02 BOPs / Woods D.R 2008 rules-of-thumb-in-Engineering-practice (epdf.tips)

Table 6.8 Continued.

Type of reaction	Example	Products/	Temperature, hC	Catalyst life	Cause of decay	Max. temp., hC	Comments about
	catalysts	reactants					regeneration
Amination	Ni,
via reduction	Cu & Cr oxides
Ammoxidation	Pt Pt–Ru
	mixed oxides,	acrylonitrile	220	1–3 years
	Fe, Sb, Sn	ex ammonia, pro-
		pylene
	mixed metal	acrylonitrile	450	1–3 years	volatalization,		periodic addition
	molybdates or	ex propene			attrition		of Mo ammo-
	antimonates						nium salt as
							vapor

none

Carbonylation, homogeneous hydroformylation, FT

Ni or Pd on C or Co Rh or Ni carbonyl or Fe carbonyl

	Cu	methanol	200–300	2–8 years	slow sintering,	325
					poisons
Chlorination	none
	ferric chloride

201 Guideline General 2.6

Table 6.8 Continued.

Type of reaction	Example	Products/	Temperature, hC	Catalyst life	Cause of decay	Max. temp., hC	Comments about
	catalysts	reactants					regeneration
Condensation	Fe/alumina	ammonia ex hy-	450–470	10–15 years	slow sintering,	625
	[K2 O]	drogen, nitrogen			poisons
	Rh/Ba/MgO or
	iron oxide or alkali
	earths or homoge-
	neous
Cracking, cat	Co/Mo S or Si
	alumina or Pd or
	oxides of V, Mn, Fe,
	Cu, Mo, W
	zeolite	refinery oils	500–560	1–2 s	rapid coking,	595	regenerate con-
					poisons		tinuously in sepa-
							rate fluidized bed
Dehydration	Ta silica
Dehydrochlorination	none
	silica alumina or
	NaOH or metal
	chloride
Dehydrogenation	none
	Cu–Cr; Cu–Zn–	2 butanone		2 years			regenerate every
	bronze						2–3 months
	Ni or Pt
	oxides of Cr,	butadiene ex bu-	600–680	i 0.5 year			regenerate for 1 h
	Ni, Fe	tene or butane					every 1–7 days

Reactors 6 202

Table 6.8 Continued.

Type of reaction	Example	Products/	Temperature, hC Catalyst life	Cause of decay	Max. temp., hC	Comments about
	catalysts	reactants				regeneration
Dimerization	tripropylalumi-
	num
Disproportionation	CdI or zeolite
	Rh/alumina	propylene from	50			regenerate every
		ethylene				10 days
Epoxidation	homogeneous
Esterification	IX resin or
	sulfuric acid
Ethynylation	Cu acetylide or
	KOH
FT	Ni or Cu with
	ZnO or carbides of
	Fe, Mo, W
	Co/ZrO2/SiO2	alcohols	5 years			regen. once per
						year.
Halogenation	none
Hydration	homogeneous
	cupric chloride
	sulfuric or
	phosphoric acid or
	Ta on silica

203 Guideline General 2.6

Table 6.8 Continued.

Type of reaction	Example	Products/	Temperature, hC	Catalyst life	Cause of decay	Max. temp., hC	Comments about
	catalysts	reactants					regeneration
Hydrodealkylation	none
	Cr or Mo oxides or
	Pt on zeolites
Hydroformylation	Co or Ru or Rh or
	Co or Pd on C or
	Cu, Ni, Zn or Fe
	pentcarbonyl
Hydrogenation	Pt or Cu on Si or
	Ni Cr or Pd on C or
	Raney Ni or Pt-Co/
	C
	Cu on ZnO	alcohols ex	220–270	0.5–1 year	slow sinter
		aldehydes
	Cu-Cr oxides	furfuryl alcohol	150–200		coke		regenerate with
		ex furfural					oxidation
	CuO/ ZnO
	sulfided Ni	aniline ex	300–475		coke		decoke via air
		nitrobenzene					burn at 300 with
							later reduction via
							hydrogen

carbides of Fe, Mo, W on alumina

Reactors 6 204

Table 6.8 Continued.

Type of reaction	Example	Products/	Temperature, hC	Catalyst life	Cause of decay	Max. temp., hC	Comments about
	catalysts	reactants					regeneration
Hydrolysis	sulfuric acid
	Pd or Co Mo
Hydrotreat
	sulfided	light distillate	300	10 year	poisons V, Ni,	450	regenerate in situ
	Co–Mo/g-				Fe; coke		by burning coke
	alumina						about 2 times via
	[Bo, P, K, SiO2]						combustion with
							steam–air. For
							poisoning, regen.
							externally with
							aqueous leach
	sulfided Ni
Isomerization	zeolites
	Pt	xylenes ex ethyl	400–480	3–5 years			regenerate every
		benzene					6–18 months
	oxides of V, Mn, Fe,
	Cu, Mo, W or oxi-
	des of rare earths
Nitration	Co naphthaneate
	mixed acids

Guideline General 2.6

205

Table 6.8 Continued.

Type of reaction	Example	Products/	Temperature, hC	Catalyst life	Cause of decay	Max. temp., hC	Comments about
	catalysts	reactants					regeneration
Oxidation	Cu Ag or Fe Mo
	Ag/a-alumina	ethylene oxide	200–270	1–3 years	poisons, Cl and S;		regen. via peri-
	[alkali metals]	ex ethylene, air			some coking		odic addition of
							Cs/methanol
							solution
	Pt Rh gauze	NO ex ammonia	800–900	0.1–0.5 years	loss of Pt, fouling
	Pd Cd/alumina	vinyl acetate ex	140–180	2 years	sintering
		ethylene, acetic
		acid
	V/K sulfate	sulfur trioxide	525	5–10 years	plugging, sintering
		ex sulfur dioxide

mixed oxides; metallic oxides or AgO

homogeneous: Mn, Cu or Co acetate

Reforming	zeolite
	metals, Ni/	steam reform	500–850	2–4 years	sintering, coke,	875
	alumina	ex methane			sulfur poison
	Pt/treated		460–525	0.01–0.5 years	coking	550	frequent regen-
	alumina						eration needed

Reactors 6 206

		6.2 General Guideline		207
Table 6.9 How the solid catalyst affects choice of reactor.
Conditions	Consider the following	Constraints
	type of reactor
Catalyst retains activity	fixed bed	if I 3 months use fluidized,
i 3 months:		moving, or slurry
Operating pressure high	fixed bed	if catalyst strength
		i 3 kg/ particle
Reaction is highly exothermic	fluidized bed	if the attrition I 1 %/day
Very short contact time	monolith with low pressure
available	drop or fluidized/loop
	reactors with recycle
Enzyme catalyst	fixed bed	if immobilized
	CSTR	if pH and temperature control
		are vital or if mass transfer
		controls

6.2.4

Specific Guidelines for Gases Reacting with Solid

When the solid reacts with gas, usually heat transfer controls because these are highly exothermic or endothermic reactions. Particle size and size distribution are critical. These reactions may follow different patterns:

xfor multigranule reactions the time for reaction = independent of particle diameter.

xfor shrinking core without ash reactions, the time for reaction

/particle diameter (for reaction controlled), / D1.5 (for fluid diffusion controlled with fluidized, fixed or moving beds),

/D2 (for fluid diffusion controlled with transported bed)

xfor shrinking core plus ash, the time for reaction / Dp

(for reaction controlled), / Dp (for fluid diffusion controlled),

/Dp2 (for fluid diffusion through the ash controlled).

6.2.5

Bioreactors

The two general types of bioreactors are anaerobic and aerobic: anaerobic means no oxygen is present; aerobic means oxygen must be supplied with 17 kJ of heat released/g O2 utilized = 13 kJ/g of cell mass generated.

The three types of biotechnical products are:

1.cell biomass

2.metabolic products of cells: anaerobic: alcohols, organic acids, hydrogen, carbon dioxide; for aerobic: citrate, gluta-

208 6 Reactors

mate, lactate, antibiotics, hydrocarbons and polysaccharides, yeast and single cell protein SCP

3. enzymes that can be used as catalysts.

In selecting an aerobic bioreactor, there is a tradeoff between the degree of mixing and the required rate of oxygen transfer, OTR. How this affects the type of reactor is illustrated in Table 6.10.

Some useful definitions are:

microorganism: microscopic, living organisms including prokaryotes (e.g. bacteria) and eukaryotes (fungi).

bacteria: microorganisms that are cells without a fully differentiated nucleus. May be round, rod-like, spiral or filamentous of diameter 0.5–3 mm.

fungi: microorganisms that are cells with membranes with a discrete nuclei (eukaryotes). They are saprophytic and parasitic plants that lack chlorophyll. Fungi are typically filaments that are 5-15 mm diameter and 50–500 mm long.

Table 6.10 Some bioreactions with different types of microorganisms.

Factors related to the reactor	Microorganisms
	Bacteria	Fungi		Mixed culture
		yeasts	fungi, moulds
Amount of biomass, kg/m3	10–50 (0.001–0.01)	10–50	10–50	5
(volume fraction)
Biomass viscosity, mPa s	low, I 100	low, I 100	high, 100–1500	low, I 100
Oxygen consumption, g/s m3	0.2–1	0.2–1	0.2–1	0.002–0.01
kLa, gas–liquid, 1/s	0.05–0.2	0.05–0.2	0.01	0.003
kLa, liquid–solid, 1/s	0.01–0.5	0.25 bubble	0.1–1 jet loop
Metabolic heat production,	3–15	3–15	3–15	0.03–0.14
kW/m3
Sensitivity to shear	relatively insensi-	sensitive (animal and plant cells)		fluidized
	tive, robust
Choice of reactor configuration	aerated STR;	bubble reactors: jet loop or air lift		fluidized
if fermentation requires high	0.1–1.7 kW/m3
OTR; e.g. antibiotics, acetic
acid, SCP
Choice of reactor configuration	aerated STR;	packed column, immersed wick,
if fermentation requires low	0.1–1.7 kW/m3	immersed column
OTR;
Example rate of growth of	1–2.5	0.5–1.2	0.3–0.5
biomass for the production

of Single cell protein, SCP, g/s m3 of broth

6.2 General Guideline 209

yeast: a fungus of the family Saccharomycetacea that is typically 1–50 mm in diameter.

enzyme: a protein produced by living cells that catalyzes the metabolic process. For mixed culture microorganisms used for biological treatment of waste water, the general range of values used are: BODu z 1.43 q BOD5; Approximately

1.4–1.5 kg O2/kg BOD5.

Microorganisms use the organic substrate, characterized by BOD5 or COD, for growth and for endogenous respiration. Illustrative reaction rate terms include, for COD at 20 hC:

Substrate concentration at which the specific growth rate is 0.5 maximum, Ks, = 25–100 mg COD/L.

Maximum specific utilization rate of the substrate, k, = 6–8 kg COD/day kg VSS.

Biomass lost to endogenous respiration per unit time per unit biomass, Kd, = 0.05–0.1 1/day.

Yield of biomass produced per unit of substrate removed, YT, = 0.35–0.45 mg VSS/mg COD.

6.2.6

Reactors for Supercritical Conditions

When a fluid is compressed and heated above the critical conditions (or to supercritical conditions, sc), the differences between gas and liquid disappear. For carbon dioxide, this occurs for temperatures above 31 hC and pressures above 7.3 MPa. For reactions (such as alkylations, aminations, hydroformylations, hydrogenations and Fischer Tropsch synthesis) occurring in supercritical fluids, the reaction rate is often increased dramatically because of improved desorption of heavy molecules; minimizing the oxygen and hydrogen solubility limitations, improved heat transfer, and improved selectivity by a catalyst by minimizing pore diffusion limitations.

6.2.7

Reactors for Polymerization

The options for polymerization reactions, the types of polymerizations and the reactor configurations are given in Table 6.11.

For suspension or emulsion polymerization, the shaft power can be estimated as follows:

Shaft kW = 16 (Di/107)(N/170)3 n where

N = rpm

n = number of impellers on the shaft. Di = diameter of impeller in cm Motor kW = 1.3 q shaft kW

2106 Reactors

Table 6.11 Polymerization reactions.

Liquid phase reactors	Types of polymerizations	Reactor options
1. Bulk: liquid monomer with	condensations, LDPE, PS,	STR (3–6 kW/m3 mixer; tip
initiator in absence of diluent	Nylon, PMMA, acrylics,	velocity 0.2–0.3 m/s), long
or solvent.	polyacetals	tubular reactors, screw extru-
		ders

2. Solution: monomer dispersed in solvent with soluble catalyst

free radical, ionic, Zeigler Nichols; thermosets, acrylics, PVAlc, PVC, polybutadiene, polypropylene, melamine phenolic resins, polyisoprene, polycarbonate, chlorinated polyesters

glass or s/s STR (2–8 kW/m3 mixer; heat transfer area = 1–4 m2/m3 depending on the volume of the reactor with small area associated with large volumes).

3.Suspension with large drops 10–1000 mm of insoluble monomer and catalyst suspended in water.

4.Emulsion: small drops 0.5–10 mm of insoluble monomer suspended in water with water-soluble catalysts in micelles

Gas phase reactors

free radical addition, styrenic IX resins, vinyl polymers, PVC, styrene-acrylo- nitrile, polypropylene

industrial polymers, synthetic rubbers, polybutadience, PVC, latex paints, adhesives, coatings, ABS, PV acetate, styrene butadiene

batch STR (1–6 kW/m3 mixer; tip speed 6–9 m/s; 3-retreated blades at 46–120 rpm; heat transfer area = 1–4 m2/m3 depending on the volume of the reactor with small area associated with large volumes); continuous PVC

batch STR; 2–3 kW/m3 mixing @ 155–230 rpm; shear number 9000–12 000; impeller diam./tank diam. = 0.25–0.3, U = 110 W/m2 K; heat transfer area = 3.3 m2/m3.

Gaseous monomers,	heterogeneous solid cat.	fluidized beds; horizontal with
continuous	PE, PP;	weirs & rotating paddles;
		helically stirred tanks.

Since polymerizations are exothermic, it is essential that the heat of reaction is removed. Since polymer and scale tend to build up on any heat exchange surfaces contacting the reactants the preference is to have jacketed reactors. The amount of internal surface area varies from 4 m2/m3 for 1 m3 vessels and reduces to 1.5 m2/m3 for a 35 m3 volume reactor. Cooling fingers or external coolers can be used. The overall heat transfer coefficients are in the range 60–350 W/m2 K. In general, to account for the fouling on the walls, the internal heat transfer coefficient for polymerization reactions is about half that expected.