Design for high purity oxygen absorption and nitrogen stripping for fish culture

Aquacultural Engineering 7 (1988) 201-210

Design for High Purity Oxygen Absorption and Nitrogen Stripping for Fish Culture R. E. Speece, N. Nirmalakhandan and Young Lee I)rcxcl University, Philadelphia, PA 19104, USA (Received 1 June 1987: accepted 21 September 1987)

A BS TRA ( ' 7 H~q,h tmrio oxygetz is economicalh, ]ustifiable for enhancement of dissoh'ed oA3'gen (1)O) in fish hatchepv watetw. Since 0.14 kg (0"3 lb) of ox31~en is required for the production o1"0"45 kg (1 lb) offish weight gain, the added cost is about 504)4 kg / (50"02 lb J) offish production. This is negligihh" compared to the htbot/r and amortization costs of conventional hatche O,,[acililies which amotmt to 53"30 to 54"40 kg i (S1"50 to 52"00 lh l). Hatchery production could therefore be doubled itz an existing (acilio,, without capital expansion, for a tz,latively minor cost if the DO in the hatchei 3, water was SUpl~lemented above air saturation concentrations with high l~ttrio ' oxygen, ht addition, troublesome dissoh'ed nitroge~t couhl he simultaiwouslv stripped during this o'pe of oaten addition. Maintemmce of high DO aho offers the pote~ltial for more efficient feed com'er~i(m, reduced disease threat, and to[era/ice o[" higher ammonia coHcentrations altd teml)erature, t;i[]icient oxygett absorption is imperative to good hatchely production. The [ollowin g design procedures will be presented in the l'otvH of a ttomogral)h offering art economical ahernative to l)resettt techniques.

INTRODUCTION In the natural habitat, fish live, under optimum conditions, in saturated water containing approximately 10"5 mg liter l dissolved oxygen (DO) and 18 mg liter i dissolved nitrogen (DN) at 13°C and sea level (Sawyer and McCarty, 1967). Thus, in hatchery fish culture, it became 'conventional wisdom' to attempt to maintain saturated water at the inlet. This has been c o m m o n practice, but there is a severe restraint associated with it. In trout production, it is desirable to maintain a minimum of 6 mg 201 Aquacuhural Engineering 0144-8609/88/S03.50 - © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain

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R. E. Speece, N. Nirma&khandan, Y. Lee

liter-J DO. This results in only about 4-5 mg liter- 1 DO being available for fish production. Two factors are commonly present. First, there is the pressure from management for increased hatchery production. This puts a severe strain on DO resources. Secondly, there is often a problem with slightly supersaturated DN concentrations in the source water. This is particularly a problem with well supplies, but is also a problem with surface supplies which go through a diurnal temperature cycle. Logically, this scenario has led the senior author for the last 18 years to advocate the use of high purity oxygen in fish culture. Use of high purity oxygen would provide elevated DO levels in the hatchery water. An increase of just 4.5 mg liter- ~ DO above air saturation values would approximately double the fish carrying capacity of a fixed water supply (Speece, 1981). An integrally related feature of the absorption of high purity oxygen into the water is that DN can be simultaneously stripped, whereas when DN is stripped in a vacuum degasifier, DO is also stripped. If DN is to be stripped, some gas must be wasted from the absorber, and if gas is wasted from the absorber, some of the oxygen must be wasted also, precluding the possibility or desirability of achieving 100% oxygen absorption efficiency. Realistically 75 to 90% efficiency can be realized, however. Thus, enhanced DO concentrations and reduced DN concentrations are realized when high purity oxygen is absorbed in fish hatchery water. In order to capitalize on the double advantage of using high purity oxygen in hatchery fish culture, two criteria need to be met. First, the price of the high purity oxygen must be reasonable. Second, the technology for efficient absorption of the high purity oxygen into the water must be available. To address the first criterion of economics, the oxygen requirement for fish production is 0-3 kg 02 kg-J (0"3 lb O, lb ~) fish weight gain (Speece, 1973). In addition, the cost of on-site high purity oxygen production using the modular pulsed swing adsorption system (PSA) commercially available and used by auto body shops for on-site oxygen production to replace bottled oxygen is approximately S0"I 1 kg J (S0"05 lb-~). (Xorbox and Buderhus make such units in the US and Europe, respectively.) About half of this cost is comprised of amortization and the remaining half is electrical operating costs. Thus, the added cost would be about S0"04 kg- 1 ( 8 0 " 0 2 Ib- 1) offish production if 90% oxygen absorption is achieved -- a relatively small cost when one considers that the trout production is nominally $3-30 kg-~ (S1-50 lb-~ at State and federal hatcheries when amortization of the facility is included with labor and feed costs. Thus, the cost of using high purity oxygen in hatchery fish

Oxygen absorption and nitrogen stripping fi~rfish culture

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culture is minimal. Monsanto markets a membrane oxygen enrichment system whose capital costs are three to four times that of the PSA systems, but whose electrical consumption is about one-third of that for PSA systems. The topic of this paper addresses the technology available and design of appropriate oxygen absorption/nitrogen stripping facilities for hatchery fish culture. Oxygen, even high purity oxygen, is relatively insoluble. Thus, the design for the absorption of gaseous oxygen in aquaculture systems is therefore a subject of concern for all hatchery managers considering the use of high purity oxygen absorption/nitrogen stripping. Elimination of DN supersaturation was a major consideration in Michigan's decision to install high purity oxygen units in their State fish hatcheries (H. Westers, pers. comm., 1984). Care needs to be exercised in the proper selection, design and operation of the oxygen absorption system. Two questions must be addressed: 1. How can efficient oxygen absorption be achieved and what effluent DO concentration is predicted? 2. What DN stripping will occur simultaneously with the DO addition.'? It is readily possible to design for efficient oxygen absorption, but in order to avoid 're-inventing the wheel' the abundance of information in the chemical engineering literature must be exploited as in the case of the nomograph included in the paper. OBJECTIVE This paper will describe the design of counter-current flow packed columns (Fig. 1) for absorption of pure oxygen and stripping of DN. Counter-current flow packed columns are efficient, have abundant applications in chemical engineering processes and have been very thoroughly investigated and modeled. Unfortunately, it is only suitable for use with virgin hatchery water, because excessive maintenance is required to remove the troublesome slime growth which develops on the packing with "used" hatchery water. DESIGN OPTIONS The relative insolubility of oxygen in water limits the types of oxygen absorption systems which can be effectively utilized. Since any oxygen absorption system using high purity oxygen can simultaneously strip DN,

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Oxygen absorption and nitrogen stripping for fish culture

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this counter-current flow scheme, the water exiting the column contacts the highest oxygen composition possible in the absorber effluent, and the most effective stripping of DN occurs. The outer monolayer of water molecules in a droplet of water is saturated within 10- 7 s after the droplet forms. Therefore, new surfaces must be continually formed to achieve effective oxygen absorption. This is accomplished by filling the column with packing which continually creates new surfaces. (An inclined surface is a poor oxygen absorber, but a free-fall cascade is much better.) In summary, a counter-current flow packed column is the most effective system for absorbing high purity oxygen into water because it maximizes the DO deficit ( C~,t - C~ct) and maximizes the rate of formation of air/water surface area ( KLa ). DO/time = KLa ( C~at - Cact). The design of a counter-current flow packed oxygen absorption column is rather complex because the gas phase is continually changing due to simultaneous absorption of oxygen from the gas phase and stripping of nitrogen from the liquid phase. Consequently, it requires a computer program with an iterative solution. The development of the design equations and the solution procedure have been detailed elsewhere (Nirmalakhandan et al., 1988). We have used such a program to construct a homograph (Fig. 2) which can be used to design a countercurrent flow packed column oxygen absorption system. This nomograph greatly simplifies the design procedure without sacrifice in the design rigor and can be used by hatchery personnel. The procedure for use of this nomograph is described in this paper.

Packed column design example: This section illustrates the use of the analytical equations and the nomograph by applying them to a typical design problem: System temperature ( T ) Hydraulic loading rate (u) Specific area of packing (a~) Expected gain in DO Expected oxygen absorption efficiency Price of oxygen Price of electricity Packing type selected

13°C 5 cm s-~ (75 gallons min-~ ft -2) 213 m 2 m -3 (65 ft 2 cuft -3) 31 mg liter 75% S 1O0 tS0"075 k w h 1-inch pall rings

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R. E. Speece, N. Nirmalakhandan, Y. Lee

Step 1: Estimation of mass transfer coefficient K~ a:(s ~)

Using Onda's correlation: Specific wetted area, a w(m2 m-~) = a t[ 1-exp[ - 1 "45( oc/ol. ),w~(L m/a~/~ 1.)~ I

× ( x (Lm)-(a,/p-~.)(g

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where a t = c r i t i c a l surface tension of packing (kg s -~) (=0"033), o~= surface tension of water (kg s - 2 ) ( = 0.0735), L m = l i q u i d mass flux (kg m - 2 s - ~ ) = (u)(p~./100)( = 50),/~ c = liquid viscosity (kg s - ~ ) ( = 0"00110, at 13°C), pl = l i q u i d density (kg m 3)( = 1000), and g = a c c e l e r a t i o n due to gravity (m s z)( = 9-81). If we assume that a microbial film would eventually cover the packing surface, then o c = o L. Therefore, substitution of the above values in the equation aw = 183.6 m -~m -~ (56 ft -~ft -)) Liquid film transfer coefficient, k t (m s- ~) = 0"005 I[{/ALg/pL}I/3{

Lm/aw//AL)}2/3{BL./fiLDL,l(,:tdp}o4]

where D E = diffusivity of oxygen in water (m 2 s J)( = 2.0 x 10 -') at 13°C), and dp= nominal diameter of packing (m) ( = 0"0254). Therefore, k L= 0"00034 m sOverall mass transfer coefficient, KLa = awkl. =

(183"6)(0.00034

= 0.0624 s-J Using the n o m o g r a p h :

Step 1: E n t e r the hydraulic loading axis at (1) with u = 5 cm s-J(75 g p m ft-2) and project vertically u p w a r d s to intersect appropriate t e m p e r a t u r e curve c o r r e s p o n d i n g to 13°C. F r o m this intersection point m o v e horizontally (2) to intersect packing type line (B) c o r r e s p o n d i n g to l-inch pall rings: N o w d r o p vertically to read K La = 0'065 at (3). Step 2: Estimation of packing height, z( m ). E n t e r the lower left h a n d scale at the desired D O gain of 31 mg liter -j at (4) and extend vertically upwards to t / = 75% oxygen utilization efficiency level. T h e required oxygen-to-water volumetric ratio is f o u n d as r = 0"03 at (5); now, extend horizontally to the right to intersect r = 0"03 curve again at (6) and proceed vertically upwards to meet the hydraulic loading curves at (7) with

Oxygen absorption and nitrogen strippingfor fish culture

209

u = 5 cm s-1; the KLa'Z value is now read by extending horizontally to intersect the KLa'Z axis at (8) where, KLa'Z = 16 cm s-1. Now continue horizontally to the left until the vertical lines, through (the previously found) Kna are intersected: From the packing height family of lines select the one which passes through this intersection point: therefore, packing height, z = 2-5 m (8 ft). Of course, the nomograph can also be used to predict the outlet D O / D N for a given height, water loading rate and oxygen injection ratio. Step 3." Estimation of p u m p horse power per unit tower area, HP ft-2(kW m - 2). Using analytical method:

Pump horse power (lip) (Head) x (Specific gravity)x (gal rain ~)x (8"33) (Efficiency) x (33 000) Assuming efficiency = 75% for a centrifugal pump and using a 5% friction loss factor, pump horse power per unit tower area: HP = 0" 197 HP/ft- 2(1 "64 kW m - 2) Step 4." Estimation of oxygen required per unit tower area. Using analytical method:

Oxygen required (02 density).

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load)x

Find r using results in step 2, and use in above equation. Oxygen required

= (0.0 3) x ( 5 cm 3 cm - 2 s - 1) x ( 0 " 0 1 5 = 0.00021 g cm -2 s- l = 8.1 kg h - l m-2 (1.62 lb h - l ft 2)

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Step 5." Estimation of the pumping cost per unit tower area. Analytical method:

Pumping cost

= (Pump power) x (Power price) = (1-62 kW m -z) x (S0"075 kW h-1) = S 0 . 0 8 1 h -1 m - 2 (S0"007 h -1 ft - 2 )

Step 6: Estimation of oxygen cost per unit tower area. Analytical method:

Oxygen cost

= (Oxygen required) x (Oxygen price) ($100/2000 lb) = S0.90 h -t m-2 (S0.09 h -I ft -2)

= (8-1 x 2 . 2 ) x

210

R. E. Speece, N. Nirmalakhandan, Y. Lee REFERENCES

Nirmalakhandan, N., Lee, Y. H. & Speece, R. E. (1988). Optimizing oxygen absorption and nitrogen desorption in packed towers. Aquaculmral Engineering, 7 (in press). Sawyer, C. N. & McCarty, R L. (1967). Chemist O, for Sanita O, Engineers, McGraw Hill, New York. Speece, R. E. (1973). Troui metabolism characteristics and the rational design of nitrification facilities for water reuse in hatcheries. Trans. Am. Fish. Soc., 102, 323-34. Speece, R. E. (1981). Management of dissolved oxygen and nitrogen in fish hatchery waters. In Bio-Engineering Symposium .for Fish Culture, FCS Publication 1, American Fish. Soc., pp. 53-62.