ph measurement in flue gas scrubber fulfills the most optimistic expectations

Transcription

1 6 ph measurement in flue gas scrubber fulfills the most optimistic expectations Disposal of today's enormous volumes of waste on a both economically and ecologicallysound basis calls for the deployment of highly-developed systems and equipment. The wastefired, combined heat and power production plant in Burgkirchen, Bavaria / Germany is using successfully the InPro 4802 ph-electrode to control the difficult, continuous measurement of highly acidic flue gas scrubber water at outstanding accuracy. The Waste Disposal Association Southeast Bavaria (ZAS) was formed in 1984 with the chief intention of realizing thermal treatment of non-avoidable and nonrecyclable residual waste. The area to be covered by the waste disposal project amounted to seven districts with approximately inhabitants. The wastefired, combined heat and power production plant (MHKW) started operation in The MHKW Burgkirchen consists of two incineration lines, each with a two-stage reciprocating grate, three vertical flues and one horizontal flue. The capacity of the plant is t waste per annum. Flue gas cleaning The flue gases contain on average a HCl concentration of 1600 mg/nm 3, SO 2 concentration of 300 mg/nm 3, and NO x - concentration of 450 mg/nm 3. First of all the gases are freed from particulate matter by an electrostatic precipitator and then fed to a three-stage scrubber. Depending upon the concentration of pollutants in the water cycle, a certain volume of wash water is removed from the circuit and fed to the effluent treatment plant. Continuous monitoring of the emission values takes place in an analysis chamber directly before the stack. ph measurement in the scrubber During flue gas cleaning in the wet scrubbing process, the ph value has to be measured at two different locations. The gas washing process itself comprises two washing stages, namely a preliminary stage and a two-step main stage. The preliminary (venturi washer) stage in partic-

2 Contents Schematic diagram of a flue gas cleaning plant. e-filter Elimination of NOx ph measurement in flue gas scrubber fulfills the most optimistic expectations Inductive conductivity for chemicals concentration control Reliable ph measurement system with long service life even in the harsh process of chloralkali control Protection from gas explosion during centrifuge inertization through oxygen measurement with InPro 6800 Imprint Publisher Mettler-Toledo GmbH Process Analytics CH-8902 Urdorf, Switzerland Editorial office process.marketing@mt.com Production Marcom CH-8902 Urdorf, Switzerland Illustrations Archives of Mettler-Toledo GmbH, Urdorf (CH) 2 Subject to technical changes. Mettler-Toledo GmbH 06 / 05 Printed in Switzerland. Radialbeam / Venturi washer ular, places severe demands on the sensor technology relative to ph-measurement. In the main (radial flow washer) stages, sulfur dioxide is absorbed in an alkaline scrubbing liquid (e.g. limestone/lime based). In the venturi stage, acidic constituents and heavy-metals are washed out of the flue gas in a water cycle. The abovementioned components are readily soluble and can be removed from the flue gas without undue difficulty. In the venturi nozzle, wash water and flue gas are subject to intense mixing and the soluble components thereby transfer to the aqueous phase. Among the main acidic constituents of the flue gas are HCl and HF, which in aqueous solution form hydrochloric and hydrofluoric acid. This leads to an extremely low ph value of the scrubber water. Through the continuous addition of sodium hydroxide solution to the wash water, the ph can be maintained at a value of ph 0±0.2. The temperature of the wash water lies between 60 and 65 C. electrode enables continuous ph measurement Previously, the ph value of the wash water used in the venturi stage was measured by hand every eight hours in a complex and laborious procedure. The ph value serves to establish when it is necessary to add sodium hydroxide solution to the wash water, and the exact amount needed. Due to the particular chemical composition and characteristics of the exceedingly acidic wash water (high proportion of HOK- Filter hydrofluoric acid, extremely low ph value, elevated temperatures), it had in the past not been possible to measure the ph value on a continuous basis. The electrodes available on the market were unable to endure the extreme operating conditions sufficiently long. The ph electrode InPro 4802 from however, with its particular design specification for such severe measurement conditions, proved to be able to provide continuous and highly accurate ph measurement values at an extended service life. As a result, it was possible to distinctly reduce the maintenance effort and, at the same time, to substantially increase process reliability.

3 Inductive conductivity for chemicals concentration control Chemicals used as reagents are usually stored for economical reasons in high concentrations and have to be diluted before usage depending on the process at hand. For this dilution a control strategy is needed. Conductivity in a broad number of cases is a function of concentration, and can therefore be used to control the makeup system. Conductivity [s/cm] HCi HCl KOH N a O 4 H 2 SO 4 HNO Concentration [%] by weight Process First of all it has to be determined whether conductivity is a function of concentration for the chemical in question. That is usually the case for any acid, alkaline, or salt. Sometimes marker ions are used with non-conductive chemicals in order to lend the concentration conductivity properties, since conductivity itself is a very easy and reliable measurement. Secondly, the conductivity vs. concentration curve for the particular chemical has to be taken into consideration, mainly to clarify the following: a) Is the working range fully to the left or right of the peak, allowing unambiguous measurement? If it is only the peak that is not embodied in the working range, then conductivity can clearly be related to concentration. b) Is the slope within the working range sufficiently steep to ensure the desired accuracy? The accuracy of the conductivity system can be expressed in µs/cm, for example. The slope tells us the change in concentration per µs/cm. The accuracy in µs/cm times the slope then gives the accuracy in concentration that is theoretically possible. Tight concentration control means using the correct concentration in the process. Wasting valuable chemicals through the use of excessive concentrations, means wasting money. Concentrations no higher than those necessary, also means that there will be less burden on recovery and/ or wastewater treatment systems. On the other hand, concentrations not too low, contribute to the efficiency of the process. Tightly controlled concentrations enhance both process repeatability and quality. solution The best solution for this application is an inductive conductivity sensor. Modern inductive systems have the sensitivity to detect very small changes in conductivity, important for accurate measurement. Inductive conductivity sensor InPro 7250 series PEEK is an excellent sensor material, with very good chemical and mechanical resistance. However, material compatibility should be checked for each individual application. Conductivity transmitter Cond Ind 7100 e The best choice is the Cond Ind 7100e. Simple in operation, precise and reliable measurements, as well as low cost of ownership, together reduce overall effort and expense. With its unique user interface (pictographs) and continuous transmitter and sensor diagnostics, this instrument can be employed in all process applications. InPro 7250 PEEK. Cond Ind 7100 e. 3

4 Reliable ph measurement system with long service life Processes involving the use of basic chemicals such as chlorine, caustic soda and hydrogen, and containing byproducts such as hydrochloric acid and chlorine bleach, require highperformance liquid-type ph electrodes with two electrolyte chambers. These electrodes act as real problem-solvers, with their long service life, these (bridge-type) electrodes are especially suitable for monitoring tasks in processes with particularly harsh operating conditions. Akzo Nobel Base Chemicals GmbH in Bitterfeld (Germany), a company of the Akzo Nobel Group, utilizes a highly modern, low-energy and low-space requirement membrane process in its five-year old chloralkali plant. The process is outstanding for its exceptional ecological compatibility, and is now being increasingly employed throughout the world. Chlorine and caustic soda as raw materials Chlorine and caustic soda are basic chemicals used in the manufacture of a whole variety of products, including many different polymers, dyestuffs and pharmaceutical products as well as crop-protection agents. Caustic soda is used to dissolve wood fibers or to neutralize acidic media. Modern membrane technology at Akzo Nobel In the electrolysis cell, the very heart of this novel membrane technology process, while electrolytic chlorine is being produced at the anode, sodium hydroxide (lye) and hydrogen are created at the cathode. A membrane located between the two electrodes keeps the substances produced completely separate from one another. This ion exchange membrane is practically impervious to liquids and gases. However, it is purposely permeable for sodium ions but suppressing the passage of chloride ions to the cathode sector, and of hydroxide ions to the anode sector. Successful ph measurement in a medium with strong chlorine content The process is optimally controlled through online monitoring of the ph value. The process conditions during the electrolysis phase are extremely harsh: Temperature approx. 85 ºC/185 ºF. Drop in ph value from 4.5 to between 1 and 2. Presence of free chlorine (some hundred ppm) with strong effect on the life of the electrode and on the actual housing material. 4

5 even in the harsh process of chloralkali control The chlorine exposure compels the use of an electrode reference system of special design. The ph electrodes from METTLER TOLEDO have two electrolyte chambers and are particularly appropriate for operation in a process with a chlorine environment. electrodes are also very suitable for eliminating measurement disturbances caused by interference voltages, a major problem during electrolysis. The ph electrodes with liquid electrolyte or InPro 2000EB with integrated temperature sensor can be pressurized in order to prevent the ingress of process media via the diaphragm. Housings suitable for pressurization are available in various material qualities. Titanium and Kalrez O-rings are recommended for such difficult pressure conditions. The electrodes are equipped with two chambers for liquid electrolyte, with an additional internal diaphragm situated between the two chambers in order to prevent poisoning of the reference system. In this way, despite strong chlorine exposure, the electrode located downstream of the electrolysis stage has a lifespan of approximately two months. The other electrode positioned following lowering of the ph value through the addition of hydrochloric acid (approx. ph 2) prior to dechlorination, has a life of approximately one month. Advantages and benefits of the high-performance ph electrodes and InPro 2000EB for customers These electrodes are real problem solvers and designed for use under situations of utmost stress. They offer extended service life as they are refillable with electrolyte, and able to withstand even the most severe operating conditions for a long period. Mounted in a suitable housing at overpressure, it is almost impossible for process medium to penetrate into the electrode, thereby ruling out any chance of poisoning of the reference system. The series includes alternative types of electrodes, for instance versions with single or double electrolyte chambers. These electrode provide high-accuracy measurements and are designed to withstand elevated temperatures (up to 130 ºC/266 ºF). With these special liquid-filled ph electrodes and a whole range of other highperformance electrode types, METTLER TOLEDO has at its disposal a portfolio of products able to meet up to the most stringent demands of customers in the chemical industry. ph electrode InPro 2000 EB filled with KCl, as successfully used by Akzo Nobel in their chloralkali production plant. Technical specification of the ph electrode InPro 2000EB ph 0 to 14 Temperature range 0 to 140 ºC (32 to 284 ºF) Pressure range up to 6 bar (87psi) Glass membrane high-alkali glass Reference electrolyte liquid, KCl or options Diaphragm ceramic Glass body Ø 12 mm Cable connection VarioPin (VP) Lengths in mm 120, 150, 200, 50, 450 Certificates ATEX / FM 5

6 Protection from gas explosion during centrifuge inertization through oxygen One of the top-ten Dimethyl Terephthalate (DMT) producers was faced with periodic deficiencies of their oxygen measurement system during centrifuge inertization processes. The use of a competitive solution led to improved efficiency. Background DMT is a primary ingredient used in the manufacture of polyesters and industrial plastics. It is used in automotive parts, films, fishing lines, and food packaging materials. The production of high-purity DMT requires accurate recrystalization processes. At the same stage, DMT is dissolved in methanol. Centrifuges play a key role in separating DMT from methanol to obtain the purified DMT crystals. Safety and efficiency must always be a top priority in the separation process. Under the wrong conditions, centrifuges can turn into an ignition source, and consequently become a risk factor. An inert nitrogen gas atmosphere serves to reduce a centrifuge s oxygen concentration below the Limiting Oxygen Concentration (LOC). Oxygen measurement requirements During the separation of DMT from methanol, entry of oxygen into the centrifuge has to be inhibited. If it comes to an oxygen concentration level greater than 11 %, then persons and equipment are dangerously exposed to a potential explosion. The risk of ingress of oxygen is particularly high since phase separation is carried out at a negative pressure of 0.8 bar/ 11.6 psi. Through a controlled feed of nitrogen, the oxygen concentration should be limited to remain below a 5% limit level. The temperature during the separation process is held at approximately 55 C/131 F. How was the application originally handled? Originally, this application was process-controlled by a system measuring pressure and nitrogen flow. Additionally, an expensive multi-channel paramagnetic oxygen system was used to monitor the oxygen concentration at a negative pressure of 0.8 bar/11.6 psi of six centrifuges in sequence. This multichannel system allowed just one measurement at a time. Five centrifuges were outside control while one was being measured. A costly sampling system was required to ensure reliable measurements. In certain cases methanol could not be retained by the sampling system and was able to cause damage to the paramagnetic oxygen system. Such shortcomings resulted in costly repairs and major process downtimes for all centrifuges. Decanter Nitrogen supply Nitrogen discharge Methanol drain DMT power Controlled feed of nitrogen keeps the centrifuge s / decanter s oxygen concentration below the LOC. 6

7 measurement with InPro 6800 InPro 6800 Gas. InTrac 777 e. Use of a test system Due to those negative experiences with the existing installation, the client decided to test the oxygen measurement system consisting of an InPro 6800 oxygen gas sensor, an InTrac 777 e retractable housing, and a M 700 oxygen transmitter. In an initial step, the test system was set up in parallel to the installation already in place. In this instance however, conditioning of the measurement gas has become redundant and the sensor was built directly into the headspace of the centrifuge. The diagram illustrates the arrangement. Customer s benefits Within the test period of three months, good practical experience was gained with the features of the new system. The comparison of both systems showed excellent measurement correlation. A more detailed analysis of the measurement values resulted into two more advantages of the solution. First, the response time and the absolute accuracy of the measurement could be improved because the measurement is directly built in. Second, long gas sampling tubing increases the possibility of oxygen diffusions, which results in false high readings compared to actual ones. The client defined a two-week maintenance and calibration cycle for the InPro 6800 oxygen sensor. By employing the InTrac 777 e retractable housing, the separation process did not need to be interrupted during maintenance work. Continuous production was guaranteed. Decision for On the basis of the positive findings surrounding the use of the METTLER TOLEDO system and in particular the fact that it does not need a costly gas sampling system, a total of 6 centrifuges were subsequently equipped with this new system. Solution InPro 6800 oxygen gas sensor Easy maintenance within two minutes Approved for use in hazardous areas ATEX II 1/2G EEX ia IIC T6/T5/T4 FM IS, CIass 1, Div 1, Group A,B,C,D Rugged IP68 rated VarioPin (VP) connector InTrac 777 e retractable housing Ability to retract sensor during continuous process operation Supports easy maintenance of oxygen sensor Approved for use in hazardous areas ATEX II 1/2G EEx ia IIC T6/T5/T4 M700 XC oxygen transmitter Easy installation and operation Simultaneous monitoring of oxygen and temperature Redundant measurements with one transmitter Approved for use in hazardous areas ATEX II 2 (1) G EEx ib [ia] IIC T4 FM IS, CIass 1, Div 1, Group A,B,C,D 7

8 Get online support on Visit our website for fast and competent information available 24 / 7. The latest and most updated product and support documentation is available in many different languages. Support center with easy and free downloads Declaration of conformities Certificates Device descriptions User manuals / data sheets For online contact simply click on: Contact us Request more information Get a quote Country specific information Pre-select your country for additional local support. Search tool Find your answer by entering your keyword. Find and download product and application documents Product News Industry Brochures Application News Mettler-Toledo GmbH Process Analytics Im Hackacker 15 CH-8902 Urdorf Switzerland INGOLD Leading Process Analytics 8