Selecting the Right Vacuum System. for your petrochemical or chemical process

Transcription

1 Selecting the Right Vacuum System for your petrochemical or chemical process

2 About the Authors Eric J. Boyles Market Manager, Chemical, Oil, & Gas - Americas Gardner Denver Nash A graduate of Edinboro University with a specialization in Physics and the University of Pittsburgh with a bachelor of science in Mechanical Engineering. Mr. Boyles is a member of AIChE and has 18 years of experience in industrial applications, with expertise in electrical, mechanical and chemical applications. He has been actively involved with international chemical/pharmaceutical/petrochemical businesses for the past nine years and is responsible for the chemical and petrochemical markets in the Americas. His extensive design and troubleshooting experience provides in-depth knowledge across the various vacuum and compressor technologies. Sandeep Hadap Sales Manager, South East Asia, Taiwan and South Asia Gardner Denver Nash Singapore Pte Ltd. Formerly with Siemens Singapore (Vacuum Pump Division), Mr Hadap has held a variety of positions in Business Development & Sales. He has a bachelor degree in Production Engineering and an MBA from Manchester Business School - University of Manchester, UK. Mr Hadap has over 16 years of expertise in rotating equipment and has spent the last nine years working in vacuum technology. He has worked on projects for the power generation, energy, process and biofuel industries. Mr Hadap is a "Member of Energy Institute, UK" (M EI) & "Affiliate Member of Marketing Institute of Singapore". Hans Wijsman Market Manager, Chemical, Oil & Gas - Europe, Asia, Africa Gardner Denver Nash Benelux B.V. Mr. Wijsman has a bachelor degree in Mechanical Engineering. An employee of Nash since 1993, he is currently Global Marketing Director for Gardner Denver Nash with responsibility for the Chemical and Petrochemical industries in Europe and Asia. He resides in the Netherlands. NASH Div. of Gardner Denver 9 Trefoil Drive Trumbull, CT U.S.A. tel: NASH fax: nash@gardnerdenver.com

3 Selecting the Right Vacuum System Selecting the right vacuum system for your petrochemical or chemical process is one of the most important and difficult steps for making consistent, high quality products without unscheduled down time. Among the fifteen vacuum producing technologies available today, only steam ejectors, vane pumps, dry pumps, and liquid ring pumps are common for chemical/petrochemical applications. Each has a certain fit depending on the gas composition, demands of the application, flow rate and vacuum level needed. When selecting a vacuum system, there is always a trade off between capital cost & operating and maintenance costs (combined, the "life cycle cost"). The capital cost (initial purchase price) should be only one of the many things considered when looking at new equipment; the operating cost (the day to day running costs, down time, and maintenance costs/frequency over the entire life of the equipment) is often much higher than the capital cost. Electricity, water and environmental compliance all involve direct power consumption - a constant expense which relates to plant operation costs, energy audits and quotas. This means that for a small incremental capital cost, higher quality equipment will quickly pay for itself through high reliability and minimized operating cost. In this article, we will consider the top ten factors used to determine the most appropriate technology and factors affecting vacuum system design: 1 Normal vacuum system specifications a Normal vacuum level (mm HgA or mbar A) b Actual volumetric flow requirements (capacity in Kg/Hr and/or m3/hr) c Scope of supply d High temperature limits for process e Normal cooling water limits or air-to-water exchanger limits f All controls, communication and safety requirements g Space layout and/or limitations h Plant utilities limitations (i.e. steam, power, water, oil, etc.) i Materials of construction 2 Actual process requirements: Hazardous area? Toxic chemical? Lethal service? 24/7 run time required? Capacity control? Evacuation time? Special materials of construction? 3 Specific cost of the equipment (Life Cycle Cost) 4 High efficiency (low utility consumption) 5 Environmental impact 6 Assessment of equipment manufacturer: capabilities, knowledge, length of experience, service and support 7 Specifications and drawing requirements (AutoCAD, 3-D drawings & piping) 8 Maintenance requirements 9 Service 10 After-sale support When performed as a batch process, vacuum tumble drying requires a gradually increasing vacuum to draw out solvents at the maximum sustainable rate. Applying too high a vacuum initially robs the dryer of vapor needed for heat transfer into the mass of solid particles. During later portions of the drying cycle, an ejector stage that uses solvent vapor as its motive fluid raises the vacuum level to extract residual solvents. ~ 3 ~

4 Vacuum Systems Vacuum is still the best and most efficient way to perform most chemical reactions because energy is saved by not having to heat the material to high reaction temperatures or boiling points. Stripping off unused product or reactants can also save money and vacuum can create a continuous processing system to maximize production and minimize investment. This is especially true with solvent recovery in the bulk chemical and pharmaceutical industry. Many factors can affect vacuum levels, including piping design, vacuum pump design, carryover, temperature fluctuations, air in-leakage, valve selection, changes in molecular weight, discharge pressure, etc. These factors make it very important to use a manufacturer that has the experience, technical knowledge and long term service/support capability to ensure that any unforeseen problems can be solved quickly, and your process continue to run profitably. There is no need to be a vacuum expert, since you can use your supplier's expertise. had been a batch reactor, using different solvents during each batch, then the dry pump may have been a good solution. For an application with easy service and no carry-over, a rotary vane is the low cost option. Each of the project drivers may direct which technology makes the most sense for the application. Life Cycle Cost All costs, including those for capital investment, maintenance, lost production time, utilities and waste cleanup, should be considered when determining the true cost of each technology. This analysis typically uncovers some surprising results. One chemical plant found that their maintenance costs were so high for a misapplied vane technology, that they were spending over $1.4 million in unnecessary maintenance costs. Actual Process Requirements Based on your specifications, a supplier will be able to provide a quotation for your project and recommend options to help you evaluate the merits of the system proposed. For example, a 2000 gallon (7500 liter) reactor needs 706 SFM (1200 m 3 /hr) at 3 inhg (75 torr) at the reactor head for the final processing step. This could be accomplished using steam ejectors, an ejector/liquid ring hybrid system, a liquid ring vacuum system, a rotary vane system or dry pumps. The reliability of each system is shown in the chart below: 100% 80% 60% 40% 20% 0% Steam Jet RELIABILITY Time Between Maintenance Cycles Liquid Ring PUmps Rotary Vane Pumps Dry Vacuum Pumps In our example, the customer stated that the plant's boiler was low on available steam, so steam ejectors were ruled out. There was carry-over on an upset basis, so rotary vane pumps were ruled out and the dry vacuum pump system had capital and maintenance costs that were too high. In the end, they purchased a low cost, liquid ring system with high reliability. If the capacity had been 1200 SCFM (20,000 m 3 /hr) or more, then a steam ejector/liquid ring hybrid system would have been the only viable solution. If the above example Utility Costs Energy efficient motors and proper sizing of the vacuum system will ensure the most efficient control of utility costs. Electricity, steam, water and gas can all be quantified to help determine the lowest lifecycle cost for the system. It is very important to identify and specify what the plant's limitations are. You may not have much chilled water capacity but have readily available tower cooling water, or perhaps air cooled exchangers are needed in hot and arid areas. The quality of resources supplied by utilities has a significant effect on the operation and maintenance costs of a vacuum system. Having cooling water with the proper ph is important for reducing calcification, plugging and wear. Normally a water ph of 7 is available, but water carrying more minerals, calcium, chlorine, and/or sediment will cause scaling, which reduces the efficiency of a vacuum system and can damage the machine. The resulting limited heat transfer, choking off of the water flow and mineral buildup will shorten the product's life. ~ 4 ~

5 Any variation in the power supply, beyond specified tolerances (in terms of voltage & frequency (hertz)), can have a detrimental effect on electrical parts, including motors, switching elements, variable speed drives, PLC, etc. Utility supplies should be monitored and controlled because they can have a considerable effect on the over all operating efficiency and cost of a vacuum system. Environmental Impact Every technology will have its own type of environmental impact and care should be taken to meet any local regulations. Typically, the exhaust of the vacuum system is where treatment or capture begins. For phenolic chemistry, high temperature steam ejectors are ideal and dry pumps can also be considered to keep the gas in a gaseous state. For low temperature chemistry, where the product could degrade in high temperatures, liquid ring vacuum systems are a good fit because they perform an almost isothermal compression. For processes with vacuum lower than 1 in HgA (25 mm HgA), a booster-hybrid, an oil sealed vane pump or a dry pump can be used. For applications with carryover, toxic chemicals, hazardous gases, or minimal supervision, a simple liquid ring system is a good choice since they can be designed specifically for these types of challenging services. Assess Manufacturer's Experience For existing vacuum systems, it is best to have a top tier vacuum pump supplier inspect your system. You should provide the supplier with the original design conditions, plus any new ones, and purchase the replacement vacuum pump or system from them. Why? Because they are investing in research & development, have many years of expertise and will perform studies and vacuum audits to accurately determine the current status of your vacuum systems and any improvement needed. This will translate into a savings of 30 to 50% on system capacity and energy efficiency. Specifications Your vacuum system provider will supply drawings based on the specifications you provide. These can include 3D CAD drawings and piping diagrams. CAD software is used to create a "virtual" system design that depicts the package to be manufactured. There are several benefits to having these virtual systems before the design hits the factory floor. One advantage is that the CAD model can be tested by applying loads to the structure using simulation software. This, in turn, can be used to optimize the structural components, so that the optimum amount of material goes into the product. Another benefit of 3D CAD is that a greater number of design variations can be explored. Typical Applications Process Industries Reactor Vacuum Vacuum Distillation Evaporator Columns VCM Recovery Vacuum Filtration Explosive & Corrosive Gas Compression A vacuum system's typical scope of supply includes: motor, vacuum pump, heat exchanger, separator, valves and flow indicator switch. Material of construction should match the needs of the process. Controls are another area where collaboration is required with the vendor to meet the needs of the process. PLC, VFD, and transmitters/switches should be specified to meet the plant's level of control or the feedback desired and a hazardous operation review process, HAZOP, should be performed to ensure safety. Maintenance, Service & Support In regards to maintenance and reliability, a premium supplier's equipment runs 200 to 300% longer, with more uptime and better productivity. This information is based on several different vacuum pump manufacturers' systems running over the past 40+ years. Copied product manufacturers often mislead customers by offering a low capital investment cost, but the real problems begin when the system is installed and underperforms, has high power consumption cost, high maintenance frequency cost and overall limited lifespan of the vacuum system itself. Copycat products consistently have a shorter life and less capacity because the technical expertise remains with the original equipment manufacturer. Thus, copycat manufactur- Solvent Recovery Drying Extrusion Vacuum Degasifiers Dowtherm Vent Systems Vacuum Pump Polyester Production Ejector Compressor Hybrid System ~ 5 ~

6 Above: Steam ejector liquid ring pump vacuum system Left: Blower liquid ring pump vacuum system ers are not able to manufacture them to the appropriate tolerances, develop new patented improvements, determine the condition of the existing equipment, and use the correct metallurgy. They do not know what is needed to return a pump or system to its original efficiency or if it needs to be replaced to meet the current demands -which would save time, money and aggravation. Preventive maintenance is the key to having an efficient, well running vacuum system and also to keeping down time as low as possible. Regional support by the vacuum system supplier is of significant importance. If you don't work on these systems on a regular basis it is best to use a supplier that does. Summary Use your resources wisely by utilizing high quality vacuum systems - they will pay long term benefits for you and your company. A very well designed and selected vacuum system is crucial not only for keeping the overall life cycle cost low, but also for keeping the environment green, consuming less energy (which results in lower CO/ CO2 emissions consume less energy) and ultimately helping to reduce any impact on the environment. Safety and proper engineering are critical for a successful plant, and the supplier should be able to meet industry standards such as ANSI, BIS, and ASME code B31.3 piping specification for chemical/refinery plants. They should also be able to meet API specification 681 & 682. You should require that your supplier provide ISO certificates for any manufacturing plant or service shops. This helps ensure you are getting a high quality supplier with measurable standards. NASH Div. of Gardner Denver 9 Trefoil Drive Trumbull, CT U.S.A. tel: NASH fax: nash@gardnerdenver.com