Unbleached pulp has a brown colour due to the remaining lignin of the mechanical pulp. As a result, it must be bleached to achieve the required quality for producing white paper.
Bleaching is a process which improves the purity and brightness of fibre suspensions using chemicals.
The process
Usually, a bleaching plant comprises the reaction container and machines, the piping, the equipment for refilling, storing and conditioning the bleaching chemicals, and the control system for pulp bleaching. A bleaching plant is normally made up of several bleaching stages, for economic reasons.
A conventional multi-stage bleaching plant comprises several bleaching towers and peroxide stages with downstream washing units. Using multi-stage systems on one hand reduces the expenditure regarding chemicals and heat, but on the other hand requires specific bleaching chemicals and processing conditions to achieve some targets (e.g. a high degree of whiteness combined with a low stability loss of the fibres).
Coloured pulp is conveyed from the digester via the delignification to the first stage of the bleaching system. The pulp reaches the first bleaching tower, where a hydrostatic pressure transmitter and electromagnetic flowmeters provide for constant levels and optimal production rates. Additionally, the chemicals are supplied to the bleaching tower.
Electromagnetic flowmeters are used for dosing control. Pressure transmitters, temperature sensors and pH measuring systems monitor the chemical reactions and reduce both quality variations and the energy cost.
After this, the pulp reaches the washing unit, where the unrequested bleaching products are removed by a wash filter. Conductivity measuring systems ensure an optimal washing process and provide for constant balancing of the salt content in the process. Filter and sieve monitoring for long-term efficient filtration is realised using differential pressure transmitters.
ABB Instrumentation
The process described here is repeated in the following oxygen and peroxide stages.
ABB offers instrumentation which is used in every phase of the bleaching process. Electromagnetic flowmeter FSM400 is suitable for multi-phase liquids, providing high performance due to DSP technology and short response time and undistributed output. It also offers local indication of measuring value and integrated monitoring and diagnosis functions.
ABB’s temperature sensor TSP121 with integrated head-mounted transmitter TTH300 features a universal design with various process connections. One of the advantages is that measuring inserts can be replaced during operation.
The 261G pressure transmitter offers a wide range of possible spans, and is compact with high operational reliability. It also allows for various process connections and is optimised for harsh process conditions. It comes in a robust stainless steel housing.
When the process requires a high-performance transmitter for the smallest measuring spans, ABB offers the 266 differential pressure transmitter. It has a high base accuracy of 0,04% and guarantees long-term stability.
pH/Redox measuring system with transmitter TB82pH and sensor TB557 is recommended due to the high-performance, long-life cycle and the minimum maintenance that it requires. The client has the benefit of selecting from a range of electrode models which are resistant to deposits.
Conductivity can be measured by using ABB’s TB82EC and TB465 sensor, which is a robust sensor and easy to install. It has a high pressure and temperature resistance which makes it ideal for seven paper bleaching processes.
For more information contact ABB South Africa, +27 (0)10 202 5000, [email protected], www.abb.com/measurement
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