OPTIMIZATION OF COMPRESSOR STATION IN K. M. M. L

International Journal of Industrial Engineering & Technology (IJIET) ISSN 2277-4769 Vol. 3, Issue 3, Aug 2013, 45-54 © TJPRC Pvt. Ltd. OPTIMIZATION OF COMPRESSOR STATION IN K. M. M. L S. JULYES JAISINGH & MOHAMMED ZIAD S Department of Mechanical Engineering, St. Xavier‟s Catholic College of Engineering, Nagercoil, Tamil Nadu, India ABSTRACT Eco-friendly & socially committed, KMML is the only integrated Titanium Dioxide facility having mining, mineral separation, synthetic rutile and pigment-production plants. Apart from producing rutile grade Titanium Dioxide pigment for various types of industries, it also produces other products like Illmenite, Rutile, Zircon, Sillimenite, Synthetic rutile etc. Country's first titanium sponge plant (TSP) was opened in KMML as a joint venture of KMML, Vikram Sarabhai Space Centre (VSSC) and the Defence Metallurgical Research Laboratory. This project is mainly focused on the compressor station of utility dept., KMML. The present system is provided with five reciprocating compressors, each of capacity 45.4 NCM/min. The compressor station is of great importance for functioning of other plants. KMML requires both wet and dry air at predetermined pressures. For dry air supply, three dryer systems with electric heaters are provided in compressor station itself. I have mainly concentrated on the efficiency improvement of the present compressor station. For this purpose, an energy efficient new dryer system is designed by eliminating the use of electric heaters, which will reduce the energy consumption and energy cost of the dry air production. This project aims to design a new dryer system which recover the heat of compression, which is generally considered as the waste. This waste heat is bypassed from the compressor discharge before it enters the aftercooler and use it effectively to dry the air dryer and their by eliminating the electric heater. KEYWORDS: KMML, Heat of Compression Compressor Station, Dryer System, Payback Period INTRODUCTION Compressed air is one of the major source of inefficient energy use in an industry. It remains a mainstay of many installations for a variety of reasons. “Compressors are better at producing heat than compressed air,” says Tom Taranto of Data Power Services. This heat from compressed air can be extracted and used wisely. A considerable amount of rejected heat can be recovered using recent advanced heat recovery systems for heating purposes [4] Compressor station at KMML comprises of 5 double acting horizontally balanced reciprocating air compressors. These compressors, compresses the air at normal atmospheric conditions to a pressure of 10 kg/cm 2and nearly a temperature of 180°C. The pressure of 10 kg/cm2is achieved in two successive stages. Each compressor is provided with an after cooler that reduces the temperature of the compressed air from 180°C to a temperature range of 40-50°C. This air is then passed through a common header to three air receivers. The industrial use needs wet air as well as dry air. For making the wet air dry a dryer system is used. The compressor station at K.M.M.L, delivers compressed air at 10 kg/cm 2and at 40ºC-50ºC to the air receivers through after coolers. The dryer system is employed at the dry air supply line. The working of a dryer system, with two vessels consists of two phases at a time.   Working phase (dry air production) Regeneration phase 46 S. Julyes Jaisingh & Mohammed Ziad S Figure 1: Present Dryer System In working phase, the wet air from the air receiver at the compressed condition flows into the distributor. The wet air passes through the working dryer and wetness get adsorbed by the adsorbent (silica gel). It then passes to the dry air header through the second four way valve and the secondary filter. A bypass is also provided with the secondary filter for the continuous supply of dry air, in case of blockage in secondary filter The working hours of a vessel is preset for six hours. The dry air is required to be produced at -40°C for instrumentation purpose. After six hours of working, the adsorbing capacity of silica gel decreases and the quality of the dry air also reduces. Hence regeneration of adsorbent is required to regain the adsorbing capacity. In regeneration process, temperature of silica gel is raised up to 180°C and then cooled to 50°C. For the heating purpose, wet air is used, which comes at the lower part of the distributor. It passes through an electric heater and gets heated to 180°C. This hot air passes to the regeneration vessel through a four way valve. After regeneration, this hot air passes through the second four way valve to a water cooled heat exchanger and the temperature of the hot air is reduced to 40-50°C. A moisture separator separates the condensate from this wet air and supplies it to the upper part of the distributor, from where it flows to the working vessel. The working hours and regeneration hours are set at six hours. In regeneration phase, four hours are needed for heating the adsorbent and two hours for natural cooling. The vessel changeover is done automatically, after every six hours. The major challenges of the dryer system are the electric power it consumes for the regeneration of the dryer beds. The wet air from the distributor at 40-50°C is heated to 180°C by passing through the electric heaters. The electric heaters work 16hrs a day consuming 1360 units of electric power. If any alternative can be adopted for the electric heater a huge amount of money can be saved. The air compressors used are 35 year old reciprocating compressors. It is high time to change the compressors. ALTERNATIVES TO IMPROVE THE EFFICIENCY Compressing air generates heat. In fact, industrial-sized air compressors generate a substantial amount of heat that can be recovered and put to useful work. More than 80 percent of the electrical energy going to a compressor becomes available heat. Heat can be recovered and used for producing hot water or hot air.[2] HOC dryer uses heat already Optimization of Compressor Station in K. M. M. L 47 generated by the air compression process which is normally considered a waste. A standard HOC model consumes less than 150 watts, which is equivalent to energy consumed by one light bulb.[3]. Operation Cycle Compressed air, directly from Air compressor discharge is taken to Air dryer inlet through insulated pipelines, at 120˚C (minimum) temperature. This hot air is passed through one drying vessel where saturated desiccant is regenerated by this hot air. The wet air passes through the second drying vessel where moisture gets adsorbed and Dry air comes out. Cycle time is 4 hours regeneration and 4 hours drying. After 4 hours the changeover of vessels takes place automatically. In regeneration cycle heating of the bed is for 2 hours.    No power requirement for regeneration No loss of compressed air Negligible operating cost. It neither requires Electric power for regeneration, nor there is any loss of compressed air. Air Dryers Adsorption occurs when the water vapor is condensed and collected on the surface of the dehydrating agent (solid desiccant). In adsorption, the water molecule physically sticks to the surface of the solid material.[1] Molecular Sieves can be used as new dryer, This is the most versatile adsorbent because it can be manufactured for specific pore size, depending on the application. It is:    Capable of dehydration to less than 1ppm water content. The overwhelming choice for dehydration prior to cryogenic processes. Excellent for H2S removal, CO2, dehydration, high temperature dehydration, heavy hydrocarbon liquids, and highly selective removal.   More expensive than silica gel and alumina, but offers greater dehydration. Requires higher temperature, for regeneration. [1] A new dryer system can be designed using HOC instead of electric heater. DESIGNED DRYER SYSTEM In order to improve the efficiency of the existing compressor station the present dryer system has been modified. The new system eliminates the electric heater and the compressed air at 180 C is bypassed from the discharge line of each compressor, before it passes through the aftercooler. For the proper working of the each dryer system, minimum of 12000m3/ hr air is required. This much air is bypassed and allowed to flow through the dryers. In this system the distributor is replaced by a Hot Air Header, which supplies hot air to the regenerating vessels. The modifications made in the existing systems are;  Five flow control valves with pressure regulators for bypassing the required quantity of hot air to the hot air header and remaining compressed airflows to the Wet Air Header through the after coolers. 48 S. Julyes Jaisingh & Mohammed Ziad S    Hot Air Header for maintaining constant flow through the dryer system Heat Exchanger and Moisture Separator for the safety of the system A two state solenoid valve is used to determine the flow of hot air to regenerating vessels. The bypassed air at the compressed condition flows to the regenerating vessel, when the solenoid valve is in ON position. Here, the hot air at 1800C is directly bypassed through the vessel; hence it takes only less time for regeneration. If the working vessel is set for hrs working, the regeneration of the vessel will be completed around 4 hrs. at that condition the solenoid valve changes it state to OFF position to cut off the hot air supply to the regenerative vessel. Since the other vessel is still working the hot air cannot be supplied to it. As a result a pressure rise in the system occurs and when it exceeds the safe limit, it flows through the designed heat exchanger to the Wet Air Header. When the working vessel changes to its regenerating phase, the solenoid valve again comes to ON state and permits the flow of hot air to it. Figure 2: Layout of Designed Compressor Station The system is designed for all the five compressors at working. The flow control valves should be carefully chosen to maintain the required flow. Globe type needle valves can be used as pressure regulators. I.F.C (Intelligent Flow Control) system can be adopted to supply wet air at 4.7 kg/cm2 and dry air at 8.7 kg/cm2 in this design. DESIGN OF COMPONENTS Heat Exchanger Design Compressed air at 10 kg/cm2 and 180°C with a flow rate of 200NCM/min is required to be cooled by cooling water supplied at an inlet temperature of 320C. The temperature rise of the cooling water is limited to 10 0C, since the cooling water inlet temperature is maintained at 420C. The temperature of the compressed air at the exit of is taken as 40 0C. The heat exchanger designed is a shell and tube counter flow heat exchanger with hot air through the tubes. Let, Entry temperature of the Hot Fluid, T1 = 180°C Entry temperature of the Cold Fluid, t1 = 32°C Exit temperature of the Hot Fluid, T2 = 40°C Exit temperature of the Cold Fluid, t2 = 40°C Optimization of Compressor Station in K. M. M. L 49 L.M.T.D (Logarithmic Mean Temperature Difference), (T1 – t2) – (T2 – t1) ln [(T1 – t2) ÷ (T2 – t1)] ∆Tlm = (1) (T1 – t2) = 180 – 40 = 140°C (T2 – t1) = 40 – 32 = 8°C ∆Tlm = = 46.11 ~ 46°C Heat lost by the hot fluid = Heat gained by the cold fluid Q = mh × ch × (T1 – T2) = mc × cc × (t2 –t1) (2) mc = mass flow rate of cooling water mh = mass flow rate hot water = 200 m3 cc = specific heat of cooling water = 4178 J/ kgK ch = specific heat of hot fluid = 1022 J/ kgK mh = mh‟ × ρ (3) ρ = density of air at 180°C = 7.541 kg / m3 mh‟ = 200 / 60 = 3.33 m3/ s mh = 25.1153 kg/ s Q = mc × cc × (t2 –t1) mc = 3592951 ÷ (4178 × 8) = 107.496 = 107.5 kg/s Assuming overall heat transfer co-efficient, U = 175 W/m2K (water to compressed air) Q = U×A×LMTD A = area = (4) = 445 m2 Assuming, Tube length, L = 5m (Standard length) Tube thickness, t = 2mm and tube diameter, D o = 25mm (Standards) Total number of tubes, N = = (5) = 113.3 = 114 tubes. For two tube passes, P = 2 N = n× P n = number of tubes / pass (6) 50 S. Julyes Jaisingh & Mohammed Ziad S n= = 57 tubes / pass. A shell and tube heat exchanger with following parameters is required; 5m length, 2mm tube thickness, 25 mm tube diameter, 114 tubes and 57 tubes per pass. The cooling water is to supplied at 107.5 kg/s Pipe Design for Compressed Air A pipe is designed for the flow of compressed from the discharge of compressor at a temperature of 180 oC, pressure of 10kg/cm2 , flow rate of 200 m3 /min and a flow velocity of 42 m/s to the dryer system . Pressure, P = 10kg/cm2 = 1.47 N / mm2 Pmax = 15kg/cm2 = Q = 200 m3/min Velocity, V = 42 m/s = 2520 m / min Inside Diameter of Pipe Q=A×V (7) D= (8) = 1.13 × D = 1.13 × = 318.3 mm Pipe Thickness Selecting the pipe material as Carbon Steel, C – 30 (0.25 -0.35 % of Carbon and 0.6 -0.9% of Manganese) Tensile strength = 500 N /mm2 Factor of safety = 8 (for varying load) Safe working stress = 500 / 8 = 62.5 N /mm2 Considering Hoop Stress T= +C t = thickness of pipe, (9) = working stress C = 3 for steel t= + 3 = 6.73 ~ 7 mm = 0.2755” Considering Longitudinal Tensile Strength t= +C (10) 51 Optimization of Compressor Station in K. M. M. L t= + 3 = 4.87 mm From pipe standard dimensions conforming to requirements ANSI B16.25, schedule 40 pipe of thickness 0.28 inches and inside diameter 6.065 inches s selected. Ceramic wool with aluminum cladding after heat resistant painting on outer side of the pipe is also to be provided as insulation. RESULTS Cost Analysis of the Suggested Dryer System The cost of suggested system comprises of    Material cost Labour cost Over – head cost Material Cost Average cost of 1 flow control valve = Rs. 14850 For 5 flow control valves = Rs. 74250 Average cost of 5 flow regulators = Rs. 42500 Material cost of pipe = Rs. 180000 Insulation cost of pipe = Rs. 7000 Cost of heat exchangers = Rs. 500000 Cost of moisture separator = Rs. 80000 Cost of air header = Rs. 18000 Total material cost = 74250 + 42500 + 180000 + 7000 + 500000 + 80000 + 18000= Rs. 9,01,750 / Labour Cost Total labour cost = Rs.1,00,000 /Over- Head Cost Total expenses = Rs. 50000/Total Expenditure Total cost = material cost + labour cost + over head cost = Rs. 10,51,750 /This is the cost of single dryer system, The cost of three dryer system = 1151750 × 3 = Rs. 31,55,250/Energy Cost Analyses of Present Dryer System Consumptions of electrical energy per day =85kW x 16hrs = 1360 units 52 S. Julyes Jaisingh & Mohammed Ziad S Cost of one unit = Rs. 7 Total energy cost per day= Rs. 9520 Total energy cost per year = Rs. 34,74,800 /The suggested system will a give a pay- back within 1 year. In the redesigning of the dryer system the required heat exchangers as well the piping system is also designed. A shell and tube heat exchanger with following parameters is required, 5m length, 2mm tube thickness, 25 mm tube diameter, 114 tubes and 57 tubes per pass. The cooling water is to supplied at 107.5 kg/s. for the piping system, schedule 40 pipe of thickness 0.28 inches and inside diameter 6.065 inches s selected from pipe standard dimensions conforming to requirements ANSI B16.25.Ceramic wool with aluminum cladding after heat resistant painting on outer side of the pipe is also to be provided as insulation. Figure 3: Energy Cost of Present Dryer System Installaton cost of new dryer system Energy cost of present dryer system Figure 4: Cost Analysis of Present Dryer System and New Dryer System Optimization of Compressor Station in K. M. M. L 53 DISCUSSIONS To improve the efficiency of the existing compressor station the present dryer system has to be modified. Eliminates the usage of electric heater and heat of compressed air is recovered at 180ºC. The detailed drawing of the suggested system is produced. The cost analysis of the suggested dryer systemic calculated and the payback period is is found out. The total cost of the suggested is obtained as Rs. 31. 5 lakhs The energy cost analysis of the present dryer system is found out as Rs.34.75 lakhs per year And the payback period of the new system is within 1 year. The present compressor station spends around 35 lakhs per year as the energy cost of the electric heater. The new suggested system without the electric heater costs only below 32 lakhs, including the installation cost. More over the electricity charges is also increasing day by day, so the company have to spend more than 35 lakhs in the coming years for the present system. By eliminating the electric heater a considerable amount and which will increase the profit of the company by saving an unnecessary spending of a huge amount. The plotted graph clearly shows the yearly increasing energy cost of the present compressor system. CONCLUSIONS A detailed study of compressor station at KMML is carried out and for the optimization and a better performance of the compressor station. The measures suggested are  Implement the suggested dryer system which use the heat of compression for the regeneration of the dryers for maximum efficiency of the compressor station   Use of better alternative for the existing reciprocating compressor and the adsorbent currently used Implementation of energy saving devices like Intelligent flow control and energy air, for higher energy efficiency Since mainly focused on the modification of the existing dryer system, a detailed drawing of the suggested dryer system is also produced and also designed a heat exchanger and the piping system. The approximate cost of the suggested system is calculated and it is found out that it is very much lesser than the cost for running the existing system and it has also the minimum payback period. ACKNOWLEDGEMENTS Authors are greatful to KMML, Kerala and .St,Xavier‟s catholic college of engineering, Nagercoil, Tamilnadu, India for providing support and assistance to carry out this work. 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