Calculating column relief loads

9 Pages·0.61 MB·English

by  Haribabu Chittibabu, Amudha Valli and Vineet Khanna.

Calculating column relief loads
Conventional, steady-state and dynamic simulation techniques are compared in
a study of relief loads for failure modes applied to a distillation column
Haribabu CHittibabu, amudHa Valli and VinEEt KHanna Bechtel india PVE Ltd
dipanjan bHattaCHarya Bechtel Corporation
E
mergency relief in the process because of compositional changes method for determining relief load
industries aims to protect along the column height. The under abnormal conditions.
equipment, the environment conventional method of estimating This article considers different
and operating personnel from relief load (unbalanced heat methods for estimating relief load
abnormal conditions. Appropriate method) is normally conservative for a distillation column — a debu-
estimation of relief loads under and leads to bigger relief valves taniser in this case — and discusses
extreme conditions is important for and flare headers, but it is the the strengths and weaknesses of
the correct sizing of relief valves approach most widely practised. each method. There are many emer-
and flare headers, and for the selec- With increasing computing speed gency cases that apply to a
tion of disposal media. In addition, and software reliability, process distillation column, and estimation
during debottlenecking or revamp- simulation is increasingly used as of the maximum possible relief load
ing of process units, adding a new an important tool for estimating requires an understanding of plant
relief valve and modifying the relief relief load and properties. Steady- behaviour and identification of the
system can be very costly and, in state simulation can also be used to worst case.
terms of construction, difficult to estimate the relief load within limi-
implement. tations and can overcome some of Case study: a debutaniser
Estimating accurate relief loads the assumptions envisaged in the The debutaniser column separates
for distillation columns under vari- conventional method. Dynamic liquified petroleum gas (LPG)
ous conditions is more complex simulation provides an alternative components from light naphtha.
PDC
To flare, R 135°F PC
174 psia Off gas
P = 214 psia
set
CWS CWR
FC
LC
LC
Debutaniser
196000 lb/hr Reflux Reflux Sour water
drum
pump
Feed, F FC
673700 lb/hr, 301°F Distillate, sour LPG, D
Feed TC FC 58120 lb/hr, 104°F
pump
Reboiler
LC
412°F Steam
Condensate
391°F
391°F FC
178 psia Bottom, naphtha product, B
615600 lb/hr, 391°F
CWS CWR
Product
pump
Figure 1 Distillation column (debutaniser)
www.digitalrefining.com/article/1000487 PTQ Q2 2010 55
The overhead includes a reach and act upon the top tray
cooling water total R liquid
condenser, reflux drum • There is enough top tray liquid
and off-gas valve, which available to generate vapour during
Q
C
is normally closed. The upset conditions.
debutaniser operates at To determine Q , the first
unbalanced
174 psia and relief is set step is to develop a sketch around
Reflux
at 214 psia. The debutan- drum the affected system (see Figure 2)
Top tray
iser bottom is heated by and perform a mass and energy
a thermosyphon reboiler balance in line with the upset
Excess heat
utilising medium-pressure condition:
D, h
F, h D
steam. Figure 1 shows a F
Debutaniser Q = F h - B h - D h + Q - Q - (F - B - D) h
flow diagram of the debu- unbalanced F B D R C L
R = Q / λ
taniser under evaluation. unbalanced (excess)
Major relief conditions or where
plant situations identified Q F = Debutaniser or column feed
R
for the debutaniser are rate at relief
loss of reflux, loss of feed h = Specific enthalpy of feed at
F
and site-wide power relief
failure. B = Debutaniser or column
B, h
B
bottom rate at relief
Conventional method h = Specific enthalpy of bottom at
B
The conventional approach Figure 2 Distillation column: unbalanced heat envelope relief
is also known as the D = Debutaniser distillate rate at
unbalanced heat method, column is available in various liter- relief
where a mass and energy balance is ature1 and hence is not covered in h = Specific enthalpy of distillate
D
developed under relief conditions, detail here. at relief
based on the scenario under consid- There are several assumptions in Q = Reboiler heat input at relief
R
eration, to determine if there is any determining relief loads: Q = Condenser duty at relief
C
unbalanced or excess heat. The • Feed, products, reflux and top (generally, the design duty can be
unbalanced heat is divided by the tray liquid compositions are unal- considered)
latent heat of vapourisation of the tered during the relief condition h = Specific enthalpy of top tray
L
top tray liquid to give the relief • Feed, product, reflux and strip- liquid
load: ping medium will continue at the λ = Latent heat of vapourisation
normal rate unless the hydraulics at of top tray liquid
Relief load = Q / λ the relieving condition determine R = Relief load
unbalanced (excess)
otherwise Credit may be taken for reboiler
The conventional method for • Enthalpy is balanced on the top pinch. At relieving pressure, the
determining the relief load of a tray and all unbalanced heat will column temperature rises and the
reboiler temperature difference may
fall, leading to lower heat input to
(cid:25)(cid:16)(cid:16) the column. This is reboiler pinch.2
Assume that the volume of the
(cid:24)(cid:16)(cid:16)
sump is sufficient to maintain a
(cid:23)(cid:16)(cid:16) constant reboiler circulation rate
and to re-rate the reboiler to obtain
(cid:72)(cid:82)(cid:22)(cid:16)(cid:16) duty at relief condition. If there was
(cid:15)(cid:66)(cid:76)(cid:0)(cid:16)(cid:16)(cid:16)(cid:21)(cid:16)(cid:16) (cid:38)(cid:69)(cid:69)(cid:68) are bosiilgenr ifidcuatnyt atr erdeulicetfi,o nth e inli ghttheer
(cid:65)(cid:12)(cid:17)(cid:0)(cid:12)(cid:69)(cid:84)(cid:20)(cid:16)(cid:16) Relief valve (cid:34)(cid:50)(cid:69)(cid:79)(cid:70)(cid:84)(cid:76)(cid:84)(cid:85)(cid:79)(cid:88)(cid:77)(cid:83) components would begin travelling
(cid:82)(cid:0)(cid:87)(cid:79) Reflux stops opens (cid:36)(cid:73)(cid:83)(cid:84)(cid:73)(cid:76)(cid:76)(cid:65)(cid:84)(cid:69) towards the bottom, causing the
(cid:76)(cid:38)(cid:19)(cid:16)(cid:16) (cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70) duty to rise again. Many designers
Reflux drum fills (cid:47)(cid:86)(cid:69)(cid:82)(cid:72)(cid:69)(cid:65)(cid:68) re-rate the reboiler with feed
(cid:70)(cid:82)(cid:79)(cid:77)(cid:0)(cid:67)(cid:79)(cid:76)(cid:85)(cid:77)(cid:78)
(cid:18)(cid:16)(cid:16)
composition instead of bottoms
(cid:17)(cid:16)(cid:16) composition in these circumstances,
to maintain a more conservative/
(cid:16) realistic reboiler duty at relief.
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16) (cid:20)(cid:21) (cid:21)(cid:16) (cid:21)(cid:21) (cid:22)(cid:16)
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
loss of reflux
Figure 3 Loss of reflux: flow vs time • Reflux stops immediately
• The reflux drum and the
56 PTQ Q2 2010 www.digitalrefining.com/article/1000487
column sump level, and finally
(cid:18)(cid:21)(cid:16) reaches zero
• The column overhead vapour
(cid:18)(cid:20)(cid:16)
rate decreases, the reflux drum level
Peak pressure
(cid:18)(cid:19)(cid:16) drops, and the distillate rate
(cid:18)(cid:18)(cid:16) decreases to maintain the condenser
(cid:65)(cid:73)(cid:83)(cid:80) (cid:18)(cid:17)(cid:16) level and finally becomes zero.
(cid:0)(cid:69)(cid:12)(cid:82)(cid:85) Therefore:
(cid:83)(cid:83)(cid:69) (cid:18)(cid:16)(cid:16) O O O OO O
(cid:80)(cid:82)
(cid:70)(cid:73)(cid:0)(cid:69)(cid:76)(cid:69)(cid:50) (cid:17)(cid:25)(cid:16) Qunbalanced = F hF - B hB - D hD + QR - QC - (F - B - D) hL
(cid:17)(cid:24)(cid:16)
(cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
(cid:17)(cid:23)(cid:16) Site-wide power failure (SWpF)
(cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:86)(cid:65)(cid:76)(cid:86)(cid:69)(cid:0)(cid:83)(cid:69)(cid:84)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
(cid:17)(cid:22)(cid:16) (cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:86)(cid:65)(cid:76)(cid:86)(cid:69)(cid:0)(cid:65)(cid:67)(cid:67)(cid:85)(cid:77)(cid:85)(cid:76)(cid:65)(cid:84)(cid:69)(cid:68) • All electrical equipment fails,
(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
therefore the feed pump, the debu-
(cid:17)(cid:21)(cid:16)
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16) (cid:20)(cid:21) (cid:21)(cid:16) (cid:21)(cid:21) (cid:22)(cid:16) taniser bottom pumps and the
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78) reflux pumps stop
• Assuming all cooling water
Figure 4 Loss of reflux: relief pressure vs time pumps are electrically driven, the
condensing duty is also immedi-
ately lost
• Steam is assumed to flow contin-
(cid:17)(cid:18)(cid:16)
uously to the reboiler. Therefore:
Reflux drum fills
O O O O O O
(cid:17)(cid:16)(cid:16)
Q = F h - B h - D h + Q - Q - (F - B - D) h
unbalanced F B D R C L
(cid:24)(cid:16)
(cid:5)
(cid:12)(cid:76)(cid:0)(cid:69) dynamic simulation of relief
(cid:69)(cid:0)(cid:86)(cid:76)(cid:80) (cid:22)(cid:16) conditions
(cid:85)
(cid:68)(cid:76)(cid:79) Chemical plants and refineries are
(cid:40)
never truly at a steady state and
(cid:20)(cid:16)
this is the case during relief. The
transient behaviour of a column is
(cid:50)(cid:69)(cid:66)(cid:79)(cid:73)(cid:76)(cid:69)(cid:82)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80)
(cid:18)(cid:16) (cid:50)(cid:69)(cid:70)(cid:76)(cid:85)(cid:88)(cid:0)(cid:68)(cid:82)(cid:85)(cid:77) best studied by means of dynamic
(cid:35)(cid:79)(cid:76)(cid:85)(cid:77)(cid:78)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80) simulation, which has gained in
(cid:16) importance since the 1990s and has
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16) (cid:20)(cid:21) (cid:21)(cid:16) (cid:21)(cid:21) (cid:22)(cid:16) been used increasingly successfully
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
as the reliability of simulation soft-
ware has increased. The equations
Figure 5 Loss of reflux: holdup level vs time for material, energy and composi-
tion balances include an additional
condenser flood, restricting the O O O accumulation term, which is
overhead vapour path and pressu- Q = F h - B h - D h + Q - Q - (F - B - D) h differentiated with respect to time.
unbalanced F B D R C L
rising the column The inclusion of an accumulation
• The feed is pumped and suffi- term enables the dynamic model to
cient head is available to maintain loss of feed rigorously calculate compositional
the feed flow rate at relief • Feed stops immediately changes at each stage and to modify
condition • After some time, when the vapour/liquid equilibrium over
• Bottom product continues at the column level drops, the bottom time.
same rate. Therefore: product decreases to maintain the Unlike steady-state simulation,
dynamic simulation works within a
relief load calculated by conventional method Pressure-Flow (P-F) network with
two basic equations: resistance and
volume balance. The resistance
upset condition relief load, lb/hr temperature, °F molecular weight
equation defines flow between pres-
Loss of reflux 124 980 164 49.28
sure hold-ups, and the volume
Loss of feed 43 650 164 49.28
Site-wide power failure 342 796 164 49.28 balance equation defines material
balance at pressure hold-ups.
For the case under consideration,
table 1 the accuracy of dynamic simulation
58 PTQ Q2 2010 www.digitalrefining.com/article/1000487
provides extra inputs compared
with steady-state simulation: (cid:23)(cid:16) (cid:17)(cid:18)(cid:16)
• Dimensions, especially volumes,
(cid:17)(cid:17)(cid:24)
for all static equipment; column (cid:22)(cid:16)
bottom and reflux drum levels are Pinched reboiler duty (cid:17)(cid:17)(cid:22)
set to normal to simulate hold-ups (cid:21)(cid:16) (cid:17)(cid:17)(cid:20)
•r•ve alASlavpt eiveoscen in(sfiChdcvio p rcvs o cafnuoludrrve ur)eco tftfaoaontrri c nppeg rr eefesosqssruuu irrcpeeom nflfletoornowwtl (cid:36)(cid:85)(cid:84)(cid:89)(cid:12)(cid:0)(cid:34)(cid:52)(cid:53)(cid:15)(cid:72)(cid:82) (cid:19)(cid:20)(cid:16)(cid:16) (cid:17)(cid:17)(cid:17)(cid:17)(cid:17)(cid:16)(cid:18)(cid:16)(cid:24) (cid:87)(cid:79)(cid:84)(cid:72)(cid:71)(cid:73)(cid:69)(cid:0)(cid:82)(cid:65)(cid:76)(cid:85)(cid:67)(cid:69)(cid:76)
relationships, and an actuator mode (cid:45)
and rate for valve actuator (cid:18)(cid:16) (cid:17)(cid:16)(cid:22)
dynamics (cid:17)(cid:16)(cid:20)
(cid:50)(cid:69)(cid:66)(cid:79)(cid:73)(cid:76)(cid:69)(cid:82)(cid:0)(cid:68)(cid:85)(cid:84)(cid:89)
• Detailed exchanger thermal (cid:17)(cid:16) (cid:35)(cid:79)(cid:76)(cid:85)(cid:77)(cid:78)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80)
(cid:77)(cid:79)(cid:76)(cid:69)(cid:67)(cid:85)(cid:76)(cid:65)(cid:82)(cid:0)(cid:87)(cid:69)(cid:73)(cid:71)(cid:72)(cid:84) (cid:17)(cid:16)(cid:18)
design for calculation of pressure
drop and heat transfer coefficient. If (cid:16) (cid:17)(cid:16)(cid:16)
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16) (cid:20)(cid:21) (cid:21)(cid:16) (cid:21)(cid:21) (cid:22)(cid:16)
detailed design is not available, a (cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
resistance term for the pressure
flow relationship and overall UA Figure 6 Loss of reflux: reboiler duty and molecular weight vs time
can be specified
• Actual tray information such as
diameter, flow path, distributor (cid:24)(cid:16)(cid:16)
details, weir length and height are
(cid:38)(cid:69)(cid:69)(cid:68)
required for column hydraulic (cid:23)(cid:16)(cid:16) (cid:34)(cid:79)(cid:84)(cid:84)(cid:79)(cid:77)(cid:83)
performance (cid:50)(cid:69)(cid:70)(cid:76)(cid:85)(cid:88)
(cid:22)(cid:16)(cid:16) (cid:36)(cid:73)(cid:83)(cid:84)(cid:73)(cid:76)(cid:76)(cid:65)(cid:84)(cid:69)
• Controller for determining (cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)
cCornetdriot l isa cntiootn tsa kdeunr ifnogr tthraen sciotniotnrosl. (cid:82)(cid:72)(cid:15)(cid:0)(cid:16)(cid:66)(cid:76)(cid:21)(cid:16)(cid:16) Feed stRoeplsief valve (cid:47)(cid:70)(cid:82)(cid:79)(cid:86)(cid:77)(cid:69)(cid:82)(cid:0)(cid:72)(cid:67)(cid:69)(cid:79)(cid:65)(cid:76)(cid:85)(cid:68)(cid:77)(cid:78)
alocatido;n , fowr hiecxha mrepdleu, cesth et hec olruemlienf (cid:16)(cid:17)(cid:84)(cid:65)(cid:16)(cid:0)(cid:12)(cid:69)(cid:20)(cid:16)(cid:16) open Relcielof svealve
bottom temperature controller (cid:38)(cid:82)(cid:0)(cid:87)(cid:79)(cid:76)(cid:19)(cid:16)(cid:16) Bottoflmow & zdeirsotillate
reduces the steam flow rate when
the column bottom temperature (cid:18)(cid:16)(cid:16)
Relief flow
rises at the relief condition.
(cid:17)(cid:16)(cid:16)
loss of reflux condition
(cid:16)
The reflux pump is stopped in five (cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16)
minutes (see Figure 3). The level in (cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
the reflux drum starts to increase
(see Figure 5). The overhead vapour Figure 7 Loss of feed: flow vs time
from the column continues to flow
through the condenser and fill the
reflux drum. After 17 minutes, the
(cid:18)(cid:21)(cid:16)
reflux drum floods and the flow to
the condenser is blocked; the (cid:18)(cid:20)(cid:16)
column pressure starts to increase
(cid:18)(cid:19)(cid:16)
Peak pressure
(see Figure 4). When the column
(cid:18)(cid:18)(cid:16)
reaches the set pressure, after
(cid:65)
about 21 minutes, the relief valve (cid:73)(cid:83)(cid:80) (cid:18)(cid:17)(cid:16)
(cid:12)(cid:0)(cid:69)
starts to open. Note that the (cid:82)(cid:85)
pressure did not reach the maxi- (cid:83)(cid:83)(cid:69)(cid:82)(cid:80) (cid:18)(cid:16)(cid:16)
mum accumulated pressure for (cid:76)(cid:0)(cid:70)(cid:69)(cid:73)(cid:69) (cid:17)(cid:25)(cid:16)
(cid:50)
the given orifice area of the relief
(cid:17)(cid:24)(cid:16)
valve. (cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:86)(cid:65)(cid:76)(cid:86)(cid:69)(cid:0)(cid:83)(cid:69)(cid:84)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
(cid:17)(cid:23)(cid:16)
Initially, the level in the column (cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
(cid:50)(cid:69)(cid:76)(cid:73)(cid:70)(cid:0)(cid:86)(cid:65)(cid:76)(cid:86)(cid:69)
bottom sump decreases as the (cid:17)(cid:22)(cid:16) (cid:65)(cid:67)(cid:67)(cid:85)(cid:77)(cid:85)(cid:76)(cid:65)(cid:84)(cid:69)(cid:68)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
reflux is stopped, and the bottoms
(cid:17)(cid:21)(cid:16)
product level control valve closes to (cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16)
maintain the column sump level. (cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
The feed continues at a constant
rate, since its pressure upstream of Figure 8 Loss of feed: relief pressure vs time
www.digitalrefining.com/article/1000487 PTQ Q2 2010 59
the control valve is higher than the
(cid:17)(cid:18)(cid:16) relief pressure.
Reboiler sump level drops
Figure 6 shows the reboiler duty
and column sump molecular weight
(cid:17)(cid:16)(cid:16)
during this relief condition. As soon
as the reflux is stopped, the molecu-
(cid:24)(cid:16) lar weight in the column sump
Column sump level drops
(cid:5) (cid:50)(cid:69)(cid:66)(cid:79)(cid:73)(cid:76)(cid:69)(cid:82)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80) increases, leading to an increase in
(cid:0)(cid:12)(cid:76)(cid:69)(cid:86) (cid:50)(cid:69)(cid:70)(cid:76)(cid:85)(cid:88)(cid:0)(cid:68)(cid:82)(cid:85)(cid:77) the boiling temperature of the
(cid:69)(cid:76)(cid:0)(cid:80)(cid:85) (cid:22)(cid:16) (cid:35)(cid:79)(cid:76)(cid:85)(cid:77)(cid:78)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80) column bottoms, finally resulting
(cid:68)
(cid:79)(cid:76)(cid:40) in reduced reboiler duty.
(cid:20)(cid:16) After 17 minutes, when the path
for the overhead vapour was
blocked (condenser flooded), lighter
(cid:18)(cid:16)
components started to fill the
column sump and reboiler duty
(cid:16) again started to increase. After 21
(cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16)
minutes, when the relief valve
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
started to open, reboiler duty
Figure 9 Loss of feed: holdup level vs time settled, based on the column sump
composition at relief condition.
(cid:23)(cid:16) (cid:17)(cid:17)(cid:20) loss of feed condition
(cid:50)(cid:69)(cid:66)(cid:79)(cid:73)(cid:76)(cid:69)(cid:82)(cid:0)(cid:68)(cid:85)(cid:84)(cid:89) (cid:17)(cid:17)(cid:19) The feed pump stops after five
(cid:22)(cid:16) (cid:35)(cid:79)(cid:78)(cid:68)(cid:69)(cid:78)(cid:83)(cid:69)(cid:82)(cid:0)(cid:68)(cid:85)(cid:84)(cid:89) minutes (see Figure 7). After 10
(cid:35)(cid:79)(cid:76)(cid:85)(cid:77)(cid:78)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80) (cid:17)(cid:17)(cid:18)
(cid:77)(cid:79)(cid:76)(cid:69)(cid:67)(cid:85)(cid:76)(cid:65)(cid:82)(cid:0)(cid:87)(cid:69)(cid:73)(cid:71)(cid:72)(cid:84) minutes, the column sump level
(cid:21)(cid:16) (cid:17)(cid:17)(cid:17) drops (see Figure 9) and the bottom
Pinched reboiler duty
flow is reduced to maintain the
(cid:15)(cid:82)(cid:72)(cid:53) (cid:20)(cid:16) (cid:17)(cid:17)(cid:16) column sump level. As the column
(cid:52)
(cid:12)(cid:34)(cid:0)(cid:89) (cid:17)(cid:16)(cid:25) overhead vapour starts to decrease
(cid:84)(cid:36)(cid:85) (cid:19)(cid:16) Condenser (cid:17)(cid:16)(cid:24) (see Figure 7), the reflux drum level
duty
decreases and the distillate flow
(cid:18)(cid:16) (cid:17)(cid:16)(cid:23)
reduces to maintain the reflux drum
(cid:17)(cid:16)(cid:22)
level. After 20 minutes, when distil-
(cid:17)(cid:16)
(cid:17)(cid:16)(cid:21) late and bottoms stop completely,
only the vapour generated by the
(cid:16) (cid:17)(cid:16)(cid:20)
(cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16) reboiler is condensed by the
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78) condenser. Figure 10 shows the
pinched reboiler duty, condenser
Figure 10 Loss of feed: reboiler duty and molecular weight vs time duty and column sump molecular
weight.
During loss of feed, the column
(cid:19)(cid:16)(cid:16) sump molecular weight increases,
resulting in reduced reboiler duty.
(cid:38)(cid:69)(cid:69)(cid:68)
(cid:34)(cid:79)(cid:84)(cid:84)(cid:79)(cid:77)(cid:83) Since the top reflux is maintained
(cid:18)(cid:21)(cid:16)
(cid:50)(cid:69)(cid:70)(cid:76)(cid:85)(cid:88) at normal flow, the lighter compo-
Site-wide power failure (cid:36)(cid:73)(cid:83)(cid:84)(cid:73)(cid:76)(cid:76)(cid:65)(cid:84)(cid:69)
nents start migrating towards the
(cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)
(cid:82)(cid:72)(cid:15)(cid:0)(cid:16)(cid:66)(cid:76) (cid:18)(cid:16)(cid:16) (cid:47)(cid:70)(cid:82)(cid:79)(cid:86)(cid:77)(cid:69)(cid:82)(cid:0)(cid:72)(cid:67)(cid:69)(cid:79)(cid:65)(cid:76)(cid:85)(cid:68)(cid:77)(cid:78) bboecttoommi.n gT hlieg hctoelru amnnd pthreo fitleem sptearrats-
(cid:16)
(cid:16) ture profile starts lowering. This
(cid:17) (cid:17)(cid:21)(cid:16)
(cid:84)(cid:65)(cid:0)(cid:12)(cid:69) also results in the lower molecular
(cid:82)(cid:0)(cid:87)(cid:79) Relief valve open weight of the column overhead
(cid:38)(cid:76) (cid:17)(cid:16)(cid:16)
vapour. After about 11 minutes, the
condenser is not able to fully
(cid:21)(cid:16) condense the overhead vapour due
to its lower molecular weight,
resulting in a rise in column pres-
(cid:16)
(cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16) sure (see Figure 8). When the
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78) column reaches the set pressure,
after about 23 minutes, the relief
Figure 11 Site-wide power failure: flow vs time valve starts to open. Note that the
60 PTQ Q2 2010 www.digitalrefining.com/article/1000487
pinched reboiler duty at this time is
higher because of the lower molec-
(cid:18)(cid:21)(cid:16)
ular weight in the column sump.
(cid:18)(cid:20)(cid:16)
After about 35 minutes, all non-
condensable or lighter components (cid:18)(cid:19)(cid:16)
Peak pressure
exit the column, reboiler duty
(cid:18)(cid:18)(cid:16)
reduces again to about 42% of (cid:65)
(cid:83)(cid:73)(cid:80) (cid:18)(cid:17)(cid:16)
normal, and the column stabilises (cid:0)(cid:12)(cid:69)
(cid:82)(cid:85)
at total reflux mode. (cid:83)(cid:83)(cid:69) (cid:18)(cid:16)(cid:16)
(cid:82)(cid:80)
Site-wide power failure condition (cid:0)(cid:70)(cid:69)(cid:73)(cid:76)(cid:69) (cid:17)(cid:25)(cid:16)
(cid:50)
(cid:17)(cid:24)(cid:16)
Assume that site-wide power fail-
(cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:86)(cid:65)(cid:76)(cid:86)(cid:69)(cid:0)(cid:83)(cid:69)(cid:84)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
ure occurs after five minutes (see (cid:17)(cid:23)(cid:16)
(cid:50)(cid:69)(cid:76)(cid:73)(cid:69)(cid:70)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
Figure 11). During the power fail- (cid:50)(cid:69)(cid:76)(cid:73)(cid:70)(cid:0)(cid:86)(cid:65)(cid:76)(cid:86)(cid:69)
(cid:17)(cid:22)(cid:16) (cid:65)(cid:67)(cid:67)(cid:85)(cid:77)(cid:85)(cid:76)(cid:65)(cid:84)(cid:69)(cid:68)(cid:0)(cid:80)(cid:82)(cid:69)(cid:83)(cid:83)(cid:85)(cid:82)(cid:69)
ure, the feed pump, column bottom
(cid:17)(cid:21)(cid:16)
pump, reflux pump and cooling
(cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16)
water pump stop, and their respec- (cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
tive flows become zero immediately.
The column sump level increases Figure 12 Site-wide power failure: relief pressure vs time
immediately as the tray inventories
are dumped to the bottom (see
Figure 13). (cid:17)(cid:18)(cid:16)
As the flows of feed, distillate,
bottoms and cooling water are cut,
(cid:17)(cid:16)(cid:16)
the vapours generated by the
reboiler cause the column pressure Column sump
level increases
to increase (see Figure 12). After 11 (cid:24)(cid:16)
minutes, the relief valve opens. (cid:5)
Initially, there is mass transfer (cid:0)(cid:12)(cid:76)(cid:69)(cid:86)
between the vapours from the (cid:69)(cid:76)(cid:0)(cid:80) (cid:22)(cid:16)
(cid:85)
(cid:68)
reboiler and the residual liquid on (cid:76)(cid:79)
(cid:40)
the trays; progressively, as the trays (cid:20)(cid:16)
dry up, the temperature and molec-
ular weight of the overhead
(cid:18)(cid:16) (cid:50)(cid:69)(cid:66)(cid:79)(cid:73)(cid:76)(cid:69)(cid:82)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80)
(relieving) vapour increase. The
(cid:50)(cid:69)(cid:70)(cid:76)(cid:85)(cid:88)(cid:0)(cid:68)(cid:82)(cid:85)(cid:77)
bottoms progressively become (cid:35)(cid:79)(cid:76)(cid:85)(cid:77)(cid:78)(cid:0)(cid:83)(cid:85)(cid:77)(cid:80)
heavier, resulting in a continuous (cid:16)
(cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16)
decrease in the reboiler duty (see
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
Figure 14). As the pinched reboiler
duty carries on decreasing, the
relief valve will eventually close. Figure 13 Site-wide power failure: hold-up level vs time
During power failure, the relief
load is relatively low compared
with the loss of feed condition (cid:23)(cid:16) (cid:17)(cid:18)(cid:16)
because the pinched reboiler duty (cid:45)(cid:79)(cid:76)(cid:69)(cid:67)(cid:85)(cid:76)(cid:65)(cid:82)(cid:0)(cid:87)(cid:69)(cid:73)(cid:71)(cid:72)(cid:84) (cid:17)(cid:17)(cid:24)
(cid:22)(cid:16) (cid:50)(cid:69)(cid:66)(cid:79)(cid:73)(cid:76)(cid:69)(cid:82)(cid:0)(cid:68)(cid:85)(cid:84)(cid:89)
is much less due to the high molec-
(cid:35)(cid:79)(cid:78)(cid:68)(cid:69)(cid:78)(cid:83)(cid:69)(cid:82)(cid:0)(cid:68)(cid:85)(cid:84)(cid:89) (cid:17)(cid:17)(cid:22)
ular weight in the column. During
loss of feed, continuing reflux (cid:21)(cid:16) Increasing column (cid:17)(cid:17)(cid:20)
mTcohaleuk metsimn t heies tcamoklueuncmh t noh rigpelhraeetsrisv uienrlye t hluiegp h ltotehsres. (cid:82)(cid:72)(cid:15)(cid:34)(cid:52)(cid:53)(cid:0)(cid:12)(cid:89) (cid:20)(cid:16) sump molecular weight (cid:17)(cid:17)(cid:17)(cid:17)(cid:16)(cid:18) (cid:69)(cid:82)(cid:76)(cid:84)(cid:72)(cid:71)(cid:73)(cid:87)(cid:0)(cid:65)(cid:85)
of feed scenario because the (cid:85)(cid:84)(cid:36) (cid:19)(cid:16) (cid:17)(cid:16)(cid:24) (cid:67)(cid:69)(cid:76)(cid:79)
(cid:45)
condenser is available, compared to
(cid:18)(cid:16) (cid:17)(cid:16)(cid:22)
the loss of power condition, Reboiler duty
decreases (cid:17)(cid:16)(cid:20)
where condensing duty was lost
(cid:17)(cid:16)
immediately. (cid:17)(cid:16)(cid:18)
(cid:16) (cid:17)(cid:16)(cid:16)
Summary
(cid:16)(cid:14)(cid:16) (cid:21)(cid:14)(cid:16) (cid:17)(cid:16)(cid:14)(cid:16) (cid:17)(cid:21)(cid:14)(cid:16) (cid:18)(cid:16)(cid:14)(cid:16) (cid:18)(cid:21)(cid:14)(cid:16) (cid:19)(cid:16)(cid:14)(cid:16) (cid:19)(cid:21)(cid:14)(cid:16) (cid:20)(cid:16)(cid:14)(cid:16)
loss of reflux condition (cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
Figure 15 shows a comparison of
relief load values obtained for loss Figure 14 Site-wide power failure: reboiler duty and molecular weight vs time
www.digitalrefining.com/article/1000487 PTQ Q2 2010 61
for the top tray at bubble point and
relief pressure when compared to
(cid:17)(cid:20)(cid:16)
dynamic simulation, which simu-
Conventional method
lates reflux failure, resulting in a
(cid:17)(cid:18)(cid:16) higher temperature and molecular
weight.
(cid:17)(cid:16)(cid:16) In a dynamic simulation of loss of
(cid:82)(cid:72) reflux, the column almost reaches a
(cid:15)(cid:66)
(cid:76)(cid:0)(cid:16)(cid:16) (cid:24)(cid:16) new steady-state condition after 25
(cid:16)
(cid:17) minutes. The rectifying section of
(cid:0)(cid:12)(cid:68) Steady-state simulation
(cid:69)(cid:65)(cid:79)(cid:76)(cid:0)(cid:70) (cid:22)(cid:16) the column goes dry and only the
(cid:73)(cid:76)(cid:69) stripping section is involved in
(cid:50)
mass transfer. This new steady state
(cid:20)(cid:16)
Dynamic simulation can also be reasonably simulated
using a steady-state simulator (see
(cid:18)(cid:16)
Steady-state simulation to obtain
relief load and properties).
(cid:16)
There is a marginal difference in
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16) (cid:20)(cid:21)
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78) the relief load obtained by steady-
state simulation and dynamic
simulation because, in steady-state
Figure 15 Loss of reflux: relief load vs time simulation, the column pressure has
been raised to an accumulation
pressure (set pressure +10% or
relief load calculated by dynamic simulation
+16% based on the scenario),
whereas in dynamic simulation the
upset condition relief load, lb/hr temperature, °F molecular weight pressure safety valve starts opening
Loss of reflux 90 800 310 62.5 at its set pressure and the pressure
Loss of feed 93 500 117 44.2
does not reach the maximum accu-
Site-wide power failure 29 250 290 76
mulated pressure for the selected
orifice area. Note that the conven-
table 2 tional method and steady-state
simulation are not time dependent,
of reflux. According to the conven- assumption is that all of the unbal- so the relief load appears constant
tional method, the predicted relief anced heat will vapourise the top in comparison with the dynamic
load is higher than the value tray liquid, which has a lower simulation relief load.
obtained by dynamic simulation. In specific enthalpy. The molecular
the conventional method, the weight and temperature are lower loss of feed
Figure 16 shows a comparison of
relief load obtained for loss of feed.
The relief load calculated by the
(cid:17)(cid:16)(cid:16) conventional method is lower than
by dynamic simulation. In the
(cid:25)(cid:16)
conventional method, the condenser
(cid:24)(cid:16) duty equals the design duty and
the cooling effect is predominant.
(cid:23)(cid:16)
(cid:82)(cid:72) In dynamic simulation, the
(cid:76)(cid:15)(cid:66)(cid:0)(cid:16) (cid:22)(cid:16) condenser duty is not fixed and the
(cid:16)
(cid:16) hold-up of the individual compo-
(cid:17)
(cid:0)(cid:12)(cid:68)(cid:65) (cid:21)(cid:16) Conventional method nents in the column determines the
(cid:79)
(cid:76)(cid:0)(cid:70)(cid:69) (cid:20)(cid:16) behaviour of the condenser.
(cid:76)(cid:73)(cid:69)
(cid:50) Initially, during loss of feed, the
(cid:19)(cid:16)
Dynamic reboiler duty decreases due to
simulation
(cid:18)(cid:16) pinch and the lighter components
subsequently travel to the bottoms
(cid:17)(cid:16)
and the whole column profile
(cid:16) becomes lighter. Eventually, the
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16)
reboiler duty again starts to raise
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
due to the decrease in molecular
weight. This phenomenon cannot
Figure 16 Loss of feed: relief load vs time be evaluated with the conventional
62 PTQ Q2 2010 www.digitalrefining.com/article/1000487
relief). This results in a conservative
estimate. The effect of hold-up
(cid:20)(cid:16)(cid:16) volumes and time taken to pressu-
rise is normally ignored.
(cid:19)(cid:21)(cid:16)
The conventional method is the
Conventional method
most conservative and requires less
(cid:19)(cid:16)(cid:16)
effort during design. Steady-state
(cid:82)(cid:72) simulation to determine the relief
(cid:66)(cid:15) (cid:18)(cid:21)(cid:16)
(cid:76)(cid:0)(cid:16) load has limited applicability. For
(cid:16)
(cid:16)(cid:17) grassroots designs, the conventional
(cid:0)(cid:12)(cid:68) (cid:18)(cid:16)(cid:16)
(cid:65)(cid:79) method may be the most appropri-
(cid:0)(cid:69)(cid:73)(cid:76)(cid:69)(cid:76)(cid:70) (cid:17)(cid:21)(cid:16) ate, as detailed design and/or
(cid:50)
complete vendor information may
(cid:17)(cid:16)(cid:16) not be available at the time of the
relief system’s design. It also helps
(cid:21)(cid:16) Dynamic simulation to build in inherent design margins
for any possible future expansion/
(cid:16) debottlenecking operation, and to
(cid:16) (cid:21) (cid:17)(cid:16) (cid:17)(cid:21) (cid:18)(cid:16) (cid:18)(cid:21) (cid:19)(cid:16) (cid:19)(cid:21) (cid:20)(cid:16)
minimise changes during the late
(cid:52)(cid:73)(cid:77)(cid:69)(cid:12)(cid:0)(cid:77)(cid:73)(cid:78)
stages of the project due to any
unforeseen design development.
Figure 17 Site-wide power failure: relief load vs time Dynamic simulation models the
system rigorously and tends to
method, but validates the hypothe- much lower than by the conven- provide more accurate results,
sis that, if the pinched duty is too tional method. In reality, during taking into account actual system
low, the designer should re-evalu- this condition, after the trays dry dynamics and configuration. It tries
ate the reboiler duty, assuming up the column simply acts as a boil- to emulate plant behaviour, which
lighter composition in the column ing pot without mass transfer. usually results in lower relief loads.
bottoms. The reboiler duty continuously Dynamic simulation also provides
decreases as the contents become relief loads based on time, which
Site-wide power failure heavier with time. According to the can be further analysed for optimis-
Figure 17 shows a comparison of conventional approach, reboiler ing the relief system’s design.
relief load obtained for site-wide duty and relief rate are calculated Dynamic simulation can be particu-
power failure. In dynamic simula- at one instant, which is at the start larly useful in unit revamps, to
tion, the relief load obtained is of the emergency (not at the start of limit the capital cost involved in
relief system modifications.
Steady-state simulation to obtain
relief load and properties
To relief
Debutaniser • Simulate the distillation column
To condenser Off gas into three sections: column, column
overhead system and reboiler
CWS CWR
Reflux system
drum
Recycle • The column can also be simulated
Reflux
Sour water as a reboiled column (column with
a reboiler) with theoretical stages
Reflux
Feed Total liquid from pump and normal operating pressure
column bottom stage Distillate
(internal stream) ■ Define a reflux stream and
Internal Bottom feed it to the top tray
energy
stream ■ Define the feed stream and
assign an appropriate feed location.
Set
Twinned Give a normal pressure drop across
column bottom
the column
■ Fix the normal reboiler duty to
External
To external reboiler the energy stream and normal boil-
reboiler
up ratio (as a specification)
Internal energy duty = external reboiler duty ■ Converge the column
Steam Condensate
• The column overhead system
includes a pressure safety valve
(PSV), cooling water condenser and
Figure 18 Distillation column – steady-state simulation – relief condition reflux drum
64 PTQ Q2 2010 www.digitalrefining.com/article/1000487
■ Split the overhead vapour pressure + normal ∆P), the bubble where it is expected that the reliev-
from the column “to relief” and “to point of the column bottom ing scenario could approach the
condenser”, and set the “relief increases. The temperature differ- steady-state condition.
flow” rate to zero ence across the external reboiler
■ Simulate the condenser as a reduces, leading to lower external references
1 Sengupta M, Staats F Y, A new approach to
shell and tube exchanger with cool- reboiler duty (pinch). The calcu-
relief valve load calculations, May 1978.
ing water on the tube side and lated duty of the external reboiler
2 Rahimi Mofrad S, Tower pressure relief
overhead vapour totally condensed. should be equal to the energy
calculation, Hydrocarbon Processing, Sep 2008.
Simulate the reflux drum, reflux stream attached to the column
pump, distillate product and reflux (internal energy stream). Iterate the
Haribabu Chittibabu is an Engineering
■ The reflux from the reflux column internal energy stream so Specialist in the Advanced Simulation and
pump should be same as the that it matches the external reboiler Analysis group at Bechtel India. He has a
defined reflux stream to the top duty. Even though the LMTD tends bachelor’s degree in chemical engineering
tray, so connect them through a to increase in the condenser, many from University of Madras and a master’s in
recycle block designers tend to restrict the maxi- petroleum refining and petrochemicals from
• The reboiler system should be mum condenser duty to design Anna University, India.
Email: [email protected]
simulated as a separate shell and duty due to uncertainties in the
amudha Valli is an Engineering Specialist in
tube heat exchanger (external calculation. For this exercise, the
the Advanced Simulation and Analysis group
reboiler) in order to study reboiler condenser duty is limited to the
at Bechtel, India. She has a bachelor’s degree
pinch at relieving conditions design duty only.
in chemical engineering from Coimbatore
■ Create an internal stream of Now the column is at relieving Institute of Technology, India, and a master’s
the total liquid from the bottom pressure, giving an idea of the in chemical engineering from Anna University,
stage in the column. The internal reduced reboiler duty and the India. Email: [email protected]
stream minus the column bottoms amount of overhead vapour. The Vineet Khanna is Project Engineering Manager
is the feed to the external reboiler, next step is to simulate the cause of with Bechtel India. He has a bachelor’s degree in
so split the internal stream to the overpressure to the maximum chemical engineering from the Indian Institute
external reboiler and twinned convergence of the column. For loss of Technology, Delhi, India.
Email: [email protected]
column bottoms. Set the column of reflux, increase the flow “to
dipanjan bhattacharya is an Engineering
bottoms flow rate to the twinned relief”, so that flow to the condenser
Specialist in the Advanced Simulation and
column bottoms stream is reduced and, ultimately, the flow
Analysis group at Bechtel, Houston. He has
■ Specify the normal UA to the to the reflux is reduced.
a bachelor’s degree in chemical engineering
external reboiler Simultaneously reduce the distillate
from Jadavpur University, India, and master’s
■ Specify the hot side of the flow step-wise as the reflux pump in chemical engineering from University of
external reboiler. For the case under is stopped. At the same time, keep Oklahoma. Email: [email protected]
consideration, the hot-side inlet is iterating the column internal energy
steam at its saturation condition stream so that it matches the exter-
and the hot-side outlet is total nal reboiler duty. Ultimately, when linKS
condensate the reflux and distillate are zero, all
• Increase the column pressure to the overhead vapour from the more articles from the following
relief pressure (PSV set pressure + column is the relieving flow. category:
allowable accumulation). Since the The above methodology can also process modelling & Simulation
bottom pressure is higher (relief be extended to other emergencies,
www.digitalrefining.com/article/1000487 PTQ Q2 2010 65