PROCEAS H2 IN NEARLY PURE CL2

Transcription

1 1 APPLICATIONS REASON FOR BEING PROCEAS H2 IN NEARLY PURE CL2 Chlorine (Cl2) production is an electrolytic process. Very high electrical current densities (e.g. 1.7 ka/m2 for diaphragm cells) are used to drive an electrolytic cell against its natural current direction. Cl2 is produced as the Cl- ions present in a strong salt brine feed their electrons to a nickel coated steal mesh anode in the anode half of the cell. The electrons are then discharged into a second half of the cell (on a cathode) to produce Hydrogen gas (H2) and OH-. Being that the brine in the anode and cathode sides are separated by a permeable membrane, the sodium Na+ ions (if the salt used is NaCl) pass through the membrane to join the OH- ions available on the cathode side to produce NaOH (caustic soda). To speed the diffusion of the Na+ ions through the membrane a carefully controlled hydrostatic pressure is maintained (difference in brine levels) between the two cell halves with the higher pressure on the anode side. See diagram below: Diagram: Courtesy of Euro Chlor The diaphragm and hydrostatic pressure keeps the H2 gas produced on the cathode side from mixing with the Cl2 gas produced on the anode side. 1/14

2 Without the diaphragm to isolate them, the hydrogen and chlorine would spontaneously ignite and the caustic soda and chlorine would react to form sodium hypochlorite (NaClO), with further reaction to produce sodium chlorate (NaClO3) [Kirk-Othmer, 1991]. Hydrogen is produced in a fixed ratio of 28 kg per tonne chlorine produced. When the Cl2 leaves the electrolytic cell it has a temperature between C and is saturated with H2O. The saturated Cl2 is not absolutely pure. It contains several non-condensable gases like N2, H2, O2 and CO2. Of these gases the most dangerous is H2. If the H2 concentration exceeds 4% it can explode. To preclude any possibility of this happening the H2 concentration in the nearly pure Cl2 gas must be monitored. The H2 concentration in the Cl2 stream must be monitored continuously to ensure that it never approaches a level that puts personnel and assets at risk. The system described in this document has its primary utility in monitoring online the H2 gas concentration in the Cl2 production stream as it leaves a Cl2 dryer. A photo of a Cl2 production site is shown below. Photo: Euro Chlor During chlorine gas compression and cooling, most of the chlorine gas is condensed. However, the noncondensable gases (H2, CO2, O2, and N2) increase in concentration. By diluting the remaining chlorine gas with 2/14

3 air, the concentration of hydrogen can be kept below the explosion limit. This allows additional liquefaction of chlorine gas. The residual gases after liquefaction (so-called "tail gas") have to be purged from the system. The tail gas still contains a significant amount of chlorine, and the gas is therefore normally led to the chlorine destruction/absorption unit. Production of hydrochloric acid Instead of diluting the remaining gases after partial condensation of the chlorine gas, the hydrogen can be removed from the system by means of a reaction with chlorine gas in a column. This removes virtually all the hydrogen and yields gaseous hydrochloric acid, which exists harmlessly with the chlorine gas and can be recovered in a hydrochloric acid unit. The remaining chlorine gas can now safely be further condensed. The tail gases with some chlorine gas and the remaining non-condensable gases (CO2, N2, and O2) will be passed through a hydrochloric acid unit. This solution can be chosen if HCl is a saleable product or if it can be used as a feedstock for downstream production, such as ferric chloride Hydrogen leaving the cells is highly concentrated (>99.9% by volume) and normally cooled to remove water vapor, sodium hydroxide and salt. The solution of condensed salt water and sodium hydroxide is either recycled as brine make-up or treated with other waste water streams. Hydrogen may be distributed to users using booster fans or fed to the main compression plant. The main hydrogen compression plant usually comprises a number of compressors and a gas holder (surge chamber). The hydrogen gas holder is incorporated into the system to minimize fluctuations in the gas pressure from the primary stage. The hydrogen product gas stream is always kept pressurized to avoid ingress of air. All electrical equipment taken into the hydrogen compression plant area must be "intrinsically safe" (ATEX). A relief valve is normally provided within the system to relieve high pressure to atmosphere. Hydrogen is normally analyzed for oxygen content; the compression will shut down automatically in critical situations. ProCeas is also available in an ATEX compatible form to allow online O2 concentration monitoring during the hydrogen compression process. The hydrogen is in general used for on-site energy production. It is burnt as a fuel, either by the company operating the chlorine plant or by another company to whom it has been sold as a fuel. Some or all of it can also be used on site in the case of integrated sites or sold to other companies as chemical feedstock (production of hydroxylamines, hydrochloric acid, hydrogen peroxide, sodium sulphite, for example). 2 OVERALL SOLUTION CONFIGURATION The analyzer operates continuously and measures automatically H2 and CO2 in the Cl2. The measurement results are be sent to the control room via 4-20mA where an operator monitors them from the control display. ProCeas can provide a new measurement every second. 3/14

4 H2 will be measured in the range and with the level measurement limit (LOM) shown below. Molecule Measurement range Limit of Measurement (LOM) 1 H2 0-5 % less than 20 ppm 2 CO ppm 50 ppm The ProCeas analyzer will be housed in a pressurized and continuously purged cabinet. If no cooling is needed, the compressed instrument air 5-6 bar purge rate will be around 100 l/hr. If cooling is needed (ambient temperature above 40 C) a vortex cooling unit will be installed and the purge rate will be increased to 300 l/hr. The analyzer IP54 rated stainless steel housing is shown below. 4/14

5 The exact dimensions of this housing are shown in the drawing below. 3 PROCEAS SYSTEM CONFIGURATION 3.1 SYSTEM INTEGRATION The following schematic shows the various connections that can be made to the ProCeas on the mural cabinet base panel. 5/14

6 The ProCeas unit can be connected to a PLC via the 4 20 ma output. The RJ45 Ethernet connection inside the unit IP54 housing permits connection of the unit to the Web via an onboard modem. The modem plus a 3G key for the local GSM network provides interactive remote system support by our service engineers. The 3G dongle must be provided by the customer. The USB port can be used to connect a laptop PC or a USB stick for stored data dumps. This data can be useful in efforts to understand the events leading up to any possible analyzer functional failures or perhaps to verify the validity of certain measurement results. To the left of the unit are the gas connections. The following connections are available: inputs: - sample stream from sample extraction point. - dry oil free instrument air for cabinet purge and vortex cooling. - standard gas reference for confirmation of measurement (regulated to 3 Bar gauge).. It is not necessary to calibrate the ProCeas analyzer. This input is intended to provide a simple check to confirm proper analyzer function with a reference gas of known composition and concentration. outputs: - single exhaust port for both sample and reference gas stream (connected to waste collection system less than -10 mbar relative). All gas connections are 1/4 inch Swagelok fittings (see photo ofconnections below). 6/14

7 The necessary connections and switches for the cable heating trace are provided at the cable entrance point inside the ProCeas cabinet. 3.2 PROCESS DETAILS PROCESS STREAM DETAILS To avoid exposing the ProCeas analyzer to damaging corrosive attack, the gases provided to the instrument must not have absolute water (H2O) content no higher than 20 ppm. Being that the Cl2 leaving the electrolysis cell is saturated with water, the Cl2 sample extraction point must be downstream of an adequate drying system. For reliability reasons there is no sample pump in the ProCeas. Process pressure of 2 bar is sufficient to move the sample from the extraction point to the analyzer efficiently. The sample flow rate should be between 10 l/hr and 30 l/hr. 3.3 SAMPLING SYSTEM A sample flow continuously from a point downstream of the Cl2 production stream dryer. For corrosion control purposes, it is very important to ensure that moisture levels do not exceed the specified 20 ppm level. At the extraction point the pressure is between 1 and 2 bar gauge. The sample can therefore flow through the analyzer without a sample extraction pump. Experience in past installations have identified the sample pump as a major factor in system reliability. The nearly pure chlorine deteriorates quickly the pump seals. By avoiding the need for a sample pump the maintenance interval has been significantly increased. 7/14

8 The system reliability is optimized by the fact that there are no moving parts other than the flow meter SAMPLE EXTRACTION HARDWARE To control the sample stream flow a sonic nozzle is used. The nozzle orifice diameter (100 µm) determines the sample flow, which is largly unaffected by upstream pressure variations. The photo below shows the sonic nozzle component with the orifice at its extremity SAMPLE TRANSPORT LINE 4/6 The tube used for sample gas transport between the process extraction point and the analyzer is a plastic PFA tubing ¼" O.D. 8/14

9 4 MEASUREMENT WAVELENGTH GENERATION To provide the Near Infrared (NIR) wave lengths needed for a given analysis up to two laser modules each with one or two lasers installed can be used. For the application H2 in pure dry Cl2 a system with a single laser module installed in a ProCeas IP 54 mural cabinet is proposed (see photo below). In this case, the module contains a single laser and mirrors that are chosen specifically according to the wavelengths absorbed by H2 and CO2, the molecules present in the matrix (Cl2, N2, O2 and trace H2O) the accuracy needed (LOD less than 10 ppm) and the H2 measurement range (0 5%). It is possible to add one other cell to measure H2O concentration. An opened laser module is shown below. 9/14

10 5 USER INTERFACE There are several ways to communicate with the Proceas unit. 5.1 FRONT PANEL TOUCH SCREEN The ProCeas 19 inch rack has a built in touch screen user interface on the rack front panel. This is a fully interactive interface that provides both graphical and numerical information in real time. With the touch screen front panel interface no external PC is necessary to operate the system while taking full advantage of all of the capabilities that the ProCeas concept offers. 10/14

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12 5.2 EXTERNAL PC VIA USB PORT The side panel of the IP65 has a USB port. This port can be used to dump the stored onboard black box data to a USB drive or onto a laptop PC. This data is of interest in reconstituting the sequence of events leading up to a system anomaly. This feature is therefore a very useful tool in understanding ex post, how to avoid the failure in the future or which failure mode created the system failure. Through this port approximately the last 40 days of the entire system operations data capabilities is available at any time. 5.3 REMOTE INTERFACE VIA INTERNET AP2E has prepared the possibility to access the ProCeas unit per internet for remote system debug purposes. If remote debug services are desired, only a 3G dongle for the local GSM network is necessary. AP2E has already installed a modem behind the RJ45 Ethernet connector. With the GSM dongle the unit can be connected via Internet and full remote debug support by AP2E experts becomes possible. Included in the offered commissioning price is 6 months of remote service support from AP2E engineers. 6 OVERALL SYSTEM CONFIGURATION The system will be configured as follows: power supply: VAC / 2A 50/60 Hz standard Mural IP 65 cabinet dual and single laser module versions sonic nozzle for sample gas extraction 4 20 ma measurement output RS 485 serial data output Ethernet with modem installed for remote technical support (RJ45) inside the unit housing USB connection on side panel 12/14

13 7 ANALYTICAL SOFTWARE All software necessary for the operation of the ProCeas unit and the support of its data interfaces is provided. 8 SYSTEM MAINTENANCE ProCeas has very low maintenance requirements 9 AFTER SALES SERVICE AP2E offers service contracts for all of its systems. Once the system has been validated and the permanent installation location has been identified we would be happy to provide an offer for full service coverage. 10 ONLINE RECORDINGS The graph below shows the measurement recordings during the startup of the Cl2 production at a chlorine production site. In the initial period shown on the graph the Cl2 production is stopped. At precisely 10:00 on 07/10 the production was started. The H2 concentration (in red) rises sharply from its residual levels between 0 and 100 ppm to production levels between 200 and 380 ppm. Cl2 production is an electrolytic process and the H2 volumes produced are directly dependent on the current passed through the cell. For this reason there is instantaneous increase in the H2 production. 13/14

14 The H2 production also occurs in a fixed relationship of 28kg /ton of Cl2. The H2 graphs during production therefore corresponds directly to the relative Cl2 production levels over time. In the graph above, the seemingly rapid variations in H2 concentration that almost look like noise are actual H2 level variations. The ProCeas is generating a new measurement every 10 seconds in this application. An entire day of temperature recordings are compressed into a few centimeters. If we expand the graph in time we see the stability of the measurement levels (even at low concentrations approaching 10 ppm). Interesting to note is the accuracy and coherence of the H2 production levels (in blue) before Cl2 production begins. Here we can see that H2 measurements can be made into the low tens of ppm. Evolutions in the H2 concentration can be easily confirmed. Being that there is a cabonization reaction associated with the electrolytic production of Cl2, CO2 is also measured and appears in parallel to the H2 concentration. With ProCeas all gases are measured from the same physical sample. This ensures perfect synchronization of the two measurements. Any correlation between the two becomes clearly visible. 14/14