Page 166 - Chemical engineering design

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FLOW-SHEETING
and column packing. The frequency of shut-downs, and the consequent loss of production
time, will depend on the nature of the process. For most chemical and petrochemical
processes the plant attainment will typically be between 90 to 95 per cent of the total
hours in a year (8760). Unless the process is known to require longer shut-down periods,
a value of 8000 hours per year can be used for flow-sheet preparation.
Scaling factor
It is usually easiest to carry out the sequence of flow-sheet calculations in the same order
as the process steps; starting with the raw-material feeds and progressing stage by stage,
where possible, through the process to the final product flow. The required production
rate will usually be specified in terms of the product, not the raw-material feeds, so it
will be necessary to select an arbitrary basis for the calculations, say 100 kmol/h of the
principal raw material. The actual flows required can then be calculated by multiplying
each flow by a scaling factor determined from the actual production rate required.

mols product per hour specified
Scaling factor D
mols product produced per 100 kmol
of the principal raw material

4.3.2. Flow-sheet calculations on individual units
Some examples of how design constraints can be used to determine stream flows and
compositions are given below.

1. Reactors
(i) Reactor yield and conversion specified.
The reactor performance may be specified independently of the detailed design
of the reactor. The conditions for the optimum, or near optimum, performance may
be known from the operation of existing plant or from pilot plant studies.
For processes that are well established, estimates of the reactor performance can
often be obtained from the general and patent literature; for example, the production
of nitric and sulphuric acids.
If the yields and conversions are known, the stream flows and compositions can
be calculated from a material balance; see Example 2.13.
(ii) Chemical equilibrium.
With fast reactions, the reaction products can often be assumed to have
reached equilibrium. The product compositions can then be calculated from the
equilibrium data for the reaction, at the chosen reactor temperature and pressure;
see Example 4.1.

2. Equilibrium stage
In a separation or mixing unit, the anticipated equipment performance may be such that
it is reasonable to consider the outlet streams as being in equilibrium; the approach to
equilibrium being in practice close enough that no significant inaccuracy is introduced