An overview of options for reducing energy use in equipment, processes and utilities
Opportunities for energy conservation in processing plants exists primarily in three areas: equipment, unit operations and processes, and utilities. Fired heaters, heat exchangers and shaft work systems are part of the equipment area. Separation and reaction systems are part of the unit operations and processes area. The utilities area includes the steam/power system, .cooling system and fuel system. Suggestions are provided in each of these areas to help focus efforts on proven techniques for reducing plant energy consumption.
EQUIPMENT
Fired heaters. Methods to improve the efficiency of fired heaters focus on minimizing the loss of useful heat to the surroundings from the hot flue gases. This can be done in. two ways. The quantity of the flue gas can be reduced (within limits set by the stoichiometric requirements for combustion) and the temperature of the flue gas can be reduced by recovering more heat. The quantity of air supplied is set by the oxygen requirements for complete combustion of the fuel. Practically, a slight excess in air is required to ensure complete combustion since fuel and air mixing at the burner is never totally complete. At low excess air, losses result from unburned fuel, while at high excess air the losses result from the heating and release of the excess air. The optimum operating point can be determined from measurements of the CO content in the flue gas. Excess air should be monitored and reduced until the CO concentration in the flue gas just begins to increase.
High flue gas temperatures can be reduced by recovering additional heat to preheat either the combustion air and/or a boiler's feed water. Air preheaters and economizers are usu ally used to recover heat from hot flue gases. The quantity of heat which can be recovered with either is set by the acid dew point of the flue gas. Flue gas energy recovery usually requires additional investment and is principally done during initial design since retrofit in the limited space around a process furnace or boiler is often difficult.
In a boiler it is possible to reduce the fuel required to make steam by raising the temperature of the boiler feed water. There are usually many low temperature hot process streams in a plant which are not hot enough to transfer heat to cold process streams economically but can be used as a source of heat for boiler feed water. This heat leads to a direct reduction in fuel use. Another method to reduce the fuel consumption in boilers is to flash the boiler blowdown to produce low pressure steam. The desirability of this will depend on the current plant requirements for low pressure steam.
Depending on the fuel burned, furnace and boiler tubes can become coated with deposits. This increase the resistance to heat transfer resulting in higher flue gas temperatures. To maximize the use of available heat transfer surface, soot blowing frequency and decoking of tubes should be optimized.
Heat Exchangers. Optimizing of heat recovery involves maximizing the transfer of heat from hot process streams which need to be cooled to cold streams which need to be heated. Targets for heat recovery can be obtained by using "pinch" technology. This technique considers what quantity of heat recovery from hot to cold process streams is possible if the streams could be broken into incremental pieces. Some advanced applications of the "pinch" technique also allow the preliminary identification of heat-exchanger services which are improperly placed and, therefore, may be operating inefficiently.
Efficiency of a heat recovery system normally decreases with time due to increased fouling of the heat exchange surface. Fouling can be reduced by periodic chemical or mechanical cleaning of the exchanger surface, and by the addition of antifoulants. The economic impact of fouling is considerable because in addition to the loss of heat recovery capability, there .are significant maintenance arid energy costs associated with the disassembly and cleaning of the heat exchangers. Therefore, there is a need to balance these costs and set an optimum exchanger cleaning schedule based on minimizing the total annual cost of all the fouling related expenses.
A whole new generation of heat exchange equipment is available. These new exchangers, including spiral and plate/frame, provide several advantages. In many applications, they are less prone to fouling and more easily cleaned. More importantly is the application of these exchangers to services where there is a very small temperature difference between the hot and cold process streams. Because of their configuration, they can effectively transfer heat with a much lower approach temperature than a shell/tube heat exchanger.
Where fouling is a recurring problem, it can be economical to clean the heat exchanger without disassembly. On-line mechanical cleaning of exchangers, once limited to water services, is now being applied to hydrocarbon services. These include brushes and balls which repeatedly pass through the heat exchanger tubes, cleaning the surfaces.
A high heat transfer coefficient is important in maximizing the efficiency of a heat recovery system. For highly viscous fluids, gas flows, or systems operating at turndown, the heat transfer coefficient is often the key limiting factor. In these cases, it may be possible to install turbulence promoters to increase the heat transfer coefficient for the limiting side.
Power providers. Substituting high efficiency motors when replacing standard motors, or as an alternative to the repair of existing motors can lead to immediate savings.
For power needs which consistently vary, or are well below capacity, there are two alternatives. The first is to use variable speed drivers to more closely track the requirements. For long-term turndown operation, impellers can be trimmed avoiding the need for pressure reduction or recycle in addition to reducing energy consumption.
TABLE 1 - Fired heaters/boilers
· Monitor CO and excess air to reduce rejected energy and improve efficiency.
· Consider installation of economizers and air preheaters to recover additional heat from flue gas.
· Preheat boiler feedwater with available low temperature process streams to reduce fuel consumption.
· Maximize use of heat transfer surface by optimizing sootblowing frequency and decoking of tubes.
· Flash blowdown to produce low pressure steam if required.
TABLE 2 - Heat exchangers
· Retrofit heat exchanger networks to maximize heat recovery from existing process streams.
· Use "pinch" techniques to set energy recovery targets and identify inefficient heat exchanger services.
· Determine optimum heat exchanger cleaning schedule using total fouling related expenses method.
· Use new generation heat exchangers such as plate-frame and spiral for increased heat transfer and closer approach temperatures.
· Consider "on-line" mechanical cleaning of exchangers where fouling is a problem.
· Use turbulence promoters in laminar flow and gas phase services and where turndown has significantly reduced the heat transfer coefficient.
TABLE 3 - Power providers
· Use high efficiency motors when replacing or repairing existing installations.
· Substitute variable speed drivers in services where operation is frequently below capacity.
· Trim impellers for continuous turndown operation. . Reduce recycle in compressor operations.
· Consider expanders on FCCU regenerator flue gas and other pressurized streams.
TABLE 4 - Separation systems
· Optimize heat integration between feed and product streams and also between towers in sequence.
· Maximize removal of heat at the highest temperature in pumparounds by use of mid-condensers.
· Optimize the use of the lowest level of heat through the use of mid-reboilers.
· Minimize tower pressure to reduce reboiler duty.
· Replace inefficient tower internals by installing packing and higher efficiency trays.
· Optimize feed location to take full advantage of entire tower.
· Consider rearranging light-ends system for more efficient operation.
· Replace steam jet-ejectors in vacuum pipestills with vacuum pumps.
· Minimize overfractionation when subsequent mixing occurs downstream.
· Replace overhead condensers with extended surface bundles to allow decreased tower temperature.
· Eliminate utility cooling on tower pumparounds.
Numerous processes operate at elevated pressures. Power can often be extracted from the product streams of these units. Typical of such power recovery systems is the use of an expander on a fluid cat cracker regenerator. Although this decreases the quantity of steam which can be generated in a CO boiler, it is more than offset by the additional power provided.
UNIT OPERATIONS AND PROCESSES
Separation systems. Separation systems otter many opportunities for heat integration due to the usually large number of product streams which need to be cooled. In addition, better efficiencies can be realized by improving the contacting.
The atmospheric and vacuum pipestills are among the refinery units which have been the subject of most energy conservation efforts. The principle focus of these efforts is to maximize the heat transfer from the hot product streams to the entering crude oil. This will result in a direct savings in furnace fuel firing. There are numerous techniques and computer programs available to optimize the heat recovery network. Many of these determine theoretical heat recovery targets, establish the optimum network configuration, and size the heat exchangers.
The most energy efficient fractionation tower would provide heat at the lowest temperature and remove it at the highest. It would have condensers for each tray above the feed and reboilers for each tray below the feed. In such a configuration, a higher level of heat can be extracted by some of the condensers while a lower level of heat can be provided to some of the reboilers. Unfortunately, such an arrangement of reboilers and condensers for each tray is not practica1. It is possible, however, to place mid-condensers and mid-reboilers at several locations.
Another way to reduce reboiler duty is to lower the tower operating pressure. Not only will this result in less required energy, but it will also lower the temperature at which the heat is required providing greater opportunity for heat integration.
Improving the contacting within the tower can effectively reduce the amount of energy used. Substituting packing for trays has long been practiced for sidestream strippers and vacuum pipestills. A whole new generation of "structured" tower packing is now available which can result in maximizing energy use by producing higher yields with the same amount of energy.
Quite often, parts of a tower are not used efficiently be- cause the feed is introduced on the wrong tray. When feed is incorrectly introduced, the useful tower height is significantly reduced due to the effect on the concentration gradient. Some analysis is necessary to correct this problem and locate the optimum feed location, but the improvement in yield and the potential reduction in energy use are well worth the effort.
One retrofit for a vacuum pipestill which can significantly reduce steam energy consumption involves replacing the steam jet-ejectors with vacuum pumps. This can be a significant steam saver.
In light-ends systems, several of the towers. are usually heat integrated. It is often possible to adjust tower pressure to facilitate this integration and maximize the heat recovery. In addition, it may be advantageous to consider alternate sequences for the separation. Such changes may increase the amount of heat recovery thus lowering the energy required for tower reboiling.
One way to reduce the amount of energy used for separations is to separate only what you need and do it only once. There are often several streams which are recovered separately from a fractionator only to be subsequently remixed. For example, two sidestreams from a vacuum pipestill may both become feed to a catalytic cracker. In this case, is it not worth using energy to ensure that product specification is achieved for each of these streams. It is not uncommon in a complex plant to have streams separated, remixed, and then partially separated again.
In many cases, decreasing the tower pressure is limited by the temperature of the cooling utility and the duty of the overhead condenser. This limitation often can be overcome by allowing a lower approach temperature in the condenser by using extended surface enhanced heat exchanger tubes. Bundle replacement with low-fin tubes is relatively inexpensive since new foundations or piping are not-required.
Finally, the amount of energy which is literally "thrown away" should be minimized. This includes any streams which are cooled by utilities at temperatures where useful heat recovery is potentially economic. Often there is no heat recovery from the atmospheric pipestill overhead even though this represents a large quantity of heat, albeit at a rather low temperature. It is possible to use this low level energy to start heating the entering crude or boiler feed water. Of greater concern is utility trim cooling on tower mid-condensers and pumparounds. This is usually higher level heat which can be used easily to reboil other fractionators or make steam.
Conversion systems. Many conversion systems, such as hydrotreaters and crackers, produce product streams at significantly higher temperatures than their feed stream. By recovering this heat, less fuel will be required to heat the feed up to the reactor inlet temperature.
Many conversion units are operating at reduced thruput due to excess capacity. One way to save energy in these situations is to decrease the operating severity while keeping the same conversion. This can result in significant energy savings from such things as longer times between catalyst regeneration, and lower recycle compression.
In some cases new, more energy efficient processes can be substituted for existing units. The new separator technology is an example. It consumes much less energy than other methods of recovering hydrogen and can be retrofit easily into a plant. Process changes are also possible, such as substitution of newer solvents for gas treating where 'improved operation as well as energy credits result.
Using higher concentration hydrogen also can result in energy savings in several processes. In hydrotreaters the total feed, including treat gas, needs to be heated. A higher hydrogen concentration reduces the amount of feed and thus the heating requirements.
UTILITIES
The utilities area consists of three energy systems: the steam/power system, the cooling system and the fuel system.
For steam systems, the system balance should be optimized using one of the many computer programs. These allow analysis of the trade-offs between ways of producing shaft work and providing steam heating, and can result in significant boiler fuel savings. One of the largest sources of energy losses in steam systems is from the steam traps. Individually the losses may be small, but there are hundreds of traps throughout a plant. A documented maintenance program should be established to repair or replace leaking or malfunctioning traps.
TABLE 5-Conversion systems
· Maximize feed/effluent heat recovery to reduce furnace fuel requirements.
· Decrease operating severity during turndown to reduce energy consumption at same conversion.
· Minimize recycle gas in reformers to save both compression energy and furnace fuel.
· Consider replacing H2 plant with separators.
· Increase concentration of hydrogen in gas feeds to hydrotreaters to reduce fuel for gas heating.
· Recover pressure energy from FCCU regenerators and cokers using gas expanders.
· Substitute newer solvents in amine gas treating units.
TABLE 6- -Utility systems
· Optimize steam/power balances using computer software.
· Establish steam trap maintenance program to reduce steam leaks.
· Consider electrical tracing of process lines where only part-time heating is required.
· Maximize cogeneration through use of backpressure steam as a heat source.
· Minimize peak electrical power demand since this is a major rate setting factor.
· Replace inefficient cooling tower internals to reduce available temperature of cooling water.
· Maximize efficiency of cooling utility by allocating coldest water to compressor suctions and tower overheads.
· Maximize sale of components from fuel gas to allow for additional energy conservation measures.
In many cases, tracing of process lines is only necessary at night or in cold weather. Steam tracing systems often are not able to respond quickly to changing ambient conditions and are left on all of the time resulting in wasted energy. An alternative is to install electrical tracing which can be controlled much more easily with changes in ambient conditions.
The highest energy efficiency for steam results when it is used more than once. Using backpressure turbines, not only can power be extracted, but the rejected steam can be used for lower level process heating. This cogeneration of power and heat should be maximized to reduce total energy costs.
The cost of electrical power is often set based on peak demand. Minimizing this by using standby gas turbines often can result in a significant savings in overall plant power costs.
Cooling towers can be upgraded by replacing the internals which can improve contacting and result in lower temperatures. Proper allocations of the coldest water to compressor suctions and tower overheads optimizes use of the utility.
The fuel system balance can have a major effect on energy conservation efforts. Savings which result in additional gas flaring are not savings at all. The energy value of the fuel is still expended. Attempts to maximize the recovery of salable products from the plant gas system can help to alleviate this "gas containment" problem.
Tables 1 through 6 provide a checklist of energy saving opportunities in the areas discussed.
