TECH NOLOGY
Goodyear Closes Loop on SBR Process Company expects analog computer system to boost production of reactor line 3 to 6% and improve product quality With about a month of closed loop operation under its belt, Goodyear Tire & Rubber finds that analog computer control at its Houston styrene-butadiene rubber plant is working "reasonably well." The company expects the computer, which is being used on one of six reactor lines, to increase productivity of the 90 million pound-per-year line by 3 to 6 # , pay for itself in one year (C&EN, May 15, page 5 1 ) . The plant is one of the first in the chemical industry to go in for analog computer control of an entire process. It uses a computer manufactured by Goodyear Aircraft Corp. Justification for the capital expense is based solely on expected productivity gains, but product quality is a major goal, too. Although either analog or digital computing techniques could be used on the process, the company chose analog for two main reasons, Goodyear's K. G. Roquemore and Goodyear Aircraft's E. E. Eddey told a joint meeting of the Chemical Institute of Canada's chemical engineering divi-
sion and the American Institute of Chemical Engineers in Cleveland. First, there isn't much need for memory or decision-making capabilities. In addition, the company felt that plant personnel would need less training for the analog than they would for a digital. The computer actually controls just the polymerization phase of the whole operation, which also includes materials preparation, recover)', and finishing. Initial studies, Mr. Roquemore says, showed that polymerization was the area most adaptable to computer control, also offered the best chance for improving over-all plant performance. In addition, polymerization has a big effect on the following operations and is the most difficult to stabilize.
Conversion Affects Quality.
Main
variable in the process is percentage conversion of monomer to rubber, and this variable affects both quality and productivity. Polymer processability depends to a large extent on average molecular weight and molecular
Analog Simulation Controls SBR Reactor Chain Using Forward, Backward Feed Conversion (density) from first group of reactors. 30% 20%" ^ • • • • • • • i H M a i a i
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Hours 54
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C&EN
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1961
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weight distribution. And average molecular weight, in turn, depends on conversion, since modifier is consumed during polymerization. Thus, better control of conversion leads to better product. Productivity gain comes from operating the process closer to the plant's limits without exceeding them. Conversion depends on catalyst concentration and reaction temperature, which is controlled by a cooling system. But temperature of conversion at a maximum deviation from average can't exceed the cooling system capacity. Another limit is recovery system capacity. Unreached monomer flow at the maximum deviation below an average conversion can't exceed this capacity. Thus, with closer control, deviation in either direction will be less, and productivity can be boosted. Simulate Pilot Plant. In arriving at a control concept for the process, Goodyear worked up a mathematical model by apphing a material balance along with relationships between reactivity, temperature, and density (a measure of conversion). These differential equations were then used to simulate a nine-reactor pilot plant on a general purpose analog computer. The simulated process divides the pilot plant into three groups of three reactors each. Simulated reactor transients compared with actual pilot plant reactor transients showed the mathematical model was within the probable error of pilot plant measurements. So Goodyear then simulated various control systems for the simulated process, finally arrived at a control concept using forward as well as backward feed of density information to adjust temperature set points. This generalized scheme was then adopted for production plant control. On-stream density measurement is critical for good control, Mr. Roquemore explains. Goodyear picked a gamma ray densitometer to make the measurement, redesigned the elec-
Tackability... Nevillac makes adhesives more tenacious Prominent among the attributes which make Nevillac, Neville Chemical Company's line of hydroxy resins, a valuable addition to adhesive formulation is its inherent tackifying ability. When correctly compounded, it improves surface adhesion, promotes quick initial grab and improves bond strength. It literally makes adhesives more tenacious. But there are other excellent reasons for including the Nevillac resins in your formulations. They are so highly compatible and soluble with a host of other materials that they act as an agent in bringing together other materials which are normally incompatible. This co-compatibilizing and
®
NEVILLE
natural solubilizing ability pertains to their use with most elastomers, plasticizers, and other resins. The Nevillac resins can lower the molten viscosity of many adhesive formulations and accentuate resistance to water, acids and alkali. They also possess generally good light stability, permanent thermoplasticity and ease of emulsification. If you formulate cold cut, hot melt, or latex blended adhesives, it will pay you to investigate further. Write for Technical Service Bulletins Number 72 and 75.
Neville Chemical Company, Pittsburgh 25, Pa.
Neville Products Resins—Coumarone-lndene, Hydrocarbon (Thermoplastic and Heat Reactive), Hydroxy • Oils—Plasticizing, Neutral, Rubber Reclaiming, Shingle Stain • SolventsAromatic (Refined and Crude), Semi-Aromatic (Refined and Crude). • Antioxidants—Non-Staining Rubber • High Purity lndene*lndene Derivatives*Crude Naphthalene.
tronic circuit to get accuracy to within x />2CU conversion. In operation, error signals feed back temperature changes to reactors upstream of each densitometer to correct the conversion error. Temperature signals also feed forward to cancel errors already generated. Another correction is also availablechanging the point at which short stop (to stop polymerization) is injected. Final conversion may still deviate from normal after temperature control limits have been reached. If this error hits 3 to 49r, short stop injection is automatically switched to another of three available points. Auxiliary Functions. The computer, Mr. Roquemore explains, has a number of auxiliary functions built in. These include: • Manual inputs for ratio control of butadiene, styrene, and soap solution flow volumes. Manual input of monomer purity analyses also adjusts the flows. And a manual input, master flow control dial controls the total flow of the three streams without changing the ratio. Readouts for flows and flow controllers are also located at the computer. • Readout and manual control for the set points of each of the computer controlled reactor temperature controllers. Also at the computer are readout and manual input for the percentage that conversion deviates from normal at any one of the densitometers. • Refrigeration load indication. To get the load, manual input of heat load from precooling input streams and losing heat to surroundings is added to flow, temperature, and conversion information already available to the computer. A number of safeguards are also built into the computer. All reactor temperature set points, for example, have certain limits. And alarms are designed to detect any computer system malfunction—"even the failure of a single resistor in a computing unit." If this happens, operation automatically transfers to conventional, manually set pneumatic controllers. Some of the instrumentation and process equipment had to be changed for computer control. For example, rotary pumps, adaptable to feedback control, replaced pulsating pumps, and blind pneumatic force balance controllers along with electric-to-pneumatic transducers were substituted in the temperature control system. 56
C&EN
MAY
2 2,
1961
UOP Extends Range of Molex Process Process makes product with up to 9 5 % Cio to C2 normal paraffins from kerosine and light gas oils Universal Oil Products has extended the range of its process for separating normal paraffins from hydrocarbon mixtures using molecular sieves. Tested in a continuous pilot plant, UOP's Molex process is now producing concentrates containing up to 95(/c C 10 to CL..j normal paraffins from kerosine and light gas oil fractions of petroleum. The process was offered originally as a method of separating straight chain paraffin hydrocarbons from other hydrocarbons in nonolefinic stocks boiling within the gasoline range (C&EN, March 30, 1959, page 39). In the Molex process, molecular sieves adsorb normal paraffins from the hydrocarbon mixture in the liquid phase. Operation is isothermal and continuous; feed and product streams enter and leave a bed of sieves at a constant rate and composition, UOP s Dr. D. B. Broughton told the 26th Midyear Meeting of the American Petroleum Institute's Division of Refining in Houston. Operating pressures are moderate, and temperatures do not exceed the atmospheric boiling points of the charge stocks. Charge stocks must be hydrorefined and low in olefins to present a rapid decline in effectiveness of the molecular sieves. Loss of sieve activity may be overcome by burning off adsorbed carbon. But, Dr. Broughton says, pilot plant experience shows that the process will operate with an onstream efficiency of about 9 0 r r . Ultimate sieve life is expected to exceed two years. Operating and overhead costs, excluding payout, will run 31 cents per bbl. of charge stock for a unit handling 4350 bbl. per day of either a kerosine or a gas oil feedstock containing 23 r r by volume straight chain molecules. This cost equals 58 cents per lb. of normal paraffin product. Such a unit would produce 850 bbl. per day of normal paraffins, says Dr. Broughton. Erection cost of a unit of this size, including initial needs of sieves and chemicals, would be about $2.5 million. Dr. Broughton estimates. As a tool to upgrade refinery streams, the process makes higher boil-
ing paraffins with several potential uses. When chlorinated, the paraffins could go into flameproofing agents, plasticizers, and lube oil additives. Dr. Broughton says that investigators have found that chlorinated waxes made from normal paraffins are more stable to heat and ultraviolet light than are conventional chlorinated waxes. The largest potential use for normal paraffins, Dr. Broughton says, may exist as a raw material for making the socalled "biologically soft" detergents. These detergents are destroyed by bacteria in streams. Detergents made with branched chain materials are "biologically hard" and persist indefinitely in streams and waterways, often causing foam problems.
Molex takes a
Gives a
light gas oil
product
with this
with this
distribution
distribution
% by volume
c9 Cio
c„ Li2
cl:l c„ C13 CK, C,7 ClS Cl9 V/20
C21 L>22
0.1 0.2 0.2 0.3 0.5 0.9 1.6 3.4 4.8 4.5 3.6 1.9 0.4 0.1
TOTAL 22.5
0.3 1.0 1.6 1.4 2.0 3.3 7.3 16.3 25.8 20.3 13.0
4.9 1.3 0.1 98.6
