of many distillates as well as for example in the case of the gasoline fractions decreasing the lead response; Sulfur exists in petroleum as a wide variety of compounds but in the petroleum distillates the sulfur exists in relatively simple compounds, such as mercaptans, sulfides, disulfides, and heterocyclic compounds such as thiophenes. It should also be remembered that sulfur exists in many distillates as free elemental sulfur. If the petroleum refiner is to produce a vendable product it is essential that the sulfur impurities in the products are reduced below certain critical limits.

Over the past 50 years, considerable efiorts have been expended on devising methods of sweetening petroleum products. Extraction of the petroleum product with aqueous solutions of alkali has become an established extractive sweetening process used in the art. This process involves intimately contacting the petroleum product with an aqueous solution of sodium hydroxide.

By virtue of their acidic properties mercaptans tend to form the sodium salt and pass into the aqueous phase. Even though this process, often referred to as the slurry process, has been in operation for many years, it is associated with several disadvantages. The mercaptan removal is incomplete since the sodium mercaptides of the higher mercaptans are extensively hydrolysed in water. Also the utilisation of the caustic is loW and the utilisation is often of the order of 40 to 60%, i.e. to say unused caustic is rejected with the spent liquor. Furthermore the process will only remove acidic sulfur bodies and would not remove significant amounts of sulfur from a sulfur-containing feed at ambient temperatures. As an example of the excessive caustic utilisation of the slurry process the applicants investigated the sweetening of a light virgin naphtha by a conventional caustic soda wash and by the process of this invention. By the process of the present invention the mercaptan number was reduced to 0.4 corresponding to an efiiciency of 44%. By means of the slurry process using 15 B. sodium hydroxide the mercaptan number was reduced to 13, corresponding to an efficiency of 4.6%. The light virgin naphtha had a mercaptan number of 16.

By means of the present invention as hereinbefore described efiicient use of caustic alkali is obtained, furthermore the process provides a means for decreasing the sulfur content of feeds.

During investigations on methods of sweetening petroleum products the applicants investigated a process in which the sour feed was passed over a fixed bed of sodium or potassium hydroxide. The applicants discovered that when the feed contained certain added concentrations of free sulfur and mercaptans a reaction took place in the bed which resulted in the removal of mercaptan and sulfur from the feed. It was discovered that sulfur could be removed from feeds by adding specified amounts of mercaptan and percolation over solid sodium or potassium Patented Sept. 19, 196i hydroxide and that it was preferably to add C mercaptans.

The process of the present invention comprises a process for desulfurizing a mercaptan-containing petroleum distillate which comprises adjusting the water or C to 0.; alcohol content of the distillate to a value between 0.01 to 5 wt. percent, percolating the distillate so obtained over a fixed bed of solid sodium hydroxide or potassium hydroxide and isolating the desulfurized distillate so obtained.

The process is preferably carried out by determining the water or C to 0.; alcohol content of the distillate. If this content is not within the defined range then additional water or C to 0,; alcohol is added. Many of the distillates to be desulfurized may be wet and contain water. Provided that the water content is within the defined range additional C to 0.; alcohol within the defined range may be added to the wet distillates before percolation over the solid sodium or potassium hydroxide.

In a further preferred embodiment of the present invention soluble mercaptans or free sulfur are dissolved in the distillate so that the ratio of free sulfur to mercaptan sulfur is greater than 2.

An unexpected advantage derived from the use of the process with light petroleum gas and potassium hydroxide is that the product is dry and a further drying stage is not required.

The preferred ratio of free sulfur to mercaptan sulfur is from 2 /2 to 4 /2. Within reasonable limits the process of the invention may be carried out when the aforementioned ratio is greater than 4 /2, but it will be understood that excess mercaptans present in the feed will utilise further caustic alkali. In the main therefore it is desirable to adjust the aforementioned ratio to within the range of 2 /2 to 4 /2. The applicants have discovered that for the maximum efficiency with light petroleum gas the ratio of 2.9 to 3.5 was preferred and that with a light virgin naphtha or cracked stocks a ratio of 2.8 to 3.6 was preferred.

The process may be applied to mercaptan-containing or free sulfur-containing petroleum distillates such as light petroleum gas, light virgin naphtha, cracked stocks and distillate heating oils and fuels, such as diesel fuels. The process is most advantageously applied to distillates which contain from 5 to 35 p.p.m. of free sulfur and/or have a mercaptan number less than 30, preferably less than 25.

The size of the solid caustic used in the fixed bed, is not critical since the size of the particles is decreasing during the operation of the process. The applicants have used fixed beds made up of lumps from in diameter up to 3" in diameter. During the operation of the process the alcohol or water is adsorbed on to the solid surface and a solution comprising caustic alkali and sulfur bodies trickles down the column. This solution may be removed from the process as a lower layer. It is therefore preferred that the feed descends from the fixed bed.

While our process is operable with sodium hydroxide or with potassium hydroxide, we have found that potassium hydroxide is to be preferred. It has been observed that potassium hydroxide has a greater tendency to absorb either alcohol or water from the distillate. As previously explained this absorbed water or alcohol tends to keep the surface of the alkali clean. We have found that our process may be advantageously operated when the fixed bed consists of an intimate mixture of potassium hydroxide and sodium hydroxide. By an intimate mixture we mean an intimate mixture of finely divided solid alkalies or a mixture obtained by [fusing the two alkali together and then grinding or pelleting the solidified mixture. It is preferred that the mixture consists of 1 part of potassium hydroxide to 1 to 4.3 parts of sodium hydroxide.

The process is conveniently operated under ambient conditions namely from 15 to 25 C. However, a faster reaction will take place at higher temperatures and column temperatures up to 60 C. may be used. Under ambient temperature conditions from /2 to 1 /2 b./ d. is a suitable rate of passing a feed having a mercaptan number from 4 to 20 or containing 1 to 5 mgs. of free sulfur per 100 cc. over 1 kg. of sodium hydroxide or potassium hydroxide. Therefore the linear velocity of the feed through the column may Well depend on the quantity of caustic alkali present in the column. Linear velocities from 0.2 to 2 ft. per minute may be used, and linear velocities from 0.05 to 1.5 ft. per minute are preferred. According to the process of the present invention the water or C to C monohydric alcohol content of the feed, is adjusted to a particular value before passage over the fixed bed of caustic alkali. The desired amount of water may be introduced by passing steam into the distillate. It will be realised that in many cases the water will exist in the distillate in the form of a haze. Methanol is the preferred alcohol for inclusion in the feed. The water and/or alcohol adjustment may be carried out before the adjustment of the free sulfur to mercaptan sulfur ratio.

Without wishing to restrict the invention the following theoretical explanation is included since this explanation may help to describe the best method of performing the present invention.

The free sulfur removal process is probably based on an oxidation reaction of the mercaptans by free sulfur according to a scheme of the following type:

Equation 3.

RSH-l-KOH-a RSK+H O (3) We have shown that the reaction undergone by the mercaptan is dependent on its molecular weight.

It is preferred to add C to C mercaptan since with these mercaptans the Reaction 3 is slow so that the added mercaptan will not be utilised before Reactions 1 and 2 have gone to completion. Furthermore the corrosion danger of permitting an excess of mercaptan to remain in the feed is not so great since the applicants have shown that the corrosivity of C mercaptans is less than for lower mercaptans. lf e.g. ethyl mercaptan were added, excess of this mercaptan remaining in the product would cause high corrosion and it would be necessary to prolong the reaction until all the excess ethyl mercaptan had been adsorbed. Consequently the addition of a C mercaptan would require a greater reaction time and caustic utilisation.

The corrosion ratings referred to in this specification were determined by the method described by A.S.T.M. D. 130.

' Consider now the case when the present invention is used to remove sulfur from a sulfur-containing feed. The analysis of the sulfur content of the feed will indicate the amount of mercaptan to be added to bring the ratio of free sulfur to mercaptan sulfur within the prescribed range. If the added mercaptan is below any mercaptan remaining after the sulfur removal reaction will be adsorbed by the solid bed. However for economic reasons, the addition of a useless excess of mercaptan is not desired due to the wastage of alkali.

When the added mercaptan is a (3 mercaptan, any excess mercaptan remaining after the sulfur reaction will be slowly adsorbed by the caustic bed and therefore the product may contain impurities of this mercaptan. Fortunately, the applicants have found that C mercaptans remaining in the product do not give increased copper strip corrosion figure. But of course excess mercaptan in the feed would cause the product to fail the doctor test. However, for the removal of free sulfur from light petroleum gases the addition of C mercaptans and above is preferred.

During the process of our invention, the alcohol or water contained in the feed is absorbed on the surface of the caustic alkali. The acidic sulfur bodies retained on the surface of the caustic alkali are washed by this absorbed layer and a concentrated aqueous or alcoholic solution of alkali mercaptides drips from the bottom of the column. This aqueous or alcoholic solution is readily separated from the petroleum product. In order to maximise the effect of the C to C alcohol or water used in this process, it is preferable to utilise the apparatus and process illustrated in the drawing accompanying this application. The fixed beds of potassium hydroxide or sodium hydroxide are contained in the towers l and 2. The mercaptan-containing feed flows continuously through the apparatus in the following sequence, viz., via pipes 3, 4, tower 1, separator 5, pipe 6, tower 2, separator '7. The water or alcohol may be dispersed in the distillate in the form of a haze. The alcohol or water added via pipe 8 mixes with the distillate in pipe 6 and the mixture flows down through tower 2. In separator 7 the distillate is separated from the aqueous or alcoholic solution containing the mercaptides and any unused caustic alkali. The pure product is removed by pipe 9. The aqueous or alcoholic solution or suspension collected in separator 7 is recycled via pipe 10 to mix with the fresh feed, viz., mercaptan-containing distillate entering the apparatus via pipe 3. The mixture is then contacted over the alkali in tower 1 and the aqueous or alcoholic solution or suspension of mercaptides issuing from column 1 is collected and separated in separator 5. The aqueous or alcoholic solution collected in separator 5 is run to waste or to a recovery system by pipe 11. Columns 1 and 2 have means by which make-up of sodium or potassium hydroxide may be added during the operation of the process.

EXAMPLE 1 A light virgin naphtha having the following inspection: Distillation:

FBP F 214 Specific gravity 0.664

Mercaptan number 16 EXAMPLE 2 An aqueous caustic washed propane containing free sulfur having a corrosion rating of 4 was mixed with a mercaptan and contacted as a liquid phase with a solid bed of potassium hydroxide. The process was carried out as follows: a cylindrical drum 71" high and 17" diameter was filled with 225 kg. of potassium hydroxide. The particles of the potassium hydrom'de have an average diameter of 1". A liquid propane stream was processed. The propane stream gave a copper strip corrosion number greater than 4 and contained 1.55 mgs. of water per litre of gas. 0.15 g. of ethyl mercaptan was added to each barrel of propane before percolation through the bed of potassium hydroxide. The propane stream was obtained and thereby containing required amount of water and the required ratio of free sulfur to mercaptan sulfur was percolated through the bed at a rate varying between 100 to 200 b./d. After percolation the product had a copper strip corrosion number less than 1. The bed of caustic alkali had a life of about 15 days and the potassium hydroxide consumption was 150 gr. KOH/bbl.

The water content of the product was 0.3 mg. per litre of gas. The drying of the product is another advantage of the process of this invention and the conventional alumina drying of the product is not required.

As shown in Table I the corrosion number was improved from 4 to less than 1 thereby demonstrating the removal of the sulfur.

discovery that by the inclusion of water or a C to C alcohol in the feed, the reaction will take place by percolating the feed over the alkali.

Since it has already been stated that the removal of mercaptans C and below may be efiected by percolation over solid potassium hydroxide by the process of the present invention, it will therefore be realised that the present invention may be advantageously applied to a feed that has previously been extracted in either aqueous alkali or by percolation over solid alkali. By this means, the lower mercaptans will be removed from the feed by adsorption upon the solid caustic and then the unabsorbed higher mercaptans will react with added sulfur Table I by the process described. It is preferred that the first stage of this preferred embodiment be carried out to give Injected mer- C a product with a corrosion number of less than 2 beca tan Corro- Conorro VOL 1 b f lioH ttact g fore treatment by the second stage. Product Litre eore gr. ime 2. er

Ethyl, Amy, test min. test EXAMPLE 4 ili tl'c ibllbe This example demonstrates an application of the process to sweetening of a light virgin naphtha having a mercs LPG 1.0 1.0 J-9 3.0 5 less captan number of 6.5. Free sulfur as indicated by Table E 111 was added and the naphtha-containing sulfur shaken o3 LPG 1.0 1.0 M 3.0 5 Do. with solid potassium hydroxide. The results obtained are recordedinTable I[[.

Table III Volume, Mer- KOH, Treating Free S, Contact Doctor Corrosion Product cc. captan gr. agent mgr. time, test number number min.

100 6.5 20 KOH 20 2 DP Greater than 4. 100 6.5 20 E011 20 1 DN]? 100 6.5 20 KOH 19 2% DP 2. 100 6.5 20 KOH 1s 3 DP 100 6.5 20 KOH 1s 2 BP 2. 100 6.5 20 KOH 17 6 DP 100 6.5 20 KOH 17 5 BP lesls than 100 6.5 20 K011 17 4 DNP 100 6.5 20 KOH 16 16 100 6.5 20 X011 16 14 DNP 100 6.5 20 K011 15 DNP Do.

1 The LVN is thoroughly mixed with the treating agent by way of a shaking machine.

EXAMPLE 3 The method of reporting the doctor test as given in Table II demonstrates the interaction of free sulfur and amyl-mercaptan in a heptane solution by shaking the solution with solid potassium hydroxide. It will be observed that the product obtained gave a satisfactory corrosion rating.

Heptane solutions containing 1 mg. of free sulfur per 100 cc. gave corrosion ratings of 4. (Use of pressure with light petroleum gas sweetening.) In this example the reactants were shaken together. Without shaking the surface of the solid alkali became coated and reaction ceased. The applicants invention is based on the Table I11 is by the method desrcibed in Standard Methods for Testing petroleum and Its Products, Institute of Petroleum, 30/56.

The light virgin naphtha used in this example was produced by topping an Aramco crude to give a sour naphtha of the following inspection data:

The sour naphtha was caustic washed to give a mercaptan number of 6.5 and then used in the experiment of this example.

In this example the reactants were shaken together. Without shaking the surface of the solid became coated and reaction ceased. The applicants invention is based on the discovery that by the inclusion of water or a C to C alcohol in the feed the reaction will take Place by percolating the feed over the alkali.

The applicants have observed that after the process. has been in operation without the use of water or alcohol the surface of the alkali becomes dirty, and the efficiency of the process decreases. It has been found that the process of the present invention is dependent on the in-- 5 clusion of a C to 0.; alcohol and/or Water in the feed.

mixture. The water may be introduced into the feed mixture in the form of a haze, for example by passing steam into the feed. It is preferred that the concentration of alcohol or water added is sufficient to remove the imconsisting of potassium hydroxide, sodium hydroxide and mixtures thereof, and recovering a petroleum distillate reduced in sulfur impurities.

2. The process of claim 1 wherein said sulfur impuripurities on the surface of the alkali but without causing ties are free sulfur and soluble mercaptans are added to the bed of alkali to disintegrate or fuse into a solid mass. said distillate prior to passing said distillate over said fixed We have found that concentrations from 0.05% volume bed. to 1% volume and preferably from 0.07% to 0.15% of 3. The process of claim 2 wherein said soluble meralcohol to be suitable. The desired amount of alcohol captans are (3 -0 mercaptans. or water may be added batchwise or continuously to the 4. The process of claim 2 wherein the ratio of free feed. Methanol is the particularly preferred alcohol sulfur to mercaptan is within the range of about 2.5 :1 to which may be injected continuously. 4.521. I

We have found that it is particularly advantageous to 5 The process of claim 1 wherein said sulfur impuriinclude the solid alkali in the water or the alcohol which ties are mercaptans and free sulfur is added to said dis is added to the feed. The alcohol or water is preferably 15 tillate whereby the ratio of free sulfur to mercaptan is saturated with potassium hydroxide or sodium hydroxide. greater than 2:1 prior to passing said distillate over We have discovered that by the addition of further alkali said fixed bed. to the bed by this means prolongs the life of the fixed 6. The process of claim 5 wherein the ratio of free caustic bed. sulfur to mercaptan is within the range of about 2.521

The water or alcohol included in the feed causes a to 4.5 :1. solution containing alkali and adsorbed sulfur impurities 7. The process of claim 1 wherein said water content to separate out as droplets from the bottom of the colis adjusted by passing steam into the distillate. umn. A. similar solution separates when the added water 8. The process of claim 1 wherein said alcohol is or alcohol contains sodium or potassium hydroxide. This methanol. insoluble liquidshould be collected at the bottom of the 9. The process of claim 1 wherein said fixed bed concolumn and run to waste. Alternatively mercaptans may tains from 1 to 50 wt. percent of activated charcoal. be recovered from this liquid and used in further proces- 10. The process of claim 1 wherein said sulfur imses. The alkali may also be recovered. purities are free sulfur and mercaptans and wherein the The product obtained from the foregoing process may ratio of free sulfur to mercaptans is greater than 2: 1. be further purified by percolating the feed over a bed of 11 A Process f removing Sulfur p ri i m a a mixture of active charcoal with the sodium or potassium petroleum distillate which comprises adding to said dishydroxide. It is preferred that from 1 to Wt. percent tillate a liquid compound selected from the group conactive charcoal be mixed with the solid alkali. The par- Sisting of water and C to C alcohols whereby said disticle size of the active carbon may be above 0.05 but tillate contains in the range of about 0.01 to 5 wt. percent preferably is from 0.8 to 2.0 mm, 5 of said compound, passing said distillate over a fixed bed EXAMPLE 5 of a solid alkali metal hydroxide selected from the group consisting of potassium hydroxide, sodium hydroxide and By ing a N mple With miXfilre 0f K H mixtures thereof, and recovering a petroleum distillate pellets and active carbon, a doctor pass product with good reduced in sulfur impurities. corrosion is obtained. Table V shows how the quantity 40 12. The process of claim 11 wherein said liquid comof active carbon, the presence of water and/ or sulfur afpound contains solid alkali metal hydroxide selected from fects the rate of sweetening reaction and the life of the the group consisting of potassium hydroxide and sodium feed. The contact time indicated the minimum time hydroxide. required to obtain a DP product. i 13 The process of claim 11 wherein said sulfur im- Table IV 7 Volume, Mer- Water Free S, KOH, Act. Contact Doctor Product cc. captan Content, mgr./100 gr. Carb., time, test number cc. cc. gr. min.

LVN 300 20 20 10 3 DP. LVN 300 20 a 20 1 12 DP LVN 300 20 2 3 20 1 7 DP LVN 300 20 2 20 1 9 DP What is claimed is: V purities are free sulfur and rnercaptans and wherein the 1. A process for removing sulfur impurities from a ratio of free sulfur to mercaptans is greater than 2:1. petroleum distillate which comprises adjusting the cornposition of said distillate whereby said distillate contains References cued m the file of this patent in the range of about 0.01 to 5.0 wt. percent of a com- UNITED STATES ENTS pound selected from the group consisting of water and 2,577,824 Stine Dec. 11, 1951 C to C alcohols, passing said distillate over a fixed bed 2,725,339 Browning et a1. Nov. 29, 1955 of a solid alkali metal hydroxide selected from the group 2,748,059 Nixon et a1 May 29, 1956

Claims (1)

1. A PROCESS FOR REMOVING SULFUR IMPURITIES FROM A PETROLEUM DISTILLATE WHICH COMPRISES ADJUSTING THE COMPOSITION OF SAID DISTILLATE WHEREBY SAID DISTILLATE CONTAINS IN THE RANGE OF ABOUT 0.01 TO 5.0 WT. PERCENT OF A COMPOUND SELECTED FROM THE GROUP CONSISTING OF WATER AND C1 TO C4 ALCOHOLS, PASSING SAID DISTILLATE OVER A FIXED BED

Images