CHAPTER – 3
FLOW SHEET SYNTHESIS
Once the process has been described, process engineer has to synthesize the flow sheet which will give an overview of the whole process. Synthesizing flow sheet is also not a very simple task. For it, process engineer has to consider various points. This whole task of flow sheet synthesis has to be decided in different sub tasks. Generally these sub tasks are considered as ‘decisions at different levels’ during flow sheet synthesis. These different level decisions includes ‘Level-1 to Level-5’ decisions. After discussing these various level decisions, flow sheet can be considered as well synthesized. In general these level-1 to level-5 decisions are categorized as below:
Level-1 Decision: Batch v/s Continuous Process
Level-2 Decision: Input-Output Structure of Flow sheet
Level-3 Decision: Recycle Structure of Flow sheet
Level-4 Decision: Separation System for Flow sheet
Level-5 Decision: Heat Exchanger Network for Flow sheet
Before discussing these different level decisions, it is needed to input some necessary and important information to the process in order to take correct and proper decisions. This input information required is as below:
Input Information
The starting point for any project / design problem is a data bank which defines the basis of design. This information includes:
1. Reactions involved and their conditions
2. The desired production rate
3. The desired level of product purity or some information about price v/s purity
4. The raw materials required and knowledge about their price v/s purity
5. Information about reaction rate and catalyst deactivation rate
6. Any processing constraint
7. Data about other plant site
8. Physical properties of all components
9. Information concerning the safety, toxicity, and environmental impact of the materials involved in the process
10. Data revealing the costs of by-products equipment and utilities
The degree of detail and depth required from each of the above heads is now discussed below:
1. Reaction Information:
In general, several different reactions take place in any given process to be designed. For better design, one should be familiar with all the relevant parameters of these reactions such as
· The stoichiometry
· The range of industrially feasible temperature and pressure for carrying out the reactions
· The phase system of the reaction
· Information regarding the product distribution v/s conversion & if possible, the knowledge about preferred reactor temperature, molar ratio of reactants, reactor pressure
· Relationship between conversion and space velocity or residence time
· For a catalyzed reaction process, it is necessary to have knowledge about state of catalyst (like homogeneous, slurry, packed bed, power state etc.), deactivation rate, regenerability of catalyst and the method of regeneration (coke burn / solvent wash etc.)
While a majority of this information should be available from patent literature as well as from in-house data bank, it is also a good idea to involve the in-house R & D / Chemists for completing the above base.
2. Side Reactions:
All possible side reactions that occur and lead to the formation of by-products should be known. In case of a plant with a recycle loop, the by-products will build up to high levels unless a method for their continuous removal is devised. Armed with the complete knowledge of the possible reactions that occur and their consequences, it is a relatively easy task to synthesize a separation system and to avoid economic penalties as result of negligence to these side reactions.
3. Maximum Yield:
While designing any process, the focus is always on trying different catalysts to identify the most suitable one & establishing conditions which enables the process to operate at maximum product yield. A word of caution: the operation at maximum yield condition, isn’t always one corresponding to the optimum economic conversion.
During a reaction scheme which involves concurrent reactions the strategy of achieving the maximum amount of desired product may be accompanied by a significant production certain undesired products that may have no use other than as fuel. In such cases, a large amount of expensive reactant is consumed and converted into a considerably low value product (equivalent to fuel). Instead, in such a case, a better strategy could be to operate the process under such conditions as, which produce a considerable amount of desirable product (but not necessarily at its maximum yield) and considerably less amount of undesirable product. While the overall conversion may be less with consequent need to separate and recycle a larger portion of unconverted reactant with a certain price for this incremental recycle, the specific consumption of the expensive reactant is minimized.
At this time we can define certain terms which will be used in the discussion to follow: Selectivity & Selectivity loss as below:
Selectivity = (Moles of desired product produced) / (Moles of reactant converted)
Selectivity Loss = (Moles of undesired product formed) / (Moles of reactant converted)
Optimum economic conversion is fixed by an economic trade off between large selectivity losses & large reactor costs at high conversions on the one hand balanced against large recycle costs at low conversions on the other. In general, therefore,
(Optimum economic conversion) ≤ (Conversion corresponding to maximum yield)
Designer should try to estimate the economic incentive for determining the economic optimum conversion even if such an approach requires some additional experiments rather than just designing a process to operate at maximum yield.
4. Catalyst Deactivation:
This data is many times not available at the early stages of design. Also, to find the best catalyst and then its deactivation rate is a time consuming process as some catalysts have an operating life of 1-2 years before their deactivation begins requiring reactivation / replacement. Thus to achieve the highest potential profitability, designer should carry out sensitivity analysis of the total product cost to such uncertainty (catalyst deactivation rate), and further experimental development program should be prepared with help of these results.
5. Production Rate:
The rated design capacity of a plant depends upon the market conditions which continually change. Before embarking on the design exercise, it is thus necessary, to assess the target market share and corresponding business risk. Based on this risk and opportunity analysis the rated design production capacity can be ascertained and the project work can get underway.
The large production rates will require large size plant which in turn will require larger investment, difficulties / restrictions during transportation of these large size plant and equipments, higher risks during development of new technologies and larger management costs.
6. Product Purity:
The price of any product will change according to its purity. During early stages of development of a new process, high costs will be incurred for producing a high purity product, and this should be informed to the marketing department of the company so that it does not raise the customer expectations to the unrealistic levels.
7. Raw Materials:
Trace amount of impurities in raw material can build up to large values in recycle loop, unless some mechanism is built in to purge these regularly. The knowledge of commercially available quality of raw materials is thus vital to enable the designer not only to estimate the impurities brought in, their characteristics and effect on the reaction and / or final product of interest (inert, unacceptable due to detrimental effects, toxic, catalyst poison etc.) but also to incorporate an appropriately designed purification / purge system.
8. Constraints:
While designing the process the designer is required to consider the constraints like, the processing conditions operating within the explosive limits, materials polymerizing and fouling the plant equipments, materials forming the coke and hence deactivating the catalyst, materials causing the corrosion etc.
9. Other Plant & Site Data:
While erecting the new process at existing plant site, the design of the new process should be compatible with the existing facilities at plant site. For this, costs of utilities such as fuel supply, levels of steam pressure, inlet and outlet temperatures of cooling water, refrigeration levels, electric power etc. as well as waste disposal facilities should be available.
10. Physical Property Data:
The conceptual designs are aimed to produce the new materials. In such design exercise, physico-chemical data such as molecular weights, boiling points, vapor pressures, heat capacities, heats of vaporization, heats of reactions, liquid densities, fugacity coefficients should be collected as these are sensitive to the total processing costs.
11. Cost Data:
Capital costs of the pieces of equipments should be gathered.
This is the input information that must be provided during initial stage of project development.
Now let’s discuss various level decisions.
Level-1 Decision: Batch v/s Continuous Process
Continuous Process: Every unit is operating 24 hrs / day, 7 days / week throughout the year before the plant is shut down for the maintenance purpose. They may be having very few batch units, else otherwise operate continuously with large production rates.
Batch Process: It is started and stopped frequently where, units are filled with material, perform the specified function for a specified time, then are shut down and drained before being cleaned for the next cycle to begin. However, a few units here may be continuous one such as when the products from batch process are stored for a while and then fed to train of distillation columns which operate continuously.
Guidelines for Selecting Batch Process:
1. Production Rates:
As a rule of thumb, continuous plants have capacity of greater than 50000 TPA while it is less than 5000 TPA for batch process. Hence continuous plants operating at high capacity require more accurate data base and incur higher design engineering costs. The batch processes are simpler and being flexible, a variety of products can be produced in the same processing equipment.
2. Market Forces:
Batch processes are suitable to produce the seasonal products, due to which the same equipment can be further used to produce another product in next season which is economical one rather than to use a continuous process for producing the seasonal product which incurs large inventory cost in storing the product.
3. Operational Problems:
Generally, batch processes are more suited to slow reactions. Also for processes involving material which tend to foul equipment needing frequent clean-ups, batch processes may be in order since the plant is idle after every batch is drained and thus cleaning down time can be factored in easily.
4. Multiple Operations in a Single Vessel:
It is often possible to accomplish the several operations in a single batch vessel, while an individual vessel is needed for each single operation in continuous plant. Also single large vessel is required when multiple operations are carried out in single vessel, with this we can obtain the economy of scale, while separate vessels are required when individual vessel is used for an individual operation such as in continuous process.
Diagrammatically, this difference is shown below by taking one example.
Continuous Process
Batch Process
Therefore in brief batch processes are selected if:
Production Rate Basis
· Sometimes batch if less than 50,000 TPA
· Usually batch if production rate is 5000 TPA or less
· Batch if more than one product is planned (Multiple plants)
Market Forces Basis
· Product is seasonal
· Products having short life span
Scale-up / Operational basis
· Very long reaction times (very slow processes)
· Handling slurries at low flow rates
· Rapidly fouling materials
Design Steps for Batch and Continuous Process:
For developing a conceptual design for a continuous process, following steps should be followed:
· Selecting the process units;
· Choosing the interconnections among these units;
· Identifying the process alternatives;
· Listing the dominant design variables;
· Estimating the optimum processing conditions;
· Determining the best process alternative;
For batch process, in addition to these steps, decisions should be taken for:
(i) Which units in the process should be batch and which should be continuous?
(ii) What processing steps should be carried out in a single vessel versus using an individual vessel for each processing step?
(iii) When it is advantageous to use the parallel batch units to improve the scheduling of the plant?
(iv) How much intermediate storage is required, and where should it be located?
Level-2 Decision: Input-Output Structure of Flow Sheet
Considering the overall process, the input is nothing but the raw material fed to process and output is the product obtained from the process. Before adding other details to design, it is necessary to calculate the raw material cost as it amounts to anywhere between 83 to 85% of the total processing cost.
While developing the flow sheet, the thumb rule should be followed that, it is desirable to recover more than 99% of all valuable materials. Basically, two different alternatives for flow sheet always exist: One in which all reactants are first completely recovered and then they are recycled, while in the other, where reactants are consumed by side reactions due to presence of impurities while process occurs, the reactants are recycled meanwhile and inerts are purged out. These two flow sheet alternatives are as shown below: