1
J. A. SHERRED, Monsanto Chemical Co., New Orleans, La. J. R. FAIR, Monsanto Chemical Co., Dayton, Ohio
High Purity Propylene
- A New Problem
Newer uses in the chemical industry require propylene of 99% purity, higher than usually available. Here is a discussion of propylene supply and demand, together with economics and technology for producing high-purity material
Pmnum
occupies a n almost unique position as a chemical raw material. in that i t is produced economically only as a by-product or coproduct. from refinery Operations or ethylene manufacture. I t \vas first used as a chemical raw material in 1920. when isopropyl alcohol from prop)-lene became commercial. (Frior to 1920. off-gases from refinery operations ivere burned as fuel. an economically wasteful practice accompanied by furnace firing problems.) I n the mid-1930’s. following development of catalytic polymerization processes. propylene \vas polymerized to high-octane gasoline. a use that today accounts for over 907c of the propylene consumed in this country. Until recently. demands of chemical processes and gasoline production were satisfied by the propane-propylene mixture that is readily separated from refinery off-gases. Today. ho\vever. growth of the petrochemicals industry and increased emphasis on new chemical uses for propylene focus attention on the supply and demand and means and costs of producing propylene of sufficient purity to meei the ne\v requirements. Propylene Sources Propylene is a by-product of t\vo processes: catalytic or thermal cracking of petroleum to yield gasoline. and cracking of propane and heavier hydrocarbons to yield ethylene. T h e former accounts for 90Yc of total propylene production. Production in petroleum cracking operations cannot be avoided. nor can its yield be substantially increased. as it \vould be uneconomic to intensify cracking operations to prodme more propylene. This poses no problem for the chemical industry; refinery operations can more than meet its needs, both now and at the reduced levels anticipated when gasoline is supplanted by “jet fuel“ as the primary petroleum product. I t is estimated that 1 6 to 17 billion pounds of propylene are recovered from refinery off-gases each year; 20 billion pounds propylene are produced, 80 to 85% of which is recovered from off-gases.
T h e extent of recover)- is governed principally by petroleum economics: Until recently recovery cost \vas balanced against the difference in value between propylene as a refinery fuel and as gasoline raw material. \$’it11 increased demand for ethylene. economics play a role. because refiners who recover off-gas ethylene must also recover propylene. Xvhich can be used as credit on the ethylene operation. In general. 40 to 90YG of the off-gas propylene is recovered; large refiners recover morc than small ones. as cost is related to size of operation as well as extent of recovery. From the off-gases. propylene is recovered admixed with propane. the niixture containing 40 to 607~; propylene. Catalytic cracking leads to more propylene. the amount varying with the catalyst as well as cracking conditions. This mixture can be used directly for conversion to polygasolines. by either polymerization or alkylation. T h e amount of propylene produced as a by-product of ethylene manufacture is hard to estimate. as some ethylene manufacturers use ethane feed (no propylene) ; others use propane (byproduct propylene from which depends on depth of cracking and propylene recycled to feed) ; and still others use butane. lvhich gives a higher-purity ( 9 0 + 5 , ) propylene. In hlonsanto experience, the ethylene stream from propane cracking contains 45 to 55’3 propylene, comparable to the stream produced in refineries. ..\n annual production of 0.8 to 1.0 billion pounds of pro-
Table I.
pylene from ethylene manufacturing is a reasonable estimate. All of this is not available for further use. however. as propylene can be economically recycled for cracking to ethylene. Prop)-lenr produced from butane crackinq is much less than this. It is estimated that the total propylene by-product from ethylene manufacture is 0.8 to 1.1 billion pounds annually. Table 1 shows estimates of propylene supply and demand today. Little propurity is produced. pylene of 90+5; Petroleum refiners dominate the propylene market both as consumers and as manufacturers; in fact. they burn mor? prop)-lene as fuel gas than the cheinical industry uses. .Although not a commercial process. production of propylene by catalytic dehydrogenation of propane. analogous to production of butylene and butadiene from butane, shoulcl not be overlooked. ‘l’echnical and operational problrms resulting from the need to use higher temperatures Lvith propane than \vith buLane have not yet been solved. Because propylene must be produced as a by-product. a company using propylene for chemical purposes has virtually n o chance of establishing a basic ra\v material position unless i t is i\illing to expand into parallel-e.%.. t-thylrnebased-produc ts. Propylene Demand l l a j o r uses for propylene a i r con\eision to gasoline. which accounts for 9W.i
Propylene Supply and Demand
(.ill figures in hillions of pounds per year) Little propylene of 90+
Productioii From refineries From ethylene plants Total
92 purity is m o d e Disponition
20.0 0.8 0.2
to 1 . 0 ” t o 0.3‘
Refinery use Chemical use Refinery losses
21.0 to 2 1 . 3
19.5 to 2 2 . 1
’’ .Is low purity 135 t o 457,) from propane crarking. Includes propylene reryvled t o .Is high purity (90+ %) from butane and heavier.
ethylene plants.
VOL. 51, NO. 3
,
14.8 to 1 6 . 1 1 . 7 to 2 . 0 b 3.0to 4.0
MARCH 1959
249
130
I
I
I
I
I
PRESSURE, p.s.i.a.
-10 W E
1.00
I
I
I
0
20
I
40
I
I
60
80
70
J 100
MOLE PER CENT PROPYLENE Figure 1. Relative volatility of propylene-propane mixtures is an index of the difficulty of separation
I
I
20
30
50
OPER AT I N G
I
I
L
200
100
P R E S S UR E,
300 p.s.i,a,
Figure 2. Theoretical tray requirement for propylene-propane separations Operating pressure level exerts a significant effect
of the total consumed, and chemical processes (Table I). Use for gasoline manufacture is difficult to estimate, because some refiners polymerize butylene as well as propylene, while others alkylate a t least part of their propylene. I t is estimated that the refining industry uses about 18 billion pounds of propylene feed per year, based on a polygasoline capacity in the U. S. of 180:000 barrelsper day. Eighteen billion pounds per year is far less than the potential demand for gasoline manufacture. Estimates of uses for propylene by the chemical industr). are summarized in Table 11. .Although supply presents no
Table II.
Estimated Usage of pylene in Chemicals
Pro-
Supply presents no problem, but purity of available propylene may not meet chemical industry needs QuailtltS,
Primary Produc t
Tetramer
Fnd C > e
Detergents, lube oil additives Trimer Oxo alcohols, detergents Isopropyl alcohol Acetone, solvent Propylene glycol Resins, plastics Poly( propylene Resins, plastics glycol) Allyl chloride Glycerol Cumene Phenol Epichlorohydrin Epoxy resins Cd alcohols and Plasticizers, aldehydes solvents Miscellaneous, including acrolein, allyl alcohol, and chlorinated products Total
250
bl h l L h ’1r 500 105 800 66 8 59 51 8
100 10
1707
problem, i t is questionable whether purity of available propylene can meet chemical industq needs. The propylene-propane mixtures are satisfactory for older uses-cg.. polygasolines. trimer. tetramer. cumene. isopropyl alcoholpropane introduces no complicating reactions nor problems of separation. In some processes propane diluent may in fact be useful. as, for example. to remove heat \vhen high heats of reaction are involved. In somr o f the newer uses for propylene. such as acrolein and allyl chloride processes. a high-purity propylene (997c) is demanded. \Vere propane present it \ \ o d d m t e r into the reaction and introduce problems of product purification as well as economic loss. I n other processes--e.g.. oxo process for alcohols and aldehydes-cost of cycling an inert diluent can be prohibitive. .4nd for polypropylene. only recently introduced commercially- and the subject of considerable research and development. high-purity monomer appears mandatorv. Propylene-Propane Separation Technology
A C 3 fraction can be isolaced in a refinery or a chemical plant convenientl>by fractional distillation. Alternative techniques involving selective adsorption and absorption have been described but are not of commercial importance. Typically. two distillation steps in series, each involving 30 to 40 actual stages and moderate reflux ratios, are surficient to isolate a propylene-propane mixture \vith less than 27c C B and C : contaminants. h-ear-complete de-ethanization is generally necessary. Special end uses of the product high-purity propylene may require fore- or afterremoval of trace contaminants such as
INDUSTRIAL AND ENGINEERING CHEMISTRY
water. carbon dioxide. and methl-l acetylene. An index of the difficulty of separating t\+’o materials by distillation is their volatility ratio, or “relative volatility.” For the propylene-propane system, relative volatility values calculated from literature data on vapor-liquid equilibria (7: 2) are presented in Figure 1. These values are low in comparison with many commercial distillation separations. particularly at 2.50 to 300 p.s.i.a. operating pressure required for condensation of reflux lvith cooling water. The volatility ratio may be altered by adding a selective solvent-i.e.. an “extractive distillation agent”-to the s p tem. In Table 111 are listed several such agents and corresponding volatility ratios (3). Thrse materials reverse the normal volatility ratio; as a result the propane-rich stream \\.odd be carried overhead and the propylene-rich stream out the bottom of an extractive distillation column. Despite effective separation. the additional expense of
Table Ill. Solvent Effect on Relative Volatility of Propylene-Propane at
82’ F. The additional expense of handling the extrclctive agent is not usually justified
Mixtuie. hlolp %
Volatility. C~HR/ (.JH~
100 40 15-18
0.83-0.87 1.03 1.19
12-15 26-34 14-19
1.23 1.29 1.35
15-25
1.72
C,’s $017 ellt
(None) Acetone Furfural Furfural with 47, water Acrylonitrile Acetonitrile 50-50 acrylonitrileacetonitrile
III
PURE PROPYLENE handling the extractive agent is not USUally justified. I n addition to the difficulty of separating these two CB hydrocarbons, specifications for high purity of the propylene product add to the separation requirements. “High purity” is here defined as 99c0purity on a propylene-propane basis, the specification used in allyl chloride manufacture. Contributing to the general problem of C 3 separation is the percentage of propylene in the feed mixture, to be recovered as high purity product. For a fced Ca mixture containing 50% propylene. the following relative stage requiremenrs are typical (30% recovery = 1 .00) : I
