Precision Control in Chemical Reactors

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Jeffrey Nicholson
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1 Precision Control in Chemical Reactors

Precision Control in Chemical Reactors Industrial Park Wolfgang Treats its Own Wastewater Demanding Control of ph Value Figure 1: Control Center and Water Basin at IPW Water is our planet s most

The requirements that have to be fulfilled by control systems in the field of water treatment are becoming stricter and stricter.

2 Precision Control in Chemical Reactors Industrial Park Wolfgang Treats its Own Wastewater Demanding Control of ph Value Figure 1: Control Center and Water Basin at IPW Water is our planet s most valuable resource, and at the same time it s scarce. Great importance is thus not only placed upon water saving measures, but rather on the treatment and regeneration of water and wastewater as well. The requirements that have to be fulfilled by control systems in the field of water treatment are becoming stricter and stricter. For example, precisely defined limit values for acids and alkalines contained in wastewater may not be exceeded. Valves and pumps by means of which these chemicals are fed to the process must be accordingly monitored and regulated. GMC-I Messtechnik GmbH, with headquarters in Nuremberg, has proven its ability to manage highly demanding and individual control tasks of this sort at Industrial Park Wolfgang in Hanau, Germany. Water is a Precious Resource The Earth is not referred to as the blue planet without good reason: 71% of the Earth s surface is covered with water. However, this doesn t means there s an overabundance of water. 97.5% of all worldwide water resources occur in the form of saltwater in the oceans, and cannot be used directly as drinking water or for agricultural. Of the remaining 2.5% fresh water, 70% is bound up as ice on the North and South Poles, and in the world s glaciers. Water consumption is enormous, especially in the industrialized nations: Daily per capita consumption in the European Union is just under 150 liters. Alone for a single shower, roughly 40 liters of water are consumed. Experts have calculated that more than 5000 liters of water are required for the production of one kilogram of pork, and up to 400,000 liters of water are necessary in order to produce an automobile. Water is not only a scarce resource, it s distributed extremely unevenly as well, both geographically and seasonally. According to the United Nations, roughly 20% of all human beings have no access to clean drinking water, and roughly 40% of the world s population suffers from water shortages. And thus dealing with our most essential means of life in a responsible manner is of utmost importance. In addition to reduced water consumption, this above all includes protecting water from dangerous contamination.

Strict Wastewater Laws and Statutes Professional environmental management, and thus water management as well, is imperative for today s business enterprises.

And making a commitment to the basic essentials of life including air, water and soil goes without saying.

environmentally sound economic development. Each individual state, county and city has its own wastewater laws or statutes.

3 Strict Wastewater Laws and Statutes Professional environmental management, and thus water management as well, is imperative for today s business enterprises. It provides the opportunity of exploiting competitive advantages, and contributes to a secure future. And making a commitment to the basic essentials of life including air, water and soil goes without saying. Strict regulations are stipulated by politics as well: The German federal environmental agency sees secure, well functioning water supply and wastewater disposal as a fundamental prerequisite for environmentally sound economic development. Each individual state, county and city has its own wastewater laws or statutes. These include regulations which, for example, stipulate protective measures for sewage systems and discharge conditions for wastewater treatment plants. The wastewater statutes of the city of Hanau, which are relevant for Industrial Park Wolfgang, also stipulate defined limit values for the discharge of non-household wastewater. The physical parameters include a maximum temperature of 35 Celsius, a ph value of between 6.5 and 10, and a value of not more than one millimeter of precipitable matter per liter of water. Maximum values are also specified for organic matter and solvents, as well as for inorganic matter such as ammonium, nitrite, nitrate, cyanide and sulphates. Wastewater which exceeds the limit values and may not be discharged, must be caught and disposed of in a legally approved fashion. Wastewater Treatment at Industrial Park Wolfgang At Industrial Park Wolfgang, where ten industrial companies are located, industrial wastewater is treated, after which it can be discharged into Hanau s city sewer system. On-site treatment necessitates competent employees who possess the required know-how and are well acquainted with the regulations and maximum values, and who continuously adjust the treatment process in accordance with the requirements, and monitor and document the process as well. Figure 2: Chemical Reactor for Neutralization

4 All of the wastewater from the industrial park is initially diverted to a catch basin. From there it s fed to two chemical reactors (so-called dosing stations), each of which has a capacity of two cubic meters. Chemical reactors are part of the system within which chemical processes take place, for example the dosing of leaching agents and acids, and within which chemical transformations take place. There are two basic types of chemical reactors: the stirrer tank and the plug flow reactor. The special feature of the plug flow reactor is the internal layout of the deflector plates which cause turbulence within the inflowing liquid. After treatment, wastewater is discharged from the chemical reactors into a precipitation tank, whose spillway empties into the city of Hanau s sewer system. In light of the rising costs of regenerating and treating water and wastewater, more and more exacting demands are also placed upon a highly diverse, top quality control system at Industrial Park Wolfgang. And system controllers from Gossen-Metrawatt are already being used at different points within the system. A recent task involved the development of a regulating module for precise and reliable control of the ph value of the wastewater, and integration of the Siemens S7-400 controller. Beyond this, the existing PCS 7 user interface was also adapted. The goal of control is to achieve a ph value of 9.6 for the industrial wastewater by adding hydrochloric acid (HCl) and sodium hydroxide (NaOH), thus allowing for discharge into the city sewer system. Complicated ph Control The ph value is a measure of the strength of the acidic or alkaline action of an aqueous solution. The term itself is derived form the Latin pondus hydrogenii: pondus means weight and hydrogenium means hydrogen. The ph value is one of the most important chemical quantities. Its ascertainment and control are standard in many industrial applications including wastewater treatment and quality control for liquids. The desired ph value of a liquid can be precisely adjusted by influencing the concentration of acids and leaching agents. If a liquid needs to be neutralized, especially great demands are placed upon the reliability and the accuracy of the regulating process. Input (ph-value) Alkaline Neutral Acidic Figure 3: Titration Curve (neutralization curve) Output (linear quantity for control)

The greatest challenges of ph value control include the unusually large measuring range which encompasses 14 powers of ten, and the long dead time.

the chemical reactors at Industrial Park Wolfgang, in order to neutralize it, i.e. to achieve a ph value of 9.6. At first, the ph value hardly reacts to the altered mixture at all.

The non-linear curve by means of which this process can be represented is called the titration or neutralization curve.

5 The greatest challenges of ph value control include the unusually large measuring range which encompasses 14 powers of ten, and the long dead time. In concrete terms, due to the fact that the concentration of acids and leaching agents in a solution determine its ph value, this means that acids or leaching agents are added to the wastewater in the chemical reactors at Industrial Park Wolfgang, in order to neutralize it, i.e. to achieve a ph value of 9.6. At first, the ph value hardly reacts to the altered mixture at all. The controller has no point of orientation for regulating the process for a time period of approximately ten seconds, which is known as dead time. The non-linear curve by means of which this process can be represented is called the titration or neutralization curve. This curve also demonstrates the fact that in close proximity to the neutral point, i.e. close to 7, the ph value reacts most sensitively to changes in the mixture. The controller must respond within this short reaction time. The transition from the acidic to the alkaline range is thus not gradual, but rather takes place abruptly in the form of a ph jump. Stable ph value control depends primarily upon the size, the layout and the number of dosing stations (chemical reactors), the feed systems (flow influx and quantity), as well as the position of the ph probes (measuring sensors). Regulator Setup and Start-Up In order to fulfill this demanding regulating task, the decision makers at Industrial Park Wolfgang decided in favor of a temperature controller supplied by Gossen Metrawatt. As far as hardware was concerned, the 4-channel R355A controller module and the VIPA IM353-1DP01 Profibus slave were installed in the control cabinet in a decentralized fashion. Both were directly connected to the Siemens S7-400 controller via Profibus-DP. Figure 4: Decentralized Layout for R355 with Profibus Slave

This corresponds to a ph value of 0 to 14.

6 With regard to software, the handling blocks were incorporated into the project and linked. Values at the measuring inputs of the ph probes are written directly as scaled values within a range of 0 to 14,000 to the data blocks as external actual values. This corresponds to a ph value of 0 to 14. Analog actuation of the valves for the sodium hydroxide and the hydrochloric acid is accomplished by outputting a manipulating factor directly from the channel data blocks via the controller depending upon momentary actual values. Figure 5: 355Remote Tool There are three ways to configure the controller module and set its parameters: Direct access via the interface at the CPU is possible with the 355Remote initial start-up, remote diagnosis and remote maintenance tool. The RS 232 interface at the controller can be used with the 355Config (configuration tool). Beyond this, communication is also possible by means of reading and writing directly to and from the data blocks. A unique ph value curve was developed for the application at Industrial Park Wolfgang. It s used to linearize the measured value for the calculation of system deviation, and to take the long dead time of approximately ten seconds into account. In consideration of all of the existing process variables such as the non-linearity of the titration curve, the long process dead time and measurement after a completely mixed reaction, the control parameters can be ascertained, which include, for example, proportional band, delay time and cycle time. In the meantime, the system at Industrial Park Wolfgang has been placed into operation successfully. Michael Roick, Nuremberg, 21 January 2009

355A Controller Module The standard version of the system-compatible R355 controller module is distinguished by high levels of functionality.

Adaptation of the control parameters can be started for each channel at any desired point in time.

The R355 is also equipped with hot runner control including start-up and booster circuits, as well as assignment to groups for control zones for synchronous heating.

7 355A Controller Module The standard version of the system-compatible R355 controller module is distinguished by high levels of functionality. For example, it s equipped with an integrated data logger, an alarm history with time stamp and power limiting, and it takes several inputs into consideration for ascertaining the controlled variable. Adaptation of the control parameters can be started for each channel at any desired point in time. The module can also be used as a limit transducer, as well as a cascade, differential, ratio, ph value and switching controller. Controller inputs, as well as the outputs, can be freely configured. The R355 is also equipped with hot runner control including start-up and booster circuits, as well as assignment to groups for control zones for synchronous heating. Numerous monitoring functions are also provided, for example channel and device-specific alarms, and read-back outputs. Quick and easy initial startup is assured thanks to data, function and organization blocks. Industrial Park Wolfgang (IPW) Industrial Park Wolfgang in Hanau near Frankfurt presents itself as a high performance, innovative high-tech location, which provides companies with an ideal environment for intelligent, lasting growth. It s an innovative production and research center for materials technology, chemistry and pharmaceuticals in the Rhine-Main region. The park is operated by Industrial Park Wolfgang GmbH. Young companies work side by side along with well established, international corporations at the Hanau location. Companies like Evonik Industries, Umicore and Dentsply International maintain important R&D centers and production facilities at Industrial Park Wolfgang. The ten companies located within the park employ a total of 4500 people. The companies are guaranteed the provision of all necessary services on-site.

8 Printed in Germany Subject to Change without notice 1/2.09 Order no GMC-I Messtechnik GmbH Südwestpark 15 D Nürnberg Telefon: Telefax: info@gossenmetrawatt.com

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