J. J. QUI",
J R .
THE ECONOMICS OF S P R A Y DRYING Critical costs analysis should increase the Process engineer's acceptance
OJ' spray drying,
even f o r the conventional drying problems. he technique of spray drying has bee11 fully described
Tin the technical literature and is accepted as a standard drying method in unit operations courses. Still, the chemical process engineer, faced with a drying problem, is often hesitant to investigate, or even consider, spray drying as a possible solution. I n adopting this attitude, he may conclude, without proper justification, that a spray drying installation will require a greater investment than some alternate, and that spray drying is extremely inefficient, resulting in high operating costs. He may further conclude that, while spray drying is ideal for those applications where specific product characteristics are required and where operating economy and process efficiency are not prime considerations, it is too exotic for most processing applications where traditional drying methods are accepted. Failure to consider spray drying, however, may result in the engineer's missing an opportunity to obtain not only a profitable process but also one which can provide additional dividends in collateral advantages and improved product quality (7). The purpose of this discussion is an examination of the traditional concepts of drying economics in the light of recent technical and economic developments. initial investment
O n the basis of initial equipment costs, spray drying has substantially improved its competitive position in the past 10 years. The improvement results from: -the development of standard pilot and semi-works dryers which are not subject to custom design costs and can, thus, be made available at low prices. Ten years ago there was no standardization of design for equipment larger that laboratory models. -general simplification of overall design, including ancillary equipment. This simplificatioli Pkfers to air heaters, product recovery systems, and atomization methods. -the trend to high capacity units which results in greater design confidence. The trend has encouraged still larger installations with even lower costs per unit of production.
As a result of these factors the normal increase in operating and materials costs during the past decade has been partially offset, and increases in equipment costs have been less that for other types of processing equipment.
Efficiency
The misconception that spray drying is, inherently, inefficient is probably rooted in the idea that it is suitable only for true solutions, slurries of low solids contents, or low viscosity materials. As practiced today, spray drying is not only efficient but also economical. There are four areas in which progress has contributed to the efficiency of spray drying. By new developments in atomization techniques and in the handling of viscous feeds, and by the successful use of deflocculants and dispersants in the feed, it is now possible to spray dry materials previously considered impossible. We are now able to accommodate, certain thixotropic press cakes containing as much as 70% solids. I n the usual spray drying installation there are only two moving parts, the main circulating fan and the energy source for atomization, usually either a high pressure pump or a centrifugal disc. All maintenance costs are centered in these two items. I n most cases maintenance costs are low and the technique naturally lends itself to continuous operation. The simplicity of spray drying systems invariably results in low labor costs. We shall consider this in greater detail later. By the very nature of the process, it has always been possible to operate a spray dryer at much higher inlet temperatures than with conventional equipment where the product comes in contact with heated metal surfaces. This characteristic has been further exploited in recent years and has resulted in more efficient heat utilization and lower fuel costs. Inorganic materials are handled in spray dryers with inlet temperatures as high as 1200 F. and even certain organics can be subjected to inlet temperatures u p to 700" F. The traditional approach to drying is to place great emphasis on the thermal efficiency of the process, and to choose one process over another on the basis of comparative fuel requirements. However, the present availability of low cost fuels, and increases in labor costs have changed the criterion for selection, even in considerations of mechanical dewatering versus thermal drying. I n certain areas of the United States, it is now possible to obtain low cost natural gas which brings heating costs below 30 cents per million B.t.u.'s. I n other applications, where whiteness or brightness are not prime conVOL. 5 7
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35
siderations, air heaters can now be provided which operate on YO.6 fuel oil at a considerable saving in fuel costs over other methods of heating.
T o better understand the effect of all these components on the total operating costs, a set of theoretical costs has been computed for five different capacities from 500 to 12,000 lbs. 'hr. The assumptions made in arriving at these costs are: -amortization will be 217, of the total installed cost -fuel will be available at 50 cents [million B.t.u's -electrical power is available at 11!4 cents!kwh -labor is available at $3.00 'hr. -the unit will operate for 6000 hr. 'yr. -the equipment will be standard in design and will have a direct fired heater using natural gas, an atmospheric system with centrifugal atomization, a simple product recovery system, and all contact parts of both liquid and solid product will be of Type-304 stainless steel. Figure 1 shows the hourly operating cost, including amortization, fuel, power, and labor for three temperature differences. Although different cost factors were used here, the curves are similar to those reported elsewhere (2). The data indicate that costs will vary from about 1 cent'lb. of water evaporated at a capacity of 500 lb./hr. to about 0.25 cent/lb. at a capacity of 12,000 lb./hr,, with a significant drop in operating costs at a capacity of 2000 lb./hr. I t is interesting to note that,
in terms of total operating costs, differences in the temperature increments are not a significant factor in determining operating costs. I n Figure 2 the curve representing a 500" F. temperature increment has been reproduced as a 50% solids line for an inlet temperature of 700' F. and an exit temperature of 200" F. The cost per pound of product, rather than cost per pound of water evaporated, was also used. From this plot it can be seen that there is a much more significant selection criterion than the temperature increment. Interesting techniques have been developed for using deflocculants and dispersants, and for handling thixotropic filter cakes without additives. With the application of these techniques, it is possible to handle materials at SOYG solids and higher. Xote that changes in concentration make much more significant cost fluctuations than do changes in temperature. Even at concentrations as low as 20yG, we can still spray dry at a cost of one cent or less per pound of product at capacities above 5500 lb./lir. At the high solids concentration of 65%, and a capacity of 12,000 lb./hr., we are refering to a cost of 0.1 cent/lb. of product. If the temperature increment were 800" F., the cost would be even less. T o clearly understand the significance of the sharp break in the curves representing total operating costs after capacities of 2000 lb.,'hr. have been reached, we must consider the component costs. Labor is one of the significant factors. Generally, it is recommended that only one man be employed to operate the dryer, regardless of size. I n a small installation the operator is able to also handle the packaging of the product and a certain amount of processing upstream from the dryer. I n all these cases the labor cost was assumed to be $3.00,/hr. and only at the lower capacity was this a significant portion of the total cost. The labor component of the total cost becomes insignificant as the size of the drying installation increases. This can be seen in Table I, in which the fuel cost becomes progressively a greater fraction of the total cost at higher capacities. If the actual cost of the fuel were doubled to $l.OO/million B.t.u., it would amount to about z / 3 of the total operating costs at the high capacities, Yet, at the higher temperature increments, it
Figure I ( l e f t ) . Hourly operating cost for hjpothetical spray dryer. A'umbers on the grid refer to the temperaiure diyerences present in
the drjer. Figure 2 ( r i g h t ) . Houri) drying cost for the sume drjer as a function of evaporative capacit?
Operating Costs
Spray drying will always have a low labor factor and a low maintenance requirement. Consequently, savings in labor handling, and maintenance costs can offset any advantage an alternative process might have in therinal efficiency. SVith the greater availability of low cost fuels, the factor of tlzermal efficiency becomes even less important than other factors of the total operating costs. There is also a certain compounding of the improvement and economy in the past decade. Two of the components of operating costs just described, viz., high solids feeds and high temperature differences within the dryer, in turn, result in smaller units for a given capacity. This results in smaller amortization charges. cost Analysis
2.4
t3 3 0
2.0
'
1.6
ZE! 03 -g 1.2 W IAZ
Z O
z'
0.8
L
k2 0.4
5
d CI -1
x o 36
2
4
6
8
10
12
EVAPORATIVE CAPACITY, THOUSANDS OF POUNDS PER HR.
INDUSTRIAL AND ENGINEE,RING CHEMISTRY
0
TABLE I.
HOURLY FUEL COST AS A PERCENTAGE O F TOTAL HOURLY COSTS
Temperature Increment ’ F.
Evafiorative Capacity 500 lb./hr.
500
12.5% 8.2%
800
9.4%
200
I
I j
12,000 lb./hr.
47.5% 47.5%
200
I
25.6%
29.401,
50.0%
would be still less than 0.3 cent per lb., total cost including amortization, power, labor, and fuel. Table I1 indicates that amortization, surprisingly, stays below 30Ye of total operating costs, regardless of the capacity of the equipment or the temperature range. Assume that amortization charges are doubled. At the higher capacity, and temperature increment, the total cost would be increased by 22%. But even in this case the total amortization cost would only be about ’ / a of the total cost. Whatever the relationship to other types of equipment, this would seem to minimize the validity of our initial statement that spray drying equipment is inclined to be high in first cost. Conclusions
The economy of spray drying increases as capacity increases and we attribute the increase to three factors: the labor requirement is little changed whatever the size of the system, thereby decreasing the cost per pound as capacity increases; high capacity spray dryers are usually built as single units, thus minimizing the rate of increase of first cost and amortization as capacity increases; the advantages of continuous operation become more apparent at the high capacities. When evaluating spray dryers in relation to other types of equipment, there is a criticaI capacity for each application. Therefore, favorable application of spray drying at 10,000 lb./hr. might be unfavorable in other equipment at 1000 lb./hr. The exact location of this critical point varies for each type of application but the efforts of industry in the past decade have lowered the unit capacity at which the critical point occurs. To make the cost data more readily understandable, we have simplified to some extent the design and arrangement of the hypothetical installation. However, as shown in Table 11, even doubling the capital cost would not materially affect the overall economics. The equipment assumed was standard without unusual collection equipment. Using combination burners to fire either No. 6 oil or natural gas would increase the capital cost by about 10% but would probably be more than offset by the decrease in operating costs due to less expensive fuel. For some applications the products of combustion are harmful to the dried product and indirect heating
J . J . Quinn, Jr., is Vice President and Sales Manager of Bowen Engineering, Inc., specialists in spray drying equipment. AUTHOR
TABLE 11. HOURLY A M O R T I Z A T I O N COST AS A PERCENTAGE O F TOTAL HOURLY COSTS
would be required. When the inlet temperature can be obtained from steam coils at the available pressure, this presents no problem and capital cost would be no greater than that for direct fired equipment. However, for higher inlet temperatures there is a substantial increase in capital and operating costs alike. The increase would be even greater if temperatures in excess of 600” F. are desired. Another factor having a great effect on the cost for spray dryer installations is the type of product collection system used. I n the examples, a single bank of high efficiency cyclone collectors is indicated with no secondary collectors. However, where cloth bag collectors are indicated, or with stainless steel construction, the cost of the collector may approach 50% of the dryer installation. If bag life is short, the operating cost a t the exit end of the system may be a major factor. I n several processes, spray drying can eliminate other processes both before and after drying. The spray drying of ceramic bodies, for use in automatic pressing of electronic components, illustrates the point. The traditional method required eight separate operations with considerable handling between each. These operations included ball milling, blunging, filtering, tray drying, grinding, classifying, milling, and the final pressing. The use of spray drying eliminated the need for all but ball milling, blunging, drying, and pressing. I n such a situation the savings in the drying operation itself are dwarfed by the savings in the elimination of the unneeded equipment. There are other situations where the inherent advantage of the spray dryer render even the overall operating costs insignificant. I n one instance, the control of product quality with spray drying was so great that very large savings were realized by the elimination of rejects. The extent of these fringe benefits will depend on specific applications and no equipment manufacturer is in a position to single out the advantages of spray drying. Each processor must analyze his own situation and depend on his own judgment. The figures presented here demonstrate that spray drying is not necessarily expensive and is competitive with other forms of drying, particularly at the higher capacities. However, when consideration is given to only some of the cost components, the restricted economic view isn’t always favorable to spray drying. REFERENCES (1) Belcher D W. Smith, D. A,, Cooke, E . M.,Chem. Eng. 70 (20), 8 3 (1963); 70 ( Z l ) , $.01’(19L3), (2) Marshall W R. Jr. “Atomization and Spray Drying,” Chem. Eng. Progr. Monopraph’Ser, ’50, k o . 1954.
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