T 0538/99 (Acetic acid production/BP CHEMICALS) 03-12-2003

Reasons for the Decision

1. The appeal is admissible.

2. The sole issue to be dealt with in the present case is the assessment of inventive step in view of the cited documents.

3. In deciding whether or not a claimed invention meets this criterion, the Boards of Appeal consistently apply the problem and solution approach, which essentially involves identifying the closest prior art, determining in the light thereof the technical problem which the claimed invention addresses and successfully solves, and examining whether or not the claimed solution to this problem is obvious for the skilled person in view of the state of the art.

4. The Board considers, in agreement with the parties to the proceedings, that the closest state of the art with respect to the claimed subject-matter of the patent in suit is the disclosure of document (1).

This document is concerned with a process for the production of acetic acid, which substantially corresponds to the process of Claim 1 of the patent in suit. It teaches that by using relatively high amounts of methyl acetate and lithium iodide of preferably between 2-5 wt% and 10-20 wt%, respectively, the acetic acid production can be carried out at a low water content of preferably 1-4 wt%, while maintaining a stable catalyst system and a high productivity (see, in particular, page 6, lines 27 to 33; page 8, lines 14 to 21; and Table II on page 18).

The use of a low water content has the advantages that there is a great reduction in the rate of formation of by-product propionic acid (see Example 1, in particular, page 26, lines 17 to 22, and Table III on page 27 indicating a propionic acid content of 91 ppm), and that the expenditure in the recovery-purification system needed to obtain dry acetic acid can be reduced (see page 11, lines 28 to page 12, line 5).

The reaction system which can be employed comprises (a) a liquid-phase carbonylation reactor, (b) a so-called "flasher", and (c) a so-called "methyl iodide-acetic acid splitter column" (see page 9, lines 7 to 13). In this context, it is indicated in Example 1 that the liquid reaction product from the carbonylation reactor is fed to a single-tray flasher operated at a head pressure of about 2.4 atmosphere absolute and that about 35% is distilled overhead for further distillation in the methyl iodide-acetic acid splitter column (see page 23, last paragraph). The residue stream from this splitter column is then drawn off as crude acetic acid containing about 4-7 wt% of water and can be purified further by conventional methods outside the scope of the described invention (see page 25, second paragraph, and lines 19 to 20).

4.1. The Appellant submitted that the process of Claim 1 of the patent in suit differed from that of document (1) by maintaining the water content of the flasher overhead up to 8. wt% and by using a single distillation zone in recovering the acetic acid product from the flash zone overhead. On the other hand, the Respondent submitted that the use of said single distillation zone would be the sole difference between the processes, since under the reaction conditions as indicated in Example 1 of document (1) involving the use of a low water content in the reaction composition of 4 to 5 wt%, the flasher overhead would have a water content of less than 8. wt%.

4.2. The Appellant referred in support of his submission that the water content of the flasher overhead was a distinguishing feature to calculations filed on 23 February 1999 showing that the water content of the flasher overhead obtained according to Example 1 of document (1) could be 11.4%, i.e. well above the upper limit as claimed in Claim 1 of the patent in suit.

4.3. However, having regard to the fact

(a) that said calculations have been made with the assumption that all the water in the feed to the flasher went overhead,

(b) that a complete transition of the water into the flasher overhead is unlikely in view of the distillation conditions (see point 5 above, last paragraph), and

(c) that said assumption is in contradiction with the patent in suit indicating that under comparable distillation conditions the liquid fraction from the flasher contains water (see page 7, lines 31 to 37), and that the water content of the flasher overhead (distillation column feed) is lower than the water content of the reaction composition (see all the examples, and in particular Example 1 indicating 3.8 wt.% water in the flasher overhead and 5.6 wt% water in the reaction mixture),

it is the Board's position that the Appellant's submissions in this respect cannot be accepted, and that there exists no doubt that the water content of the flasher overhead achieved according to the preferred embodiment of the process of document (1) applying a low water content in the reaction composition such as indicated in Example 1 falls within the range of up to 8 wt% as indicated in Claim 1 of the patent in suit.

4.4. Thus, in these circumstances, the Board concludes that - as submitted by the Respondent - the process of Claim 1 of the patent in suit only differs from that of the preferred embodiment of the process of document (1) applying a low water content in the reaction composition, such as the process of Example 1, by the use of a single distillation zone in recovering the dry acetic acid product from the flasher overhead.

5. Having regard to the closest prior art document (1), the Appellant considered that the process of Claim 1 of the patent in suit had the advantage that the recovery system for achieving the dry acetic acid was improved.

Thus, in the light of the closest state of the art, the technical problem underlying the patent in suit can be seen in the provision of a process for preparing acetic acid with an improved recovery system (see also page 2, lines 45 to 46, of the patent in suit).

According to Claim 1 of the patent in suit this technical problem is solved by recovering the acetic acid product from the flash zone vapour fraction by use of a single distillation zone.

Furthermore, in view of the examples of the patent in suit, the Board is satisfied that the technical problem as defined above has indeed been solved. This has not been disputed by the Respondent.

6. The question now is whether the solution of the technical problem underlying the patent in suit by the process of Claim 1 would have been obvious to the skilled person in view of common general knowledge and the cited prior art.

7. As indicated above (point 4), document (1) discloses that the liquid reaction product from the carbonylation reactor is fed to a single-tray flasher, that the flasher overhead is further distilled in the methyl iodide-acetic acid splitter column, and that subsequently the crude acetic acid can be purified further by conventional methods outside the scope of the described invention.

Therefore, document (1) as such does not provide an incentive to the skilled person to replace the methyl iodide-acetic acid splitter column and the subsequent recovery method by a single distillation zone.

8. However, in the Board's judgment, the skilled person faced with the technical problem underlying the patent in suit and being informed by document (1) that the use of a low water content has the advantage that the expenditure in the recovery- purification system needed to obtain dry acetic acid can be reduced (see page 11, lines 28 to page 12, line 5) would find in document (4) a clear pointer to the solution of the technical problem as claimed.

Document (4) discloses namely a process for the purification of crude acetic acid, produced by the reaction of methanol with carbon monoxide in the presence of a catalyst system of a group VIII noble metal component and an iodide by using a flasher and subsequently a single distillation unit (see column 1, lines 11 to 31; Claim 6; and Example I). In particular, it discloses in said Example I that by flashing a stream of acetic acid, water, hydrogen iodide and methyl iodide, and distilling the overhead from the flasher in a single distillation unit as illustrated by Figure 1 a dry acetic acid product containing 83-132 ppm of water (see Table I) can be obtained.

9. In this context, the Appellant submitted that the skilled person would not combine the teaching of document (1) with that of document (4), since the flasher overhead compositions were quite different in view of the high water content of about 18 wt% in the composition of document (4) and the relatively high methyl acetate concentration in the composition of document (1). Moreover, he submitted that even if the skilled person had combined both documents, he would have found a clear incentive in document (4) to use the distillation system consisting of 2 distillation units as illustrated by Figure 2 in view of the presence of a iodide salt in the flasher overhead of document (1).

However, the Board does not accept these submissions for the following reasons:

Although it is true that the flasher overhead in the process of document (4) comprises more water than the flasher overhead in the process of document (1), the skilled person considering the suitability of the distillation unit as illustrated by Figure 1 of document (4) would rather conclude that said distillation unit would even be more appropriate for removing a lower amount of water. Moreover, like the Respondent, the Board does not see any reason why the single distillation unit as indicated in Figure 1 of document (4) would not be appropriate for purifying a crude acetic acid feed containing some methyl acetate. In fact, the Appellant could not provide any support for his contention in this respect.

Furthermore, the Board finds that the Appellant's submission that in view of the presence of lithium iodide in the flasher overhead in the process of document (1) the skilled person would rather select the distillation system of Figure 2 instead of the single distillation unit of Figure 1 (see document (4), column 4, lines 1 to 5) also fails, since lithium iodide is involatile, so that the skilled person would rather expect that the lithium iodide content in the flasher overhead being fed to the distillation unit would be negligible. This finding is also confirmed by the patent in suit indicating that the involatile lithium salt stabiliser forms part of the liquid fraction from the flasher (see page 7, lines 35 to 37).

10. Thus, in view of these considerations the Board concludes that the solution of the above defined technical problem as claimed in Claim 1 of the patent in suit is obvious to the skilled person, and consequently does not involve an inventive step within the meaning of Article 56 EPC.

Claims 2 to 14 fall with Claim 1, since the Board can only decide on the Appellant's request as a whole.