1. Chapter Four: PART 1 Mole Balances in Terms of Conversion
4.1 Design Structure for Isothermal Reactors
One of primary goals of this chapter is to solve chemical reaction engineering (CRE) problems by using logic rather than memorizing which equation applies where.

2. It is author’s experience that following this structure, shown in Fig
It is author’s experience that following this structure, shown in Fig. 4-1, will lead to a greater understanding of isothermal reactor design

5. Begin by applying our general mole balance equation (level 1) to a specific reactor to arrive at design equation for that reactor (level 2).

6. If feed conditions are specified (e. g
If feed conditions are specified (e.g., NA0 or FA0), all that is required to evaluate design equation is rate of reaction as a function of conversion at same conditions of temperature & pressure of reactor.

7. When -rA =f (X) is known or given, one can go direct from level 3 to level 9 to determine either batch time or reactor volume necessary to achieve specified conversion.

8. If rate of reaction is not given explicitly as a function of conversion, proceed to level 4 where rate law must be determined by either finding it
in books or journals or
by determining it experimentally in laboratory.

9. After rate law has been established, one has only to use stoichiometry (level 5) together with conditions of system (e.g., constant volume, temperature) to express C as a function of X.

10. For liquid-phase reactions & for gas-phase reactions with no pressure drop (P = P0), combine information in levels 4 & 5, to express rate of reaction as a function of conversion & arrive at level 6.

11. Now possible to determine either t or V necessary to achieve desired conversion by substituting relationship linking conversion & rate of reaction into appropriate design equation (level 9).

12. For gas-phase reactions in packed beds where there is a pressure drop, need to proceed to level 7 to evaluate pressure ratio (P/P0) in concentration term using Ergun eq. (Section 4.5).

13. In level 8, combine equations for pressure drop in level 7 with information in levels 4 & 5, to proceed to level 9 where equations are then evaluated in appropriate manner (i.e., analytically using a table of integrals, or numerically using an ODE solver).

14. Although this structure emphasizes determination of a reaction time or reactor volume for a specified conversion, it can also readily be used for other types of reactor calculations, such as determining conversion for a specified volume.

15. Different manipulations can be performed in level 9 to answer different types of questions mentioned here.

16. Fortunately, by using an algorithm to formulate CRE problems.
Step 1 is to begin by choosing appropriate mole balance for one of three types of reactors shown.
Step 2 we choose rate law, &
Step 3 we specify whether reaction is gas or liquid phase.

17. Finally, in Step 4 we combine Steps 1, 2, & 3 & either obtain an analytical solution or solve equations using an ODE solver.

18. Suppose that we have, as shown in Fig
Suppose that we have, as shown in Fig. 4-2, mole balances for three reactors, three rate laws, & equations for concentrations for both liquid & gas phases.

20. In Fig. 4-2, see how algorithm is used to formulate equation to calculate PFR reactor volume for a first-order gas-phase reaction.

21. Pathway to arrive at this equation is shown by ovals connected to dark lines through algorithm.
Dashed lines & boxes represent other pathways for solutions to other situations.

22. Algorithm for pathway shown is
1. Mole balance, choose species A reacting in a PFR
2. Rate law, choose irreversible first-order reaction
3. Stoichiometry, choose gas-phase concentration
4. Combine Steps 1, 2, & 3 to arrive at Eq. A
5. Evaluate. Combine step can be evaluated either
Analytically (Appendix A1)
b. Graphically (Chapter 2)
c. Numerically(Appendix A4 ), or
d. Using software (Polymath).

23. In Fig. 4-2 we chose to integrate Eq
In Fig. 4-2 we chose to integrate Eq. A for constant temperature & pressure to find volume necessary to achieve a specified conversion (or calculate conversion that can be achieved in a specified reactor volume).

24. For isothermal operation case with no pressure drop, able to obtain an analytical solution, given by eq. B, which gives reactor volume necessary to achieve a conversion X for a 1st-order gas-phase reaction carried out isothermally in a PFR.

25. However, in majority of situations, analytical solutions to ordinary differential equations appearing in combine step are not possible.

26. Consequently, include Polymath, or some other ODE solver such as MATLAB, in our menu in that it makes obtaining solutions to differential equations much more palatable.

27. 4.2 Scale-Up of Liquid-Phase Batch Reactor Data to Design of a CSTR
One of jobs in which chemical engineers are involved is scale-up of laboratory experiments to pilot-plant operation or to full-scale production.

28. In past, a pilot plant would be designed based on laboratory data
In past, a pilot plant would be designed based on laboratory data. However, owing to high cost of a pilot-plant study, this step is beginning to be surpassed in many instances by designing a full-scale plant from operation of a laboratory-bench-scale unit called a microplant

29. To make this jump successfully requires a thorough understanding of chemical kinetics & transport limitations.
In this section, show how to analyze a laboratory-scale batch reactor in which a liquid-phase reaction of known order is being carried out.

30. After determining specific reaction rate, k, from a batch experiment, use it in design of a full -scale flow reactor.

31. 4.2.1 Batch Operation
In modeling a batch reactor, we have assumed that there is no inflow or outflow of material & that reactor is well mixed.
For most liquid-phase reactions, density change with reaction is usually small & can be neglected (i.e., V = V0),

32. In addition, for gas-phases reactions in which batch reactor volume remains constant, also have V = V0. Consequently, for constant volume (V = V0) (e.g., closed metal vessels) batch reactors mole balance
written in terms of concentration.

33. Generally, when analyzing laboratory experiments, it is best to process data in terms of measured variable.

34. Because concentration is measured variable for most liquid-phase reactions, general mole balance equation applied to reactions in which there is no volume change becomes

35. Let’s calculate time necessary to achieve a given conversion X for irreversible second-order reaction

37. This time is reaction time t (i. e
This time is reaction time t (i.e., tR) needed to achieve a conversion X for a 2nd-order reaction in a batch reactor.

38. Table 4-1 shows algorithm to find batch reaction times, tR, for both 1st-& a 2nd-order reactions carried out isothermally.
Obtain these estimates of tR by considering 1st-& 2nd-order irreversible reactions of form:

41. Note that if 99% conversion had been required for this value of kCA0, reaction time, tR, would jump to 27.5 h.

42. Table 4-2 gives order of magnitude of time to achieve 90% conversion for 1st-& 2nd-order irreversible batch reactions.

44. Total cycle time in any batch operation is considerably longer than reaction time, tR, as one must account for time necessary to fill (tf) & heat (te) reactor together with time necessary to clean reactor between batches, tc:

45. In some cases, reaction time calculated from Eq
In some cases, reaction time calculated from Eq. (4-5) may be only a small fraction of total cycle time, tt.

46. Typical cycle times for a batch polymerization process are shown in Table 4-3.
Batch polymerization reaction times may vary between 5 & 60 hours.

48. As reaction time is reduced (e. g. , 2
As reaction time is reduced (e.g., 2.5 h for a second-order reaction with kCA0 = 103 s-1), it becomes important to use large lines & pumps to achieve rapid transfers & to utilize efficient sequencing to minimize cycle time.

49. Usually one has to optimize reaction time with processing times listed in Table 4-3 to produce maximum number of batches (i.e., pounds of product) in a day.

50. In next four examples, describe various reactors needed to produce 200 million pounds per year of ethylene glycol from a feedstock of ethane.
Begin by finding rate constant, k, for hydrolysis of ethylene oxide to form ethylene glycol.

51. Example 4-1 Determining k from Batch Data
Desire to design a CSTR to produce 200 million pounds of ethylene glycol per year by hydrolyzing ethylene oxide.

52. However, before design can be carried out, it is necessary to perform & analyze a batch reactor experiment to determine specific reaction rate constant, k.

53. Because reaction will be carried out isothermally, specific reaction rate will need to be determined only at reaction temperature of CSTR.

54. At high temperatures there is a significant by-product formation, while at temperatures below 40°C reaction does not proceed at a significant rate; consequently, a temperature of 55°C has been chosen.

55. Because water is usually present in excess, its concentration may be considered constant during course of the reaction. Reaction is first-order in ethylene oxide.

56. In laboratory experiment, 500 mL of a 2 M solution (2 kmol/ m3) of ethylene oxide in water was mixed with 500 mL of water containing 0.9 wt % sulfuric acid, which is a catalyst.

57. Temperature was maintained at 55°C.
Concentration of ethylene glycol was recorded as a function of time (Table E4-1.1).
Using data in Table E4-1.1, determine specific reaction rate at 55°C.

67. 4.3 Design of Continuous Stirred Tank Reactors (CSTRs)
Continuous stirred tank reactors (CSTRs), such as one shown here schematically, are typically used for liquid-phase reactions.

68. This (space time) equation applies to a single CSTR or to first reactor of CSTRs connected in series.

69. 4.3.1 A Single CSTR
4.3.1 A Single CSTR
combine mole balance Eq. (4-7), rate law & concentration, Eq. (3-29):

70. Also combine Eqs. (3-29) & (4-8) to find exit reactor concentration of A, CA.
For 1st-order reaction, τk is often referred to as reaction DamkӪhler number, Da, a dimensionless number, give us a quick estimate of degree of conversion that can be achieved in continuous flow reactors. DamkӪhler number is ratio of rate of reaction of A to rate of convective transport of A at entrance to the reactor.

71. 2nd -order irreversible reaction
1st -order irreversible reaction
2nd -order irreversible reaction

72. A value of Da = 0.1 or less will usually give less than 10% conversion & a value of Da = 10.0 or greater will usually give greater than 90% conversion:
Eq. (4-8) for a 1st-order liquid-phase reaction in a CSTR can also be written:

73. 4.3.2 CSTRs in Series
A first-order reaction with no change in volumetric flow rate (υ= υ0) is to be carried out in two CSTRs placed in series (Fig. 4-3).

76. Instead of two CSTRs in series, n equal-sized CSTRs connected in series:

77. A plot of conversion as a function of number of reactors in series for 1st -order reaction is shown in Fig. 4-4 for various values of Da

78. Observe from Fig. 4-4 that when, Da ≥ 1, approximately 90% conversion is achieved in two or three reactors; thus cost of adding subsequent reactors might not be justified.
When Da ≈ 0.1, conversion continues to increase significantly with each reactor added.
Rate of disappearance of A in nth reactor is

79. Balance on any reactor, say i, gives:
4.3.3 CSTRs in Parallel
Consider case in which equal-sized reactors are placed in parallel rather than in series, & feed is distributed equally among each of reactors (Figure 4-5).
Balance on any reactor, say i, gives:

82. This result shows that conversion achieved in anyone of reactors in parallel is identical to what would be achieved if reactant were fed in one stream to one large reactor of volume V.

83. 4.3.4 A Second-Order Reaction in a CSTR
For a 2nd-order liquid-phase reaction being carried out in a CSTR, combination of rate law & design equation yields

85. Minus sign must be chosen in the quadratic equation because X cannot be greater than 1. Conversion is plotted as a function of Da in Fig. 4-6.
At high conversions (say 67%) a 10-fold increase in reactor volume (or increase in specific reaction rate by raising temperature) will increase conversion only to 88%. This observation is a consequence of fact that CSTR operates under condition of lowest value of reactant concentration (i.e.. exit concentration), & consequently smallest value of rate of reaction.

87. Example 4-2 Producing 200 Million Pounds per Year in a CSTR

98. 4.4 Tubular Reactors
Gas-phase reactions are carried out primarily in tubular reactors where flow is generally turbulent.

99. By assuming that there is no dispersion & no radial gradients in either temperature, velocity, or concentration, model flow in reactor as plug-flow.

100. Differential form of PFR design equation
must be used when there is a pressure drop in reactor or heat exchange between PFR & surroundings.

101. In absence of pressure drop or heat exchange, integral form of plug-flow design equation is used.

106. Look effect of change in number of moles in gas phase on relationship between conversion & volume. For constant T & P, Eq. (3-45) becomes:

107. fluid moves through reactor at a constant volumetric flow rate as conversion increases.

108. When there is a decrease in number of moles ( δ < 0, ε < 0) in gas phase (e.g., 2A → B), volumetric gas flow rate decreases as conversion increases; for example,
Consequently, gas molecules will spend longer in reactor than they would if flow rate were constant

109. If there is an increase in total number of moles (δ > 0, ε> 0) in gas phase (e.g., A → 2B, then volumetric flow rate will increase as conversion increases; for example,
& molecules will spend less time in reactor than they would if volumetric flow rate were constant.

110. Fig. 4-8 shows volumetric flow rate profiles for three cases just discussed.
We note that, at end of reactor, virtually complete conversion has been achieved.

111. Example 4-3 Producing 300 Million Pounds per Year of Ethylene in a Plug Flow Reactor: Design of a Full-Scale Tubular Reactor
Determine plug-flow reactor volume necessary to produce 300 million pounds of ethylene a year from cracking a feed stream of pure ethane. Reaction is irreversible & follows an elementary rate law. Achieve 80% conversion of ethane, operating reactor isothermally at 1100 K at a pressure of 6 atm.

117. 4.5 Pressure Drop in Reactors
In liquid-phase reactions, concentration of reactants is insignificantly affected by even relatively large changes in total pressure.
Consequently, we can totally ignore effect of pressure drop on rate of reaction when sizing liquid-phase chemical reactors.

118. However, in gas-phase reactions, concentration of reacting species is proportional to total pressure; consequently, proper accounting for effects of pressure drop on reaction system can, in many instances, be a key factor in success or failure of reactor operation.

119. This fact is especially true in microreactors packed with solid catalyst. Here channels are so small that pressure drop can limit throughput & conversion for gas-phase reactions.

120. 4.5.1 Pressure Drop & Rate Law
For an ideal gas, recall Eq. (3-46) to write concentration of reacting species i as

121. Determine ratio P/P0 as a function of volume, or catalyst weight, W, to account for pressure drop.
Then combine concentration, rate law, & design equation.

124. 4.5.2 Flow Through a Packed Bed
Majority of gas-phase reactions are catalyzed by passing reactant through a packed bed of catalyst particles.
Equation used most to calculate pressure drop in a packed porous bed is Ergun equation:
Term 1 is dominant for laminar flow, & Term 2 is dominant for turbulent flow.

126. In calculating pressure drop using Ergun equation, only parameter that varies with pressure on right-hand side of Eq. (4-22) is gas density, ρ.

134. 4.5.3 Pressure Drop in Pipes
Normally, pressure drop for gases flowing through pipes without packing can be neglected.
For flow in pipes, pressure drop along length of pipe is given by

135. Friction factor is a function of Reynolds number & pipe roughness
Friction factor is a function of Reynolds number & pipe roughness. Mass velocity, G, is constant along length of pipe. Replacing u with G/p , & combining with Eq. (4-23) for case of constant temperature, T, & total molar flow rate, FT, Eq. (4-35) becomes

137. Example 4-4 Calculating Pressure Drop in a Packed Bed
Plot pressure drop in a 60 ft length of 11/2-in. schedule 40 pipe packed with catalyst pellets 1/4-in. in diameter. There is lb/h of gas passing through bed. Temperature is constant along length of pipe at 260°C. Void fraction is 45% & properties of gas are similar to those of air at this temperature. Entering pressure is 10 atm.

143. 4.5.4 Analytical Solution for Reaction with Pressure Drop
We will first describe how pressure drop affects our CRE algorithm. Fig. 4-9 shows qualitatively effects of pressure drop on reactor design.

145. These graphs compare concentrations, reaction rates, & conversion profiles for cases of pressure drop & no pressure drop. We see that when there is pressure drop in reactor, reactant concentrations & thus reaction rate for reaction (for reaction orders greater than 0 order) will always be smaller than case with no pressure drop.

146. As a result of this smaller reaction rate, conversion will be less with pressure drop than without pressure drop.

150. Example 4-5 Effect of Pressure Drop on the Conversion Profile

151. (a) First, calculate conversion in absence of pressure drop.
(b) Next, calculate conversion accounting for pressure drop.
(c) Finally, determine how your answer to (b) would change if catalyst particle diameter were doubled

152. Entering concentration of A is 0.1 kmol/m3 & specific reaction rate is

157. By increasing particle diameter we decrease pressure drop parameter & thus increase reaction rate & conversion.

158. Example 4-6 Calculating X in Reactor with Pressure Drop
Calculate catalyst weight necessary to achieve 60% conversion when ethylene oxide is to be made by vapor-phase catalytic oxidation of ethylene with air.

169. PART 2 Mole Balances Written in Terms of Concentration & Molar Flow Rates
There are many instances when it is much more convenient to work in terms of number of moles (NA , NB) or molar flow rates (FA, FB , etc.) rather than conversion.
Membrane reactors & multiple reactions taking place in gas phase are two such cases where molar flow rates are preferred rather than conversion.

170. We now modify our algorithm by using concentrations for liquids & molar flow rates for gases as our dependent variables.

171. The main difference between conversion algorithm & molar flow rate/concentration algorithm is that, in conversion algorithm, we needed to write a mole balance on only one species, whereas in molar flow rate & concentration algorithm, we must write a mole balance on each & every species.

172. This algorithm is shown in Fig. 4-11.
First, write mole balances on all species present as shown in Step (1).
Next, write rate law, Step (2), & then
relate mole balances to one another through relative rates of reaction as shown in Step (3).

175. Steps (4) & (5) are used to relate concentrations in rate law to molar flow rates.
In Step (6), all steps are combined by ODE solver (e.g., Polymath).

176. 4.7 Mole Balances on CSTRs, PFRs, PBRs, & Batch Reactors
4.7.1 Liquid Phase
For liquid-phase reactions in which there is no volume change, concentration is preferred variable. The mole balances for generic reaction are shown in Table 4-5 in terms of concentration for four reactor types we have been discussing.

177. From Table 4-5 that we have only to specify parameter values for system (CA0, ν0, etc.) & for rate law parameter, (e.g., kA , α, β) to solve coupled ordinary differential equations for either PFR, PBR, or batch reactors or to solve coupled algebraic equations for CSTR.

179. 4.7.2 Gas Phase
The mole balances for gas-phase reactions are given in Table 4-6 in terms of number moles (batch) or molar flow rates for generic rate law for generic reaction Eq. (2-1).

182. Molar flow rates for each species Fj are obtained from a mole balance on each species, as given in Table 4-6.
For example, for a plug-flow reactor.

185. 4.8 Microreactors
Microreactors are emerging as a new technology in CRE. Microreactors are characterized by their high surface area-to-volume ratios in their microstructured regions that contain tubes or channels.

186. A typical channel width might be for 100 μm with a length of 20,000 μm (2 cm).
Resulting high surface area-to volume ratio reduces or even eliminates heat & mass transfer resistances often found in larger reactors.

187. Consequently, surface catalyzed reactions can be greatly facilitated, hot spots in highly exothermic reactions can be eliminated, & in many cases highly exothermic reactions can be carried out isothermally.

188. Microreactors are also used for the production of specialty chemicals, combinatorial chemical screening, lab-on-a-chip, & chemical sensors.
In modeling microreactors, we will assume they are in plug flow for which mole balance is
For plug-flow case, algorithm is described in Fig

189. Example 4-7 Gas-Phase Reaction in a Microreactor-Molar Flow Rates

191. Although this particular problem could be solved using conversion, shall illustrate how it can also be solved using molar flow rates as variable in mole balance.

192. First write reaction in symbolic form & then divide by stoichiometric coefficient of limiting reactant, NOCl.

198. 4.9 Membrane Reactors
Membrane reactors can be used to increase conversion when reaction is thermodynamically limited as well as to increase selectivity when multiple reactions are occurring.

199. Thermodynamically limited reactions are reactions where equilibrium lies far to left (i.e., reactant side) & there is little conversion.
If reaction is exothermic, increasing temperature will only drive reaction further to left, & decreasing temperature will result in a reaction rate so slow that there is very little conversion.

200. Term membrane reactor describes a number of different types of reactor configurations that contain a membrane.

201. Membrane can either provide a barrier to certain components while being permeable to others, prevent certain components such as particulates from contacting catalyst, or contain reactive sites & be a catalyst in itself.

202. Like reactive distillation, membrane reactor is another technique for driving reversible reactions to right toward completion in order to achieve very high conversions.

203. These high conversions can be achieved by having one of reaction products diffuse out of a semipermeable membrane surrounding reacting mixture.

204. As a result, reverse reaction will not be able to take place, & reaction will continue to proceed to right toward completion.

205. Two of the main types of catalytic membrane reactors are shown in Fig
Reactor in Fig. 4-13(b) is called an inert membrane reactor with catalyst pellets on feed side (IMRCF).

206. Here membrane is inert & serves as a barrier to reactants & some of products.
Reactor in Fig. 4-13(c) is a catalytic membrane reactor (CMR).

209. Catalyst is deposited directly on membrane, & only specific reaction products are able to exit permeate side.
For example, in reversible reaction

210. Hydrogen molecule is small enough to diffuse through small pores of membrane while C6H12 & C6H6 cannot.
Consequently, reaction continues to proceed to right even for a small value of equilibrium constant.

212. Hydrogen, species B, flows out through sides of reactor as it flows down reactor with other products, which cannot leave until they exit reactor.

213. In analyzing membrane reactors, only need to make a small change to algorithm shown in Fig. 4-11.
Choose reactor volume rather than catalyst weight as our independent variable for this example.

214. Mole balances on chemical species that stay within reactor, namely A & C, are shown in Fig. (4-11) & also in Table 4-6.
Mole balance on C is carried out in an identical manner to A, & resulting equation is

215. However, mole balance on B (H2) must be modified because hydrogen leaves through both sides of reactor & end of reactor.

216. Perform mole balances on volume element ∆V shown in Fig. 4-13(c)
Perform mole balances on volume element ∆V shown in Fig. 4-13(c). Mole balance on hydrogen (B) is over a differential volume ∆V shown in Fig. 4-l3(d) & it yields
Balance on B in catalytic bed:

217. where RB is molar rate of B leaving through sides of reactor per unit volume of reactor (mol/dm3.s). Dividing by ∆V & taking limit as ∆V → 0 gives:
Rate of transport B out through membrane RB is product of molar flux of B, WB, & a, surface area, per unit volume of reactor.

218. Molar flux of B, WB in (mol/m2
Molar flux of B, WB in (mol/m2. s) out of reactor is a mass transfer coefficient times concentration driving force across membrane.
Where k’C is overall mass transfer coefficient in m/s & CBS is concentration of B in sweep gas channel (mol/dm3).

219. Overall mass transfer coefficient accounts for all resistances to transport: tube side resistance of membrane, membrane itself, & on shell (sweep gas) side resistance.

220. In general, this coefficient can be a function of membrane & fluid properties, fluid velocity, & tube diameters.

221. To obtain rate of removal of B, need to multiply flux through membrane by surface area of membrane in reactor.

222. Rate at which B is removed per unit volume of reactor, RB , is just flux WB times surface area membrane per volume of reactor, a (m2/ m3); that is,

232. Use of Membrane Reactors to Enhance Selectivity
In addition to species leaving through sides of membrane reactor, species can also be fed to reactor through sides of membrane.
For example, for reaction:

234. 4.10Unsteady-State Operation of Stirred Reactors
In this section, we discuss two other aspects of unsteady operation: startup of a CSTR & semibatch reactors. First, startup of a CSTR is examined to determine time necessary to reach steady-state operation [see Fig. 4-14(a)], & then semibatch reactors are discussed.

236. In each of these cases, we are interested in predicting concentration & conversion as a function of time. Closed-form analytical solutions to differential equations arising from mole balance of these reaction types can be obtained only for zero-& first-order reactions. ODE solvers must be used for other reaction orders.

237. There are two basic types of semibatch operations
There are two basic types of semibatch operations. In one type, one of reactants in reaction
(e.g., B) is slowly fed to a reactor containing other reactant (e.g., A), which has already been charged to a reactor such as that shown in Fig. 4-14(b).

238. This type of reactor is generally used when unwanted side reactions occur at high concentrations of B or when reaction is highly exothermic.

239. Other type of semibatch reactor is reactive distillation & is shown schematically in Fig. 4-14(c).
Here reactants A & B are charged simultaneously & one of products is vaporized & withdrawn continuously.

240. Removal of one of products in this manner (e. g
Removal of one of products in this manner (e.g., C) shifts equilibrium toward right, increasing final conversion above that which would be achieved had C not been removed.

241. In addition, removal of one of products further concentrates reactant, thereby producing an increased rate of reaction & decreased processing time.
This type of reaction operation is called reactive distillation.

242. Startup of a CSTR
The startup of a fixed volume CSTR under isothermal conditions is rare, but it does occur occasionally. We can, however, carry out an analysis to estimate time necessary to reach steady-state operation.

243. For case when reactor is well mixed & as a result there are no spatial variations in rA, we begin with general mole balance equation applied to Figure 4-14(a):

246. Semibatch Reactors
One of best reasons to use semibatch reactors is to enhance selectivity in liquid-phase reactions. For example, consider following two simultaneous reactions. One reaction produces desired product D.

248. & guides us how to produce most of our desired product & least of our undesired product. From instantaneous selectivity that we can increase formation of D & decrease formation of U by keeping concentration of A high & concentration of B low. This result can be achieved through use of semibatch reactor, which is charged with Pure A & to which B is fed slowly to A in the vat.