pH Measurements for Corn Mash Slurry (Liquefaction)

Yokogawa pH / ORP Sensors

Reprinted with permission from Yokogawa Corporation of America

Application Description

Many Ethanol plants running today are using a combination style pH electrode with a non-flowing reference to measure pH in the Mash Slurry transfer line from the Mash slurry mix tank to cook. The Mash is being pumped out of the Mash Slurry tank is at approximately 82 °C and 2 to 4 bar (180 °F and 40 to 60 psig). The original pH electrode systems that were installed during plant construction are online retractable assemblies and are mounted in orientations from completely horizontal to completely vertical and everywhere in between.

The Problem

The combination probe that is being used will typically drift out of calibration very quickly. Also, the probe is damaged sometimes from excessive removal from the process. The reason this probe drifts out of calibration is due to the fact that the non-flowing reference system plugs and becomes fouled by the mash passing by it. pH measurements are only as good as the reference required to make this measurement. If the reference is not doing its job, the measurement electrode will drift.

Product Recommendations

Yokogawa manufactures a multi-probe holder called the FF20 – flow through fitting or the FS20, which is pH chamber assembly with 1⁄2” NPT process connections. With these holders we use a combination electrode, part number: SC21C-AGC55 for measurement and reference and a separate temperature sensor part number: SM60-T1. The Yokogawa electrode system works due to the fact that the SC21C-AGC55 combination probe uses a pressurized reference system. By using plant air regulated to a KCl reservoir, the SC21C-AGC55 utilizes a positive flowing reference that does not foul.

Plants using this system typically check the pH measurement against a grab sample and only make adjustments if the sample and the online measured values are more than 0.2 pH difference from one another. Typically, the system will not need daily or weekly calibrations. Most plants will pull the electrodes once a month for cleaning and calibration in a standard 4 and 7 buffer solutions. 

pH Probe Locations

Installation Considerations

The Yokogawa pH system is not retractable from the process. It is usually best to put the Yokogawa pH electrodes in a by-pass or recirculation line that you can add isolation valves for isolating the probes from the process for maintenance and calibration. The probe assembly should be mounted downstream of the Slurry Tank transfer pump. Ideally it will be in a recirculation line going back into the tank or into the suction side of the slurry pump.

The picture below shows an installation that is actually flowing from left to right. The arrows indicate the direction of the mash flow through the recirculation line and back into the suction side of the pump. You will get an idea of the installation of the Yokogawa probes and the pressurized reference KCl reservoir from this picture. The reservoir pressure is typically set 1 to 2 psig above the slurry line pressure. The KCl reservoir will require refilling every 2-3 months for most applications.

For more information contact:

Power Specialties, Inc.
9118 E. 72nd Terrace
Raytown, MO 64133
Toll Free: (800) 432-6550
Phone: (816) 353-6550
Fax: (816) 353-1740

www.powerspecialties.com

Fundamentals of pH Control in Industrial, Municipal, and Commercial Applications

Handheld pH Meter & Sensor
(courtesy of Yokogawa)

Analytical measurement and control of pH within a system is necessary for many processes. Common applications include food processing, wastewater treatment, pulp & paper production, HVAC, power generation, and chemical industries.

To maintain the desired pH level in a solution, a sensor is used to measure the pH value. If the pH is not at the desired set point, a reagent is applied to the solution. When a high alkaline level is detected in the solution, an acid is added to decrease the pH level. When a low alkaline level is detected in the solution, a base is added to increase the pH level. In both cases the corrective ingredients are called reagents.

Accurately applying the correct amount of reagent to an acid or base solution can be challenging due to the logarithmic characteristics a pH reaction in a solution. Implementing a closed-loop control system maintains the pH level within a certain range and minimizes the degree to which the solution becomes acidic or alkaline.

An example of an automatic pH level control system is a water treatment process where lime softened water is maintained at a pH of 9, using carbon dioxide as a reagent. As the untreated water (or influent) enters the tank, the pH is continuously monitored by the pH sensor. The sensor is the feedback device to the controller where the setpoint is compared to the control value. If the values are not equal, the controller sends a signal to the control valve that applies carbon dioxide to the tank. The reagent is applied to the tank at varying rates to precisely control the pH level. With the pH level at 11 detected by the sensor, the controller commands the control valve to open and introduce more carbon dioxide. As the increased carbon dioxide mixes with the influent, the pH is lowered in a controlled manner. Reaching the setpoint, the carbon dioxide flow is minimized and the process is continually monitored for variation. The effluent is the treated water that is discharged out of the tank. The process continues to provide the lime softened water at the desired pH level.

Basics of pH Measurement

pH of Common Items
(image courtesy of
Wikipedia)

pH is a numeric scale used to designate the acidity or alkalinity of an aqueous solution. Solutions with a pH below 7 are acidic and solutions with a pH higher than 7 are basic. Pure water is considered neutral and are neither an acid nor a base.

pH measurement is very important in many industries including medicine, chemistry, agriculture, food science, environmental science, chemical engineering, water & wastewater treatment, and many others.

pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.

2 wire transmitter (Yokogawa)

pH meters, analyzers and transmitters are used for continuous process monitoring of pH to ensure water/product quality, monitor effluent discharge, batch neutralization, pulp stock, scrubbers, cooling towers, chemical, water/wastewater treatment and many other applications.

Four Wire Analyzers: 4-wire analyzers separate the power supply from the measurement output. As a minimum we need two wires for the power supply and two wires for the current output of the analyzer.

Two Wire Analyzers: 2-wire analyzers combine the wires for current output, power supply, and digital communication.

The document below (courtesy of Yokogawa) provides a comprehensive understanding of pH/ORP measurement and how to achieve reliable results. Basic information on the principles of measuring pH/ORP, the construction of the sensing elements and their basic use in process applications are provided.

The Basics of pH Measurement from Power Specialties, Inc.

For more information, contact:

Power Specialties, Inc.

9118 E. 72nd Terrace
Raytown, MO 64133
Toll Free: (800) 432-6550
Phone: (816) 353-6550
Fax: (816) 353-1740

www.powerspecialties.com