Distillation column heat pump process

Cryogenic distillation has been used extensively ia the processiag of natural gas for nitrogen removal and for helium recovery (22—23). Two basic processes are now used for nitrogen rejection from natural gas— the single-column heat-pumped process and the double-column process. Eadier processes utilized multistage flash columns for helium recovery from natural gas (24). 

An NGL plant was selected to analyze several distillation assisted heat pump processes when compared to conventional distillation. The depropanizer column which is the third column of the NGL plant was suitable for retrofitting by heat pump systems. This conventional process, along with top vapour recompression, bottom flashing and absorption heat pumps, were simulated using the Aspen Plus software, in order to determine economically the best alternative. Distillation with both top vapor recompression and bottom flashing heat pumps allows reduction of operation (energy) costs by 83.3% and 84%, respectively. This improves the economic potential (incorporating capital costs) by 53% and 54%, respectively. 

In the factor methods for cost estimating, first calculate the purchased or delivered cost of all major equipment, for example, distillation columns, reactors, pumps, heat exchangers, etc. Then multiply the total equipment cost by factors to estimate the various other components of the depreciable capital cost given in Equation 2.2, such as piping and electrical wiring. Thus, we arrive at the cost of installing all the equipment and supplying all the services needed to produce an operational process. 

Figures 1.1 and 1.2 indicate increasing interest in process retrofit/revamp from chemical engineering researchers and substantial retrofit/revamp activity in chemical process industries. However, there seems to be only a few books and book chapters devoted to process retrofit and revamp. The book by Lieberman (2010) has many practical strategies to reuse and also improve the performance of existing equipment (such as pumps, compressors, heat exchangers, heaters and distillation columns in chemical process industries) and of several...

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

Heat pumps are increasingly finding applications in the process industries. A typical application is the use of the low grade heat from the condenser of a distillation column to provide heat for the reboiler see Barnwell and Morris (1982) and Meili (1990). Heat pumps are also used with dryers, heat being abstracted from the exhaust air and used to preheat the incoming air. The use of a heat pump with an evaporator is described in Volume 2, Chapter 14. 

For heat pumping to be economic on a stand-alone basis, it must operate across a small temperature difference, which for distillation means close boiling mixtures. In addition, the use of the scheme is only going to make sense if the column is constrained to operate either on a stand-alone basis or at a pressure that would mean it would be across the pinch. Otherwise, heat integration with the process might be a much better option. Vapor recompression schemes for distillation therefore only make sense for the distillation of close boiling mixtures in constrained situations3. 

Example 1.3. Our third example illustrates a typical control scheme for an entire simple chemical plant. Figure 1.5 gives a simple schematic sketch of the process configuration and its control system. Two liquid feeds are pumped into a reactor in which they react to form products. The reaction is exothermic, and therefore heat must be removed from the reactor. This is accomplished by adding cooling water to a jacket surrounding the reactor. Reactor elHuent is pumped through a preheater into a distillation column that splits it into two product streams. 

The dynamic behavior of processes (pipe-vessel combinations, heat exchangers, transport pipelines, furnaces, boilers, pumps, compressors, turbines, and distillation columns) can be described using simplified models composed of process gains, dead times, and process dynamics. 

Internal energy (through the enthalpy, defined in Sec. 2.5) is useful for the calculation of heat and work quantities for such equipment as heat exchangers, evaporators, distillation columns, pumps, compressors, turbines, engines, etc., because it is a state function. The tabulation of all possible Q s and W s for all possible processes is impossible. But the intensive state functions, such as specific volume and specific internal energy, are properties of matter, and they can be measured and their values tabulated as functions of temperature and pressure for a particular substance for future use in the calculation of Q or W for any process involving that substance. The measurement, correlation, and use of these state functions is treated in detail in later chapters. 

The bottoms from the DIB contains most of the nC4, along with some iC4 impurity and all of the heavy isopentane impurity. Since this heavy component will build up in the process unless it is removed, a second distillation column is used to purge out a small stream that contains the isopentane. Some n C4 is lost in this purge stream. The purge column has 20 trays and is 6 ft in diameter. The distillate product from the second column is the recycle stream to the reactor, which is pumped up to the required pressure and sent through a feed-effluent heat exchanger and a furnace before entering the reactor in the vapor phase. 

The flowchart shown here depicts a multi-unit separation process. Three liquid streams are mixed adiabatically the product stream is pumped through a heater to a distillation column, and the overhead product from the column is partially condensed to yield liquid and vapor products. Using the blocks MIX (mix two streams to form a third), PUMP, HEAT, DISTILL, and CNOS, construct a block diagram for the simulation of this process. 

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