Chemical Engineering December 2021 - 31

STEP
Component
Valve A
Valve B
TABLE 4. CALCULATION/ESTIMATION RESULTS FOR STEPS 1 TO 5
1
2
Cv,base
180.0
40.6
ΔPbase
1.90 bar
4.06 bar
FIGURE 10. The graph shows the ΔHbase and ΔHrevamp estimate,
as discussed in Example 3. A composite pump curve
(blue) represents parallel operation
strated in the previous example, the
designer can apply the same method
to determine the flow coefficient for
the main control valve (Cv,main) and
bypass valve (Cv,bypass). Accordingly,
one can estimate the total flow coefficient
for each scenario (Cv,total) from
Equation (6).
Cv,total = Cv,main + Cv,bypass
(6)
Besides liquid-phase applications,
Equation 6 also applies to other applications,
such as gas phase or
steam. During the test run, the bypass
valve was opened to explore
the maximum wash-water rate at
this injection point. Table 5 summarizes
the test-run data and calculation
results.
Although opening a bypass valve
can achieve the revamp flowrate,
it can also lead to undesired situations,
including vessel overflow.
As such, a design engineer has no
choice but to upgrade the existing
1-in. control valve. In this case,
a full-bore 2-in. valve could be a
good fit. From Table 5, the total
flow coefficient of the revamp
case (Cv,total,revamp) is 9.14. Consequently,
the travel of the new
control valve (%Travelrevamp)
would be 47, as highlighted in
Figure 8.
3. Increased diesel production.
Diesel has the highest value
among all products from a middledistillate-oriented
hydrocracker. Primarily,
the petroleum refinery in this
example tries to minimize the cut
point between kerosene and diesel
to maximize its diesel production.
Unfortunately, the existing diesel
pump limits the diesel production at
199.4 m3/hr from a high motor load.
According to the refinery economist,
the diesel production capacity
should be expanded to 226.0 m3/
hr (at the pumping temperature).
Possible solutions include either revamping
both existing diesel pumps
or adding a third larger pump.
Installing one pump is obviously a
less expensive solution than replacing
two pumps. In the end, the refinery
decided to install the third larger
pump (Figure 9). This new pump will
be the main workhorse, with the existing
pumps as spares, and will be
offline only when there is low diesel
load.
As pointed out earlier, diesel is the
most
expensive
refinery
product.
often
Therefore,
the
refinery
FIGURE 11. The illustration shows the crude distillation overhead system in Example
4. One can consider reducing the suction pressure drop as a fast-track solution
to unload the overhead compressor, as highlighted here
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ran both existing
pumps
in parallel to
maximize diesel
production.
The only
problem with
this operation
is that the existing
pumps
cannot be
DECEMBER 2021
3
ΔPrevamp
0.40 bar
2.56 bar
4
Cv,revamp
391.8
51.1
5
%Travelrevamp
54.0
12.6
offline for regular maintenance. Furthermore,
the liquid level inside the
diesel-side stripper was sometimes
unstable when the diesel hydrotreater
was run at high throughput (two diesel
streams from both units join together),
forcing the refinery to reduce
diesel production, due to the unstable
level control valve.
As the worst-case scenario, the
design engineer conducted the test
run at the design revamp flowrate
(Qrevamp, 226.0 m3/hr) with the diesel
hydrotreater at its maximum throughput.
During the test run, both diesel
pumps were in parallel operation, with
%Travel of the level control valve at 95
(less than 80 is preferable). Based on
Qrevamp, the baseline pump's head
(ΔHbase) is 159.7 m, as graphically
determined from Figure 10.
This diesel rundown system consists
of long and complex pipelines,
plus multiple heat exchangers, thus
requiring challenging and expensive
demolition work during the turnaround
if it was modified. Hence,
the new diesel pump will be sized
so %Travel of the level control valve
is around 60, by including a design
margin (ΔHmargin) as per Equation 7.
ΔHrevamp = ΔHbase + ΔHmargin
(7)
According to the study, the revamp
pump's head (ΔHrevamp) must
be 175.7 m to ensure good operability.
In this case, a new relief valve is
unavoidable due to higher shut-off
pressure, but the additional cost is
trivial considering the benefits.
4. Improved diesel recovery. A
crude-petroleum distillation unit suffered
from high pressure, resulting
in lower diesel recovery. As per the
design, the suction pressure of the
overhead compressor directly sets
the column pressure (Figure 11).
The
pressure-control
system
varies
%Travel of the discharge control
valve to maintain a constant accumulator
pressure. When the %Travel
reaches 100%, for example, at a
31
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Chemical Engineering December 2021

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