1 reply to this topic

[Aaron Guerra]

HI

I'm asking for help to know what kind of trays or pack i need for a precut of fatty acids, we work with tallow as raw material and the only thing that we need is to remove color, and odours from the fatty acids, or only the light end. We have a column of 9 mts that we want to fix to this process so that's what i'm asking this.

If you have some aclaration or article or text to share i'll be grateful.

AG

[wnovaes]

Dear AG,

First of all, let me introduce myself. I’m working in design of fatty alcohol unit and fatty acids unit from Palm Kernel Oil (PKO) and Palm Stearin (PS). Now we are in the construction stage. So, I don’t have any practical experience in operation of precut and total distillation of fatty acids yet.
As you were mentioned, color and presence of smelting compounds are inherent problem in fatty acids specification independent of raw material is used for. The tendency of product to darkness is associated to the thermal decomposition of fatty acids due high temperatures and residence time manly at distillation column bottom. So, is important to bear in mind, to design a distillation column for low operation pressure and low pressure drop through internals, therefore, high efficiency structured packing is recommended for this kind of service. The operation pressure and maximum pressure drop allowable will be dependent of the maximum bottom temperature permissible to avoid thermo decomposition of your fatty acids. For fatty acids from PKO and PS the maximum temperature is around 235 oC. Another point to be taking into account is the reboiler design. For thermal-sensitive compounds, are important to not keep a high temperature difference between the bottom re-circulating feed through reboiler and heat medium, as well, avoid a high retention time for column bottom product. In my case, should be avoid a temperature difference higher than 40 oC. For reboiler design, should be used a falling-film reboiler or a flooded reboiler with high recirculation flow rate, both approaches using forced recirculation. Another important thing to do is to maintain the column feed air free. So, is important to deaerating the column feed stream first, heating up to 105 oC and flash it at 70 mmHg should be enough.
The smelting characteristics are due to presence of incondensable compounds in fatty acids, in literature we find out like unknown compounds; however, I think they likely should be aldehyds from oxidation reactions during processing. For smelting treatment, should be provided a steam injection sparger at precut column bottom for helping to stripping the smelting compounds using low pressure steam injection and purged for the column overhead to the vacuum system. In the most cases, very low flow of steam at 3 barg steam is enough. However, after plant start-up field tests are recommended.
For the light ands, you need to perform a phase equilibrium calculations to match your required fatty acids cut and determine the number of stages needed for. In PKO and PS fatty acids precut column, the light ends are draw-off as a mixture of C6-C10 fatty acids. The main reason to cut off the lighter fatty acid is because they very unstable. I don’t know the specification of your final product, however, together with fatty acids come too not splitted oils or fats from the hydrolysis reactor and depending of the final product specification should be required a final purification to remove oils or fats. So, after the precut section should be required a purification column to remove oils or fats and until fatty acids with high chain length. For example, if your product has a specification with a fatty acids distribution of C12-C18, and the crude fatty acids produced by hydrolysis have longer chain than C12-C18. The design of final purification column should be taking into account the same guidelines used for precut distillation column as described before. One more thing is regarding the bottom heavy byproduct (the PITCH). In the most cases, for the final distillation column, do not possible to recovery all fatty acids product from the bottom because for that will be required a high bottom column temperature and therefore a final product with high color as well. So, fatty acids product will be lost from column bottom and depending of the amount will be required a secondary recuperation for recovery the fatty acids into the PITCH. A kettle heat exchanger should be reasonable as a secondary distiller. Normally, 1% of oils or fats are lost like not splitted matter and the PITCH byproduct has around 40%-50% of fatty acids. You could recovery up to 90% of the fatty acids lost at the final fatty acids purification column.
Beyond the separation stages, I recommend you to evaluate possibility of using two more packing sections, one for condensation and another one for scrubber. I’m recommending that, because likely the columns will operate at almost full vacuum and the pressure drop allowable through condenser should be very low, a choice for using a heat exchanger may be very expensive. The scrubber section will be to wash the incondensable gases to the vacuum system to avoid discharge of fatty acids to wastewater treatment system.
From the safety point overview, because the high temperature operation of these columns, leakages or air intake into the column will burn the fatty acids and leading a equipments damages and/or serious personal injury. So, to minimize potential hazards and operation problems, a good HAZOP and operability study should be conducted. A good practice, it’s to provide the most columns connections as welding than you can. It’s important too, make a right choice of material of equipment construction. In the most cases are used SS 317L. Another good idea is to provide a nitrogen injection at column bottom above maximum liquid level to make possible brake down the column vacuum to shutdown the column without air intake from overhead, as well, in case of vacuum system failure.
Take care with steam trace and insulation of the piping and equipments because the fatty acids could freeze at relatively high temperature. The PITCH from PKO and PS fatty acids, for example, may freeze about 90-100 oC. Should be required to provide a coil at inside bottom of all equipment may accumulate fatty acids during start-up and shutdown procedures.
I hope may help you. In a short future, I’d like we could share ours experiences in fatty acids process and operation.

Kind Regards,
WSN.