First Assignments for 57138 – Computers & Applications in Chemical Engineering

Distribution date: 27 March 2014Submission date: 27 April 2014

(1) Given a flash separator for separating ammonia-water mixture as shown

below:

a. Develop a steady state model. (Note: The model include all the

equations that will be needed to determine the flowrates, compositions

and temperatures of the streams)

b. Analyse the number of degree of freedom (DOF).

c. Determine which sets of variables to assign to make the simulation

work and explain your decision.

(2) Refer to the Second Lab Demonstration Document – Case 2.1 Propane

Refrigeration System. Answer the following questions:

a. What are the pressure, temperature and phase for each stream?

b. Based on the simulation in HYSYS, analyse how the system works

physically.

c. How does the simulation work numerically?

(3) Refer to the Second Lab Demonstration Document – Case 2.2 Separation of Ammonia and Water. Answer the following questions:

a. What are the compositions for each stream?

b. What is the purpose of such a process? Which factors will influence

the function of this process? Explain these in basic theory.

(NH3, H2O)

(NH3, H2O)

(NH3, H2O)

c. Use Databook in HYSYS to do case studies to validate your analysis

above. Only one factor should be used as independent variable in

each case study.

(4) Refer to the Second Lab Demonstration Document – Case 2.3 Natural Gas Liquids (NGL) Fractionation Train. Answer the following questions:

a. Analyse how DC1 column works and how to improve the purity of the

top product. Use HYSYS case study to validate your analysis.

b. In DC2 column, check the reflux flowrate to the first stage (or tray) and

explain why the reflux flowrate is this value.

c. Analyse the impact of the reflux ratio in DC2 column and use case

study to validate your analysis.

(5) 200 kg/hr liquid ethyl mercaptan (C2H6S) at 25OC and 1bar is pumped

to a pressure of 130bar and mixes with 8 kg/hr of hydrogen gas feed at same

temperature and pressure before entering a conversion reactor. The

components react as follows:

C2H6S + H2 → C2H6 + H2S

Conversion of the base reactant is 99.99%. The reactor product is cooled to

35OC. The pressure drop of the cooler is 500 kPa. The pressure is dropped

further across a valve before entering a flash separator (S1). The bottom

product of the S1 is further separated in a distillation (DC1) (distillation

column sub-flowsheet). The default entry stage of the feed stream must be

retained and the total number of stage must be selected in such a way to

achieve 100% H2S mass composition in the bottom stream. The condenser

and reboiler pressure will be respectively 300 kPa and 500 kPa and also the

distillate must be refluxed 100%. Overhead product of DC1 and vapour

product of S1is mixed and then the stream split into two streams according

to their composition. One of the streams must be 100% ethane. The other

stream with other components is further separated in a flash separator (S2).

The inlet pressure to S2 must be 50kPa. Decide what you unit that is needed

to achieve this and explain your decision.

The bottom product of S2 is mixed with bottom product of DC2. Part of the

top product of S2 is purged and the other recycled and combined with the H2

feed stream. Ensure that the pressure of the recycle stream is the same as

that of the hydrogen stream before combining the two streams.

Based on this information, attempt the following questions:

(i) State with reason the appropriate base component (limiting

reactant) in the equation above.

(ii) What unit operation do you need to reduce or increase the pressure

of the stream before mixing the stream with the H2 feed stream.

Explain

(iii) Prepare a complete flowsheet for the process described using

ASPEN HYSYS. Explain your selection the various unit operations

used in your flowsheet.

(iv) Determine the amount of the top product of S2 to be purged to

ensure that the H2S mass fraction in reactor inlet stream lies within

<0.2. What is the impact of increasing or decreasing the purge

flowrate on material accumulation within the system? Comment.

(v) What is the impact of increasing or lowering inlet pressure on mass

fraction of ethane in top product of DC2?