The stripper

Steam strips light components out of the separator liquid. What leaves overhead rejoins the mixing zone as stream 5; what does not becomes the stripper's own liquid, stream 12, which leaves as the product, stream 13. The vessel is treated as liquid only: it has no vapour space of its own in the model.

Source: teprob.f:614-662 for the column, teprob.f:677-678 for the reboiler, and teprob.f:778-782 for the energy balance.

Equations

The feed is the mixed A/C feed plus the separator underflow (teprob.f:635-639). Note that stream 4 arrives here directly rather than through the mixing zone:

\[ f_i = \dot n_{i,4} + \dot n_{i,11} \]

A vapour-to-liquid ratio sets how hard the column strips, scaled by a temperature factor (teprob.f:622):

\[ \Lambda = \frac{F_4}{F_{11}} \, \tau(T_c) \]

Each condensible then strips according to a Langmuir-shaped saturating function of that ratio (teprob.f:623-627):

\[ s_i = \frac{k_i \Lambda}{1 + k_i \Lambda}, \qquad i \in \{D \ldots H\} \]

and the split is simply (teprob.f:643-644)

\[ \dot n_{i,5} = s_i f_i, \qquad \dot n_{i,12} = f_i - \dot n_{i,5} \]

Both product streams leave at the stripper's own temperature (teprob.f:652-653), and their enthalpies are taken on different bases: stream 5 with ITY = 1, the vapour basis, and stream 12 with ITY = 0, the liquid one (teprob.f:654-655).

The temperature factor has a pole at 177 C

\[ \tau(T) = \begin{cases} T - 120.262 & T > 170 \\ 0.1 & T < 5.292 \\ \dfrac{363.744}{177 - T} - 2.22579488 & \text{otherwise} \end{cases} \]

from teprob.f:615-621. The middle branch diverges at 177 C, which is inside the range the two outer branches leave for it only if TCC exceeds 170, and it does not: the T > 170 branch takes over first. So the pole is unreachable by seven degrees, and the two branches are continuous to within 0.1% at 170.

The adversarial state catalogue built for Tier 2 places a state at 176 C anyway, to sit near the pole and confirm that it stays on the linear branch. That state is coverage of the guard, not of the pole.

FTM(11) > 0.1 switches the whole block

Below that threshold the column is not really running, and teprob.f:629-633 substitutes five fixed stripping factors (0.9999, 0.999, 0.999, 0.99, 0.98) rather than evaluating the correlation. The reason is visible in the arithmetic: \(\Lambda = F_4 / F_{11}\) diverges as \(F_{11} \to 0\).

Both sides are covered by the adversarial catalogue, which places a state exactly on FTM(11) = 0.1. Since the test at teprob.f:614 is .GT., that state takes the fixed-factor branch.

SFR(1..3) are never recomputed

teprob.f:623-627 and 629-633 both write slots 4 through 8 only. Slots 1, 2 and 3 are set once in TEINIT (teprob.f:1126-1128) and are read at teprob.f:643 on every evaluation, so A, B and C strip at a fixed 99.5%, 99.1% and 99.0% no matter what the column is doing.

That is the intended physics rather than an oversight: the non-condensibles are gases, they leave overhead essentially completely, and no temperature or flow ratio in the plant's range would change that. It is worth stating because the loop at teprob.f:643 runs I=1,8 and looks as though all eight factors come from the branch above it.

The reboiler

Steam is at 100 C, so above that there is nothing to transfer and the original sets the duty to zero rather than letting it go negative (teprob.f:677-678):

\[ Q_c = \begin{cases} U A_c \, (100 - T_c) & T_c < 100 \\ 0 & \text{otherwise} \end{cases} \]

The coefficient UAC is not computed here. It is a valve-lagged capacity with a disturbance drift factor, set at teprob.f:572:

\[ U A_c = \frac{v_9 R_9 (1 + d_9)}{100} \]

which is the point at which IDV(16), published as "Unknown", enters the model.

The nominal trajectory sits near 65 C, so the cutoff is the branch at risk of never being exercised. B-0021 measured 300 of 300 nominal states below 100.

The reactor inlet is an alias, and this is where it is made

teprob.f:656-661 copies flow, enthalpy, temperature, composition and component flows from stream 6 to stream 7 wholesale. There is no mixing, no pressure drop and no heat loss between them: stream 7 exists so that the reactor's balance at teprob.f:763-772 can name its own inlet. It lives in the stripper block for no reason other than that is where the original put it.

Balances, and an asymmetry between them

The component balances are a straight pass-through of the two streams the column produced (teprob.f:762-770), while the energy balance names the column's inputs instead (teprob.f:778-782):

\[ \frac{dn_i}{dt} = \dot n_{i,12} - \dot n_{i,13} \]

\[ \frac{dE}{dt} = h_4 F_4 + h_{11} F_{11} - h_5 F_5 - h_{13} F_{13} + Q_c \]

That is what the source does, and the two forms describe the same vessel: the component split at teprob.f:643-644 conserves moles by construction, so streams 4 and 11 in, less stream 5 out, is stream 12.

Variables

Two of the eight shutdown conditions belong to this vessel: stripper liquid volume above 8 or below 1 cubic metre (teprob.f:709-710). Its level measurement is also the odd one out among the three, because its span is the vessel volume VTC itself rather than a separately hard-coded range (teprob.f:693).