Ammonia Plant Steam System Optimization

Ammonia Plant Steam System Optimization Steam systems use complex arrangements of turbine drivers, heat exchangers, fans and blowers requiring careful heat exchanger engineering and process engineering services oversight to sustain optimal plant performance.  There are countless ways to design and arrange steam system components to supply the necessary energy requirements to drive rotating equipment, provide heating needs and furnish process steam to the plant.  This article concentrates on improving steam system performance.  The article illustrates a base case plant operation, followed with improvement in Vacuum Surface Condenser performance, improving Steam Superheat, and using the most efficient combinations of turbines in operation and examples of the economics for improvement.

Ammonia Plant Steam System Optimization The steam system for an Ammonia plant is the "heart" of the facility.  It has been said that Ammonia plants are steam producers that also happen to make a little ammonia on the side.  Typically, ammonia facilities generate 4 to 5 times as much steam as the product ammonia they produce.  Thus, the steam system is vital to an Ammonia plant and the selection and arrangement of steam system equipment is key to the plant efficiency.  There are countless ways to design and arrange steam system components to supply the necessary energy requirements to drive rotating equipment, provide heating needs and furnish process steam to the plant.  In this brief article, we shall concentrate on improving steam systems, rather than fundamental design.  Figure 1 on Page 2 illustrates a typical modern 1800 STPD Ammonia plant steam system employing several levels of steam generation as "supply" headers operating in the Base Case at nominal 1500 Psi, 550 psi and 50 psi pressures. In this steam system design, an extraction turbine, (103-JAT), recovers mechanical work from all of the high pressure steam generated in the plant partially driving the synthesis compressor, while providing 550 psi steam, which is then further let down through back-pressure turbines or a pressure control valve furnishing 50 psi steam for process heating and other uses.  A large portion of the 550 psi steam from the extraction turbine is used as Process Steam, with the remainder being used to generate power in vacuum turbine driven rotating equipment applications, or supplies the 50 psi header as shown on Figure 1. Improving Surface Condenser Performance In the Base Case example in Figure 1, the main (largest) vacuum turbine surface condenser, (101-JC), which recovers the exhaust steam from the process compressor drivers is operating at 4 Inches HgA pressure (25.9 Inches Mercury Vacuum.) The operating pressure may be excessively high due to fouling, mechanical problems or overload.  With cleaning, modification or replacement, the operating pressure can be restored to normal design value of 3 Inches HgA.  As illustrated in Figure 2, the vacuum has been restored by renovation or replacement to 26.9 Inches Mercury, (3 Inches HgA pressure) and the steam balance improves as shown.  In this case, high pressure steam generation and boiler water pumping requirements resulting from replacement of the condenser have reduced by about 1.3 %, resulting in energy savings, (at 82% HHV furnace efficiency), of 13.7 MM Btu/Hr (HHV), equivalent to about $346,000 per year savings, based upon an energy cost of $3.00/MM Btu (HHV).  Thus even if equipment replacement were needed to resolve the condenser pressure problem the economics are very favorable to upgrade it.

3 Raising Steam Superheat In general however from actual plant experiences, it usually does fairly closely match the limiting flow for the lower (original) steam superheat condition.  For abbreviation of Figure 3, the details of the "blocks" of back-pressure and vacuum exhaust turbines shown in Figure 1 are not given in the Revised cases (Figures 2, and 3).  The efficiencies of these respective turbines were maintained constant for all revised calculation cases. Higher Steam Superheat Advantages It has long been understood by most companies that increased steam superheat is key to minimizing energy consumption of the steam system and plant, improves capacity of the turbines and reduces load on boiler feed-water pumping.  In this example illustrated in Figure 3, the boiler feed-water pumping load is reduced by 2.2% for 30 Deg F higher 1500 psi steam superheat.  Energy savings at 82% (HHV) furnace efficiency is 13.3 MM Btu/Hr, HHV, equivalent to $335,000 per year savings, based on $3.00/MM Btu (HHV) energy cost.  The economic benefits of improving steam superheat for this example can be illustrated in the following table of economics. Higher Steam Superheat Advantages Generally, plants that are consistently operating at high pressure steam superheat conditions 20-30 Deg F (or more) below maximum allowable or original design basis can benefit from improving the steam system performance by replacing or modifying the furnace steam superheater coil, for the efficiency improvement alone.  Additional benefits for 30 Deg F of high pressure steam superheat improvement for this example include down-firing of the auxiliary boiler associated with the furnace, (about 16%), expanding the capacity of large medium pressure header turbines driving process compressors, (about 1.6%) and potentially some increased plant capacity, if rotating equipment is the limitation due to a power constraint, (about 1-2%.) Utilizing the Most Efficient Turbine Combinations In most steam systems, critical rotating equipment is spared to prevent plant-outages and the spare equipment is not always driven with the same type of turbine.  (ie, back-pressure and vacuum exhaust turbines are sometimes both used for specific rotating equipment items, such as fans and pumps) Examples include lean amine solvent pumps, ID/FD furnace fans, Boiler Feedwater pumps, demin water pumps, lube/seal oil pumps, product pumps etc.  Usually the large centrifugal compressors are not spared, because equipment costs would become prohibitive.  Therefore, the opportunity arises to select a more optimal combination of turbine driven equipment to minimize steam system load and maximize overall plant efficiency. The third example, a "turbine swap" is illustrated in Figure 4 on Page 5.  The steam balance calculations show how the swap from a vacuum exhaust lean amine pump turbine driver for a back-pressure turbine driven lean amine pump improves overall steam system load and plant efficiency.  In both cases the pump load is fixed (Base Case is Figure 1) and the efficiencies of all turbines remain constant. Page 5 Utilizing the Most Efficient Turbine Combinations What changes dramatically is the overall high pressure steam generation, the 50 psi let-down steam to supply the low pressure steam header and the 102-JC surface condenser steam load.  It always makes sense to use a suitably sized back-pressure turbine in place of a vacuum driver for the same rotating equipment service, if an excessive amount of low pressure steam is being generated by let-down, and operating the additional back-pressure service turbine does not result in over-supply of low pressure steam, which could further result in venting to control the low pressure header operating pressure. This example of an optimized turbine selection to maximize the overall steam system performance results in a huge savings of 24.0 MM Btu/Hr, HHV of energy, (0.320 MM Btu/T NH3 HHV), equivalent to $605,000 per year to the bottom line of profits, because no additional equipment is even required.  The equipment is already on the plant-site.  It just needs to be used. Sometimes steam balances change from modifications to the plant and resultant improvements over time.  The old operating philosophies need to be looked at to be sure they are truly best operating practice.  In some instances there are some rare opportunities to save substantial energy without capital investment that are being continuously overlooked. If steam let-down though a valve to the low pressure header is substantial, say 5,000-10,000 pounds per hour (or more), then savings can result from swapping a small vacuum exhaust turbine drive, optimally selected over to back-pressure service.  If the drive does not exist, (ie, two vacuum exhaust drivers in a particular rotating equipment service), then an economic opportunity could exist to create a project to replace one turbine with a back-pressure exhaust selection.  In certain instances, it may make more sense to operate a vacuum exhaust turbine, instead of a back-pressure driver.  The reasoning would fall in-line with the previous discussion.  Usually there would be an excess of low pressure steam, which cannot be fully utilized in the process and thus must be vented.  Such steam may be vented into a surface condenser to recover the water or to a muffled vent stack.  Operating an optimally selected vacuum service turbine in the plant as an alternative could eliminate the excess steam situation, thus saving energy. 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