2．2 Sensitivity to operation condition changes
Fig．6 shows the transient responses O two end
products in four different purity systems after the feed
composition，Zf，is disturbed ±15％ from its steady
state values．W e can see that the non—linearity of re—
sponses to feed composition is strongly intensified as
purity increases．Increased non—linearity in high—and
very high—purity system leads to different sensitivities
of two end products to Zf．In these systems，the bot—
Fig．5． The asymmetric responses to q in very high—purity
tom product composition iS extremely sensitive to the
positive disturbance of Zf and less sensitive to the
negative disturbance of Zf．On the other hand，the
situation is opposite to the overhead product composi—
tion． Besides， the higher the purity is， the more
sluggish the response is．Hence，during the design of
control systems，special attention should be paid to
the rejection of feed composition disturbances that
have great influence on both overhead and bottom
products among all four different purity systems．
Fig．6 The responses lo±15％step changes of Zf．(a)Low purity；(b)moderate purity；(c)high purity；(d)very high—purity
Fig．7 shows the overhead and bottom responses
to ± 15％ disturbance of feed flow rate， F， from
steady state values．The degrees of non—linearity are
not obvious in lOW— and moderate—purity systems，
while severe non．．1inearity and extremely sluggish re．．
sponses of high--and very high--purity systems are ob--
served．As a result．both product compositions will be
less influenced than feed composition in low and
moderate—purity systems．
However， intensified
responses in the high—and
non—linearity and sluggish
very high--purity system in--
dicate that rejection of feed flow rate should be taken
into account when designing tight control schemes．
The responses to Zf and F give US the insight
that high——and very high--purity ITCDIC is more sen--
sitive to external disturbances of operation conditions
than lOW—and moderate—purity systems．The degree
of sensitivity to operation conditions is reinforced by
internal heat integration． In ITCDIC system， heat
integration transfers the effect of any external distur—
bance of operation condition from one column to an—
other column．SO that external disturbances affect the
rectifying and the stripping sections simultaneousl
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