System Integration & Control Systems Design


Measuring pH at the headbox in the paper making process

March 2000 System Integration & Control Systems Design

Monitoring and controlling hydrogen ion activity (pH) of pulp stock is critical in the paper making process.A narrow pH range must be maintained during various phases of production. One area in which ph has a pronounced effect on final paper quality is at the headbox.

Background

The headbox is used to physically prepare the pulp stock before it is converted into paper by the paper machine. The pulp stock supplied to the headbox has a consistency of between 2 and 5% solids. Today's advanced paper machines can produce over 650 tons of paper per day. The paper is made in a continuous sheet which is rolled for shipment and eventually cut to size.

The front section of the headbox, called the stock distributor, receives the pulp flow from one or more inlet pipes and spreads the stock uniformly to a width equal to the final paper width. The central portion of the headbox dampens excessive pulp flow turbulences and inlet cross flows from the stock distributor, corrects flow inequalities and directs the stock into the slice which is the rear portion of the headbox. The slice controls the distribution of stock onto the paper machine. The stock flows between a stationary lip and a movable lip to ensure an even velocity across the width of the machine and to provide a constant, random fibre distribution.

Application

Preparation of pulp stock is crucial in achieving desired paper qualities. A pulp bleaching process may be used to change brown paper (its natural colour) to white paper. Also, the stock may be unbeaten or beaten. Paper made from unbeaten stock is generally not suitable for most uses, whereas paper made from beaten stock has a high density and good strength. Bleaching and beating are an integral part of stock preparation with both substantially affecting the final product.

The pH of the stock does not affect the final product as dramatically as bleaching or beating but it is critical in obtaining desired printability. Most paper produced will have printing ink applied to it. If the paper is too acidic, it will not take printing inks very well.

Pulp stock pH also affects the distribution of paper fibres. At very low or very high pH levels, paper fibres tend to flocculate (form in clumps) which adversely affects paper quality. Other factors affected by pH include:

1. Surface characteristics.

2. Foaming.

3. Brighteners.

4. Shade and dye retention (each dye has an optimum pH range for maximum tinctorial value).

The pH of the stock is measured in the pulp stock line which feeds the stock distributor. Depending on the design of the processing equipment, a variety of hardware is available for mounting the pH sensor into the line. Since paper making is a continuous process which cannot be interrupted, insertion mounting is the preferred method. An alternate approach is to flow-through mount the sensor in a bypass line which has isolation valves to allow routing cleaning and replacement if necessary. Most plants require a continuous 4-20 mA signal proportional to the pH because they typically have control centres that also accept other important monitored parameters from locations throughout the plant.

Summary

The pH measurement at the headbox is the final pH check and the last chance to alter the pulp stock before it becomes paper. pH is one of many variables that can significantly influence the characteristics of paper, so it is important that an accurate, reliable and continuous measurement is made.

Instrumentation

A number of different types of sensors can be used for this application. Selection criteria include cost, convenience, mounting style and personal preference.

p System 1 (insertion mounting):

- Model 692P3F5A7N pH transmitter (accepts differential sensor input).

- Model 6070P0 insertion pH sensor.

- MH718 insertion hardware (316 SS).

p System 2 (flow-through mounting):

- Model 692P3F5A7N pH transmitter (accepts

differential sensor input).

- Model 6028P0 pH sensor (LCP).

- MH376 flow-through mounting hardware (PVC).

PREI Instrumentation

(011) 448 2172



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