mini project handout sem i 2015-2016

Mini Project (Integrated Project Design)

BKF2453 Chemical Reaction Engineering I

1 Introduction

The mini project of reactor design in the first semester of 2015/2016 session requires submission of a report by a group of 4 or 5 of the students from each section. The respective lecturers would coordinate the project based on the flowchart (Appendix A) and to evaluate the students’ achievement by using the rubric (Appendix B).

The broad coverage of a chemical process synthesis, particularly at the reaction level (in this case, up to level 3), necessitates integration of the knowledge from other subjects such as thermodynamics of chemical engineering, chemistry, material and energy balance and engineering mathematics. Besides engineering and science, all decisions made are also to be optimized in the perspective of economic potential for the process. The basic concept of applying economic analysis would therefore be introduced in the subsequent descriptions as it is the judgment tool for the students to obtain practical decisions.

The students must be able to identify all variables involved in the process and to analyze accordingly their degree of freedom in order to know the key variable (bases of calculation) for their material balance calculation. The material balance and perhaps including energy balance are to be elaborated at each unit and junction of the process stream, either for mixing or splitting to ensure the feasibility of the process. The design of the reactor in the process is additionally estimated. Runge Kutta 4 th

degree is advisable to be used as the numerical method for calculations involving differential equations by using the Polymath software.

2 Description of Reactor Design for a Process Plant

As the most important unit in a process plant, reactors must be properly designed to obtain as high as possible the profit based on the desired product sales per year (a rate that is normally obtained from a market survey which would be given in this mini project) without neglecting their safety and environmental effect. In chemical engineering, the calculation of the profit cannot simply be made from the maximum conversion because of the multiplicity of involving reactions, nonlinearity of reaction behavior due to conditions and catalyst difference besides resulted reactor volume from calculation which normally follows exponential increment alongside the conversion.

In reality, undesired side reactions usually consume the valuable reaction components. The expensive reactants would be probably wasted for unnecessary products though their conversions are collectively high. In some cases it may consume the main product too. Figure 6-2 and Figure E6-41 illustrate the effect of the component concentrations at various space times of a pack bed

reactor, an example taken from Chapter 6 of the Fogler’s chemical engineering book. This phenomenon would create optimum conversion at far lower than unity. In the view of a chemical process, the feasibility of a process can be seen when the capital investment and operational cost of units and cost of materials are deducted from the revenue. Thus, the maximum profitability would be no longer at high conversion but at a tradeoff point.

In the chemical reaction engineering subject, the students are taught to design reactors for high productions. If some reactions may appear as a single reaction with high yield and high conversion, this does not merely mean they can have unrealistic size reactors as illustrated in Figure(a) and (b) for a maximum throughput. The design must also consider the cost as almost all expenditures increase proportionally with key design variables as exemplified in Figure(c).

2Conceptual Design of Reactor for a Chemical Process – Mohd Sabri Mahmud

A B

A C

(a) 100 m3 CSTR

(b) Three 378 m3 CSTRs in series

(c)

Figure 1: Comparison between (a) typical industrial reactor and (b) gigantic reactors illustrated from the reactor size probably designed for conversion, X close to 1. (c) Cost and installation time of jacketed and stirred reactors (Peters & Timmberhaus, 1991b)without considering the cost of

agitation and other accessories

Douglas (1988) and Biegler (1997) stated 5 hierarchical steps of making decision for a process synthesis by integrating the material and energy balance with the cost of operational utility and equipment. Decisions of a successful plant are highly relying on the potential of economics at various levels of process decision except level 1 of which the mode of operation will be decided based on the capacity of the process and nature of the reaction involved. Not just conventional calculation of unit operations, all the decisions including the ones which were made based on intuition and experiences of plant managers before can actually be made through these economic potential analyses. Details of heuristic designs and guidelines can further be obtained from Seider et al. (2002)and other plant synthesis textbooks.

As the heart of a process plant, reactive unit material balance and reactor design is conducted from level 1 until level 3. Economic potential (EP) calculation starts at level 2 where the material balance begins to be considered and the EP calculation must be conducted until level 5. If the result of each economic potential of a level (from 2 to 4) shows feasibility (EP>0), the plant synthesis can then proceed to the next level of decision and again the next EP would be obtained. Otherwise, other


possible alternatives must be considered to replace the current set of process architecture and then the process synthesis is started back from the beginning.

Level 1: Mode of Operation

The process would be run either in batch, continuous or semi-batch mode as a whole or in modules, is subject to the production capacity, difficulty of the reaction and variability of raw materials and products. The heuristics given by Turton (2008) stated that the rate greater than 5,000 ton per annum is worth for continuous mode. If reactions involve very careful monitoring and control, highly sensitive biocatalyst and too many raw materials and seasonal-demanding products, the mode of choice is normally the batch one.

It is optional to calculate the profit margin by using typical value of yield and conversion or as an ideal system with 100% yield, 100% conversion and 100% separation efficiency in order to ensure the least profitability of the synthesis route. Simple material balances along with their costs or prices would be computed in order to see whether the selected globally-available feedstocks can give profit to the process or not.

Level 2: Input-Output Structure

At this stage, the input-output structure of a chemical process would be drawn as a simple block flow diagram as in Figure 2. The input consists of the desired feedstocks that must be managed. Raw materials are available in the global market with specific grade and purity. Usually, the technical grade one contains some other outlet components. If the feedstocks contain impurities which are problematic to the process, they must then be separated in a pretreatment process that would incur cost in the economic potential. Even, if they are inert but their amounts are more than 40%, separation is also chosen as it is the rule of thumb. Heuristics of impurity management includes that if the impurity is (Douglas, 1988):-

1. inert and present in small amount, do not separate it2. difficult to separate, do not separate it.3. able to foul and poison catalyst, separate it4. reactant to form difficult-to-separate material or hazardous product, separate it.5. required if it can stabilize problematic products6. required if it can enable separation or minimizing side reactions7. required if it can control exothermic reactions8. required if it can control equilibrium

Apart from the impurities, the destinations of components which exist in the reactor outlet must also be decided and can be two possibilities as shown in Figure 2(a) and (b). The stream line will illustrate the individuality of the components or combination.


Figure 2: Block Flow Diagram at Level 2: Input-Output Structure

If a stream is for a component that means the purity of the component is 100% and the decisions, recycle, recycle and purge, venting, dispose as waste, burned or sale, are made at this level based on heuristics and market demand. The destinations are decided as follows:-

Table 1: Destination Codes of Component of the Output (Douglas, 1988)

Destination Code Component ClassificationVent Gaseous by-products and feed impuritiesRecycle and Purge Gaseous reactants plus inert gases and/or gaseous by-productsRecycle Reactants

Reaction intermediates Azeotropes with reactants (sometimes) Reversible by-products (sometimes)

None Reactants – if complete conversion or stable reaction intermediateExcess-vent Gaseous reactant not recovered or recycledExcess-waste Liquid reactant not recovered or recycledPrimary product Primary productValuable by-product Separate destination for different by-productsFuel By-products to fuelWaste By-products to waste treatment

After the structure of input-output is finalized, the variables appears from the material balance of the structure must be computed under the basis of the design variable (if any) as a result of preliminary discrimination using the degree of freedom analysis. Usually, conversion is the design variable for the continuous operation and number of reactor cycle (and schedule) for the batch operation. Other variables are depending on the decisions made thereof.

The economic potential of the process can be calculated as follows

EP2 (RM/year) = Revenue – Cost of Feedstocks – cost of feedstock pretreatment (if any) (1)

Where Revenue = sales of product + energy tariff exported from waste/side product incineration (if any)


Cost of Feedstocks = flow rates x cost of respective materials, where the flow rates can be as the algebraic function of conversion and/or other variables.

Level 3 Recycle Structure

At this level, the recycle streams would be decided and the reactor parts would be detailed as shown in Figure 3. If there is a need to use a compressor for the recycle stream, the cost of the unit installation and operation must be considered because it is classified as an expensive mass transfer unit.

Figure 3: Example of Recycle Structure of Flowsheet (Compressor is normally for high pressure gaseous reactor).

The specification method of system and condition of the reactors can be referred to the Fogler’s textbook in Chapter 6 (Multiple Reaction) and other related chapters (such as Chapter 7 for nonisothermal reactors). The decision would be made for:-

1. The number of reactor systems required.2. The type of the reactor (CSTR or plug flow or else)3. Operating modes and conditions4. Heat management5. Number of recycle streams6. Whether a gas recycle is required and thus the needs of using of a compressor.

Again, here the students need to analyze the degree of freedom on the reactor system block including the mixing junctions of the recycle stream. As a result, feed ratio of the reactor is usually the basis of the calculation at this level.

The economic potential of the process can be calculated as follows:-

EP3 (RM/year) = EP2 – Cost of reactor installation – cost of catalyst (if any) – cost of compressor installation (if any) – cost of compressor operation (if any) (2)

Where

Cost of reactor, CR, comprises of expenditure required for purchasing and installation depends on the type of the reactor. Mechanical features of the reactor basically come in the form of tanks (for batch reactors or CSTRs), large cylinders (for fluidized bed reactors or PFRs) or


multiple tubes inside a cylindrical container (for PFR when special needs exist for temperature control). At the conceptual level, the costing of the reactor is handled in a manner similar to that for regular mixing and pressure vessel. The cost of the reactor based on the features are expressed as follows (Peters & Timmberhaus, 1991b)

CR=index ratioofyear of referenceyear of design

× purchased cost× (1+installation cost percentage )

where index ratio can be either Chemical Engineering Plant Cost Index (CEPCI: year 1990 = 355.4 and year 2015 = 2000) or Marshall & Swift Index (M&S) and the installation cost percentages are shown below

Table 2 Installation cost for equipment as a percentage of the purchased equipment cost

Tank type, the purchased and installation cost (no need to refer Table 2) can be referred from Figure 1(c)

Cylindrical type, the purchased cost can be obtained from estimation of tubes for heat exchangers (or insulated tubes) from chapter 15 (Peters & Timmberhaus, 1991b). For the sake of simplicity assuming all tubular reactor costing has the same properties: the tube material is stainless steel, the internal diameter following ¼” schedule 40 pipes, for the pressure not more than 40 bar and projected to year 2015 considering inflation using M&S index of 2000. The cost correlation can be seen in Figure 4. This correlation cannot be used for a real conceptual design calculation.


0 2 4 6 8 10 12 14$0

$500,000

$1,000,000

$1,500,000

$2,000,000

$2,500,000

f(x) = 125933.031764 x + 651507.800973004R² = 0.996093663768609

Volume, m3

Purc

hasin

g, in

stal

latio

n an

d M

aint

enan

ce

Cost

, $

Figure 4: Simplified correlation between the cost of tubular reactor and its volume for year 2015

When there is no cost for a piece of equipment due to the size, the estimation can be done for scaling up using (Peters & Timmberhaus, 1991a)

Cost of equipment A=cost of equipment B( capacity equipment Acapacity equipment B )∝

(3)

Where is typically based on heuristics of which the values are as follows


Table 3 Typical exponents for equipment cost versus capacity (Peters & Timmberhaus, 1991a)

Detail design of the reactor especially when particular conditions are required for the reaction would be commenced after the level 5 decision.

Cost of catalyst can be obtained from the supplier. Usually they would advertise their price via the internet. If the prices are quoted for the previous year, the index used in the calculation of the equipment can also be applicable here to find the current year price.

Cost of compressor, Cc, , can refer the following chart with the M&S index of 2000 (Peters & Timmberhaus, 1991b), or by using the equation

Installed Cost = where in this case Fc can be 1 (for simplicity). Brake horsepower is assumed 90% of efficiency.


Cost of compressor operational cost, Cco = electric tariff (normally for industrial grade, USD0.1/kW) x brake power of compressor efficiency of the compressor (typically 80%).

The optimum conversion obtained would probably not be the same as the one in EP2 because of different level of cost consideration.

Tasks

Each group has to refer to the process in Appendix C and to design accordingly reactor in level 3. You must construct analyses from level 2 up to level 3 and calculate economic potential in order to get optimum designs. Make sure that the degree of freedom is done properly before any material balance is computed.

The report should contain as following chapters and subchapters:

1. Table of Content2. Introduction: explaining task of each member in the group and organization of chapters in

the report3. Level 2 Decision

i. Material balance Block Flow Diagram (with symbols of variables) Mole Balance in Term of Extent of Reaction and Degree of Freedom Reaction Selectivity, Yield and Stoichiometry (analysis to determine optimum feed

ratio and limiting reactant) Degree of freedom analysis

ii. Economic Potential


4. Level 3 Decisioni. Recycle stream analysis and decision

ii. Block flow diagram (detailing recycle stream and reactor along with symbols of variables)

iii. Mole balance for level 3 Degree of Freedom

iv. Degree of freedom analysis (where to start material balance calculation)v. Reactor design and Costing

Thermal effect (plot XEB versus Treaction to identify whether adiabatic, diabatic or non-isothermal diabatic condition for the reactor)

Levenspiel plot of reactor (under preferred thermal effect: adiabatic or diabatic to determine type of reactor: stirred tank or tubular reactor)

Heat Management (applicable for diabatic reactor only where your have to design heat exchanger and calculate necessary heat for the reactor. You need to calculate the cost of utility) and Reactor Scheme (based on selectivity analysis)

Design of reactor (based on varied feed and fixed production rate) Cost of the reactor and catalyst (if any)

vi. Compressor design and costing (applicable to the reactor compressed at greater than 3 bar) Installation cost Operating cost

vii. Economic Potential5. Conclusion

References

Biegler, L. T., Grossmann, I. E., & Westberg, A. W. (1997). Systematic Methods of Chemical Process Design: Prentice-Hall.

Douglas, J. M. (1988). Conceptual Design of Chemical Processes. Sydney: McGraw-Hill Book Company.

Lewin, D. R., Seider, W. D., & Seader, J. D. (2002). Integrated process design instruction. Computers & Chemical Engineering, 26(2), 295-306. doi: 10.1016/s0098-1354(01)00747-5

Peters, M. S., & Timmberhaus, K. D. (1991a). Cost Estimation. In J. J. Carberry, J. R. Fair, W. P. Schowalter, M. Tirrell & J. Wei (Eds.), Plant Design and Economics for Chemical Engineers (4th ed.). New York: McGraw Hill.

Peters, M. S., & Timmberhaus, K. D. (1991b). Mass Transfer and Reactor Equipment - Design and Costs. In J. J. Carberry, J. R. Fair, W. P. Schowalter, M. Tirrell & J. Wei (Eds.), Plant Design and Economics for Chemical Engineers (4th ed.). New York: McGraw Hill.

Turton, R., Bailie, R. C., Whiting, W. B., & Shaeiwitz, J. A. (2008). Analysis, synthesis and design of chemical processes: Prentice Hall.


mini project handout sem i 2015-2016

Documents