Patent Publication Number: US-2005135970-A1

Title: Hydrogen sulfide monitoring system

Description:
TECHNICAL FIELD  
      This invention relates to the analysis of chemical compositions in general and, more particularly, to a system for measuring and analyzing the concentration of hydrogen sulfide (H 2 S) in a sulfur dioxide (SO 2 ) environment or combinations of sulfur dioxide with water vapor/carbon monoxide/carbon dioxide/nitrogen/oxygen in a SO 2  environment.  
     BACKGROUND OF THE INVENTION  
      Flash smelting sulfide ores generates large quantities of sulfur dioxide gas which is subsequently captured and treated. It often is converted into liquid SO 2  and sulfuric acid (H 2 SO 4 ). However, due to the incomplete oxidation of the sulfur entrained in the ores, quantities of water (H 2 O), and under the right conditions, considerable quantities of hydrogen sulfide gas may also be formed.  
      In the presence of SO 2 , H 2 S gas decomposes into elemental sulfur which adversely impacts plant equipment, plant performance and the eventual downstream quality of the liquid SO 2  and sulfuric acid by-products. Factors affecting the formation of the SO 2  gas include natural gas, coke quality and quantity, low oxygen (O 2 ) partial pressures, innate furnace design, feed quality, etc.  
      To help alleviate the undesirable formation of H 2 S, roof mounted oxygen lances and downstream afterburners are installed in the flash furnaces to oxidize the resultant H 2 S. Knowing the exact concentration of H 2 S close to the source enables furnace operators to monitor and regulate the H 2 S oxidizing equipment more efficiently by modulating the oxygen required to oxidize the H 2 S.  
      At Inco Limited&#39;s Ontario Division (Copper Cliff, Ontario), oxygen lances were installed in the roof of a flash furnace to more fully oxidize the H 2 S. In order to control the amount of oxygen injected into the furnace, an H 2 S analyzer is required. Over-oxidizing, that is, using too much oxygen, results in various problems.  
      For example, in the furnace a shoulder buildup of oxides of feed concentrate in the uptake necessitates the furnace to be shut down for about six hours every two weeks so this material can be physically cleaned and removed. In addition, the production and routing of pure oxygen for various processes is costly, somewhat limited and requires close supervision. Better efficient modulation of the oxygen that is actually introduced into the lances can result in a substantial usage savings—up to 50%. For example, when the demand for oxygen exceeds the supply, the local extensive copper circuit is cut off resulting in lost productivity. By more closely monitoring and controlling the usage of oxygen, rather than excessively supplying it in a somewhat haphazard manner, additional precious pure oxygen is available for more pressing needs such as on-line metal production.  
      As far as the inventors are aware, there are no commercially available on-stream analyzers that are able to measure parts per million levels of H 2 S in a 40-60% SO 2  gaseous environment. There are H 2 S detectors/analyzers for use in paper mill stacks that use solid-state semiconductor technology or rotating tapes impregnated with lead acetate solutions. Unfortunately, these devices fail in the highly corrosive SO 2  environment.  
      As a result, furnace operators have used a somewhat crude manual stain test where the SO 2  gas is passed through a membrane impregnated with silver nitrate (AgNO 3 ). H 2 S present in the gas forms a dark silver sulfide (Ag 2 S) spot whose darkness level corresponds to the H 2 S concentration in the gas. Experienced operators are able, with careful timing and control of the SO 2  gas flow, to roughly estimate the quantity of H 2 S entrained in the SO 2  gas stream.  
      As noted above, this rough and ready measurement regimen leaves much to be desired. There is a need for a simple robust apparatus and method for accurately measuring the quantity of H 2 S in a SO 2  gas stream.  
     SUMMARY OF THE INVENTION  
      There is provided an automated H 2 S stain test analyzer. A measured volume of sample process gas is introduced into a measured volume of AgNO 3  solution. The resulting color of the solution is analyzed by a colorimeter which subsequently provides a measurement reading to an operator and/or to a subsequent oxygen injection control device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic of an embodiment of the invention.  
       FIG. 2  is a graph depicting H 2 S concentrations.  
       FIG. 3  is a graph depicting H 2 S concentrations as a function of furnace conditions.  
       FIG. 4  is a graph depicting H 2 S concentrations.  
       FIG. 5  is a graph depicting H 2 S concentration as a function of furnace conditions. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION  
       FIG. 1  is a schematic representation of the hydrogen sulfide monitoring system  10 .  
      The system  10  is designed to operate with moisture in the sample process gas, typically up to about 100 ml/min continuous flow of H 2 O(1) although the system  10  is not so limited, and under fluctuating vacuum levels. The system  10  operates continuously and provides an analysis, in parts per million (“ppm”), at selected periodic intervals. The read-out rate is adjustable but it is preferred to produce the ppm analysis every 2.5 minutes.  
      The system  10  includes a ganged sample conditioning system  12  and an H 2 S analyzer section  14 .  
      For ease of non-limiting discussion, the system  10  is arbitrarily divided into the sample conditioning system  12  and the hydrogen sulfide analyzer section  14 . However, as will become evident in the following discussion, these arbitrary constructs are not meant to be physical limitations of the system  10 . Various combinations of components may be arranged in different physical permutations.  
      The “heart” of the system  10  utilizes a reaction cell  16  communicating with a colorimeter  18 . The colorimeter  18  in turn communicates and exchanges intelligence and instructions with an appropriately configured process logic controller (“PLC”)  20 .  
      The colorimeter (or chromometer)  18  is an apparatus that measures the concentration of a selected component in a solution by comparing the colors of known concentrations in that solution.  
      In the embodiment shown the PLC  20  is an Allen Bradley Micrologix™ 1200 model and the colorimeter  18  is a Brinkmann™ PC  910  model. Naturally, similar components made by different or the same manufacturers may be used as well.  
      The basic chemical reaction that occurs in the reaction cell  16  is: 
 
H 2 S (GAS) +2AgNO 3(AQ) →Ag 2 S (PPT) +2HNO 3(AQ)  
 
      The insoluble precipitated silver sulfide is so fine that it is uniformly distributed in the solution. The darkness of the solution (absorbance) is directly proportional to the hydrogen sulfide concentration.  
      The colorimeter  18  includes a two centimeter long probe  22  and a 420 nm filter (not shown).  
      Due to the desirably short sampling time of the system  10  and the high acidity of the AgNO 3  solution, the reaction cell  16  remains free of any Ag 2 S or Ag 2 SO 3  residue.  
      Process gas to be sampled from a furnace sample source port  24  is drawn by a gas pump  26  and routed to a gas filter/condenser  28 . The gas filter/condenser  28  includes an internal impinger that draws the liquid out of the gas. Condensate is directed to a condensate sump  30 . Trapped gases entrained therein will egress back to process for subsequent handling in drain  68 .  
      Sample process gas emerges from the filter/condenser  28  and is heated by heater  32 . A gas bypass waste gate  34  routes the sample process gas to the drain  68  or to a high precision gas flow control  36  (AEM Systems, Model 135, High Precision Sample Pressure [Flow] Controller) which meters the correct quantity of gas to the reaction cell  16  or to the drain  68 . A solenoid valve  38  after the high precision flow control  36 , switches gas flow between the reaction cell  16 , and the drain  68  at timed intervals. Excess gas is sent to the drain  68  via the valve  34 .  
      Gas flow parameters are measured by system pressure gauge  40  and sample pressure gauge  42 . Flow rates and process calibrations are measured by detector  44  (AEM Systems, Model 136, Sample Flow Display with Low Flow Alarm Output). The detector  44 , as well as all the other relevant components, are electrically connected to the PLC  20  for process operations and safety considerations in a manner known to those in the art. Some communication lines are shown as being solid, others are dashed and some are not shown for the sake of simplicity.  
      AgNO 3  solution is supplied to the reaction cell  16  from AgNO 3  source  46  via pump  48 . Similarly, waste solution from the reaction cell  16  is drawn off by pump  50  and dumped into waste sump  52 .  
      A source of  50  ppm H 2 S gas for calibration purposes is stored in tank  54 . The H 2 S calibration gas is directed through the heater  32  and goes through the same path as the process gas. It passes through the high precision gas flow control  36  and into the reaction cell  16  via the solenoid valve  38 . Process gas and excess H 2 S gas are forced out by the waste gate  34  due to a pressure differential.  
      A valve  56  allows the H 2 S gas to flow in a timed sequence (controlled from the PLC  20 ) when calibration button  62 C is pressed. The H 2 S gas then floods/purges the system to allow for calibration to occur. A flow detector  64  indicates the flow rate of the calibration gas from the tank  54 .  
      A cooler  58  provides cooling for the analyzer  14 &#39;s components and provides a positive pressure to keep dust out of the system enclosure (not shown). Cooler  58  cools the gas pump  26 , AgNO 3  pump  48  and waste pump  50  as well as PLC  20 , colorimeter  18 , electronics, etc.  
      A series of color-coded warning and status lights  60  ( 60 A,  60 B,  60 C) provide information to an operator.  
      Push button panel  62  ( 62 A,  62 B,  62 C) allows the operator to start/run, stop and calibrate the system  10 . Both the lights  60  and the panel  62  electrically communicate with the PLC  20 .  
      The PLC  20  communicates with a monitor  66  and displays selected parameters. Indeed as noted previously all of the control components, valves, instruments and pumps are electrically connected to the PLC  20 .  
      The operation of the system  10  is now discussed as follows:  
      Initially, the system  10  must be powered up and calibrated from a cold start.  
      The operator presses the start button  62 A on the panel  62  and the sample conditioning module  12  electronics and heater  32  power up. The gas flow control  36  and the solenoid valve  38  receive power and the gas vacuum pump  26  starts. The sample-conditioning module  12  is now acquiring sample process gas from the source port  24  and conditioning it for the analyzer section  14  for analysis. While system  10  is powered up, pressing calibration button  62 C puts the system  10  into calibration mode for one cycle (cycle=2.5 minutes) to allow calibration of flow rate to the reaction cell  16  to be set via a needle valve (not shown) for the gas flow controller  36 .  
      Calibration Cycle:  
      1. The operator presses the calibrate button  62 C and the associated calibration light  60 C energizes indicating the calibration routine is now activated. Alternatively, this step, as well as most of the operations, may be automated.  
      2. The waste pump  50  starts and removes any waste solution that may be in the reaction cell  16 .  
      3. The AgNO 3  solution pump  48  commences operation and fills the reaction cell  16  for about 25 seconds to produce a volume of about 4 mls in the cell  16 . This covers the calorimeter probe  22 .  
      4. The colorimeter  18  is energized and is ready to zero itself on the first bubble of calibration gas to ensure that there is zero drift in the readings. (The colorimeter  18  measures the absorbance of the solution in the reaction cell  16 ). 
          5. The colorimeter  18  takes about ten seconds to power up and zero itself so the calibration solenoid valve  56  is opened about three seconds before the colorimeter  18  zeros. The calibration gas from the tank  54  floods the entire system  12  and forces out the SO 2  process gas based on a pressure difference. The process gas runs between 5/psi (34.5 kPa) and 1 5 /psi (103.4 kPa) and the calibration gas runs at a higher pressure than the greatest process gas pressure indicated on the system pressure gauge  40 . This technique conforms to calibration standards.        

      6. The dry 50 ppm H 2 S calibration gas (the remainder is nitrogen) is heated by the heater  32  and is introduced to the reaction cell  16  by the controller  36  and then by the solenoid valve  38  as the colorimeter  18  zeros itself. The gas flows into the cell  16  for about forty-four seconds and the high precision gas flow controller  36  that works on a differential pressure principle controls the flow.  
      7. After about forty-four seconds, the solenoid valve  38  stops the flow of gas to the reaction cell  16  and the signals representing concentration of H 2 S in the cell  16  are captured by PLC  20 , conditioned, then sent to a visual display such as a digital control system  66  where it is graphically displayed and the data logged for operators to see in the control room.  
      8. The waste pump  50  subsequently turns on and drains the cell  16  at which time the operator can decide whether or not to run the calibration routine again.  
      To adjust the calibration of the analyzer  14 , the needle valve (not shown) is adjusted to control the pressure on the outlet of the gas flow controller  36 . This changes the flow into the reaction cell  16 , which changes the concentration of H 2 S in the cell  16 . The change in concentration is directly related to the absorbance by a linear relationship. The relationship between H 2 S and absorbance is linear up to an absorbance of 0.800 A (representing 200 ppm H 2 S).  
      The Process Gas Test Cycle:  
      The process gas test cycle is similar to the calibration cycle above except that the process gas sample from the furnace  24  flows to the reaction cell  16  (through essentially the same tubing as the calibration gas) instead of the calibration gas.  
      1. The waste pump  50  starts and removes any waste solution that may be in the reaction cell  16 .  
      2. The AgNO 3  solution pump  48  starts and fills the reaction cell  16  for about twenty five seconds to produce a volume of about 4 mls in the cell  16 . This covers the colorimeter probe  22 .  
      3. The colorimeter  18  is energized and zeros itself on the first bubble of sample process gas to ensure that there is zero drift in the readings.  
      4. The process gas sample generally fluctuates between 5/psi (34.5 kPa) and 15 psi (103.4 kPa) coming into the sample conditioning system  12  and is continuous so that any particulate matter does not deposit in the tubing or any other analyzer parts. Moreover, keeping the gas flowing continuously allows the entire system to operate under steady-state conditions. If there is condensate in the gas, it will be forced out by the filter/condenser  28  (impinger design) along with most of the moisture, and up to about 100 ml/min liquid water. This separates the gas from any condensate, where the condensate is removed out at the bottom of the condenser  28 , and the gas travels through the heater  32  and over to high precision gas flow control  36 .  
      5. The process gas is heated by the heater  32  to keep any remaining moisture in the gas phase and is introduced to the reaction cell  16  by the valve  38  as the colorimeter  18  zeros itself. The gas flows into the cell for about forty-four seconds and the high precision gas flow controller  36  which works on a differential pressure principle controls the flow.  
      6. After about forty-four seconds, the solenoid  38  stops the flow of gas to the reaction cell  16  and the solution is allowed to reach equilibrium. Following equilibrium, the 4-20 mA signals generated by the probe  22  representing the ppm H 2 S in the cell  16  are sent to the PLC  20  for signal conditioning and then to the display  66  to be graphically displayed and data logged for operators to see in the control room. This intelligence may be routed to an automatic oxygen injector control.  
      7. The waste pump  50  subsequently turns on and then drains the cell  16  and the cycle repeats at a relatively predetermined rate.  
      Experimental and actual operations testing demonstrated the efficacy of the system  10 .  
       FIGS. 2 and 3  show H 2 S data collected by the system  10  and the flash furnace conditions that contributed to the H 2 S formation respectively. The data was collected over a sequential three-day period (day “A”, “A+1”, and “A+2”).  
      The vertical spikes in  FIG. 2  indicate the presence of H 2 S in the process gas stream sample. Each spike correlates and agrees with a simultaneous conventional “patch” test using paper impregnated with AgNO 3  placed in the process gas sample stream for a measured period of time and flow rate. The higher the spikes on the system  10  graph ( FIG. 2 ), the darker the patch on the AgNO 3  paper.  
       FIG. 3  illustrates actual operating conditions (as does  FIG. 2 ) in Inco Limited&#39;s Ontario Division Number  2  flash furnace during a two day (“A” and “A+1”) interval. The graph shows that the total oxygen to the afterburners and roof lances was zero. This caused a spike in the H 2 S gas detected by the system  10 . The deficiency in oxygen in the furnace uptake caused the H 2 S gas to leave the furnace unoxidized. The furnace conditions support the system&#39;s  10  reading of H 2 S gas.  
      The following symbols shown in  FIG. 3  (and  FIG. 5 ) are defined as follows: 
          Δ signifies tonnes/hour of petroleum coke times 1000 (to fit in the graph)     ◯ signifies natural gas/10 (to fit in the graph)     □ signifies filter plant H 2 S readings as measured by the system  10  in parts per million.     ⋄ signifies total tonnes/hours oxygen going into the furnace through two roof lances divided by tonnes/hour of dry solid charge (“DSC”) times 1000 (to fit in the graph).     ▭ signifies total tonnes/hour of oxygen going into the furnace through two roof lances and four floor after burner business lances divided by tonnes/hour of DSC times 1000 (to fit in the graph).        

       FIGS. 4 and 5  illustrate conditions in the flash furnace about a month later than those depicted in  FIGS. 2 and 3 .  FIG. 4  depicts three consecutive days (B, B+1, B+2). The corresponding furnace operating conditions are shown in  FIG. 5  during the single (second) day (“B+1”).  
      The data shown in  FIG. 4  is the H 2 S detected by the system  10 .  FIG. 5  indicates that the furnace decreased the total oxygen to the afterburners and lances and increased the amount of natural gas. This caused a spike in H 2 S gas that corresponds to the detection by the system  10 .  
      The  FIGS. 2-5  demonstrate that the H 2 S level in the process gas can be accurately monitored on an automatic continuous basis. The system  10  introduces efficiencies whereas the prior conventional detection process is a laborious manual batch technique.  
      The above discussion essentially relates to a wet basis analysis. Alternatively, the sample conditioning system  12  may be bypassed by bypass  72  in the event of a malfunction or maintenance. The bypass  72  includes a bypass (third) pump similar to the pumps  48  and  50  and drying crystals. The bypass pump draws a gas sample off the gas pump  26  and sends the sample through the drying crystals and then to the reaction cell  16 .  
      This admittedly less desirable dry analysis bypass alternative provides a less accurate reading since the bypass pump does not deliver the same flow precision (measured volume) that the high precision gas flow control  36  does, especially under fluctuating vacuum conditions. Moreover, the drying crystals must be changed frequently when lots of water (condensate) is present in the gas. However, the system  10  and related technique are adaptable for continuous monitoring in a pinch.  
      While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention. Those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.