OPTIMISATION OF ANGLO PLATINUM S ACP ACID PLANT CATALYTIC CONVERTER Anglo Platinum, ACP, Kroondal, South Africa ABSTRACT Anglo Platinum s Waterval smelting complex based in South Africa operates the Anglo Converting Process (ACP) that produces SO 2 offgas. This SO 2 offgas is treated in a doublecontact, double-absorption acid plant allowing for conversion efficiencies higher than 99.90 %. The catalytic converter used in the acid plant to convert the SO 2 to SO 3 is a 4-bed reactor loaded with a combination of cesium-promoted and conventional vanadium pentoxide catalysts. In June 2008 there was a shutdown, in which one of the main activities was to screen the catalyst. Before this shutdown, the operator would control the converter pass temperatures manually, via the SCADA, especially during plant start-ups after which some of the automatic temperature controllers were used. After the shutdown, the control philosophy was revisited and all temperature controllers were put on auto mode during plant start up. Furthermore the set points of the pass inlet temperatures were reduced to maximize conversion in the first two passes. The result has been a reduction of SO 2 emissions, from a daily average of 50 70 ppm to as low as 20 ppm for similar sulphur dioxide and oxygen concentrations. Further work has been done to maintain high bed temperatures in the third and fourth passes at low SO 2 strengths by splitting the gas stream between pass 1 and 2 thus increasing the SO 2 content in the gas to pass 2. This increases the temperature of the gas exiting the second pass, which results in higher temperatures in passes 3 and 4. Comparison of the converter performance prior to and after the changes is made. The benefits of partially bypassing the first pass to increase the SO 2 content in the gas to the second pass are shown. Conclusions and scope for further work have been drawn from the comparisons. KEYWORDS Catalytic converter, SO 2 offgas, conversion, sulphur fixation, DCDA, heat exchanger. Page 137
1. INTRODUCTION Anglo Platinum s Waterval smelting complex, located to the North-west of Johannesburg in South Africa, operates the Anglo Converting Process (ACP) that produces a high SO 2 strength offgas which is treated in a Monsanto Enviro-Chem double-contact, double-absorption (DCDA) Acid Plant before the stripped tail gas is vented into the atmosphere. The DCDA section was designed for a gas flow of 70,700 Nm 3 /h of dry gases with a sulphur dioxide concentration of 10.0% by volume and oxygen concentration of 11.0% by volume to achieve a sulphur fixation of 99.91%, which equates to emissions from the stack of less than 100 ppm SO 2. A variable speed driven blower provides the motive force used to draft the offgas from the ACP through the gas cleaning, dehydration and DCDA sections and then into the atmosphere via a stack. In the cleaning section entrained solids are removed from the gas stream by scrubbing with weak acid in two quench towers followed by a jet scrubber. The scrubbed gas then enters the 1 st stage wet electrostatic precipitators (ESPs) where the finer particles are removed. The dust-free and watersaturated gas at 55 O C is routed to the dehydration section where a cooling tower is used to condense moisture from the gas by contact with chilled water which lowers its temperature to less than 30 O C. A 2 nd stage of wet ESP s is located downstream of the cooling tower to remove moisture and acidic aerosols leaving an optically clear gas, which then enters the pre-drying tower that uses 65% sulphuric acid to remove approximately 70% of the moisture. The partially dried gas is then contacted with 96% sulphuric acid in the post-drying tower before entering the blower, the discharge of which enters into the DCDA section of the plant. The catalytic converter used in the acid plant to convert the SO 2 to SO 3 is a 3:1 four-bed reactor loaded with a combination of cesium-promoted and conventional vanadium pentoxide catalysts. The top half of the first pass bed is cesium-promoted catalyst and conventional catalyst makes up the bottom half, whilst the second pass bed consists only of conventional catalyst. The top 20% of the third pass bed has cesium-promoted catalyst and conventional catalyst makes up the remaining 80% whilst the pass four bed consists only of cesium-promoted catalyst. The required operating temperatures for conversion in all the beds are achieved using the array of shell and tube heat exchangers that interchange heat from gas streams exiting the passes with cooler streams from the blower and interpass absorption tower (IPAT). This report will focus on how usage of the automated temperature controllers coupled with lower inlet temperature set points has resulted in a reduction of SO 2 emissions. The benefits of partially bypassing pass 1 to generate more heat for the remaining three passes will also be discussed. As per design of the DCDA section (Figure 1), the gas stream exiting the blower at 120 0 C is heated to 240 O C in the cold heat exchanger by indirect contact with hot gas exiting the fourth pass, enroute to the final absorption tower (FAT). This partially heated gas is further heated to 420 O C in the hot interpass heat exchanger Page 138
using heat from the gas exiting the first pass going into the second pass. Approximately 70% of the SO 2 in the gas stream is converted to SO 3 releasing heat which increases the gas exit temperature to a maximum of 614 O C. The gas exiting pass 1 is cooled to 435 O C in the hot heat exchanger before entering the second pass where a further 20 % conversion of the SO 2 increases the exit gas temperature up to approximately 503 O C. The gas exiting the second pass is cooled to 435 O C in the hot interpass heat exchanger before entering the third pass. The conversion in the third pass increases the gas temperature to 447 O C and at this point 97% overall conversion of SO 2 to SO 3 has occurred. According to Le Chatelier s principle, the high content of SO 3 will inhibit further oxidation of SO 2 so the exit gas is routed to the IPAT where all the SO 3 is absorbed into circulating 98.5% sulphuric acid. Bed 4 024 From post-drying tower COLD Cold 011 HOT Bed 3 Bed 2 HOT INTER PASS 020 COLD INTER PASS From IPAT To IPAT Blower To SO 3 cooler and FAT 015 013 Bed 1 KEY IPAT FAT Heat Exchanger Interpass Absorption Tower Final Absorption Tower Temperature Control Valve Figure 1: ACP Acid Plant DCDA section process flow sheet Prior to entering the IPAT, gas is cooled to 200 O C in the cold interpass heat exchanger. The cool SO 2 gas stream exiting the IPAT is heated in the shell sides of the cold and hot interpass heat exchangers, before entering the fourth pass at 425 O C where up to 97% of the SO 2 is converted, bringing the overall conversion to 99.91%. The temperature in the fourth pass rises to approximately 435 O C and this gas is cooled down in the cold heat exchanger and SO 3 cooler before absorption of Page 139
the SO 3 occurs in the FAT, ensuring the gas vented to atmosphere contains less than 100 ppm SO 2. 2. DCDA OPERATIONAL CONTROL PHILOSOPHY 2.1 PREVIOUS PROCESS CONTROL The gas inlet temperatures into all four passes were controlled manually by the SCADA operator although the provision for automatic control was available (Figure 1). Pass 1 inlet temperature was maintained in the range of 450 500 O C by ensuring none of the cold and hot heat exchanger bypass valves were opened. This allowed all the heat generated in pass 1 to be used to heat the incoming gas. When the inlet temperature exceeded 500 O C, the cold heat exchanger bypass valve -011 was manually opened resulting in cooler gas entering the hot heat exchanger, which in turn would lower pass 1 inlet temperature. Once this was lowered to 450 O C, the bypass valve would be closed. Pass 1 outlet temperature was controlled when the temperature exceeded 610 O C. Control was achieved by manually opening the pass 1 bypass valve -013 and directing cooler gas into the pass 1 exit stream. Reducing the amount of SO 2 into the first pass meant less conversion and less heat generation so lowering the gas outlet temperature. As a result of running pass 1 inlet temperatures high, the gas entering pass 2 would not exceed 435 O C, thus no control was effected. Pass 3 and 4 inlet temperatures were also not controlled as temperatures did not exceed 435 O C and 425 O C respectively. 2.2 PROCESS CONTROL MODIFICATIONS After the June 2008 annual shutdown, where all the catalyst beds were screened and losses topped up, the set points and operational modes of all five temperature indicator controllers (TIC) were revised and adjusted as summarised in Table 1. During this shutdown, 10 m 3 of conventional catalyst in the third pass was replaced with cesium-promoted catalyst to enable conversion of SO 2 to start at lower temperatures and thus reduce occurrences of high emissions. The first adjustment was to maintain all temperature controllers in auto mode during plant operations thereby eliminating manual control by the SCADA operator. Pass 1 inlet temperature is now controlled by TIC-015 with a set point of 420 O C. Control is achieved by opening or closing the hot heat exchanger -240 bypass valve - 015 (Figure 2). When the temperature is less than 420 O C, -015 remains closed but at temperatures greater than 420 O C, the valve will open. This allows cooler gas that has bypassed the hot heat exchanger to join the gas stream into the first pass. Page 140
This lowers the temperature till the set point is achieved after which the valve closes again. Table 1: Summary of previous and current temperature controllers and set points for converter passes Control Point Pass 1 inlet control Pass 1 outlet control Pass 2 inlet control Pass 3 inlet control Pass 4 inlet control Temperature Setpoint Valve Position Valve Position Controller (ºC) ( PV > SP) ( PV > SP) previous previous previous previous previous TIC-011 TIC-013 TIC-015 TIC-020 TIC-024 435 590 435 435 425-011 open -013 open -015 open T V-020 closed -024 open -011 closed -013 closed -015 closed -020 open -024 closed current current current current current TIC-015 TIC-011 420 620 420 420 400-015 open -011 open -015 closed -011 closed Definitions: TIC- Temperature indicator controller - Temperature valve SP- Set point PV- Process variable The second controller TIC-013 has a set point of 620 O C and is used to control pass 1 outlet temperature using pass 1 bypass valve -013. At temperatures less than 620 O C, the valve remains closed, but once the set point is exceeded the valve opens allowing cooler gas to bypass both the hot heat exchanger and pass one. By reducing the amount of SO 2 into pass 1, less conversion will occur and therefore less heat generation will lower the outlet temperature. Once the temperature is below the set point the valve closes again. Pass 2 inlet temperature is controlled by TIC-011 with a set point of 420 O C and control is achieved by opening or closing the cold heat exchanger -225 bypass valve -011. At temperatures less than 420 O C the bypass valve remains closed, but once the set point is exceeded, the valve opens allowing cooler gas discharged from the blower to enter the hot heat exchanger. This means gas exiting pass 1 enroute to pass 2 is indirectly contacted with a much cooler gas resulting in a lower pass 2 inlet temperature. Once the set point is achieved, the valve closes. Controller TIC-020, with a set point of 420 O C, is used to control pass 3 inlet temperature by opening or closing the cold inter-pass heat exchanger -250 bypass valve -020. At temperatures below 420 O C, the valve opens reducing the amount of cooler gas flowing on the shell side of the hot interpass heat exchanger. This means the gas exiting pass 2 will lose less heat, therefore the inlet temperature into pass 3 will increase. Page 141
400 O C TIC-024 Pass 4-024 420 O C -011 TIC-011 420 0 C Pass 3 Hot IP -020 Cold IP Gas from postdrying tower Cold Hot 420 0 C Pass 2 TIC-020 from IPAT to IPAT Blower To SO 3 cooler -015 TIC-015 Pass 1 620 0 C Key Main gas flow Bypass flow TIC-013 Controller link -013 IP Interpass Figure 2: Temperature set points and controllers for all converter passes Controller TIC-024, with a set point of 400 O C, controls pass 4 inlet temperature by opening or closing hot inter-pass heat exchanger -255 bypass valve -024. At temperatures below 400 O C, the valve remains closed. At temperatures higher than the set point, -024 opens allowing gas from the IPAT to bypass the cold interpass heat exchanger resulting in a lower temperature gas entering the hot interpass heat exchanger and eventually the fourth pass. 3. RESULTS AND DISCUSSION Graphical comparisons are made between days prior to and after the process control modifications when the SO 2 gas strengths and volumetric gas flows were similar (Figures 4 and 5). 3.1 PASS 1 AND 2 OPTIMISATION Prior to automated control, Figure 4 shows how the inlet to pass 1 was operated at temperatures as high as 500 O C. Manual control of the first pass inlet temperature would also lower the outlet and second pass inlet temperatures. Page 142
650 100 600 90 80 Pass 1 and 2 temperature ( O C) 550 500 450 400 350 70 60 50 40 30 20 10 SO2 emissions (ppm) 300 05:00 08:00 13:25 15:30 19:00 21:05 23:10 01:15 04:10 Time (minutes) 0 1st Pass Inlet Gas Temp 1st Pass Outlet Gas Temp 2nd Pass Inlet Gas Temp Gas to Stack - SO2 ppm Figure 4: Converter operations prior to operating philosophy changes (12 th May 2008) 650 100 600 90 80 Pass 1 and 2 temperature ( O C) 550 500 450 400 350 70 60 50 40 30 20 10 SO2 emissions (ppm) 300 0 06:00 08:55 11:55 14:00 16:05 19:35 21:40 23:45 01:50 03:55 Time (minutes) 1st Pass Inlet Gas Temp 1st Pass Outlet Gas Temp 2nd Pass Inlet Gas Temp Gas to Stack - SO2 ppm Figure 5: Converter operations after changes to operating philosophy (10 th August 2008) Page 143
Once the second pass inlet temperature dropped to less than 400 O C, the SO 2 emissions increased above the limit as the temperature is lower than the catalyst strike temperature. The emissions only reduced once the second pass inlet temperature was again above 410 O C. Figure 5 shows automated control of the first and second pass inlet temperatures at the set points of 420 O C. Pass 1 gas outlet temperatures of greater than 600 O C are still achieved. This results in a higher percentage of SO 2 being converted, thus reducing the load on the remaining passes. Increasing the pass one outlet limit from 590 O C to 620 O C generates extra heat in pass 1 that is distributed to pass 2, allowing its inlet to be controlled at higher inlet temperatures of 420 O C as shown in Figure 5. Control on this pass was rarely effected as temperatures did not exceed 420 O C. Figure 6 shows the reduction in monthly averages for SO 2 emissions from the stack recorded between January 2008 and March 2009 against a monthly average of 100 ppm. As can be seen from the graph, after July 2008 the monthly averages have been on a downward trend. 240 550 220 500 200 450 SO2 (ppm) 180 160 140 120 100 80 60 400 350 300 250 200 150 Contact Plant running hours 40 100 20 50 0 Jan-08 Feb-08 Mar-08 Apr-08 May-08 Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 Dec-08 Jan-09 Feb-09 Mar-09 SO2 60.7 134.7 57.2 64.4 76.9 49.7 91 40.2 55.6 25.6 50.0 42.7 48.6 27.4 Running Hours 233.0 251.0 218.5 311.5 320 190.2 379.7 527.3 510.4 518.8 416.5 424.9 347.1 195.2 339.3 0 Time (months) Figure 6: Total running hours and average monthly SO 2 emissions out of the Acid Plant stack (November excluded for reasons of abnormal operation) Page 144
3.2 PASS 3 AND 4 OPTIMISATION During operation of the converter with relatively low gas strengths of less than 8 % SO 2, it was observed that the last two passes tend to operate at lower temperatures resulting in high emissions as the catalyst was not always at the strike temperature. This is the reason Cesium-promoted catalyst was put into the third pass to start conversion of SO 2 at temperatures less than 400 O C. This deficiency was overcome, by incorporating in the plant operating procedure, that at low gas strengths the SCADA operator must put the temperature controller TIC-013 in manual mode to enable manual operation of pass 1 bypass valve - 013. Opening the valve results in part of the incoming SO 2 gas bypassing pass 1 and is sent directly to the second pass. This results in higher temperatures as the amount of SO 2 being converted increases (Figure 7). 500 100 Converter Temperatures ( O C ) 480 460 440 420 400 380 360 90 80 70 60 50 40 30 20 10 % opening of pass 1 bypass valve -013 340 0 18:00 20:05 22:10 00:15 02:20 04:25 06:30 08:35 10:40 12:45 14:50 17:45 Time (minutes) 2nd Pass Outlet Gas Temp 3rd Pass Inlet Gas Temp 4th Pass Inlet Gas Temp -013 % opening Figure 7: Manual operation of pass 1 bypass valve to heat up the third and fourth passes (8 th March 2009) With reference to Figure 7, as the bypass valve is opened in 2 5% incremental steps, the temperature of the gas exiting the second pass increases. This in turn heats up the third and fourth passes achieving of the desired operating temperatures. Once Page 145
pass 4 inlet temperature is above 385 O C, the bypass valve is either partially or fully closed to maintain the temperature between 380 385 O C, in order to maximize the benefit of cesium-promoted catalyst. This operation of partially bypassing pass 1 has resulted in the converter maintaining its pass temperatures for several hours at SO 2 gas strengths lower than the autothermal limit of 7%. 4. CONCLUSIONS The reduction of pass 1 and 2 inlet temperature set points has resulted in less SO 2 emissions for comparable plant conditions. Operating the temperature controllers in auto mode, effectively cutting out manual control by operators, has also contributed to lessening the incidence of high SO 2 emissions. Partially bypassing pass 1 when processing low strength gas has resulted in maintaining the desired operating temperatures for pass 3 and 4, which in turn has reduced the incidence of high emissions. 5. ACKNOWLEDGEMENTS Anglo Platinum for permission to publish this paper and the ACP Acid Plant Metallurgical Production Engineer, Mr Sean Heukelman, for providing operating guidelines. Page 146