Because in the oleochemical production process, the crude fatty acid obtained usually contains impurities such as odor, pigment matter and short-chain fatty acid. The presence of these substances in the fatty acid will affect its odor, color and use purpose. They can be distilled The method to be removed. Since unsaponifiable matter is formed under high temperature, long time and exposure to oxygen, the distillation and purification of fatty acids must be carried out under high vacuum. For this reason, the continuous distillation process of fatty acid distillation processing and the corresponding production equipment have emerged to obtain tasteless and light-colored products. At the same time, a heat recovery system is specially designed to reduce energy consumption.

The process mainly includes four sections: degassing, light end separation, mixed fatty acid distillation and residue distillation to recover fatty acids. The advantages of this process are:

In order to prevent the oxidation of fatty acids in further processing, the crude fatty acids are efficiently dried and degassed.

The low boiling point, colored and odorous substances are removed at a high reflux rate in the pre-cutting tower to reduce the loss of fatty acids.

Two-step continuous fatty acid distillation: In the first step, about 90% of the fatty acids are evaporated in this step. Due to the mild distillation conditions, the unsaponifiable form (polymerization and decomposition of fatty acids) in the main distillation vessel is greatly reduced. Under this process conditions, a distilled fatty acid product with high quality and purity that meets the high requirements of the international fatty acid market will be obtained. . In the second step, the residue will be squeezed to reduce the free fatty acid content to less than 20%. Due to the high heat load during the distillation, the quality of the distillate from the residue distillation will be lower than that of the distillation step 1. Therefore, the distillate 2 from the residue distillation 2 will be collected and discharged separately. To ensure that the quality of the distillate is better and more stable.

With the help of high-efficiency mist traps, non-volatiles can be prevented from being mixed into fatty acid products.

By applying high vacuum and using a falling film evaporator, the distillation conditions are mild and the product is prevented from overheating.

The equipment adopts clamp flange connection, fully enclosed or magnetic drive pump to ensure that there is no oxygen contact with the product during the production process.

By recovering the heat of condensation in the gasified fatty acid, 0.3MPa steam is produced in a main condenser to maximize heat recovery. In addition, the fatty acid distillate is cooled by heating the crude fatty acid. Through this operation, the process achieves the maximum heat recovery effect.

1.2 Process flow description

1.2.1 Degassing and drying

The crude fatty acid mixture is sent from the tank area or pre-equipment to the degassing and drying equipment, where it is heated to the process temperature with steam in the heat exchanger. Any moisture in the feed will flash volatilize in the degassing drying tower, and the unpleasant impurities, moisture and air contained in the volatile gas will be condensed and discharged by the vacuum equipment. The dried, degassed crude fatty acid accumulates at the bottom of the degassing/drying tower and is pumped into the pre-cutting tower.

1.2.2 Pre-cutting tower

Before the crude fatty acid enters the pre-cutting tower, it is heated by the heat exchange of the main distilled fatty acid fraction from the condenser (heat recovery) in the heat exchanger, and at the same time, it is further heated by the heat-conducting oil in the heat-conducting oil heater to cut and separate. temperature. The pre-cutting tower is a structured packing tower, which contains two sections of stainless steel structured film packing. The lower part is called the stripping section, and the upper part is the adjustment section. The crude fatty acid enters the tower at the cutting temperature and is distributed on the packing by the distribution plate. The cut crude fatty acid is collected in the bottom liquid of the tower, and circulated by the falling film evaporator heated by the heat transfer oil by the pump. The evaporated fatty acid rises through the packing section to the top of the tower.

The fatty acid vapor is cooled in the condenser. The condenser is cooled by temperature-controlled cooling water (hereinafter referred to as TCW). The distillate, or pre-cut light end, is mainly composed of low molecular weight fatty acid light end and unsaponifiable matter, which can be pumped into the tank area with a piston pump. In order to effectively separate the light ends, a certain amount of distillate in the tower should also flow back into the top of the tower. The entrained long-chain fatty acid gas is recovered by condensation. The cut crude fatty acid is pumped into the fatty acid flash tower with a circulating pump.

1.2.3 Fatty acid flash tower (first distillation)

The flashed material liquid is collected at the bottom of the fatty acid flashing tower, and circulated through the falling film evaporator through a circulating pump. The falling film evaporator is heated by heat transfer oil and maintained at the distillation temperature and vacuum required by fatty acids. The pressure of the system is about 0.5kPa-0.6kPa, and the temperature is 220°C-230°C. By means of the instantaneous volume expansion of the mixed fatty acid heated in the falling film heater in the flash tower, the flashing effect of the fatty acid is achieved.

The rising fatty acid vapor is separated by the demister at the top of the fatty acid flash tower and enters the main condenser. The main condenser can also be called a so-called steam-producing boiler device. The fatty acid vapor from the distillation tower is condensed in the shell side by the natural circulating cooling water in the tube side. It means that the evaporation heat of the fatty acid vapor is transferred to the tube-side cooling water of the steam-producing boiler, and low-pressure steam of about 0.3MPa is generated. Through the associated pressure relief tank, the steam is separated from the added steam generating boiler cooling water (BFW) and can be used in the required low-pressure steam system. Fatty acid vapors that fail to be condensed in the main condenser will be condensed in the final condenser. The final condenser is cooled by TCW. Since the fatty acid II cooled by the cooler is relatively low in purity, it is not returned as a product to the degassing/drying tower for re-distillation.

The fatty acid distillate I from the condenser enters the heat exchanger to recover heat and is discharged. The final cooling of the distillate takes place in a fatty acid cooler by means of TCW. And from this, the fatty acid distillate product is transported to the tank storage tank. The residue with high boiling point is continuously removed from the circulating residue of the fatty acid flash evaporator under the control of the computer program, and is continuously and quantitatively sent to the falling film evaporator to recover the residual fatty acid contained.

1.2.4 Residue Distillation (Second Distillation)

The residue liquid diverted from the fatty acid flash evaporator enters from the upper end of the falling film evaporator. It is mixed with the circulating residue in the residue distiller. The liquid film flows down from the tube of the falling film evaporator. The evaporated fatty acid and the non-evaporating residue material in the residue distiller are separated. The fatty acid vapor from the residue still is cooled in the condenser. This condenser is also a so-called boiler steam generating device, similar to the main condenser. The steam from the residue still is cooled in the shell side by natural circulation boiler cooling water (BFW) in the tube side.

The non-condensate from the condenser is pulled into the final condenser by the vacuum system for final cooling, and the non-condensate enters the vacuum device. In order to maintain the vacuum of the flash distillation tower (the first distillation) and the residue distillation tower (the second distillation) and prevent fluctuations, the vacuum of each section can also be designed to consist of two separate steam injection systems. The fatty acid distillate II from the condenser is returned to the degassing/drying tower to re-distill fatty acids. The residue is quantitatively discharged from the residue product circulating in the residue distiller by means of a computer control system. The discharged residue is first collected in the residue receiving tank, and then effectively circulated and cooled by the circulating pump through the heat exchanger, and then the residue is discharged into the residue collecting tank.

1.2.5 Vacuum system

The vacuum of different sections will be completed by a multi-stage steam jet vacuum system. The cooling water of the atmospheric condenser of the vacuum system is circulated by the pump from the hot well through the plate heat exchanger cooled by the cooling water tower. In order to ensure the continuous operation of the process, two plate heat exchangers are installed for standby. When fatty acid fouling occurs, the other can be cleaned up during one operation. This certain indirect vacuum cooling device prevents the water pollution of the cooling tower and ensures the minimum waste water.

1.2.6 Heat transfer oil system

The energy of fatty acid distillation is supplied by the thermal circulation of the heat transfer oil system through the falling film evaporator and preheater (indicated by HO).

1.3 Indexes and consumption of raw and auxiliary materials for continuous distillation of crude fatty acid

1.3.1 Quality of raw materials

All data given are based on processing crude fatty acid mixtures obtained by high- or medium-pressure continuous hydrolysis, with impurities and moisture content of less than 1%. For fatty acids obtained by soapstock hydrolysis or intermittent hydrolysis, various indicators may vary due to their low quality. But it is undoubted that this process can be processed.

1.3.2 The quality of finished distilled crude fatty acids

After the equipment is started, when the quantity and quality of the auxiliary materials meet the requirements and the supply is continuous and uninterrupted, the device can obtain a purity greater than 99.5% under the conditions of self-adjustment and continuous operation; the color (Lovibond 133.4mm) is less than R1.0 , Y10; Distilled fatty acid products with an acid value of about 196mgKOH/g-198mgKOH/g (related to the fatty acid composition of the oil itself).

1.3.3 Consumption indicators of excipients

The consumption index of auxiliary materials is calculated based on processing 1t crude fatty acid: steam (1MPa gauge pressure) 50kg, steam (0.3MPa gauge pressure) 30kg, electricity 20kWh, cooling water 25m3, fuel oil 20kg, compressed air maximum 8Nm3/h, boiler water approximately 180kg, 140kg of steam produced (0.3MPa gauge pressure).

2 Results and discussion

A large number of modern control and measurement technologies are used in this process, such as Coriolis mass flow meters, pressure level gauges, and PLC systems. The use of these technologies not only saves the manpower investment in operating equipment, but also enables the production process The achieved technical parameters are more stable and accurate, eliminating the instability of process parameters caused by human factors. In actual continuous production, only two people are required to monitor the system, and remove the residue regularly and add exhausted auxiliary materials.

Since the purity of the distilled fatty acid is closely related to the cooling temperature, the automatic control of the steam boiler cooling water (BFW) control technology is used to control the cooling temperature of the fatty acid, which not only improves the purity of the distilled fatty acid, but also satisfies the requirements for the heat recovery of the system. Greatly reduce production and operating costs. The use of two sets of boiler steam-producing cooling devices can use the generated 0.3MPa auxiliary steam for the preheating of crude fatty acids, so that heating steam is no longer needed in the continuous production of distillation, and there is surplus steam that can be used for other factory production aspect.

The process is carried out in a continuous manner, and its advantages are not only the large equipment processing capacity, which can be 20t/d-350t/d as required, but more importantly, the continuous production is extremely conducive to the design and use of heat recovery and utilization. Not only that, because it is a continuous production, all distillation parameters are always consistent, and a consistent quality of distilled fatty acid fractionation can be obtained. The fatty acid fraction recovered to ensure the yield is continuously re-injected back into the system, which will not cause the loss of yield due to the emphasis on the quality of the finished product, which is an unavoidable shortcoming in the fatty acid batch distillation method.

A falling film evaporator is used for the distillation of fatty acid residues, which not only has high evaporation efficiency, but also helps to avoid fatty acid polymerization and decomposition under severe evaporation conditions. Judging from the consumption of auxiliary materials, the consumption of energy and auxiliary materials per ton of fatty acid in this method is much lower than that of the intermittent method. All incompletely cooled fatty acids and fatty acids recovered from residues are cooled and recovered by the final temperature-controlled cooling water (TCW), which not only ensures the distillation and purity of the process fatty acid product I under mild conditions, but also ensures the distilled fatty acid of the entire production process Yield.

In summary, no matter from the perspective of improving product quality or reducing operating costs and improving the economic benefits of the plant, the continuous distillation process of crude fatty acid has an incomparable advantage over the intermittent fatty acid distillation method. It is an advanced fatty acid distillation production process worth adopting.