I
L.
K. HERNDON and H. C. CLAFLIN,
Jr.
Olin Mathieson Chemical Corp., 460 Park Ave., New York,
N. Y.
High Energy Fuels
-
The chemical engineer has taken high energy fuels from the chemical laboratory and tamed them to the environs of an economic chemical process
A S has developed, there has been a conT H E H I G H E N E R G Y FUEL PROGRAM
tinual refinement of cost estimates as actual operating cost data are generated by facilities of increasing size. A far more realistic sense of facility costs and operating costs is possible from extrapolations of plants actually existing. Facility costs were initially estimated with considerable dependence on Chilton curves, which are well known to the cost estimating engineer. Special cost curves have been developed for certain unusual equipment identified with high energy fuel processes. Cost estimates of this type are brought to a common basis by means of the appropriate correction for the Engineering News Record Index. Quotations from local vendors, in the final analysis, is the only way to establish a firm basis for equipment costs. I n order to speak intelligently with a vendor the process design engineer must crystallize his needs and define the operating conditions required. Assuming the process design is “frozen,” an experienced cost estimator can predict installed equipment cost to within plus or minus 10%.
unit can be met by minor compromises of supply to other process areas with less pressing demands a t a particular time. T h e design engineer choosing equipment for a centralized refrigeration system may select the best equipment for the plant as a whole. T h e standardization on temperature levels of brine and the use of larger units permit appreciable cost reduction in both the original purchase of equipment and its subsequent operation. Less personnel are required to service the various pieces of refrigeration equipment housed in a common building. Also control instrumentation is simplified, and dependable supply-on-demand service is afforded. Deep refrigeration is obtained from liquid nitrogen which is vaporized to absorb its heat of vaporation a t temperatures dependent on the working pressure of the respective heat exchangers. Gas phase nitrogen formed in this manner is collected and employed as a n inert gas for purging the process equipment of undesirable hydrides and for blanketing highly reactive process chemicals to
Facility Costs Refrigeration. CeI tain process areas require appreciable refrigeration to prevent undesirable side reactions of intermediate boron hydrides. For instance diborane is a gas comparable to ethane in volatility and boiling point, but diborane is very different in that a t elevated temperatures it can generate copious amounts of hydrogen upon decomposition. When such reaction occurs, highly pyrophoric by-products are formed which constitute a serious fire hazard to plant and personnel. As refrigeration is required in several process areas and a t different temperature levels down to liquid nitrogen temperature, central refrigeration is employed almost exclusively. Principal advantages afforded by central refrigeration are : plant-wide flexibility, greater dependability, lower facility cost, and lower operating cost. By treating refrigeration as a common utility and pumping brine from a central compressor plant. the flexibility of the system is greatly enhanced in contrast to isolated independent units serving separate process areas. A temporary overload in one area or complete failure of a
process 8
PLAN1 CAPACITY, tonr/day
Advancing technology leads to lower boron-based high energy fuel costs Both facility and production costs are reduced for given size plants
avoid contact with air or moisture. A small liquid nitrogen plant costing approximately $1.OOO,OOO is employed in the Air Force high energy fuel plant. Ninety-five per cent gaseous oxygen is provided from the same air fractionating equipment for use as a process chemical. Expedient use of this oxygen will permit more production capacity at a lower cost from the operation, thus justifying a simultaneous oxygen credit to the nitrogen facility. Decontamination. Decontamination of process equipment containing air sensirive hydrides is a major problem. Plugging of process lines is minimized by vigilant observance of pressure drop across the equipment so that accumulated solids may be flushed free Lvith solvents before complete blockage occurs. Eventually the piping must be broken into, however. This is done only at the considerable expense of long and careful decontamination procedures, first by solvent action where possible and then by passing steam and water through the equipment to react thoroughly with residual solids. Hot oil heat transfer media are employed in many areas where highly reactive hydrides or alkali metal must be heated. Such indirect techniques require appreciably more costly facilities than simple steam or hot water systems. Outdoor Construction. Insofar as possible, outdoor construction is utilized for high energy fuel plants. Especially in the smaller plants, many areas are exposed to the elements a t least on one side or sometimes on all four sides. I n addition to the obvious cost advantages, such open or partially open construction has other very important benefits. The boron-based fuels and their intermediates are extremely toxic. Highly ventilated areas serve to dissipate any escaping gasesand prevent accumulations. Personnel are required to wear gas masks in hazardous areas. Even disregarding these reasons, a good case can be made for outside location of many types of equipment, particularly fractionation equipment Process investigations have shown that distillation throughputs and yields arr higher with a low ambient temperature. Blast walls are common practice for smaller compact research facilities to provide the protection from fire and VOL. 51, NO. 9
SEPTEMBER 1959
991
explosion. Larger production facilities depend upon excellent ventilation afforded in part by external location of critical operations. The process areas are also isolated considerably to reduce the effect of fire upon one another. Centralized Instrumentation. Central control rooms are becoming a n integral part of many new plants, especially those where hazardous materials are processed. All high energy fuel plants are built with central control rooms in order to provide immediate, effective action in the event of an emergency. Under normal conditions, one control room operator can oversee an entire process area and the foremen and process engineers can control operations as necessary. Such a setup provides a more dependable and economical control system than scattered local control setups, will require less maintenance, and also provide an excellent means for training of new personnel. High Energy Fuel Facility Costs Summarized T h e major process equipment accounts for 27% of the total plant cost. Factors commonly used for estimating total plant costs based on FOB process equipment cost range from a value of three for solid process plants to nearly five for fluid process plants. T h e factor of four obtained by dividing 27 into 10@70 seems to confirm the usual estimating practice. I n actuality, the high energy fuel processing plants must deal with both fluid and solid operations in addition to combinations of fluids and solids. The FOB equipment cost comprises roughly 85% of installed equipment cost. This is somewhat higher than the 70% frequently assumed by chemical plant estimators. O n the other hand, piping costs for the plant as a whole amount to only 27% of the installed equipment cost rather than 50% or higher for fluid plants. Obviously process piping costs for certain process areas are much higher and in fact exceed 70% in some areas involving fractionation for purifying the final products. Engineering costs for high energy fuel facilities are excessive, owing in part to undeveloped design. As experience is gained in constructing these plants and the design becomes stabilized, engineering and design of future plants will cost proportionately less. Many safety features and certain overdesign in critical areas have been incorporated to ensure production capability. T h e effect of advancing technology on high energy fuel facility costs is depicted in the top of the figure for several hypothetical fuel processes. T h e various alternate routes represent different degrees of processing simplicity. Process .4 might be considered as a present-day process and the present capacity if far to the left on the very steeply sloped
992
portion of the curve. As the state of the art progresses, it will be possible to drop sharply to process B and even to a very direct route, process C, which has but two or three very simple steps and begins with a very simple form of starting material. In addition to lower facility costs a t low production capacities, the more straightforward process C should permit continued savings potential with ever-increasing size more readily than the complex process .4,whose ultimate economy is limited by inherently highcost process equipment. Scale-up of processes B and C \+ill be much more rewarding from the standpoint of the greater simplicity of equipment and its susceptibility to enlargement rather than duplication in kind and size. If by doubling the size of a given piece of equipment the production may be doubled, the cost of the equipment is roughly (2)0.fior but one and one-half times the cost of the original. If on the other hand the well-known 0.6 factor is not appropriate owing to processing limitations. two units at essentially twice the cost of a single unit may be required to double the capacity. According to a popular correlation sometimes used as a yardstick, the facility cost must not exceed the market value of the annual production. !Vide divergence from this concept prevails for the high energy fuel facilities constructed to date. Smaller facilities utilizing expensive process chemicals produced in other plants as starting materials turn out to cost only a fraction of the value of their annual production. This is true because the present starting materials for these high energy fuel facilities are rather high-priced specialt); chemicals. The cost of the larger? completely integrated high energy fuel facility is three times the annual production cost of the product. To make a more comparable case relating facility to the sales value of the annual production, the latter total annual production cost should be doubled to account for depreciation, taxes, insurance, general and administrative expenses, and profit. Thus the ratio of facility cost of the integrated high energy fuel plant is one and one-half times the fair sales value of the annual production even though the production cost is reduced to 20 or even as loiv as i%,compared to much smaller plants. Production Costs The chief components of the cost for producing any chemical are: raw materials, utilities, labor, repair and maintenance, and overhead items including taxes, insurance, and depreciation. Ultimately, raw material cost is likely to be the most important cost factor. As the production capacity of a high energy fuel product is increased to permit economic recycle of chemical intermediates and economic treatment of a very
INDUSTRIAL AND ENGINEERING CHEMISTRY
basic form of starting material, the ra\v material costs for producing a pound of fuel are drastically reduced. As utilities, labor, and maintenance costs tend to reduce a t a n excclerating rate \vith increasing production, the effect of raw materials cost is predominant a t high levels of production. T h e relative costs of three hypothetical process schemes a t varying plant sizes are shown a t the bottom of the figure. The sharp change in slope of each curve is due to the production rate. which permits economic production of captive raw materials and intermediates with complete integration of process steps. For example, solvents would be synthesized rather than purchased, byproducts would be recycled, and process equipment would be justified to utilize very cheap basic starting materials, such as the ore. Subsequent reductions in cost for still larger size operations would be anticipated to result from slightly lower utility rates, freight rates, savings in labor, and overhead. Minimum Production Cost. For a given plant size there is usually a n optimum rate of operation which will yield a maximum value of product per unit time. I n commercial operation this need not be the rate which provides the lowest production cost per unit of product but rather is usually a somewhat higher rate with a correspondingly higher unit cost and greatly increased total value. If production capacity is forced to higher levels, increased costs may be incurred by overtime pay, increased maintenance, and lower yields. Also, for a given process there is an optimum size beyond which it is more economic to build a n alternate facility a t a separate location rather than to build a single plant of larger size. The figure (bottom) shows production cost curves for high energy fuel production as a function of plant capacity. There is an optimum size plant for each process. The optimum size becomes greater, however, as the process itself becomes simpler and more economic to operate. Hence for the three processes A, B, and C, which actually represent the technological advance in the art of making high energy fuel products, there are three minimum points which form a series of descending production cost minima as processing experience and state of the art progress. This experience is portrayed by the ever increasing number of large-scale industrial operations in attempts t o meet the challenge of competition. RECEIVED for review April 17, 1959 ACCEPTED April 27, 1959 Division of Industrial and Engineering Chemistry, Symposium on Plant Costs and Economics of the Chemical Process Industry, 135th Meeting, ACS, Boston, Mass., April 1959.
