Roots-water ring vacuum pump set in butyl acrylate unit

Roots – water ring vacuum pump in butyl acrylate plant of application

Refining unit of vacuum system for existing butyl acrylate device using the steam jet vacuum pump, operation cost is higher, and the influence factors of vacuum fluctuations caused more the situation, to study the feasibility of using other vacuum equipment replacement, mainly studied the application of roots – water ring vacuum pump, and economy analysis, provide a reference for devices of the future.

Because of butyl acrylate (BA) containing vinyl this property, when external conditions change, such as temperature, residence time increases, rising concentrations, phase change, initiator, etc.) are prone to aggregate, which is especially great for the influence of the polymerization, the rise of temperature, BA refined system adopt the reduced pressure distillation, such as pressure BA’s boiling point is 146 ℃, while 2.0 kPaA for 43 ℃, the boiling point of the slash effectively inhibit the aggregation. Due to the relatively low vacuum requirements, the existing BA refining system adopts steam jet pump as vacuum pumping equipment.

With the continuous increase of steam cost, the operating cost of BA device also rises. Besides, steam jet pump is adopted, and there are many factors affecting vacuum fluctuation (such as steam pressure fluctuation, nozzle blockage, atmospheric leg, etc.). In this paper, the scheme of using roots – water ring vacuum pump group to vacuum the refining system of BA unit is studied.

1. Vacuum requirements for the refining system of BA device
1.1 design value

The refining system of BA unit has two distillation columns, which are alcohol extractor column (C440) and purification column (C450). The vacuum design value of BA unit refining system is shown in table 1.

1.2 actual operating value

Shanghai huayi acrylic acid co., LTD. Now has 3 sets of BA production units, which are U400 unit, U2400 unit and U4400 unit respectively.

2. Water ring pump principle and ultimate vacuum degree

2.1 working principle of water ring vacuum pump

The impeller of liquid ring vacuum pump is installed eccentrically in the pump shell. Pour a specified height of liquid (working fluid) into the cylinder of the pump before starting. When the impeller rotates clockwise according to the arrow shown, due to the centrifugal force, the liquid is thrown to the pump body wall. The rotation of the impeller forces the working liquid to form a closed ring of approximately equal thickness along the inner wall of the pump shell, which is determined by the shape of the pump cavity. At this point, an airtight “air chamber” is formed between two adjacent blades, impeller hub and the inner surface of the liquid ring. Because the impeller in the pump shell is eccentric configuration, so the inner surface of the liquid ring and the impeller hub between the formation of a crescent shape space, it is divided into a number of blade volume is not equal to the small room, the volume of each small room with the impeller rotating action cycle to expand and shrink, when the small room volume gradually expanded, gas inhalation from the outside; As the volume of the chamber shrinks, the previously inhaled gas is compressed and expelled. In this way, the volume of the chamber between the blade and the blade changes once every rotation of the impeller. The liquid between the two blades moves back and forth like a liquid piston, continuously sucking gas to achieve the purpose of vacuuming. See figure 1 for details.

2.2 limit vacuum degree of water ring vacuum pump

The ultimate vacuum of water ring pump is limited by the saturated vapor pressure of water. When the temperature of water is less than 10℃, the ultimate vacuum can only reach 1.23kpaa. Although BA refining system can barely operate, C450 has little room for adjustment. Cavitation will damage the impeller, pump body and disc of vacuum pump.

Therefore, the continuous and stable operation of BA refining system cannot be guaranteed by using single-stage water ring pump.

3. Operating principle of roots pump

In the pump chamber, two “8” rotors are mounted perpendicular to each other on a pair of parallel shafts, which are driven by a pair of gears with a transmission ratio of 1 to rotate synchronously against each other. Between the rotor, between the rotor and the inner wall of the pump shell, maintain a certain gap, can achieve high speed operation. Because roots pump is a vacuum pump without internal compression, usually the compression ratio is very low, so high, medium vacuum pump needs the front stage pump. The ultimate vacuum of roots pump depends not only on the structure and manufacturing accuracy of the pump itself, but also on the ultimate vacuum of the front stage pump. Roots pump can be used in series in order to improve the vacuum limit of the pump. The operating principle of roots pump is similar to roots blower. Due to the continuous rotation of the rotor, the extracted gas is inhaled from the air inlet to the space V0 between the rotor and the pump shell, and then discharged through the exhaust port. Since the V0 space is completely closed after inspiration, the gas in the pump chamber is not compressed or expanded. However, when the top of the rotor turns the edge of the exhaust port and the V0 space is communicated with the exhaust side, due to the high pressure on the exhaust side, a part of the gas rushes back to the V0 space, making the gas pressure suddenly increase. As the rotor continues to rotate, the gas is expelled from the pump.

4. Feasibility study on the application of roots – water ring vacuum pump group in BA device

Roots vacuum pump cannot be used alone, must grade vacuum pumps in series with a front, to be the pressure in the pump system is the front-end of the roots vacuum pump vacuum pump to allow inlet pressure, the roots vacuum pump can begin to work, and in the general case, roots vacuum pump work do not allow the high pressure difference, otherwise it will overload and overheating and damage, so using the roots vacuum pump must be reasonably choose the former vacuum pump, ultimate vacuum requirements between 133 ~ 1333 pa can choose reciprocating vacuum pump or liquid ring vacuum pump as the backing pump. Roots water ring vacuum unit is the roots pump as the main pump, water ring pump as the front pump series.

The ultimate pressure of a root pump and a water ring pump in series vacuum unit is about 400Pa. If two roots pumps are combined with water ring pumps in series, the ultimate pressure of the unit can be greatly increased to 25PaA. The vacuum degree of BA unit refining system is less than 1000PaA, which can meet the production requirements. Therefore, the BA unit refining system only requires a roots pump and a water ring pump in series.

5. Implementation plan
Because of the use of single-stage water ring vacuum pump, even if the pump sealing water temperature is less than 10℃ (the sealing fluid chooses frozen A water), the ultimate vacuum degree is 1.23kpaa. In order to further reduce the vacuum degree to below 1kPaA, it is proposed to consider the use of water ring pump and roots pump in series, and the vacuum degree can reach below 1kPaA to meet the production requirements of BA refining system. Now consider C2440, C2450 separately using A set of roots – water ring vacuum pump, water ring pump seal liquid chooses A frozen water), such C2440 noninterference, C2450 vacuum, 2 for 1, it is proposed in the actual operation to test A set of vacuum unit C2440, C2450 vacuum at the same time, to save operating cost, provide the basis for other devices of the future.

6. Run loss analysis of effective components in tail gas
The running loss of organic matter in the tail gas is mainly determined by the temperature and flow rate of the tail gas. The temperature determines the partial pressure of organic matter, namely the concentration, and also the concentration of condensate. The main components of exhaust gas in the original design of U400 unit are BuOH, BA, water and AIR. The flow rate and composition are shown in table 3.

7, the conclusion
(1) roots – water ring vacuum pump unit can meet the production requirements of butyl ester refining system.

(2) operation cost of roots – water ring vacuum pump unit is better than steam jet pump.