Preliminary Database Creation

Preliminary Process Synthesis

Development of the Base-Case Design

Understand how to go about assembling design data and creating a preliminary data base. Be able to implement the steps in creating flowsheets involving reactions, separations, and T-P change operations. In so doing, many alternatives are identified that can be assembled into a synthesis tree that contains the most promising alternatives. Know how to select the principal pieces of equipment and to create a detailed process flowsheet, with a material and energy balance and a list of major equipment items. On completing this part of the course, you should:

Preliminary Database Creationto assemble data to support the design.Experimentsoften necessary to supply missing database items or verify crucial data. Preliminary Process Synthesistop-down approach.to generate a synthesis tree of design alternatives.illustrated by the synthesis of processes for the manufacture of VCM and tPA (refer to text book, pg. 94).Development of Base-case Designfocusing on the most promising alternative(s) from the synthesis tree.

Thermophysical property dataphysical propertiesphase equilibria (VLE data)Property prediction methods

Environmental and safety datatoxicity dataflammability data

Chemical Pricese.g. as published in the Chemical Marketing ReporterCompanies carry out market studies and have a basis for projecting market size and chemical prices.Chemical market reporter, newspaper provide up-to-date for chemical commerce, but not reflect the market situation, just give a good starting point.To get the better estimation, contact manufacturers directly.For utility prices; steam, cooling water, electricity, it is desirable to estimate it during process creation.

Design concepts result from experimentsAdditional experiments at other conditions of compositions, temp, pressures and representative solvents.Rates of reactions and catalyst life estimations.Aid in the selection and preliminary design of separation operations.

Synthesis of chemical processes involves:Selection of processing mode: continuous or batchFixing the chemical state of raw materials, products, and by-products, noting the differences between them.Process operations (unit operations) - flowsheet building blocks Synthesis steps -Eliminate differences in molecular typesDistribute chemicals by matching sources and sinksEliminate differences in compositionEliminate differences in temperature, pressure and phaseIntegrate tasks (combine tasks into unit operations)

Decide on the raw material and product specifications (states):Mass (flow rate)Composition (mole or mass fraction of each chemical species having a unique molecular type)Phase (solid, liquid, or gas)Form (e.g., particle-size distribution and particle shape)TemperaturePressure

Chemical reactionTo effect differences in the molecular types between raw material and product streams.Positioning in the flowsheet involves many considerations (conversion, rates, side reactions, reactions in the reverse direction), related to T and P at which the reaction are carried out.Methods to remove or supply energyCatalysts (competitive reaction rates and selectivity). Separation of chemicalsneeded whenever there is difference between the desired composition of a product stream or intermediate stream and the composition of its source. (separation is needed when there is impurities in raw materials, or when products, byproducts and unreacted raw materials coexist in effluent stream). Choice of separations operations depends on the phase of the mixture and differences in the physical properties of chemical species.Distillation (volatilities are large differrent) (vapor-liquid separation); Crystallization (melting point s are large different) (solid-liquid separation)Liquid liquid separation (difference in volatilities and melting points are small): use solvent which is selective for some components (e.g. liquid-liquid extraction).Gases separation: adsorbent, absorbent, membrane separation

Phase separation - Flash drums (vapor-liquid separation)- Decanters (liquid-liquid separation)- Filters and centrifuge (liquid-solid separation)Throughout a chemical process, there are often:- Change of temperature- Change of pressure- Change of phase when enter or leave a process operation (e.g. reaction and separation operations) or during Mixing and splitting of streams and branches

Synthesis StepEliminate differences in molecular typesDistribute chemicals by matching sources and sinksEliminate differences in compositionEliminate differences in temperature, pressure and phaseIntegrate tasks (combine tasks into unit operations and decide between continuous or batch processing)Process OperationChemical reactionMixingSeparationTemperature, pressure and phase change

Continuous Dominant in chemical process industry (large-scale) for manufacture of commodity chemicals, plastic and etcCombined chemical fed, processing and product removal continuouslyWhen production rates are low, it is difficult to justify construction of continuous plant comprised of small vessels and pipesBatchSmall scale production rates, production of many specialty chemical using batch processing Batch chemical fed before and product are removed after processing Fed-batch combined fed and process continuously, product removed after processing is finishedBatch-product removal chemical fed before processing. Then product is removed continuously as processing occursFed- batch and batch-product removal semicontinuous process The challenge in designing batch processing time and size of vessel

Continuous and Batch

Example 1:

Vinyl Chloride Manufacture

Process CreationDESIGN AND ANALYSIS II - (c) Daniel R. Lewin*Chemicals participating in VC Manufacture:

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

Chemical

Molecular

weight

Chemical

formula

Chemical

structure

Acetylene

26.04

C2H2

Chlorine

70.91

Cl2

ClCl

1,2-Dichloroethane

98.96

C2H4Cl2

Cl Cl

| |

HCCH

| |

H H

Ethylene

28.05

C2H4

Hydrogen chloride

36.46

HCl

HCl

Vinyl chloride

62.50

C2H3Cl

_931450118.doc

H H

C = C

H H

_1044717498.doc

H H

C = C

H H

_1044717674.doc

H - C ( C - H

_1044717736.doc

H H

C = C

H H

_931450120.unknown

_931450117.doc

H Cl

C = C

H H

Direct chlorination of ethylene:

Advantages:Attractive solution to the specific problem denoted as Alternative 2 (analysis of primitive problem in chapter 1).Occurs spontaneously at a few hundred oC.Disadvantages:Does not give a high yield of VC without simultaneously producing large amounts of by-products such as dichloroethyleneHalf of the expensive chlorine is consumed to produce HCl by-product, which may not be sold easily.

Hydrochlorination of acetylene:

Advantages:This exothermic reaction is a potential solution for the specific problem denoted as Alternative 3 (analysis of primitive problem in chapter 1). It provides a good conversion (98%) of C2H2 to VC in the presence of HgCl2 catalyst impregnated in activated carbon at atmospheric pressure. These are fairly moderate reaction conditions, and hence, this reaction deserves further study. Disadvantages:Flammability limits of C2H2 (LFL = 2.5; UFL = 100%)

Thermal cracking of C2H4Cl2 from chlorination of C2H4:

Advantages:Conversion of ethylene to 1,2-dichloroethane in exothermic reaction (2.3) is 98% at 90oC and 1 atm with a Friedel-Crafts catalyst such as FeCl3. This intermediate is converted to vinyl chloride by thermal cracking according to the endothermic reaction (2.4), which occurs spontaneously at 500oC with conversions as high as 65% (Alternative 2). Disadvantage:Half of the expensive chlorine is consumed to produce HCl by-product, which may not be sold easily.

Thermal Cracking of C2H4Cl2 from Oxychlorination of C2H4:

Advantages:Highly exothermic reaction (2.5) achieves a 95% conversion to C2H4Cl2 in the presence of CuCl2 catalyst, followed by pyrolysis step (2.4) as Reaction Path 3. Excellent candidate when cost of HCl is lowAlso a solution for specific problem denoted in Alternative 3.Disadvantages:Economics dependent on cost of HCl

Balanced Process for Chlorination of Ethylene:

Advantages:Combination of Reaction Paths 3 and 4 - addresses Alternative 2.All Cl2 converted to VCAll HCl produced in the pyrolysis reaction is consumed in the oxychlorination reactionNo by-products!

Chemical Bulk PricesReaction Path is eliminated due its low selectivity.This leaves four alternative paths, to be compared first in terms of Gross Profit.

Chemical

Cost (cents/lb)

Ethylene

18

Acetylene

50

Chlorine

11

Vinyl chloride

22

Hydrogen chloride

18

Water

0

Oxygen (air)

0

Gross profit = 22(1) + 18(0.583) - 18(0.449) - 11(1.134) = 11.94 cents/lb VC

Reaction path (

C2H4

+

Cl2

=

C2H3Cl

+

HCl

lb-mole

1

1

1

1

Molecular weight

28.05

70.91

62.50

36.46

lb

28.05

70.91

62.50

36.46

lb/lb of vinyl chloride

0.449

1.134

1

0.583

cents/lb

18

11

22

18

Reaction Path

Overall Reaction

Gross Profit

(cents/lb of VC)

(

C2H2 + HCl = C2H3Cl

-9.33

(

C2H4 +Cl2 = C2H3Cl + HCl

11.94

(

C2H4 + HCl + O2 = C2H3Cl + H2O

3.42

(

2C2H4 + Cl2 + O2 = 2C2H3Cl + H2O

7.68

800 MM lb/year @ 330 days/y 100,000 lb/hr VCOn the basis of this principal sink, the HCl sink and reagent sources can be computed (each flow is 1,600 lbmol/h)Next step involves distributing the chemicals by matching sources and sinks.

Process Flowsheet?

Raw Materials

C2H4, Cl2

C2H3Cl, HCl

Products

A conversion of 100% of the C2H4 is assumed in the chlorination reaction.

Only 60% of the C2H4Cl2 is converted to C2H3Cl with a byproduct of HCl, according to Eqn. (2.4). To satisfy the overall material balance, 158,300 lb/h of C2H4Cl2 must produce 100,000 lb/h of C2H3Cl and 58,300 lb/h of HCl. But a 60% conversion only produces 94,980 lb/h of VC and HCl.The additional C2H4Cl2 needed is computed by mass balance to equal: [(1 - 0.6)/0.6] x 158,300 or 105,533 lb/h. Its source is a recycle stream from the separation of C2H3Cl from unreacted C2H4Cl2, from a mixing operation, inserted to combine the two sources, to give a total 263,833 lb/h.

The effluent stream from the pyrolysis operation is the source for the C2H3Cl product, the HCl by-product, and the C2H4Cl2 recycle.

Reactor pressure levels: Chlorination reaction: 1.5 atm is recommended, to eliminate the possibility of an air leak into the reactor containing ethylene.

Pyrolysis reaction: 26 atm is recommended by the B.F. Goodrich patent (1963) without any justification. Since the reaction is irreversible, the elevated pressure does not adversely affect the conversion. Most likely, the patent recommends this pressure to reduce the size of the pyrolysis furnace, although the tube walls must be considerably thicker and many precautions are necessary for operation at elevated pressures.

The pressure level is also an important consideration in selecting the separation operations, as will be discussed in the next synthesis step.

The product of the chlorination reaction is nearly pure C2H4Cl2, and requires no purification.In contrast, the pyrolysis reactor conversion is only 60%, and one or more separation operations are required to match the required purities in the C2H3Cl and HCl sinks.One possible arrangement is given in the next slide. The data below explains the design decisions made.

Boiling point (oC)

Critical constants

Chemical

1 atm

4.8 atm

12 atm

26 atm

Tc,(C

Pc, atm

HCl

-84.8

-51.7

-26.2

0

51.4

82.1

C2H3Cl

-13.8

33.1

70.5

110

159

56

C2H4Cl2

83.7

146

193

242

250

50

There may be other, possibly better alternative configurations, as discussed in the following chapter.

Summary of synthesis steps:Reaction pathDistribution of chemicalSeparations Temperature, pressure and phase change Task integration

Heuristics are the rules of thumb which must be obeyed for designing a processHeuristics are used commonly by design team to expedite the generation of alternative flowsheet in preliminary process synthesisDetailed for heuristics will be covered on topic 4

Develop one or two of the more promising flowsheets from the synthesis tree for more detailed consideration.

Block flow diagram (BFD) Represent the main processing sections in terms of functional blocksDiagram indicates overall material balances and condition at each stage where appropriate The detail is helpful to summarise principal processing section and appropriate to be used in early design stages, where alternative processes are usually under considerationProcess flow diagram (PFD)Provide more detailed view of process Display all major processing unit in the process (including HE, pump, compressor), provide stream info, include main control loopsPreliminary PFD constructed using process simulator, then more detailed prepared using software such as AUTOCAD and Microsoft Visio

Processing unitIcon that represent units are links by line that represent the process stream Unit for icon are taken from accepted standard (ASME: American Society for Mechanical Engineers)Labeling refer textbook pg 103 (new version); pg 97 (old version)Stream information Directed lines represent stream, with flow direction from left to right wherever possibleBy convention, when streamline cross, the horizontal line shown as continuous, vertical line brokenMass flowrate, P, T may appear on PFD directly, but more often are placed in the stream table instead, for claritySee table 4.6 pg 104 (new version); table 3.6 pg 100 (old version)UtilitiesVarious utility streams are utilised for heating or cooling the process stream For ex. See table 4.7 pg 105 (new version); table 3.7 pg 100 (old version)

Equipment summary table Provides info for each equipment item in PFD (See table 4.8 pg 105 (new version); table 3.8 pg 101 (old version)Materials of construction (MOC) and operating T and P, required for all units Piping and Instrumentation diagram (P&ID) Design document transmitted by process design engineers to engineers responsible for plant construction To support start-up, operation of process and operator trainingContains item that do not appear in PFD such as location and type of measurement and control instruments, positioning of valves, size, schedule, material of construction of piping etc.

Calculation supporting flow diagram The important calculation in flow diagram is mass and energy balance for each processOther than that, engineer should know how to calculate number of stages and reflux ratio for distillation towersThe calculation could been completed using process simulatorsComplete simulation not justified until design team is ready to begin detailed designGradually, additional detail is added to the simulation model. For ex.: number of stages and reflux ratio are selected for the distillation column and; material and energy balance are completed with recycle streams that are not assumed to be pure

Process Flow Diagram (PFD) development is central to the design task

The PFD depicts the process route, showing the flows of material and energy between those process units that make up the plant. It therefore

defines the role/task and operating conditions of each section or unit in the process line gives an overall view of the process route allows an insight into the overall operability of the process provides an initial assessment of potential sources of hazardsforms the template for subsequent material and energy balances

A well thought out PFD simplifies unit process design and allows more accurate assessment of the overall viability of the plant

There are usually two stages to developing the PFD, namely:

operational specification functional specification

Here, you decide upon the sections of the process that would be needed to manufacture the product. Typically, a proposed plant is divided into the following sections:

raw material storage feed preparation manufacturing separation purification effluent and waste treatment product packaging and storage

These are shown in the following figureIn most cases, the required sections would have been dictated by the chosen process route. It is during this stage of the design that you should be alerted to:

where recycles may be required to minimise wastage materials handling and transport requirements, and where loss containment systems (control) have to be installed The result at this stage is the process flowsheet

Next, using process knowledge together with information obtained from the literature, you will have to provide details about the kind and number of main process units that are required to perform the various operations defined by the main sections of the plant. By convention, you should include only those units where composition, temperature or pressure changes occur. To do this, you will need data on reaction kinetics, physical and thermodynamic properties of the materials being handled at each section of the process and to call upon your knowledge about the capabilities of each type of processing equipment.Thus, it is during this stage of the PFD development, that you get an indication of the:probable materials of construction for each process unit utilities (steam, water, electricity) that will be required potential hazards that may occur at unit level impact of each unit on other units in the plant

Create a detailed database by refining and adding to preliminary databaseA pilot-plant is constructed to confirm that the equipment items operate properly and provide data for detailed data bank and simulation model is preparedIt is common to add transport and kinetics data, as well as data concerning the feasibility of separation, the identity of any forbidden matches in HE, heuristic parameters and data for sizing equipmentAdditional data for sizing equipment are typically maximum pressure drops, tube lengths and baffle spacing in HE, surface tension and drag coefficients for estimating flood velocities, specification for tray spacing in multistaged towers and residence times in flash vessel and surge tanks

As detailed database is assembled, the needs for pilot-plant testing become quite evidentFor manufacture new chemical, pilot plant can produce quantities of product suitable for testing and evaluation by potential customersVery few processes that include reaction steps are constructed without some form of pilot plant testing prior to doing detailed design calculation. Though it is an expensive and time consuming step.In connection with the need for laboratory experiments, pilot plant test also help to identify potential problem that arise from small quantities of impurities in feed stream and etcPilot plant can also verify separation schemes developed during process design

When part of simulation model exist, it is common for design team to assemble a more comprehensive model, one that enable team to examine the effect of parametric changes on entire processWhen process simulator have not been used for design, simulation model is created for comparison with pilot plant data and parametric studies

Preliminary Database Creationneeded to provide data to support the design.Experimentsoften necessary to supply missing database items or verify crucial data. Preliminary Process SynthesisSelection of processing modeFixing the chemical stateProcess operationSynthesis steps: - Top-down approach. - Generates a synthesis tree of design alternatives.Development of Base-case DesignCreating block flow diagram (BFD)/process flow diagram (PFD)focusing on the most promising alternative(s) from the synthesis tree.

Symbols (or elements of symbols) for use in conjunction with other symbols

Symbols (or elements of symbols) for use in conjunction with other symbols

Pumps and Compressors