Optimizing control of a continuous stirred tank fermenter using a neural network

Feedforward neural networks are a general class of nonlinear models that can be used advantageously to model dynamic processes. In this investigation, a neural network was used to model the dynamic behaviour of a continuous stirred tank fermenter in view of using this model for predictive control. In this system, the control setpoint is not known explicitly but it is calculated in such a way to optimize an objective criterion. The results presented show that neural networks can model very accurately the dynamics of a continuous stirred tank fermenter and, the neural model, when used recursively, can predict the state variables over a long prediction horizon with sufficient accuracy. In addition, neural networks can adapt rapidly to changes in fermentation dynamics.List of Symbols F Dimensionless flow rate (F/ V₀) - F m³/h Flow rate - F ₀ m³/h Inlet flow rate - J Objective cost function - K _i Dimensionless constant in Eq. (3) (k _i /s₀) - k _i kg/m³ Substrate inhibition constant in Haldane model - k _m Dimensionless constant in Eq. (3) (k _s /s₀) - k _m kg/m³ Substrate inhibition constant in Haldane model - n prediction horizon - S Dimensionless substrate concentration (s/s₀) - s kg/m³ Substrate concentration - t h Time - v Dimensionless volume (V/V₀) - V m³ Liquid volume in fermenter - W _ij , W _jk Weight matrices in neural network - X Dimensionless biomass concentration - x kg/m³ Biomass concentration - Y Biomass/substrate yield coefficient - Weighting factor in Eq. (4) - Dimensionless specific growth rate (/ ) - 1/h Maximum specific growth rate - 1/h Specific growth rate - Dimensionless time ( t)     

Variation in stable isotope signatures of seston and a zooplanktivorous fish in a eutrophic Chinese lake

Temporal and spatial changes in ¹³C and ¹⁵N of seston (mainly phytoplankton) and isotopic relationship between seston and the lake anchovy (Coilia ectenes) were studied in the large eutrophic freshwater Lake Chaohu in China. Much of the spatial and temporal variation in ¹³C of lake anchovies was explained by variation in seston, indicating a strong link between pelagic primary production and higher order consumers. Because the lake is shallow, there were no significant differences in ¹³C and ¹⁵N of seston between surface and overlying waters. Spatially, the relatively high ¹³C and ¹⁵N of seston in the western part of the lake might be due to high levels of anthropogenically derived N and C introduced from the surrounding cities through sewage drainage systems. The trophic position of the lake anchovy in the food web of Lake Chaohu was estimated to be 2.9–4.1 (3.5 ± 0.4), which agrees well with the previous stomach content analysis suggesting that the lake anchovy fed both on zooplankton and small planktivorous fishes.     

Paramagnetic spin catalysis of a radical recombination reaction

Recombination of the triplet state radical pair consisting of two hydrogen atoms catalysed by molecular oxygen is considered as a simulating example of a paramagnetic-exchange catalytic process. Intermolecular exchange interaction in the collision complex between the H₂ and O₂ molecules is calculatedab initio in STO-6G and 6–31 G^* basis sets with complete active space configuration interaction. Calculations are done at a fixed O–H distance (3 Å), scanning the H–H bond length from 0.6 till 12 Å at the linear geometry of collision. The mixture of the triplet (T)³ _u ⁺ and singlet (S)¹ _g ⁺ states of the hydrogen moiety is possible because both states have the same triplet symmetry in the collision complex with O₂ (³ _g ^– ). A strong mixture of the S (¹ _g ⁺ , H₂ +³ _g ^– , O₂) and T) and T (³ _u ⁺ , H₂ +³ _g ^– , O₂) states is actually obtained even at large H–H distances. The quintet and singlet states^5,1(³ _u ⁺ , H₂ +³ _g ^– , O₂) are also considered for comparison of the exchange potentials. Atr(H–H)4.4 Å the S-T splitting is approximately constant (12 cm^-1 in the STO-6G basis set; 55.5 cm^-1 in the 6–31 G^* basis set) and is determined by the exchange interaction between O₂ and the nearest hydrogen atom in the O–O...H fragment. The paramagnetic catalyst can accelerate radical recombination through the triplet-singlet nonadiabatic transition to the lowest S reactive state when the radical encounter takes place in the vicinity of the catalyst. Though we do not consider the radical dynamics in a real solvent, which modulates the exchange potentials and the T-S transitions, the nature of this mechanism of spin catalysis is obvious. The electric polarization and charge transfer are important in the analysis of the exchange interaction and radical recombination potentials for all multiplets. In accordance with the concept of spin catalysis, the electronic spin-uncoupling mechanism, induced by O₂ perturbation, has the same nature as other known catalytic processes of paramagnetic-exchange type.     

Evaluation of sensors for the control of a continuous ethanol fermentation

A method is presented for the evaluation of sensors used in the control of continuous fermentations. Simulations of open-loop response to input disturbance provided a starting point for the choice of sensor type. This was evaluated quantitatively through a sensitivity ratio. It was shown that in the case of ethanol fermentation, there existed three regions where different sensors could be used for the process control depending on the inlet sugar concentration. Sugar sensors were preferable above an inlet sugar concentration of 50 kg/m³, while ethanol sensors were preferable below 25 kg/m³. In the intermediate region, sugar and ethanol sensors demonstrated equally good performance. A controllability study of a continuous ethanol fermentation was also made. A single-stage continuous stirred-tank fermentor was simulated operating at a dilution rate of 0.1 1/h and inlet glucose concentration of 160 kg/m³. The outlet glucose concentration was controlled with a PI controller. Mean square error of the controller input signal during the first five hours after introducing input disturbance was taken as a measure of the controllability. This was studied in the relation to the two key sensor characteristics, sampling time and accuracy.List of Symbols c _p kg/m³ ethanol concentration - c _p kg/m³ fermentor ethanol concentration corresponding to c _si and D - c _s kg/m³ substrate (glucose) concentration - c _s kg/m³ fermentor glucose concentration corresponding to c _si and D - c _si kg/m³ inlet substrate (glucose) concentration - c _si kg/m³ inlet glucose concentration value used for sensitivity evaluation - c _sm kg/m³ glucose concentration — measured value - c _ss kg/m³ glucose concentration setpoint value - c _x kg/m³ biomass concentration - D 1/h dilution rate - D 1/h dilution rate value used for sensitivity evaluation - D _i 1/h dilution rate at ith sampling interval - D ₀ 1/h dilution rate at steady state - K _c m³/kgh controller gain - K _p kg/m³ product inhibition constant - K _s kg/m³ Monod constant - n ₁, n ₂ random numbers - r _p kg/m³ h ethanol production rate - r _s kg/m³ h substrate (glucose) consumption rate - r _x kg/m³ h biomass growth rate - vector of independent variables - y _i ith dependent variable - Y _ps ethanol yield - Y _xs biomass yield - parameter vector - _j jth parameter - _ij sensitivity of y_i with respect to _j - _p sensitivity of fermentor ethanol concentration - _s sensitivity of fermentor glucose concentration - sensitivity ratio - c _p kg/m³ ethanol concentration difference corresponding to a change of c _si by 5% - c _s kg/m³ glucose concentration difference corresponding to a change of c _si by 5% - c _si kg/m³ concentration difference added to c _si - _i kg/m³ error at ith sampling interval - 1/h specific growth rate - _m 1/h maximum specific growth rate - _s kg/m³ standard deviation of monitored glucose concentration - _I h min kg/m³ integral time - _s min sampling period The Swedish Ethanol Foundation and the National Board for Technical Development (NUTEK) are kindly acknowledged for the financial support of this project. The authors wish to thank Peter Warkentin for the linguistic advice.     

Control of basidiospore production by a nuclear gene in the fungus Coprinus congregatus

Summary The genetical control of basidiospore production by sporophores of the fungus Coprinus congregatus was studied. This species is characterized by a bipolar compatibility control, and homokaryons with complementary alleles A1 and A2 can be distinguished apart. We confirmed that the pale mushroom phenotype of the fungus is determined by a nuclear gene symbolized pal. This gene also controls a sporeless character and segregates independently of the mating-type locus. Dikaryons homoallelic for the pal ^– allele produce typical pale and sporeless sporophores, while heteroallelic (pal ⁺, pal ^–) and homoallelic (pal ⁺, pal ⁺) dikaryons produce normal or almost normal sporulating sporophores. In order to segregate homokaryons homoallelic for the pal gene (A1, pal ^–; A1, pal ⁺, A2, pal ^–; A2, pal ⁺), the following protocols were used: (a) the dikaryotization of stock homokaryons containing the pal ⁺ allele and of each mating type, A1 or A2, by dikaryotic mycelia homoallelic for the pal ^– allele; (b) the culturing of homokaryotic mycelia issuing from the germination of basidiospores from sporophores produced by dikaryotic mycelia heterokaryotic for the pal gene; (c) the culturing of mycelia grown from protoplasts obtained from dikaryons homoallelic for the pal ^– allele (D6 strain), and from homokaryons heteroallelic for the pal gene (H8), or homoallelic for pal ^#x002B;+ allele (H7). These techniques enabled us to segregate homokaryons of the four types defined above and were indispensable in the segregation of the pal ^– homoallelic homokaryons as no basidiospores were produced by typical pale mushrooms.     

Detection of a 65 kDaras binding protein in rat and sheep brain cytosol using a chemical cross linking agent

The ability of aras protein to associate with proteins present in rat brain cytosolin vitro was investigated using chemical cross-linking agents and the¹²⁵I-labelled v-H-ras protein. Two iodinated protein complexes with apparent molecular weights of 40 and 85 kDa were observed when a mixture of rat brain cytosol and [¹²⁵I]ras was treated with the cross-linking agent disuccinimidyl suberate and subjected to SDS-PAGE. Formation of the [¹²⁵I] 85 kDa complex was enhanced by a high concentration of EDTA while generation of the 40 kDa species was abolished by this treatment. Formation of the [¹²⁵I] 85 kDa complex was inhibited by unlabelledras protein, GTP, GTPS, and GDP but not by ATPS and GMP.Chromatography of the cross-linked brain cytosol-[¹²⁵I]ras mixture on DEAE cellulose partially resolved the [¹²⁵I] 85 kDa complex from the [¹²⁵I]ras protein. The [¹²⁵I] 85 kDa complex (formed using ethyleneglycolbis (succinimidylsuccinate) as the cross-linking agent) could be immunoprecipitated using a rabbit anti-ras polyclonal antibody. Treatment of the immunoprecipitate with hydroxylamine to cleave the cross-link yielded [¹²⁵I]-labelledras. A substantial enrichment of the proportion of the [¹²⁵I] 85 kDa complex in the cross-linked extract was achieved by preparative SDS-PAGE. It is concluded that thein vitro chemical cross-linking approach employed here has detected tworas binding proteins in rat brain cytosol: a 65 kDa heat-sensitive and a 20 kDa heat-stable protein. The possibility that the 65 kDaras binding protein is aras regulatory orras effector protein which has not so far been characterised is briefly discussed.Abbreviations DSS disuccinimidyl suberate - EGS ethyleneglycolbis (succinimidylsuccinate) - GTPS guanosine 5-[-thio] triphosphate - ATPS adenosine 5-[-thio] triphosphate     

Gas holdup and mass transfer characteristics of carboxymethyl cellulose solutions in a bubble column with a radial gas sparger

The gas phase holdup and mass transfer characteristics of carboxymethyl cellulose (CMC) solutions in a bubble column having a radial gas sparger have been determined and a new flow regime map has been proposed. The gas holdup increases with gas velocity in the bubbly flow regime, decreases in the churn-turbulent flow regime, and increases again in the slug flow regime. The volumetric mass transfer coefficient (k _La) significantly decreases with increasing liquid viscosity. The gas holdup and k _La values in the present bubble column of CMC solutions are found to be much higher than those in bubble columns or external-loop airlift columns with a plate-type sparger. The obtained gas phase holdup ( _g) and k _La data have been correlated with pertinent dimensionless groups in both the bubbly and the churn-turbulent flow regimes.List of Symbols a m^–1 specific gas-liquid interfacial area per total volume - A _d m² cross-sectional area of downcomer - A _r m² cross-sectional area of riser - d _b m individual bubble diameter - d _vs m Sauter mean bubble diameter - D _c m column diameter - D _L m²/s oxygen diffusivity in the liquid - Fr Froude number, U _g/(g D_c)^1/2 - g m/s² gravitational acceleration - G _a Galileo number, gD _c ^{3 2}/² _app - H _a m aerated liquid height - H _c m unaerated liquid height - K Pa · sⁿ fluid consistency index - k _L a s^–1 volumetric mass transfer coefficient - n flow behavior index - N _i number of bubbles having diameter d _bi - Sc Schmidt number, _app/( D _L) - Sh Sherwood number, k _L a D _c ² /D_L - U _sg m/s superficial gas velocity - U _gr m/s superficial riser gas velocity - V _a m³ aerated liquid volume - V _c m³ unaerated liquid volume - N/m surface tension of the liquid phase - _g gas holdup - _app Pa · s effective viscosity of non-Newtonian liquid - kg/m³ liquid density - ý s^–1 shear rate - Pa shear stress     

Start-up sensitivity to the initial state of a batch bioreactor for a recombinant Escherichia coli in a complex medium

The sensitivities with respect to the initial state of five key variables describing the performance of a batch bioreactor have been computed from an experimentally validated kinetic model. The system has a recombinant Escherichia coli strain containing the plasmid pBR Eco gap, which codes for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in a complex medium. Since previous studies have shown the start-up sensitivities to be particularly important, the initial 10% of the duration of fermentation was chosen as the time span. The sensitivities of the cell mass, GAPDH and acetate increased with time while those of glucose and yeast extract remained practically constant.Acetate has a crucial role as it functions as both a product and a reactant. With no acetate in the inoculum, the sensitivities of acetate increased an order of magnitude faster than other sensitivities. However, upon addition of acetate through the inoculum, its sensitivities decreased the fastest and stabilised beyond a starting concentration of about 1 g/l whereas other sensitivities stabilised after 5 to 6 g/l of initial acetate. A three-dimensional envelope in the space of acetate concentration-time-relative sensitivity shows a locus of concentrations for minimum time-dependent acetate sensitivity; this may be maintained through fed-batch operation.List of Symbols a A/A₀ - A g/l initial concentration at any time - A ₀ g/l initial acetate concentration - e E/E₀ - E g/l yeast extract concentration at any time - E ₀ g/l initial yeast extract concentration - g G/G₀ - G g/l glucose concentration at any time - G ₀ g/l initial glucose concentration - k _A ^A g/l inhibition constant for acetate-dependent growth during the acetate phase - k _A ^G g/l inhibition constant for acetate-dependent growth during the glucose phase - k _M ^A 1/h rate constant for acetate phase - k _M ^G 1/h rate constant for glucose phase - K _A g/1 affinity constant for acetate - K _G g/1 affinity constant for glucose - m ^A 1/h coefficient of maintenance in acetate - m _m ^A 1/h maximum value of m ^A - m ^G 1/h coefficient of maintenance in glucose - m _m ^G 1/h maximum value of m ^G - n empirical constant - P P/P₀ - P U/ml GAPDH concentration at any time - P ₀ U/ml initial GAPDH concentration - s _c (i,j) sensitivity of y _i to y _j(0) for A ₀=c - t h time - x X/X₀ - X g/l cell mass concentration at any time - X ₀ g/l initial cell mass concentration - y ₁ x - y₂ g - y₃ a - y₄ e - y ₅ p - y _x/A ^A g/g yield coefficient for cell mass per unit mass of acetate during acetate phase - y _x/A ^G g/g yield coefficient for cell mass per unit mass of acetate during glucose phase - y _x/G g/g yield coefficient for cell mass per unit mass of glucose - y _E/x ^A g/g yield coefficient for yeast extract per unit cell mass during acetate phase - y _P/x ^A g/g yield coefficient for yeast extract per unit cell mass during glucose phase - y _P/x ^A U/g yield coefficient for GAPDH per unit cell mass during acetate phase - y _P/x ^G U/g yield coefficient for GAPDH per unit cell mass during glucose phase Greek Letters ₀ proportionality constant for plasmid loss probability - ₁ 1/h maximum rate of plasmid replication - ₂ 1/h saturation constant of the host component of plasmid replication - regulation function (0 or 1) - regulation function (0 or 1) - exponent of growth inhibition term for acetate during the acetate phase - exponent of growth inhibition term for acetate during the glucose phase - ^A 1/h specific growth rate during acetate phase - _m ^A 1/h maximum value of ^A - ^G 1/h specific growth rate during glucose phase - _m ^G 1/h maximum value of ^G - _c (i,j) ratio of sensitivities, s _c (i,j)/s ₀(i,j) - nondimensional time, t _m ^G      

Analysis of D2 d : a D-region class I gene

The mouse major histocompatibility complex is composed of several genes arranged into the K, D, Qa, and Tla regions. The D region of the BALB/c mouse includes genes D2 ^d , D3 ^d , and D4 ^d , in addition to H-2D ^d and H-2L ^d . We have determined the DNA sequence of the D2 ^d gene and compared it with the known sequences of several class I genes. The exon/intron structure of the D2 ^d gene is similar to other class I genes. It also contains similar 5 regulatory elements. A frameshift occurs in exon seven, resulting in a gene product with a truncated cytoplasmic tail. To examine the surface expression of the D2^d molecule, we generated an exon-shuffled construct containing the promoter and exons 1–3, encoding the signal peptide, 1, and 2 external domains of the D2 ^d gene linked to exons 4–8, encoding the 3, transmembrane and cytoplasmic domains, of the H-2D ^d gene. The construct was transfected into mouse L cells, and a protein was detected at the cell surface by a monoclonal antibody (mAb) specific for the 3 domain of H-2D^d, as well as by other class I-specific mAbs. Although D2^d is expressed at low levels, it may be a functional class I gene that most probably evolved from a Qa region gene.     

Mixing time in a down-flow jet loop bioreactor

Mixing time was determined in a down-flow jet loop bioreactor with Newtonian and non-Newtonian fluids. It was observed that the mixing time decreased with an increase in linear liquid velocity, superficial gas velocity, draft tube to column diameter ratio, nozzle diameter and shear thinning of media. The optimum draft tube to column diameter ratio was found to be about 0.44. Correlations were presented for prediction of mixing time.List of Symbols A m² cross sectional area of the column - C kmol/m³ local tracer concentration - A _D m² flow area,A _D =/4 (D _Z ² -D _TO ² ) - D m column diameter - D _E m draft tube diameter - D _TO m outside diameter of the air tube - D _TFL m equivalent flow diameter,D _TFL =(D _Z ² -D _TO ² )^0.5 - D _z m nozzle diameter - g m/s² gravitational acceleration - h % inhomogeneity - H m height of the column - H _B m distance between the lower edge of the draft tube and the impact plate - H _T m distance between the upper edge of the draft tube and the liquid nozzle - K Pa.sⁿ consistency index in power-law model - L _E m length of the draft tube - n flow index in the power-law model - Re _j jet Reynolds number,Re _j =(D _TFL×w₁×_L)/ _eff - t _M s mixing time - t _sg m/s superficial gas velocity based onA - W _l m/s linear liquid velocity based onD _D Greek Letters N/m² shear stress - s shear rate - kg/m³ density of liquid - N/m surface tension of the liquid - Pa.sⁿ viscosity of liquid Indices X concentration at infinite time maximum value of tracer concentration - eff effective - L Liquid - obs observed - pred Predicted     

Inactivation of a newly transferred gene in recombination-deficient recipients of Escherichia coli

Summary The expression of a newly transferred lacZ ⁺ gene in lacZ recipients carrying various mutations in the recA and recB genes was studied by measuring the rates of induced synthesis of -galactosidase in zygotes formed after mating with either F or Hfr donors. The ability to synthesize -galactosidase decreases with time in both recA and recB zygotes when the lacZ ⁺ gene is transferred from an Hfr donor, but not when the lacZ gene is transferred from an F donor. There is no such inactivation of the newly transferred lacZ ⁺ gene in Rec⁺ zygotes. We conclude that the functioning of the transferred DNA is progressively inactivated in rec recipients unless the DNA is contained in an episome such as F.     

Rates of decay for the survival probability of a mutant gene

If qk is the extinction probability of a slightly supercritical branching process with offspring distribution P _kr : r = 0, 1, 2,..., then it is shown that if sup _r r ³ p _kr , < , inf ² _k > 0, and m _k 1, then 1 – q _k 2(m _k –1) _k ^–2, where m _k = _r rp _kr , ² _k = ^k _r r ² p _kr – m _k ². This provides a simple set of sufficient conditions for the validity of a conjecture of Ewens (1969) for the survival probability of a slightly advantageous mutant gene.Research supported in part by NSF grants DMS-8803639 and DMS-9007182     

Sodium-transport NADH-quinone reductase of a marineVibrio alginolyticus

The respiratory chain of a marine bacterium,Vibrio alginolyticus, required Na⁺ for maximum activity, and the site of Na⁺-dependent activation was localized on the NADH-quinone reductase segment. The Na⁺-dependent NADH-quinone reductase extruded Na⁺ as a direct result of redox reaction. It was composed of three subunits, , , and , with apparentMr of 52, 46, and 32 KDa, respectively. The reduction of ubiquinone-1 to ubiquinol proceeded via ubisemiquinone radicals. The former reaction was catalyzed by the FAD-containing subunit. This reaction showed no specific requirement for Na⁺. For the formation of ubiquinol, the presence of the subunit and the FMN-containing subunit was essential. The latter reaction specifically required Na⁺ for activity and was strongly inhibited by 2-n-heptyl-4-hydroxyquinolineN-oxide. It was assigned to the coupling site for Na⁺ transport. The mode of energy coupling of redox-driven Na⁺ pump was compared with those of decarboxylase- and ATP-driven Na⁺ pumps found in other bacteria.     

Existence and uniqueness of a sharp travelling wave in degenerate non-linear diffusion Fisher-KPP equations

In this paper we use a dynamical systems approach to prove the existence of a unique critical value c ^* of the speed c for which the degenerate density-dependent diffusion equation u _ct = [D(u)u _x ] _x + g(u) has: 1. no travelling wave solutions for 0 < c < c ^*, 2. a travelling wave solution u(x, t) = (x - c ^* t) of sharp type satisfying (– ) = 1, () = 0 ^*; '(^*–) = – c ^*/D'(0), '(^*+) = 0 and 3. a continuum of travelling wave solutions of monotone decreasing front type for each c > c ^*. These fronts satisfy the boundary conditions (– ) = 1, '(– ) = (+ ) = '(+ ) = 0. We illustrate our analytical results with some numerical solutions.     

Optimization and control of a continuous stirred tank fermenter using learning system

A variable structure learning automaton is used as an optimization and control of a continuous stirred tank fermenter. The algorithm requires no modelling of the process. The use of appropriate learning rules enables to locate the optimum dilution rate in order to maximize an objective cost function. It is shown that a hierarchical structure of automata can adapt to environmental changes and can also modify efficiently the domain of variation of the control variable in order to encompass the optimum value.List of Symbols f Random number - F Dimensionless flow rate (F/V ₀) - F m³/h Flow rate - F ₀ m³/h Inlet flow rate - J Objective function - K _i Dimensionless constant in Eq. (3) (k _i/s₀) - k _i · kg/m³ Substrate inhibition constant in Haldane model - K _m Dimensionless constant in equation (3) (k _s/s₀) - k _m kg/m³ Substrate inhibition constant in Haldane model - L Number of levels of the hierarchical system of automata - N Number of possible control actions - p Probability - S Dimensionless substrate concentration (s/s ₀) - s kg/m³ Substrate concentration - T Dimensionless sampling period - t h Time - v Dimensionless volume (V/V ₀) - V m³ Liquid volume in fermenter - W Input to the stochastic automaton - X Dimensionless biomass concentration - x kg/m³ Biomass concentration - Y Biomass/substrate yield coefficient - Weighting factor in Eq. (4) - Dimensionless specific growth rate (/ ^*) - ^* h^–1 Maximum specific growth rate - h^–1 Specific growth rate - Dimensionless time ( t)     

Photosynthesis Genes and LH1 Proteins of Roseospirillum Parvum 930I,a Purple Non-Sulfur Bacterium with Unusual Spectral Properties

The purple bacterium Roseospirillum(Rss.)parvum 930I, like most other purple bacteria, contains bacteriochlorophyll (BChl)-a as a LH chromophore, but exhibits an extremely red-shifted Q _y absorption maximum centered at 909 nm. The puf operon encoding the LH1 and other pigment-binding proteins was cloned and sequenced, revealing the gene structure pufBALMC. Comparative analysis of predicted amino acid sequences of the - and -core LH polypeptides (PufA and PufB) revealed five amino-acid substitutions at positions that are highly conserved in other purple bacteria. In the primary structure, these residues are in close proximity to the BChl-a ligating histidine residue ( ⁺³, ⁺⁵, ⁺⁶ and ^–4, ⁺⁹, with respect to the central histidine, His⁰). The nature of the enormous red-shifts to the near-infrared region of the light were attributed to those residues, with special emphasis to cysteine residues rarely present in other purple bacterial antenna polypeptides. Three-dimensional structural models of the Rss. parvum LH1- and - polypeptides suggest that the thiol groups of Cys⁺³ and Cys^–4 are oriented toward the BChl-a macrocycle in the native antenna complex. Because of their predicted close proximity to BChl, these thiol groups are likely to modulate the spectral properties of the LH1 complex. Phylogenetic comparisons of LH1 polypeptides indicate that Rss. parvum-like amino-acid substitutions in proteobacterial LH1 complexes arose independently during evolution.This revised version was published online in October 2005 with corrections to the Cover Date.     

Verification studies of a simplified model for the removal of dichloromethane from waste gases using a biological trickling filter

The removal of dichloromethane from waste gases in a biological trickling filter was studied experimentally as well as theoretically within the concentration range of 0–10,000 ppm. A stable dichloromethane elimination performance was achieved during two years of operation, while the start-up of the system only amounted to several weeks at constant inlet concentrations. The trickling filter system was operated co-currently as well as counter-currently.However, experimental and theoretical results revealed that the relative flow direction of the mobile phases did not significantly affect the elimination performance. Moreover, it was found that the gas-liquid mass-transfer resistance in the trickling filter bed applied was negligible, which leaves the biological process inside the biofilm to be the rate limiting step.A simplified model was developed, the Uniform-Concentration-Model, which showed to predict the filter performance close to the numerical solutions of the model equations. This model gives an analytical expression for the degree of conversion and can thus be easily applied in practice.The dichloromethane eliminating performance of the trickling filter described in this paper, is reflected by a maximum dichloromethane elimination capacity EC _max=157 g/(m³ · h) and a critical liquid concentration C _lcr=45 g/m³ at a superficial liquid velocity of 3.6 m/h, inpendent of the gas velocity and temperature.List of Symbols a _s m²/m³ specific area - a _w m²/m³ specific wetted area - A m² cross-sectional area - C _g g/m³ gas phase concentration - C _go g/m³ inlet gas phase concentration - C _gocr g/m³ critical gas phase concentration - C _g ^* C_g/C_go dimensionless gas concentration - C _l g/m³ liquid concentration - C _lcr g/m³ critical liquid concentration - C _lcr ^* mC_lcr/C_go dimensionless critical concentration - c _li g/m³ substrate concentration at liquid-biofilm interface - C _l ^* mC_l/C_go dimensionless liquid concentration - C _o g/m³ oxygen concentration inside the biofilm - C _oi g/m³ oxygen concentration at liquid-biofilm interface - C_s g/m³ substrate concentration inside the biofilm - C _si g/m³ substrate concentration at liquid-biofilm interface - D _eff m²/h effective diffusion coefficient in the biofilm - D _o m²/h effective diffusion coefficient for oxygen in the biolayer - E mu_g/u_l extraction factor - E _act kJ/mol activation energy for the biological reaction - EC g/(m³· h) K _o a _w : elimination capacity, or the amount of substrate degraded per unit of reactor volume and time - EC _max g/(m³ · h) K _o a_w: maximum elimination capacity - f degree of conversion - h m coordinate in height - H m height of the packed bed - K ₀ g/(m³ · h) _maxX_b/Y zeroth order reaction defined per unit of biofilm volume - k _og m/h overall gas phase mass transfer coefficient - K ^* dimensionless constant given by Eq. (A.5) - K _l ^* dimensionless constant given by Eq. (A.6) - K ₂ ^* dimensionless constant given by Eq. (A.6) - m C _g /C_l gas liquid distribution coefficient - N g/(m² · h) liquid-biofilm interfacial flux of substrate - N _og k_oga_wH/u_g number of gas phase transfer units - N _r k_o a_w H/u_g C_go number of reaction units - OL g/(m³· h) u _g C _go /H organic load - r _s g/(m³ ·h) zeroth order substrate degradation rate given by Eq. (1) - R _s g/(g TSS ·h) specific activity - T K absolute temperature - u _g m/h superficial gas velocity - u _t m/h superficial liquid velocity - X _b g TSS/m₃ biomass concentration inside biofilm - X _s g TSS/m₃ liquid suspended biomass concentration - x m coordinate inside the biofilm - Y g TSS/(g_DCM) yield coefficient Greek Symbols dimensionless parameter given by Eq. (2) - m averaged biofilm thickness - biofilm effectiveness factor given by Eqs. (7a)–(7c) - m penetration depth of substrate into the biofilm - _max d^–1 microbiological maximum growth rate - v _o stoichiometric utilization coefficient for oxygen - v _s stoichiometric utilization coefficient for substrate - dimensionless height in the filter bed - h H/u _g superficial gas phase contact time - _o (K ₀ /DC _ii )^1/2 - _o C _o /C _oi dimensionless oxygen concentration inside the biofilm - _s C _s /C _si dimensionless substrate concentration inside the biofilm Experimental results, verifying the model presented will be discussed Part II (to be published in Vol. 6, No. 4)     

Modelling of alcohol fermentation in a tubular reactor with high biomass recycle

Fermentation in tubular recycle reactors with high biomass concentrations is a way to boost productivity in alcohol production. A computer model has been developed to investigate the potential as well as to establish the limits of this process from a chemical engineering point of view. The model takes into account the kinetics of the reaction, the nonideality of flow and the segregation in the bioreactor. In accordance with literature, it is shown that tubular reactors with biomass recycle can improve productivity of alcohol fermentation substantially.With the help of the computer based reactor model it was also possible to estimate the detrimental effects of cell damage due to pumping. These effects are shown to play a major role, if the biomass separation is performed by filtration units which need high flow rates, e.g. tangential flow filters.List of Symbols Bo d Bodenstein number - c kg/m³ concentration of any component - CPFR continuous plug flow reactor - CSTR continuous stirred tank reactor - d _h m hydraulic diameter - D _eff m²/s dispersion coefficient - f residence time distribution function - K _s kg/m³ monod constant for biomass production - K _s kg/m³ monod constant for alcohol production - p kg/m³ product concentration - P _i kg/m³ lower inhibition limit concentration for biomass production - p _i kg/m³ lower inhibition limit concentration for alcohol production - p _m kg/m³ maximum inhibition limit concentration for biomass production - p _m kg/m³ maximum inhibition limit concentration for alcohol production - q _p h^–1 specific production rate - q _p,max h^–1 maximum specific production rate for alcohol production - q _s h^–1 specific substrate consumption rate - Q _L m _gas ³ /m³h specific gas rate - r _p , r _s , r _x kg/(m³ · h) reaction rate for ethanol production substrate consumption and cell growth, respectively - S _F kg/m³ substrate concentration in feed stream - s kg/m³ substrate concentration - t h time - x kg/m³ biomass concentration - x _max kg/m³ maximum biomass concentration for biomass production - Y _p/s yield coefficient - h^–1 specific growth rate - _max h^–1 maximum specific growth rate - dimensionless time (t/) - h mean residence time - _s glucose conversion     

Recombination in the lambda repressor gene: evidence that very short patch (VSP) mismatch correction restores a specific sequence

Summary The mutation am6 in the cI gene of bacteriophage is identified as a CT transition in a 5CC _T ^A GG sequence. In four-factor crosses of am6 with nearby mutations in cI, the frequencies of cI⁺ recombinants are much higher than expected from the physical distances. A very short patch (VSP) mismatch repair system is presumed to recognize am6/am ⁺ mispairs in the heteroduplexes that accompany recombination between the outside markers. Mutation am6 is corrected to am ⁺; correction of am ⁺ to am6 was not detected. Clear-plaque mutation 1-1 in cI is a TC transition in a 5CTTGG sequence, resulting in the sequence 5CC _T ^A GG. When 1-1 was crossed with nearby mutations in gene cI, there were no excess cI⁺ recombinants, which would result from repair of CCTGG (1-1) to CTTGG (cI⁺). However, in crosses of cI⁺ phages with mutation 1-1, there was an excess of cI^- recombinants, indicating that cI⁺ was repaired to 1-1. Preferential repair does not require adenine or cytosine methylation: when repairing a mismatch, the VSP repair system apparently identifies specific mispaired bases by sequence alone.     

Genetic architecture of a heterotic cross between two populations of maize

Summary The nature and magnitude of variability in the interpopulation cross of Mezcla Amarillo Selection (MAS), an introduction from CIMMYT, Mexico, and J607, a population developed in India using indigenous, American, and Yugoslavian germplasm, were studied. Interpopulation progenies developed by following the North Carolina Design I were evaluated at two locations. The additive genetic variance component in interpopulation cross, _A(12) ² , and in one population assuming the other population as tester, _A12 ² and _A21 ² were significant for all the traits evaluated, namely ear length, ear girth, kernel rows and days to silk, with one exception. For kernel rows, the dominance variance component, _A(12) ² , was also significant but it was smaller than _A(12) ² . The variance component due to dominance X location interaction, _DL(12) ² , was significant for all traits except kernel rows. In the case of ear length and ear girth, _DL(12) ² was greater than the other components. _AL(12) ² , _AL12 ² and _AL21 ² were not significant for any trait. Expected genetic advance indicated a superiority of half-sib reciprocal recurrent selection over full-sib reciprocal recurrent selection.