pH Measurement for Cobb Control in Pulp and Paper Industry

Cobb Control in Pulp and Paper with Effective pH Measurement

Industry:   Pulp and Paper
Product:    pH Process Liquid Analyzers

Introduction

Paper is used in a broad array of products, which are essential to everyday life. From newspapers, books, and magazines to cardboard boxes, disposable plates, and napkins, we are constantly surrounded by paper products.

Printing and writing are important uses of paper. The quality of paper for printing or writing should be high and it depends on many parameters. Cobb is one of the parameters, which requires control.
 

What is Cobb Control?

Cobb control is the control of quality and binding of pulp in such a fashion that prevents ink on one side of a sheet of paper from penetrating through to the other side.

How is Cobb variation minimized?

Before processing, the raw pulp pH ranges between seven and eight. To minimize Cobb variation, this must be maintained in an acidic range between five and six pH.
This is essential for the following reasons: 

• It ensures proper binding of the fiber.
• Cobb is controlled.
• Further, it also helps in maintaining paper machine run ability at the headbox.

How is Cobb control performed?

Cobb control is done by the addition of alum, which is in the range between two and three pH, and rosin to the pulp. When alum and rosin are mixed with pulp, the pH of the mixture is measured. If it is not within the desired range, the transmitter will set the alum dosing via a controller to maintain the pH of the pulp. Rosin, on the other hand, has no such controlled action. It is dosed to the pulp continuously in a  specific quantity. It is the alum whose dosing is controlled in a manner that depends upon pH variations.

Process Overview

By installing a pH loop in the process, not only is Cobb control performed but the end-user can achieve the following:

• Realize a 20% saving in alum consumption.
• Eliminate corrosion in pipelines, pumps, and other equipment that would result from too much alum dosing.

Field Notes

Measurement points:  After the rosin tank

Typical problems: Cleaning of the sensor in such installations

Solution

Using a retractable fitting, this optimum pH level can be achieved by using Yokogawa's 4-wire pH Analyzer with a suitable, retractable fitting and sensor. The manual retractable fitting, PR10, with a titanium ball valve is designed for use in pulp industry applications where frequent cleaning of the sensor is required. For cleaning, one must remove the pH sensor, close the ball valve so that the sample will not come out, clean the sensor, open the ball valve, and put the clean sensor in place. The pH sensor will be a combination version with double-junction electrodes, which provide long-term stability and a prolonged lifetime. In the 4-wire pH analyzer, on the second mA output, there is a PI control facility. The speed of the alum dosing pumps can be controlled per pH variations with this feature.

Product Recommendations

Measurement System
Process Liquid Analyzer: 

• 2-wire FLEXA pH/ORP Analyzer
• 4-wire FLXA402 pH/ORP Analyzer

 

 

 

 

Features

• Dual sensor measurement on a 2-wire type analyzer and up to five sensor measurements on  a 4-wire type analyzer
• Indication of sensor health status
• Easy touchscreen operation 
• Trending display up to two weeks 
• Advanced process temperature compensation 

Sensor Selection:
pH retractable fitting with sensor: PH97/DP solid-state reference pH sensor in retractable fitting model PH87 series with ball valve.

 

 

 

Digital SMART Option:
The reusable smart adapter, SA11, offers the full measuring parameter functionality of analog sensors equipped with a Variopin connector and Yokogawa ID chip. The SA11 automatically recognizes the installed sensor and prepares the appropriate configuration.

The adapter can be integrated on top of the all-in-one style FU20 and placed into the PR10 retractable assembly. 

 

 

Key Advantages

Since a separate controller is not required for the dosing pump, this solution provides a cost-saving.

Note: For additional information on this application, please contact Yokogawa.