production of synthetic rubber exceeded Thethatworld of all natural rubber products for the first
BASF Process for Production of Pure Butadiene U. WAGNER H. M. WElTZ
time in 1962 (6). At the same time, the proportion of polybutadiene in synthetic rubber greatly increased; therefore, butadiene has become a n important petrochemical base material. I n the USA, the bulk of the butadiene-namely about 850jo-is obtained by the dehydrogenation of butanes and butenes. However, in Western Europe and Japan the main base material for the production of butadiene is the C4 hydrocarbon fraction obtained in cracking naphtha to yield ethylene. T h e amount of butadiene produced in this process is about one seventh of the amount of ethylene. Table I indicates that the butadiene content of the cracked fractions is between 40 and 5oOj0, the actual amount depending on the sew erity of cracking, and is thus decidedly higher than that of dehydrogenation fractions ( 2 ) . The content of acetylenes--i.e., up to 1.5%-is greater than that in the dehydrogenation fractions by a factor of about 10. A modern process of recovering butadiene from C4 hydrocarbon mixtures must give a good yield of butadiene with the high degree of purity required for the production of stereospecific butadiene rubber. The guaranteed values for the purity of butadiene of “polycis quality” are also listed in Table I ; accordingly the content of 1,3-butadiene should be a t least 99.5%. T h e catalysts for the stereospecific polymerization contain metalorganic compounds, and the content of reactive impurities in the monomeric butadiene should be very low-e.g., less than 25 ppm for acetylenes and 50 ppm for allenes. This requirement must be met, even
TABLE I. TYPICAL COMPOSITION OF C, FRACTIONS CONTAINING BUTADIENE AND GUARANTEED VALUES FOR BUTADIENE PRODUCT (POLY-CIS-QUALITY)
Compound Propane propene iso-Butane n-Butane iso-Butene 1-Butene trans-2-Butene cis-2-Butene 1,3-Butadiene Propadiene 1,2-Butadiene Propyne 1-Butyne 2-Bu tyne Vinylacetylene Diacetylene C.K+ hydrocarbons
+
BASF method uses N-me thylpyrrolidone as solvent of proved high selectivity, low separation costs
Cracked fraction, % VOl ca. 0.2 1 .o 4.7 23.5 12.6 6.4 5.4 44.6 0.1 0.2 0.3 0.2 Traces 0.8 Traces Traces
VOL. 6 2
Dehydrogenation fraction, % wt
80-35
20-45
Product butadiene (guarantee values)
2
>99,50J, vol 2:
NO. 4
< 0.5y0 vol
< 50 ppm voI
APRIL 1970
43
if the feedstock is the Cq cracked fractions obtained in modern high-severity cracking processes and thus have a very high content of C4 acetylene-i.e., greater than 3Yc. T h e large amount of acetylenes and other highly unsaturated compounds in the cracked fractions incurs the risk of polymer formation in separation plants and thus of fouling and choking of columns, equipment, fittings, and pumps. A number of processes have been described for avoiding these difficulties by chemical pretreatment of the Cg fraction or by partial hydrogenation ( 4 ) . However, these demand additional capital investment and utilities. Although in thc second method the higher unsaturated compounds and part of the Cd acetylenes are selectively hydrogenated, the effect is usually accompanied by a not insignificant loss of butadiene. I n a process for recovering butadiene from C,cracked fractions, the butadiene content in the butene mixture should be so lom-e.g., less than lyo-that this iriixture can be used for further chemical purposes or, for example, for the recovery of pure 1-butene.
Figure 7.
Absorption process:
straight-through scrubbing
Figure 2. Absorption process: counter-current scrubbing or extractive distillation 44
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Principle of the Process The most important processes for recovering butadiene from ( 2 4 hydrocarbon mixtures are those employing selective solvents--i.e., extractive distillation or countercurrent scrubbing. I n the BASF process for recovery of butadiene, :V-methylpyrrolidone (NMP) containing water is used as the selective solvent (7). The solvent has also proved successful in the recovery of acetylene, the extraction of aromatics, and various other separation processes. In this paper it will be demonstrated that NMP more than meets all the requirements imposed on a selective solvent for the recovery of pure butadiene from crude Cq cracked fractions. T h e separation of gas mixtures with the aid of physical selective solvents exploits the different solubilities of the ingredients in the solvent. For example, carbon dioxide can be removed from synthesis gas by scrubbing with water under pressure. Only small amounts of the other ingredients are contained in the water, because their solubility in water is much less than that of carbonic acid. However, if there is little difference in the solubilities, it is still possible to obtain the less soluble component in the pure form a t the head of the absorber, but the solvent runs off at the bottom of the absorber and will always contain a mixture of the two components (Figure 1). Natta (5) was the first to recognize in 1938 that, in analogy to distillation, the more readily soluble component can be obtained in gas scrubbing if a zone of desorption fractionating is inserted between the absorption fractionating and the degassing zones (Figure 2). This is the principle of countercurrent scrubbing or extractive distillation, which is the basis of all butadiene processes employing physical solvents. T h e only differences between the individual processes are the arrangestages and the method Of ment Of the recovery.
___
TABLE I I .
Compound Propane iso-Butane Propene n-Butane iso-Butene 1-Butene Propadiene trans-2-Butene cis-2-Butene 1,3-Butadiene Propyne 1,2-Butadiene 1-Butyne 2-Butyne Vinylacetylene Diacetylene
PHYSICAL DATA OF HYDROCARBONS T O BE SEPARATED
BP, OC -42.07 -11.73 -47.70 - 0.50 - 6.90 6.26 -34.5 0.88 3.72 - 4.41 -23.22 + l o . 85 8.1 +26.9 5.1 +10.3
-
+ +
+ +
Bunsen solubility coeficient a! at I atm and 40 O C , m3 NTP/m3 atm 3.08 4.87 5.37 9.5 15.42 15.6 18.4 20.4 25.1 41.5 43 .O 78.0 102 206 226 2,200
T o give a n idea of the separation task to be accomplished, the components of the C4 fraction are arranged in Table I1 in sequence of increasing solubility in Nmethylpyrrolidone. A distinction can be made between three groups of compounds. (1) Hydrocarbons whose solubility is less than that of 1,3-butadiene. These, together with the butanes and butenes, represent the bulk of the other components of the feedstock and are withdrawn from the head of the main scrubber. (2) Compounds whose solubility is greater than that of butadiene-1,3. These consist of small amounts of C 4 acetylenes and 1,2-butadiene impurities. They are separated in a second absorber from the 1,3-butadiene, which in this case is withdrawn a t the head of the column. This group of compounds also includes some that are not listed in the table but are usually present in small amounts in Cd cracked fractions-namely Cg hydrocarbons (mainly pentanes and pentenes), carbonyl compounds such as acetaldehyde, and sulfur compounds such as methylmercaptan. (3) Compounds whose solubility does not differ
U . Wagner and H . M . Weitz are with Badische Anilin- €3 Soda-Fabrik AG, Ludwigshafen/Rhine, West Germany. This paper is from the Symposium on Novel Processes and Technology of the European and Japanese Chemical Industry presented at the 158th Meeting of the American Chemzcal Society, New York, N . Y., September 7-12, 1969. AUTHORS
13.5 8.52 7.73 4.37 2.69 2.66 2.29 2.03 1.65
*
Main scrubber (tops)
,
very much from that of butadiene. Here this only concerns propyne, but because its boiling point differs by ca. 20 "C from that of 1,3-butadiene, it can be separated by distillation quite easily. T h e ingredients of the mixture whose solubilities differ least from that of 1,3-butadiene are referred to as key components. They govern the amount of solvent required in the stripper for separation. They are cis2-butene and 1,2-butadiene. However, other components could also be present in high concentrations in the feedstock, but present in only small amounts in the end product. A schematic diagram showing the separation of the three groups of materials from 1,3-butadiene is presented in Figure 3. Comparison between Various Solvents
T h e greater the solubility of the gas, the less is the amount of solvent primarily required for physical scrubbing. However, the closer the selectivity S, which is defined by CQ: solubility of 1,3-butadiene = aB'aK CYK: solubility of key component
{
lies to 1-i.e., the less the selectivity-the more recycle gas to be returned from the desorption fractionating to the absorber zone (corresponding to the reflux in normal distillation) has to be used, and this increases the amount of solvent required. A measure of the amount of solvent required is given by the efficiency factor F, which allows for both solubility a n d selectivity. VOL. 6 2
NO. 4 A P R I L 1 9 7 0
45
Figure 3. Schematic diagram of a Cq separating with N-methylpyrrolidone
F
=
cyB(OLB/oLK
- 1)
= aB(S
- 1)
I n this equation the selectivity, S, for the key components should be used, because these require the greatest amount of solvent. It is a common source of error in comparing various solvents for the separation of butadiene to use the selectivity for a component that can be readily separated--e.g., 1-butene-or to use the selectivity for all butenes in general. A few efficiency factors for the separation of 1,3butadiene and cis-2-butene are listed in Table 111. From these it is clear that N-methylpyrrolidone shows the highest value. Similar conditions pertain to the separation of the components that are more readily soluble than butadiene. Thus vinyIacetylene, which is a component that can be readily separated with N-methylpyrrolidone (NMP), becomes a key component in acetonitrile. Separation with selectivities below 1.6-corresponding to a boiling point difference of about 10 OC-in normal distillation can be considered as difficult. It demands either large amounts of solvents or a large number of trays or even both. There are a number of other aspects that are important for the assessment of a selective solvent besides
TABLE I I I.
COMPARISON OF VARIOUS SOLVENTS
Acetonitrile 82 AT-methylpyrrolidone 206 Dimethylformamide 153 Dimethylacetamide 168
46
80
86 83 85
1.47 1.66 1.40 1.40
38 57 33 35
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
these thermodynamic criteria. A particular advantage of AV-methylpyrrolidone is its very high hydrolytic and thermal stability. Thus it does not give rise to corrosion in any part of the plant, and all equipment can be made of normal carbon steel. The low vapor pressure a t normal temperature (Pzs = 0.4 mm Hg) facilitates the recovery of the solvent vapors from the product stream; owing to the ready solubility of NMP in water, this operation is most easily accomplished by scrubbing with water or alternatively with hydrocarbons. Inasmuch as the boiling point of N-methylpyrrolidone is greater than that of water, it is quite simple to maintain the water balance in the solvent circuit. All of these properties entail very low solvent losses. Another factor worth stressing is the fact that *V-methylpyrrolidone is physiologically harmless and can be readily biologically degraded. DC
Practical Realization of Process
T h e following steps were required to develop the separating process to technical maturity : (1) T h e whole separation process had to be minutely investigated in a pilot plant, and all important operating data had to be measured exactly. (2) T h e separation process had to be described mathematically, and the calculated results had to be compared with the empirical results obtained in the pilot plant. T h e mathematical expressions had then to be adjusted until agreement was obtained. T h e principle of butadiene separation in the main scrubbing stage is shown schematically in Figure 4. T h e gaseous C4 mixture is introduced in the center of the column, which is subdivided into scrubber and butene stripper. The descending stream of solvent preferentially absorbs the more readily soluble butadiene, whereas the butenes leave the head of the scrubber. 1,3-Butadiene, together with 1,2-butadiene and Cq acetylenes, which behave very similarly to butadiene in this section of the plant, is boiled off from the solvent.
Figure 4. Separation of a butene-butadiene mixture by countercurrent scrubbing
An amount equivalent to that in the feed mixture is withdrawn as crude butadiene, and the rest is returned as recycle gas to the stripper. The crude butadiene thus obtained is still contaminated with 1,2-butadiene and the acetylenes, from which it is freed in a completely analogous second stage. These two steps can be combined to save equipment. T h e flow diagram of a technical plant (Figure 5) shows that the second scrubber stage is operated as a side stream column of the main scrubbing stage. This diagram also shows the removal of propyne by distillation in the pure butadiene column. One compressor is required for a standard plant operating a t optimum economic efficiency ( 3 ) . Nevertheless, the number of compressors, the arrangement of the columns, and other design considerations in the separation process can be modified for given local conditions. A special computer program was designed for the separating process. With the aid of this program, the concentrations of all hydrocarbons a t the top and bottom of the columns and on each theoretical tray can be calculated. A feature of this calculation is equating the solubility of each individual component in terms of the pressure, temperature, and concentration of the other hydrocarbons in the solvent. The pilot plant had a monthly capacity of 15 tons of hydrocarbon feedstock, and the diameter of the column was 200 mm. A knowledge of the tray efficiency is required for the technical realization of a separation process employing mass transfer trays. At present, it cannot be calculated with a sufficiently high degree of reliability from the properties of the system, particularly under the conditions of extractive distillation or countercurrent gas scrubbing. Even extrapolation of results of mass exchange efficiency obtained with similar systems is unreliable. This applies particularly if high demands
Figure 5. B A S F process for recovery of butadiene. Flow diagram of a commercialplant Symbols: a, 0 vaporizer; 6 , scrubber; c, butene stripper; d , second scrubber; e, recycle gas compressor; fpdegaser with off-gas scrubber; g , pure butadiene distillatton; h, solvent heat exchanger
VOL. 6 2
NO. 4
APRIL 1 9 7 0
47
a r e imposed on the purity, in which case a large number of theoretical trays are required. For this reason, extensive experinients were carried out on a bubble-cap column with 10 practical trays. T h e relationship of tray efficiency to all important engineering and design parameters was investigated. T h e results were evaluated by the computer program mentioned above. Collating the tray efficiency and solubility model allowed correct design of the technical plant. T h e tray efficiency determined was 20 to 40%, which are reasonable
r Figure 6.
[m3 N TP/rn3cP] Cm/y
Tray eflcicncj-system
NMP/H20-C( hjdrocarbons
Symbols: 7, tray eficiency; C ,, concentralion of hydrocarbons in solvent; cosity of solvent-hydrogen mixture (cP)
TABLE IV.
Company
p,
dynamic vis-
PLANTS PRODUCING BUTADIENE BY BASF PROCESS
Site
Erdolchemie Cologne (Germany) IC1 Wilton (England) Industrialimport Pitesti (Rumania)
TABLE V.
Metric tons of butadiene per year 75,000 80,000 15,000
Taken onstream March 1968 June 1968 July 1969
BASF PROCESS FOR RECOVERY O F BUTADIENE
C1 crack fraction with ca. 45% wt butadiene Feedstock Product Butadiene of poly-czs quality Butadiene losses 3y0
Utilities per ton butadiene product 2.1 tons Steam 250 kWh Electricity Cooling water 150 m3 0 . 2 m3 Condensate ,V-Rlethylpyrrolidone < 0 . 2 kg Chemicals ca. 0 . 2 Dhl
48
figures for extractive distillation. According to our measurements, the tray efficiency increases with rise in temperature and thus with decrease in the viscosity of the liquid. I t also increases with the hidrocarbon content of the solvent. I n Figurc 6 the tray efficiencies, 7, calculated from the measurements, are presented as suggested by O’Connell (7)-z.e., they are plotted against the magnitude B C?,J,LL, where C, is the concentration of hydrocarbons in the solvent (m3KTP/m3) and p is the dynamic viscosity of the solvent-hydrocarbon mixture (cP). Within the limits of error--- z.e., f 15%-most of the measured points fall on the curve representing the serniempirical correlation formulated by O’Connell. T o discover methods of preventing fouling, the formation of polymers, obtained essentially frorn butadiene, was investigated during operation of the pilot plant. T h e deposits can choke columns and equipment and are particularly disturbing when they are formed in the heat exchangers. I n a special apparatus the rate of contamination of heat exchangers was measured under the operating conditions a t various parts of the plant. T h e successful avoidance of breakdowns in the plant was finally achieved and fouling of the heat exchangers was suppressed (eveii by crude untreated Cd fractions) by suitable design of the equipment in which polymerization occurred and in particular by discovering a very effective antifouling agent.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Results Obtained with Commercial Plant
T o date, three plants for the recovery of butadiene by the BASF process have been erected. T h e sites and capacities of these plants are listed in Table IV. T h e thrce plants were taken on-stream rapidly and ivithout any trouble. Within a short time, butadiene of rnarketable quality was being produced, and since then the plants have been operating without breakdown. All the results, in particular the high degree of purity of the butadiene product, fulfilled expectations. Fouling was so slight that all heat exchangers kept operating without change for more than one year. Other production units will be erected in the near future. By virtue of the beneficial properties of the selective solvent AY-methylpyrrolidone, the capital and utilities costs for the separation process are low. Typical values for utilities are given in Table V. Hence, the BASF process is a very economic means of producing butadiene of high quality from crude C4cracked fractions. RE FER ENCES (1) O’Connell, Trans. rimer. Inrt. Chern. En:., 42, 741-6 (1946). (2) Kirk, R., and Othrnrr, D., “Encyclopedia of Chemical Technology,” Vol. 3, 2nd ed, New York, 1964, p 784 A . (3) Klein, I f . , Hjdiocarboii Process. Petrol. Rejizer, 47 ( l l ) , 135-8 (1965). (4) Kronig, W., EtdoelKohle, 21, 140-8 (1965). ( 5 ) Natta, G., Italian Patent 364,723 (1938). (6) T h e Secrctariat of the International Rubber Study Group, Rubber Statistical Buiietzn, 2 2 ( l l ) , 2, 20-1 (1968). ( 7 ) Weitz, H. hi., tt‘agner, K., and Schmidt, 0. I%.,Chern.-lng.-Ttck.,32, 796-801 (1960). (8) KrBper, H., Weitz, H. hf., and Lt‘agner, I?., Pelrol. Rejner, 4 1 ( l l ) , 191-6 (1962). ( 9 ) Weitz, H. M., and Wagner, U., 6th World Petrol. Congr., €‘roc., June 19-26, 1963, Section IV, Paper 41. (10) KrBper, H., and LVeitz, H. M., Oil Gni J., 65 (Z), 98-104 (1967).
