Pure triglycerides are odorless, but the oils obtained from different processes have different levels of odor, some smell of oil is loved by people, such as the fragrance of sesame oil and peanut oil, whereas others like rapeseed oil & rice bran oil’s offensive odor are disliked by people, and the offensive odor even affects the oil quality which should be removed. Some of these odors are natural, and some are generated in the production of oil and the process of oil storage. Odor components in natural oils are 0.1~1% and the odorous material is related to the amount of free fatty acids(FFA): the odor will be eliminated if the FFA in oil is reduced to 0.01~0.03%.
Deodorization makes use of the difference of odor and triglyceride volatility (vapor pressure) in oil to remove oil’s offensive odor by steam distillation under high temperature and high vacuum. Oil deodorization process can not only remove oil’s odor, enhance the smoke point of oil, improve the flavor of the edible oil, but also improve the stability & chroma & quality of the oil. Because it also removes FFA, peroxides, some heat-sensitive pigments, the rotten oil protein volatile decomposition products and pesticide residues. Thus, deodorization of oil is highly valued in the production of high-grade oil products and oil deodorization is the final process of edible oil refinery processing.
Steam distillation, also known as water vapor distillation or stripping, is a key step of oil deodorization. Its principle is small molecules odor components in oil evaporate more easily than triglycerides at high temperatures & under negative pressure. When inputting steam to pass through the odorous oil, the steam-liquid surface is in contact, and volatile odor components in oil would escape to the steam at the ratio of its partial pressure, then the steam is saturated with odors and would be discharged from exhaust outlet, so as to achieve the purpose of removing odor components.
Deodorizing equipment such as intermittent deodorization pot and continuous deodorization tower mainly adopt the principle of steam distillation to deodorize oil, but the intermittent deodorization machine has the shortcoming of long operation time, high steam consumption, and low heat utilization, which is about to be eliminated. Thus, continuous edible oil deodorizing tower is the commonly used deodorizer machine in oil refinery factory.
Continuous Deodorization Tower Introduction
The continuous deodorizing tower is the best choice especially for large-scale oil refinery factory with advantages of compact structure, high steam utilization, and easy maintenance. The type of continuous deodorizer tower has developed from plate deodorization tower (single shell) to tray deodorization tower (multiple-layer shell), while in recent years the packing deodorization tower and combined deodorization tower have also been introduced into production.
The tray deodorization tower is equipped with a vertical cylindrical shell and a series of trays to ensure all processes (heat recovery, final heating, and cooling, deodorization) are carried out in specific layers of trays, respectively. But with the growing demand for lowering the residence time at high temperature, a new process of tray deodorization tower combining with packing deodorizer method is developed, and it is particularly suitable for refinery factory with frequent oil variety replacement to process oil which has a complex source, poor quality and deep color, such as rice bran oil, tea seed oil.
Continuous Type Combined Deodorization Tower
The upper part of combined type deodorizer is packing section and the lower part is the tray section. The oil enters from the top of the tower and distributes evenly on the packing through the distributor pan to form an oil film. From top to bottom, oil flows by steam-liquid contact with the counter-current heated steam on the packing. The fatty acids and part of odor components are firstly distilled off in the packing section and drawn off through the vacuum, then the oil flows through the pipe into the tray section. The steam in the deodorizing tray is spared from the oil in bubble form and is contacted again with the oil for stripping. In the tray section, oil is deodorized by high vacuum reduction of odor components’ boiling point and by steam distillation, what’s more, the oil would be stranded for thermal decolorization, finally, it is discharged from the bottom of deodorization tower.
(1.) Low steam consumption
Since most of the fatty acids and volatile odor components are removed in the packing section, less distillation takes place in the plate section and the retention time of the oil in the plate section is also greatly shortened which is about 30 minutes. Thus, the steam consumption of new combined deodorization tower is very low which is about 0.5–1% of oil weight, only equivalent to 30% of the plate tower’s steam consumption.
(2.)Good product color
When the oil is stranded in the plate section, some of the thermosensitive pigments that can’t be removed by adsorption method are decomposed under high temperature & high vacuum to achieve the effect of thermal decoloration. Since the residence time is not long, the combined tower is less easy to lead to the emergence of new pigments than the plate tower. Therefore, the new combined deodorization tower produces a lighter color of oil than that of ordinary plate tower.
(3.) Good deacidification effect
The combined deodorization deacidification tower adopts the principle of physical deacidification. Most fatty acids are rapidly removed in 2~5 minutes in the packing section of the combined tower, therefore, there is no need to do alkali treatment in the pretreatment process, which avoids the loss of alkali refining process and reduces the refining consumption.
(4.) Flexible operation, low use & maintenance costs
The combined deodorization tower’s packing section and plate section are completely separated. People can adjust the most suitable temperature, amount of steam and residence time according to the oil quality, effectively avoiding the waste. For example, when processing low acid value oil that the acid content of refined oil is not concerned, you can directly put the oil into the plate section, not through the packing section, which saves the cost of deacidification.
First of all, the bleached oil is pumped into the deaerator at a constant flow rate through the pump and the flow of oil is regulated by the flow meter. In the deaerator, a small amount of air in the oil would be released under high vacuum degree of 3mmHg’s absolute pressure.
Then, the oil is pumped from the deaerator to an oil-to-oil heat exchanger, where exchanges heat with hot deodorized oil and is heated to at least 180℃.
After that, the oil flows from the oil-to-oil heat exchanger to the heat-treating facility and is heated to 260℃.
Next, the heated oil is pumped from the heat-treating facility to the packing section of combined deodorization tower, where fatty acids and other relatively volatile odor in oil is removed by stripping, and then the oil flows into the plate section for deodorization & thermal decolorization.
Afterwards, the deacidified and deodorized oil is discharged from the combined deodorization tower, and pumped into the heat exchanger for heating the oil that needs to be deodorized, then the oil enters the cooler and is cooled by water, after cooling, the temperature of the oil is not higher than 70℃.
Finally, the oil passes through the fine filter for assurance of the oil cleanliness, and then the refined oil is stored in the sealed storage tank.
|Deodorizing vacuum degree||3.28mbar|
|Oil intake flow||25.5t/h|
|Oil inlet flow temperature||225℃|
|Deodorized oil Flow||24.5t/h|
|Liquid level of deodorization tower||37.8%|
|Fatty acid temperature||60.1℃|
|Finished oil outlet flow temperature||39.7℃|
In the entire oil refinery process, deodorization is not a completely independent process and it depends on the coordination of other processes to achieve the ultimate goal of refining. For example: before deodorization, the oil need be bleached in the bleaching tower at the temperature of 110℃, and the temperature of filtration should be 100℃-105℃.
Then, if the bleached oil need be processed in the packing section of the deodorization tower, the oil shouldn’t contain bleaching clay or it will lead to bad gelatinization reaction in the deodorization tower, causing the coking of packing material, which not only affects the oil quality and flavor but also shorten the packing material’s service life.
Please note: When the deodorizing tower isn’t in use for a long time, the oil in the deodorization tower shall be completely drained & steam-cleaned to avoid residual film sticking on the surface of packing material & clogging the oil. In order to keep the capacity of deodorization, people should always check and remove residue from the bottom of the tower. The packing material should be removed for cleaning every 6 months to 1 year, and the maintenance is extremely easy to use.