Waste plant designed for tertiary treatment Economic evaluation leads to state-of-the-art plant using activated carbon
of computers, a scheme that will soon appear throughout the chemical processing industries. The next rung would be a refinerywide computer followed by another for all of the company's manufacturing. The top rung would be the corporate computer. Each successive rung in the hierarchy would feed its senior unit with an appropriate diet of facts and figures. Since the hierarchy would parallel the management structure of the company, each managerial level would have available those data necessary for its operation in a form and with a frequency that would be optimal. Skeleton. This idea of a computer hierarchy isn't as "blue-sky" as it might at first appear. According to Mr. Nelson, the skeleton for the hierarchy already exists at Sun Oil. Sun's economics department has already constructed a mathematical model of the entire corporation. The model enables the management to simulate a year's operation of the company in 14 seconds and is used to prepare budgets and for predicting the impact of alternate investment plans on the company as a whole. Sun has also developed a five-refinery model incorporating operations at Marcus Hook, Toledo, Ohio, Corpus Christi, Tex., and at Tulsa and Duncan, Okla., refineries. It is inevitable that industrial managements would question the value of DDC over conventional analog control. This is particularly true since apparent improvement on a unit-for-unit basis isn't really as great as many control people would like to claim. With conventional analog control, each loop requires a separate control channel and control scheme. A DDC system, however, can handle many control loops kwith a single central computer arrangement. Even more important are the added flexibility provided by DDC and the ability to adapt to unseen condi-
tions in the future without the necessity for redesigning the entire system. There are some practical drawbacks to DDC that make it less than perfect. Operating personnel must be upgraded to the mysterious world of software in a hard hat. As one old timer at Marcus Hook was overheard to say, "You can't just send a man out to diddle a set point on a controller any more." Operators as well as designers and managers must now adopt a genuine systems viewpoint. Although most of the DDC experience has been gained in petroleum refineries, other types of plants have made notable progress as well. The pioneering installation at Monsanto's ethylene p 1 o ^t in Texas City in 1962 is s o m e t h i n g s ! a milestone in control history. Another is the Dow-Foxboro joint project begun in 1965 at Midland, Mich. The Dow-Foxboro project employed DDC in a complex of a reactor, a gas stripping column, four distillation columns, and ancillary equipment where conventional analog control had already been established. A side-byside comparison was, thereby, possible. At the last notification the Dow-Foxboro project indicated that DDC was all that it was cracked up to be and the project might be extended. Probably the most significant thing about DDC is that it represents a bona fide approach to the unification of many diverse disciplines that had previously been labeled "systems design." Modeling of operating plants, both with physical pilots and mathematical approximations, now is not only possible but appears to be progressively more desirable as a means of designing plants and operating them. When the ability to integrate day-to-day nuts and bolts operating decisions into a managerial hierarchy is achieved, as Sun suggests, then indeed another milestone in plant engineering will have been reached.
Design of a tertiary waste water treatment plant using granular activated carbon and based on state-of-the-art technology has resulted from studies at M. W. Kellogg Co. and SwindellDressier Co., both divisions of Pullman, Inc. The plant is designed to handle 10 million gallons of secondary treatment (activated sludge) effluent per day and to produce a product water with chemical oxygen demand (COD) of 7 mg. per liter or less, turbidity of 3 Jackson turbidity units or less, and suspended solids concentration of less than 1 mg. per liter. Kellogg's A. E. Cover and SwindellDressler's C. D. Wood and J. F. Skelly provided details of the design and an economic evaluation of activated carbon treatment at the 67th national meeting of the American Institute of Chemical Engineers in Atlanta. The studies were carried out under contract to the Federal Water Pollution Control Administration ( FWPCA ).
In the process under study, feed water flows through two adsorbers in series, where organics in the water are adsorbed on activated carbon to provide the IOW-COD product water. The lead adsorber is periodically taken off stream for carbon regeneration, the second adsorber switched to the lead position, and another adsorber containing regenerated carbon brought on stream in the second position. Carbon to be regenerated is sent to a multiplehearth, Herreshoff-type regeneration furnace. Overflow water from the conveyor returns to settlers in the primary treatment plant. In an independent operation, the lead adsorber is backwashed whenever pressure drop builds up because of suspended solids on the top layer of the carbon. Base case. In deciding on a base case on which to carry out economic evaluations, Mr. Cover explains, the engineers chose a feed water containing 60 mg. per liter COD, 70% dissolved and 30% suspended. Other feed water characteristics were assumed to be the same as those at a pilot plant operated jointly by the Los Angeles County Sanitation Districts and FWPCA in Pomona, Calif. The Pullman engineers provided for a residence time of 50 minutes in two packed-bed carbon adsorbers with minimum bed depth-to-diameter ratios of 1. MARCH 2, 1970 C&EN 33
Tertiary treatment system uses activated carbon Backwash to primary' ^settlers
Adsorbers Effluent ; from secondary treatment i
Engineers project heat rejection requirements Call for new energy policy based on thrift rather than consumption
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Flue gas Dewatering screw conveyor
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On the basis of these and other assumptions, the engineers studied the economic effect of plant size; velocity; contact time; carbon loss, capacity, and cost; type of contactor; number of contacting stages; and certain combinations of these variables. Among their findings: • Comparing 1, 10, and 100 million gallon-per-day plants, investment charges and operating labor are substantially higher for the former, tvpical for small plants. However, ι ^^rction in investment charges going from 10 to 100 million gallons per day are not very great, since multiple equip ment trains are required for the larger plant. Total operating costs on a com parable basis run 30.80 cents per 1000 gallons for the 1 million gallonper-day plant, 10.21 cents for the in termediate plant, and 6.82 cents for the large plant. • Velocity has little effect on eco nomics, once it is above 7 gallons per minute per sq. ft., assuming velocity doesn't affect adsorber performance. If, however, velocity affects the re quired contact time, effect of velocity on economics is quite significant—op erating cost of 9.42 cents per 1000 gallons at 10 gallons per minute per sq. ft., compared to 12.66 cents per 1000 gallons at 4 gallons per minute per sq. ft. • Carbon capacity has a strong ef fect on operating cost but very little effect on investment. As capacity changes from 0.25 to 0.87 pound COD per pound carbon, operating cost 34
C&EN MARCH 2, 1970
Regenerated carbon slurry
drops 2 cents per 1000 gallons to 9.67 cents per 1000 gallons. • Based on a comparison of costs for one-, two-, three-, and four-stage sys tems, a two-stage system turns out to represent a true optimum number of contacting stages. Economies. Designing a state-ofthe-art plant, Mr. Wood points out, the engineers chose a two-train, twostage, pressurized vessel downflow ad sorption system with regeneration. Total cost of operating the 10 mil lion gallon-per-day plant is 12.6 cents per 1000 gallons. The plant, Mr. Wood says, hasn't been fully proven in practice, but he feels it offers econ omies over any other plant that has been successfully demonstrated. Costs for the plant are based on lo cation in the Gulf Coast area to estab lish environmental conditions, labor costs, and freight shipment rates. Prices of materials, equipment, and la bor are those of March 1969, and the 1969 City Cost Index for the location is 85. It is also assumed that one major contractor has overall responsibility. On this basis, fixed capital invest ment, including original carbon charge, is $2.1 million. Fixed operating costs—makeup carbon, power, fuel gas, and labor—come to 4.88 cents per 1000 gallons. Costs affected by policy decisions—amortization, maintenance and insurance—come to 7.74 cents per 1000 gallons. Amortization in this case is figured at 8.02% fixed capital investment per year for 20 years at 5 % interest.
"With projections of total energy re lease to the environment from the con terminous United States exceeding 190,000 trillion B.t.u. per year by the year 2000, with release rates in the Boston-Washington megalopolis projected to exceed 30% of the inci dent solar energy by the same year, with electrical consumption of a single new building in New York soon to ex ceed 80,000 kw., the time has come for a serious examination of national energy policy on a broad front/' Thus did R. T. Jaske, J. F. Fletcher, and K. R. Wise of Battelle Memorial Institute, Pacific Northwest Labora tory, Richland, Wash., sum up the im plications of an estimate by Battelle of public and industrial heat rejection re quirement through the year 2000. Their findings were presented at the 67th national meeting of the American Institute of Chemical Engineers in At lanta. With growing current awareness of the potential for thermal pollution from electric generating plants, there is a tendency to think of heat rejec tion as a cooling water problem limited to such plants. As sophistication of industry increases, Mr. Jaske points out, a growing portion of the total heat released to the environment will orig inate from electrical generation and use. But currently only about 20% of total environmental heat loads comes from this source. Doubling. On a per capita basis, the Battelle engineers project an over all doubling during the next 30 years of the gross consumption and release of energy. Per capita consumption of electrical energy will increase by a fac tor of 5.2. This growth, Mr. Jaske says, will come mainly from use of electricity to replace traditional heat sources in the conversion industries and for space heating. A considerable amount of energy release comes from private autos. Even conversion to a total electric automobile would only cut total energy release to the environment by a factor of 2, Mr. Jaske says. In electrical generation, he points out, 50% thermal efficiency appears a barely achievable goal at projected efficiency growths in the electrical industry under current energy policies. (Efficiency currently averages about 35%.)
