Homogeneous Reactors

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Process Engineering Guide: GBHE-PEG-RXT-809

Homogeneous Reactors

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CONTENTS Page 0 INTRODUCTION / PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 DESIGN STEPS 2

4.1 Residence Time and Flow Pattern 3 4.2 Modeling of Ideal Reactor Types 4 4.3 Costing 6

5 EQUIPMENT SELECTION SUMMARY 6 6 EQUIPMENT EXAMPLES 8

6.1 Gas Reactors - Plug Flow 8 6.2 Gas Reactors - Backmixed or Batch 9 6.3 Liquid Reactors - Low Viscosity 10 6.4 Liquid Reactors - High Viscosity 12

7 MEASUREMENT OF HOMOGENEOUS REACTION KINETICS 13

7.1 Gas Phase Reactions 14 7.2 Liquid Phase Reactions 14

8 NOMENCLATURE 15



TABLES 1 EQUIPMENT SUMMARY 7 FIGURES 1 LOOP REACTOR 9 2 BACKMIXED GAS REACTOR 9 3 IN-LINE FLOW MIXERS 10 4 SPINNING CONE THIN-FILM REACTOR 10 5 STIRRED VESSEL REACTOR 11 6 JET-MIXED REACTOR VESSEL 11 7 EXTRUDER 12 8 SCRAPED-FILM REACTOR 12 9 Z-BLADE MIXER 13 10 CONTINUOUS FLOW 14 11 STOPPED FLOW 15

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 16



0 INTRODUCTIONS / PURPOSE

This Guide is one of a series of Guides produced by GBH Enterprises, C2PT Catalyst Process Technology Consultancy.

1 SCOPE

This Guide sets out the key steps in the design of gas or liquid phase homogeneous reactors and suggests appropriate types of reactor according to the required residence time, flow pattern and heat transfer duty.

A liquid phase reactor with a vapor product is covered only if the vapor removal rate does not affect the overall rate. If it does, consider thin-film reactors.

2 FIELD OF APPLICATION

This Guide applies to the process engineering community in the GBH Enterprises.

3 DEFINITIONS

For the purposes of this Guide, the following definition applies: Homogeneous Reactions having only one phase (gas or liquid) in the Reactions reactor or, if there is another phase, it has no effect

on the reaction, on the fluid flow or on the temperature.

With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms.



4 DESIGN STEPS

A logical progression of events for the design of a homogeneous phase reactor are: (a) Identify the important reaction products and by-products, and

the reaction steps. Specify the downstream equipment for separation and treatment of the product, and for effluent disposal if any. Decide the relative yield required for satisfactory economics.

(b) Decide on solvents or diluents, temperature, concentrations and pressure. Reiterate from (a) if necessary.

(c) Measure the kinetics (rate constants and orders of reaction)

of these steps, see Clause 7 for measurement methods.

Note any reversible (equilibrium) steps).

(d) If the reaction scheme is complex and the economics justify it, a computer model may be required to describe the interactions between reactions; see GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling.

(e) Measure the heat of reaction; see GBHE-PEG-RXT-804

Decide on the materials of construction. (f) Recycle from (b) if necessary. (g) Decide on batch or continuous operation; see GBHE-PEG-

RXT-800. Decide on residence time, and flow pattern (residence time distribution) ideally required in the reactor, see below and GBHE-PEG-RXT-802.

(h) Make an initial selection of equipment from Table 1 (see

Clause 5). If this is not possible or impractical, consider the sensitivity of the reaction operational yield and selectivity to the factors in steps (b) and (g) and make a new compromise.



(j) Calculate an approximate size for the reactor. Assess how closely the flow is likely to approach the ideal required. It may not be very close (especially if the reactor is large), in which case, if the costs are sensitive to it, it will be necessary to computer model the reaction within the flow pattern (see 4.1). If a semitech or pilot scale plant is being designed, its flow pattern should be made to copy that of the proposed plant.

(k) Obtain the flowsheet and approximate sizing of downstream

equipment. (l) Obtain an approximate costing of the system. Annualize the

capital cost and add raw materials and operating costs. If the cost is unacceptable, reconsider the decisions of steps (b), (d), (g) and (h) to find a new compromise. Examine how sensitive the costs are to reactor flow pattern and revisit (j) if necessary.

4.1 Residence Time and Flow Pattern

GBHE-PEG-RXT-802 gives the basic effects of residence time distribution on reaction rate and conversion, see also GBHE-PEG-RXT-802. This implies that, for simple reactions of the order > 0, a backmixed reactor will be larger than the plug flow reactor for the same conversion (e.g. by a factor of 3.9 for 90% conversion, or 21.5 for 99% conversion with a first order reaction). For multiple reactions, the reaction selectivity will also be affected, for example, with the reaction:

A B C high selectivity to B can only be obtained with plug flow whereas backmixed flow favors conversion to C; or for:

A + B R R + B S

high production of R requires plug flow, (and sufficiently rapid mixing of A and B). Whereas formation of S is favored by backmixing.



Sometimes reactions of widely differing rate have to be accommodated together and a combination of reactor types is used. This can also be done for temperature effects, for example a backmixed zone with an exothermic reaction can be placed before a plug flow reactor in order to bring reactants up to temperature quickly [Ref. 1]. The models used to simulate the performance of the basic ideal reactor types are described in 4.2. Non-ideal and more complicated cases are dealt with in many textbooks, for example [Ref. 2], [3] and [4]. Residence time distribution in continuous reactors refers to mixedness in the main flow direction. In plug flow, for any but first order reactions, the degree of cross or radial mixedness is important: the extremes are often referred to as premixed or segregated feeds; see GBHE-PEG-RXT-802. Sometimes rapid radial mixers (e.g. turbulent jets) are used at the entry to tubular plug flow reactors. Obviously the theory can only be used for real reactors if the fluid dynamics provide a reasonably close approach to one of the ideal residence time distribution. Often this is not so, and more complex RTD models are used, either: (a) Using networks of interlinked ideal reactors of appropriate size to

model a measured RTD (this descriptive method is touched on in GBHE-PEG-RXT-802.

Or (b) Computing the flow patterns and reaction progress using

computational fluid dynamics (CFD) programs, which are basically predictive since they use fine-detail fundamental calculations without specific empirical input? GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling.



4.2 Modeling of Ideal Reactor Types

This section illustrates the sort of equations which arise in describing the ideal reactor types. They may be useful for initial design and approximate reactor sizing. Chemistry: Consider for example the general reaction scheme:

Where A, B, D, and E are chemical species, and R1, R2 and R3 are the reaction rates (in moles/volume, time).

The rates are functions of concentration, temperature and sometimes pressure and are defined by:

where C is a concentration in moles/volume, and t is time. Each reaction has an associated heat of reaction ∆H1, ∆H2 and ∆H3 in energy/mole, (negative for exotherms).



4.2.1 Ideal Batch Model

The above differential equations must be solved, often with R non-linear in C, and certainly non-linear if temperature varies, in which case a heat balance equation is required:

4.2.2 Ideal Plug Flow Reactor Model

Again a set of differential equations arises; the first of these is

These equations can be solved by writing a program using a standard routine (e.g. from NAG Library) to do the integration from x=0 to x=reactor length, see GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling.



4.2.3 Ideal Backmixed Reactor Model

This model gives a series of algebraic equations, e.g.:

The heat balance equation if the reactor is non-isothermal is:

4.3 Costing The item cost of the reactor may be obtained from in-house engineers if it is a vessel, tube, heat exchanger, etc., or from manufacturers if it is proprietary device (such as a Sulzer mixer, Buss Loop Reactor, or high viscosity mixer). This must be multiplied by factors (consult GBH Enterprises, Engineering) for instrumentation, installation and design charges to give the installed cost. Downstream equipment could be specified approximately from short-cut design methods (see [Ref. 5], and costed as above. Its cost may be substantially greater than that of the reactor.

5 EQUIPMENT SELECTION SUMMARY Table 1 lists a wide selection of designs, classified by Gas, Low Viscosity Liquid, or High Viscosity Liquid, then according to flow pattern and heat exchange system. Low Viscosity refers to liquids which can be practically processed in turbulent flow; say Reynolds Number,

Re = ρDU / µ > 1000 or viscosity µ > 1 Pas (1000 cp).





6 EQUIPMENT EXAMPLES 6.1 Gas Reactors - Plug Flow

Thermal crackers are typical examples; generally multiple tubular reactors in furnaces. High process side heat transfer coefficients and close approach to plug flow require turbulence, hence high velocities and short residence times. Pressure drop consideration may require a compromise on the tube velocity. Multiple tubes in parallel are often used to increase heat transfer area.

For a given process, throughput Q and mean residence time t are specified i.e.:

An empty tube is cheap and easy to clean, but has a longer mixing length (95% mixed in about 100 diameters, if turbulent) than a jet mixer or static mixer (95% mixed in about seven diameters). An empty tube in laminar flow provides no mixing. With a static mixer, flow is more pluggish and process side heat transfer coefficient is about four times that in an empty tube.

Adding a swirl component to the jet flow in a coaxial jet mixer improves the mixing substantially (common in combustor technology).



GBH Enterprises examples are: (a) Naphtha cracking; (b) Arcton cracking; (c) EDC cracking. Stirred tube reactors were used on the polythene process to get near the plug flow without having to use turbulent flow (the gas had a density of about 1000 kg/m3) at the cost of mechanical complexity. Inert-solids fluidized beds can be used for heat transfer to gas reactions, with heat input via jacket, induction heating, internal heaters (preferably short vertical plate heaters), or submerged combustion. Gas superficial velocity would be around 1 m/s. Gas turbines can be used for very rapid reactions with positive volume change, to provide mechanical work.

6.2 Gas Reactors - Backmixed or Batch

Any of the above plug flow reactors can be used within a loop to provide a batch reactor, or a good approach to a continuous backmixed reactor (if the recirculation rate > 10 x throughflow rate). A blower or compressor will be required to drive the recirculation flow (Fig. 1), unless there is sufficient feed pressure to re-induce it by means of an eductor. Where the kinetics are appropriate a backmixed reactor can also act as the preheater for exothermic reactions. FIGURE 1 LOOP REACTOR



For short residence times an approximately backmixed vessel can be used, with jet mixing provided by the input stream (or a recycle stream) (Fig. 2). Typically a draught tube is used to organize the flow pattern. FIGURE 2 BACKMIXED GAS REACTOR

Early work by Bush and Shires attempts to model the degree of recirculation: now it is preferable to use CFD methods. Examples are: Chloromethanes reactors; HCl generators. Internal combustion engines are a special case for very rapid reactions with large volume increase (e.g. combustion) where shaft work is to be extracted.

6.3 Liquid Reactors - Low Viscosity 6.3.1 Plug Flow

The above remarks for gas tubular and stirred tube reactors apply here also. Coaxial or side entry jet mixers in turbulent flow provide very rapid mixing between streams, as do turbulent static mixers, which also give near-plug flow (Fig. 3). If velocities or tube lengths preclude these, the oscillating-flow baffled tube (M Mackley et al, Cambridge University) could be considered: a pilot scale example is used in the CANDID project. Examples are: Burn Hall plant scale jet mixer; reaction injection moulding; nitroglycerine reactor; CANDID reactor.



FIGURE 3 IN-LINE FLOW MIXERS

Centrifugal thin-film reactors have been developed to semi-tech scale by the GBH Enterprises Process Technology Group. The preferred type is the spinning cone in which a thin, turbulent liquid film flows up the inside of a rotating cone and is collected for recirculation at the top (Fig. 4). Heat transfer (by evaporation or rotating jacket) is intense. FIGURE 4 SPINNING CONE THIN-FILM REACTOR

6.3.2 Backmixed or Batch The most common near-backmixed reactor for liquids is the stirred vessel (Fig. 5); it is versatile and flexible (mixing varied independently of throughput). Heat transfer via jacket must often be supplemented by internal coils or recirculation through external heat exchangers; boiling and reflux must be resorted to for highly exothermic processes, though this is limited by liquid entrainment by the exit vapor (maximum vapor superficial velocity 1.5 m/s). Recommended agitators are axial flow hydrofoil types for overall blending or disk turbines for high local (micro-) mixing. Liquid mixing is often promoted by wall baffles (though mixing per unit power input is claimed to be higher without). It is difficult to predict (varied correlations from unreliable data);



CFD could help here. 95% mixing times are around 5 sec for a lab vessel to 100 sec for a large plant vessel, so an ideal mixing assumption cannot be made with rapid reactions. If the reaction is sensitive to mixing CFD (MIXFLOW) modeling is required. Local shear stress is highly non-uniform.

FIGURE 5 STIRRED VESSEL REACTOR

Thermal instabilities (multiple steady states) can occur by interaction between reaction thermodynamics and heat transfer rate. Capital cost depends on vessel size and drive torque but is often an insignificant fraction of plant cost (beware of agitator manufacturers excessively paring down shaft sizes). Operating cost depends on drive power. Jet-mixed vessels (Fig. 6) are a somewhat less vigorous and flexible alternative, useful mainly when a feed or recirculating stream of sufficient momentum is to hand. They are also useful where a rotating stirrer shaft seal would be a problem (but not the pump seal!), and where exotic materials of construction are necessary. FIGURE 6 JET-MIXED REACTOR VESSEL



6.4 Liquid Reactors - High Viscosity 6.4.1 Plug Flow

Static mixers may be used up to very high viscosities; limited only by practicalities of pumping. Again flow is much more pluggish that an empty tube (N.B. laminar flow). Heat transfer area per unit reaction volume is high. Extruders are common for very high viscosity transport and heat transfer. Mixing of streams is poor in a conventional single-screw extruder (Fig. 7), but is good when mixing zone (e.g. Cavity Transfer Mixer) is added, or if a twin-screw extruder is used (expensive!). FIGURE 7 EXTRUDER

Scraped-surface thin film machines (Fig. 8) offer very high heat transfer per unit liquid volume (via either jacket or evaporation), with near-plug flow, so are suitable for temperature-sensitive reactions or materials. FIGURE 8 SCRAPED-FILM REACTOR



6.4.2 Backmixed or Batch For the moderate viscosity range (up to say 104Pa.s) stirred vessels with helical ribbon, helical screw (both for shear-thinning or yield-stress fluids), anchor, or bent-anchor stirrers are recommended. The standard anchor provides only poor mixing, but good heat transfer to the wall. The bent-anchor was devised to improve the axial turnover and mixing. With higher viscosities mechanical stresses and power intensities are high, and the specialized stirred chamber mixers are used. These have rotating blades (of various types) which cover the entire volume of the enclosed chamber. Common blade shapes include twin concentrate helices, Z-blades (Fig. 9), double naben blades, and intermeshing cams. Twin-screw extruders also come into this category. Some theory is available (see the GBHE Mixing and Agitation Manual) but design is limited to trials with the manufacturers' specific machines.

EQUIPMENT SUPPLIERS: APV-Baker-Perkins Ltd. (D Todd,USA). Werner-Pfleiderer GmbH. Banbury Mixers Ltd.

FIGURE 9 Z-BLADE MIXER

7 MEASUREMENT OF HOMOGENEOUS REACTION KINETICS

Elucidation of reaction pathways and measurement of kinetics in sufficient detail is the key to the design of efficient and predictable reactors. The measurements are often hampered by the limitations of quantitative analysis of intermediates or products, so the analysis methods should be established before any kinetics measurements are contemplated. If unsteady-state (batch) measurements are to be made, the response of the measurement instrument or sampling and quench system must be much more rapid than the characteristic time of the reactions under study.



Wherever possible all reactants and products should be measured so that a mass balance can be established. This is a valuable check on the accuracy of the concentration measurements. Generally isothermal conditions are required for ease of interpretation, except where the rate of temperature rise in an adiabatic system is being used to measure overall reaction rate. The kinetics measuring reactor should preferable operate without mixing effects influencing the reaction progress, or at least mixing influences must be comparatively minor and modellable. The reactor can be either used in batch or continuous steady-state mode. Steady-state measurements are easier to interpret but batch experiments are often more convenient, especially for slower reactions. If a continuous reactor is used it should approximate well to either an ideal plug-flow or backmixed reactor for ease of interpreting the results. Plug flow reactors can be operated in differential (low conversion) or integral (high conversion) mode; see GBHE-PEG-RXT-805 for a discussion of these. The kinetics of most homogeneous reaction steps are represented by one of the following rate equations (CA and CB are reactant concentrations for liquids or facilities for gases):

Sometimes it is not possible to elucidate a step in full detail, and rates must be modeled using non-integral apparent orders.



Complex interactions of species can be modeled with GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling. Fitting of parameters to models is covered in GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling.

7.1 Gas Phase Reactions

Steady-state measurements are made using a small-diameter (<10 mm) tubular reactor (a microreactor) installed in an appropriate constant temperature environment. Integral or differential experiments can be carried out according to ease of concentration measurement and temperature control. as described in GBHE-PEG-RXT-805. Reactor diameters of less than about 10mm are recommended to minimize the effects of mass and heat transfer on the kinetic results. [Ref. 6] (Section 1.6) gives some guidance for handling complex reactions.

Batch methods available include temperature jump and pressure jump methods, in which the well-mixed reactants are placed in a closed vessel at non-reacting conditions, which are suddenly adjusted to the desired reaction conditions. A suitable transient (pressure, temperature, concentration) is measured and analyzed by fitting rate expressions to it. Very rapid reactions can be studied in this way. See [Ref. 7].

7.2 Liquid Phase Reactions

Removal of diffusion limitations and mixing effects is more difficult than with gas reactions.

For steady-state isothermal work a tubular microreactor can be used but it must be of very small diameter (Goddard and Deans recommend 0.2 - 0.5 mm [Ref. 13]) or run at high velocity to achieve adequate radial mixedness to match a fully turbulent plant reactor. Radial mixing through the reactor, and approach to plug flow, can be improved by using Static mixer (if a sufficiently small one can be found). Initial mixing is often achieved using a turbulent jet mixer of coaxial or T-jet design (Fig. 10) which for low viscosity systems enable reaction times down to 10 msec to be studied. See Reports [Refs 8], [9], [10] and [11]. Experiments should be repeated at different velocities to check for absence of mixing effects.



FIGURE 10 CONTINUOUS FLOW

In the Integral form, extra information can be gathered by on-line measurement (e.g. spectrophotometry, NMR, or small temperature changes) or sampling (with rapid quench) at intervals along the tube. For slower reactions a continuous stirred vessel reactor could be useful if the backmixing is near ideal. Stirrer speed should be varied to check for mixing effects. A batch method is available for rapid aqueous reactions (reaction time 5 milliseconds) at <70°C and 1atm. This is the stopped flow technique (Fig. 11). Reactants flow through a small jet-mixed glass mixing cell, the flow is suddenly stopped and the concentrations followed versus time by spectrophotometry, or some other rapid response technique. The equipment is available commercially; see Report [Ref. 12].



FIGURE 11 STOPPED FLOW

For slower reactions (reaction times > 5 sec for low viscosities; slower for higher viscosities) a batch stirred vessel is commonly used. Experiments must be repeated at various stirrer speeds to confirm freedom from mixing limitations. Guidance on design can be obtained from GBHE-PEG-MIX-701; if the design is unconventional it may be modeled using the GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling, computer program to calculate mixing time, etc. see GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling. Temperature is kept constant by means of a jacket or immersed coil (which must not interfere with the mixing), or by boiling and reflux, in which case care must be taken to design for the minimum unmixed holdup in the condenser. 8 NOMENCLATURE D Characteristic length; tube diameter; agitator diameter. U Characteristic velocity; tube mean velocity. Re Reynolds Number = uDU/o. L Tube length. n Number of tubes. Q Volumetric throughput of reactor. ρ Mean fluid density. µ Mean fluid viscosity. t Mean residence time in reactor. ∆p Pressure drop over reactor tube.





DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE

This Process Engineering Guide makes reference to the following documents: GBHE-PEG-MIX-700 Experts on Mixing and Agitation

(referred to in 7.2)

GBHE-PEG-MIX-701 Mixing of Miscible Liquids (referred to in Clause 5 and 7.2)

GBHE-PEG-MIX-702 Gas Mixing (referred to in 5.1)

GBHE-PEG-RXT-800 How to use the Reactor Technology Guides (referred to in Clause 4 and 4.1)

GBHE-PEG-RXT-802 Residence Time Distribution Data

(referred to in Clause 4 and 4.1)

GBHE-PEG-RXT-804 Physical Properties and Thermochemistry for Reactor Technology (referred to in Clause 4)

GBHE-PEG-RXT-805 Solid Catalyzed Reactions (referred to in Clause 7 and 7.1)

GBHE-PEG-RXT-810 Gas-Liquid Reactors (referred to in Clauses 0 and 1)

GBHE-PEG-RXT-800 Series Proprietary Tools for Reactor Modeling.

Reactor Dynamics Control and Safety

Tools for Reactor Modeling



REPORTS Use of a Semi-Tech Rapidly Mixed Tubular Reactor for a Preliminary Study of the Methacrylamide Reaction System (referred to in 7.2 and Clause 9) A Rapidly Mixed Continuous Flow Tubular Reactor Rig for the Study of Fast Liquid Phase Reactor Systems (referred to in 7.2 and Clause 9) The Measurement of Fast Liquid Phase Reaction Rates by Kinetic Spectrometry in a Stopped Flow Apparatus (referred to in 7.2 and Clause 9) A Rapidly Mixed Continuous Flow Tubular Reactor Rig for the Study of Fast Liquid Phase Reaction Systems at High Temperatures and Pressures (referred to in 7.2 and Clause 9) The Design of an Experimental Rig to Study the Nitration of Chlorobenzene in Liquid Hydrogen Fluoride (referred to in 7.2 and Clause 9) Integrated Reactor Systems Procedure (referred to in 4.3 and Clause 9) Reactor Design (referred to in 4.1 and Clause 9).

