Limestone Engineering Consultants Ltd.
To:
Methanol Project Team
From: Blair Schroeder, Methanol Project Manager
Optimization of a Distillation Column
Now that we have the basic flowsheet defined we should optimize the design
of the expensive pieces of equipment. I propose that we start with the Lights
column, and determine the optimum number of trays.
Attached is a set of notes describing the technique for doing this.
By doing a series of HYSYS runs with different numbers of trays we can
obtain data on a number of useful variables:
 sizing of the column
 heat loads
 temperatures in the heat exchangers
The "rules of thumb" contain a wealth of information which is useful in
converting the data from HYSYS into the values required by CAPCOST. Also,
Table 6.3 contains typical utility costs.
Assume that the plant runs at nominal capacity (that shown in the
simulation) for 8000 hours per year.
Turton (page 177) suggests a factor of 1.18 to go from the “Bare Module” costs
obtained from CapCost to the total capital cost.
In CapCost
 For the column use SS valve trays, CS shell.
 For the exchangers use CS shell and tubes, S/T fixed head (or kettle
reboiler)
In HYSYS, use all the defaults for tray design (see the section “Tray
Hydraulics/Packing”.
Assume a “payback period” of three years.
As you go through this exercise, keep track of the reflux ratio values
(optimum tray location runs only). When finished, estimate the minimum
number of trays and the minimum reflux ratio. Perry (chapter 13) discusses
the Gilliland correlation which describes how the trays vs. reflux ratio
relationship normally behaves. Compare your results with this (the file
106743854
- 2 “Gilliland Correlation.xls” can be used to generate a graph that will show a
comparison).
The attached note describes the procedure for doing this optimization.
- 3 -
Optimization of Distillation Column
This is an exercise in doing a "quick and dirty" or "back of the envelope"
calculation to optimize an important design parameter.
We could decide to use the equipment sizing capability of HYSYS to work up
and cost a set of cases, and do a more accurate optimization. In fact, that
might be a good thing to do in the final stages of the design process. But in
many situations where we are working on a conceptual design, the extra
work is not justified.
At the other extreme, we could just use the "rules of thumb" and select the
case in which the reflux ratio is 1.2 times the minimum, or the number of
stages is twice the minimum.
We will use a technique of intermediate accuracy.
Trays vs Reflux
One of the most common optimizations is the trade-off between trays and
reflux in a distillation column.
Given the specifications for stream purity at the top and bottom, there are an
infinite number of configurations that will produce the desired separation.
The case we have modeled is just one of many.
At one extreme, we could operate with an extremely large number of trays,
and the minimum reflux ratio. At the other end, we could operate with the
minimum number of trays and an extremely high reflux ratio.
Neither of these is a good way to run. The former will have a very high
column cost, the latter will have very high condenser, reboiler and utilities
costs.
In examining a series of options with varying numbers of trays it is
important to ensure that all of the cases have the feed going in at the
optimum location. Otherwise we would not be comparing the cases on a
consistent basis.
Economic Analysis
- 4 -
In many cases this comes down to a trade-off between capital costs and
operating costs. To handle this we need to know our company's policy with
respect to the expected return on capital. This does not imply that we should
do the type of formal analysis required to assess the economic viability of the
overall project.
The important things to remember in this type of economic optimization are:
 we need to know which is the best case
 we do not need to know how good it is
 in assessing a group of cases we can ignore all costs that are the same in
all cases
 we should focus our attention on the costly items and ignore the minor
ones
 almost all policies on expected return on capital can be reduced to a rule of
the form $1 of incremental capital is equivalent to $X of
incremental operating costs
All but the final item should be obvious. To understand the capital/operating
cost equivalence, consider the following simplified case. I am neglecting the
impact of taxes on profits, and assuming that the process will run forever.
Suppose we can spend an extra $1 in 1998 and reduce our operating costs by
$X in 1999, 2000, 2001, 2002 ........ forever. Also, suppose that the company
policy is to evaluate investment on the basis of Net Present Value (NPV) at
the time the capital is spent, calculated from discounted cash flow with a
discount rate of D%. The best choice is the one that has the highest NPV.
The way the arithmetic works out, the operating cost benefit will just balance
the capital expenditure if X = 100/D. Other policies will generate similar
results.
We can think of X as a payback period (even if our company policy does not
use this term), and do our optimization on the basis of:
(Incremental Capital Cost) + (Payback Period)*(Incremental Operating
Costs)
Costing
First, we need to decide which cost factors are likely to change significantly
from case to case. A reasonable list is:
 column
 condenser
- 5 


reboiler
steam (for reboiler)
cooling water (for condenser)
Some smaller items will also change (e.g. the reflux pump and its electrical
supply), but we will assume that the cost variations here are small enough to
ignore.
For these estimates we will use the CAPCOST program on the CD-ROM
which comes with Turton. Instructions for the use of the program are
available in another document. This requires that we specify design
parameters such as:
 column height and diameter
 heat transfer area
 pressures
 materials of construction
We will assume that, if we take all our costs from Turton, they will all be on
the same basis ($US, same year, same location). Therefore, accept the default
value of the plant cost index in CapCost.
Procedure
In summary, the steps to determine the optimum design are:
 select a series of values for the number of trays (I try to go from the
lowest value that will work, up to the point at which raising the
number of tubes 20% lowers the reflux ratio less than 5%)
 for each tray configuration, vary the feed tray location so that it is
optimum
 using the optimum feed tray location, collect thermal and tray design
data from HYSYS
 with that data, and other parameters calculated from it, use CapCost
to get the costs of the column, condenser and reboiler
 also, work out the utility cost
 finally, calculate an effective cost for each configuration (capital cost +
payback period * operating cost)