Ultimate Guide to Power Efficiency
J 4.1.1 Classification of Energy Saving Techniques j Energy saving techniques can be classified into two categories achieving large energy savings, and are mainly suitable for new i plants or major revamps. The techniques generally involving considerable amount of capital j expenditure are discussed in Section 4.2, while those involving a smaller amount of capital expenditure are outlined in Section 4.3 I Finally, Section 4.4 surveys some of the literature which I specifically describes energy saving techniques, pointing out the strengths and major topics dealt with in each literature source. 4.2 ENERGY SAVING TECHNIQUES FOR NEW PLANT AND REVAMPS
A designed system for the reduction of CO2 emission from coal-fired power plants is presented (28). Microalgal cultivation using CO2 in gas emitted from power plants has been described. In this system, CO2-fixed product by mi- croalgal culture is used as biomass fuel, which will substitute for fossil fuel (Fig. 3). CO2 recycling in coal-fired power plants will achieve the reduction of CO2 emission from power plants. 1. Plant size emission gas from a 500 MW power plant a. One hundred percent of the CO2 produced by the power plant is fed to biological systems, and 90 of the CO2 fed to the system is utilized during daylight summer hours. b. Direct biofixation of CO2 is applied where CO2 in emission gas from the power plant is directly fed to algal ponds for photosynthesis. 4. Operation of the plant The power plant is operated at 70 operation factor. That is to say, the plant is operated at 100 of rated capacity for 18.8 h (70 of 24 h) during the day and is shut down for 7.2 h (30 of 24...
Lactic acid fermentations are very energy efficient, generally requiring no heating or cooking either before or after fermentation. A prime example of lactic acid vegetable fermentations is the sauerkraut fermentation. Fresh cabbage is shredded and mixed with 2.25 w w salt (sodium chloride). The salted cabbage is placed in a crock and covered with a lid or a plastic cover that allows no penetration of air. The natural fermentation (no inoculum required) begins with the development of Leuconostoc mesenteroides. L. mesen-teroides produces both lactic acid and carbon dioxide, which flushes out any residual oxygen, helping to maintain anaerobic conditions. The second organism that develops is Lactobacillus brevis, which produces additional lactic acid and carbon dioxide. This is followed by Lactobacillus plantarum, which produces additional acid. The last organism to develop is Pediococcus cerevisiae, which produces additional acid. The final product has an acidity of about 1.7 to 2.3...
Changes here can cause changes in boiling rate. The best preventive is a steam-to-feed ratio control system combined with temperature and pressure compensation of steam flow. A high pressure drop across the steam valve favors smooth control but velocity-limiting trim may be required to minimize noise and plug and seat wear. A high pressure drop is also undesirable for energy conservation. If the pressure drop is high enough, sonic flow through the valve results, and reboiler steam-side pressure has no effect on flow rate. The design should avoid having sonic flow corresponding to low feed rates and nonsonic flow corresponding to high feed rates, since required controller gain changes make tuning very difficult.
Most chemical processes involve two important operations (reaction and separation) that are typically carried out in different sections of the plant and use different equipment. The reaction section of the process can use several types of reactors continuous stirred-tank reactor (CSTR), tubular, or batch and operate under a wide variety of conditions (catalyzed, adiabatic, cooled or heated, single phase, multiple phases, etc.). The separation section can have several types of operations (distillation, extraction, crystallization, adsorption, etc.), with distillation being by far the most commonly used method. Recycle streams between the two sections of these conventional multiunit flowsheets are often incorporated in the process for a variety of reasons to improve conversion and yield, to minimize the production of undesirable byproducts, to improve energy efficiency, and to improve dynamic controllability. Economic and environmental considerations have encouraged industry to focus on...
In addition, the energy consumption of this process is greater than that of extractive distillation, but less than that of extractive distillation. So it is concluded that this process is especially suitable for the separation of requiring high purity of product. For instance, in the case of separating acetic acid and water, the concentration of acetic acid in water below 20ppm is required in industry.
A reactive distillation process for ETBE synthesis would utilise the same principles as the MTBE process and should yield the same benefits of increased conversion, increased energy efficiency and reduced capital cost. The majority of the reaction (say, 80 ) would be performed in an isothermal, tubular reactor operating at moderate conditions of temperature (around 90 C) and pressure (1500-2000 kPa to ensure all components remain in the liquid phase). The feed to the reactive distillation column would, therefore, be rich in ETBE but still contain some ethanol and isobutene. The products from the bottoms and overheads of the reactive distillation column would be ETBE with some ethanol, and non-reactive hydrocarbon with a small amount of isobutene and ethanol, respectively. The distillate product may or may not require further processing depending on its composition and the refinery configuration. This scheme is shown in Figure 6.4 without ethanol recovery equipment.
It is usually undesirable to put everything through the feed tank, since this typically operates at a lower temperature than Process Unit No. 1. Cooling all the process stream and then feeding it to Process Unit No. 2 normally increases energy consumption in process Unit No. 2 significandy. The control system shown in Figure 5.10A is a simple technique for minimizing energy consumption. The two valves on the makeup and purge lines from and to the feed tank are split ranged so that both valves cannot be open at the same time. This guarantees that a minimum amount of material is going to or coming from the tank.
The chemical system considered in previous chapters featured the classical quaternary two-reactant, two-product A + B , C + D reversible reaction. Some interesting phenomena were discussed. In particular, the effect of the number of reactive trays on energy consumption was demonstrated to be counterintuitive, that is, there is an optimum number of reactive trays that minimizes energy consumption.
The effect of the isobutene feed concentration in ETBE reactive distillation contrasts with MTBE processes where an isobutene concentration of around 60 was found to be optimal for conversion and energy efficiency. A consequence of this result is that, when the isobutene concentration in the hydrocarbon feed is low, ETBE synthesis may be more favourable than MTBE synthesis for some column configurations. However, the presence of significant azeotropes in the MTBE system, between methanol and various butenes, means that isobutene conversion and MTBE product purity can essentially be increased much further by adding stages and or increasing reflux. This does not necessarily apply to ETBE systems.
In dead end filtration, the applied pressure drives the entire feed through the membrane filter producing a filtrate which is typically particle-free while the separated particles form a filter cake. The feed and filtrate travel concurrently along the length ofthe filter generating one product stream for every feed. In CFF, one feed generates two product streams, retentate and permeate. Per pass recovery in through-flow mode is almost 100 (since only the solids are removed) whereas in the cross-flow mode the per pass recovery typically does not exceed 20 and is often in the 1 to 5 range. Recirculation of retentate is thus necessary to increase the total recovery at the expense of higher energy costs.
The easier the separation, the fewer trays required and the lower the required reflux ratio (lower energy consumption). 3. There is an engineering tradeoff between the number of trays and the reflux ratio. An infinite number of columns can be designed that produce exactly the same products but have different heights, different diameters, and different energy consumptions. Selecting the optimum column involves issues of both steady-state economics and dynamic controllability. 1. The farther the VLE curve is from the 45 line, the smaller the slope of the rectifying operation line. This means a smaller reflux ratio and therefore lower energy consumption. A fat VLE curve corresponds to large relative volatilities and an easy separation.
In most distillation columns, the major operating cost is reboiler energy consumption. Of course, if refrigeration were used in the condenser, this heat removal expense would also be quite large. For our propane isobutane example, the pressure was deliberately set so that cooling water could be used in the condenser. Therefore, reboiler heat input is the quantity that should be minimized. The simulation is run using different feed stages. The purities of both products are held constant. The feed stage that minimizes reboiler heat input is the optimum. Table 3.1 gives the results of these calculations. Feeding on stage 14 gives the minimum energy consumption.
(ii) Reducing reflux and reboil - energy savings. j EXAMPLES If the reason for a revamp is increasing capacity or reducing energy consumption, classification into the above categories is straight forward. The effect on utilities is also important. For instance, when a refrigerated column which has been operating at low rates is revamped with valve trays, reflux rate is reduced because of the higher efficiency. If the refrigeration compressor also operates at low rates, it may need to operate on its minimum flow by-pass after the revamp, in which case, little or no energy savings will result.
In extractive distillation, an additional solvent (i.e. separating agent) is used to alter the relative volatility of the components to be separated. In this way, it is possible to obtain one pure component at the top of one column and the other, together with the solvent at the bottom, which may be separated easily in a secondary distillation column due to the high boiling point of the solvent. The solvent doesn't need to be vaporized in the extractive distillation process. However, in azeotropic distillation (discussed in chapter 3), which is also often used for the separation of close boiling point or azetropic mixtures, both the solvent and components must be vaporized into the top of an azeotropic distillation column. Moreover, the amount of azetropic solvent is usually large, which leads to large energy consumption
For pressure-swing distillation to be practical, the azeotropic composition must vary at least 5 percent, (preferably 10 percent or more) over a moderate pressure range (not more than ten atmospheres between the two pressures). With a very large pressure range, refrigeration may be required for condensation of the low-pressure distillate or an impractically high reboiler temperature may result in the high-pressure column. The smaller the variation of azeotrope composition over the pressure range, the larger will be the recycle streams between the two columns. In particular, for minimum-boiling azeo-tropes, the pressure-swing distillation approach requires high energy usage and high capital costs (large-diameter columns) because both recycled azeotropic compositions must be taken overhead. Often one lobe of an azeotropic VLE diagram is pinched regardless of pressure and, therefore, one of the columns will require a large number of stages to produce the corresponding pure-component...
Consider a vacuum column with 10 theoretical stages, operating at 70-mbar top pressure. The bottom pressure will be 170 mbar with trays, but only 90 to 110 mbar with packings. The packed tower will have a much better relative volatility in the lower parts, thus reducing reflux and reboil requirements and bottom temperature. These translate to less product degradation, greater capacity, and smaller energy consumption, giving packings a major advantage.
Unnecessary restraints should not be imposed on the pressure drops permitted across the water side of condensers. All too often, specified design values for pressure drop are too low and much higher values are realized when the unit has been installed and is operating. Not only does this result in more expensive equipment, but frequently the water flow rate is not monitored and cooling water consumption is excessive, increasing operating costs. Because cooling water consumption is governed by factors other than energy conservation and because cooling water velocities must be maintained above certain values, tempered water systems can be effectively used at locations where There are many ways to waste energy in pumping systems. As energy costs have continued to climb, it has often been found that a complete pumping unit's initial investment can be less than the equivalent investment value of one electrical horsepower. Calandria circulating pumps require a certain available NPSH. This...
In this chapter we study distillation columns that have more than the normal two product streams. These more complex configurations provide savings in energy costs and capital investment in some systems. Sidestream columns are used in many ternary separations, and the examples in this chapter illustrate this application. However, a sidestream column can also be used in a binary separation if different purity levels are desired. For example, two grades of propylene products are sometimes produced from a single column. The bottoms stream is propane, the sidestream is medium-purity propylene, and the distillate is high-purity polymer-grade propylene.
We would expect that a single reactive column that is operated neat will have lower capital investment and energy costs than a two-column system. The purpose of this chapter is to give a quantitative comparison of these two alternative processes. One example of an industrial system with excess reactant is the ETBE system. A 10 -20 excess of ethanol is fed to the column. If the excess ethanol can be included in the ETBE bottoms product from the column and blended into gasoline with the ETBE, there may be no economic penalty and no need for recovering the excess ethanol. However, in other systems, the excess reactant must be removed from the product and recycled. This involves an additional separation step, so capital investment and energy costs are increased.
Increasing reflux ratio requires fewer trays (less capital cost) but increases energy costs. Economic optimization studies have led to the commonly used heuristic (rule of thumb) that the optimum actual reflux ratio is 1.1 to 1.2 times the minimum reflux ratio (see Figure 2.28).
Procedures for solving several types of optimization problems commonly encountered in the design and operation of conventional and complex distillation columns are presented. The continued increase in energy costs for operating distillation columns has created the need for rapid calculational procedures both for the design of new distillation columns and for the selection of optimum operating conditions for existing columns. Problems of the following types are solved (1) determination of the minimum number of stages required to effect a specified separation at a given reflux ratio, (2) optimum economic design of a distillation column, and (3) minimization of the reflux ratio for an existing column by determination of the optimum feed plate location. These problems are solved by use of a modified form of the search procedure called the complex method, which was proposed by Box.2 The primary modification consists of including the constraints in the objective function which reduces the...
Selectivity in batch ion exchange adsorption can be enhanced by judicious choice of adsorption buffer conditions (e.g., moderate salt concentration) to exploit differences in properties between the desired product and contaminants. This may be especially true in the case of recombinant proteins, which may be fundamentally different from the host organism's natural proteins. Adsorption equilibria that rely on weak physical interactions such as hydrophobic interaction, hydrogen bonding, and ion exchange are desirable for separations because these low-energy interactions can be readily reversed. Additional selectivity can be introduced during the desorption step so that adsorption processes are not without resolving power. Nonetheless, high selectivity is not the primary requirement for such an early process step.
REPLACING THE COLUMN BY A SECOND-HAND COLUMN This is another expensive revamping technique. Depending on the column available, this technique can be used for increasing capacity, reducing energy consumption, or both. ADDING AND MODIFYING HEAT EXCHANGERS This technique is commonly used for energy saving revamps, but may also be effective in capacity revamps. The main types of exchangers concerned are interreboilers, intercondensers, feed preheaters, feed precoolers and feed-bottom interchangers. Adding or modifying these exchangers is relatively cheap compared to other external hardware changes. Application of these exchangers for energy savings has been discussed in Section 4.1. In general, interreboilers and feed preheaters will significantly save energy if a heating medium cheaper than the reboiling medium is available at a temperature and flow rate sufficiently high to reduce reboiler heat duty. Similar considerations apply to the cooling medium used in a feed subcooler or an...
The final, overall objective of any process control application should always be to maximise the profitability of the process under control. This is normally achieved via a rationalisation of the value added by the process with the energy that is consumed by the process. For example, in a conventional distillation column, increasing the internal vapour and liquid flows nearly always increases the separation of key components and, therefore, increases either the product yield or its value. However, the increase in internal flow rates is only achieved at the expense of additional energy consumption in both the condenser and reboiler. With most reactors and many other unit operations, this principle often manifests itself with respect to the heating or cooling requirement, or the recycle rate. An effective control application adjusts the process operation towards an optimum where the incremental value added is just less than the incremental cost of the energy and raw materials. The...
As the flash chamber runs at steady state the evaporation rate is constant and heat flux Qx to the condenser tubes is constant, Now, as we have seen the stage heat transfer coefficient U is a constant, then if a low energy input into the brine heater is desired fin must be as near as practicably possible to imax, so that the with corresponding logarithmic mean temperature difference of 6.2 C (11.2 F). Thus the plant with the lower performance ratio will have a 45 per cent reduction in heat transfer area compared with the high performance ratio plant (given that all other factors are the same for both plants). This illustrates the relationship between energy costs and capital costs. A high performance ratio plant would be specified for a location where fuel is expensive whereas the low performance ratio plant would be used where fuel is very cheap, e.g. in Kuwait.
A control system consists of three parts a measurement a control algorithm and a process actuator. The process actuator (often a control valve) is always a direct user of energy the measurement may take energy from the process (as in the case of a head-type flow meter) and the control calculation never requires a significant energy supply. However, the correct control calculation is essential for energy-efficient operation of any process.
During each pass of a continuous column operation, the feed flow rate, feed location, vapour boilup rate, etc. will influence the recovery of component, time of operation, energy consumption, etc. Also the number of equilibrium stages and the relative volatility of the components in the feed mixture, feed location will dictate the number of passes which will be required to achieve a high recovery and high purity of a particular component. However, very little information regarding these was available in the literature until the work of Mujtaba (1997). Whether the CBD operation can be replaced by the continuous column operation without compromising the efficiency in terms of recovery of component, total time of operation and energy consumption is an open area. Mujtaba (1997) has addressed some of these issues using optimisation techniques.
In batch distillation, as the overhead composition varies during operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main-cuts are usually determined by the market or downstream process requirements but the amounts recovered must be selected based on the economic trade off between longer distillation times (hence productivity), reflux ratio levels (hence energy costs), product values, etc. Increasing the recovery of a particular species in a particular cut may have strong effects on the recovery of other species in subsequent cuts or, in fact, on the ability to achieve at all the required purity specifications in subsequent cuts. The profitable operation of such processes therefore requires consideration of the whole (multiperiod) operation.
Here we examine another application of nonadiabatic columns - to decrease energy consumption in separation. Nonadiabatic columns are widely used for this purpose in petroleum refining (heat output by pumparounds ). If it is accepted that the price for input heat is proportional to the value (1 T0 -1 Teb) and the price for output heat is proportional to the values (1 TOn -1 T0), where T0 is the ambient temperature, and amount of input or output heat is proportional to liquid flow, then the cost of energy consumption in the main and intermediate reboilers will turn out to be proportional to the hatched area in
Fermentations having a high oxygen demand must be agitated with sufficient power to maintain a uniform environment and to disperse the stream of air introduced by aeration. In an early reference it was stated that the cost of energy necessary to compress air for yeast production proved that a considerable amount (10 to 20 ) of the total production expenses was due to aeration (de Becze and Liebmann, 1944). Swartz (1979) has reported that the mixing costs in a penicillin fermentation are 15 of the total production costs. Energy consumption for a stirred aerobic fermentation to provide agitation, air compression and chilled water is approximately 8.2 kWm 3 (Curran and Smith, 1989). Assuming an electricity cost of 0.07 kW-1, a 6-day antibiotic fermentation in a 100-m3 fermenter with a 1-day turnaround would use 8,000 of power (Royce, 1993).
The control of column AP by throttling steam to the reboiler was once very popular in the chemical industry, particularly for small columns. The usual practice was to run at a boilup that would give considerably more reflux than called for by design. This would usually provide a product purer than specification. In an era when it was common practice to overdesign columns (low f factors, bubble-cap trays, and extra trays) and there was litde concern about saving energy, this approach to control did have the advantage of usually providing a good-quality product with simple instrumentation. For today's tighdy designed columns, it is technically less satisfactory, and with the rapidly rising energy costs its wastage of steam is economically unattractive. Nevertheless we still have an interest in this control technique for override purposes an override controller is now commonly used to keep column A P from exceeding the maximum value specified by the column designer or determined by plant...
A mixer applying 150 kW to the mixer shaft is operating in a batch fermentation at a cell concentration of 20gII. Associated with the mixer is a blower, which is providing air at a total expansion horsepower of 37 kW leaving the sparge ring. The cost of power is 0.50 MJ. Use an overall energy efficiency for the equipment of 0.9.
Figure 18.4 Feed tray locations and corresponding energy consumption (compared to base case) from optimization results. Figure 18.4 Feed tray locations and corresponding energy consumption (compared to base case) from optimization results. In addition to the percentage of energy savings, comparisons are also made in terms of the profiles of temperature, compositions, fraction of total conversion, and forward and reverse specific reaction rates. Figure 18.5 shows that the optimal feed arrangement (Fig. 18.5b) has a much sharper temperature profile in the reactive zone than the conventional feed locations (Fig. 18.5a). The tray temperature almost reaches 390 K in the optimal case, but barely reaches 380 K in the conventional case. Further, the profiles of the tray conversion and rate constant also take a qualitative shape similar to that of the temperature.
In the first case, the relative volatilities are aC 6, aA 3, aB 2, and aD 1. With the conventional feed arrangement (reactants fed at the two ends of the reactive zone), we have 33 more energy consumption (0.0428 kmol s) compared to that of the base case. This shows that, similar to conventional distillation, difficult separation, even between reactants A and B, requires more energy. Moreover, Figure 18.6 shows that the composition of A is higher toward the lower reactive zone compared to the base case, and this leads to a decrease in product D composition, which subsequently requires a larger vapor rate to meet the specification. The optimization procedure predicts that the optimum feed trays are NF,A 11 and NF,B 13 (Fig. 18.6b), and this configuration produces a 15.2 energy savings (from 0.0428 to 0.0363 kmol s) over the conventional feed arrangement (Table 18.2). Note that the percentage of energy savings is computed with respect to the conventional feed arrangement in each case....
From the comparison of different biofilters in use in Europe (1,37,81-83), total operating and maintenance costs ranging from 0.2 to 0.5 per 1,000 m3 of waste gas can be estimated, which includes depreciation and interest, water consumption, replacement of filter materials and personnel, wastewater treatment, and energy costs for compressors. Costs reductions showing a half of these amounts for United States installations do not include expenses for support replacement and reflect the lower cost of electricity (1).
It is not often relevant to talk of the quantity of energy consumed per unit of water produced as a basis for process comparison. What must be compared is the energy cost per unit of water, e.g. reverse osmosis will consume electrical power whereas distillation uses thermal, but one kWh of electrical energy is much more expensive than one kWh of thermal energy (approximate ratio of 4 1). A distillation plant may therefore have a specific energy consumption of three times a reverse osmosis plant and still be competitive on energy cost terms. When comparing plants of the same type, comparisons can of course be made in terms of energy consumption but sight must never be lost of the fact that it is product cost which matters and the lowest product cost for any given condition is a function of both energy and capital costs. Energy costs can be reduced in distillation plants at the expense of increased capital expenditure and a trade-off is made between the two - a theme which subsequent...
Evaporators should be designed to make the best use of available energy, which implies using the lowest or the most economical net energy input. Steam-heated evaporators, for example, are rated on steam economy pounds of solvent evaporated per pound of steam used. 2
For higher pressure-drops, a centrifugal compressor would be the best choice, however, the energy consumption would be prohibitive in many cases. If possible, it is advisable to design and operate the bioreactor and air preparation system in such a manner as to minimize the pressure drop, such that it is possible to use a fan, rather than to work with a compressor. The best strategy is to select the equipment that provides the largest pressure range for the maximum required flow-rate, this flow-rate being deduced from the energy balance model. It is then necessary to check whether the equipment will operate economically in terms of energy consumption at the required combination of pressure and flow rate. If energy consumption is too high then possibly an inferior blower will need to be selected. This may not be capable of meeting the aeration needs during the periods of peak heat generation, so the performance of the process may be deleteriously affected. As stated above, many of the...
The computing-time investigations for regions 1, 2, and 4, which are particularly relevant to computing time, were performed for the functions listed in Table 45. Each function is associated with a frequency-of-use value. Both the selection of the functions and the values for the corresponding frequency of use are based on a worldwide survey made among the power plant companies and related industries.
Table 8.11 gives the basis for equipment sizing and the costs used for equipment capital investment and energy. Design parameters, capital costs, and energy costs for the separation sections of both processes are listed in Table 8.12. The reboiler heat inputs in the two columns of the extractive distillation process are about 30 of those in the pressure-swing process. This reduces column diameters and heat exchanger areas, so the capital cost is also much smaller (about 40 lower).
In the first case, we have aC 16, aA 4, aB 2, and aD 1. With the conventional feed arrangement, the energy consumption (0.0285 kmol s) is 10.9 less than the base case because of the large relative volatility between C and A. The optimization calculations show that the optimum feed trays become NF A 12 and NF B 14 (Fig. 18.7b). Compared to the conventional feed arrangement, this corresponds to a 46.8 energy savings (from 0.0285 to 0.0152kmol s) This is a very significant energy savings by very simple means (feed rearrangement). Figure 18.7 Profiles of temperature, composition, fraction of total conversion, and reaction rate constants in the reactive zone for aC aA aB aD 16 4 2 1 system with (a) conventional feed arrangement (NF,A 6 and NFB 16) and (b) optimal feed arrangement (NF,A 12 and Nf,B 14) with 46.8 energy savings. Figure 18.7 Profiles of temperature, composition, fraction of total conversion, and reaction rate constants in the reactive zone for aC aA aB aD 16 4 2 1 system with...
The final design parameter to be studied in this chapter is the locations of the two fresh feed-streams. Up to this point we have assumed that the lighter reactant fresh feed F0A is introduced on the bottom tray of the reactive zone (NS + 1) and the heavier reactant fresh feed F0B is introduced on the top tray of the reactive zone (NS + 1 + NRX). This configuration seems like a logical choice. However, the fresh feeds could be introduced on trays inside the reactive zone. The question is how this affects the design, primarily in terms of energy consumption because this is our major economic performance criterion.
Base Case with Low Activation Energies. Figure 18.9a shows that, with the conventional feed arrangement, the profiles for the temperature, composition, and fraction of total conversion are qualitatively similar to those of the higher activation example (Fig. 18.5a). However, the profile of the backward reaction rate is smaller. The energy consumption is higher in the present example compared to one with a higher activation energy (0.0393 vs. 0.0320 kmol s). The reason for this is that energy is no longer released from the reactions, and the effect of direct heat integration disappears. The optimization calculations show that the optimal feed trays becomes NF,A 11 and NF,B 15, and this results in a 10.7 energy savings when compared to the conventional feed arrangement (see Figure 18.9 Profiles of temperature, composition, fraction of total conversion, and reaction rate constants in reactive zone for base case with low activation energy using (a) conventional feed arrangement (NF,A 8...
Besides splits without distributed components, we also discuss splits with one distributed component 1, 2, k _ 1, k k, k + 1, n. The significance of these splits is conditioned, first, by the fact that they can be realized for zeotropic mixtures at any product compositions, while at two or more distributed components only product compositions, belonging to some unknown regions of boundary elements of concentration simplex, are feasible. Let's note that for ideal mixtures product composition regions at distribution of several components between products can be determined with the help of the Underwood equation system (see, e.g., Fig. 5.4). This method can be used approximately for nonideal mixtures. From the practical point of view, splits with one distributed component in a number of cases maintain economy of energy consumption and capital costs (e.g., so-called Pet-lyuk columns, and separation of some azeotropic mixtures Petlyuk & Danilov, 2000 ).
Distillation Trajectories and Minimum Reflux Mode in Two Feed Columns with Nonsharp Separation in Intermediate Sections
Gradual heating and evaporation is used in the case of separation of mixtures with a wide interval of boiling, when heat is put in at a lower, and cold is put in at a higher temperature, compared with their input in the reboiler and condenser. This allows for a decrease of total energy consumption in separation. tie-lines liquid-vapor) and the points of top xD or bottom product xB, along with that, in accordance with Eq. (6.3) can be located only in the vicinity of sides 1-2 or 2-3 in Fig. 6.3 or facets 1-2-3 or 2-3-4 in Fig. 6.4. Hence, it follows that feasible splits for columns with one or two feeds are the same (i.e., if the flows of several feeds are mixed before separation, we can only get the same products as in a column with several feeds, but the energy consumption for separation will be bigger). the height of the column, which requires the calculation of the minimum reflux mode at different successions. For the column with two feeds, one has to begin the calculations with...
Figure 2.5 demonstrates the effect of changing the number of reactive trays NRX with all other parameters held constant at base case values. The important graph is the upper left one that shows how vapor boilup (or energy) changes as the number of reactive trays is varied. It is quite unexpected that there is a minimum in this curve, which says there is an optimum value for the number of reactive trays in terms of energy consumption. This is certainly different than in conventional distillation in which adding more trays always reduces energy consumption. Figure 2.6 gives temperature profiles with several numbers of reactive trays.
This chapter embraces a few minor processes each with their own sphere of applicability. The first is vapour compression distillation where the energy input is supplied by a compressor instead of a heat exchanger. Next is solar distillation which relies on solar energy for its operation thus entailing different design concepts from conventional methods. The opportunity will also be taken to discuss within the umbrella of this chapter one of the fringe developments in the distillation field, namely low temperature difference distillation which may grow in importance as energy costs increase.
For a binary distillation, if the pressure is fixed, a single temperature measurement will provide a reliable guide to composition. But pressure is often not very constant. Today it is sometimes deliberately allowed to float to minimize energy consumption (see Chapter 8). Even if one fixes pressure at one end of a column, it will vary at the other end as a function of boilup rate. An early approach to compensating for pressure variations was to use two temperature measurements, one usually near one end of the column and the other at an intermediate tray. One was then subtracted from the other. For binary or almost-binary distillations, this works fairly well as long as boilup does not change much. If, however, boilup does change significantly, and if the two temperature measurements are separated by a substantial number of trays, one may encounter a nonmonotonic relationship between boilup and AT. As pointed out by Boyd,3 an increase in boilup tends to decrease AT due to increased...
In this chapter the steady-state economic optimization of a distillation column is discussed. Basically, we need to find the optimum number of total stages. There are some simple approaches, and there are more rigorous approaches. The simple methods use heuristics such as setting the total number of trays equal to twice the minimum. The rigorous methods determine how the capital and energy costs change with the number of trays and find the minimum total annual cost design.
For anyone seriously interested in distillation control, two books are highly recommended. The first is an easy-to-read, nontheoretic (as far as control is concerned) work by F. G. Shinskey.11 The treatment of energy conservation alone is worth the price of the book. The second book is by Rademaker, Rijnsdorp, and Maarleveld.12 It relies heavily on conventional, single-loop control theory, and explores painstakingly a large number of possible control systems. It also contains an extensive bibliography.
The original document was Energy Conservation Seminars for Industry Texas Energy Conservation Program Distillation Column Operations by J. E Sirrine Company. Within the confines of HTML, the text has been converted to an approximation of the material. At some points it is unclear in the original document if a typographic change was made to organize the text or to simply make text fit better on a page. As closely as possible, the organization of the material has been maintained. Very few corrections have been made to the original text, even where errors may be present. The intent has been to maintain the original document.
The steady state simulation of a series of reactive distillation columns and processes for the production of ETBE and MTBE illuminated a number of important issues related to the optimal design techniques. Many of these issues are peculiar to reactive distillation and would not reasonably be anticipated without a priori knowledge of the phenomena involved. For example, the addition of theoretical equilibrium stages and an increase in the reflux ratio do not always have a directionally equivalent effect. The trade-off between energy consumption and capital cost which is the basis for most distillation designs cannot always be applied to reactive distillation. Importantly, the use of standard modelling techniques for equilibrium processes was also validated for reactive distillation design.
For a given feed composition and enthalpy, there is an optimum feed-tray location that permits making the specified separation with the least energy consumption. It is also the tray that will permit maximum feed rate without causing the column to flood. As shown in Figure 5.6, a column should generally be equipped with a number of alternative feed trays to handle changes in operating conditions from those assumed for design. The magnitude of the energy savings to be realized by changing feed tray location can be very significant in some systems (10-20 percent reduction in heat input), but in other columns the effects can be small. Each system must be examined to determine the strategy and the incentives for controlling to an optimum feed-tray location. Sometimes unexpected results occur. Luyben5 has shown that the optimum feed-tray location in some columns rises higher in the column as the feed becomes fighter (increase in more volatile component concentration), while in other columns...
The information presented herein is intended to enhance knowledge of industrial energy conservation and to provide the necessary tools to implement an energy conservation program in an industrial plant. References to specific products or ideas should not be considered endorsements of said products or ideas by the Texas Industrial Commission. TEXAS ENERGY CONSERVATION PROGRAM DISTILLATION COLUMN OPERATIONS These materials were prepared as a result of work sponsored by the Governor's Office of Energy Resources through funds provided by the Department Energy. Neither the Texas Industrial Commission, nor the sponsoring agencies, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, expressed or implied, or assumes any legal liability for the successfulness of the implementation of energy conservation techniques described. References to specific ideas, products, and services should not be construed as endorsements. It is hoped...
For most other services, either sieve or valve trays are the best choice. Sieve trays are at an advantage when the service is fouling, or corrosive, or when turndown is unimportant, while valve trays are preferred when turndown is important. With high energy costs, the energy saved during even short turndown periods usually justifies the relatively low cost difference between valve and sieve trays. This has made valve trays most popular.
In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by
Several other advantages may be recognised. There are reductions in both energy usage and labour costs per unit volume of beer produced. The relatively high ethanol concentrations formed during fermentation promote increased precipitation of polyphenol protein material and, therefore, high-gravity beers have better colloidal stability than standard gravity fermented products (Whitear & Crabb, 1977). Relatively high ethanol yields impart greater microbiological stability to beers. Yeast growth extent increases with increase in wort concentration however, the relation is not pro rata and high gravity fermentations are more efficient and produce a com-mensurately greater yield of ethanol. Nevertheless, it is necessary to increase the yeast
Level control is cascaded to flow control. With floating pressure columns and with the trend toward small control-valve pressure drops for energy conservation, this is virtually mandatory to counteract the effect of control valve up- and downstream pressure variations. The flow measurement must be linear if an orifice flow meter is used, it must be followed by a square-root extractor.
AUXILIARIES Figure 4.8b shows the same flow scheme, but with several auxiliary equipment items. These are items which may or may not be included in heat pump circuits to improve energy saving or control. At least some, but rarely all of these items are always included in heat pump schemes. When evaluating a heat pump scheme, it is important to ensure that the appropriate items are included failure to do this can sometimes lead to erroneous conclusions. Ref 10 presents a case history demonstrating that an analysis reported in a previous paper (11) reached an incorrect conclusion because it did not adequately consider an interreboiler in the heat-pump circuit.
We are now ready to summarize the graphical design technique for determining the number of trays required to achieve desired product purities, given a reflux ratio. As we will show later, the lower the reflux ratio specified, the more trays are required. Since increasing reflux ratio increases energy costs (F La + D), while increasing trays increases investment costs, distillation design involves a classical engineering trade-off between the two design variables reflux ratio and number of trays. This will be discussed further under Limiting Conditions. Given
Enough for all the sugar to have been completely used up. And then there are all those unwanted side reactions which produce the congeners such as methanol, fusel oils, etc., instead of ethanol. Another place where losses occur is in the last stages of beer-stripping where time and energy consumption require that the stripping cease long before the last drop of alcohol has been extracted. As a result, the practical yield of 95 alcohol is likely to be no better than about 5 litres which is a yield of 73 of the theoretical value. This is equivalent to 11 2 litres of gin, which is not too bad.
Microalgal production of useful chemicals and energy resources have been extensively investigated. Processes that utilize the majority of the resulting microal-gal biomass as energy sources would be desired. Such processes may allow the recycling of evolved CO2 from human energy consumption rather than a one-way emission, as is the case with fossil fuels. The following six products can be produced from microalgal biomass for use as fuels hydrogen (through biophotolysis), methane (through anaerobic digestion), ethanol (through yeast or other alcohol fermentation), triglycerides (through extraction of lipids), methyl ester fuels (through transesterification of lipids), and liquid hydrocarbon (from Botryococcus braunii).
The combination of reaction and separation within a single unit operation not only reduces the overall capital cost of a process but provides process benefits in some cases. These benefits arise from the constant recycling of reactants to the reaction zone which increases the conversion of the limiting reactant in an equilibrium limited reaction. A secondary benefit is the increased energy efficiency which results from directly utilising the heat of reaction for fractionation. Reactive distillation is usually justified on steady-state process results but the increased process complexity reduces operability and controllability compared with a conventional two-stage process. Consequently, the full steady-state benefits may not be realisable in dynamic operation, particularly if regular disturbances are likely.
Geyer and Kline1 give, as an example, a 70-tray column separating a mixture with a relative volatility of 1.4 and with specifications of 98 percent low boilers overhead and 99.6 percent high boilers in the base. If the operator adds enough boilup and reflux to increase overhead purity to 99 percent and base purity to 99.7 percent, an increase of 8 percent in energy consumption results. 6. Possible use of mechanical vacuum pumps. For vacuum columns there is some opinion1 that mechanical vacuum pumps offer energy savings over steam jets. The difference, however, is usually small.
he feed system for a column should function as a filter for incoming disturbances in feed flow rate, feed composition, and sometimes feed enthalpy. For minimum energy consumption operation, it should also send the feed to the proper feed tray. And for startup shutdown of the column being fed, it may serve to receive recycled column product streams. In the following discussion, we will assume that (1) process material-balance control is in the direction of flow, (2) feed-tank level control is of the averaging type, and (3) there is good mixing in the tank.
The cost of energy varies quite a bit from plant to plant. In some locations energy sources are plentiful and inexpensive. For example, in Saudi Arabia gas coming from an oil well is sometimes simply flared (burned). In other locations, fuel is quite expensive because it must be transported long distances. For example, in Japan some of the natural gas is shipped in from Indonesia on liquefied natural gas tankers (LNG), which are very expensive. Therefore energy costs depend on location. A value of 4.7 per kilojoule is used in the results presented below.
The TAC comparison in Figure 7.16 shows that the type II (EtAc and IPAc) flowsheet is the most expensive process, followed by the type III flowsheet (BuAc and AmAc). The type I flowsheet is the most economical process. This is because the type II flowsheet boils up both products to the top of the reactive distillation column (Fig. 7.2) and recycles an almost ternary azeotropic composition back to the decanter from the stripper. Thus, this flowsheet is energy intensive and requires significant capital investment in the heat transfer area. The difference in the TACs between EtAc and IPAc comes from the VLLE advantage of the IPAc system, which has a larger LL envelope (Fig. 7.1). The type III flowsheet, despite not boiling up both products to the top, needs to boil up some acetate alcohol along with H2O to get into the LL envelope and then recycles the organic phase totally back into the column. We would expect larger energy consumption for this type of system compared to systems without...
Because the design goals are essentially economic, the relationship of the variables shown in Figure 1 to the process costs must be kept in mind. The volume of the commercial-scale bioreactor equals the actual gas flow to be treated (a process requirement) divided by g, a large value for g translates directly into a smaller and, all other things being equal, cheaper bioreactor. The liquid flow, f should be kept as small as possible because providing clean water and disposing of any process wastewater both cost money, as do any nutrients and chemicals that must be added. The pressure drop, h, through the bioreactor may be significant, particularly on a large scale, and it determines the capital cost of the gas compressors and the energy costs for running them. Compressing the feed gas is not only expensive in itself, it heats the feed such that heat exchangers may be needed to cool it to the desired temperature.
You can make a boiler yourself (paying due attention to grounding and electrical safety, or to fire and ventilation if using open flame heating) or you may choose to buy a boiler ready-made. Small electrical water heaters are a good option because they're compact, sturdily made, and are usually well heat insulated (which saves on your electricity bill). Using a water heater for a boiler is described in more detail later, in the section on Pot Stills.
For the rectifying tray number NR we use the liquid composition right above the reactive tray for xB, and for NS we use the vapor phase composition right below the reactive zone as xD. This leaves us with the feed tray locations as the design variables. Because all tray numbers are determined, the effects of feed tray locations can be compared by simply looking at the energy consumption (i.e., vapor boilup rate).
The choice of the material is strongly influenced by the need to minimize the overall volume necessary for the reactor, optimize the removal efficiency, keep energy consumption to a minimum, and minimize maintenance. In addition, the characteristics of the carrier material impact directly on the microbial growth and activity, thus in turn affecting biofilter performance. Biofilter beds have the advantage of immobilizing the microflora on the packing material, as a result of which these organisms, forming a bio-layer, are not drained from the system, as is often the case in freely dispersed systems.
Although MD is still emphasized as a low cost, energy saving and potential alternative to traditional separation processes, special membranes only for this purpose haven't been provided yet. At present the membranes employed in MD are those made for micro filtration purposes, because most of the required specifications by MD processes are available from those membranes. Hydrophobic microporous membranes made of polypropylene (PP), polyethylene (PE), polytetrafluoro ethylene (PTFE) and polyvinyl id ene fluoride (PVDF) are the commonly used membranes in MD, These membranes are fabricated in the form of either flat sheets or hollow fibers. A micrograph of a flat sheet membrane made of PVDF is shown in Fig. 3, as well as a hollow fiber in Fig. 4. By far, there are many kinds of commercially available microporous membranes maybe feasible for MD. However, as pointed out by many researchers 4 , the requirement for MD membranes is a higher permeability, lower membrane thickness, higher liquid...
The other columns in Table 4.3 give results for columns with other total stages. If the number of stages is reduced to 24, which gives a shorter column, reboiler heat input increases. This increases column diameter and heat exchanger areas. This results in an increase in both capital and energy costs. If the number of stages is increased, the column becomes taller, but its diameter becomes smaller because reboiler heat input decreases. This decreases heat exchanger costs and energy costs. However, the cost of the vessel increases because it is longer. So the effect of increasing the number of stages is to increase the capital cost of the shell and to decrease the capital cost of the heat exchangers and energy costs. As more and more
Actually, this is a relatively simple reactive distillation column with moderate energy consumption, despite having a relatively large number of reactive trays. Having one reactant that is the HHK (reactant B) has its advantages and disadvantage. The HHK increases tray temperatures when we have significant amounts of this heavy reactant, which is advantageous for the reaction (see the temperature profile in Fig. 17.15b). The down side is that we have to react away almost all of the HHK in the reactive zone (otherwise it will end up in the column base), and this leads to a large number of reactive trays. This is clearly illustrated in Figure 17.15a, where we have a very small amount of conversion between tray 9 and tray 40. The purpose for this portion of the reactive trays is to consume the remaining heavy reactant (HHK component B). The composition of the LK reactant (component A) is kept fairly constant below the feed point to ensure the dominance of the forward reaction (Fig....
Although the control objectives have been established, the actual controlled variables and manipulated variables are still to be decided. Many choices are available but the most likely candidates for the controlled variables are the various flows, temperatures, pressures and compositions in the system. However, the final choice is not restricted to these parameters and the use of composite variables such as the column flooding factor or the energy consumption could be advantageous for some economic scenarios. The manipulated variables are effectively selected in choosing the control structure that will be used.
Although this book deals primarily with distillation control in design projects, it is pertinent to consider briefly the controls of typical, existing columns, the opportunities for their improvement, and how to troubleshoot them when necessary. Frequently encountered problems include unstable or ineffective controls, off-specification product or products, and flooding or dumping. In addition, it is fairly common practice to use excessive boilup and reflux to make sure of meeting or exceeding product specifications. This not only wastes energy it also reduces column capacity. To provide a perspective on energy savings, one may note that 100 lbm hr of steam is worth 3200 per year (basis 4.00 1000 pounds, 8000 hours per year). To save this amount of steam would probably be only a modest accomplishment for most columns. In view of the preceding comments about problem areas and likely opportunities for improvement of composition control and reduction in energy consumption, the following...
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