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Full text of "Technology of dormancy release in potato tubers."

■ a ■ Agriculture 



Canada 

Research Direction generate 
Branch de la recherche 

Technical Bulletin 1991-5E 



Technology of dormancy 
release in potato tubers 



■ * ■ Agriculture 
It! Canada 



MAR ! 1 1991 






Library / Bibliotheque,Ottawa K1A 0C5 



630.72 
C75S 

c S/-5 
o-5 




Canada 



Digitized by the Internet Archive 
in 2013 



http://archive.org/details/technologyofdorm19915cole 



Technology of dormancy 
release in potato tubers 



W.K COLEMAN 
Research Station 
Fredericton, N.B. 

G.HAWKINS 

New Brunswick Department of Agriculture 

Fredericton, N.B. 

J. MCINERNEY and M. GODDARD 
Department of Chemical Engineering 
University of New Brunswick 
Fredericton, N.B. 

Technical Bulletin 1991-5E 



Research Branch 
Agriculture Canada 
1991 



Copies of this publication are available from 

Director 

Research Station 

Research Branch, Agriculture Canada 

PO. Box 20280 

Fredericton, New Brunswick 

E3B4Z7 

Produced by Research Program Service 

© Minister of Supply and Services Canada 1991 
Cat. No. A54-8/1991-5E 
ISBN 0-662-18550-1 



Cover illustration 

The dots on the map represent 
Agriculture Canada research 
establishments. 



CONTENTS 

ACKNOWLEDGMENTS iii 

SUMMARY lv 

INTRODUCTION 1 

Tuber dormancy 1 

Dormancy release by external agents 2 

Objectives of current research 3 

BROMOETHANE 4 

Properties 4 

Toxicology 5 

Mutagenicity and carcinogenicity 7 

Flammable hazard 7 

EVALUATION OF DORMANCY RELEASE 8 

Controlled environment evaluation 8 

Field evaluation 16 

Disease detection in treated tubers 16 

EXPERIMENTAL ADSORPTION SYSTEMS 17 

Screening of adsorbents 17 

Adsorption of bromoethane 20 

Regeneration of YAO adsorbent 29 

Bench- scale experiments 30 



CONCLUSIONS 37 

Potential for environmental containment 37 

Pilot plant design 38 

REFERENCES 43 

APPENDIX I : Theoretical Analysis 47 

Theoretical Analysis 48 



ii 



ACKNOWLEDGMENTS 
This work was supported by the federal/provincial Industrial Innovation and 
Technology Development Subsidiary Agreement (grant no. 9963-121-109) between 
Canada and the province of New Brunswick. 



iii 



SUMMARY 

Potato tuber dormancy can hinder production of basic nuclear seed stock 
from greenhouse tubers , sales of Canadian seed potatoes to early export markets 
and rapid post-harvest disease testing. Tuber dormancy can be released 
immediately in a wide range of commercially important North American cultivars 
by brief treatment (1-2 days) with bromoe thane vapor. The development of a 
complete, large scale technology for bromoethane application and its safe removal 
through a capturing techinque is necessary for successful application of this 
dormancy release method. Results of screening studies for adsorbents indicate 
that both YAO and HTO activated carbon have a similar high capacity for 
bromoethane. Both compounds have a low affinity for water adsorption, and adsorb 
and desorb bromoethane quickly and easily. The more economical YAO is the 
adsorbent suggested for a bromoethane adsorption system. 

Based on the present study, a plausible design for a dormancy release 
facility is presented. This facility should allow the current objectives of the 
North American seed potato industry to be met in a safe and environmentally 
responsible manner. 

RESUME 

La dormance peut avoir un effet negatif sur la production de materiel de 
semence de base a partir de tubercules de pommes de terre provenant de serres, 
sur les ventes hatives de pommes de terre de semence canadiennes sur les marches 
exterieurs et sur la rapidite du depistage phytosanitaire apres la recolte. Une 
exposition breve (1-2 jours) des tubercules de pommes de terre aux vapeurs de 
bromure d' ethyl peut rompre la dormance immediatement chez plusieurs des varietes 
nord-americaines qui sont d' importance commerciale. L' application reussie de 
cette methode pour rompre la dormance necessite le developpement a grande echelle 
de toute la technologie requise pour 1' application du bromure d' ethyl et sa 
recapture. Les resultats de tests de selection indiquent que les charbons 
actives YAO et HTO ont des niveaux d' adsorption du bromure d' ethyl qui sont 
similaires et eleves. Les deux produits ont peu d'af finite pour l'eau par 
adsorption mais font rapidement et facilement 1' adsorption et la desorption du 
bromure d' ethyl. Par ce qu'il est plus economique que le HTO, la YAO est 
recommande pour une systeme d'adsorption du bromure d'ethyl. 

La structure de base d'une unite servant a rompre la dormance des 
tubercules en utilisant le bromure d'ethyl est presentee. Cette unite devrait 
permettre de satisfaire les objectifs de l'industrie de la pomme de terre nord- 
americaine de facon securitaire et tout en respectant l'environnement . 

iv 



INTRODUCTION 

The potato (Solanum tuberosum L.) tuber is currently among the top five 
food crops in the world in terms of production volume, dollar value of 
production, edible energy, and protein yield per hectare; it is grown in more 
countries than any other crop except maize (Horton and Sawyer 1985) . Although 
tuber dormancy is a desirable trait for the food industry in terms of high- 
quality commodity storage, there are crucial times in the potato seed industry 
when tuber dormancy can hinder production and sales . 

TUBER DORMANCY 

The potato tuber is a modified stem structure or stolon, which usually 
develops below ground as a consequence of the swelling of the subapical portion 
of the stolon with the simultaneous accumulation of starch and proteins. A tuber 
is considered dormant if the buds of this organ are unable to grow at favorable 
temperatures . 

Tuber dormancy proceeds in a cultivar- specific manner because of a 
hypothetical balance of endogenous plant growth inhibitors and promoters where 
dormancy release involves a shift in the ratio in favor of promoters and 
subsequent establishment of positive feedback between the bud and mobilized food 
reserves (Coleman 1987) . The endogenous promoters are believed to include 
cytokinins, gibberellins , and possibly ethylene; at least some of the inhibitors 
are present in an "inhibitor /3" complex of which abscisic acid may be a major 
active component. 



DORMANCY RELEASE BY EXTERNAL AGENTS 

Although numerous chemicals have been found to reduce the duration of tuber 
dormancy, commercial exploitation has been unsuccessful. Substances used to date 
have been quite wide ranging in chemistry and have varied from gasoline or 
ammonia vapors to water or carbon dioxide (Coleman 1987). Large-scale methods 
of dormancy release must be based on the following practical considerations: 
ease of application, effectiveness, cost, environmental concerns, safety, and 
health aspects. Using these criteria, the current use of Rindite vapor at room 
temperature (a mixture of seven parts by volume of ethylene chlorohydrin, three 
parts 1,2-dichloroethane and, one part carbon tetrachloride) is unsatisfactory. 
For example, ethylene chlorohydrin, the major component of Rindite, induces 
cumulative kidney and liver degeneration at concentrations of 1-20 ppm 
(Department of Health, Education and Welfare 1978; L.F. Juodeika 1981, personal 
communication). Although Rindite presents no direct flammable hazard, the 
presence of the Rindite components in a fire fueled by other materials would 
present a significant safety hazard through possible exposure to ethylene 
chlorohydrin vapor. 

First described in 1945 (Denny 1945) , commercial use of Rindite has been 
attempted with only limited success because of the highly toxic nature of its 
components. However, bromoethane appears to be a potentially suitable chemical. 
This substance was first described as an effective agent for dormancy release by 
McCallum in a brief annual report (McCallum 1909). Subsequent evaluations of 
bromoethane were carried out by researchers at the Boyce Thompson Institute 
(Denny 1926A, 1926B; Miller 1934). Miller's study suggested that bromoethane was 
ineffective as a dormancy-releasing agent for potato. However, Miller applied 
only 25% of the amount of bromoethane shown by McCallum to be necessary to break 



dormancy. Consequently, this promising agent appeared to need additional 
evaluations in the context of current requirements of the North American seed 
potato industry. 

Recent work indicates bromoethane (BE) vapor is a very suitable candidate 
because it is as effective as Rindite, easy to apply, inexpensive, and possesses 
low toxicity (Coleman 1983, 1984; Coleman and Coleman 1986; McDonald and Coleman 
1988). 

OBJECTIVES OF CURRENT RESEARCH 

Dormancy of potato tubers affects the seed potato industry in three major 
ways: 

(1) Many North American and European cultivars have been unable to penetrate 
the early tropical and subtropical export seed markets because of a cultivar- 
specific and "innate" dormancy period. In addition, Canadian seed exporters have 
experienced dormancy problems in some South American and Mediterranean rim 
countries with traditional North American cultivars . 

(2) The production of basic nuclear seed stock from greenhouse tubers is 
rapidly becoming an integral part of seed-production programs. However, 
greenhouse tubers possess an intense dormancy that appears to be more difficult 
to release than normal field- grown material. A consistent dormancy breaking 
method to maximize efficiency and turnaround time is required. 

(3) Timely postharvest test results are required by potato -seed tuber growers 
as an essential component of their marketing strategy. Currently, most rapid 
postharvest test procedures require observation of field- grown plants derived 
from tubers that have had their dormancy broken. Even if postharvest testing 
becomes greenhouse or laboratory based, there will be a requirement for a 



procedure to break dormancy but not to interfere with the detection of disease. 
At present, no existing technology in North America or Europe addresses the 
problem of potato tuber dormancy as a major limiting factor in the seed export 
industry. The present research addresses this problem in terms of fumigant 
selection, evaluation, and environmental containment. 

BROMOETHANE 
PROPERTIES 

Bromoethane (ethyl bromide, monobromoe thane , bromic ether, hydrobromic 
ether) is a very volatile, clear, and colorless liquid (Table 1). This substance 
is made commercially by refluxing ethanol with hydrobromic and sulfuric acids, 
and removing bromoethane by distillation (Blatt 1960) . A second commercial 
method uses gamma irradiation from cobalt-60 of hydrogen bromide and ethylene 
followed by a caustic scrubber system to remove unreacted hydrogen bromide 
(Harmer and Beale 1963) . This method was used by Dow Chemical in the 1960s to 
produce about 500 tonnes of bromoethane annually. The process is exothermic and 
requires the gamma radiation only to initiate the process . Minor contaminants 
of commercially prepared bromoethane can include bromide or sulfate salts, 
ethanol, water, ethyl ether (up to 0.7%), hydrobromic acid, or ethylene. 
Ethylene dibromide may be a trace contaminant in bromoethane at levels up to 
0.01% (D.A. Rickard, 1990, personal communication). Tests should be carried out 
on commercial bromoethane samples to select the lot with the lowest level of 
contaminants . 

Bromoethane is used as an ethylating agent in organic synthesis (Great 
Lakes Chemical Corp. 1981), although past uses included as a refrigerant, an 
inhalation anesthetic, and a gasoline additive. It has been used as an 



experimental fumigant to control mites in farm- stored grain in the United Kingdom 
(Bowley and Bell 1981) and was shown to be an effective sterilant of catgut 
against highly resistant spore formers when used at 8% concentration in 96% 
ethanol for 24 h at 56°C (Harmsen and Ostertag 1950). 

Table 1 Physical properties of bromoethane 



Property 



Characteristics 



Reference 



Appearance 




clear 

colorless 

liquid 


Autoignition point 




511°C 


Boiling point 




38.2°C 


Flash point 




-23.0°C 


Formula 




C 2 H 5 Br 


Freezing point 




-119. 0°C 


Molecular weight 




108.98 


Refractive index, n D (25°C) 


1.421 


RTECS number 




KH6475000 


Solubility in water 






(g/100 g at 20 8 C) 




0.914 


Specific gravity (25' 


'C/4°C) 


1.451 


Surface tension 






(dynes/cm at 25 8 C) 




23.45 


Vapor density 






(air - 1) ' 




3.75 


Vapor pressure 






(mm Hg at 25°C) 




469.0 


Viscosity 






(centipoises at 25 


'O 


0.379 



Great Lakes Chemical Corp. 1981 
Mumford and Phillips 1950 
Great Lakes Chemical Corp. 1981 

Great Lakes Chemical Corp. 1981 

Windholz 1976 

Great Lakes Chemical Corp. 1981 



Windholz 1976 

Great Lakes Chemical Corp. 1981 

Mumford and Phillips Corp. 1950 

Great Lakes Chemical Corp. 1981 

Great Lakes Chemical Corp. 1981 

Mumford and Phillips 1950 



TOXICOLOGY 

When bromoethane is compared with the components of Rindite on the basis 
of health and safety, the preference for bromoethane is apparent. However, 
bromoethane is not without its hazards, and precautions must be taken in handling 
and usage . 



Bromoethane has been described as a respiratory irritant and a hepato- and 
renal toxin. At high concentrations, it causes narcosis, which probably accounts 
for its early use as a human anesthetic (NIOSH 1978) . Applied to humans as a 
respiratory anesthetic at a concentration of approximately 100 000 ppm (10% by 
volume) , bromoethane caused some fatalities either immediately from respiratory 
or cardiac arrest or in a delayed fashion from its effects on the liver, kidneys, 
and heart. Exposure of four guinea pigs for 30 minutes to 24 000 ppm (2.4% by 
volume) was fatal to three of them within 3 days as a result of pulmonary 
congestion, centrolobular necrosis of the liver, and diffuse nephritis (Tatkin 
and Lewis 1983; NIOSH 1978). Repeated and prolonged exposure of skin to 
bromoethane can lead to dermatitis. 

Because the ether- like odor of bromoethane is detectable at concentrations 
above 200 ppm, the threshold limit value was set at 200 ppm (890 mg/m 3 ) to avoid 
narcosis and other toxic effects. Symptoms of overexposure to bromoethane 
include eye irritation, vertigo, dermatitis, or signs of liver and kidney damage 
(Oettingen 1978; Sax and Lewis 1988). Severe exposure may result in a generally 
confused state that will need to be differentiated from other causes such as 
hypoglycemia, hyperglycemia, cerebrovascular accident, transient ischemic 
episodes, head injury, postepileptic confusion, heat stroke, drug abuse, toxic 
encephalopathy, meningitis, or encephalitis. Treatment for overexposure to 
bromoethane involves removal from exposure, washing of skin areas, and irrigation 
of the eyes. People with a history of skin, liver, kidney, or chronic 
respiratory disease may be at an increased risk from exposure (Proctor and Hughes 
1978). 



MUTAGENICITY AND CARCINOGENICITY 

Although the components of RIndite are mutagenic and carcinogenic, 
bromoe thane was initially believed to be nonmutagenic (L.F. Juodeika, 1981, 
personal communication). However, a study by Barber et al. (1981) reevaluated 
the traditional Ames Salmonel 2a/microsome screening test for the detection of 
chemical mutagens using a closed, inert incubation system for testing the 
mutagenicity of volatile compounds. They noted that bromoethane probably is a 
direct-acting, base-pair mutagen. Further work supported this conclusion (Barber 
and Donish 1982; Barber et al. 1983). 

An initial study of possible bromoethane carcinogenicity found that 
multiple injections of bromoethane into mice resulted in no significant increase 
in the frequency of lung tumors (Poirier et al. 1975). Long-term animal testing 
of bromoethane for carcinogenic activity is currently taking place as part of the 
national toxicology program of the United States Public Health Service. 
Preliminary results indicate that exposure of rats and mice to 100-400 ppm 
bromoethane 6 h a day, 5 days per week, for 2 years, resulted in significant 
neoplastic lesions (Roycroft 1989; Roycroft et al. 1989). Although workers using 
this fumigant would not be exposed to similar conditions, a dormancy release 
facility that uses bromoethane should be closely monitored during and after 
operation, and workers should be protected from inhalation of, and exposure to, 
the fumigant. 

FLAMMABLE HAZARD 

Bromoethane can become explosively flammable when mixed with air over the 
concentration range of 6.75-11.25% by volume. The fumigation process requires 
bromoethane at 6.6% by volume and should not pose an unusual fire hazard although 



explosion-proof electrical utilities should be used during construction. All 
conventional extinguishing media (e.g., foams, dry powder, and carbon dioxide) 
are suitable for fire fighting. The presence of bromoe thane in fires fueled by 
other materials may generate hydrogen bromide or bromine vapors requiring 
appropriate respiratory protection. 

EVALUATION OF DORMANCY RELEASE 
CONTROLLED ENVIRONMENT EVALUATION 

Initial experiments in 1981 involved small quantities (2-4 kg) of seed 
tubers from different potato cultivars. After treatment with bromoethane vapor 
at room temperature for 24 h in airtight containers, the tubers were aerated for 
2 to 4 h and then planted directly in clay pots under greenhouse conditions (18- 
24°C) and 16 h days created with supplemental incandescent and fluorescent light. 
Emergence, growth, and number of sprouts were recorded at 1-2 week intervals and 
analysis of variance was carried out on emergence time, rate of shoot elongation, 
and number of shoots. 

When applied to whole tubers, which had been removed directly from the 
field 2 weeks after top killing in early August, bromoethane effectively broke 
dormancy and promoted multiple sprouting at a concentration of 294 000 mg/m 3 
(i.e., 0.2mL liquid BE added for each litre of container volume) provided the 
tubers possessed a mature, intact periderm or "skin." Emergence gains of 40-70% 
were observed in five cultivars when whole tubers were planted (Coleman 1983) . 
If the tubers were cut immediately after bromoethane treatment and then planted, 
emergence gains of 50-75% were observed (Tables 2 and 3; Coleman 1983). 

The rate of shoot elongation increased significantly (P < 0.05) by 
2.8 mm/day and there was no difference in bromoethane 's effect among the 

8 



cultivars (Tables 4 and 5; Coleman 1983). Because of a strong apical dominance, 
the mean number of shoots per seed piece remained low although bromoe thane 
significantly increased this number (Table 6). 

Subsequent large-scale application of bromoethane to 50-kg tuber samples 
of 14 cultivars confirmed the effectiveness of this fumigant to significantly 
promote tuber dormancy release, subsequent sprout growth rate, and number of 
emergent shoots per tuber relative to untreated control tubers (Coleman 1984) . 



Table 2 Effect of bromoethane on shoot emergence from tubers of five different 
cultivars 

Number of days 

Post- treatment to 50% emergence Emergence 

Cultivar handling* Control BE treated gain (%)** 



Russet Burbank 


whole 




cut 


Red Pontiac 


whole 




cut 


Bintje 


whole 




cut 


Kennebec 


whole 




cut 


Caribe 


whole 




cut 



100 


53 


+47% 


80 


40 


+50% 


71 


40 


+44% 


65 


24 


+63% 


58 


35 


+40% 


45 


23 


+49% 


75 


25 


+67% 


75 


22 


+71% 


90 


37 


+59% 


75 


19 


+75% 



* After treatment with liquid BE at 0.2 mL/L of fumigation chamber volume, 
tubers were either planted whole in the greenhouse or cut into seed pieces and 
then planted . 



• * 



Emergence gain 



Contzol-BE treated 
Control 



x 100%, 



10 



Table 3 ANOVA table for shoot emergence 



Source of variation D.F. M.S. F value 

Cultivars 4 422.45 43.55* 

Cutting 1 672.8 69.36* 

BE 1 8652.8 892.04* 

Cutting x BE 1 3.2 0.33 NS 

Cultivar x cutting 4 37.8 3.89 NS 

Cultivar x BE 4 166.05 17.12* 

Error 4 9.7 



* P < 0.05 



11 



Table 4 Effect of bromoe thane* on mean rate of shoot elongation and mean number 
of shoots from tubers of five different cultlvars 

Mean rate of Mean no. shoots per 

shoot elongation (mm/day) seed piece 

Post- treatment 
Cultivar handling Control BE treated Control BE treated 



Russet 


whole 


3.2 


7.3 


Burbank 


cut 


8.0 


8.5 


Red 


whole 


4.2 


8.8 


Pontiac 


cut 


4.8 


5.9 


Bintje 


whole 


8.5 


16.0 




cut 


13.0 


16.7 


Kennebec 


whole 


4.8 


5.1 




cut 


4.3 


7.4 


Caribe 


whole 


5.2 


6.0 




cut 


4.7 


7.4 



1.1 
1.1 

1.4 
1.0 
1.1 
1.0 
1.1 
1.0 
1.0 
1.0 



1.7 
1.2 
1.9 
1.1 
2.9 
1.9 
1.4 
1.1 
1.7 
1.1 



* 0.2 mL BE/L. 



12 



Table 5 ANOVA table for rate of shoot elongation 



Source of variation 



D.F. 



M.S 



F value 



Cultivars 

Cutting 

BE 

Cutting x BE 

Cultivar x cutting 

Cultivar x BE 

Error 



4 


46.86 


17.27* 


1 


6.73 


2.48 N.S 


1 


40.33 


14.86* 


1 


1.92 


0.71 N.S 


4 


2.84 


1.05 N.S 


4 


2.60 


0.96 N.S 


4 


2.71 





* P < 0.05. 

N.S. - not significant. 

Table 6 ANOVA table for number of shoots 



Source of variation 



D.F. 



M.S 



F value 



Cultivars 

Cutting 

BE 

Cutting x BE 

Cultivar x cutting 

Cultivar x BE 

Error 



4 


0.21 


12.43* 


1 


0.72 


43.10* 


1 


1.35 


80.72* 


1 


0.34 


20.18* 


4 


0.03 


1.99 N.S 


4 


0.22 


13.18* 


4 


0.02 





* P < 0.05. 

N.S. - not significant, 



13 



Table 7 Effects of chemical treatments on tuber sprouting In three potato 
cultivars under New Brunswick field conditions 



Cultivar 



Treatment 3 



Mean no. days to 
50% emergence 



Mean no. 
sprouts/tuber 



Kennebec 



Katahdin 



Russet Bur bank 



BE 

BE/BE 

BE/BE+ETOH 

BE 

BE/BE 

BE/BE+ETOH 

BE 

BE/BE 

BE/BE+ETOH 



68.1 
61.1 
33.1 
35.1 
42.9 
61.7 
41.3 
25.4 
35.7 
se 



df 



1.0 
1.1 
1.6 
1.4 
1.4 
1.0 
1.1 
2.3 
1.2 
se 



df 



Standard error of differences between: 

Two cultivar means 0.908 

Two treatment means 0.831 

Two treatment means for one cultivar 1.485 

Two cultivar means for one treatment 1.439 

1 BE - bromoethane vapor (0.3 mL/L) for 24 h. 

BE/BE - bromoethane vapor (0.3 mL/L) for 24 h, aeration for 24 h and second 

treatment with bromoethane (0.3 mL/L). 

BE/BE+ETOH - bromoethane vapor (0.3 mL/L) for 24 h and second treatment with 

bromoethane (0.3 mL/L) and ethanol vapor (0.2 mL/L). 



21 


0.0175 


21 


21 


0.0175 


21 


7 


0.0331 
0.0344 


7 



14 



Table 8 Effects of chemical treatments on tuber yield responses in three potato 
cultivars 











Mean total 


Mean i 


io. 


Mean no. 






Mean 




tuber fresh 


grade 


A 


grade B 






no . tubers 


weight 


tubers 


tubers 


Cultivar 


Treatment 1 


(x 10 5 /ha) 


(x 10 3 kg/ha) 


(x 10 : 


J /ha) 


(x lOVha) 


Kennebec 


BE 


2.139 




7.148 


4.3 




55.9 




BE/BE 


2.320 




14.845 


38.7 




77.0 




BE/BE+ETOH 


2.457 




24.024 


91.2 




75.7 


Katahdin 


BE 


2.414 




23.563 


89.9 




59.0 




BE/BE 


2.264 




20.925 


70.2 




45.6 




BE/BE+ETOH 


2.380 




13.094 


27.5 




84.4 


Russet 


BE 


4.489 




27.124 


69.3 




147.6 


Bur bank 


BE/BE 


5.139 




42.101 


137.7 




176.5 




BE/BE+ETOH 


3.701 




31.066 


89.9 




117.9 






se 


df 


se df 


se 


df se df 



Standard error of 
differences between: 

Two cultivar means 

Two treatment means 

Two treatment means 
for one cultivar 

Two cultivar means 
for one treatment 



79.2 


21 


760.7 


21 


49.4 


21 


81.0 


21 


128.4 


21 


638.5 


21 


76.6 


21 


96.4 


21 


336.0 


7 


1180.7 


7 


202.5 


7 


273.9 


7 



385.3 



1106.0 



229.8 



289.0 



1 BE - bromoethane vapor (0.3 mL/L) for 24 h. 

BE/BE - bromoethane vapor (0.3 mL/L) for 24 h, aeration for 24 h and second 

treatment with bromoethane (0.3 mL/L). 

BE/BE+ETOH - bromoethane vapor (0.3 mL/L) for 24 h, aeration for 24 h and second 

treatment with bromoethane (0.3 mL/L) and ethanol vapor (0.2 mL/L)/ 



15 



FIELD EVALUATION 

Basic nuclear stock seed tubers from the cultivars Kennebec, Russet 
Burbank, and Katahdin were top pulled on 28 May 1985 and harvested from the 
greenhouse at Bon Accord Elite Seed Potato Farm in early June 1985. They were 
immediately treated for 24 h at 24*C as follows: (a) bromoethane vapor (0.3 mL 
liquid BE per litre volume of chamber); (b) bromoethane vapor for 24 h, aeration 
for 24 h, and then a second treatment with bromoethane vapor for 24 h; and (c) 
bromoethane vapor, aeration for 24 h, and then a second treatment with 
bromoethane vapor and ethanol vapor (liquid at 0.2 mL/L) for 24 h (Coleman and 
Coleman 1986) . 

All tubers were planted on 18 June 1985 at the Fredericton Research Station 
in a randomized split plot design where the three cultivars were the main factors 
and the three treatments were the subf actors. Tubers were harvested in late 
October and number and fresh weight of individual tubers were noted. 

There was significant interaction (P < 0.01) between cultivar and treatment 
effects in terms of mean number of sprouts per tuber and time to 50% emergence 
although treated tubers began to emerge within 4 weeks (Table 7). Similar trends 
were evident in tuber number, total tuber yield, and the number of grade A and 
B seed tubers (Table 8). Two consecutive bromoethane treatments (treatment b) 
gave the best overall response when the mean results of the subfactors were 
compared. 

DISEASE DETECTION IN TREATED TUBERS 

To date (1990) , there is no evidence that bromoethane will give a different 
disease response in the treated tubers or resulting plants compared to Rindite. 
However, this situation requires further evaluation. A previous paper (McDonald 

16 



and Coleman 1984), which indicated the Rindite- treated tubers increased ELISA 
values for potato virus Y (PVY) in comparison to bromoe thane- treated material, 
has since been shown to be incorrect (McDonald and Coleman 1988) . Both fumigants 
are equally effective in breaking dormancy and increasing the ELISA values for 
PVY. Preliminary results (unpublished) suggest that the detection of potato 
mosaic virus is not differentially affected relative to Rindite. 

In evaluations of dormancy- releasing chemicals on subsequent tuber response 
to dry rot (Fusarium sambucinum f .6) , development of seed tuber breakdown was not 
affected significantly by pretreatment with either Rindite or bromoethane 
(Coleman and Murphy, 1990). 

EXPERIMENTAL ADSORPTION SYSTEMS 
A capture system using adsorption would allow temporary storage of 
bromotethane in an adsorbent before being released for subsequent treatment of 
fresh tuber samples. The current study outlines initial experiments in the 
development of an environmentally appropriate procedure for handling bromoethane 
vapor . 

SCREENING OF ADSORBENTS 

The objective of the screening studies was to select an adsorbent that 
would have a high capacity for bromoethane and a low affinity for water 
adsorption, adsorb bromoethane quickly, and desorb bromoethane easily. 

The apparatus employed in preliminary screening and detailed studies of 
adsorbents is shown schematically in Fig. 1. The apparatus consisted of two 
sections. The first facilitated generation of air or helium gas streams with 
controlled bromoethane and water vapor contents. The second section was 

17 



He 



r\ 



Temperature bath 



Flov controller 1 



He 



r\ 



® 



^k 



Thermal 
conductivity 
detector 



/N 



Packed bed 



)® 



Flow controller 2 



whmiy 



■Bromoethane 
- Temperature bath 



Gas chromatograph Integrator 



Section I - Bromoethane generation and flm control Section 2 - Adsorption and analysis 



Fig. 1 A schematic diagram of the apparatus for the 
preliminary adsorption screening studies 



18 



comprised of a packed column containing the adsorbent under study and the gas 
detection system. Using air (or helium) as the carrier gas, the gas generation 
system functioned as follows. 

Streams of compressed air were passed through gas wash bottles containing 
bromoethane and water, respectively. The temperature of the bottles was 
controlled by two Haake N3 constant temperature baths. The pressure of the inlet 
gas and the temperature fixed the bromoethane and water vapor concentration in 
the outlet streams. The rate of gas flow in the combined outlet streams from 
this section was controlled by two Matheson model 8200 mass flow controllers. 

Gas entering the second section passed through a packed bed fabricated from 
12.7 mm (1/2") O.D. copper tubing with 12.7 mm to 3.2 mm (1/8") Swagelok union 
fittings used as end caps. A Varian 3400 gas chromatograph equipped with a 
thermal conductivity detector was used to maintain the temperature of the packed 
column and measure gas concentrations in the outlet stream. The response of the 
detector was monitored with a Varian 4290 Integrator. 

Four adsorbents were screened for adsorption of bromoethane from helium. 
They were: 

B0C - a carbon molecular sieve (British Oxygen) 
Amberlite AXD-4 - a polyethylene polymer (Rohme and Haas) 
HTO - a high-capacity activated carbon (Carbon and Filtration Canada) . 
YAO - a medium - capacity activated carbon (Carbon and Filtration Canada) . 
The adsorbents were crushed and sieved to produce a 40-60 mesh material. 
Each adsorbent was tested in the "as received" condition and after regeneration 
with helium at 150°C. In the screening test, a concentration step 
chromatographic technique (Kumar 1978) was employed. A given weight of adsorbent 
was used to fill the packed bed (volume approximately 8 cc) . The packed bed was 

19 



placed in the chromatograph oven, connected to the gas generation section, and 
then maintained at 33 °C. The bromoe thane generator was held at a pressure of 
2.76 x 10 5 Pa (40 psia) and a temperature of 23°C to produce a stream of helium 
containing bromoethane at 22.1 mol%. In this set of experiments, water vapor was 
not added to the helium dilution stream. The mass flow controller on the outlet 
of the gas generation section was set to allow air to flow at 150 ml/min. The 
concentration of bromoethane in helium was increased in a step -wise fashion by 
varying the set-point on the other mass-flow controller, i.e. , by decreasing the 
helium dilution factor. Because of bromoethane by the adsorbent, no immediate 
response to the step -change was detected with the chromatograph. The response 
of the detector was monitored until a complete breakthrough curve was achieved. 
The step increases in concentration were repeated until a bromoethane 
concentration of approximately 6.0 mol% was reached. The chromatographic 
response curves were analyzed to obtain retention times and equilibrium 
concentrations. The packed bed was weighed and the total amount of adsorbed 
bromoethane determined. 

ADSORPTION OF BROMOETHANE 

Pure component equilibrium adsorption isotherms over a wide range of 
concentrations extending to near saturation at a partial pressure of 0.062 atm 
were determined chromatographically for a number of possible adsorbents. A 
summary of the results of these experiments is presented in Table 9. 



20 



Table 9 Summary of adsorbent screening studies 



Adsorbent Type 



Avg. 
Particle 

Size 
Tested 



Avg. 




Wt% 


Pore 


Typical 


Bromoethane 


Diameter 


Cost 


adsorbed at 


(A) 


(1989$/kg) 


33 *C and at 
6.3 kPa BE 


2.5 


10 


< 1 


3.20 


3.5 


59.34 


3.20 


7.0 


59.32 


40.60 


15 


17.40 



BOC carbon 442 

YAO carbon 507 

HTO carbon 475 

XAD4 polystyrene 580 



The carbon-molecular sieve (BOC) was the first of the three carbon-based 
adsorbents studied. Only small molecules can be admitted into the structure of 
this sorbent because of the small size of the pores. Results of the adsorption 
equilibria indicated that less than 1 wt% was adsorbed at 33 °C suggesting that 
bromoethane could not penetrate the pores of the adsorbent. Clearly, an 
adsorbent with a larger pore size would be required to achieve a higher 
bromoethane capacity. 

Another adsorbent that showed a small capacity for bromoethane was a 
synthetic resin, XAD-4. At 33 °C, the maximum capacity after regenerating the 
sample at 150°C was 17.4 wt% bromoethane. The high cost, coupled with the low 
capacity and a high affinity for water, would make this an unsuitable adsorbent 
for an adsorption system (Fig. 2) . 

Equilibrium isotherms for two activated carbon samples, YAO and HTO, were 
determined for samples both as they were received from the manufacturers and 
after regeneration at 150° C. Fig. 3 shows that for YAO, the capacity for 
bromoethane is higher throughout the range of partial pressures used in these 



21 



experiments . The difference in the adsorbed phase concentration may result from 
water present on the adsorbent when it is received. By regenerating the samples 
at 150'C, the water is removed and a higher capacity for bromoe thane is achieved. 
Fig. 4 shows little difference in adsorption equilibrium for the high surface 
area HTO when compared with YAO, suggesting that either would be suitable 
adsorbents. Because HTO costs more, this sample was eliminated as a possible 
adsorbent, and further studies on the economically attractive YAO were conducted. 
The adsorption of bromoe thane on YAO was also determined at 50 °C to 
estimate the heat of adsorption. In the limiting case, when the concentration 
of bromoethane approaches zero, the heat of adsorption is approximately 
5.4 kcal/mol indicating that bromoethane is only weakly held by the adsorbent at 
low concentrations . The practical implications of this value for the heat of 
adsorption is that heating (< 150 °C) or mild heating with vacuum will remove the 
bromoethane from the adsorbent. Fig. 4 shows the adsorption equilibria 
determined at two temperatures. 



22 



Wt % BE ADSORBED 




XAD-4 



2 4 6 

PARTIAL PRESSURE ( xlOE-3 MPa ) 



8 



Fig. 2 Adsorption equilibria for bromoethane on YAO, HTO and 
XAD-4 adsorbents at 33°C 



23 



Wt %BE ADSORBED 




As Received 



-*K- Regenerated 



2 4 6 

PARTIAL PRESSURE ( xlOE-3 MPa ) 



8 



Fig. 3 Adsorption of bromoethane on the "as received" sample 
of YAO and after regeneration at 150°C 



24 



Wt %BE ADSORBED 




Temp 33C 



-HS- Temp 60C 



2 4 6 

PARTIAL PRESSURE ( X10E-3 MPa ) 



8 



Fig. 4. A comparison of the adsorption equilibria for 
bromoethane on YAO at 33°C and 50°C 



25 



For the pure component adsorption isotherms, helium was used as a 
nonadsorbing carrier gas. However, in a facility for exposing seed potatoes, 
bromoethane would be recovered from a mixture of air, bromoethane, water, and 
compounds released by the seed potatoes. A series of experiments were conducted 
to determine how the bromoethane concentration on YAO would be affected with air 
as a carrier gas and water present in the mixture. 

Adsorption isotherms presented in Fig. 5 showed only a slight decrease in 
the adsorptive capacity for bromoethane when air, compared with helium, is used 
as a carrier gas. The introduction of water in the mixture reduced the capacity 
from 57 to 56 wt% for bromoethane. 

Shown in Fig. 6 are breakthrough curves of bromoethane as a function of 
time using helium and air as carrier gases, as well as an experiment in which 
water vapor was present at 40-50% relative humidity. The breakthrough curve for 
the experiment using the helium carrier is delayed compared with the curves for 
air and air + water. In the air and air-water experiments, bromoethane has 
insufficient time to adsorb onto the surface of the activated carbon and will 
easily pass through the adsorption column (i.e., it has an early breakthrough). 
For the experiments using helium, bromoethane adsorption occurs quickly and so 
the breakthrough is delayed. Note that although the detector response for the 
air and air-water breakthrough curves are not normalized (the attenuation 
settings were different for each experiment) , the curves indicate similar early 
breakthrough of bromoethane. 

To summarize, these data suggest that even at high concentrations of water, 
YAO- activated carbon does not have a strong affinity for water, and the capacity 
for bromoethane is maintained at the values measured in the absence of water. 
Analysis of the breakthrough curves indicates that a longer period (i.e., a 

26 



Wt %BE ADSORBED 




2 4 6 

PARTIAL PRESSURE ( X10E-3 MPa ) 



8 



Fig. 5. A comparison of the adsorption equilibria for 
bromoethane from a mixture of BE-helium and BE-air on 
YAO at 33°C 



27 



V 

on 

C 


a 

00 

a 
X 

u 



- 
o 

V 

*J 

V 

a 




Time (minutes) 



Fig. 6 Breakthrough curves of bromoethane from three mixtures 
of bromoethane - helium (a), bromoethane - air - water 
(b) and bromoethane - air (c) 



28 



longer residence time) is required for equilibrium to be achieved when 
bromoethane is adsorbed from air, or from a mixture with water vapor. 

REGENERATION OF YAO ADSORBENT 

Several techniques were used to regenerate YAO adsorbent, including: 
(i) regeneration by air purge at 33°C or 150°C 
(ii) regeneration by vacuum alone 
(iii) regeneration by vacuum plus heating 

The results with these methods are summarized in Table 10. As expected, 
the air purge and heat (150°C) treatment is the fastest method of regeneration. 
Unfortunately, this technique is not possible for a treatment facility, because 
only air from the chamber at room temperature is available to blow over the bed. 

Vacuum desorption was effective at room temperature but only for the finer 
particle size (40 x 60) mesh. For the 4x8 mesh material, it appears that 
diffusion processes make desorption by vacuum effective only when the adsorbent 
bed is at temperatures greater than 100°C. 



29 



Table 10 Summary of regeneration technique for YAO 



activated carbon 



Method Particle size 


Duration 


Vt% 


bromo* 


i thane 


Pressure 


Temp. 


( 


mesh) 


(hrs) 


Initial 


final 


(pm Hg) 


CO 


Air purge 


40x60 


16 


57.3 




11.3 


760 K 


33 


Air purge 
and heat 


40x60 


4 


57.3 




0.0 


760 K 


33 


Vacuum 


40x60 


4.5 


57.3 




3.6 


31 


23 


Vacuum 


4x8 


12 


15.0 




13.4 


60 


23 


Vacuum and 
heat 


4x8 


12 


27.5 




0.0 


140 


> 100 



BENCH- SCALE EXPERIMENTS 

The objective of this set of experiments was twofold: firstly, to 
determine the concentration of bromoe thane in the exposure chamber after the gas 
had been circulated through a bed of YAO- activated carbon and equilibrium had 
been achieved. In this way a check of the results of the screening experiments 
could be made using an independent experimental method. Secondly, by monitoring 
the chamber contents as a function of time, the kinetics of adsorption could also 
be measured. 

A schematic diagram of the bench-scale apparatus is presented in Fig. 7. 
The apparatus consisted of an airtight chamber to which bromoethane could be 
added and subsequently removed in adsorption. An attached blower recycled the 
air in the chamber through the adsorbent bed. Characteristics of the bench- scale 
adsorption setup are give in Table 11. 



30 



Table 11 Characteristics of the bench- scale adsorption apparatus 



Chamber Volume 295 litres 

Blower Flowrate 164 L/min (5.79 cfm) 

Adsorbent YAO - activated carbon 

Size 4x8 MESH 

Quantity 260 g 

Bed Diameter 135 mm 

Bed Depth 32 mm 

Face Velocity 1.44 m/s 



Initial bromoethane concentrations in a range of two to six mole percent were 
prepared in the exposure chamber by vaporizing a measured quantity of liquid 
bromoethane on a pre -heated surface. After complete vaporization an initial gas 
sample of 0.25 mL was taken with a gas-tight syringe. The blower motor was 
started causing the vapor in the chamber to circulate through the packed bed of 
YAO-activated carbon. Analysis of a gas sample for bromoethane, water and air 
with the chromatograph required approximately six minutes. This allowed sampling 
of gas in the chamber at only six to eight minutes intervals. Sampling was 
continued until equilibrium was achieved between the adsorbent bed and the air 
in the chamber. At the end of the test, the adsorbent was removed and weighed 
in order to determine the uptake of bromoethane. With 260 g of 4 x 8 mesh YAO- 
activated carbon a possible maximum uptake of 45 wt% could be achieved. Used 
adsorbent was regenerated by vacuum plus heating at 150°C. 

Table 12 is a summary of experimental results designed to evaluate the 
concentration of bromoethane in an exposure chamber after the vapor had been 
circulated through a bed of YAO-activated carbon. Included in this table is the 
initial concentration of bromoethane in the chamber and the concentration after 
adsorption, the adsorbed phase concentration and the time to reach equilibrium 

31 



Packed bed (YAO) 



N/ 



Evaporator 



Bromoethane liquid 




/\ 



Exposure chamber 



Blover motor 



Fig. 7 A schematic representation of the apparatus used for 
the bench-scale adsorption experiments 



32 



at the experimental temperature. Plotted in Fig. 9 is the concentration of 
bromoethane in the chamber (mol%) as a function of time. 

Fig. 8 shows the good agreement of the equilibrium isotherms obtained using 
the small bench- scale apparatus at 23 °C and the isotherm obtained from the 
screening experiment at 33°C. The small difference between the two isotherms at 
the point they overlap may be accounted for by the difference in temperatures. 

Mixtures of bromoethane in air and in air with water at 90% relative 
humidity were prepared for experiments 3 and 6. The results show that the 
adsorbed phase concentration of bromoethane is reduced slightly from 21.9 to 
20 wt% by the presence of water and is similar to the results obtained in Che 
screening studies. 

Analysis of the kinetic results indicates that the rate of adsorption is 
reduced by the presence of water. In experiment 6, the equilibrium concentration 
in the chamber was achieved in 27 min. and in experiment 3, 16 min. was required 
for the bromoethane concentration in air to achieve equilibrium. 

The practical implications of these results suggest that, although the 
adsorbed phase equilibrium may only be reduced slightly by a high concentration 
of water, the slow rate of adsorption must be allowed for and a longer residence 
time in the adsorption bed is required. 



33 



Table 12 Summary of small scale adsorption tests 



Ept. 


Bromoe 
injec 

(g) 


than 
ted 


e Bromoe thane 
cone, in air 


Bromoethane 
adsorbed 


Equilibrium 
reached ** 

(min) 


Temp. 




initial equil. 
(mol%) (mol%) 


initial 
(wt%) 


equil . 
(wt%) 


CC) 


1 


26.78 




2.220 <0.001 


0.0 


9.9 


15 




23 


2 


57.50 




5.320 0.158 


9.9 


30.1 


20 




23 


3 


57.06 




4.014 0.055 


0.0 


21.9 


16 




23 


4 


39.97 




2.780 0.018 


0.0 


15.0 


12 




23 


5 


82.85 




5.750* 0.556 


15.0 


42.7 


20 




23 


6 


55.70 




3.890* 0.081 


00.0 


20.0 


27 




27*** 



* Estimated initial concentration. 

** Time at which first minimum bromoethane concentration reading reached. 

*** Water vapor presence at 90% relative humidity. 



34 



70 



Wt %BE ADSORBED 




i 
0"- 



— r- 23C Chamber 

~$~ 33C Chromatograph 



0.06 0.1 0.15 0.2 

BE CONCENTRATION ( MOLE % ) 



0.25 



Fig. 8 Adsorption equilibria isotherms for bromoethane on YAO 
at 23°C determined using the small bench-scale 
apparatus and at 33°C determined using the screening 
studies apparatus 



35 



10 



BE CONCENTRATION ( MOLE % ) 



8- 




— r— 2.78 mole % 
-#- 3.81 mole % 
-B- 6.80 mole % 



— E3 



40 60 

TIME ( MIN ) 



80 



100 



Fig. 9 Bromoethane concentration in the chamber of the bench- 
scale apparatus as a function of time for different 
initial concentrations of bromoethane in air 



36 



CONCLUSIONS 
POTENTIAL FOR ENVIRONMENTAL CONTAINMENT 

The development of a complete, large-scale technology for bromoethane application 
and its safe removal through a capturing technology is necessary for the 
successful application of a dormancy release technology. 

The present study indicates that dormancy in seed potato tubers can be 
released immediately in a wide range of commercially important North American 
cultivars by treatment with bromoethane vapor. Based on the present study, a 
plausible design for a dormancy release facility can be presented (see "Pilot 
plant design" section below) . This facility should allow the current objectives 
of the North American seed potato industry to be met in a safe and 
environmentally responsible manner. 

Because treated tubers will not be used for human consumption, any residues 
(e.g., bromide) within the tubers should not be a problem. 

Bromoethane appears to be as effective as Rindite in breaking tuber 
dormancy. However, bromoethane is superior in terms of method of application, 
environmental concerns, safety and health aspects, and cost. Although there is 
no evidence that bromoethane will give a different disease response in the 
treated tubers or plants compared with Rindite, further evaluations should be 
carried out (see "Disease detection in treated tubers"). 

The adsorption system is an appropriate technology for handling bromoethane 
in a treatment facility designed to break the dormancy period of seed potatoes. 
Results of the screening studies for adsorbents indicate that both YAO - and 
HTO - activated carbon have a high capacity for bromoethane that is essentially 
the same. The more economical YAO is the adsorbent suggested for a bromoethane 
adsorption system. 

37 



The release of water vapor from the tubers should saturate the surrounding 
air in the treatment facility during the dormancy release treatment, which may 
last 2-3 days at room temperature. Present results suggest that water vapor in 
a mixture of bromoethane and air only reduces the equilibrium concentration of 
adsorbed bromoethane slightly. However, the rate at which bromoethane 
equilibrium is achieved will be reduced by the presence of air and water. In 
view of the superior results with helium, the replacement of air with an inert 
gas such as nitrogen (an acceptable low cost alternative to helium) would improve 
the equilibrium rate and reduce the flammable hazard associated with bromoethane . 
No detrimental effects on dormancy release would be expected although a small 
scale experiment to test this supposition would be necessary. 

The low heat of adsorption (5.4 kcal/mol) indicates that heating or mild 
heating coupled with vacuum will desorb bromoethane from the activated carbon. 
Because the effects of compounds released from the tubers on the adsorption 
capacity are not well understand or the flow rate of bromoethane has not been 
determined to achieve an efficient use of the adsorption process, a pilot plant 
is recommended. 

PILOT PLANT DESIGN 

The pilot plant is envisaged as two fixed beds of activated carbon with 
regeneration by vacuum and heating. Both beds in the system will be of equal 
size with an overdesign allowance of 25% for bromoethane capacity. All functions 
of the adsorption- recovery system will be monitored and regulated by a 
computerized control system. A schematic diagram is presented Fig. 10. 



38 



£ 



1 

■8 

1 
I 



Blover 





Bed h 
Absorbent 



V 



*g>- 



(V) Vacuum Pump 



>£ 




r^\ 



Bed 2- 
Adsorbent 



V 



*g>- 



Fig. 10 A schematic diagram of an adsorption pilot plant for 
handling bromoethane 



39 



In this design, bromoe thane is introduced to the exposure chamber via an 
automated dispensing and evaporation system. The tubers in the chamber are then 
exposed for the required time in two stages. 

In the first stage of the bromoe thane adsorption cycle, air from the 
chamber is blown through bed #1 until the concentration of bromoethane in the 
chamber reaches equilibrium, i.e., about 85% of the bromoethane is recovered. 
At this point, bed #1 is isolated and bed #2 is opened to the system. Air is 
circulated through bed #2 until the bromoethane concentration in chamber falls 
below the 200 ppm threshold level. Then bed #2 is isolated from the chamber. 

The treated tubers are removed and the chamber is reloaded. Release of the 
bromoethane to the chamber is accomplished by heating of the bed to 150 °C and 
imposing a vacuum. Any additional bromoethane required because of leakage or 
adsorption by the tubers is added by the automated dispensing system. Spent 
activated carbon after a number of adsorption cycles may be transferred 
pneumatically (using the blower) to containers for disposal. 

A summary of the costs of materials for a pilot plant system is presented 
in Table 13 and the basis for this calculation is found in Table 14. It should 
be noted that quoted costs do not include delivery charges , installation and 
labor and technical assessment costs. 

In the context of the proposed fumigation facility, future R&D areas that 
should be addressed include the following: (i) determining the appropriate 
concentration- time product to optimize the bromoethane treatment of seed tubers; 
(ii) determining the amount and type of residues associated with tubers and 
their containers; and, (iii) developing an acceptable treatment and post- 
treatment protocol for safely handling the seed potatoes . 



40 



Table 13 Materials and estimated costs for demonstration- scale pilot plant 



Item 



Detail 



Quantity 



Unit cost 
(1989$) 



Price 
(1989$) 



Adsorbent system 



Activated carbon 

Blower 

Compressor 

Piping 

Circulating fans 

Air filter system 

Control system 

Br omoe thane 
dispenser 

Bromoe thane 
detector 



Stainless steel, 0.915 m 
(3 ft) dia. x 1.22 m (4 ft) 
chamber stands 250 mm SS vac. 
valves, 40 SS vac. valves 
67 . 9 m 3 /h (40 cfm) vac . pump 
heaters , thermocouples 
controller vac. sensors 
and display system controller 

YAO 6 x 16 mesh 

2038 m 3 /h (1200 cfm), FRP 

for pneumatics 

to adsorber beds, circulation 

for air mixing in chamber 

for adsorption system 

computer + relays 

pump, control valve, tank, 
evap. 

B & K model 1302 08 
to 80 000 ppm 



4 








1 








1 






100 000 


150 kg 


3. 


50 


1 225 


1 


5 600 




5 600 


1 


1 000 




1 000 


1 


9 000 




9 000 


2 


1 500 




3 000 


1 


5 000 




5 000 
15 000 


1 


4 500 




4 500 


1 


34 700 




34 700 


com]: 


)onent total 


$179 025 



Note: Because of time restrictions that may be placed upon construction and 
commissioning of the plant, the major components, i.e., adsorbent, beds, 
valves, and vacuum system, are priced as a manufactured unit. Because the 
high capacity of activated carbon for most organic compounds and the 
flexibility of the vacuum-heat regeneration regime, the facility should be 
able to handle other agricultural fumigants. 



41 



Table 14 Basis for pilot plant calculations 



BASIS BROMOETHANE: 
BROMOETHANE @ 4 mol% 
CHAMBER VOLUME: 



55.2 kg 

(34 L of BE/L of chamber) 

284 m 3 (10 017 ft 3 ) 



Bed #1 For adsorber bed assume 40 wc% adsorbed on bed #1 and equilibrium BE 
concentration of 0.60 @ 23*C bed designed with 25% additional 
capacity. 



bulk density of adsorbent 

weight of adsorbent in #1 

volume adsorbent 

bed diameter 

bed length 

face velocity 

blower capacity 

air changes/hour (chamber) 



440 kg/m 3 
172 kg 
0.391 m 3 
0.76 m 
0.85 m 
0.51 m/s 
0.47 m 3 /s 
5.99 



(13.8 ft 3 ) 
(2.5 ft) 
(2.8 ft) 
(100 fpm) 
(1000 cfm) 



Vacuum system: 

target vacuum 50 /jm Hg 

volume BE adsorbed 11 344 L 

capacity 850 L/min 

pump down time to target vacuum 2.67 h 



Bed #2 Bed size is the same as #1, which is probably cost effective if 
vessels are custom built. The second bed must have enough capacity 
to remove remaining bromoethane from chamber after bed #1 reaches 
equilibrium and is switched out of the recirculating mode. The 
remaining bromoethane in the chamber when bed #2 is switched in 
should be < 10 wt% of the adsorbent in bed #2. Based on laboratory- 
scale test the remaining bromoethane should be < 15% of the 
original, which would be 8.28 kg making a minimum bed size of 82 kg. 
A reasonable safety margin is thus maintained by making bed #1 and 
#2 of equal size. 



42 



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measurements of the microbial mutagenicity of volatile liquids. Tice, 

R.R. , Costa, D.L., Shaich, K.M.,eds,, In Genotoxic effects of airborne 

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Barber, E.D.; Donish, W.H. , Mueller, K.R. , 1981. A procedure for the 

quantitative measurement of the mutagenicity of volatile liquids in the 

Ames Salmonella/microsome assay. Mutat. Res. 90:31 - 48. 
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growth and reversion in the Ames Salmonella plate incorporation assay. 

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Blatt, A.H. ed. , 1960. Organic syntheses. Collective volume I. 2nd. ed. , John 

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species of mites infesting grain. J. Stored Prod. Res. 17:83 - 87. 
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in Solanxxm tuberosum L. Am. Potato J. 60:161 - 167. 
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tuber dormancy. Am. Potato J. 61:587 - 589. 
Coleman, W.K. , 1987. Dormancy release in potato tubers: A review. Am. Potato 

J. 64:57 - 68. 
Coleman, W.K. ; Coleman, S.E., 1986. The effects of bromoethane and ethanol on 

potato (Solanum tuberosum) tuber sprouting and subsequent yield responses. 

Am. Potato J. 63:373 - 377. 



43 



Coleman, W.K. ; Murphy, A. , 1990. Effect of dormany releasing chemicals on 

subsequent tuber response to Fusarium samhucinum. Am. Potato J. 

67:133 - 136. 
Denny, F.E., 1926A. Hastening the sprouting of dormant potato tubers. Am. J. 

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46 



APPENDIX I: THEORETICAL ANALYSIS 



47 



THEORETICAL ANALYSIS 

When the vapor of a compound is allowed to come to equilibrium with a solid 
surface , the concentration of the compound is always found to be greater on the 
surface than in the free gas. This process forms the basis of an equilibrium 
adsorption system for removing an adsorbable compound (a sorbate) using a porous 
solid adsorbent. 

For a component of a mixture, at a fixed temperature, the adsorbed phase 
concentration of that component is a function of the partial pressure in the gas 
phase. Experimentally we can determine the adsorbed phase concentration as a 
function of partial pressure of that component - the resulting plot is the 
adsorption equilibrium isotherm. Isotherms show two distinct regions. At low 
concentrations , the amount adsorbed is directly proportional to the partial 
pressure: 

q - Kp 
where: q is the adsorbed phase concentration of a component 

p is the partial pressure of the component in the gas phase 
K is an equilibrium constant. 

At high concentrations, there is a region where the amount adsorbed is not 
a strong function of the partial pressure. For very high partial pressures, the 
amount adsorbed reaches a maximum called the saturation limit. When adsorption 
occurs, the heat adsorbed is called the heat of adsorption. Knowing the 
equilibrium constant as a function of temperature, the heat of adsorption can be 
calculated using the following equation: 

K - Ko exp (-AH/RT) 
where: K is the equilibrium constant 
T is the absolute temperature 

48 



Ko is the pre -exponential constant 

R is the Universal Gas Constant (cal/gmol- °K) 

AH is the heat of adsorption. 

The strength of the sorbate/adsorbent interaction is quantified by the heat 
of adsorption. For large values, the energy required to remove the adsorbed 
compound is also large. 

Adsorption equilibrium isotherms can be determined using gravimetric or 
volumetric methods. For sorbates that have high heats of adsorption and are 
adsorbed quickly, a constant temperature may be difficult to maintain using these 
methods, which may influence the equilibrium data. A chromatographic technique 
is a practical alternative to these more conventional techniques because 
intrusion of heat transfer resistances are essentially eliminated. 

In a chromatographic experiment, an adsorption column is subjected to a 
perturbation in the sorbate concentration of the inlet stream and the response 
of the outlet concentration is monitored. This response plotted as a function 
of time is the breakthrough curve. The perturbation can either be a pulse or a 
step input in concentration - the same information can be obtained from either. 
In this study a step was used. 

For a symmetrical breakthrough curve, the mean residence time t B> is equal 
to the mid-point time, i.e. 

t B - t, C/C - 0.5 
where: C is the outlet sorbate concentration at time t 
Co is the initial concentration at the inlet. 

For each breakthrough curve we can obtain the effective equilibrium 
constant, K, according to: 



49 



where: m - e/(l - O. the void volume per unit solid volume in the bed 
L - length of the bed 
V - interstitial velocity in the bed, 

- volumetric flow . A is cross sectional area of the bed 

A • €. 

The effective equilibrium constant is the sum of the equilibrium constants 
for the helium gas carrier and the adsorbable component: 

K — ^l^i + £ 2 X 2 

where: K x is the helium equilibrium constant 
K 2 is the sorbate equilibrium constant 
x : is the mole fraction of helium 
x 2 is the mole fraction of the sorbate. 
The amount of carrier gas (in most cases, helium) adsorbed is generally 
considered to be negligible compared to the sorbate concentration, i.e. K x » K 2 . 
For binary systems, as x x -» and x 2 -» 1 (helium is component 1), the above 
equation is true and for strongly adsorbed species as Xj_ -* 1 and x 2 ■* 0, the 
equation will also hold true. 



50 



UBRARY/BIBLIOTHEQUE 



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