Optimisation of WSA technology - Integration of energy and adaptation to the Chinese market

Transcription

1 Otimisation of WSA technology - Integration of energy and adatation to the Chinese market Annette Wendt Deartment of Chemical Engineering, Lund University Abstract The aim of this Master thesis was to otimise the WSA technology in two areas; integration of energy and to adat the design for the Chinese legislation demands. The rocess contains combustion, catalytic oxidation, hydration and condensation, which are all exothermic reactions. The inch technology was used to investigate if there could be imrovements in the heat recovery. The result illustrated that there could be imrovements in the heat recovery if the outlet stack gas temerature was lowered and the excess heat was used for more steam roduction. In order to decrease the heat exchanger area and thereby lower the investment cost, the steam suerheater could be laced before the waste heat boiler. China is a growing market for the WSA rocess. The oerating conditions should therefore be adated to meet the Chinese legislation demands in a more economic way. In order to find the otimal design the combustor temerature, the concentration of H 2 S in the inlet acid gas and the caacity of the WSA lant were studied. The result was that the investment cost decrease with a decreased combustor temerature, a decreased concentration of H 2 S and a decreased caacity. If the investment cost was studied er ton roduced sulhuric acid, it decreased with a decreased combustor temerature, an increased concentration of H 2 S and an increased caacity. Introduction This master thesis has been carried out at the Deartment of Chemical Engineering in cooeration with the comany Haldor Tosøe A/S. The environmental WSA (Wet gas Sulhuric Acid) lant has been otimised in two areas. The rocess is used for cleaning off-gasses from various industries with a content of sulhur. The heat released by the exothermic reactions is utilised for steam roduction and rocess-torocess heat recovery. In the first art of the otimisation the main focus was to investigate if there could be imrovements in the heat recovery using the Pinch technology. In the second art an economical otimum should be determined for design of the rocess to meet the demands in the Chinese legislation. China is a large market in the desulhurization area and it continues to grow. A more economic rocess adated to their legislation demands would be more cometitive and could thereby lead to an increased market share. The WSA rocess A box diagram over the various stes in the WSA lant is illustrated in figure 1 [1]. Figure 1: A box diagram over the various stes in the WSA rocess. The inlet sulhurous off-gases are led to a combustor were they are mixed with combustion air. In the combustor the hydrogen sulhide is converted to sulhur dioxide according to the following exothermic reaction: H 2 S + 3/2 O 2 H 2 O + SO kj/mole (1) The hot rocess gas leaving the combustor is cooled in a waste heat boiler and then it is cooled once again in a steam suerheater. The cooled SO 2 containing gas is led to the SO 2 converter where it is converted to SO 3 in three adiabatic catalytic beds [2]. In the converter the SO 2 is oxidized to SO 3 according to: SO 2 + 1/2 O 2 SO kj/mole (2)

2 The reaction is a temerature and ressure deendent equilibrium reaction. In order to achieve a high conversion the reaction is cooled in two interbeds. After the third catalyst bed the rocess gas is cooled by boiler water. During this cooling art of the formed SO 3 reacts with the water vaour resent in the rocess gas according to the following exothermic hydration reaction [2] : SO 3 (g) + H 2 O (g) H 2 SO 4 (g) kj/mole (3) After the SO2 converter the rocess gas is led to the WSA condenser were it is cooled by ambient air. During the cooling the remaining hydration reaction and condensation of the sulhuric acid takes lace: H 2 SO 4 (g) H 2 SO 4 (l) kj/mole (4) The condensed hot sulhuric acid is further cooled in a heat exchanger cooled by cooling water. The acid is of commercial quality. The clean rocess gas leaves the condenser at the to. The heat released by all the exothermic reactions in the WSA lant is recovered in the form of steam. [2] The concet of inch analysis The Pinch technology is a strategic method based on fundamental thermodynamic for otimising the heat exchange between hot and cold streams in a rocess lant. In that way the need for external heating and cooling is minimised. [3] A helful way of visualise the cooling and heating demand of a stream is the temerature/heat content-diagram, most often called comosite curve, see figure 2. Figure 2. A comosite curve visualising the minimal energy demands and the inch temerature. Since the change in heat content, Q, during the heat exchange is the only thing that is interesting, a given stream can be lotted anywhere on the heat content axis roviding it has the correct sloe and goes from the suly temerature to the target temerature. [4] The differential heat flow added or reduced to a stream during heat exchange, will increase or reduce the heat content on the stream. The change in heat content from a suly temerature to a target temerature is calculated by equation 1. TT Q = m& C dt = m& C (T T ) (1) TS The sloe of the line in the T/H diagram reresents the stream and is calculated by equation 2. dt dq 1 1 = = (2) m & C CP When the heat content in each temerature interval is summarized, multile streams are hand able. This is calculated by equation 3. ( m C ) + (m& C ) ) ( T T ) & (3) line1 line2 This method for generating comosite curves and achieves the minimal energy targets were alied on the WSA rocess. Method A inch analysis was alied on the rocess. The data used in the analysis was extracted from a flowsheet and was assumed to be fixed data. 1. The first inch analysis was carried out the studied flowsheet. The studied flowsheet contained a lot of heat in the clean outlet rocess gas. The roduced acid was cooled by recirculation of cold acid and then heat exchanged to the target temerature. 2. The second attemt had a decreased temerature of the outlet clean rocess gas and only heat exchange of the roduced acid of was investigated. 3. A third attemt had also a decreased temerature of the outlet clean rocess gas but the same acid cooling rocedure as in the studied flowsheet was carried out in order to comare the solutions. Area calculations for the heat exchangers in the three attemts were the carried out. Results Pinch analyse The comosite curve for the first inch analysis is seen in figure 3. There it is received that there is no overhang at the right side of the curve and thereby no need for hot utility. On the left side T 2 S 1

3 there was an overhang and that overhang reresents the demand for cold external utility. Figure 3. Comosite curve for the first inch analysis. That gave the following result in the first inch analysis: Pinch temerature: 1170 C Minimum cold utility: 727 kw In the second attemt the decreased temerature of the clean outlet rocess gas increased the overhang at the left side, see figure 4. Figure 4. Comosite curve for the second inch analysis. The energy targets became the following: Pinch temerature: 1170 C Minimum cold utility: 3073 kw The increased energy demand indicated that there could be an increase in steam roduction. This hot stream could be used for reheating the inlet boiler water that was used for cooling the gas several times in the rocess and thereby generate suerheated steam. In the third inch analysis the difference by carrying out the cooling with recirculation gave the following results: Pinch temerature: 1170 C Minimum cold utility: 2939 kw The difference between direct cooling in a heat exchanger and cooling by adding cold acid and then heat exchange was small. There were less heat that could be used for an increased steam roduction in the third attemt but the decrease was not very noteworthy. When the areas was calculated for all the heat exchangers the result was that the area decreased with 50 % if the heat exchanger for suerheating the steam was laced before the waste heat boiler instead of after it. Chinese legislation The Chinese legislation that covers the studied rocess lant is GB The WSA lant is counted in category of lant that roduces sulhuric acid and lants that has alications of nitrogen oxides. That results in the following legislation demands: < 960 mg SO 2 / Nm 3 < 0 mg NO x / Nm 3 Method The arameters varied to find an economic otimum for the Chinese market was: The combustor temerature, 1000 C; 10 C; 00 C Concentration of H 2 S, %; 25%; 30% Caacity of the lant, 10 Nm 3 /h; 00 Nm 3 /h; 4800 Nm 3 /h In each simulation the amount of SO 2 emitted was otimised to the legislation demand. The varied oerating conditions demanded a changed design of the rocess equiment. The equiment that was cost estimated after each simulation was: heat exchangers SO 2 converter WSA condenser The amount of catalyst was also included in the cost estimation. Results adatation to Chinese market The investment cost as a function of the caacity of the WSA lant is received in figure 5. The investment cost er ton roduced sulhuric acid decreases with a decreasing caacity along with a high combustor temerature and high concentration of H 2 S.

4 Cost in ercentage of base case/ ton roduced H2SO4 Caacity (Nm3/h) 30%, 1000 C 25% 1000 C % 1000 C 30%, 10 C 25%, 10 C %, 10 C 30%, 00 C 25%, 00 C %, 00 C Figure 5. Investment cost er ton roduced sulhuric acid as a function of the caacity of the lant. The investment cost er ton roduced sulhuric acid as a function of the concentration of H 2 S in the inlet gas is received in figure 6. From the figure it is received that the higher concentration the lower investment cost er ton roduced sulhuric acid. The trend is weaker when the concentration increases. Cost in ercentage of base case /ton roduced H2SO Concentration (% H2S) 1000C 10Nm3/h 10C 10Nm3/h 00C 10Nm3/h 1000C 00Nm3/h 10C 00Nm3/h 00C 00Nm3/h 1000C 4800Nm3/h 10C 4800Nm3/h 00C 4800Nm3/h Figure 6. Investment cost er ton roduced sulhuric acid as a function of the concentration of H 2 S The investment cost er ton roduced sulhuric acid as a function of the combustor temerature is showed in figure 7. Cost in ercentage of base case /ton roduced H2SO Combustor temerature (C) 30% 10Nm3/h 25% 10Nm3/h % 10Nm3/h 30% 00Nm3/h 25% 00Nm3/h % 00Nm3/h 30% 4800Nm3/h 25% 4800Nm3/h % 4800Nm3/h Figure 7. Investment cost er ton roduced sulhuric acid as a function of the combustor temerature. The investment cost er ton roduced acid increases with an increased combustor temerature. In this case as well the large caacities and concentrations resulted in the lowest cost. For high caacities the trend is broken between 10 C and 00 C were the cost deceases for high combustor temeratures in a coule of cases. The total investment cost was the oosite for the caacity and concentration of H 2 S, the lower concentration and caacity the lower investment cost. A low combustor temerature did however still lead to the lowest investment cost. Conclusions The conclusion of the inch analysis is that there are winnings ossible to do in the WSA rocess. There is excess heat when the temerature of the clean outlet rocess gas is decreased. That heat could be used for an increased steam roduction, which would imrove the heat recovery. The investment cost for the heat exchanger network, could be lowered if the steam suerheater was laced before the waste heat boiler. The conclusion from the otimisation of the oerating conditions to achieve an economical rocess design was: 1. For the investment cost er ton roduced sulhuric acid: otimal oerating conditions are identified at low combustor temeratures, high concentrations of H 2 S and high caacities of the WSA lant. 2. For the total investment cost: otimal oerating conditions are identified at low combustor temeratures, low concentrations of H 2 S and low caacities of the WSA lant. Prosective Investigate the inch if electricity was roduced from the suerheated steam. Vary less oerating conditions in the economic otimisation. That would make it easier to understand the economic trends for the single equiments. Study the SO 3 content in the outlet of the converter. Acknowledgements This work has been carried out at Haldor Tosøe s environmental deartment with Jane Albertus Steenberg and Morten Mejlholm as suervisors, they are gratefully acknowledged along with Prof. Stig Stenström, De. of Chem. Eng., Lund University who was examiner.

5 Table of Symbols = c Secific heat caacity P ( kj / kg / C) CP = heat caacity flowrate ( kw / C) m& = mass flow ( kg / h) T = temerature ( C) Q = heat flow ( kw ) References 1 Haldor Tosøe A/S, (07); VK Series Sulhuric acid Catalysts for Today and for the Future; brochure 2 MOHN, (07); Process Descrition; Job S-003; Haldor Tosøe 3 Andersson, P., (1998), Otimisation of heat exchanger networks, Course material for KET010, Institution of Chemical Engineering, Lund, 3 Kem, I.C., (07), Pinch Analysis and Process Integration, Elsevier Ltd, second edition, chater 1-4, Wendt, A., (08), Otimisation of WSA technology, Deartment of Chemical Engineering, Lund University