The invention relates to a continuous process for recovery of solvent from off-gases exhausted from a process for the production of ammonium sulfate that is co-produced with caprolactam.

Ammonium sulfate ((NH ₄ ) ₂ S04) is produced on a large scale. This inorganic salt has a number of commercial uses, but is used mainly as fertilizer in agriculture to provide nitrogen and sulfur. It contains 21 % nitrogen as ammonium cations, and 24% sulfur as sulfate anions. Ammonium sulfate crystals for this use are classified according to crystal size.

Caprolactam is the precursor to Nylon 6, a widely used synthetic polymer. Several processes are known for its production. Large-scale industrial processes employ cyclohexanone, cyclohexane, or toluene as starting materials. In most processes cyclohexanone oxime is obtained as an intermediate. Examples of these processes are (modified) Raschig process using ammonium salts and cyclohexanone as starting materials, ammoximation of ammonia, hydrogen peroxide and cyclohexanone, and Hydroxylamine-Phosphate Oxime Process (HPO ^® ) of DSM and NO based hydroxylammonium sulfate routes of BASF or Inventa with ammonia, hydrogen and cyclohexanone as raw materials. The produced cyclohexanone oxime is via Beckmann rearrangement, which includes opening the cyclohexyl ring, converted into caprolactam.

Ammonium sulfate is co-produced in almost all commercial caprolactam production processes. The ammonium sulfate might be produced during the formation of the intermediate cyclohexanone oxime and/or during the Beckmann rearrangement of cyclohexanone oxime into caprolactam. In the Beckmann

rearrangement reaction of cyclohexanone oxime, either sulfuric acid or oleum or S0 ₃ is used as rearrangement medium. The reaction gives the sulfate of caprolactam in excess sulfuric acid, which is then neutralized with ammonia or ammonia water, giving also ammonium sulfate. The resulting reaction mixture is then separated into an aqueous crude caprolactam phase and an aqueous ammonium sulfate comprising phase. However, typically the separation is not perfect. Accordingly, traces of ammonium sulfate remain in the crude caprolactam phase, and traces of caprolactam remain in the aqueous ammonium sulfate comprising phase.

J P54128589 describes a method of recovering caprolactam from a neutralized Beckmann rearrangement mixture, by separation into organic and aqueous phases followed by extraction of caprolactam into an organic solvent and subsequent distillation.

Ammonium sulfate is typically crystallized for further use. In order to obtain on the one hand good quality crystalline ammonium sulfate and on the other hand to increase the overall yield of caprolactam, dissolved caprolactam is extracted with an organic solvent from the aqueous ammonium sulfate comprising phase before being crystallized. In the case of agricultural use, it is important to reduce the organic content of the ammonium sulfate which is to be crystallized.

US 3,264,060 describes such a process for the manufacture of lactams, including caprolactam. After the separation of aqueous crude lactam and aqueous ammonium sulfate, the aqueous ammonium sulfate solution is crystallized by evaporation of water and the remaining mother liquor diluted and returned to the neutralization step. Lactam from the crude lactam phase is extracted into organic solvent for further processing.

Dissolved organic solvent remaining in the aqueous ammonium sulfate comprising phase is in general recovered by stripping with either steam or an inert gas. By (steam) stripping a mixture of organic solvent and water goes overhead, which is condensed in a condenser. The recovered organic solvent might be reused in the process. Exhaust gases produced in the extraction with organic solvent of the aqueous ammonium sulfate comprising phase and the subsequent stripping of the aqueous ammonium sulfate comprising phase and the handling with the organic solvent are in general combusted to reduce the emission of organic solvents to the atmosphere.

US4804473 (also published as EP02741 1 1 ) describes a process for cleaning an off-gas containing a water immiscible solvent by washing with water containing caprolactam. It goes on to describe that the off-gas contains benzene or toluene and is obtained in the extraction of caprolactam from crude caprolactam. The aqueous caprolactam solution which contains dissolved benzene or toluene is preferably fed into the crude extraction stage.

A problem exists in known processes for the production of ammonium sulfate co-produced with caprolactam, in that off-gases are generated in each of the steps of extracting caprolactam into organic solvent, and stripping the organic solvent from the ammonium sulfate. Typically these off-gases are burned and optionally heat is recovered therefrom. The present inventors have however, found a way to recover desirable materials therefrom, and emit only cleaned gases to the environment. The invention has the particular advantage that a lower amount of organic solvent is consumed in the process, because organic solvent is recovered, and may be reused in the process. Accordingly, it is neither released to the environment, nor burned, generating harmful waste products.

The present invention provides a continuous process for the production of ammonium sulfate in combination with caprolactam, said process comprising,

a) neutralizing with ammonia a mixture comprising caprolactam and sulfuric acid to form an aqueous ammonium sulfate phase and an aqueous crude caprolactam phase;

b) separating the aqueous ammonium sulfate phase from the aqueous crude caprolactam phase;

c) extracting into an organic solvent caprolactam present in the aqueous

ammonium sulfate phase to form a caprolactam-containing organic phase and a purified aqueous ammonium sulfate phase;

d) stripping organic solvent from the purified aqueous ammonium sulfate phase; and

e) withdrawing organic solvent-containing gas produced in, or subsequent to, any one of steps c) and d),

characterized in that

f) organic solvent is recovered from said organic solvent-containing gas.

As used herein, production of ammonium sulfate in combination with caprolactam means that both ammonium sulfate and caprolactam are produced by the process.

A mixture comprising caprolactam and sulfuric acid is typically derived from the Beckmann rearrangement of cyclohexanone oxime to form

caprolactam, carried out in the presence of sulfuric acid, S0 ₃ or oleum.

Caprolactam present in the aqueous ammonium sulfate phase refers to trace amounts of caprolactam. The separation of aqueous ammonium sulfate and aqueous crude caprolactam phases is not perfect. Typically, trace amounts of caprolactam are present in the aqueous ammonium sulfate phase, and trace amounts of ammonium sulfate are present in the aqueous crude caprolactam phase.

Organic solvent-containing gas is produced because the organic solvent itself is volatile. The organic solvent in the organic solvent-containing gas is itself gaseous. The amount of gas produced increases with increased agitation.

Conventionally said gas is flushed from the system for reasons of safety. As used herein, the term subsequent to, means a step later in the process. The organic solvent-containing gas need not be withdrawn immediately on its formation. For example subsequent to step d) might be condensation of the stripped gas to form a liquid organic phase and a liquid aqueous phase, which are then separated. Organic solvent-containing gas may be generated in such steps.

Preferably the recovered organic solvent is reused in the process. This has the advantage of avoiding the need for transporting the solvent to another location, or removing trace impurities which may still be present. The recovered organic solvent may be reused either in the production of ammonium sulfate, the production of caprolactam or both. The recovered organic solvent may be returned to the process, in other words recycled, to any point where organic solvent is already present. It is preferred to return the organic solvent to the extraction of the aqueous crude caprolactam phase separated in step b). In another preferred option the organic solvent is returned to step c).

Preferably, the stripping of step d) is steam stripping.

Typically, the process further comprises step d') condensing organic solvent-containing gas produced in step d) and separating said condensed gas into a condensed organic phase and a condensed aqueous phase. In this way organic solvent is recovered from organic-solvent containing gas produced in step d).

As mentioned above, organic solvent-containing gas may be produced in, or subsequent to, any one of steps c) and d). Typically, organic solvent- containing gas is produced in several steps in the process. Preferably, the organic solvent-containing gas is produced in each of steps c) and d). Preferably, the organic solvent-containing gas is produced in each of steps c), d) and d').

Typically, step e) comprises withdrawing organic solvent-containing gas wherein said organic solvent is evaporated from at least one of: the organic solvent defined in step c), the caprolactam-containing organic phase defined in step c) and the organic solvent defined in step d).

Preferably, step e) comprises withdrawing organic solvent-containing gas wherein said organic solvent is evaporated from the organic solvent defined in step d) and at least one of the organic solvent defined in step c) and the caprolactam- containing organic phase defined in step c).

The term "evaporated from" means that the gaseous organic solvent in the organic solvent-containing gas originates from the (liquid) organic solvent of at least one of the organic solvent defined in step c), the caprolactam-containing organic phase defined in step c) and the organic solvent defined in step d), Recovery of the organic solvent is by any suitable means. Typical techniques are absorption, adsorption or condensation. Absorption or adsorption is preferred. Most preferably absorption is used.

The organic solvent may be recovered into any suitable media.

Typically, the organic solvent is recovered into a medium in which it may be easily reused in the process. Preferably said medium is itself used in the process. Preferably, the organic solvent is recovered by absorption into a caprolactam-containing absorbent. More preferably, the caprolactam-containing absorbent comprises at least a fraction of the aqueous crude caprolactam phase. In this way the organic solvent may be recovered by passing a stream of organic solvent-containing gas over a stream of the aqueous crude caprolactam phase. This is particularly advantageous as it avoids the need for any additional absorption agent.

The caprolactam-containing absorbent may optionally comprise water further to that present in the aqueous crude caprolactam phase. Typically, the water content of the caprolactam-containing absorbent is from 10 to 70 wt.%. Preferably, the water content of the caprolactam-containing absorbent is from 25 to 40 wt.%. More preferably, it is from 30 to 35 wt.%.

Typically, the ratio of the mass flow rate of the caprolactam- containing absorbent into step f) to the mass flow rate of the organic solvent-containing gas into step f) is less than 3.5 %. Preferably it is less than 3 %; more preferably less than 2 %, for example 1 %.

The majority of the organic solvent-containing gas is the atmosphere under which the operations are carried out. This is typically nitrogen, for example nitrogen from air, or air. Typically, the organic solvent-containing gas comprises more than 1000 ppm by weight organic solvent. Preferably it comprises more than 2000 ppm by weight organic solvent; more preferably more than 3000 ppm.

The organic solvent has a low miscibility with water and a high affinity for caprolactam. Typically, the organic solvent is toluene, benzene, a chlorinated hydrocarbon or a mixture thereof. An example of a chlorinated hydrocarbon is trichloroethylene. In general these organic solvents have a boiling point that is much lower than that of caprolactam (for example, the atmospheric boiling points of toluene and benzene are approx. 1 1 1 °C and 80.1 °C, respectively, whereas the boiling point of caprolactam is 271 °C). This makes it easier for the solvent to be evaporated from caprolactam later in any subsequent step for the purification of caprolactam.

Typically, the process further comprises step b') extracting into a second organic solvent caprolactam from the aqueous crude caprolactam. Typically, the second organic solvent is the same as the organic solvent. This is beneficial, not only for the convenience of having only one type of solvent present in the system, but also that the organic solvent and second organic solvent can be mixed. Further caprolactam has the same solubility in each of the first and second solvents.

Typically, the second organic solvent is recovered by absorption into a caprolactam-containing absorbent and the resulting caprolactam-containing absorbent is mixed with the aqueous crude caprolactam that undergoes step b'). The mixing might be before the aqueous crude caprolactam is extracted into a second organic solvent in step b'). However, it is preferably mixed during the extraction. In this embodiment organic solvent is used to extract caprolactam from the aqueous ammonium sulfate; and the part of the solvent which evaporates in the processing is recovered by absorption into crude aqueous caprolactam phase; which itself undergoes extraction with a second organic solvent. Accordingly, it is particularly advantageous if the organic solvent and second organic solvent are the same.

Typically, the second organic solvent is itself partly evaporated during the extraction of caprolactam from the aqueous crude caprolactam phase into the second organic solvent in step b'). It is also beneficial, for the reasons mentioned above, to recover said evaporated second organic solvent. Further it is beneficial to reuse said recovered second organic solvent in the process. This has the advantage of avoiding the need for transporting the solvent to another location, or removing trace impurities which may still be present. The recovered second organic solvent may be reused either in the production of ammonium sulfate, the production of caprolactam or both. The recovered second organic solvent may be returned to the process, in other words recycled, to any point where organic solvent is already present. It is preferred to return the second organic solvent to the extraction of the aqueous crude caprolactam phase separated in step b). In another preferred option, in case the second organic solvent and the organic solvent are the same, the second organic solvent is returned to step c). Typically, a second organic solvent-containing gas is produced in step b'); the second organic solvent is recovered from said second organic solvent-containing gas; and said second organic solvent is reused in the process.

Where said second organic solvent is the same as the organic solvent, it may be treated in an identical manner to the organic solvent. Recovery of the second organic solvent is by any suitable means. Typical techniques are absorption, adsorption or condensation. Absorption or adsorption is preferred. Most preferably absorption is used. The second organic solvent may be recovered into any suitable media. Typically, the second organic solvent is recovered into a medium in which it may be easily reused in the process. Preferably said medium is itself used in the process. Preferably, the second organic solvent is recovered by absorption into a caprolactam-containing absorbent. More preferably, the caprolactam-containing absorbent comprises at least a fraction of the aqueous crude caprolactam phase. In this way the second organic solvent may be recovered by passing a stream of second organic solvent-containing gas over a stream of the aqueous crude caprolactam phase. This is particularly advantageous as it avoids the need for any additional absorption agent.

The caprolactam-containing absorbent may optionally comprise water further to that present in the aqueous crude caprolactam phase. Typically, the water content of the caprolactam-containing absorbent is from 10 to 70 wt.%. Preferably, the water content of the caprolactam-containing absorbent is from 25 to 40 wt.%. More preferably, it is from 30 to 35 wt.%.

Typically, the ratio of the mass flow rate of the caprolactam- containing absorbent into step f) to the mass flow rate of the second organic solvent- containing gas into step f) is less than 3.5 %. Preferably it is less than 3 %; more preferably less than 2 %, for example 1 %.

The majority of the second organic solvent-containing gas is the atmosphere under which the extraction is carried out. This is typically nitrogen, for example nitrogen from air or air. Typically, the second organic solvent-containing gas comprises more than 1000 ppm by weight second organic solvent. Preferably it comprises more than 2000 ppm by weight second organic solvent; more preferably more than 3000 ppm.

The second organic solvent has a low miscibility with water and a high affinity for caprolactam. Typically, the second organic solvent is toluene, benzene, a chlorinated hydrocarbon or a mixture thereof. This makes it easier for the solvent to be evaporated from caprolactam later in any subsequent step for the purification of caprolactam.

Typically, more than 95 wt.% of the organic solvent present in the organic solvent-containing gas is removed therefrom. Preferably, more than 98 wt.% of the organic solvent present in the organic solvent-containing gas is removed therefrom. More preferably, more than 99 wt.%, for example 99.5 wt.% of the organic solvent present in the organic solvent-containing gas is removed therefrom. Typically, more than 95 wt.% of the second organic solvent present in the second organic solvent-containing gas is removed therefrom. Preferably, more than 98 wt.% of the second organic solvent present in the second organic solvent-containing gas is removed therefrom. More preferably, more than 99 wt.%, for example 99.5 wt.% of the second organic solvent present in the second organic solvent-containing gas is removed therefrom.

Accordingly, particularly preferably more than 95 wt.% of the organic solvent or, where present, of the second organic solvent present in the organic solvent- containing gas or in the second organic solvent-containing gas, respectively, is removed therefrom. More preferably more than 98 wt.% of the organic solvent or, where present, of the second organic solvent present in the organic solvent-containing gas or in the second organic solvent-containing gas, respectively, is removed therefrom. More preferably, more than 99 wt.%, for example 99.5 wt.% of of the organic solvent or, where present, of the second organic solvent present in the organic solvent- containing gas or in the second organic solvent-containing gas, respectively, is removed therefrom.

The present invention will be more fully explained with reference to the following drawings.

FIG. 1 describes the prior art. FIG. 2 illustrates an embodiment of the process of the present invention. FIG. 3 illustrates a preferred embodiment of the process of the present invention.

In FIG. 1 a neutralization zone [A] is supplied with a mixture comprising caprolactam and sulfuric acid [1 ], for example from the Beckmann rearrangement of cyclohexanone oxime, ammonia [2] and an aqueous solution [3]. In the neutralization zone [A], a neutralization mixture containing aqueous ammonium sulfate solution and aqueous crude caprolactam is formed. The neutralization mixture is passed through line [4] to a separation section [B] in which the aqueous ammonium sulfate phase and the aqueous crude caprolactam phase are separated through phase separation. The aqueous crude caprolactam phase is discharged through line [5]; and the aqueous ammonium sulfate phase is passed through line [6] to extraction section [C]. Organic solvent is added through line [7]. The remaining aqueous ammonium sulfate phase passes through line [8] to stripping section [D]. An organic stream comprising organic solvent, caprolactam and organic contaminants is discharged through line [9]. Stripping section [D] comprises one or more strippers and one or more condensers for condensing stripped out organic solvent. Steam is fed through line [10]. The ammonium sulfate phase from which organic solvent has been stripped is discharged through line [1 1 ] for further processing. This is typically crystallization.

Stripped out organic solvent is, after being condensed, discharged through line [12]. Organic solvent containing gas produced in extraction section [C] and in stripping section [D] is withdrawn and fed via line [13] to a flare [E] where organic solvent present in the organic solvent containing gas is burnt. The resulting exhaust gases that contain combustion products of organic solvent are discharged to the atmosphere though line [14]. In this way, emissions of organic solvent into the atmosphere are limited. Optionally, heat may be recovered from the flare [E].

FIG. 2 represents an embodiment of the process according to the present invention. It incorporates the same features as FIG. 1 , except for flare [E]. Instead, organic solvent containing gas produced in extraction section [C] and in stripping section [D] is withdrawn and fed via line [13] to a recovery section [F]. There, organic solvent from the organic solvent-containing gas is recovered by absorption, adsorption or condensation, and is fed via line [15] for further use. Exhaust gas is fed to the atmosphere through line [14]. Conversely with the situation of FIG. 1 , the combustion products of organic solvent are not present in the exhaust gas, because no solvent is burned.

FIG. 3 represents a preferred embodiment of the process according to the present invention. It incorporates all the features of FIG. 2. In addition, the recovery section is elaborated. A portion of the aqueous crude caprolactam in line [5] is directed to recovery section [F] via line [5B]. Water is added to this portion of aqueous crude caprolactam via line [16], and the diluted aqueous crude caprolactam passes through line [17] to recovery section [F]. Here, organic solvent in the organic solvent- containing gas is absorbed into the diluted aqueous crude caprolactam. The diluted aqueous crude caprolactam is therefore a caprolactam-containing absorbent. The absorbed organic solvent comprising caprolactam-containing absorbent is discharged through line [15] to extraction section [G], where they are combined with a portion of aqueous crude caprolactam phase charged through line [5A]. A second organic solvent is charged to the extraction section [G] through line [18]. Caprolactam is extracted into the second organic solvent and these are together discharged through line [19], for further processing. Typically the second organic solvent is subsequently distilled from the caprolactam. An aqueous phase, typically also containing some impurities is discharged via line [20].

The invention is illustrated by the following examples, without being limited thereto. In the Comparative Example and in the Example ammonium sulfate was co-produced in a commercial continuous process for the production of

caprolactam.

With reference to FIG. 1 and FIG. 3, to neutralization section [A] a mixture of caprolactam and sulfuric acid, aqueous ammonia and water were fed via lines [1], [2] and [3], respectively. In the neutralization section [A], a mixture of caprolactam and sulfuric acid, produced from the Beckmann rearrangement of cyclohexanone oxime, was neutralized with aqueous ammonia whereby an aqueous crude caprolactam phase and an aqueous ammonium sulfate solution that contains dissolved caprolactam were obtained. The obtained aqueous crude caprolactam phase and the aqueous ammonium sulfate solution that contains dissolved caprolactam were charged to a gravity settler in a separation section [B] via line [4]. From separation section [B] an aqueous crude caprolactam phase was discharged via line [5] and charged to extraction section [G] (not shown in FIG. 1 ).

The aqueous crude caprolactam phase was extracted with benzene in extraction section [G] whereby a benzenic caprolactam-comprising solution was formed. This benzenic caprolactam comprising solution was further worked-up to nylon-6 grade caprolactam.

From separation section [B] an aqueous ammonium sulfate solution was charged to extraction section [C] via line [6]. In an extraction column of the extraction section [C], caprolactam was extracted into benzene from the aqueous ammonium sulfate solution. The benzene was fed to the bottom section of the extraction column via line [7] and the aqueous ammonium sulfate solution was fed via line [6] to the top section of the extraction column. An organic stream comprising benzene, caprolactam and organic contaminants exited the top section of the extraction column via line [9] and was re-used for the extraction of caprolactam from aqueous crude caprolactam in extraction section [G] (not shown in FIG. 1 and FIG. 3). The aqueous ammonium sulfate solution from which caprolactam was extracted, was discharged from the bottom section of the extraction column and was fed to the stripping section [D] via line [8].

In the stripping section [D] aqueous ammonium sulfate solution, which also contained some organic solvent was steam stripped in a stripping column. As stripping column a packed bed column was used that was equipped with a steam heated reboiler and a water cooled condenser. The steam was charged to the reboiler via line [10]. The aqueous ammonium sulfate solution was fed to the top section of the stripping column via line [8] and the stripped aqueous ammonium sulfate solution was discharged through the bottom section of the stripping column via line [1 1 ]. Stripped out benzene is, after being condensed, discharged through line [12].

The remaining aqueous ammonium sulfate solution was charged to a crystallization section via line [1 1 ] where ammonium sulfate crystals were produced by evaporative crystallization (not shown in FIG 1 and FIG. 3). The produced ammonium sulfate crystals were separated from the mother liquor by centrifugation and afterwards dried. The water vapor produced in the crystallization was partly re-used in the neutralization section. In order to remove impurities from the crystallization section a part of the ammonium sulfate solution was purged.

The flow rate and the composition combined benzene containing gas exiting the extraction section [C] and the stripping section [D] via line [13] were:

Flow rate benzene containing gas: 0.35 Nm ³ /hr

Benzene content in benzene containing gas: 24 % by weight

Comparative Example

A set-up was used as depicted in FIG. 1. The combined exhaust gases exiting the extraction section and the stripping section were burned in flare [E] and as a result of that the benzene emission to the atmosphere was almost nil.

Example

A set-up was used as depicted in FIG. 3. The combined benzene containing gas [13] exiting the extraction section [C] and the stripping section [D] were sent to an absorption section [F]. The absorption section [F] was equipped with an absorption column packed with structured packing (250 m ² /m ³ ) and which was operated in a counter-current mode. The combined benzene containing gas were fed to the bottom section of the absorption column and the cleaned exhaust gases discharge from the top section of the absorption column via line [14]. The fresh absorbent entered the top section of the column via line [17] and the absorbent loaded with benzene exited the bottom section of the column via line [15]. As absorbent, aqueous crude caprolactam originating from the neutralization section, cooled to 40°C was applied. No water was added to the crude caprolactam originating from the neutralization section via line [16]. The weight ratio of absorbent to exhaust gases that were fed to the absorption column was 3 to 100.

The benzene absorption column was modeled with AspenPlus software, with phase equilibria are based on the NRTL model (non-electrolyte). The applied binary interaction parameters were derived from the PhD thesis 'Caprolactam extraction in a pulsed disc and doughnut column with a benign mixed solvent' (ISBN: 90-365-2223-4) by M.L. van Delden (University Twente, The Netherlands, 2005). The simulations show that almost 99 % by weight of the benzene in the exhaust gases could be removed by absorption.

Optionally, the exhaust gases treated in the absorption section could be sent afterwards to a burner for removal of the final traces of benzene.

This example clearly shows that by applying an absorption column in absorption section [F] that is fed with aqueous crude caprolactam originating from the neutralization section, virtually all benzene could be recovered from the exhaust gases. The recovered benzene dissolved in the aqueous crude caprolactam can be fed as such to the extraction section [G] where the aqueous crude caprolactam phase originating from the neutralization section [B] is extracted with benzene. As a result of the recovery of benzene in absorption section [F] less fresh benzene is consumed in the process for the production of caprolactam whereby ammonium sulfate is co- produced.