Optimize Batch Distillation

Proper design depends upon an understanding of key relationships.

By John E. Edwards, P & I Design

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The reflux ratio

R = L/D (6)

relates to the slope of the operating line,m, via:

m = L/V = R/(R+ 1) (7)

The overall mass balance at the top of the column is:

D/V= 1/(R+ 1) (8)

The mass balance demonstrates that the top composition is established by the D/V ratio, which depends upon the reflux ratio. If the D/V ratio is high, separation will be low and withdrawal of distillate must be stopped while a relatively high value of the mvc mole fraction, XD, remains, i.e., light ends recovery will be poor. If theD/Vratio is reduced to enhance recovery, the distillation may consume an uneconomic amount of time and energy.

A mass balance on the mvc yields the following relationship, known as the Rayleigh equation:

(dS)XC = d(SXS) (9)

ln (S0/S1) = (10)

The overall mass balance for the system gives:

S0 – S1 = C(11)

A mass balance on the mvc yields:

S0XS0 – S1XS1 = CXC (12)

and by transposing:

C/S0 = (XS0 – XS1)/(XC – XS1)(13)

S1 = S0(XS0 – XC)/(XS1 – XC) (14)

The Fenske equation uses a separation factor, F, to establish the minimum number of theoretical stages, NMIN, required at total reflux to achieve a specified separation of a binary mixture with near-ideal behavior:

NMIN ln α = ln F(15)

F = [XD/(1 – XD)][(1 – XS)/XS] (16)

where XD and XS are the mole fractions of the mvc in the distillate and still compositions, respectively, and α is the relative volatility of the two components.

If a given column can achieve the required separation at total reflux, the next step is to determine the minimum reflux ratio, Rmin, using Underwood’s equation for a binary system:

Rmin = [1/(α - 1)][(XD/XS) – α(1 – XD)/(1 – XS)](17)

When the distillate must contain the mvc at high purity, i.e., XD >0.995 mole fraction, Eq. 17 simplifies to:

Rmin = 1/(α - 1)XS(18)

For a high separation factor, a minimum relative volatility of 1.5 is considered reasonable, thus setting a top limit of Rmin at 2/XS. Batch distillations should start with Rmin equal to that required for a continuous split; Rmin increases as the amount of the mvc in the still decreases.

The total reboiler heat input to reduce the reactor contents from S0 to S1 moles for a variable top composition, achieved by setting a fixed reflux ratio, is:

Q = λ(S0 – S1)(R + 1) (19)

where λ is the latent heat of vaporization.

The reboiler heat input for a fixed top composition, achieved by varying the reflux ratio to maintain a fixed top temperature at constant pressure, is:

Q = λ(S0 – S1) (20)

Both relationships indicate that the reflux ratio must be kept to a minimum, subject to satisfying the requirements for the desired separation specification, to minimize the heat input.

The boil-up rate,V, without reflux is:

V = Q/λ (21)

At total reflux, the reboiler heat duty is:

Q = V(λ + Ht (22)

where H is the liquid specific heat andtsub is condenser subcool in °C.

The batch time, θbatch, at constant reflux ratio is given by:

θbatch = [(R +1)/V](S0 – S1) = λ/V (23)

BATCH STILL HEAT TRANSFER

The boil-up rate achievable with stirred jacketed reactors depends upon many factors, including operational temperature difference, jacket heating media and heat transfer considerations [2, 6].

The fundamental equation for heat transfer is:

Q = UAΔtmean = fjHj(t2 – t1)(24)

where Q is the still heat duty, U is the overall heat transfer coefficient, A is the heat transfer area, Δtmean is the mean temperature difference, fj is jacket fluid flow and Hj is jacket fluid specific heat.

Δtmeanwith a still bottoms temperature tbotand jacket-fluid inlet and outlet temperatures t1 and t2 is approximated by:

Δtmean = (t2 – t1)/2 – tbot(25)

The overall heat transfer coefficient is the sum of the individual resistances, i.e.:

1/U = 1/hi + 1/hfi + 1/(kw/x) + 1/ho + 1/hfo (26)

where hi is the inside film coefficient, hfi is the inside fouling coefficient, kw is the thermal conductivity of the still wall, x is the vessel wall thickness, ho is the outside film coefficient and hfo is the outside fouling coefficient.

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