hews

.Lecture 3

*

A Hierarchical Decomposition for Process SynthesisTo guide the selection of process alternatives, Douglas formalized a Decision Hierarchy as a set of levels, where more detail in the process flowsheet is successively added to the problem. These levels are classified according to the following process decisions:Level l: Batch versus continuousLevel 2: Input-output structure of the flow sheet Level 3: Recycle structure of flow sheetLevel 4: Separation system synthesis4a: Vapor recovery4b: Liquid recoveryLevel 5: Heat recovery network

*

First level: we consider batch processes only if at least one of the following holds.We must get the process operational in a few months. The product is one where the first company to market wins an enormous competitive advantage. We need only a few days production for a year's supply. We have little design information and the process is sensitive to upsets and variations.The product will likely have a total lifetime of one to two years before some other product will come out that replaces it.The value of the product overwhelms the cost to manufacture it.*

level 2: we consider the number of raw material and product streams and their overall relation to the process. We also consider the presence of by-products and inert components in the process and how they participate in the reaction chemistry. An important question is the recovery of these compounds. At this level, a process recycle may be needed for the reactor, and the designer needs to consider the addition of purge streams to avoid the buildup of inert components or by-products.

*

Level 3 Explores the recycle structure of the flow sheet and focuses more closely on the reactor itself. We consider the number of separate reactor networks in the flowsheet and their interactions through recycle streams. We also consider the effects of reactor conditions on the rest of the flow sheet. These could include the effect of inerts as a diluent in the reactor feed and the effects of equilibrium in choosing pressure, excess components, and adiabatic operation for the reactor.

*

Level 4 is divided into two decision stages: vapor and liquid recovery. Raw materials from this step will be recycled to the reactor while products and by-products are generally processed further and removed. In vapor recovery, the more expensive stage, we also need to consider the effect of purge streams and the removal of components based on their value and their effect on the reactor if they are recycled. In the liquid recovery stage, we prefer to use distillation, as this is often the least expensive separation. Design decisions at this stage include sequencing of the separators and determining their operating conditions.

*

Level 5 deals with the heat recovery network (HEN) once all of the other flowsheeting decisions have been made.

*

Case study: Ethylene and Water to EthanolWaterEthyleneMethanePropyleneREACTORMethane (Waste)Ethylene (Recycle)Propylene (Waste)Diethyl ether (Bi product)Ethyl alcohol (Product)Iso propyl alcohol (waste)Water (Recycle)535-575 K, 68 atm5-7% conversionW/E-4/1 but due to low conversion per pass, we choose small water ratio 0.6-110% M excess prevent cokingExcess water push equilibrium towards ethanol production and back to reactant (ethanol) Propylene to IPA 0.5-0.7% conversion

Maximum Potential ProfitDepending on the purity and composition of raw material, product and market rate with inflations , we calculate the maximum profit potential for the setup.Estimate the gross profit : Depending on the market price of product and requirement and purityCalculate the cost of raw materialProfit =gross profit-cost of raw materialEquipment cost/3 + annual operating cost profit(assume payout time and depreciable life)

Physical property data for species Chemical formula, MW, Sp gravity, Melt Pt., enthalpy, VP (using antoine eq parameters)M, E-ME, PL- E

BP (oC)Tcritical (oC)Pcritical (atm)Water100374.4217.6Ethyl alcohol78.4243.163.1Ethylene-103.79.650.7Di ethyl ether34.6193.835.5Methane-161.5-82.145.8Propylene-47.791.445.4Isopropyl alcohol82.4235.1647.0

Ethylene + waste gasesWaterDiethyl etherEthanolWater + wasteWaterEthyleneAbsorberDCDCDCFlashReactorCompressorCompressor4. Condensible and non condensible3. Recycle structure of flow sheetPurge stream

SepnSepnREACTORSepnSepnSepnREACTORREACTORWELPLMWWELPLMELPLMELPLMEL MPLELDEEEAWELPLMDEEEAWELMDEEEAWELWELWELWMM PL IPAEADEEEADEEEADEEAlternative separation Schemes

GENERAL PROCEDURE FOR MATERIAL-BALANCE PROBLEMSProcedureStep 1. Draw a block diagram of the process.Show each significant step as a block, linked by lines and arrows to show the stream connections and flow direction.Step 2. List the available data.Show on the block diagram the known flows (or quantities) and stream compositions.Step 3. List all the information required from the balance.Step 4. Decide the system boundaries.

Step 5. Write out the chemical reactions involved for the main products and byproducts.Step 6. Note any other constraints, such as specified stream compositions, azeotropes, phase or reaction equilibrium, tie substancesStep 7. Note any stream compositions and flows that can be approximated.Step 8. Check the number of conservation (and other) equations that can be written, and compare with the number of unknowns. Decide which variables are to be design variables; Step 9. Decide the basis of the calculation.

calculate the stream flows for a production rate of 10,000 kg/h.

Chemical Process DesignAfter economical and technical feasibility; mass and energy balance are taken care.Synthesis of process involves two stepsIndividual steps are selectedThese steps are interconnectedLeads to flowsheet structure of the processNow the simulation of process is carried out using mathematical models to predict the flow rates, compositions, temperature and pressure of product

*