No worries, this is common reaction system -and easy to get up and running. You can get it done by reading this: http://www.doiserbia.nb.rs/img/doi/1451-9372/2016/1451-93721600010K.pdf
Use of heterogeneous catalyst for biodiesel production requires a lot of care. First, the CaO must be synthesized properly and stored air-tight before use; otherwise, the calcination/synthesis would be reversed. Then, what you had as CaO would become Ca(OH)2 or carbonate. I would like to know how you synthesized the catalyst (at what temperature and for how long did you calcine? How did you cool and store before use?)
Secondly, I feel that the reaction condition used may not be appropriate. The catalyst concentration used (7.5g with just 50 ml oil = more than 10 wt.% of the oil) is too high. I suggest you reduce it to about 8%. The methanol to oil ratio also appears to be too high. How long did you allow for reaction? It should be up to 150 minutes.
Finally, ensure that the oil is treated till the acid value drops below 2 mgKOH/g and limit moisture content of the oil to almost 0.1%. You would encounter slight bubbling after about an hour of stirring if the method is effective; that's not necessarily soap formation. After the reaction, your catalyst would come out in colloidal form; that is also not soap. So, review your method and let us know.
The production of heterogeneous alkaline catalytic biodiesel is always more complicated than the homogeneous alkaline one. In order to succeed in this operation with a higher biodiesel yield the catalyst must be "well activated" (muffle activation at a temperature of about 400 to 500 ° C) and the residual oil must have a refining purification similar to that applied to the crude. Articles shown in the links on this portal can help you in this task.
Waste cooking oil should have very high acid value. Check the acid value first. If it is greater than 4 you need to use acid catalyzed esterification process. The molar ratio of oil and methanol should be 1:12 (roughly gm:gm). The amount of acid (H2SO4/HCl) should be 1% v/v of oil. This should be followed by transesterification. Same molar ratio for oil and methanol. For CaO catalyst you should use 1% wt/wt of oil. Put CaO beads in methanol and stir vigorously so that it mix well. Put the oil in the reaction chamber for few minutes to heat up properly then mix the mixture. The reaction should initiate instantly.
Thanks.
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You could think about the option of carrying out a homogeneous catalysis with which you would obtain good results and reduce the level of complexity. However, the cooking oil must be previously esterified to reduce the free fatty acid content. This guarantees that the transesterification yield will not affected by the soap formation (saponification reaction).
Dealing with heterogeneous catalyst is very tough. The catalyst must be prepared with care. But the most important thing to optimize any biodiesel is to optimize the chemicals for the reaction. You must use different quantity of alcohol and catalyst in order to find out the right combination. Kindly do that. I thing it would give you some result.
Did u synthesize d CaO ? If yes, check d process very well cos during the transfer of the calcined catalyst for cooling, reaction might have taken place to covert it to slake lime or chalk. This will definitely not activate the reaction wen used.