Twin Roll Press VS Vacuum Drum Washer: Which Is Cost-efficient?

The twin roll press washer is favored by paper pulp mills because of its large capacity, wide adaptability, high washing efficiency, and small floor space. However, its high price has made some companies hesitate, especially during the construction period. The investment amount has become the focus of the company’s attention. It is often neglected or not carefully studied the operating costs after the completion of the project, and instead uses a cheaper one – vacuum washer. CNBM compare and analyze the consumption of water, electricity, and steam in the production process and the discharge of wastewater, and for new construction, reconstruction and equipment upgrade reference.

Bleaching process

The bleaching process based on the vacuum washer is shown in Figure 1. The pulp in the cooking digester is filtered and then washed into three series vacuum drum washers. The washed pulp is passed through a medium-concentration pump, a steam heater and the pulp and oxygen mixer enters the oxygen reaction tower for the delignification reaction. After the reaction pulp is discharged to the blow tank through the top, and the pulp is diluted to a concentration of 3.5% and then sprayed to the series of vacuum washers, the washed pulp enters the pulp tower, and then washed into a bleaching section by a vacuum washer. The bleaching process uses D0-E op-D1 three-stage bleaching, and each section is equipped with a vacuum drum washer. The bleached pulp is pumped to the pulp tower for use in the next section.

The bleaching process based on the two roll press washer is basically similar to that based on the vacuum washer, only the position and quantity of the washer are different. The process is shown in Figure 2. Two sets of two-roller press washers are provided before and after the oxygen delignification, and each section of the bleaching section is equipped with a two-roll press washer.

Comparison of production costs

The production cost generally includes raw materials, chemicals, water, electricity, steam, and labor. In order to compare the production costs of vacuum washers and twin-roll washers in a better way, this paper only deals with the analysis of water, steam and electricity consumption, while considering wastewater treatment and environmental issues, the discharge of wastewater from the pulp bleaching section was also analyzed. Filters that are common to two processes are not included in this comparison.

Comparison of clean water consumption

The water consumption in the paper pulping process is large, and in order to reduce the water consumption during the washing process, the pulp is usually washed in a countercurrent manner. According to the process flow of this paper, there are three points of dilution water: the first point is at the last washing machine after oxygen delignification, and the light condensate is added as washing water (when the amount of light sewage is insufficient, then add hot water as washing water), the dilution factor is 2.5m3/AD t; the second point and the third point are respectively in the Eop section and the D1 section washer, and the filtrate is diluted water, the dilution factor is 2.0m3/ADt .

Oxygen delignification: The washing water of the last washing machine as shown in Figure 3. The dilution factor is 2.5m3/ADt, and the concentration of the inlet and outlet of the vacuum washer is respectively 1.5% and 12%. The calculated amount of washing water is 9.1m3/ADt, while the working concentration of the twin roll press washer is high. The inlet and outlet pulp concentrations were 7.0% and 32%, respectively, and the amount of washing water was calculated to be only 4.4 m3/AD t. Similarly, the amount of dilution water used in other addition points is shown in Table 1.

table1

It can be seen from Table 1 that when the dilution factor is 2.0m3/AD t in the E op segment and the D1 segment, the amount of washing water of the two-roll press washer is 3.9m3/AD t, and the amount of washing water of the vacuum washer is 8.6m3/AD t. The total amount of washing water of the two-roll press washer is 12.2 m3/AD t, and the total amount of washing water of the vacuum washer is 26.3 m3/AD t. The consumption of clean water of the two-roll washer is reduced by 14.1 m3/ADt.

Comparison of steam consumption

During the pulp bleaching process, the consumption of steam is mainly used for the heating of the pulp. The reaction of the pulp and the chemical needs to be carried out at a certain temperature, and the pulp in some stages of the pulping process needs to be heated by steam to reach a suitable reaction temperature, such as an oxygen delignification section and an Eop section.

Taking the oxygen delignification section as an example, as shown in Fig. 4, the vacuum washer is dehydrated by vacuum, in order to prevent the filtrate from vaporizing in the water leg, the operating temperature of the equipment does not exceed 85 ° C, that is, the pulping temperature of the vacuum washer is lower than that. 85 ° C, usually oxygen desorption temperature is 95 ° C, you need to increase 10 ° C to reach the oxygen desorption temperature; the twin roll press washer relies on displacement and press dehydration, operating temperature is higher than 90 ° C, the pulping temperature is about 90 ° C, the pulp temperature only needs to be increased by 5 °C. According to this calculation, the double-roll press washer can save about 60kg of steam per ton of pulp compared to the vacuum drum washer.

Comparison of power consumption

The power consumption is also an important indicator in the production cost. The installed power of the vacuum drum washer and the twin-roll press washer is shown in Table 2. Since the roller of the twin-roll press washer is driven by a hydraulic motor, the power of the hydraulic system motor is 110 kW, and the drum of the vacuum drum washer is rotated by the motor drive gearbox, and the motor power is small, only 37 kW. After comprehensive calculation and comparison, the total installed power of a twin-roll press washer is 62.8kW higher than that of the vacuum washer.

table2

Except for the washing equipment, the pulp washing requires other auxiliary equipment, such as a stirrer, a mixer, a filtrate pump, etc. Take the D1 segment as an example to compare the total installed power of the D1 segment, as shown in Table 3. In the auxiliary equipment, the medium-concentration pump and the pulp pump of the twin-roll press washer are equipped with a large power of 160 kW and 200 kW, respectively. Because of the equipment requirements, the vacuum washer has a lower pulp concentration of only 1.5%, while the bleaching tower has a pulp concentration of 12%, which requires a large amount of filtrate for diluting the pulp, so the filtrate pump has a higher power. Up to 305kW; comparatively speaking, the twin-roll press washer has a pulp concentration of 7% and a small amount of dilution water. The required filtrate pump has a small flow rate and low installed power. It can be seen from Table 3 that the installed power of the D1 section is 106.2 kW lower than that of the vacuum washer.

table3

Although the installed power of the twin-roll press washer is high, only 7 sets are required in the production line, and 9 sets of vacuum washers are equipped with other auxiliary equipment. After preliminary calculation, the total installed power of twin roll press is lower than that of vacuum drum washer, about 1500kW, and the electricity consumption of is about 35kW.h/ADt.

In conclusion

According to the preliminary comparison, the washing and bleaching production line equipped with twin roll press is better than that of the vacuum washer. It not only reduces the production cost of the enterprise but also reduces the bad influence on the environment.

In addition, the twin roll press has a small volume, a small construction area, and a filtrate tank volume, and the floor height and load requirements are lower than that of the vacuum washer, which reduces the construction cost of the plant.