WO2013076385A1

WO2013076385A1 - Improved process and improved device for extracting sulphur-containing compounds from a hydrocarbon-based cut by liquid-liquid extraction with a solution of sodium hydroxide

Info

Publication number: WO2013076385A1
Application number: PCT/FR2012/000420
Authority: WO
Inventors: Fréderic AUGIER; Arnaud Baudot; Jérémy GAZARIAN; Damien Leinekugel Le Cocq
Original assignee: IFP Energies Nouvelles
Priority date: 2011-11-24
Filing date: 2012-10-16
Publication date: 2013-05-30
Also published as: FR2983207B1; FR2983207A1

Abstract

Process for extracting sulphur-containing compounds from a hydrocarbon cut (petrol, LPG, etc.) by liquid-liquid extraction with a solution of sodium hydroxide requiring a unit for the pretreatment of the feedstock to be treated consisting of N pretreatment tanks arranged in parallel, each tank operating in batch mode while complying with a programming law for replenishing the solution of sodium hydroxide.

Description

IMPROVED METHOD AND APPARATUS FOR EXTRACTING SOFTENED COMPOUNDS FROM HYDROCARBON CUTTING BY LIQUID LIQUID EXTRACTION WITH SODA SOLUTION.

Field of the invention

The invention relates to the field of the extraction of sulfur compounds such as mercaptans, COS and H ₂ S from a hydrocarbon fraction. This selective extraction is done by contacting the hydrocarbon feedstock in the liquid phase with a sodium hydroxide solution. Prior art:

The extraction of sulfur compounds from a hydrocarbon fraction (gasoline, LPG, etc.) by liquid-liquid extraction with a sodium hydroxide solution is well known in the state of the art. When the majority of the sulfur species are mercaptans, or thiols, a type of widespread process consists in carrying out an extraction of the sulfur species using a looped soda solution in the process, as described in US Pat. 4.081, 354. The sulfur species of the mercaptan type dissociate in sodium thiolates in sodium hydroxide. After extraction, sodium hydroxide loaded with sodium thiolate is oxidized in the presence of a dissolved catalyst, for example based on cobalt phthalocyanine. Thus, sodium thiolate species are converted to disulfides. The disulfide-rich soda solution is brought into contact with a hydrocarbon phase, which makes it possible to extract the disulphides and thus regenerate the soda which can be reused at the top of the liquid-liquid extraction column. The parameters associated with the oxidation are chosen so as to oxidize almost all the sodium thiolates present in the sodium hydroxide. The process thus allows partially or completely desoldering a hydrocarbon cut, and generates another organic effluent heavily loaded with sulfur species.

A problem inherent in this type of process lies in the fact that certain chemical species such as COS or H ₂ S irreversibly form salts in the presence of sodium hydroxide, salts that accumulate in the soda loop. Too much salt in the soda loop eventually limits its performance. For this reason, purges and regular additions are operated on the loop. Another widespread practice is to pretreat the hydrocarbon upstream of the extraction column in a chamber containing a solution of soda. This pretreatment has the effect of consuming a portion of the sulfur species, especially the species forming salts. The soda solution used in the pretreatment is not regenerated. This pretreatment step can be carried out in a separate enclosure, or in the same enclosure as the extraction column, if the latter is partitioned into 2 separate capacities, as described in US Pat. No. 6,749,741.

Thus, the extraction of sulfur species is generally carried out in two stages:

• the pre-treatment stage: extraction of COS and residual H ₂ S;

The mercaptan counter-current continuous extraction step: a stage situated downstream of the pre-treatment step.

The pretreatment is generally discontinuous, and consists in injecting the charge into a capacity filled with a soda solution which is periodically changed. Due to the discontinuous operation of the pretreatment, the concentration of sodium hydroxide decreases with time, as well as its extraction performance. When the pretreatment performance is too low, the aqueous phase containing the sodium hydroxide is renewed, which can be carried out for example between 1 and 10 times per month depending on the methods and the size of the enclosure used for pretreatment. The initial concentration of sodium hydroxide is generally set at a content of between 2% and 10% by weight.

The soda-countercurrent extraction of the hydrocarbon phase leaving the pretreatment can be carried out in different types of extraction columns. Many technologies are known, such as those reported in the Handbook of Solvent Extraction (Krieger Publishing Company, 1991). These columns are generally designed to generate at least 2 theoretical stages of extraction. A technology of extraction column often encountered is that of the perforated plates with weirs, because the counter-current extraction with soda is often carried out with a soda flow much lower than the flow of hydrocarbon. The ratio between the flow rates of hydrocarbon and soda can vary between 0 and 40. The sodium content in the loop is generally set at a content of between 15% and 25% by weight. The discontinuous operation of the pretreatment has the advantage of maximizing its performance with respect to continuous operation in a perfectly stirred type reactor. As a result, the contents of COS and H ₂ S are on average greatly reduced by the pretreatment step. On the other hand, the sulfur species leaving the pretreatment, including the majority species of mercaptan type, have fluctuating concentrations depending on the age of the soda solution used in the pretreatment capacity. The fluctuations in total sulfur can thus for example vary from single to double at the counter-current extraction column inlet.

Fluctuations in total sulfur concentrations pose several problems:

1) When the soda used at the pretreatment is at the end of its life, the quantity of mercaptans leaving the pretreatment can be as high as at the pre-treatment inlet, or even higher because of a salting out of mercaptans linked to the previous accumulation of a large amount of sodium thiolates and the low concentration of sodium hydroxide. Thus, waves of high concentrations of total sulfur may be present at the inlet of the countercurrent extraction column, which can potentially generate losses of liquid-liquid extraction efficiency in the column if the flow of sodium hydroxide in the loop is not sufficient to treat the highest concentrations. In addition, waves of mercaptans in the hydrocarbon then generate waves of sodium thiolates in the bottom soda of the extraction column. The excessive concentration of sodium thiolates in the oxidizer can lead to a partial conversion to disulfide and thus a sodium thiolate referral in quantity in the regenerated sodium hydroxide at the top of the extraction column. This can also decrease the performance of the extraction column.

2) Conversely at the beginning of the pretreatment cycle, the hydrocarbon entering the countercurrent extraction column contains little sulfur, so the concentration of sodium thiolate in the soda at the bottom of the extraction column is low. In the oxidizer, the amount of air is then in excess. The dissolved oxygen in the sodium hydroxide is not consumed by the residual sodium thiolates, and is directly returned to the extraction column with the regenerated sodium hydroxide. The oxygen present in the regenerated sodium hydroxide can then react with the mercaptans and produce disulfides within the extractor. These disulfides are then extracted by the hydrocarbon phase to be treated directly in the extraction column, and as a result, the overall performance of the process is reduced. Thus, fluctuations in the concentration of sulfur species in the hydrocarbon fraction to be treated can potentially generate a decrease in the efficiency of the process, which results in an increase in the concentrations of sulfur species in the hydrocarbon phase leaving the extraction column. against the current.

The method according to the invention proposes to remedy the performance problems of the extraction process related to fluctuations in the contents of sulfur compounds in the stream obtained at the outlet of the pretreatment stage. The object of the invention is to carry out a pretreatment which generates fewer fluctuations in sulfur compounds than in the pretreatment described according to the prior art.

For this purpose, the hydrocarbon stream to be pretreated is divided into N fluxes, N being between 2 and 10, each being paralleled in a pretreatment capacitor operating in a batch mode. Each pretreatment capacity operates in batch mode, the capacity being partially filled with a soda solution renewed regularly.

The interest of the process is related to the fact that the renewal of N sodium hydroxide solutions is operated sequentially and not simultaneously. Thus the fluctuations induced by the changes in composition of the sodium hydroxide, and by its regular renewal, are damped, and the hydrocarbon flow resulting from the mixing of the N pretreated streams in parallel has a more stable composition than its counterpart resulting from a pretreatment. performed in a single pre-treatment chamber.

The operation of the pretreatment approaches a continuous operation, but the performances remain superior to those of a pretreatment carried out in purely continuous mode in a single perfectly stirred reactor type enclosure.

Let N be the number of capacitors used in parallel, and i the number of a capacity, ^"going from 1 to N. Let X be the frequency of renewal of each capacitor, in days. The renewal capacity is then evenly distributed on the X days to renew a capacity every XN days.

Brief description of the figures:

Figure 1 shows a version of the device according to the prior art. The pretreatment is performed in a single capacity. FIG. 2 represents a version of the invention for which the pretreatment is carried out in 3 parallel capacities. The frequency of renewal of the soda solution in each capacity is fixed at X days, and the capacities are renewed according to the following agenda: capacity 1, renewed at day D, capacity 2 renewed at day D + X / 3, capacity 3 renewed on day D + 2X / 3.

FIG. 3 shows the mercaptan (RSH), COS and H ₂ S contents coming out of the pretreatment unit according to the prior art.

FIG. 4 shows the mercaptan (RSH), COS and H ₂ S contents coming out of the pretreatment unit according to the invention.

Brief description of the invention

The process according to the present invention can be defined as a process for extracting sulfur compounds from a hydrocarbon fraction (gasoline, LPG, etc.) by liquid-liquid extraction using a sodium hydroxide solution using a sodium hydroxide unit. pretreatment of the charge to be processed consisting of N pretreatment capacitors diposed in parallel, each capacitor operating in a discontinuous manner, and the N capacitors having substantially equal volumes. By substantially equal means a volume variation between any two capacitors of the series of N capacities less than 10% of the average volume of a capacity.

The operation of each of the preprocessing capacities is carried out so that by calling N the number of capacitors, i the number of a capacitor, ranging from 1 to N, and designating by X the sodium renewal frequency of each capacitor. , expressed in days, the sodium renewal of each capacity is regularly distributed over the X days in order to renew a capacity every X / N days.

The capacities are renewed in the following order: capacity 1, renewed on day D, capacity 2 renewed on day D + X / N, capacity 3 renewed on day D + 2X / N and more generally, capacity i renewed on D + day ( i-1) X / N.

Preferably, the number N of pretreatment capacities is between 2 and 10, and the number of days X for the renewal of the sodium hydroxide solution is a multiple of N which is less than 6N. Preferably, the content of the sodium hydroxide solution used at the inlet of each pretreatment capacity is between 2% and 15% by weight, and more preferably between 3% and 7% by weight.

The process according to the present invention is more particularly applicable to the extraction of sulfur compounds contained in LPG (liquefied petroleum gas) cuts and / or to cuts with 3 or 4 carbon atoms, called C3 C4 cuts. Detailed description of the invention

The present invention relates to a process for extracting sulfur compounds present in a hydrocarbon, in the case where the majority sulfur species are mercaptans, denoted RSH, for example methanethiol CH ₃ SH, ethanethiol C ₂ H ₅ SH, propanethiols C3H7SH, and or other sulfur species are also present, such as H ₂ S hydrogen sulfide or COS carbon oxysulfide.

Figure 1 illustrates a process used to extract the sulfur species according to the prior art. This method according to the prior art should be described to fully understand the process according to the present invention.

The hydrocarbon fraction 1 enters a pretreatment chamber 2 pre-filled with a dilute sodium hydroxide solution at a concentration of between 2 and 0% by weight.

The treated hydrocarbon feedstock exits the pretreatment via the pipe 3. The soda solution in the enclosure 2 is renewed according to an operating cycle of between 3 and 30 days, and depending on the age of the soda, the pretreatment extracts a variable amount sulfur species, including mercaptans.

The hydrocarbon then enters a countercurrent extraction column 4, from the bottom of the column. The column is also fed with a regenerated sodium hydroxide solution 6 at the top of the column. The concentration of sodium hydroxide is then between 15 and 25%. The function of column 4 is to extract the majority of the mercaptans still present in the hydrocarbon. The hydrocarbon thus refined leaves column 4 via line 5.

The sodium hydroxide leaving column 4 via line 7 is charged with sodium thiolate species RS-Na corresponding to the mercaptans extracted, dissociated and recombined with Na ⁺ sodium ions. Stream 7 enters an oxidation reactor, also supplied with air via line 8. The presence of air and a catalyst dissolved in the sodium hydroxide solution promote the oxidation reaction of sodium thiolates to disulphides noted RSSR. The catalyst used may be of the family of cobalt phthalocyanines. The multiphase medium leaving the reactor via the pipe 11 is sent to a separating enclosure 12. A gasoline or other hydrocarbon cutting stream 10 is injected into the sodium hydroxide solution upstream of the enclosure 12, for example in the It can also be injected into line 7. This flow makes it possible to extract the disulphides and to recover by decantation in the chamber 12 a hydrocarbon fraction highly enriched in sulfur species 13.

The depleted air exits the settling tank 12 via line 14. The soda thus regenerated is returned to the top of extraction column 4 via line 6.

Sometimes a separating flask is added on line 6 to optimize the extraction of disulfides with the hydrocarbon cut. In this case, the hydrocarbon cut 10 used to extract the disulfides is injected into the line 6, and then decanted into the additional separation flask. The hydrocarbon cut then leaving the additional balloon is sent in line 7.

Figure 2 illustrates a particular version of the method according to the invention. The pre-treatment stage of the hydrocarbon cut to be desoldered 1 is here carried out in 3 distinct capacities, 2a, 2b and 2c, of the same volumes. The flow to be treated is divided into three equal streams. The hydrocarbon streams leaving the three capacities are remixed before entering the extraction column 4.

The dates of renewal of the solutions of soda present in the 3 capacities are regularly spaced. Thus, if the soda lifetime is X days, the first capacity is renewed every X days to a counter of a reference day J, the second capacity is renewed every X days starting from a reference day J + X / 3 and so on. If a number N of capacities are used, the capacity i is renewed on the day D + (i-1) X / N.

The process according to the present invention has the essential advantage over the process according to the prior art of limiting the sulfur fluctuations in the pre-treatment unit and in the downstream extractor. Another effect is that the relative content of mercaptans (corresponding to the ratio between the mercaptan content at the input of the pretreatment and the mercaptan content at the pre-treatment outlet) never exceeds 90% (see FIG. 4) whereas it can reach 100 % (see FIG. 3) in the method of the prior art. As a result, the extraction column can be slightly reduced in height by about 10%.

It is also possible to reduce the hydrocarbon / soda ratio at the extraction column. This ratio, which is commonly in the range of 10 to 40, can be reduced to a range of 5 to 20.

Examples:

The invention will be better understood on reading the examples which follow.

Example 1 (according to the prior art)

An extraction unit of mercaptans present in a hydrocarbon phase of LPG type, a mixture of alkanes and alkenes with 2,3 and 4 carbon atoms, is considered.

The process is in all respects similar to that described in FIG.

The pretreatment is composed of a 12 m ³ prewash flask filled 2/3 of a 6% weight soda solution, renewed every 9 days.

The hydrocarbon feedstock to be treated has a flow rate of 30 m 3 / h, and contains 146 ppm (weight S) of methyl mercaptans, 10 ppm (weight S) of COS and 7 ppm (weight S) of H ₂ S.

The composition of the hydrocarbon at the pre-treatment outlet as a function of time is obtained by simulation. The contents of RSH, COS and H ₂ S are reported in Figure 3. The content of RSH varies greatly between the beginning and the end of life of the soda, in this case over a period of 9 days, which is detrimental to overall smooth operation of the process. On the other hand, it is observed that approximately 60% of the COS and 20% of the H ₂ S are extracted during the pretreatment, which makes it possible to minimize soda consumption at the level of the extractor.

The average sulfur content in the refined LPG leaving the process, which is 2.05 ppm (weight S), is also obtained by simulation.

Example 2 (according to the prior art)

This example constitutes the continuous version according to the prior art. It is a question of replacing the pretreatment step in a discontinuous manner by a continuous step, in a perfectly stirred type reactor.

The volume of the balloon used is unchanged compared to Example 1.

The amount of sodium hydroxide also unchanged is now introduced continuously into the reactor, with a constant injection and withdrawal rate. The injected 6% sodium hydroxide flow rate is 3.7 × 10 ^-2 m ³ / hr The advantage of this implementation in the pretreatment reactor is obviously to operate in a stationary manner, ie to stabilize the concentrations at the preprocessing exit.

In this sense, this solution is relevant, since it makes it possible to significantly lower the average sulfur content in the refined LPG leaving the process.

An average sulfur content in refined LPG is obtained by simulation at 1.35 ppm (weight S).

This solution however poses a problem in terms of pretreatment efficiency, as illustrated by the SOC content in the hydrocarbon phase at the pre-treatment exit obtained by simulation. Indeed, this mode of operation is not very effective in terms of hydrolysis of SOCs, since only 50% by weight of the incoming COSs are converted in this step, ie substantially less than using discontinuous pretreatment (example 1).

This leads to increased consumption of soda at the extractor.

This continuous solution therefore does not effectively replace batch pretreatment.

Example 3 (according to the invention)

According to the invention, the pretreatment flask is replaced by 3 flasks of 4 m ³ each, all 2/3 filled with 6% sodium hydroxide and renewed every 9 days.

The renewal of the soda is out of phase of 3 days between each balloon.

The outputs of the 3 balloons are remixed before entering the countercurrent extraction column. The pre-treatment output composition after remixing the three streams is presented in FIG. 4.

The fluctuations in RSH and COS are clearly damped compared to the prior art.

This makes it possible to minimize the consumption of soda at the extractor, while operating an extraction of the RSH in the very efficient extractor. In fact, a mean sulfur content in the hydrocarbon at the process outlet, ie measured at the top of the extraction column, of 1.39 ppm (weight of S) is obtained by simulation.

This represents a 32% reduction of the sulfur output compared to the method according to the prior art (Example 1).

Claims

1. Process for extracting sulfur compounds contained in a hydrocarbon fraction (gasoline, LPG) by liquid-liquid extraction with a sodium hydroxide solution using a pretreatment unit for the charge to be treated placed upstream of the unit of extraction, the pretreatment unit consisting of N volume pretreatment capacitors substantially equal to each other, arranged in parallel, each capacitor operating in a discontinuous manner such that by calling N the number of capacitors, i the number of a capacitance , ranging from 1 to N, and denoting by X the sodium renewal frequency of each capacity, expressed in days, the sodium renewal of each capacity is then regularly distributed over the X days so as to renew a capacity every XN days, and the capacities being renewed in the following order: capacity 1, renewed at day D, capacity 2 renewed at day D + X / N, capacity 3 renewed at ur J + 2X 3 and more generally, capacity i renewed on day D + (i-1) X / N.

2. Process for extracting sulfur compounds from a hydrocarbon fraction (gasoline, LPG) by liquid-liquid extraction according to claim 1, wherein the number N of pretreatment capacities is between 2 and 10, and the number of days X for the renewal of the soda solution is a multiple of N which is less than 6N.

3. A process for extracting sulfur compounds from a hydrocarbon fraction (gasoline, LPG) by liquid-liquid extraction according to claim 1, wherein the content of the sodium hydroxide solution used at the inlet of each pretreatment capacity is between 2. % and 15% by weight and preferably between 3% and 7% by weight.

4. Application of the process for extracting sulfur compounds according to claim 1 to LPG cuts and / or coupex C3 C4.