TECHNOLOGY
Process Models Can Reduce Pilot Planting American Oil uses process model approach to optimize isomerization process; model makes scale-up more reliable 141ST
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Petroleum Chemistry
American Oil, in searching for new techniques to shorten process development time and to reduce the high costs of collecting empirical data, has turned to the process model as one approach. The company has used the process model to develop an optimized process for liquid phase isomerization of pentanes and hexanes, says it minimizes the need for pilot plant experiments. According to R. E. Dudley and J. B. Malloy of American's R&D department, developing an optimized process with minimum cost is espe-
cially difficult in the petroleum industry because of complex feeds and reaction kinetics. A typical development project involves extensive pilot plant tests based on empirical programing and interpretation of results. Statistical design of experiments has been found useful in improving efficiency, but it doesn't tackle the basic problem of complex kinetics. A process model, based on a few controlling reactions, was developed to study reaction kinetics directly. The model interprets experimental results mathematically, and reveals interactions and effects of variables such as space velocity. It makes extrapolation and interpolation of data and scale-up more reliable. The process model also defines catalyst activity, Mr. Dudley says. In catalytic processes, the
DATA STUDIES. J. B. Malloy (left), and R. E. Dudley of American Oil discuss data observed with their experimental reactor (in background) designed by using a process model approach. The model was used to calculate optimum process conditions for liquid phase isomerization of pentanes and hexanes 70
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usually extensive interactions between major variables are revealed automatically by the model. The process model approach was highly successful in the isomerization development, both in optimizing process conditions and in designing the reactor. Isomerization is especially suited to this approach because the important reactions are relatively few and have been thoroughly studied. According to Mr. Dudley, process models can be useful for more involved processes. Even though the reactions are more numerous and more complicated, the major factors which determine product quality and yield are probably controlled by only a few critical reactions. Two Reactions. Isomerization takes place in the liquid phase, using an AICI3 catalyst dissolved in a liquid AlCl 3 -hydrocarbon complex. Two reactions control the economics of the process, Mr. Dudley says. These are the conversion of hexane to neohexane (2,2-dimethyl butane) and the cracking of hexanes. Pentanes isomerize more readily and crack less readily than hexanes, hence are not critical to the economics. Rate equations are determined for these hexane reactions. The model is developed most easily by applying these rate equations to a well-stirred reactor. Equations for performance of the reactor are then developed in terms of temperature, catalyst activity for isomerization and cracking, space velocity, equilibrium constants for the isomerization reactions, the number of reactor stages, and feed composition. The equations are solved using a computer. Plots of data that result show, for example, how selectivity will vary depending on catalyst activity, temperature, or space velocity. When inhibitors such as benzene, naphthenes, and hydrogen are removed to raise carbonium ion concentration, activity increases faster for cracking than
for isomerization, and liquid yield is unsatisfactory. Raising temperature and lowering space velocity give better yields. Mr. Dudley and Mr. Malloy confirmed their model in single-stage and four-stage continuous stirred reactors and in a batch reactor. Optimizing the Process. The engineers used the model to calculate optimum process conditions. For example, balancing incremental gain from, changing a variable against the cost of the change fixes the optimum. Raising temperature is the cheapest way to raise conversion, but near thermodynamic equilibrium yield losses offset conversion gains. Optimum approach varies from 94% of equilibrium in a single-stage reactor to 98% in a fourstage reactor. Four stages were found to be optimum. After optimizing the approach to equilibrium, the only way to get higher conversion is to trade space velocity or activity for lower temperature and more favorable equilibrium. Other calculations show the maximum cost that can be paid for lowering space velocity or raising activity. Reactor cost studies establish how far space velocity can be lowered without exceeding this cost. Pilot plant studies show how high A1C13 and HC1 concentrations can be raised without exceeding the maximum cost for increasing activity. Designing the Reactor. The model also helped solve reactor design problems. Design of baffles proved critical for high stage efficiency. Using the model, Mr. Dudley and Mr. Malloy were able to discriminate between changes caused by the baffles and changes caused by other variables, thus experiments on different baffles were interpreted. Mr. Dudley and Mr. Malloy worked out an optimum temperature pattern, which maximizes reaction rate in the first stage and makes equilibrium most favorable in the last stage. A pattern of 220° F., 170° F., and 150° F. gives a reactor effluent of 45% neohexane in one three-stage reactor. If all the stages are operated at 175° F., the optimum for a uniform temperature, neohexane concentration of the reactor effluent is only 1% lower. Since the difference is so small, and the high first-stage temperature may cause high AICI3 consumption, maintaining a uniform and lower temperature gives the best over-all operating conditions, according to Mr. Dudley.
Descaling Method Uses Circulating Resin Resin carried through evaporators with sea water would hold hardness below scale point, return it to brine by final effect Nalco Chemical Co., Chicago, has applied for a patent on a method of passing ion exchange resins through multiple effect evaporators to prevent scaling (C&EN, March 26, page 41). If the process were applied to sea water evaporation, the resin would emerge from the last evaporation effect already regenerated, could be recycled directly to the first effect after removing residual concentrate from it. Calcium and magnesium scale-formers would be dissolved in the concentrate from the last effect. With the method, from 5 to 10% of a spherical cation exchange resin, Nalcite HGR-W, is introduced into preheated, deaerated evaporator feed, softening it to a point where it forms no scale in the first effect. In the second effect, resin begins to regenerate as equilibrium gradually reverses with increased NaCl concentration. But with temperature dropping and NaCl concentration increasing, solubility of calcium increases and scaling doesn't occur. In the final effect—about 10% NaCl and 119° F.-all hardness removed before the first stage has been restored to the brine. As an example of internal use of ion exchange resins in evaporators, Nalco cites tests using a synthetic sea water that contained 420 p.p.m. of calcium and 1067 p.p.m. of magnesium. After equilibrium was established by repeated cycling, using 5% resin by volume, the resin was added to the first effect water, which was at 212° F. At this temperature, calcium concentration in the sea water dropped to 320 p.p.m., magnesium dropped to 915 p.p.m. This mixture was concentrated by evaporating and, at the same time, reducing pressure. Concentrated sea water in the final effect at 130° F. contained 1200 p.p.m. calcium and 3050 p.p.m. magnesium. Costs. Operating costs of a sea water evaporating method using internal ion exchange would depend almost entirely on the amount of resin lost through breakdown, according to Louis Wirth, Jr., manager of Nalco's ion exchange products. The Nalcite HGR-
W used by Nalco is one that the company believes will be suitable for such a job. Its density—1.2—makes it heavy enough to settle from concentrated sea water in a settling device, but not heavy enough to segregate while being pumped through the system. And beads are spherical, thus flow easily and resist attrition. The amount of resin circulated would depend on the required hardness in the input sea water. Nalco estimates the amount at from 5 to 10% resin by volume. Nalco anticipates using a 20 to 50 mesh size resin, but could go to a smaller one if attrition became a problem. Smaller resins do not break down as easily as larger ones, but they are more expensive. Nalco foresees some possible problems, but believes these can be handled with proper design. Open faced impeller pumps or a similar pump that is easy on the resin would probably be necessary to keep breakage down. The other requirement—a system to remove the resin from concentrated sea water—would have to be designed. This would probably include an overflow settler, where resin could collect in a steep cone, and debris associated with sea water could overflow. The resin could be drained free of surface brine for the return to the first stage evaporator. No wash would be required since only a minor amount of concentrate would be carried back with the resin, Nalco believes. One other possible problem, that of contamination of the circulating resin through some foreign ion like iron, would have to be evaluated by test. It might prove necessary to remove portions of the resin for external cleaning, or feed in an agent periodically to scavenge the contaminant.. The Nalcite HGR-W is made for Nalco by Dow Chemical at Midland. The two companies have long been associated in the field of ion exchange chemistry. Nalco has supplied Dow with distribution and application research on Dowex and Nalcite resins used in water conditioning. APRIL
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Sunshine, Fuel Combine for Desalination Combination is more economical than direct solar distillation, more costly than fossil-fueled way 141ST
ACS
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Water and Waste Chemistry
A process for saline water conversion that combines a solar heat collector with a multistage flash evaporator looks economically more attractive than direct solar distillation. However, in California this process is not now competitive with the production of potable water from the same evaporator in which brine is heated by a
fossil-fuel, Dr. Donat B. Brice of the Department of Water Resources, Sacramento, told the Symposium on Saline Water Conversion. The relatively low temperature output from a flat plate solar heater can be used more efficiently by a multistage flash evaporator than it can by most other known conversion processes. However, Dr. Brice says, a considerable improvement in glazing materials is needed before the reflection and reradiation losses can be reduced enough to make this a generally economical wav to heat sea water.
Flash Evaporator and Solar Heat Collector Would Perform Like This: (At a terminal temperature difference of 3°F. and the indicated brine temperature)
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Dr. Brice has estimated that during a 10-month operating year the average water cost from a solar heater-flash evaporator plant runs about $1.10 per 1000 gal. Dr. Brice based his estimates on a plant designed to produce 345 X 10 7 gal. of water per year. Such a plant would cost $26 million— 60r/r for the evaporator, 40% for the solar heat collector. The conversion economics were based on the average monthly insolation at a site along the southern California coast. About 12 billion B.t.u. of heat are needed to produce 10 million gal. of water per day. This could be supplied by a solar heater with about one square mile of collector. Water Pumped. Sea water is pumped through the tubes in a 20stage flash evaporator. It is gradually heated to 132° F. by the condensing vapors outside the tubes, then flows by gravity to the solar heater. This heater is similar to a large pond. It's lined with plastic to prevent loss of hot water into the ground. Unlike a solar still, which uses the sun to vaporize water, the heater retains in the salt solution as much of the incident solar energy as possible. Five layers of plastic, separated by air spaces, cover the pond to reduce the amount of absorbed heat that is lost by atmospheric convection. As the sea water flows across the pond it's heated further until it reaches the design temperature. The brine travels about 2 miles through the solar heat collector and takes about 36 hr. 4 to complete its flow. It's then returned to the shell side of the first stage of the evaporator vessel, which is under partial vacuum. Its temperature at this point is 140° F. Here the flashing process starts. Steam results from this flashing and condenses on tubes which are being cooled by the incoming sea water. Pumping the sea water through the evaporator thus serves two purposes. By separating the tanks into several compartments and operating each at a higher vacuum than the preceding one, it's possible to vaporize water in each compartment of the 20stage evaporator. This repeated evaporation results in energy reuse. The sea water flows through the plant at a rate of 174,000 gal. per min. Pure water is pumped to the distribution system at about 10,000 gal. per min., and about 164,000 gal. per min. is returned to the ocean as blowdown.
Eastman Clarifies
Autoprep Separates Mixed C6 to Cn n-Hydrocarbons Like This
Polyallomer Structure Company has applied for patents on composition of matter, catalyst, process Eastman Chemical Products, which first introduced the term "polyallomer" Several months ago, has now defined *he structure of this class of polymers. The company has also spelled out its expected patent coverage. Applications are in for composition of matter, catalyst system, and process. Polyallomer chains, Eastman explains, consist of polymerized linear segments of each of the monomers used. Monomers can be mono- or diolefinic hydrocarbons, acetylenic hydrocarbons, or vinyl monomers. A propylene-ethylene polyallomer, for example, is composed of alternating segments of polyethylene and polypropylene. Eastman is quick to point out, however, that polyallomers are not physical blends of homopolymers. And the terms "block polymer" and "copolymer" fall somewhat short of adequately describing the compounds. The company says that full-scale techniques for commercial production of polyallomers have been developed. It describes the process as medium pressure. Crystalline. The highly crystalline structure of polyallomers is due to the ong segments of homopolymer, according to Eastman. The company adds that the new materials can be tailored to overcome many of the deficiencies found in crystalline homopolymers. Propylene-ethylene polyallomer, for example, is said to have many of the best properties of both high density polyethylene and crystalline polypropylene. Melt flow, softening point, hardness, stress crack resistance, and mold shrinkage are better than for linear polyethylene, and the polyallomer has an annealed density of less than 0.91, compared to 0.96 for linear polyethylene, Eastman says. Propylene-ethylene polyallomers are the first members of the group to become commercially available and will be marketed in the company's Tenite line. Eastman expects that polyallomers of propylene with isoprene or butene-1 may also be added to the family before long. The materials will definitely compete in fields now served by other plastics, the company feels.
Sample size: 1.0 ml. Temperature programing: 70° to 200° C. Column: 3/8 in. xlO ft. SE-30
vAA. Time 0
5
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New Chromatograph Designed for Efficiency Wilkens' Autoprep uses narrow coiled columns, operates repetitively and automatically A compact preparative gas chromatograph, designed for efficiency, has been developed by Wilkens Instrument & Research, Walnut Creek, Calif. To get high efficiency, Wilkens has gone to narrow columns. It's possible, however, to prepare large amounts of material, since the unit automatically injects samples and collects fractions in repetitive runs. Wilkens calls its chromatograph the Aerograph Autoprep A-700. The unit will make either isothermal or temperature-programed runs, operates from 0° to 300° C. Columns are coiled 3 /s-in. aluminum tubes, 10 to 20 ft. long and packed with a 30% substrate on firebrick. The Autoprep costs $2885, without recorder. The collector consists of a 6-in. diameter rotating table which carries eight collector bottles with a waste position between each. At the beginning of a run the oven turns on, and the sample (which may range in size from 0.1 to 2.0 ml.) is automatically injected. The rotating collector table is at a waste position at this point, and a
precollection timer can be set to leave it there for 0 to 15 min. in order to eliminate solvents and other unwanted low boiling material. Table Rotates. As the pen on the recorder starts to move upward on the first peak, it produces a signal which causes the table to rotate and accurately indexes the first sample bottle under the heated collector orifice. When the recorder pen completes its trace of the first peak and nears the base line again, another signal rotates the table to the waste position. The automatic indexing of the table to alternate waste and collection positions continues until samples from as many as eight peaks have been collected. After collection of the last sample, the table indexes back to the precollection position, the oven door opens, and compressed air and a circulating fan rapidly cool the column oven for any period between 0 and 15 min. that has been set on the timer. At the end of this period the compressed air turns off, the oven door closes, the oven turns back on, and APRIL
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the sample is automatically injected to repeat the entire cycle. Collector bottles are made of glass, contain glass wool scrubbers to remove aerosol materials. The carrier gas and sample enter a center tube, where sample is condensed. Any aerosols, Wilkens explains, are removed as the gas passes out of the bottle. Centrifuging then recovers the material in the bottom of the bottle. According to the company, the design philosophy behind the new instrument has been to eliminate large inefficient columns and yet provide for collecting large amounts of material with the efficiency of an analytical column. For example, a run requireing 20 min. can repeat itself 72 times during 24 hrs. If the equipment were turned on at 5:00 P.M. on Friday afternoon, it would complete 200 runs by Monday morning. If each injection were set at 0.5 ml., 100 ml. of sample could have been separated into eight fractions.
Interest in Hydrocracking Leads to New Catalyst New performance data on other catalysts also aid hydrocracking 141ST
ACS
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MEETING
Petroleum Chemistry
A new catalyst and new data on other catalysts' performance for hydrocracking of petroleum fractions come from research aimed at improved petroleum products and lower cost refining. The catalyst work is one result of the renewed interest in hydrocracking, one of the oldest cracking processes. The new hydrocracking catalyst has been developed by chemical engineers at Esso Research Laboratories, Baton Rouge, La. It is a small percentage of a noble metal on a new type of support, according to A. Voorhies, Jr., who did not disclose other details of its composition. The new catalyst has good activity and good maintenance of activity in the presence of nitrogen compounds, Mr. Voorhies claims. Additional data on performance of presently available hydrocracking catalysts have been obtained by a group of chemists at the Socony Mobil 74
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Oil Research Department, Paulsboro, N.J. In general, when processing straight run gas oils, they found that platinum catalysts give more valuable products initially, but are less stable to regeneration than are cobalt-molybdena catalysts, W. E. Garwood says. Nickel-tungsten sulfides are intermediate in initial product value and in regeneration stability. Thus, if regenerations will be frequent, cobaltmolybdena catalysts seem better. Behind renewed interest in hydrocracking are economic factors: • Availability of low cost hydrogen as a by-product of catalytic reforming. • Availability of more active catalysts which permit lower operating pressures for hydrocracking, thus lowering equipment costs. • Efforts to increase feedstocks for catalytic cracking by upgrading some refinery streams such as catalytic cycle stocks and coker gas oils. Continuous. The new catalyst operates well in continuous service, Mr. Voorhies says. During the equivalent of four months' operating time, the catalyst lost a negligible amount of its activity with a constant feed of unrefined stock containing 40 p.p.m. of nitrogen. If the same feed were hydrocracked with a conventional catalyst such as nickel sulfide on silicaalumina and using the same pressure of 1500 p.s.i., the catalyst's useful life would be one to two weeks. If the nitrogen content of the feedstock is low or about 2 p.p.m., the new catalyst will operate satisfactorily around 800 p.s.i.—lower than conventional catalysts can tolerate, Mr. Voorhies says. The Socony Mobil group studied 11 hydrocracking catalysts for their stability of hydrocracking performance, hydrogenation-dehydrogenation activity, and acid activity during continued use. All of these properties decline during operations, but at different relative rates for different catalysts, Mr. Garwood says. Superior hydrocracking activity or high yield of liquid products for fuel uses with low yield of coke and gaseous by-products requires a minimum level of hydrogenation-dehydrogenation activity. Platinum catalysts have such minimum activity. But, Mr. Garwood and co-workers Dr. C. G. Myers, Dr. R. L. Wadlinger, B. W. Rope, and Dr. W. P. Hawthorne think that the platinum supports, silica-alumina or silica-zirconia, for example, which are acidic, are rate-controlling in the over-
all hydrocracking reaction. This is because the reaction sites of platinum supply olefin intermediates faster than acid sites crack them into gasoline. As hydrocracking performance declines with time, a decline in activity of acid sites is at fault for the short term; but over the long term, less hydrogenationdehydrogenation activity may become the major factor. Cobalt-molybdate catalysts do not have as large a hydrogenation-dehydrogenation activity as platinum catalysts, Mr. Garwood says. Nor do they supply olefins as fast as platinum catalysts. Thus the dehydrogenation reactions are rate-controlling. But, the relatively long-term stability of hydrogenation-dehydrogenation activity accounts for the long-term stability of cobalt-molybdate catalysts. In all types of hydrocracking catalysts, surface area relates to catalytic activity. As surface area decreasesdue most often to steaming of the catalyst by water formed from oxygencontaining compounds in the f e e d activity falls.
BRIEFS A patent on urethane elastomers has been issued to Mobay Chemical Co., Pittsburgh, Pa. The patent (U.S. 3,016,364) covers a process to make urethane elastomers with greater hardness and at lower cost than those made from other commonly used materials. Mobay uses diphenylmethane diisocyanate (MDI) and an aromatic chain extender, a bis-hydroxyethyl ether of hydroquinone.
A TRW-330 digital computer will control a high purity ethylene unit for Petroleum Chemicals, Inc., at Lake Charles, La. The computer will calculate material and energy balances, conversions, and yields. It will also scan instruments, give an alarm when variables reach abnormal levels, and log operating data. The system is flexible and can be expanded as needed.
Titanium trichloride technology and patent rights owned by Union Carbide Plastics Co. have been licensed to Pearsall Chemical Corp., Phillipsburg, N.J. Titanium trichloride, used pri-
marily as a polymerization catalyst for polypropylene, is thus added to the line of chloride catalysts made by Pearsall. The line now includes aluminum, ferric, and antimony chlorides. Pearsall will move UCM's production unit to Phillipsburg and expand it. NEW CHEMICALS Polar polymers for use as stationary phases of gas chromatographs are offered by Analytical Engineering Laboratories, Inc., Hamden, Conn. There are 58 compositions in Series R, giving variety in molecular weight and in polarity. Combinations of dibasic acids and diols are used to change polarity, and are useful in improving efficiencies in separation and identification, according to Analabs. C 1
Tris-(2-ethylhexyl)-phosphine oxide is
offered by Chemical Procurement Laboratories, Inc., College Point, N.Y., for use in the spectrophotometry determination of tin in various alloys. C 2
Maitlieson Small Cylinders
Bromoacetic acid is offered in pilot plant quantities by Michigan Chemical Corp., St. Louis, Mich. The acid is a white, crystalline solid that is potentially useful in agricultural chemicals. C3
Further useful information on keyed C h e m i c a l i t e m s m e n t i o n e d is r e a d i l y a v a i l a b l e . . .
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MATHESON GAS DATA BOOK Third Edition. Needed wherever gases are used or stored. $8.00 postpaid in the United States and Canada.
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