Industrial Chemistry

Laboratory Chemistry vs. Industrial Chemistry There are fundamental differences between the design of a chemical synthesis for industry and that for a research laboratory. 1) Explain how industrial synthetic approaches differ from laboratory synthesis methods. 2) Evaluate possible reaction schemes based on thermodynamic, economic, and other considerations.

Differences in the Synthetic Approach e.g., formation of ethyl alcohol by hydration of ethylene: Laboratory Scale bubble ethylene into 98% H2SO4 dilute and warm the reaction mixture to hydrolyze the resultant sulfate ester Industrial Scale a stream of ethylene is mixed with steam at 325°C and 1000 psi and passed over a solid catalyst consisting of phosphoric acid absorbed on diatomaceous earth; the process is run continuously, and unreacted ethylene is recovered and recycled to the feed stream.

Different Approaches for Different Objectives Laboratory Objectives synthesize the product in the most convenient manner considering: 1) chemist’s time 2) equipment available (usually must use glassware) 3) conditions achievable (usually close to ambient pressure and between -195.8° C (N2(l)) and 132° C (chlorobenzene). Industrial Objectives produce the product at minimum total cost on a scale that will generate the maximum economic return. may use: 1) large range of temperatures and pressures 2) batch process or continuous operation 3) reactants in vapor phase or liquid phase

Evaluation of a Reaction If a chemist has an idea for an industrial scale process, what are the steps that must be taken before the process can be implemented? 1) Evaluation of the reaction: Before any serious literature search or laboratory work is started, various possible strategies are proposed. 2) Economic feasibility 3) Technical feasibility 4) Other considerations: environmental issues, etc.

Evaluation of a Reaction The chemist must consider not only the well-known, obvious approaches, but also unknown or untested approaches. e.g., the manufacture of ethylamine

Economic Feasibility Estimate the difference between the market value of the products and the reactants. First approximation, assume: 1) 100% yield 2) no costs of solvents or catalysts 3) no value for co-products These assumptions must be reassessed further on in the development stage.

Technical Feasibility There are two basic questions that a chemist or chemical engineer must ask concerning a given chemical reaction: 1. How far does it go, if it is allowed to proceed to equilibrium? (Does it go in the direction of interest at all?) 2. How fast does it progress? Question 1 is concerned with thermodynamics and amounts to evaluating the equilibrium constant (K). Question 2 is a matter of kinetics and reduces to the need to know the rate equation and rate constants (k).

Technical Feasibility Generally, the first approach is to consider the thermodynamics of the reaction. This may be done by evaluating the change in Gibbs free energy (ΔG). DGR = DHR - T DSR A spontaneous reaction has a decrease in Gibbs energy of the system. To calculate the Gibbs energy of reaction, use standard Gibbs energies of formation DGf°. DGreaction = DDGf°products - DDGf°reactants

Example: Dissociation of ethyl chloride At 298 K DHf° -26.70 12.50 -22.06 DGf° -14.34 16.28 -22.77 S° At 1000 K DHf° -30.43 9.21 -22.56 DGf° 18.60 28.85 -24.08 S° 93.80 72.07 53.25 DHrxn = DHf°CH2=CH2 + DHf°HCl - DHf°CH3CH2Cl DGrxn = DGf°CH2=CH2 + DGf°HCl - DGf°CH3CH2Cl DSrxn = S°CH2=CH2 + S°HCl - S°CH3CH2Cl

Change in Free Energy Indication -DG promising At 298 K: DHrxn = +17.14 kcal/mole DGrxn = +7.86 kcal/mole DSrxn = +31.15 cal/mole At 1000 K: DHrxn = +17.08 kcal/mole DGrxn = -14.43 kcal/mole DSrxn = +31.52 cal/mole -G at 1000 K only! Change in Free Energy Indication -DG promising small +DG worth further investigation large +DG possible only under unusual conditions

DGreaction = DGf°products - DGf°reactants DGf° kcal/mol 298 K 1000 K CH2=CH2 16.28 28.25 CH3-CH3 -7.87 26.13 CH3CH2NH2 8.91 60.96 CH3CH2OH -40.22 1.98 NH3 -3.86 14.85 H2O -54.64 -46.04 H2 0 0 N2 0 0 DGreaction = DGf°products - DGf°reactants DG298 = + 16.78 kcal/mol DG1000= + 34.83 kcal/mol DG298 = -1.65 kcal/mol DG1000= -1.91 kcal/mol

Other Considerations Evaluate: number of possible side-products (and separation difficulties) air or moisture sensitivity of reactants and intermediates commercial value of side-products environmental impact of side-products health and safety issues