Section VII - The Oxo-Synthesis
The total olefine feed (C11+) boiling from (175 to 303° C) is first fractionated into narrow cuts. This is necessary for the subsequent separation of the products into the neutral oil and the alcohols. The latter, having been lengthened in their chain by one carbon atom, boil 20 to 30° C above the corresponding olefine. All distillations in the oxo plant must be done carefully and the different unavoidable intermediate fractions are either rerun or discarded. If this rule is not followed the final alcohol product is contaminated with undesirable products.
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The C11-C17 feed is fractionated into the following four fractions: |
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C11C12 |
175-218° |
45% |
|
C13C14 |
218-245° |
25% |
|
C15C15 |
245-280° |
20% |
|
C17+ |
280-305° |
10% |
(The feed is obtained from LP and MP cobalt synthesis and thermal cracking “Gatsch”)
The fractionation was carried out under atmosphere pressure for the first two cuts and under 100-150 mm. Hg. vacuum for the last two. “Kittel” columns were used, with 10:1 reflux ratio. They were chosen because they were considered to give less pressure drop through the tower and thus allowed a bottom temperature several degrees below that required by ordinary bubble towers.
All four fractions were treated alike in the oxo plant as follows:
A known volume of feed was mixed with 3.0-3.5% by weight of catalyst. The slurry was pressured into the tubular reactor and watergas (CO:H2=1:1, purified) introduced at 150 atm. The reaction is exothermic by 45 kg. cal/kg. of olefine. The heat of reaction is removed by the cooling coil. In addition the pressure may serve as a means to control the heat release. (By lowering the pressure the reaction is slowed down).
The gas is pumped through the reactor in a closed recycle. As the reaction proceeds and gas is used up, the pressure drops. New gas is added and the reaction is complete when no further drop in pressure occurs.
Throughout the reaction a temperature between 125 and 140° C is maintained, depending on the type of feedstock. The temperature may be raised towards the end of the reaction to encourage decomposition of the carbonyl. The watergas is then released and the entire batch including the catalyst is pressured into the (2nd stage) hydrogenation chamber. The olefins are converted 100% in the “aldehyde” stage. 87-95% go to aldehyde, yielding about 100% wt. of alcohol based on olefine fed. The rest goes to Ketone and aldol, but the latter are largely broken up in the hydrogenation to give additional alcohol.
The hydrogenation is also exothermic by about 30-35 kg/cal kg/mol. hydrogen reacted. The operation is carried out in analogous fahion to the aldehyde step. Pure hydrogen is admitted at 150 atm. The H2 is recycled until no further pressure drop occurs. The temperature is held at 180° C. At the end of the reaction the H2 is released and the product withdrawn. The catalyst if filtered through a ceramic thimble.
Before final distillation the product is caustic washed to remove the acids formed in the process. The distillation is carried out in batch columns. Care is to be taken that the kettle temperatures do not exceed 180 to 200° C (about 5 mm. Hg. is required in the higher fractions). Aluminum was used in the construction of the coolers and receivers of the fractionating system (See also reference VII/15 at end of section).
The distillation of the two lower boiling feed fractions (C11,12 and C13,14) is carried out in one column each. The “neutral” oil and the alcohol are taken overhead in succession. The two higher boiling feed fractions (C15, 16 and C17) are distilled in two towers each. The neutral oil is removed first and the alcohols in the second tower. The heavy polymers from all four fractions are charged to a common evaporator. Refractive index measurements are sued in the distillation to control the overhead cut points. The difference in h.p. between alcohol and paraffin is 1.39 to 1.42.
The gas requirements for oxo-synthesis are given as follows: (they are substantially the same for all types of operation)
Basis 10,000 ton/year of alcohol:
|
Watergas |
Exit gas 1st stage |
|
|
CO2 |
6.8% |
12.9% |
|
CO |
38.0% |
27.3% |
|
H2 |
49.0% |
48.1% |
|
CH4 |
0.2% |
0.4% |
|
N2 |
6.0% |
11.3% |
| 5,145,000 m3/year | 2,715,000 m3/year * |
| Hydrogen | Exit gas 2nd stage | |
| H2 | 90% | 85% |
| N2 | 10% | 15% |
| 3,633,000 m3/year | 2,418,000 m3/year * |
* This is on once through basis. (See also reference VII/25 at end of section.)
(b) I.G. Leuna
The I.G. Leuna type of operation was continuous. It might be pointed out that RCH would probably also have gone over to a continuous operation in the near future. The fresh feed is used to pick up the catalyst from the ceramic filter and is charged to a mixer, where fresh additional catalyst can be added. The slurry is then picked up by the h.p.feed pump and pumped through a preheater in the reactor proper. Watergas is recycled through the reactor at 150 amt. Cooling tubes in the reactor are used to remove the heat of reaction. The slurry is next withdrawn and the pressure released in a gas separator. The gas is passed through a scrubber, where cobalt-carbonyl is removed by washing with fresh olefine feed. The slurry from the gas separator is fed to the hydrogenation system, which is an exact duplicate of the one described above. Pure hydrogen is used instead of watergas. The slurry from the final gas separator is filtered and the filtrate caustic washed, before refractionation.
Details of the operation may be taken from the attached documents.
The system was described in the paragraph on catalyst. It is in principle identical with the Leuna system except for the handling of the catalyst.