Separation Process Principles-2

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All separation processes must satisfy the energy balance. Inefficient separation processes require large transfer of heat and/or shaft work both into and out of the process; efficient processes require smaller levels of heat transfer and/or shaft work. The first law of thermodynamics provides no information on energy efficiency, but the second law of thermodynamics (also referred to as the entropy balance) does. The entropy balance is
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heat transfer, mass transfer, and chemical reaction must be reduced. Practical limits to this reduction exist because, as driving forces are decreased, equipment sizes increase, tending to infinitely large sizes as driving forces approached zero. For a separation process that occurs without chemical reaction, the summation of the stream availability functions leaving the process is usually greater than the same summation for streams entering the process. In the limit for a reversible process (LW=0), Eq. (3) of Table 2.1 reduces to Eq. (4), where Wmin is the minimum work required to conduct the separation and is equivalent to the difference in the heat transfer and shaft work terms in Eq. (3). This minimum work is independent of the nature (or path) of the separation process. The work of separation for an actual irreversible process is always greater than the minimum value computed from Eq. (4). The second-law efficiency is defined as (fractional second-law efficiency) = (minimum work of separation)/(equivalent actual work of separation)
2.1 Energy, entropy, and availability balances
* Most commercial separation operations utilize large quantities of energy in the form of heat and/or shaft work. For example, the energy consumption by distillation in the US is approximately $10 trillion per year (1991). Thus, it is of considerable interest to know the extent of energy consumption in a separation process, and to what degree energy requirements might be reduced. Such energy estimates can be made by applying the first and second laws of thermodynamics.
(stream availability flows + availability of heat + shaft work) entering system - (stream availability flows + availability of heat + shaft work) leaving system = loss of availability (lost work) * For any separation process, lost work can be computed from Eq. (3) in Table 2.1. Its magnitude depends on the extent of process irreversibilities, which include fluid friction, heat transfer due to finite temperature-driving forces, mass transfer due to finite concentration or activitydriving forces, chemical reactions proceeding at finite displacements from chemical equilibrium, mixing of streams at differing conditions of temperature, pressure, and/or composition, and so on. Thus, to reduce the lost work, driving forces for momentum transfer,
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At steady state, if kinetic, potential, and surface energy changes are neglected, the first law of thermodynamics (also referred to as the conservation of energy or the energy balance), states that the sum of all forms of energy flowing into the system equals the sum of the energy flows leaving the system: (stream enthalpy flows + heat transfer + shaft work) leaving system - (stream enthalpy flows + heat transfer + shaft work) eny flows + entropy flows by heat transfer) leaving system - (stream entropy flows + entropy flows by heat transfer) entering system = production of entropy by the process Note that the entropy balance contains no terms related to shaft work. * Although the production of entropy, Sirr, is a measure of energy inefficiency, it is difficult to relate to this measure because it does not have the units of energy/time (power).
Separation Process (system)
Sirr, LW Streams out n, zi, T, P, h, s, b, v
(Surroundings) T0 (Ws)in (Ws)out
Shaft work in and out
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One or more feed streams flowing into the system are separated into two or more product streams that flow out of the system. n- molar flow rates, zi- the component mole fractions T- temperature, b- molar availabilities h- molar enthalpies, s- molar entropies P- pressure, Q- heat flows in or out, W- shaft work crossing the boundary of the system
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Consider the continuous, steady-state flow system for a general separation process in Fig. 2.1.
Heat transfer in and out Qin, Ts Qout, Ts
Streams in n, zi, T, P, h, s, b, v