DE10226543A1 - Continuous process for the production of lower fatty acid alkyl esters by reaction of fatty acid glycerides with lower alcohols in a tubular reactor at a Reynolds number of less than 2300 - Google Patents

Abstract

A continuous process for the production of lower fatty acid alkyl esters by reaction of fatty acid glycerides with lower alcohols in a tubular reactor using a homogeneous alkali catalyst comprises addition of a premixed solution of catalyst and alcohol into the stream of fatty acid glyceride before entry at a Reynolds number, calculated by quasi-homogeneous analysis, at the reactor inlet of less than 2300, preferably 160. A continuous process for the production of lower fatty acid alkyl esters (I) by reaction of fatty acid glycerides with lower alcohols in a tubular reactor using a homogeneous alkali catalyst at up to 10 bar and up to 100[deg]C with an average residence time of 5-60 minutes comprises addition of a premixed solution of catalyst and alcohol into the stream of fatty acid glyceride before entry to the reactor to form a fine dispersion in the continuous phase and the flow through the reactor is such the Reynolds number by quasi-homogeneous analysis at the reactor inlet is less than 2300, preferably 160.

Description

The invention relates to a continuous process for producing lower Fatty acid alkyl esters.

When converting natural fats and oils to fatty acid alkyl esters and glycerin acidic or alkaline homogeneous catalysts are used. For use Alkaline homogeneous catalysts come with deacidified oils and fats low proportion of free fatty acids because these catalysts are free Fatty acids can be deactivated. Sodium hydroxide, Sodium methylate, potassium hydroxide and the like. a. used. For natural fats and oils with acidic homogeneous catalysts are used for a higher proportion of free fatty acids Commitment. (Freiberg Research Books, A852 Ecology, 34-61, 1999)

The production of fatty acid alkyl esters and glycerin from natural oils and fats is a chemical process that has been carried out for decades. There are already one large number of procedures. A basic overview is in Fat / Lipid, Vol. 99, p. 418-427, from 1997. From this overview it can be seen that mainly the process variants of multi-stage tubular reactor or stirred tank, at temperatures up to 100 ° C and pressures up to 10 bar.

Lurgi's continuous process works with combined mixer-settlers Units. In this two-step process, the reactants become strong mixed (turbulent) stirred tanks. The turbulence in the reaction vessel is caused by mechanical agitation instruments. After each stage Glycerin deposited and the loss of alcohol and catalyst compensated. The Process works at temperatures below 100 ° C and in normal or Low pressure area.

The process from Henkel KG provides an example of the use of a tubular reactor on shares (WO 91/05034). The reaction is at least two stages carried out. After each stage, glycerin is separated in a static one Separator.

In this process, the reactants are contacted in stages, in a first step preheated oil is mixed with alcohol and in a second mixing stage an alcohol / catalyst solution is then metered in. The amounts of alcohol and catalyst withdrawn from the reaction mixture by separating the glycerol are balanced before entering the next stage. There is strong turbulent mixing within the reactors. The Reynolds number Re is used as a key figure for the flow state in the tubular reactor. The calculation is based on the following equation:

Here ρ is the mixture density, η the mixture viscosity, c the linear Pipe speed and d the pipe diameter. If the Reynolds number exceeds 2300 the flow is called turbulent, below this limit it is laminar.

The flowing medium is a multi-phase mixture that is assumed to be pseudo-single phase in the calculations for the Reynolds number. The mixture density ρ is calculated as follows:

The quantity w _i describes the mass fraction and ρ _i the density of component i. The following formal relationship is used to determine the pseudo-single-phase mixture viscosity:

Η _{i is} the dynamic viscosity and x _{i is} the molar fraction of component i. In the process described here, the Reynolds numbers are in the range from 4,000 to 10,000, i.e. in the turbulent flow range. A turbulent flow occurs in the tubular reactor from a minimum flow velocity. This process is operated up to 100 ° C and at pressures up to 10 bar.

Mixing is necessary because the two starting materials fatty acid glyceride and Difficult to dissolve alcohol. A disadvantage of the methods used to date is the thorough mixing within the entire reaction zone around the space Increase time yield (RZA). This increase is due to a significant Bought energy use. This turbulent flow also makes separation difficult the glycerin / alkyl ester phase and leads to high sales in tubular reactors larger reaction volumes.

The technical object of the invention is to provide a continuous process for Producing lower alkyl esters with large RZA to develop which is less energy, Investment and maintenance costs required as well as easier and safer too operate as a previously known method.

This task is solved according to the invention in that the reaction in a laminar flow tube reactor is carried out. A solution from alcohol and catalyst is continuously introduced into the stream of fatty acid glycerides. The problem of the low solubility of the reactants in one another is solved by that before entering the reactor a fine dispersion of alcohol / catalyst Solution in the continuous phase (oil) is generated. By this measure the mass transfer between the two phases improves so much that on one further mixing within the tube reactor can be completely dispensed with. The value for the Reynolds number described in the introduction therefore remains in the reactor below 2300 in the laminar range.

The main advantages of the invention

The energy that is introduced into the reactor during turbulent flow changes mainly turns into heat through internal fluid friction. With laminar Flow conditions is this part of the energy dissipation through internal friction much less. This means that a significant proportion of the energy input is eliminated.

An important key figure in order to quantify the efficiency of a production plant is the space-time yield (RZA), which is defined via the mass flow of the target product ≙ _P and the reactor volume V _R.

Another advantage of laminar pipe flow is the fact that Reactor volume to achieve the same residence times compared to the turbulent one Flow, can be greatly reduced because of the flow velocity Reynolds numbers Re <2300 is considerably lower than in a reactor Reynolds numbers from 4,000-10,000. That way they can Investment costs can be reduced by the smaller size of the reactor and it the RZA is increased at the same time. Decreased with decreasing flow velocity also the performance of the pumps used, which leads to a further reduction contributes to the energy requirements of the process.

The reaction products of alkyl esters are formed when fatty acid glycerides are reacted and glycerin. These two components are difficult to dissolve and form in one another therefore independent phases as the reaction progresses. For further These phases must be separated from one another during processing. This happens in the Generally in a separator downstream of the reaction stage. The With laminar process control, deposition time can be achieved without additional Shorten devices because only very small shear forces on the newly formed Phase and can therefore grow unhindered during the reaction. If the reaction is turbulent, the newly formed phase is replaced by the flow-related shear forces repeatedly destroyed.

example

Deacidified rapeseed oil with a volume flow of approx. 1 l / h at a temperature of 50 ° C and ambient pressure in a tubular reactor implemented.

A solution of potassium hydroxide and methanol is metered into the oil stream at 25 ° C. The ratio of methanol (MeOH) to fatty acids (FS) is 1.8 mol _MeOH / mol _FS , ie 1.8 mol methanol are added per 1 mol fatty acid contained. The ratio for the catalyst is 0.05 mol _KOH / mol _FS . After introduction, this methanol / catalyst solution is divided into drops with an average diameter of d _SV = 0.3 mm using five static mixers (SMX mixer from Sulzer ChemTech AG) and distributed over the reactor cross section. The resulting droplet collective was characterized by the Sauter diameter (d _SV ), which divides the droplet collective under consideration into droplets of the same volume, so that the surface of these droplets corresponds exactly to the surface of the droplet collective. The Reynolds number of the pseudo-single-phase mixture is approximately 160 at the reactor inlet and approximately 50 at the reactor outlet. With a residence time of 14 minutes, conversions of approximately 95% were achieved with yields of approximately 93%, with partial separation of the glycerol.

Claims (2)

1. Continuous process for the preparation of lower fatty acid alkyl esters, by reacting fatty acid glycerides with lower alcohols, in a tubular reactor using a homogeneous alkaline catalyst, at a reaction pressure of up to 10 bar, at a reaction temperature of up to 100 ° C, with an average residence time of the reactants in the reactor from 5 to 60 min, characterized in that a premixed solution of catalyst and alcohol is introduced into the stream of fatty acid glycerides before the reactor enters in such a way that a fine dispersion forms in the continuous phase and the flow through the reactor is adjusted in such a way that the Reynolds number when viewed quasi-homogeneously at the reactor inlet Re <2300, preferably Re = 160. 2. The method according to claim 1, d. G., that the number of static mixing elements to produce the fine dispersion 1-10, preferably 5.