7.73 x 101
No. of cycles Days per cycle"
It is doubtful that management would agree to shutdowns every 33 days to maximize energy savings when the maximum profit occurs at 355 days (Case 2), or 143 days (Case 3). However, as the energy cost increases, the frequency of exchanger cleaning will increase for Cases 2 and 3. In a real plant, the assumptions of linear losses of heat transfer and production may not be true, but the principles of handling the decision making are still valid.
CENTRIFUGAL PUMP CHARACTERISTIC AND SYSTEM CURVE
CENTRIFUGAL PUMP CHARACTERISTIC AND SYSTEM CURVE
FIGURE 3-2 EXPANSION OF PUMPING SYSTEM
Pumping Efficiency A or B at 750 gpm = 0.50 A or B at 575 gpm = 0.45 Total head - Ft of fluid C at 1100 gpm = 0.50
Two Pumps -Pumps ABC
Pumps AB New Impellers -System Curve
New Pump C
Energy consumption for all distillation processes in the United States in 1976 was estimated at 3% of the entire national energy usage. Since distillation is considered a low efficiency process, it should be possible to improve efficiency with investments of capital and still receive a reasonable return on investment.
Investments may be made in additional exchangers for heat recovery, column revisions, better insulation, or column control. In contrast to these simple changes not requiring capital investments, the more complicated vapor recompression or heat pump changes are reviewed.
4-A. OPTIMIZATION OF HEAT RECOVERY - HEAT EXCHANGERS
The basis for optimizing heat recovery involves the first and second laws of thermodynamics. The first law covers the energy balance, the conservation of energy and the energy equivalence of work and heat. The second law develops the concept of energy level, the irreversible process, and the conversion of heat to work energy.
If one process stream must be heated and can be heated using another process stream without using energy from steam or electricity, the heat recovered saves fossil fuels. The cost savings in energy must exceed the capital investment equivalence of energy for the heat exchangers and ancillary equipment to be worthy of installation.
It is easier to design a new facility with the objective of optimizing energy use than an existing plant. According to Steinmeyer (Seminar on energy
conservation in the AIChE today Series), "----the existing plant cannot economically achieve the same low (energy) usage as a new plant. The cost to return to an existing plant and reinsulate a vessel, add heat exchangers, or increase the number of distillation trays on the basis of energy conservation alone is much higher than starting out in the design phase of a new plant. Thus, any proposed changes in an existing unit must be carefully analyzed so that no expenditure for making the change is overlooked. Changes that reduce profit because certain expenditures were overlooked will be remembered by management when additional changes are recommended.
The amount of heat that can be exchanged depends upon the fluid's temperature level and the amount available. The optimization of heat recovery involves exchanging Btu's at as high a temperature as possible. For example, a vapor product stream is condensing at 350°F in an exchanger using cooling water to remove the heat. The cooling water temperature discharges at 110°F. At this temperature level, the energy in the cooling water has no use and is totally wasted.
To give an example of the amount of heat available, assume liquid stream A is flowing at 10,000 lbs/hr at 400°F, liquid stream B is flowing at 600 lbs/hr, and 400°F too. If both streams must be cooled to 300°F, stream A has the greater availability of heat. If liquid stream C is flowing at 100,000 lbs/hr at 300°F, the heat available above 300°F for transfer is zero. Stream C could be used to heat up a cooler stream, D, to 280°F and then stream A could heat up stream D to 380°F. The method for optimizing heat recovery is described in the technical article by Huang and Elshout (see Appendix 5-C)
A heat availability diagram is shown as Figure 2 in their technical article, "Optimizing the Heat Recovery of Crude Units", by Huang and Elshout. Four streams, the overhead reflux, kerosene pump around, gas oil product, and the residuum are available for exchanging heat with the crude in a 130,000 bbl per stream day crude unit. Each exchange stream has restrictions as to the temperature range that heat can be removed, and the rate of flow. Huang and Elshout had plotted the heat available in "Enthalpy Times Mass Rate", as millions of Btu's per hour for each stream using 0 enthalpy as the lower restriction temperature for the stream available for heat exchange. Figure 4-1 is the same drawing as found in Figure 2 of the Huang and Elshout except the total heat availability curve was returned to its unshifted position.
The total heat availability curve is determined by summing the enthalpy rate for each stream at a given selected temperature. For example, at 300°, the enthalpy rate is 0 (kerosine PA) + 37 (G 0 Product) + 55 (residuum) + 320 (OVH) : 412 million Btu/hr. At 400°F, the enthalpy rate is 55 (kerosine) + 60 (G 0 Product) + 106 (residuum) + 320 (OVHD) = 541 million Btu/hr.
The total heat exchange curve as plotted is right of the crude oil heat requirement curve. At first, this would indicate that the crude can be heated to 645°F and have an excess of 60 million Btu/hr excess (675 x 106 Btu/hr at 645°F -515 x 106 Btu/hr crude requirement). This is not true because heat must be available at the required temperature level. Below 370°F, the slope of the total heat availability curve is less than the crude requirement curve. This means that sufficient heat is available at the proper temperature to heat up the crude. Above 370°F, the slope is greater than the crude curve and insufficient heat is available. Even with infinite heat transfer, the final crude heat exchange temperature must be below 645°F.
Haung and Elshout shifted the total heat availability curve to the left until the two curves touched. They said this represented the maximum amount of heat that can be exchanged with infinite heat transfer. Below the pinch point, we have already concluded that more than enough heat is available at the proper temperature to heat up the crude. Thus, the maximum amount must be represented by the end point of the total available with the shifted curve or 420 million Btu's per hr. The maximum crude temperature is 530°F. When Huang and Elshout studied the heat optimization of this unit, they studied four cases and the maximum temperature reached was 480°F. (See Case D, their Figure 4).
Bannon and Marple of Shell Oil Company presented a paper on "Heat Recovery In Hydrocarbon Distillation" (see Appendix 7-C for paper), in November 1977. They show two ways to improve the thermal efficiency of distillation columns based upon the concepts just discussed. If the overhead vapor from a column is at a temperature high enough to be useful and produces a boiling range top product, the overhead can be condensed into two stages. First, heat is removed to condense only enough of the overhead vapors to produce column reflex. The temperature of the condensation stage is at a higher level than if the entire overhead vapors were condensed in one step. Then, the remaining vapors are condensed and cooled to product conditions. Bannon and Marple described a crude oil distilling column at one of their manufacturing complexes. This column used the two stage condensation approach and transferred 203 million Btu/hr to the crude oil feed. If one stage operation, the heat recovered would only be 122 million Btu/hr, a loss of 81 million Btu/hr.
If heat can be withdrawn from a column to balance column vapor loads and improve separation, the temperature level of the heat removed and made available for exchange can be increased by designing at high circulating rates. The three factors for designing circulating reflux systems are the number of systems, the placement of the systems, and the circulation rate. These factors are described in the Bannon and Marple article.
The heat recovery efficiency of your distillation columns can be checked for possible improvements. This can be done by using the Elshout "Heat Exchanger Network Simulator" program available on the computing service bureau, United Computing Systems (UCS) or other similar programs. You can also develop your own available heat curves. Using the exchangers available in the plant as well as new exchangers, you may be able to hand calculate a fairly good heat recovery system that is economically feasible. 4-B. COLUMN REVISIONS
Many options are available for conserving energy in distillation processes. Mix, et al have outlined and also placed in tabular form guidelines for selecting energy saving options. The more attractive options found in their table and article are discussed below.
4-B-1. Additional or More Efficient Trays - According to Mix, et al, tray changes are economically feasible if:
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