GB2055888A - Process for Removing the Nitrogen Impurities From a Hydrocarbon Mixture - Google Patents

Abstract

A liquid hydrocarbon and an aqueous acid having an acid concentration between 0.01 and 5% by volume are mixed in a low volume mixer to form an emulsion of the hydrocarbons in the aqueous acid solution, by mixing during a period of time not exceeding 2 seconds, and the resultant emulsion is passed to a decantor from which a purified hydrocarbon phase is withdrawn.

Description

SPECIFICATION Process for Removing the Nitrogen Impurities from a Hydrocarbon Mixture The present invention relates to an improved process to remove the nitrogen impurities, by means of an acid, from a liquid hydrocarbon mixture. More particularly, the present invention relates to the treatment, by means of an acid, of a liquid hydrocarbon mixture containing unsaturated hydrocarbons.

Generally, in order to remove aromatic compounds therefrom, such liquid hydrocarbon mixtures are subjected to treatments such as hydrogenation or reforming.

However, it is well known that the catalysts used in the hydrogenation treatments, which are most frequently a noble metal on a support, or the catalysts used in the reforming, are poisoned by the nitrogen compounds which are present in the liquid hydrocarbon mixtures. One of the required conditions for subjecting a hydrocarbon feed to a reforming process, is a nitrogen compound content lower than 0.5 ppm. Indeed, the presence of nitrogen compounds in a hydrocarbon feed which is to be reformed gives rise to the formation of NH3 which may be either adsorbed at the catalyst level, with the resulting neutralization of the acid sites, or recombined with HCI formed during the reforming, with formation of a salt which forms a deposit on the apparatus.

The nitrogen compounds contained in the hydrocarbon feeds are present, for the major part, in the form of basic nitrogen compounds.

In order for the hydrocarbon mixtures to be suitable for reforming or for subjecting to a hydrogenation treatment in the presence of a noble metal catalyst, the basic nitrogen compound content must be reduced to less than 2 ppm and preferably to less than 1 ppm. The rest of the nitrogen compounds, inasmuch as their content does not exceed 10 ppm, may easily be eliminated by hydrogenation under operating conditions similar to those of a desulfurization.

If the basic nitrogen compound content is not reduced to less than 2 ppm, the residual basic nitrogen compounds will have to be eliminated by hydrogenation under severe conditions, and particularly under a high hydrogen partial pressure, said conditions being difficult to use in conventional refineries.

In-order to avoid these drawbacks, it has already been proposed to treat with an organic or inorganic acid those hydrocarbon feeds which have a boiling point of from 200 to 6000C, and which have to be further submitted to a thermal or catalytic cracking to form gasolines. However, the nitrogen compounds are only partially removed by this acid treatment and therefore when the hydrocarbon feed is submitted to a cracking, nitrogen compounds are still formed and these have to be removed before carrying out the reforming. Moreover the operating conditions of the acid treatment of the hydrocarbon feed before cracking are not suitable for use with the cracked products.

Indeed, the cracked products contain unsaturated hydrocarbons which can polymerize if they are submitted to an acid treatment under the condition suggested for the acid treatment before cracking.

The polymerization of these unsaturated hydrocarbons depends on the acid concentration of the solution and on the contact time of the feed with the acid solution.

The conditions of the acid treatment of the feed before cracking are an acid concentration of from 85 to 100% and contact times of from 5 minutes to 2 hours; under these conditions, the unsaturated hydrocarbon present in the feed will polymerize.

It has been proposed to directly treat the cracked products with an acid solution, particularly a sulfuric acid solution, but under specific conditions of acid concentration and contact time. Thus it has been proposed in "The Science of Petroleum", Vol. III page 1773 to use an aqueous sulfuric acid solution, having an acid concentration of from 40 to 98% with contact times such that the lower the acid concentration, the higher the contact time. Nevertheless, the polymerization of the unsaturated hydrocarbons cannot be completely avoided, even if the acid concentration is as low as 40%.

In order to reduce the contact time, and therefore the polymerization rate, it has been proposed to use more concentrated sulfuric acid solutions, but using certain specific methods of contacting the feed with the sulfuric acid solution. Examples of contacting devices, which have been suggested are mixer batteries wherein the feed and the concentrated sulfuric acid solution move in counter current flow, and a contacting column having a particular contact bed described in the U.S. Patent No. 2999807.

According to the process described in this U.S. patent, one uses an aqueous sulfuric acid solution having an acid concentration of at least 65% together with contact times in the column of from 3 to 25 seconds. This column includes a contact bed made of an inert material which may be a ceramic, coke or glass having a hydrophilic surface so that it is easily wetted by the sulfuric acid which is thereby retained on the surface, with formation of a large contact surface. However, this process does not avoid the formation of a significant amount of polymerized products.

An object of the present invention is to obviate these drawbacks.

Another object of the present invention is an improved process for removing the nitrogen impurities from a liquid hydrocarbon mixture thereby to reduce the basic nitrogen compound content of the said mixture to less than 1 ppm, and furthermore thereby to avoid the formation of polymerized products.

According to the present invention there is provided a process for removing nitrogen impurities from a mixture of liquid hydrocarbons, which process comprises continuously introducing into a low volume mixer a dilute aqueous solution of an inorganic or organic acid, the said solution having an acid concentration of from 0.01 to 5% by volume, the volume ratio of the amount of dilute aqueous acid solution to the amount of hydrocarbons being from 0.075 to 3, continuously introducing the mixture of liquid hydrocarbons into said mixer, forming in the mixer an emulsion of the hydrocarbons in said aqueous acid solution by mixing during a period of time not exceeding 2 seconds the diluted aqueous acid solution and the hydrocarbons, thereby extracting the major part of the nitrogen impurities, withdrawing the resultant emulsion into a decantation zone-where the emulsion breaks and phase separation occurs, and recovering the hydrocarbon phase from the decantation zone.

The process of the invention is applicable to a wide range of liquid hydrocarbon mixtures, and particularly to hydrocarbons having a boiling point from 30 to 3000 C.

The process of the invention is particularly applicable to liquid hydrocarbon mixtures obtained from straight run distillates, and more particularly to liquid hydrocarbon mixtures, containing unsaturated hydrocarbons, boiling in the gasoline range and obtained by thermal or catalytic cracking of heavier hydrocarbons. The liquid hydrocarbon mix turns obtained from straight run distillates are generally constituted of kerozenes or white-spirits having a boiling point in respective ranges of from 150 to 2900C and from 1 50 to 2000C. These hydrocarbon mixtures contain from 20 to 30 ppm of nitrogen compounds including from about 10 to 1 7 ppm of basic nitrogen compounds, and thus they are not suitable to be submitted to a hydrogenation in the presence of noble metal catalyst.

The liquid hydrocarbon mixtures obtained by thermal or catalytic cracking of heavier hydrocarbons contain unsaturated hydrocarbons and have a boiling point in the gasoline range. These liquid hydrocarbon mixtures contain generally from 50 to 60 ppm of nitrogen compounds including from 30 to 50 ppm of basic nitrogen compounds, and thus they are not suitable to be submitted to a catalytic reforming.

When these hydrocarbon mixtures are treated according to the process of the invention, the basic nitrogen compound content of the feed is reduced to less than 2 ppm. The rest of the nitrogen compounds is thereafter removed by a hydrogenation treatment under operating conditions similar to those of a desulfurization, in order to finally reach a nitrogen compound content of less than 0.5 ppm.

According to known methods for reducing the content of basic nitrogen compounds in such liquid hydrocarbon mixtures, the mixtures have to be treated with concentrated aqueous solutions of sulfuric acid, generally 40 to 98%, and during a period of from 3 to 30 seconds.

It has now been unexpectedly found that by use of the method of the present invention these liquid hydrocarbon mixtures can be treated with greatly diluted aqueous solutions of an inorganic or organic acid and during an extremely short period of time, lower than 2 seconds. This is believed to be due to the formation of an emulsion of the hydrocarbon in the aqueous acid solution in the mixer.

We have now found that the nitrogen compounds can be easily removed from the hereabove described hydrocarbon mixtures by treating these mixtures with an aqueous solution of an inorganic or organic acid, having an acid concentration which may be as low as 0.01% by volume, but which is generally from 0.01 to 2.5% by volume, and with extremely short contact times, which do not exceed 2 seconds or even 1 second.

When hydrocarbon mixtures containing unsaturated hydrocarbons are treated according to the operating conditions of the invention, it has been noticed that polymerized products are substantially not formed. The polymerized product content is determined in accordance with the ASTM D 381 method.

In order to carry out the acid treatment of the invention, an inorganic acid as well as an organic acid may be used. Examples of suitable acids are HCI, HBr, HF, HI, sulfuric acid, phosphoric acid, boric acid, fluorosulfuric acid, trifluoroacetic acid, trichloroacetic acid, formic acid, alkane sulfonic acids, and alkyl benzene sulfonic acids. However, sulfuric acid is generally used for reasons of storage and ease of handling.

The amount of aqueous acid solution to be used with regard to the amount of hydrocarbons to be treated for carrying out the process of the invention may vary over wide limits. Indeed the volume ratio of the amount of aqueous acid solution to the amount of hydrocarbons is usually from 0.075 to 3, more preferably from 0.3 to 2.

Examples of mixers which may be used in the process of the invention are centrifugal pumps and static mixers.

Such mixers enable an emulsion of the hydrocarbons in the aqueous acid solution to form in a very short time. Moreover, said emulsion may be withdrawn substantially immediately after its formation. As a result, the contact times between the liquids to be treated is very short and the formation of significant amounts of polymerized products is avoided.

It has unexpectedly been noticed that a substantially complete removal of the basic nitrogen compounds from the hydrocarbon mixtures can be carried out by means of the above described mixers, while using a dilute aqueous acid solution.

According to an embodiment of the process of the present invention, the aqueous acid solution and the hydrocarbon mixture are simultaneously introduced into the aspiration side of a centrifugal pump. The emulsion which is formed inside the pump is thereafter sent into a decanter wherein the rupture of the emulsion is substantially immediately achieved. The purified hydrocarbon mixture is then recovered at the top part of the decanter.

Other types of mixers can be used to form an emulsion of the contact liquids; however, these other types of mixers, for example a vessel fitted with a turbo-dispersor, or still a pulsed extraction column, have various drawbacks. For example, the contact time is generally too long, or the emulsion formed is not sufficiently homogeneous. As a result, the amount of polymerized products is too high or the extraction of the basic nitrogen compounds is still not sufficiently complete.

The following Examples illustrate the present invention.

Example 1 At the aspiration side of a centrifugal pump there was simultaneously introduced: a hydrocarbon mixture, whose characteristics are indicated in Table I, having a total nitrogen compound content of 61.3 ppm including 43.3 ppm of basic nitrogen compounds, and an aqueous sulfuric acid solution having an acid concentration of 0.2% by volume.

The polymerized product content of the hydrocarbon feed was measured according to the ASTM D381 method and was of 7 my per 1000 ml hydrocarbon.

Table I Characteristics Feed Distillation ASTM D 86 Initial point 540C End point 2030C Specific gravity 1 5/40C 0.793 Composition Paraffins (Vol. %) 27.5 Olefins (Vol. %) 33.0 Naphthens (Vol. %) 3.5 Aromatics (Vol. %) 36.0 The volume ratio of the aqueous acid solution to the hydrocarbon mixture was 1/1.

The rotating speed of the pump was 1450 t/min.

Inside the centrifugal pump, both liquids were intimately mixed, and an emulsion of the hydrocarbon mixture in the aqueous acid solution was formed. The centrifugal pump pumped this emulsion to a decanter where breaking of the emulsion was substantially immediately achieved with formation of two phases: one phase containing the purified hydrocarbons located at the top of the decanter, and an aqueous phase at the bottom of the decanter.

The contact between both liquids was estimated by the time required to pass through the centrifugal pump, i.e. 1 second.

The phase containing the hydrocarbons was analyzed to determine the nitrogen compound content.

The following results were obtained: total nitrogen compound content: 8 ppm basic nitrogen compound content: 0.8 ppm polymerized product content: 1 5 mg/1 00 ml hydrocarbon.

Thereafter a feed was prepared, which contained 75% by volume of naphtha and 25% by volume of the purified hydrocarbon mixture.

This feed was submitted to a desulfurization treatment under the following conditions: usual desulfurization catalyst: 400 ml H2 partial pressure: 12.5 kg/cm2 total pressure: 25 kg/cm2 temperature: 303.40C hourly space velocity: 4 hF1 H2/HC: 150N1/1.

No trace of nitrogen compounds was detected in the desulfurized hydrocarbon mixture.

By way of comparison, a feed was prepared containing 75% by volume of naphtha and 25% by volume of the hydrocarbon mixture which was not submitted to the acid treatment.

This feed was submitted to a desulfurization treatment under the same conditions as those hereabove described.

After desulfurization, the feed had a nitrogen content of 5 ppm. This content was too high and this feed was not suitable for a catalytic reforming.

Example 2 The process described in Example 1 was repeated, but using different hydrocarbon mixtures (whose characteristics are indicated in Table II), different acid concentrations and different volume ratios between the aqueous acid solution and the hydrocarbon mixture.

The operating conditions and the results are indicated in Table Ill.

Table II Characteristics Feed 2.1 2.2 2.3 2.4 Distillation ASTM D 86 Initial point (OC) 56 51 62 55 End point (OC) 171 168 200 172 Specific gravity 1 5/40C 0.776 0.737 0.782 0.777 Composition Paraffins (vol.%) 31.5 49 33 34 Olefins (vol.%) 36 43.6 34 34 Naphthens (vol.%) 2.5 3.5 2 2 Aromatics (vol. %) 30 3.9 31 Table III Polymerized H2S04 Basic Total product concentration nitrogen nitrogen content H20/ in water compounds compounds Experiment (mg/1000 ml) Hydrocarbons (% vol) (ppm) (ppm) 2.1 5 33.1 49.1 2.2 8 35.5 44.9 2.3 7 45.3 70.1 2.4 7 37.3 53.6 2.1 15 0.5 0.12 1.2 10.3 2.1 12 0.25 0.12 1.6 11.4 2.2 12 0.5 0.05 1.8 10.6 2.2 17 0.5 0.29 0.7 8 2.3 15 0.25 0.25 1.8 9.9 2.4 15 0.5 0.12 1.4 10.6 By way of comparison, the feed 2.2 was submitted merely to water treatment. In the treated product, 35.5 ppm of basic nitrogen compound were detected.

By way of comparison, the feed 2.2 was treated according to the hereabove described embodiment but with an aqueous sulfuric acid solution having an acid concentration of 25% by volume. The volume ratio of the aqueous acid solution to the hydrocarbon mixture was 0.5.

The polymerized product content was determined on the treated hydrocarbon mixture, according to the ASTM D 381 method, and a value of 80 mgl1 000 ml was found, that is too high.

Example 3 At the aspiration side of a centrifugal pump, there was simultaneously introduced: a hydrocarbon mixture, whose characteristics are indicated in Table IV, having a total nitrogen compound content of 55.7 ppm including 39.2 ppm of basic nitrogen compounds, and an aqueous hydrochloric acid solution having an acid solution of 0.5% by voiume.

The polymerized product content of the hydrocarbon mixture, determined according to the ASTM D 381 method, was of 5 mg/1 000 ml.

Table IV Characteristics Feed Distillation ASTM D 86 Initial point 550C End point 1750C Specific gravity 1 5/40C 0.782 Composition Paraffins (vol. %) 34 Olefins (vol. %) 36 Aromatics (vol. %) 30 The volume ratio of the aqueous acid solution to the hydrocarbon mixture was 2/1.

The rotating speed of the pump was 1450 t/min.

Both liquids were intimately mixed inside the centrifugal pump and an emulsion of the hydrocarbon mixture in the aqueous acid solution was formed. The centrifugal pump pumped this emulsion to a decanter wherein breaking of the emulsion is substantially immediately achieved with formation of two phases.

The contact time between both liquids was estimated by the time required to pass through the centrifugal pump, i.e. 1 second.

The nitrogen compound content of the phase containing the hydrocarbon mixture was the following: Total nitrogen compound content: 8.6 ppm Basic nitrogen compound content: 0.9 ppm Polymerized product content, according to ASTM D 381: 15 mug/1000 ml The run was continued as described in Example 1 and the desulfurized feed did not contain any trace of nitrogen compound.

Example 4 At the inlet of a static mixer of 4 cm diameter and 75 cm length, there was simultaneously introduced: a hydrocarbon mixture, whose characteristics are indicated in Table V, having a total nitrogen compound content of 54.4 ppm including 43.4 ppm of basic nitrogen compounds, at a rate of 0.85 m3/hour, and an aqueous sulfuric acid solution having an acid concentration of 0.5% by volume at a rate of 1.7 m3/hour.

The polymerized product content of the hydrocarbon mixture, according to ASTM D 381, was of 8 mg/1000 ml.

Table V Characteristics Feed Distillation ASTM D 86 Initial point 60 C End point 1840C Specific gravity 1 5/40C 0.767 Composition Paraffins (vol.%) 32 Olefins (vol.%) 36 Aromatics (vol.%) 30 Naphthens (vol.%) 2 The contact time in the static mixer was about 1 second.

Both liquids were intimately mixed in the static mixer, and an emulsion of the hydrocarbon mixture into the aqueous acid solution was formed. The emulsion was thereafter sent to a decanter.

wherein its breaking was substantially immediately achieved, with formation of two phases as indicated in Example 1.

The nitrogen compound content of the phase containing the hydrocarbon mixture was the following: Total nitrogen compound content: 10.5 ppm Basic nitrogen compound content: 1.4 ppm Polymerized product content, according to ASTM D 381: 17 mug/1000 ml.

The run was continued as described in Example 1 and the desulfurized feed did not contain any trace of nitrogen compound.

Example 5 At the aspiration side of a centrifugal pump, there was simultaneously introduced: a hydrocarbon mixture, whose characteristics are indicated in Table VI, having a total nitrogen compound content of 23 ppm including 12 ppm of basic nitrogen compounds, and an aqueous sulfuric acid solution having an acid concentration of 0.2% by volume.

Table Vl Characteristics Feed Distillation ASTM D 86 Initial point 1 800C End point 2610C Specific gravity 1 5/40C 0.820 Composition Paraffins (vol.%) 80 Olefins (vol.%) Traces Aromatics (vol.%) 20 The volume ratio of the aqueous acid solution to the hydrocarbon mixture was of 1/1.

The rotating speed of the pump was of 1450 /min.

Both liquids were intimately mixed inside the centrifugal pump, and an emulsion of the hydrocarbon -mixture in the aqueous acid solution was formed. The centrifugal pump pumped this emulsion to a decanter wherein the rupture of the emulsion was substantially immediately achieved with formation of two phases, as indicated in Example 1.

The contact time between both liquids was estimated by the time required to pass through the pump, i.e. 1 second.

The nitrogen compound content of the phase containing the hydrocarbon mixture was the following: Total nitrogen compound content: 6 ppm Basic nitrogen compound content: 0.5 ppm.

This purified feed was thereafter submitted to a hydrodesulfurization treatment under the operating conditions described in Example 1.

In the desulfurized hydrocarbon mixture, no trace of nitrogen compounds was detected.

Claims (11)

Claims

1. A process for removing nitrogen impurities from a mixture of liquid hydrocarbons, which process comprises continuously introducing into a low volume mixer a dilute aqueous solution of an inorganic or organic acid, the said solution having an acid concentration of from 0.01 to 5% by volume, the volume ratio of the amount of dilute aqueous acid solution to the amount of hydrocarbons being from 0.075 to 3, continuously introducing the mixture of liquid hydrocarbons into said mixer, forming in the mixer an emulsion of the hydrocarbons in said aqueous acid solution by mixing during a period of time not exceeding 2 seconds the diluted aqueous acid solution and the hydrocarbons, thereby extracting the major part of the nitrogen impurities, withdrawing the resultant emulsion into a decantation zone where the emulsion breaks and phase separation occurs, and recovering the hydrocarbon phase from the decantation zone.

2. A process according to Claim 1, wherein the liquid hydrocarbon mixture comprises hydrocarbons having a boiling point in the range from 30 to 3000 C.

3. A process according to Claim 1 or 2, wherein the hydrocarbon mixture is a straight run distillate.

4. A process according to Claim 1 or 2, wherein the hydrocarbon mixture is obtained by thermal or catalytic cracking of heavier hydrocarbons.

5. A process according to Claim 4, wherein the hydrocarbon mixture contains unsaturated hydrocarbons and has a boiling point in the gasoline range.

6. A process according to any one of Claims 1 to 5, wherein the hydrocarbon mixture is treated by means of an aqueous solution of an acid selected from the group comprising hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydroiodic acid, sulfuric acid, phosphoric acid, boric acid, fluorosulfuric acid, trifluoroacetic acid, trichloroacetic acid, formic acid, alkane sulfonic acids and alkyl benzene sulfonic acids.

7. A process according to any one of Claims 1 to 6, wherein the aqueous solution has an acid concentration of from 0.01 to 2.5% by volume.

8. A process according to any one of Claims 1 to 7, wherein the volume ratio of the amount of aqueous acid solution to the amount of hydrocarbons is from 0.3 to 2.

9. A process according to any one of Claims 1 to 8, wherein the low volume mixer is a centrifugal pump or a static mixer.

10. A process according to Claim 1, substantially as described in any one of the foregoing Examples 1 to 5.

11. A liquid hydrocarbon mixture obtained by a process as claimed in any one of the preceding claims.