Flash separator design

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

To demonstrate the effectiveness of this approach, consider the optimization of the ammonia process given in Fig. 2. This process is a single-loop design with a three-stage adiabatic flash separation. Further details of the process can be found in Lang and Biegler (1987). 

Suggest a solution order for the set of equations in Table P5.14 representing the flash separator in Fig. P5.14. The selection of the first seven of the eight design variables to be calculated (feed stream variables and the equilibrium pressure) is standard for flash separations. The last design variable is usually selected as either... 

The equilibrium flash separator is the simplest equilibrium-stage process with which the designer must deal. Despite the fact that only one stage is involved, the calculation of the compositions and the relative amounts of the vapor and liquid phases at any given pressure and temperature usually involves a tedious trial-and-error solution. 

No major changes have been made to the SCWO system, but there are some differences between the SCWO unit previously tested and the SCWO units proposed for EDS and for full-scale operation. Changes have also been made to equipment downstream of the SCWO reactor to facilitate processing of the suspended solids in the reactor effluent, especially for aluminum-rich feeds. The effluent flows from the pressure letdown valves to a knockout drum that contains a venturi scrubber, which separates liquid and suspended solids from the uncondensed vapor. The slurry is pumped to an evaporator/crystallizer system that replaces the flash separator in the original design. 

When we apply thermodynamics to industrial and research problems, we should draw fundamental ideas from Parts 1 and 11, devise an appropriate solution strategy, as in Chapter 10, and combine those with a computational technique, as in Chapter 11. Such a procedme provides values for measurables that can be used to interpret novel phenomena, to design new processes, and to improve existing processes. The procedure is illustrated in this chapter for several well-developed situations. They include conventional phase-equilibrium calculations for vapor-liquid, liquid-liquid, and solid-solid equilibria ( 12.1) solubility calculations for gases in liquids, solids in liquids, and solutes in near-critical solvents ( 12.2) independent variables in steady-flow processes ( 12.3) heat effects for flash separators, absorbers, and chemical rectors ( 12.4) and effects of changes of state on selected properties ( 12.5). 

In the section on heat effects, we emphasized how the steady-state energy balance can be used to design and analyze flash separators, absorption columns, and chemical reactors. In each application we developed a general form for the energy balance, and then we showed how it simplifies when it is applied to adiabatic and isothermal operations. We also noted that engineering calculations for process design involve the same quantities and the same equations as those for process analysis. Process design differs from process analysis only in the identities of the knowns and unknowns. 

Problem 8.2 Use the data for the system methanol-ethanol (from the previous problem) to design a separation process that takes a mixture with 20% methanol and produces a stream that contains 90% methanol by continuously flashing the vapor stream until the required purity is reached (the liquid streams are not recycled). The process is to be operated at 1 bar with a ratio V/L = 0.5 in all flash separators. 

The catalyst is continuously pre-pol5mierised before entering the main loop reactor, which is designed for supercritical conditions and typically operated in the temperature range of 80 to 100 °C and 5 to 6 MPa pressure with propylene as the diluent (bulk pol5mierisation). The slurry from the loop reactor is fed directly into the gas phase reactor without any flash separation step. The gas phase reactor is t5q)ically operated at 80 to 100 °C and 2.2 to 3 MPa. 

The following points should be considered for the design of the flash separator ... 

The flash separator can be designed using the program separator.exe. 

The pressure in the motor end is reduced to tire flash separator pressure witii some margins for frictional and static pressure drops. In some designs, rich glycol is used to generate reflux in the regenerator column, and for such... 

Flash separator pressure = 500 kPaG Separator retention time = 20 min Approach to equilibrium temperature = 4°C TEG recirculation rate = 0.025 m /kg water removed Contactor internals = bubble cup with 610-mm spacing Rich-glycol outlet temperature = 150°C (from glycol/glycol exchanger) Heat flux for fire tube design = 90,000 kJ/Cm h)... 

The most frequent application of phase-equilibrium calculations in chemical process design and analysis is probably in treatment of equilibrium separations. In these operations, often called flash processes, a feed stream (or several feed streams) enters a separation stage where it is split into two streams of different composition that are in equilibrium with each other. 

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