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Ethanol Based Biodiesel

Ethanol Based Biodiesel

OPTIMIZATION OF A BATCH TYPE ETHYL ESTER PROCESS

ABSTRACT

Conversion of rapeseed oil into ethyl esters for use as Biodiesel fuel involves transesterification of the oil triglycerides to mono-esters of the component fatty acids. To accomplish this conversion, raw rapeseed oil is treated at room temperature with ethyl alcohol in the presence of potassium hydroxide as a catalyst. During the process, the glycerol which is produced is insoluble in the ester product, and being heavier, settles out carrying most of the dissolved KOH catalyst with it.

Upon initial settling, some of the undesirable, emulsion-forming by-products may remain in the ester layer, causing problems in the washing stage. It was discovered (by tracking the process with a glycerol determination) that most of these products could be removed by simply restirring the glycerol into the ester, adding water and letting the mixture settle out again. After draining off the glycerol/water layer, the product (ethyl ester) can be easily water-washed to remove residual alcohol and potassium.

INTRODUCTION

Processing

Transesterification of rapeseed oil at the University of Idaho from 1980 to 1990 used methanol as the alcohol. Methanol is highly toxic, does not produce a visible flame when burning, can be absorbed through the skin, and is 100% miscible with water, so any kind of spill presents a serious problem. Ethanol provides the advantage of making a Biodiesel fuel produced entirely from renewable resources. The use of ethanol in Biodiesel production has not been studied as extensively as has methanol.

Water washing the ester was accomplished through sprinkling water into the tank at an approximate rate of 100 gallons per hour. As the water droplets travel through the ester, they remove the impurities. Washing would continue for 20 to 30 hours consuming as much as 3,000 gallons of water.

Oil Seed Press

Two commercially manufactured screw type oil expellers are used for extracting oil from the rapeseed for this project. The seed is manually fed into a 100 pound capacity tapered bin atop an auger. The seed is heated as it moves up the auger to the screw type press. A retention time of approximately 20 minutes is required for the seed to be heated before the oil is extracted. These two presses with augers were mounted on a movable base that were 12 feet in length, 4 feet wide, and 5 feet in height. Due to the size of these platforms and the limitations of shop space only one press could be feasibly operated at a time.

Reactor

A 290 gallon cross-link polyethylene reaction tank and a 50-gallon plastic rubbermaid barrel were used prior to this grant. The reactor tank was capable of producing 200 gallons of ester per week. A sink type drain was used with gaskets in the bottom of the reactor tank to drain the glycerol and then the wash water from the ester layer. The drain gasket was continually leaking and the cross-link polyethylene reaction tank was warping due to material compatibility with ester and glycerol. Fluids were pumped through the use of a centrifugal pump which required a hand primer pump to transfer the alcohol from the plastic barrel to the reactor tank.

MATERIAL AND METHODS

Processing

Reactants

The reactants for the transesterification process are used in the following previously determined proportions in U.S. and metric units:

Raw rapeseed oil 100 L 100 Kg 100 Gal
Anhydrous Ethanol 27.4 L 23.74 Kg 27.4 Gal
Potassium Hydroxide 1.30 Kg 1.43 Kg 10.83 lb

The input amount of raw rapeseed oil determines the batch size, and the other components are calculated from the following formulas:

EtOH = 0.2738 x RO
KOH = 0.013 x RO

where;

EtOH = amount of ethanol required, in liiters
RO = the desired amount of oil to be processed, in liters
KOH = amount of KOH required, in kg

According to these formulas ethanol is added at a 65% stoichiometric excess, or a molar ratio of 5.0: 1 EtOH to oil. The KOH is added at 1.43% of the weight of input oil.

Quality of Reactants

1. Rapeseed Oil Best when clear (filtered) because excess sediment collects on the bottom of the reaction vessel during glycerol settling and at the liquid interface during washing. This sediment interferes with the separation of liquid phases and with the washout of catalyst, and may tend to promote stable emulsion formation. Slight haziness of the oil probably does no harm. The original oil must be water-free, because every molecule of water destroys a molecule of catalyst, thus decreasing its concentration.

2. Ethanol. The nearer to absolute (200 proof), the better. Gasoline present in the alcohol as a denaturant appears to do no harm. The reaction proceeds satisfactorily in mixtures of 200-proof ethanol with 10%(v/v) or more gasoline present. However, even small quantities of water (less than 1%) can decrease the extent of the conversion reaction enough to prevent the separation of glycerol from the reaction mixture.

3. Potassium Hydroxide Catalyst. Best if it has > 85% KOH. Even the best grades of KOH have 14 to 15% water (which cannot be removed). It should be low in carbonate, because potassium carbonate does not serve as a satisfactory catalyst, and may cause cloudiness in the final ester.

Other catalysts which may be used are potassium ethoxide and sodium ethoxide, but they are prohibitively expensive. Sodium hydroxide was not a suitable catalyst because it was not aufficiently soluble in ethanol and it tends to promote undesirable gel and emulsion formation during transesterification.

The Reactions

1. The first step is to activate the ethanol by dissolving the potassium hydroxide to form potassium ethoxide. Stir vigorously in a covered container until the KOH is dissolved, approximately 20 minutes. Protect as much as possible from atmospheric CO(2) and moisture, both of which reduce the activity ofthe catalyst. The entire portion of ethanol is used here. (There is enough ethanol to accomplish the complete transesterification, with about 65% in excess.) Preparing this solution is, in effect, preparing a solution of potassium ethoxide according to the following reaction.