Emission Reduction in GTL Facility Using Spray Techniques

Transcription

1 ILASS Americas, 25 th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013 Emission Reduction in GTL Facility Using Spray Techniques G.C.Enyi, A.J.Abbas, G.G.Nasr and M.Burby Spray Research Group University of Salford Manchester, UK. In Gas-to-liquid (GTL) facility, significant quantities of CO 2 and reaction water are produced and various chemicals are used as intermediate treatment chemicals. The reaction water is contaminated by these chemicals which impair the ph and the related properties of the water. The ph has to be controlled in the effluent treatment unit before the water is re-used or released to the environment. A laboratory-scale effluent neutralization unit for ph control utilizing sprays and atomization techniques was designed and built. The apparatus is comprised of pre-designed laboratory tank. The tank is a simulated neutralization stainless steel tank where CO 2 bubbles were induced using various selected commercial atomizers into a predetermined volume (maximum 120 litres) of laboratory prepared alkaline wastewater having a ph of up to 12. The reaction of CO 2 with the alkaline wastewater reduced the ph to normal threshold of approximately The main objective of this paper is to develop an alternative approach to CO 2 emission reduction in GTL facility. The paper describes and provides the related analysis of the neutralization time of a given volume of wastewater (120 litres) using the gas and full cone atomizers to induced CO 2 bubbles in the tank. The neutralization time of the reaction reduced from minutes for full cone atomizer to minutes when gas atomizer was used. Finally it was estimated that about 83% of the daily carbon dioxide production which would have otherwise been emitted to the atmosphere was used in the neutralization process. This process is intended to reduce the emission of CO 2 in GTL facilities. Corresponding Author: G.C.Enyi@salford.ac.uk

2 ILASS Americas, 25 th Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013 Introduction The Gas-To-Liquid (GTL) technology consists of a chemical conversion of natural gas into a stable liquid by means of Fischer-Tropic (F-T) process [1]. This process makes it possible to obtain products that can be consumed directly as fuel such as diesel, kerosene and gasoline or special products such as lubricants. The products that are derived from the GTL technology have two types of economic advantages: 1. Their transport cost is much less than that of the transport of natural gas, which due to its volume (that is 1000 times more than the volume of petroleum) does not only presents high transport costs but also requires specific assets like pipeline and cryogenic LNG ship. 2. They emit less CO 2 into the atmosphere than the corresponding products from oil since they are derived from a clean fuel - natural gas. Despite the above advantages, the Fischer-Tropsch process emits large volume of CO 2 into the atmosphere. Figure 1 shows the emissions associated with various oil and gas processes including conventional oil, heavy oil, and GTL. The natural gas supply contributes about 1.61% of the total volume of produced CO 2 while the reforming and Fischer-Tropsch reactions contribute 98.39% [3]. Operational Project Operator Location Size (bpd) Sasol 1 South Africa 8,000 Sasol II/III South Africa 160,000 Shell Malaysia 12,500 Sasol/Chevron Qatar 34,000 In Progress Sasol/Chevron Nigeria 34,000 Proposed Rentech Indonesia 16,000 Rentech Bolivia 10,000 Syntroleum Australia 10,000 Syntroleum USA 90,000 Sasol Qatar 100,000 ExxonMobil Qatar 100,000 Shell Qatar 140,000 BP Alaska 100,000 Syntroleum Russia 13,000 Demo. Plant BP Alaska 300 ConocoPhilips Oklahoma 400 Expected total no of barrel of product per year 828,200 Table 1. Summary of Current and Proposed Commercial GTL Plant [4] In GTL operations, steam reforming involves reacting steam with natural gas over a bed of nickel oxide catalyst. In complete combustion reaction, one molecule of pure methane (CH 4 ) reacts with two molecules of steam (H 2 O) to produces one molecule of carbon dioxide (CO 2 ) gas and four molecules of hydrogen (H 2 ). In practice, the combustion process is not always complete, and this results in the production of carbon dioxide, hydrogen as well as carbon monoxide (CO) as shown in Equations 1 and 2. Figure 1. CO 2 emissions associated with oil and gas operations [2] The Figure shows that about 1150 kg of CO 2 is produced by a tonne of GTL product and based on over 16 commercial and proposed GTL projects around the world as shown in Table 1, the volume of produced CO 2 calls for capture to avoid environmental emissions. These large volumes of CO 2 are available in GTL plant from the following sources: 1. The natural gas supply 2. The reformer product and 3. The F-T reaction CH 4 + H 2 O CO + 3H 2 (1) CO + H 2 O CO 2 + H 2 (2) This work focuses on the in-situ utilization of produced CO 2 from the reforming and Fischer-Tropsch reactions in GTL plant for wastewater treatment. The CO 2 produced is processed and sent to the wastewater effluent neutralization basin, where it is injected into the basin containing alkaline wastewater for ph control by utilizing spray and atomization techniques. This technology eliminates the process and design problems associated with the recycle of CO 2 either as feed or low energy fuel to the reformer. It will also eliminate the current use of sulphuric acid for ph control of the effluent and therefore reduce the operating costs of GTL plant and it

3 will finally reduce the emission of CO 2 into the atmosphere. Sources of Waste Water in GTL plant The hydrocarbon-free aqueous effluents which require ph adjustment come from the following six sources within the GTL plant capacity of 34,000 barrel of products per day. Reaction water from Fischer-Tropsch unit 5216 Kg/h Waste/Reaction water from Hydrogen production unit 566 Kg/h Waste/Reaction water from Synthesis unit 5891 Kg/h Waste water from Steam leaks and blow downs 2787 Kg/h Waste water from Cooling towers Kg/h Waste water from Caustic and Sulphuric acid storage units and Raw water treatment Kg/h. These wastewaters contain a high concentration of dissolved solids with a ph range of 9 to 12. These wastewater streams are collected in the neutralization basins. The basin has two compartments known as the main and the discharge compartments. The main compartment receives the untreated water and the treatment chemicals and sends the water to the discharge compartment after treatment. The discharge compartment discharges the treated water via a discharge pump to a safe location. The main compartment occupies about 75% to 80% of the total volume of 380 m 3. The neutralization reactions take place in the main compartment. The water after undergoing series of treatment processes is released to the discharge compartment through a guillotine door. The discharge compartment has a pair pump connected to it. It receives the treated water and pumps it to a safe location. The collected water in the neutralization basin is neutralized by the addition of sulphuric acid. A typical wastewater treatment skid is shown in Figure 2. About 150,000 Kg/h of waste water flow into the treatment plant while 149,868 Kg/h of effluent is treated and discharge. The difference is lost due to evaporation. Figure 2. Schematic of conventional wastewater treatment using sulphuric acid Proposed Method The proposed treatment process will utilise the CO 2 produced from the reforming and Fischer-Tropsch units to neutralise the wastewater from the different parts of the plant, using spray and atomisation techniques to generate CO 2 bubbles in the neutralization basin instead of the conventional Sulphuric acid. The recovered carbon dioxide is channeled to the effluent unit as shown in Figure 3, to replace the sulphuric acid injection unit. The CO 2 will react with the wastewater to form carbonic acid (H 2 CO 3 ) as shown in Equation 3. CO 2 + H 2 O H 2 CO 3 (3) HH 2 CCCC 3 + NaOH NaHCO 3 + H 2 O (4) NNNNNNCCCC 3 + NNNNNNNN NNNN 2 CCCC 3 + HH 2 OO (5) The carbonic acid formed, is used for ph control by neutralizing the alkaline water as shown in Equations 4 and 5, instead of the expensive and corrosive sulphuric acid. Equations 4 and 5 show that the neutralization of the alkaline water by Carbon dioxide exhibits two neutralization points. The first neutralization point in Equation 4 produces sodium bi-carbonate salt which is very unstable. The unstable bi-carbonate salt finally neutralizes to sodium carbonate.

4 Figure 3. Schematic of the proposed wastewater treatment using CO 2 and atomizers Description of the Experimental test-rig A pre-determined volume of water (100 litres) supplied from the laboratory source was filled into 50 cm diameter and 78 cm high PVC reservoir with a capacity of 120 litres. The reservoir was mounted on struts 76.2cm (2ft 6ins) high. A given quantity (40 g) of sodium hydroxide (caustic) pearls which corresponds to the desired ph value of 12 was weighed using a weighing scale. The caustic was dissolved in the reservoir containing 100 litres of water. An electric motor operated agitator was used to properly mix the solution which represents an alkaline wastewater. The water ph should always be set to a desire value of a typical industrial wastewater (ph~9 to 13) and at an initial temperature of about 20 o C and standard atmospheric pressure. Carbon dioxide at 99.99% purity, from the CO 2 storage cylinder was regulated through a pressure regulator valve and bubbled through the alkaline wastewater via a gas atomizer. The generated CO 2 bubbles in the basin react with water to form carbonic acid which in turn reacts with the alkaline water until the water was neutralized at ph ~ This value will be indicated by the ph meter PHH-830 mounted on the experimental rig and whose ph probe is immersed in the alkaline solution inside the reservoir. The CO 2 flow was recorded using a rot meter which records the instantaneous flow Figure 4. Snap shot of the complete experimental test rig The total volume of CO 2 used to neutralize a predetermined volume of wastewater of known ph and the neutralization time were recorded. At the end of the reaction, the reservoir was drained of its content to the laboratory drain and a fresh volume of water was measured and the experiment was repeated using full cone atomizer to compare the efficiency of both atomizers in CO 2 neutralization process. The batch experiments were performed in triplicate, and the mean values of required neutralization time were taken for each set of values. Results and Discussion Comparison between two commercial atomizers The experiments on alkaline wastewater neutralization were carried out using both full cone atomizer and gas atomizer. Table 2 shows the result of the test conducted using the two commercial atomizers to neutralize effluent volume of 100 litres, at CO 2 flow rate of 25 lit/min. The effluent solution temperature was 20 o C and the atomizers angle position was 90 o which was chosen perpendicularly from the wastewater horizontal position as shown in Figure 5. The graphical representation of Table 2 is shown in Figure 6. The Figure shows the effects of gas atomizer and full cone atomizer on neutralization time at a given carbon dioxide flow rate of 25 litres/min. The neutralization characteristics show that gas atomizer achieved faster and better neutralization time of minutes than the full cone atomizer that had neutralization time of

5 made of the total CO 2 requirements of any GTL plant effluent unit, given the size and volume of the plant wastewater treatment unit. Figure 1 shows that to produce one ton (7.52 barrels) of GTL product, an equivalent of 1150 kg of CO 2 is produced. Hence a typical barrels/day (5405 m 3 /day) GTL plant will produce about 216,670 kg/hr of CO 2. Figure 7 shows the consumption of carbon dioxide at different effluent volumes. The Figure shows carbon dioxide consumption increases with an increase in effluent volume. Figure 5. Photo shot of generated bubbles Neutralization Time (min) ph Full Cone Atomizer Gas Atomizer Table 2. Comparison of Commercial Atomizers reaction time The performance of the gas atomizer could be attributed to the production of tiniest CO 2 bubbles from the 0.8mm bore of the gas atomizer compared to bigger bubble sizes produced by 1.0 mm full cone atomizer. These tiniest bubbles were rapidly absorbed by the chemical reactions that took place in the basin. This is one of the major findings of this research. Effects of effluent volume on CO 2 consumption Six sets of experiments were conducted for six different effluent volumes 30, 40, 50, 60, 70 and 100 litres. The total volume of carbon dioxide and the change in ph with time for each volume were recorded. Since both the CO 2 flow rate and the neutralization time are known, the total CO 2 consumed by each volume of wastewater could be estimated as shown in Table 3. By plotting the graph of effluent volume against the total volume of CO 2 consumed, deductions could be Figure 6. Performance curves of commercial atomizers at 90 o orientation angle and CO 2 flow rate of 25 l/min. From the experimental results, 100 litres (0.10m 3 ) of the effluent volume consumed a total of litres/min (0.433m 3 /min) of carbon dioxide. Assuming that the effluent volume is operating at a capacity of 350 m 3 and assuming the density of CO 2 is 1.98 kg/m 3, it will thus consume a total of 180,041 kg/hr of carbon dioxide. This shows that approximately 83% of the total carbon dioxide produced in a typical GTL plant could be utilized within the plant having the capacity specified above.

6 Effluent Volume (litres) Neutralization Time (min) Volume of CO 2 Consumed (litres) Table 3. CO 2 consumption at different effluent volumes which used min to achieve the same neutralization effect. Acknowledgements The lead author acknowledges the financial and technical supports of the Spray Research Group (SRG), Oxford Laser Ltd, U.K. and Petroleum and Gas Engineering Division at the University of Salford, UK for this research work. References 1. Rojey, A. Durand, B. Jaffret, C. Jullian, S and Valais, M., Natural Gas Production, Processing and Transport, Editions Technip, Paris, France, IEA. CO 2 Capture and Storage: A Key Carbon Abatement Option, Energy Technology Analysis, OECD/IEA, Paris Cedex 15, France, Chevron. Overall Operating Manual, Escravos Gas to Liquid Plant, Document Number EGTL1-MGN- OPS-OM , Chevron Nigeria Limited, Nigeria, Adegoke, A. Barrufet, M and Ehlig-Economides, C. GTL Plus Power Generation: The Optimal Alternative for Natural Gas Exploitation in Nigeria, IPTC 10523, International Petroleum Technology Conference, Doha, Qatar, November 21-23, Figure 7. Variation of effluent volume with volume of CO 2 utilized for neutralization Conclusions All the above work led to the conclusions that are listed below. The consumption of carbon dioxide increased with increase in effluent volume hence more carbon dioxide will be captured and utilised as the effluent volume increases. At least 83% of the daily carbon dioxide production from the process reactions, which would have been emitted to the atmosphere, could be used in the treatment process. This will lead to the reduction of carbon footprint. For the same experimental conditions, the gas atomiser produced superior neutralisation time of min than the full-cone atomiser