GB2088850A - Treatment of N methyl pyrrolidone - Google Patents

Abstract

N-methyl pyrrolidone (NMP) used in selective extraction of aromatics from mixtures with paraffins contains acidic, and/or chloride contaminants which can corrode plant. These contaminants can be removed from non-aqueous streams using a basic ion-exchange resin, and can be kept at a low level by continuous or semi-continuous treatment.

Description

SPECIFICATION Treatment of NMP This invention concerns the treatment of NMP (N-methyl pyrrolidone), more especially to remove corrosive contaminants.

NMP is well known as a selective solvent capable of selectively dissolving aromatic hydrocarbons from paraffinic hydrocarbons and there are several processes operated on a commercial scale utilising this property. The selectivity of NMP for aromatics is increased if about 10% of water is mixed therewith and to recover NMP for recycle to the extraction steps, the aromatic-rich NMP stream is distilled to yield an aromatic stream containing water and a substantially non-aqueous NMP stream, containing about 3% water.

We have found that in such a process, NMP is capable of causing serious corrosion of plant, and even stainless steel components are susceptible to stress corrosion. It was discovered that there was a slow but constant build up of two corrosive contaminants during normal operation of the extraction process despite the use of steam condensate or deionised make-up water. One contaminant was chloride which is thought to leak into the system, and the other was an organic acid resulting from the thermal degradation of NMP. It became clear that although it was possible to remove the contaminants by shutting down the plant and distilling the NMP under reduced pressure, this course was not a final solution because of a significant rate of build up of the contaminants. It was therefore desirable to find a method of treating NMP on a continuous or semicontinuous basis.

The invention therefore provides a method of treatment of NMP to remove acidic and/or chloride corrosive contaminants which comprises contacting the NMP with a basic ion-exchange resin.

Preferably the ion-exchange resin is a strongly basic resin, and these are commercially available.

A particularly suitable strong base resin is Amberlite (Registered Trade Mark) IRA 458. Initial research work showed that chloride could be removed by a weak base resin, such as Amberlite IRA 68, but that only a low volume of NMP (containing about 0.3% organic acid) could be passed before the resin was saturated with acid.

That is, the IRA 68 resin could reduce the 36 ppm of chloride in process NMP to approximately 3 ppm in a single pass, yielding a theoretical ability to treat 1 600 bed volumes of NMP before the bed is saturated, although practically one would probably not axceed 800-1000 bed volumes between regenerations cf the bed However, when considering NMP containing 36 meq/litre of organic acid, a single pass through IRA 68 resin reduces the amount of acid to about one-half for about 20-30 bed volumes of NMP treated, but after approximately 40 bed volumes the bed appears saturated and the acid level returns to its original value, which is clearly inadequate on a commercial scale.

Treating NMP containing 36 meq/litre of acid with a strong base resin such as IRA 458 gave a much more efficient removal of acid, reducing the acid content of the NMP down to 4 meq/litre in a single pass. Again the resin was soon saturated; only 25 bed volumes were treated before regeneration was necessary, but this was due to the relatively high concentrations of acid in the circuit NMP at the time of these trials.

Measurements have shown that the rate of organic acid formation in a commercial aromatic hydrocarbon extraction plant is only about 8 ppm per day and therefore the concentration of acid is reduced, it is possible to remove acid as it is formed without requiring frequent regeneration.

Accordingly, it is possible to incorporate the invention on-line in an aromatic hydrocarbon extraction plant. The invention, therefore, also provides a plant for the extraction of aromatic hydrocarbons from mixtures with paraffinic hydrocarbons using NMP as selective solvent, in which a basic ion exchange resin is used to remove acidic and/or chloride contaminants from the NMP before recycle to the extraction step. The invention further provides a process for the extraction of aromatic hydrocarbons from mixtures with paraffinic hydrocarbons using NMP as selective solvent, comprising treating the recycle NMP with a basic ion exchange resin to remove acidic and/or chloride contaminants.

While the invention offers particular advantages when used to provide on-line purification of the NMP, it is also possible to treat the NMP on a semi-continuous basis or as a batch process.

Regeneration of the resin may be carried out in a manner known per se, for example by using dilute (4%) caustic soda solution. Conventional operating and regeneration procedures are conveniently used, that is, regeneration, rinsing with deionised water and then use for ion exchange. Preferably, the bed is backwashed with deionised water before regeneration, to classify the bed and to remove any accumulation of particulate matter.

The invention will now be described by way of Example only.

EXAMPLE In a commercial plant using NMP to extract aromatics from naphtha, the recycle NMP was causing chemical/galvanic corrosion of mild steel plant components and there appeared a danger of stress corrosion of stainless steel. The NMP was neutralised and distilled under reduced pressure before restarting the extraction process. After three months, the NMP contained more than 10 meq/litre of acid and more than 30 ppm of chloride. A pair of columns were fitted each containing 10 kg of Amberlite IRA 458 strong base ion exchange resin, to treat the recycle NMP.

Over a period of six weeks, regenerating the columns when necessary, the amount of organic acid was reduced to 1.2 meq/litre and the amount of chloride was reduced to zero. It was noted that the columns increased in overall efficiency with increasing number of regeneration cycles and also that although acid could be detected as slipping through the columns after about 50% of the capacity was used up, it was still removed at a good practical level until the beds had reached 80% of capacity.

It was found that the Amberiite IRA resin undergoes a 100% expansion of volume on switching from the non-aqueous (NMP) stage of the operating cycle to the aqueous stage (backwashing). A certain amount of care was required in ensuring that there was no opportunity for this volume expansion to cause problems, for example in causing joints leakage.

Claims (6)

1. A method of treating NMP to remove acidic and/or chloride corrosive contaminants which comprises contacting the NMP with a basic ionexchange resin.

2. A method according to claim 1, wherein the resin is a strong base resin.

3. A method according to claim 1 or 2, wherein the NMP is contaminated with degradation products resulting from the use of NMP at elevated temperatures.

4. A method according to claim 1 , substantially as hereinbefore described.

5. A process for the extraction of aromatic hydrocarbons from mixtures with paraffinic hydrocarbons using NMP as selective solvent, comprising treating the recycle NMP with a basic ion-exchange resin to remove acidic and/or chloride contaminants.

6. A plant for the extraction of aromatic hydrocarbons from mixtures with paraffinic hydrocarbons using NMP as selective solvent, in which a basic ion-exchange resin is used to remove acidic and/or chloride contaminants from the NMP before recycle to the extraction steps.