Optimizing Chemical Process Design through Simulation

Introduction

Design process design gives an overview of the whole process in drafts, ideas and many other ways. This is very useful, it has made things easy for many industries; they are a lot of software’s that assist with process stimulation that are used industrially to make things easier for companies. Block flow diagram and process flow diagrams are useful as a start in process design.

What is the Design Process?

Process design is a systematic series of steps that helps to define, plan and produce a product desired to be built.

It allows you to be efficient, transparent and focused on creating the best product possible. In Chemical engineering, a design process is used to help make sure we don’t miss a step or a part and sequencing of units for desired physical and chemical transformation of materials.

Process design can be the design of new facilities or it can be the modification or expansion of existing facilities.

The design starts at a conceptual level and ultimately ends in the form of fabrication and construction plans. Process design is distinct from equipment design, which is closer in spirit to the design of unit operations. Processes often include many unit operations.

Importance of Process Simulation

Now, new software can perform repetitive chemical engineering calculations in a fraction of the time it takes to execute them by hand. Simulation modelling solves real world problems safely and efficiently. It provides an important method of analysis which is easily verified, communicated and understood.

Across industries and disciplines, simulation modelling provides valuable solutions by giving clear insights into complex systems. Simulation enables experimentation on a valid digital representation of a system. Unlike physical modelling, such as making a scale copy of a building, simulation modelling is computer based and use algorithms and equations.

Process Simulation Software

• CHEMCAD steady state (Aspen plus)
• CHEMCAD Dynamics (Aspen Hysys)
• CHEMCAD Batch (Aspen FlareNet)

Advantages and Disadvantages

Faster Time To Market

Project time can be dramatically shortened if fabrication, assembly, and testing of the process system occur during any site civil and facilities construction. In a traditional project timeline, site civil and facilities must be completed before any process system work can begin. Since process systems are being assembled off-site into easily transportable skids, modular process system can be developed in parallel with civil and facilities construction. Start-up time is also minimized since systems can be fully assembled and tested before they ship, reducing the amount of on-site start-up time. Weather delays are eliminated during process system development since skids are assembled indoors.

Reduced Costs

Lower labor and operational costs are achieved due to a shorter project timeline, efficient use of material, and a smaller field crew. For multi-unit projects, higher capitol efficiency is achieved by designing once and building duplicates Off-site module construction does not interrupt or shut-down pre-existing operations.

Safety Benefits

Safety risks are reduced for modular process plant personnel with fewer onsite OSHA exposure hours and smaller crew sizes. Ideal construction conditions provided by an enclosed fabrication facility further reduce safety risk for fabricators. Full modular process system testing and checkout prior to shipment identifies and corrects any potential problems before the system is delivered on-site

Case Study: Monochlorobenzene Production

Manual calculations

REACTIONS: C6H6 + Cl2 = C6H5Cl +HCl

C6H6 +Cl2 = C6H4Cl2 +2HCl

Basis: 100 kmol of benzene

Mol ratio of chlorine to benzene ratio = 0.9

Overall conversion of benzene = 55.3%

Yield of monochlorobenzene = 73.6%

Yield of dichlorobenzene = 27.3%

Molar mass of HCL = 36 kg/kmol

Molar mass of H2O = 18kg/kmol

The production of other chlorinated compounds are neglected.

Reactor inlet calculation

Chlorine feed to reactor = 100×0.9 = 90 kmol/h

Reactor outlet calculation

MCB produced =(55.3×73.6)/100 = 40.7 kmol/h

DCB produced = (55.3×27.3)/100= 15.1 kmol/h

Benzene = 100-55.3 = 44.7 kmol/h

HCl produced = 2(15.1)+40.7 = 70.9 kmol/h

Chlorine produced = 90-(2(15.1)+40.7) = 19.1 kmol/h

Total stream flow = 40.7+15.1+44.7+70.9+19.1 = 190.5kmol/h

Condenser feed stream calculation

Assumption: total condensation of chlorobenzenes and unreacted benzenes.

Condenser feed stream has the same flow as the reactor outlet

MCB fed = 40.7 kmol/h

DCB fed =15.1 kmol/h

Benzene fed = 44.7 kmol/h

HCl fed = 70.9 kmol/h

Chlorine fed =19.1kmol/h

Total stream flow = 40.7+15.1+44.7+70.9+19.1 = 190.5kmol/h

Condenser outlet stream calculation

At the condenser the is no reaction therefore the outlet stream of the condenser has equal compositions as the inlet stream.

Absorption feed stream calculation

All the HCl and Cl2 from separator is fed to the absorber

HCl fed = 70.9×36.5=2587.85 kg/h

Cl2 fed = 19.1kmol/h

Absorber Tops

98% of the chlorine fed to absorber is recycled

Chlorine recycled = 0.98×19.1 = 18.72 kmol/h

Absorber bottoms

Chlorine = 19.1-18.72 = 0.38 kmol/h

100% absorption of HCL is achieved

HCl = 70.1 kmol/h

H2O fed = H2O out =2587.85/0.3 = 8626.17kg

Distillation inlet stream

All the chlorobenzenes and unreacted benzene from separator is fed to the distillation column

MCB fed = 40.7 kmol/h

DCB fed = 15.1 kmol/h

Benzene fed = 44.7 kmol

Distillation overheads

95% of the benzene fed is recovered

Benzene recovery = 44.7×0.95 = 42.4 kmol/h

Distillation bottoms

MCB = 40.7 kmol/h

DCB = 15.1 kmol/h

Benzene = 44.7 -42.4=2.3 kmol/h

Reactor with recycle feeds

Benzene Fresh feed = recycle + feed

= 42.4 + 100

= 142.4 kmol/h

Chlorine fresh feed = recycle + feed

=18.9 + 90

= 108.9 kmol/h

Scaling Factor:

Product required=400000t.d-1 =400000/8760=45.66t/h

45.66/112.9=0.41

Fresh feed = 50.53

Produced = 40.7

Scaling flow = 0.41/40.7=0.01

Excel graph

Reactor In Out

Chlorine 90 kmol 19.1054 kmol

Benzene 100 kmol 44.7 kmol

MCB 0 kmol 40.7008 kmol

DCB 0 kmol 15.0969 kmol

HCl 0 kmol 70.8946 kmol

Condenser/Flush Drum In Vapour liquid

Chlorine 19.1054 kmol 19.1054 kmol 0 kmol

Benzene 44.7 kmol 0 kmol 44.7 kmol

MCB 40.7008 kmol 0 kmol 40.7008 kmol

DCB 15.0969 kmol 0 kmol 15.0969 kmol

HCl 70.8946 kmol 70.8946 kmol 0

Absorber In Tops Bottoms

Chlorine 19.1054 kmol 18.723292 kmol 0.382108 kmol

HCl 70.8946 kmol 0 70.8946 kmol

Distillation Column In Tops Bottoms

Benzene 44.7 kmol 42.465 kmol 2.235 kmol

MCB 40.7008 kmol 0 kmol 40.7008 kmol

DCB 15.0969 kmol 0 kmol 15.0969 kmol

Conclusion

The detailed process simulation and design for the production of Monochlorobenzene highlight the significance of simulation software in optimizing chemical engineering processes. By providing a clear visualization of process flows and enabling precise calculations of material balances, process simulation tools are indispensable in the efficient and safe design of chemical processes. The ability to predict process outcomes and refine designs through simulation leads to improved product quality, cost savings, and enhanced safety, underscoring the pivotal role of process design in modern chemical engineering practices.