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 ProcessLevel-2 Decision: Input-Output Structure of Flow sheetLevel-3 Decision: Recycle Structure of Flow sheetLevel-4 Decision: Separation System for Flow sheetLevel-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 conditions2. The desired production rate3. The desired level of product purity or some information about price v/s purity4. The raw materials required and knowledge about their price v/s purity5. Information about reaction rate and catalyst deactivation rate6. Any processing constraint7. Data about other plant site8. Physical properties of all components9. Information concerning the safety, toxicity, and environmental impact of the

materials involved in the process10. 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:

Heat Reactor SeparatorFeed

Catalyst

Product

Heat

Reactor

Separator

Stil

l

Feed

Catalyst

Product

Heat

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:

Once it is decided that process should be batch wise or continuous, the next level decision is to decide the input-output structure of the flow sheet. At this level of decision making, designer should consider the points such as-

Need to purify the feed streams before their entry in the process Removing or recycling the reversible by-product Using a gas recycle and purge stream Recovery and recycling of some reactants Predicting the possible number of product streams The design variables for this structure and economic trade-offs associated with

these variables.These points are briefly discussed below:

Purification of feeds:

While synthesizing the flow sheet, proper purification system must concurrently be designed. For designing this purification system, certain guidelines should be followed:

i) Feed impurity – if not inert and present in significant amount, should be removed to reduce raw material losses and eliminate avoidable by-products;

ii) Feed impurity – if present in gas feed, process the impurity;iii) Feed impurity in liquid stream is also a by-product or product component –

feed should be processed through the separation system;iv) Feed impurity – if present in large amount, should be removed;v) Feed impurity – if present as a azeotrope with a reactant, process the impurity;

Process

Feed

Streams

Product

By-products

Process

Feed

Streams

Product

By-products

Purge

vi) Feed impurity – if inert but easier to separate from the product than the feed, process the impurity;

vii) Feed impurity – if it is a catalyst poison, must be removed.

Process alternatives:

Whatever decision we take, if we are not sure about them, then opposite decisions should be considered, all such opposite decisions can be termed as process alternatives.

Economic Trade-Offs for Feed Purification:

Cost of the process increases when feed containing large amount of inert or active impurities is being handled as it requires design of a proper separation system whose design criterion is not fixed.

Recover or Recycle Reversible By-products:

If reversible by-product is to be recycled, all the equipments in that recycle loop should be oversized in order to accommodate the equilibrium flow of the reversible by-product. And if that by-product is removed from the process, economic penalty is required to be paid as result of the increased raw material cost of reactant which was converted to the reversible by-product.e.g.

Toluene + H2 Benzene + CH4

(Reactant) (Product)

2 Benzene Diphenyl + H2 (By-product)

So here, the decision involves economic trade-off between raw material losses to less valuable by-products and increased recycle costs.

Gas Recycle & Purge:

It should be used when a light reactant and a light feed impurity or a light by-product boil lower than propylene (- 48oC). This is because the lower-boiling point components normally can’t be condensed at high pressure with cooling water i.e. both the high pressure and refrigeration will be required. So in such cases, as gaseous reactants are less expensive than liquid reactants, it is cheaper to lose some of the gaseous reactants from a gas recycle and purge stream than to recover and recycle the reactant.

Do Not Recover and Recycle Some Reactants:

As per the design guidelines, we have to recover 99% of all valuable materials. But certain materials like, air and water, which are less valuable than organic materials, need not to be recovered and recycled. Usually, these components are used in excess so as to achieve complete conversion of other reactant. But here also, one thing should be kept in mind that, using excess amount of air means increasing the operating cost of blower used to move the air. So the extent of excess used needs to be optimized.

Number of Product Streams:

The number of product stream depends upon the components that are expected to leave the reactor, these components include the components in feed stream, reactants and products appearing in every reaction. Once these are known, certain destination codes are given to them as per their properties. This procedure can determine the number of products stream. Certain list of destination codes and component classification is as below:

Destination Code Component Classification1. Vent Gaseous by-products and feed impurities2. Recycle & purge Gaseous reactants plus inert and / or gaseous by-products3. Recycle Reactants; Reaction intermediates; Azeotrope with reactants;

Reversible by-products4. None Reactants – if complete conversion or unstable reaction

intermediate5. Excess Vent Gaseous reactant not recovered and not removed6. Excess Waste Liquid reactant not recovered and recycled7. Primary Product Primary product8. Valuable by-

productSeparate destination for different by-products

9. Fuel By-products to fuel10. Waste By-products to waste treatment

Level-3 Recycle Structure of the Flow Sheet

The process should be batch or continuous, what should be input-output structure of the flow sheet, these points were decided at level-1 and level-2 decision. Now next step in decision making is to decide the structure of recycle stream.

While taking the decision, following points should be evaluated:

1. Number of reactor systems required and need for separation between these reactor systems

2. Number of recycle streams required3. Need for using an excess of one reactant at the reactor inlet

4. Need for gas compressor and its cost5. Operation method for reactor i.e., should it be adiabatic operation, or with direct

heating and cooling, or it is a diluent or heat carrier type6. Need for shifting the equilibrium conversion and if it is then how7. Effect of the reactor cost on the economic potential

These points are discussed individually below:

(1) Number of Reactor system:

Multiple reactors are required for reactions taking place at different T & P, or involving different catalysts.

e.g. During HAD process,

Toluene + H2 Benzene + CH4

(Reactant) (Product)

2 Benzene Diphenyl + H2 (By-product)

these both reactions occurs at 1150 – 1300oF and 500 psi., i.e. at same T & P condition, hence only one reactor system is required while in the following reaction system,

Acetone Ketene + CH4

Ketene CO + ½ C2H4

Ketene + Acetic acid Acetic anhydrideIn this second process system, first two reactions occur at 700oC & 1 atm, while third occurs at 80oC and 1 atm, i.e. at different T & P conditions, hence here in this case, 2 reactor systems are required.

(2) Number of Recycle Streams:

For this reaction steps should be associated with the reactor number, then feed streams with the reactor number, and similarly recycle streams can be associated with reactor number. Also difference should be shown between liquid and gas recycle stream as gas recycle stream will require costly compressors while for liquid recycle stream, pumps are used which are comparatively cheaper one. So list of all the components leaving the reactor and list of the reactor number as its destination code should be prepared and then accordingly number of the recycle stream associated with leaving component and destination reactor should be decided.For example, consider following system where two reactor systems and two recycle streams are involved.

(3) Excess Reactants:

Use of an excess reactant can shift the product distribution. e.g. production of isobutene by butane alkylation.

Butene + Isobutane Isooctane

Butene + Isooctane C12

In this reaction, excess of isobutene can improve the selectivity of isooctane, but this excess should be optimum one so that, cost to recover and recycle the product does not outweigh the benefit of improved conversion to isooctane.

Also use of excess can be used to shift the equilibrium conversion. This equilibrium conversion can be shifted to the right side by using excess of reactant at reactor inlet. Molar ratio of reactants at the reactor inlet is often s design variable. By carrying out an economic analysis in terms of this variable alone, it is possible to arrive at the economically optimum value of this design variable.

(4) Heat Effects & Equilibrium Limitations:

For evaluation of heat effects of reactor and equilibrium calculations, reactor flows should be known.

Recycle Material balance:

Limiting Reactant Balances:

Consider the flow sheet for HDA process-

Reactor R1 Reactor R2 Separator

Acetone Feed

Acid Feed

Acetone Recycle

Acid Recycle

Product

Limiting Reactant : Toluene

FT = Flow of Toluene entering the reactor; x = conversion of toluene;FT (1-x) = the amount of toluene leaving the reactor; FFT = Fresh feed of toluene;

Hence, material balance for limiting reactant at a mixing point before reactor is:

FFT + FT (1-x) = FT

which leads to the expression for feed to reactor, FT as given by,

FT = FFT / x

Other Reactant:

For this process, other reactant is H2. Its material balance can be calculated as-

MR = molar ratio of hydrogen to toluene at reactor inlet; Fresh feed hydrogen = yFH FG;Recycle Hydrogen = yPH RG ;

Hence,yFH FG + yPH RG = MR (FFT / x)

From this recycle stream can be calculated as

RG = [PB / (Sx yPH)] {(MR / x) – [yPH / (yFH - yPH)]}

Design Heuristics:

Reactor Separator

H2 Feed

Toluene Feed

FG

FFTFT

FT (1-x)

FT (1-x)

RG yPH

Purge

Diphenyl

Benzene, PB

No rule of thumb is available for selecting the conversion, x or purge composition yPH or molar ratio, MR for complex reactions. However for single reaction, x can be considered as equal to 0.96 or 0.98xeq.

Reversible By-products:

Flow of by-product being recycled during a reversible reaction can be calculated using equilibrium expression. For e.g., for HDA process, where Diphenyl is a by-product produced by a reversible reaction:

2 Benzene Diphenyl + H2 (By-product)

Keq = [ (Diphenyl) (H2) ] / (Benzene)2

Reactor Heat Effects:

The next decision to be taken is about operating condition of the reactor i.e. to be operated adiabatically, or by direct heating or cooling, or using heat carrier. Depending upon this only, further separation system of process can be designed. So for this, estimation of reactor heat load and adiabatic temperature change is needed.

Reactor Heat Load:

For a single reaction, generally all fresh feed gets converted. So here, heat load can be calculated as

Reactor heat load = Heat of Reaction × Fresh Feed Rate

However, for complex reactions, extent of each reaction depends upon the design variables as like conversion, molar ratio of reactants, T & P etc. Hence in this case heat load is calculated as a function of these design variables.

Adiabatic Temperature Change:

This can be calculated by knowing heat load and flow rate through reactor as-

QR = F Cp (TR,in - TR,out)

Heuristic for Heat Load:

When adiabatic operation is not possible, direct heating & cooling technique is to be used. But also there is limitation for heat transfer surface area that can fit into a reactor. What should be the magnitude of this area, for this, case of high temperature gas-phase reaction is considered where heat transfer coefficient and temperature difference are

assumed as U = 20 Btu/(hr.ft2.oF) and ΔT = 50oF, and hence area of heat load of 1×106

Btu/hr will be

A = QR / (U ΔT) = 1000 ft2

The maximum heat transfer area that can fit in heat exchanger is about 6000 to 8000 ft 2. Hence, when direct heating or cooling method is used, and a single heat exchanger is to be used, the reactor heat load is limited to 6 to 8×106 Btu/hr.

Heat Carrier:

The adiabatic temperature change depends upon the flow through the reactor, and by increasing the flow rate, temperature change can be moderated. So for moderating the temperature change, recycle flow of reactant or product or by-product should be increased, and if it is not possible, extraneous component is need to be added. In some reactions, certain by-products are produced which act as heat carrier, and if such component are allowed to leave the reactor, exit temperature of the reactor will be increased & hence adiabatic reactor could not be used longer. To avoid this, recycle flow should be increased. E.g., in HDA process, methane in gas-recycle stream acts as heat carrier.

Equilibrium Limitations:

For industrial processes, equilibrium limitations are important. Hence the flows of reversible by-products are estimated when they are recycled and allowed to build up to their equilibrium levels at reactor outlet.

The process flows are first calculated as a function of design variables and then they are put into equilibrium relationship to check whether the conversion that was selected earlier is above or below the equilibrium value, if it is above then, result is of no meaning.

Level-4 Separation System for the Flow Sheet

In this level decision of flow sheet synthesis, we have to decide a separation system for the process in order to recover gaseous and liquid components.

(A)General structure of the Separation System :

To determine a general structure of the separation system, phase of the reactor effluent should be determined first. So far for vapor-liquid process, there are 3 different possibilities, which are as discussed below:

(i) If reactor effluent is a liquid, we assume that we only need a liquid separation system. This may include, distillation column, extraction unit, azeotropic distillation, etc. but in this case, normally there will not be any gas absorber or gas adsorption unit as there is no any vapor/gaseous phase present.

(ii) If the reactor effluent is a two-phase mixture, we can use the reactor as a phase splitter (or put a flash column after the reactor). Then further we send the liquid phase to liquid separation system. If the reactor is operating above cooling water temperature, we usually cool the reactor vapor stream to 100oF and phase split this stream. If the low-temperature flash liquid that we obtain, contains mostly reactants (and no product components that are formed as intermediates in a consecutive reaction scheme), then we recycle them to the reactor. However, if the low temperature flash liquid contains mostly products, then we send this stream to the liquid recovery system. The low temperature flash vapor is usually sent to a vapor recovery system. But if the reactor effluent stream contains only a small amount of vapor, we often send the reactor effluent directly to a liquid separation system (for e.g. to train of distillation columns).

(iii) If the reactor effluent stream contains all vapor, then we cool the stream to 100oF (cooling water temperature) and we attempt to achieve a phase split or to completely condense this stream. The condensed liquid is sent to a liquid recovery system, and the vapor is sent to a vapor recovery system.

These 3 alternatives can be shown diagrammatically as below:

*Fig. 1: Alternative 1: Reactor Effluent is Liquid*

Reactor System

Liquid Separation System

Feeds

Liquid Recycle

LiquidProducts

Gas Recycle

Purge

*Fig. 1: Alternative 2: Reactor Effluent is Vapor and Liquid Mixture*

*Fig. 3: Alternative 3: Reactor Effluent is Vapor*

If a phase split is not obtained, we see whether we can pressurize the reactor system so that a phase split will be obtained. If a phase split is still not obtained, then we consider the possibility of using both, high pressure and refrigeration system.

Liquid Separation System

Reactor System

Feeds

Liquid Recycle

Vapor

Products

Phase Split

Vapor Recovery System

VaporLiquid

Liquid

Reactor System

Feeds

Phase Split

Liquid Separation System

Vapor Recovery System

Gas RecyclePurge

Liquid RecycleProducts

Vapor

Liquid

(B) Vapor Recovery System (VRS) :

While synthesizing a vapor recovery system, two decisions are needed to be taken:

(i) What is the best location for VRS?(ii) What type of VRS is cheapest one?

Location of VRS: For this there are 4 choices-(i) VRS on the Purge stream(ii) VRS on Gas recycle stream(iii) VRS on Flash vapor stream(iv) None (No VRS)

The thumb rules for selecting out of these 4 choices are:

(i) Place the VRS on purge stream if significant amounts of valuable materials are being lost in the purge. The reason for this heuristic is that, the purge stream normally has the smallest flow rate.

(ii) Place the VRS on the gas recycle stream if materials that are deleterious to the reactor operation (catalyst poison etc.) are present in this stream or if recycling of some components degrades the product distribution. The gas recycle stream normally has the second smallest flow rate.

(iii) Place the VRS on the flash vapor stream if both case 1 and 2 are valid i.e. flow rate is higher, but we accomplish two objectives.

(iv) Do not use a VRS, if neither case 1 nor case 2 are important.

Type of VRS:

The most common choices for VRS are-(i) Absorption(ii) Adsorption(iii) Membrane separation(iv) Condensation-high pressure or low pressure or both

(C) Liquid recovery System (LRS) :

The decision where we need to decide the synthesizing system for liquid separation, includes:

(i) How should light ends be removed if they might contaminate the product?(ii) What should be the destination of the light ends?(iii) Do we recycle components that form azeotrope with the reactants or do we

split the azeotrope?

(iv) Which separations can be made by distillation?(v) What sequence of distillation columns should we use?(vi) How should we accomplish separations if distillation is not possible?

Light Ends:

Some light ends will be dissolved in the liquid leaving the phase splitter and normally some will be dissolved in liquid streams leaving the VRS. They must be removed if they are contaminating the product. They can be removed by using either of the following methods:

(a) Drop the pressure or increase the temperature of a stream and remove the light ends in a phase splitter

(b) Use a partial condenser on product stream(c) Use a pasteurization section on product stream(d) Use a stabilizer column before the product column

Azeotropes with reactants:

If component forms an azeotrope with a reactant, we have the choice of recycling the azeotrope or splitting the azeotrope and just recycle the reactant. Splitting the azeotrope normally requires two columns and therefore it is expensive. However, if we recycle azeotrope, we must oversize all the equipments in recycle loop to handle incremental flow of extra components. No general heuristic is available to decide between these two, hence both alternatives need to be evaluated on economical basis.

Applicability of Distillation:

It is least expensive mean of separating mixture of liquids. However, if relative volatilities of 2 components with neighboring boiling points is less than 1.1, then distillation becomes very expensive, i.e. large reflux ratio is required which corresponds to large vapor rate, large column diameter, large condenser and reboiler duties, and large steam and cooling water costs, etc. Therefore when such situation arises, then group these components together and treat this group as a single component in mixture.Use proper sequencing of distillation columns i.e. first split lightest or heaviest component and then split remaining.

Heuristics for column sequencing are:(i) Most plentiful component first(ii) Lightest component first(iii) High recovery separation last(iv) Difficult separations lat(v) Favor equimolar splits(vi) Next separation should be cheapest

Alternative to Distillation:

(a) Extraction(b) Extractive Distillation(c) Azeotropic Distillation(d) Reactive Distillation(e) Adsorption(f) Crystallization(g) Reaction

So these are the various points that we have to taken into consideration for synthesizing the separation system for the flow sheet.

Level-5 Energy Integration in Flow Sheet (To design Heat Exchanger Network for flow sheet)

This is the final level decision to be taken during flow sheet synthesis. Here, we need to identify the sources of waste heat available in process. These heat sources may be like exothermic chemical reactions, distillation columns, boilers, condensers, hot waste water, evaporator, etc. similarly, identify the process steps or unit operations, equipments, where heat is required or to be removed., such as heat exchangers, reactors, distillation columns, etc. Then heat exchanger network (HEN) should be designed in such a way that the waste heat can be utilized for the operations which requires the heat. In this way, we can reduce the utility load and the cost of utilities/fuel can be reduced to some extent.

This is the way to synthesize the whole flow sheet for a particular process.

READCTOR TRAINS: OPTIONS AND SELECTION CRITERIA

Introduction:

The carrying out chemical reactions to achieve the transformation which are desired, forms a large part of chemical engineer’s core skills. Choosing the right type of reactor for converting the maximum of the input reactants into desired products, minimizing the by-products/ wastes during reactions, deciding upon a reactor geometry which leads to minimum effluent and assure minimum environmental footprint, and finally, minimizing the energy required for this transformation are all considerations whioch go into this decision making process. A correctly selected reactor type / geometry indirectly translates into additional benefits further downstream and vice versa:

If a reactor scheme selected minimizes the generation of by-products, less effort needs to be spent in separating the product of interest from the by-products or “impurities”.

A proper choice of reactor type and conversion may save a significant cost of loss of expensive raw materials getting converted into non-remunerative and / or

expensive-to-separate by-products while paying a small price in terms of recycle of the unconverted input reactants.

A proper design of reactor would also have a positive impact on the energy requirement for a specific chemical transformation. The multi-pass catalytic reactor in a typical sulphuric acid plant as also the ammonia converter reactors are good examples of these. While in the sulphuric acid plant, the multi bed adiabatic staged conversions enables good kinetics without excessive comprises on equilibrium conversion in each stage, the very multi-bed geometry also allows profitable heat extraction between passes.

In case of the ammonia converter, the incoming gas stream flows down the annular space between the reactor shell and the catalyst bed assembly thereby simultaneouslya. Protecting the reactor shell from seeing the higher temperature at which the

reaction actually takes placeb. Using the exothermic heat of reaction to heat the incoming reactants to the

desired initial reaction temperaturec. Cooling the reaction mass to promote shift of the reversible reaction to the

right

Certain reaction conditions require the chemical engineer beyond the narrow confines of the known models of reactors to specifically design reactor geometries which are best suited for the peculiar reaction / energy considerations.

The present topic will cover the broad choice of reactor geometries, some of the heuristic considerations which go into the selection of type one or the other followed by touching upon some of the more exotic reactor set-ups used in industry today.

Reactor Geometries: Options

Most chemical engineering students would, at this point in their academics, be familiar with the basic two models: continuous stirred tank reactor (CSTR) and the plug flow reactor (PFR).

(A)Continuous Stirred Tank Reactors (CSTR)

A CSTR is best described in the conventional sense, by a mechanically stirred tank heated or cooled as required, in a time invariant steady state, wherein one or several inlet stream(s) of reactants of constant rate and composition enter, get instantly mixed with the contents of the reactor, and with no accumulation, leave as a constant stream of the exhausted reactants with composition identical to that in the reactor itself. This would typically be a batch reactor also if the inlet and outlet streams were absent and the reaction in the tank proceeded as a function of the residence or batch time.

Critical parameters to be noted here:

1. Rate of reaction: a function of outlet composition thus the lowest concentration of the reactants. Normally best for fast main reactions and slow or equilibrium type of side reactions. Typical unit processes such as sulphonation, nitration, etc.

2. Mixing as limited by the mechanical mixing power and agitator system design. Here, the recently introduced ultrasound mixing (ultrasonication) can provide extensive micro-mixing and cavitations.

3. Heat transfer area limited to wall and a portion of bottom but the heat sink is the entire contents of the reactor so better temperature control. Other variations of this basic theme include

Using reaction medium / solvent which boils at the desired reaction temperature, vaporizes, is condensed in an overheated condenser and returned to the reactor thereby providing additional evaporative cooling

A side arm heat exchanger which theoretically enlarges the heat transfer area limitlessly (along with the inventory, unfortunately)

Use of a cascade of reactors with all or some of the feed streams split and fed to the successive members of the cascade

Some of the limitations which are inherent can be substantially reduced by having a cascade of CSTRs with one, several or all the input reactant streams split and progressively added to successive reactors of the cascade. As can be easily seen, this arrangement gives a larger residence time, larger surface area for heat transfer, a smaller heat transfer need per cascade member (extraction or supply) and somewhat better rates of reactions for individual cascade member as compared to a single reactor. This arrangement is a discrete approximation of a conceptual plug flow reactor.

Under industrial conditions the concept of a CSTR is practically realized through the following geometries:

(1) Conventional Mechanically Agitated Reactor

A cylindrical or other suitably shaped vessel with or without jacket / cooling or heating coil for heating / cooling, complete with an agitator driven by a motor or other power source and with adequate number of nozzles for inlet and outlet streams, instruments, cooling utility if any etc.

(2) Bubble Column Reactor

Bubble column reactor in which a reactant in the gas phase rises after being sparged from the bottom of a tower which is filled with a liquid with a bulk downward flow, is an excellent approximation to a CSTR. Additional mechanical agitation or use of column packing bring it closer to a true CSTR in performance.

(3) Fluidized Bed Reactors

This type of reactors with reaction between the fluidizing medium and the fluidized solids also can be approximated as a CSTR. One of the most successful practical application of this idea is the ORTHO Reactor for FCC of Petroleum.

(4) Loop Reactors

Loop reactors consist of a closed loop with a re-circulating device which continuously recycles the reacted material. The fresh stream of reactants is introduced into this stream and an equivalent amount of the reacted products leave the closed loop thus maintaining the constant loop inventory.

A heat exchanger may be added into loop to add or remove heat. The relative ratio of recycle to the fresh ingredients decides the temperature rise. Additional mixing apart from the re-circulating device may be provided by inline static mixers.

(B) Plug Flow Reactors (PFR):

In industrial practice a plug flow reactor is most commonly encountered in heterogeneously catalyzed gas phase reactions wherein the reactant gas flows through a packed bed of catalyst particles. With sufficiently high NRe, the velocity profile across the bed can be assumed to be substantially flat and thus the approximation to a PFR.

A more common variation of this basic theme is a packed bed multi-tube type reactor which allows heat transfer possibilities through the tube walls. This is nothing but a shell and tube heat exchanger with tubes packed with catalyst particles, the shell side carrying heating/cooling fluid as required by the reaction.

Alternatively, side heat exchangers are employed if adequate heat transfer is not possible through walls. The multi-bed catalytic converter of the DCDA Sulphuric acid manufacturing plant is a good example with waste heat boilers / economizers / superheaters / interpass heat exchangers removing heat after the reactants, SO2

and oxygen in this case, have reacted adiabatically in the catalyst bed and a certain conversion has been achieved.

Falling film type of gas-liquid contacting reactor encountered in SO3 based sulphonation plants is another example of use of a PFR for an irreversible, exothermic, and very fast reaction. This type of sulphonation can also be carried out in a CSTR cascade.

Application of Different Reactor Geometries and Associated Heuristics:

1. Single irreversible reactions (not autocatalytic) (A) Isothermal – always use a plug flow reactor(B) Adiabatic –

(i) use plug flow if the reaction rate monotonically decreases with conversion(ii) CSTR operating at the maximum reaction rate followed by a plug flow section

2. Single reversible reactions – adiabatic

(A)Maximum temperature if endothermic(B) A series of adiabatic beds with a decreasing temperature profile if reaction is

exothermic

3. Parallel reactions – composition effects

(A)For the reaction scheme

A R (desired)A S (waste)

where the ratio of the reaction rates is rR / rS = (k1 / k2) CAa1-a2

(i) if a1 > a2, keep CA high. Also,

a. Use batch or plug flowb. High pressure, eliminate inertsc. Avoid recycle of the productsd. Can use a small reactor

(ii) if a1 < a2, keep CA low. Also,

a. Use a CSTR with a high conversionb. Large recycle of productsc. Low pressure, add inertsd. Need a large reactor

(B) For the reaction scheme

A + B R (desired)A + B S (waste)

where the ratio of the reaction rates is rR / rS = (k1 / k2) CAa1-a2 CB

b1-b2

(i) if a1 > a2 and b1 > b2, then both CA and CB high(ii) if a1 < a2 and b1 > b2, then CA low and CB high(iii) if a1 > a2 and b1 < b2, then CA high and CB low(iv) if a1 < a2 and b1 < b2, then both CA and CB low

4. Consecutive reactions – composition effects

For the reaction scheme

A R (desired) S (waste)

Minimize the mixing of streams with different compositions. Consider the option of removing “R” from partially reacted stream before attempting further conversion.

5. Parallel reactions – temperature effects

A R (desired)A S (waste)

rR / rS = (k1 / k2) f (CA CB)

(A) If E1 > E2, use a high temperature(B) If E1 < E2, use an increasing temperature profile

6. Consecutive reactions – temperature effects

A R (desired) S (waste)

Having reaction rate constants k1 and k2 respectively,

(A) If E1 > E2, use a decreasing temperature profile-not very sensitive(B) If E1 < E2, use a low temperature

SUMARIZED DESIGN HEURISTICS

1. Any feed impurity, if inert, should be removed.2. Impurity if present in gas feed stream, as a first guess process the impurity.3. Impurity in liquid feed stream, if it is a product or by-product, then feed the

process through separation system.4. Impurities present in large amount, should be removed.5. Impurity if present as an azeotrope with a reactant, then, processes it.6. Feed impurity- if inert but easier to separate from the product than the feed,

process the impurity.7. Whenever there is a light reactant and a light feed impurity or by-product (where

light components boil lower than propylene, -48oC), use a gas recycle and purge stream for the first design. Also consider a membrane separator later.

8. If O2 from air or water is a reactant, then consider using an excess amount of this reactant.

9. For single product, vapor-liquid processes, we determine the number of product streams by grouping components with neighboring boiling points that have the same exit destinations; i.e. we never separate streams and then remix them.

10. We have to assure that all products, by-products and impurities leave the process.11. Design variables those affect the product distribution and purge composition of

gas stream, are considered as significant.

12. Raw material costs are often in the range from 33 to 85% of the total costs.13. If reactions are taking place at different temperatures and pressures and/or they

require different catalysts, then, for each operating condition, separate reactor system is required.

14. Components recycled to same reactor that have neighboring boiling points should be recycled in same stream.

15. A gas recycle compressor is required if the recycled component boils at temperature lower than that of propylene.

16. If an excess reactant is required, then there should be an optimum amount of reactant.

17. If the reactor temperature, pressure and/or molar ratio are changed to shift an equilibrium conversion, then there should be an optimum values of these variables.

18. For an endothermic processes with a heat load of less than 6 to 8×106 Btu/hr, then isothermal reactor with direct heating is to be used. For larger heat loads, diluents or heat carrier can be added.

19. For exothermic reactions, an adiabatic reactor is to be used if the adiabatic temperature rise is less than 10 to 15% of the inlet temperature and if the adiabatic temperature rise exceeds this value, then direct cooling is used if reactor load is less than 6 to 8×106 Btu/hr. otherwise, a diluents or heat carrier should be introduced.

20. For a single reaction, conversion of 0.96 to 0.98 of the equilibrium conversion is to be selected.

21. The most expensive reactant is usually the limiting reactant.22. Reversible by-product should be recycled if its equilibrium constant is small.23. The recycle flow of the limiting reactant is given by F = FR ( 1 – x) / x, where FR

is the amount of limiting reactant needed for the reaction and x is conversion.24. The recycle flow of other component can be determined by specifying the molar

ratios at reactor inlet. ************************************************************************