Introduction
Process design and optimization play a vital role in creating and improving the manufacturing processes for chemicals and related products. Chemical process design includes several stages, including conceptual design, process development, detailed design, construction, and operation. The ultimate goal of chemical process design is to develop a cost-effective and safe process that can produce high-quality products at a high yield.
Importance of Chemical Process Design:
- Cost reduction: It can help reduce production costs by identifying and eliminating inefficiencies in the production process. By optimizing the process, manufacturers can produce more with fewer resources, reducing the cost of production.
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Improved product quality: Manufacturers can improve the quality of the product. The process can be designed to produce products that meet specific quality standards, resulting in fewer defects and customer complaints.
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Increased safety :By identifying potential hazards and mitigating them during the design stage, manufacturers can reduce the risk of accidents during production and ensures the safety of workers.
- Environment Sustainability : Minimize waste and energy consumption and promote the reuse of materials, manufacturers can reduce their carbon footprint and contribute to a more sustainable future.
- Competitive Advantage : Manufacturers can get a competitive advantage by improving their efficiency, quality, and sustainability. This can help them gain market share and increase profitability.
- Innovation : Manufacturers can lead to the development of new products/technologies and processes that are more efficient and sustainable.
Chemical Process Design:
Creating of manufacturing process in cost effective and safe manner involves several steps including basic engineering package, detailed engineering packages, construction and operation. Process engineers are professional who play a vital role to design a process which can produce high quality products efficiently.
Basic Engineering Design Services:
Basic Engineering Design (BED) or Front-End Engineering Design (FEED) services is most important engineering design activity for working on any Greenfield or Brownfield projects. BED will establish the specific set of process operating conditions and equipment necessary to achieve the level of reliability, efficiency, and safety required. This design phase sets the direction for the rest of the project. At the completion of this phase, a cost estimate of +40%/-20% can typically be developed for the project. BED puts great emphasis on the development of the Design Basis at the initiation of design. When the design basis is complete, we typically have the following information defined:
- Raw material specifications
- Plant capacity requirements
- Product specifications
- Critical plant operating parameters
- Available utilities specifications
- Individual unit operations performance requirements
- Process regulatory requirements
- All other operating goals and constraints desired by the plant
Once the design basis is in place, and agreed upon with the client, process engineer works to create, analyze, and optimize the many aspects of the plant design. The end result is process documentation that clearly defines the process.Typical Process Engineering Deliverables for BED package can include the following or a smaller subset of these items:
- Process design basis
- Material & Energy Balance (M&EB)
- Process Flow Diagrams (PFDs)
- Process descriptions
- Utility balances and Utility Flow Diagrams (UFDs)
- Preliminary Piping & Instrumentation Diagrams (P&IDs)
- Process control description
- Preliminary line/pipe list
- Preliminary instrument list
- Process equipment list
- Preliminary Tie-in list
- Equipment process datasheets
- Instrument process datasheets
- Hydraulic design reports.
Case studies on chemical process
Let us assume a homogeneous liquid phase non-catalytic reaction. In this reaction two organic raw materials, chemical ‘A’ and chemical ‘B’ reacts to form chemical ‘C’. This is an exothermic reaction and raw material ‘A’ is limiting reactant. Chemical ‘B’ consumption is 1.25 times of reactant ‘A’. Heat of reaction is 150 kcal/kg of reacted ‘A’.
In this process equilibrium conversion of the reaction is 85% on the mass basis for reactant ‘A’. This reaction takes place at 85 °C and atmospheric conditions. Selectivity of the reaction is 95% on mass basis. And remaining 5% of reacted ‘A’ converts into high boiling tar like material. This residue composition is as below which is sent for incineration. The calorific value for residue is 7500 kcal/kg approximately.
| Sr. No. | Component | Composition (wt%) |
|---|---|---|
| 1 | Chemical A | 1% |
| 2 | Chemical B | 2% |
| 3 | Chemical C | 2% |
| 4 | Heavies | 95% |
Physical and Chemical Properties:
For our batch reactor process calculations, we need physical and chemical properties for the chemicals are in below table.
| Sr. No. | Component | Normal Boiling Point (℃) | Heat Capacity (kcal/kg℃) | Density (kg/m3) | Heat of Vapourization (kcal/kg) |
|---|---|---|---|---|---|
| 1 | Chemical A | 70 | 0.35 | 900 | 100 |
| 2 | Chemical B | 90 | 0.35 | 900 | 100 |
| 3 | Chemical C | 100 | 0.35 | 1000 | 90 |
| 4 | Heavies | 120 | 0.35 | 1000 | – |
Reaction:
A (liq.) + B (liq.) —-> C (liq.) at 85 °C and atmospheric pressure
Process Flow Diagram (PFD)
Here,
RM – Raw Material , CWS – Cooling Water Supply, CWR – Cooling Water Return, Cond. – Steam Condensate
Material Balance for the Batch Reactor System
This process includes two steps first is reaction and second is batch distillation. The material balance for per batch will be as below.
- Charge of RM – A, 2000 kgs/batch
- Charge of RM – B, 2500 kgs/batch (since B is charged 1.25 times of A)
- Total mass of in reactor (RM – A + RM – B = 4500 kgs/batch)
- Equilibrium conversion is 85% hence unreacted RM – A in crude product = 2000*(100 – 85)/100 = 300 kg.
- Unreacted RM – B in crude will be = 300*1.25 =375 kg.
- Product C in crude will be (2000 + 2500) *0.85*0.95 = 3633 kg (since selectivity is 95% for the product C.)
- Heavies’ generation in reaction will be = (2000 + 2500) *0.85*0.05 = 181.7 kg/batch. Since the composition of heavies in residue is 95% hence residue generation per batch will be = 181.7 *100/95 = 191.3 kg.
- Loss of RM – A in residue will be 191.3 *1/100 = 1.91 kg/batch
- Loss of RM – B in residue will be 191.3 *2/100 = 3.83 kg/batch
- Loss of Product C in residue will be 191.3 *2/100 = 3.82 kg/batch
The recovered quantities from distillation based on 90% recovery will be as below
- RM – A recovered = 300 *90/100 = 270 kg/batch
- RM – B recovered = 375 *90/100 = 337.5 kg/batch
- Product – C recovered = 3633 *90/100 = 3269.7 kg/batch
- Intercut quantity of A & B = 45.5 kg/batch (66% A and 34% B) – from R&D package
- Intercut quantity of B & C = 44.0 kg/batch (50% B and 50% C) – from R&D package
Total production of product – C will be = product in crude – loss in intercut – loss in residue = 3633 – 22 – 3.82 = 3607.2 kg/batch.
Total RM – A consumed = Charged – Recovered = 2000 – 270 = 1730 kg/batch
Total RM – B consumed = 2500 – 337.5 = 2162.5 kg/batch
Energy Balance for the Batch Reactor System
Heating utility for our process is 3.5 bar steam at saturated conditions. The temperature of the steam is 139 °C and latent heat is 513.5 kcal/kg.
Steam Requirement
Heat load for reaction mass heating after charging of RM – B will be Q1 = mass RM-B * Cp * (initial temp – final temp) = 2500*0.35*(80-35) = 39375 kcal/batch. Hence steam requirement will be m1 = Q1/513.5 = 76.7 kg/batch.
Heat load and steam requirement in distillation will as follows:
For recovery of RM – A, heat load will be Q2 = mass recovered * (1 + reflux ratio) * latent heat = 270*(1 + 5) *100 = 162000 kcal/batch. Steam requirement will be m2 = Q2/513.5 = 162000/513.5 = 315.5 kg/batch.
Similarly, for RM – B recovery Q3 = 337.5*(1 + 10)*100 = 371250 kcal/batch. Steam requirement will be m3 = Q3/513.5 = 723.0 kg/batch.
For product recovery Q4 = 3269.7*(1 + 10)*90 = 3237003 kcal/batch. Steam requirement will be m4 = Q4/513.5 = 3237003/513.5 = 6303.8 kg/batch.
For first intercut Q5 = 44.5*(1 + 25)*100 = 115700 kcal/batch. Steam required will be m5 = Q5/513.5 = 225.3 kg/batch.
Heat load for second intercut Q6 = 44.0*(1 + 40)*100 = 180400 kcal/batch. Hence steam requirement will be m6 = Q6/513.5 = 351.3 kg/batch.
Therefore, total steam requirement for total batch processing will be Q = Q1 + Q2 + Q3 + Q4 + Q5 + Q6 = 76.7+315.5+723.0+6303.8+225.3+351.3 = 7995.6 kg/batch. Considering 5% steam loss actual steam requirement will be Q’ = 1.05*Q = 8395 kg/batch.
Cooling Water Requirement
Cooling water flow rate requirement will be based on when our reaction is going on and pure product draw is going on. As this will be maximum requirement any point of time during the process.
- Hence, heat load on jacket during reaction q1 = rate of addition A * heat of reaction = 500 * 150 = 7500 kcal/h (for calculation we are considering 100% conversion). Cooling water supply and return temperature are 32 and 40 °C respectively. Hence, cooling water flow will be w1 = q1/(Cpw*(40-32)) = 7500/(1*(40-32)) = 937.5 kg/h.
- Heat load during pure product draw (3269.7/10 = 327 kg/h) will be q2 = 327*(1 + 10) *90 = 323730 kcal/h. Therefore, cooling water requirement at column condenser will be w2 = q2/(Cpw*(40-32)) = 323730/(1*(40-32)) = 40466 kg/h.
- Water circulation in reactor condenser w3 = 5000 kg/h. Total flow rate for cooling water pump will be W = w1 + w2 + w3= 937.5 + 40466 + 5000 = 46403.5 kg/h or 46.4 m3/h.