The Loop Process

This is a modification of emulsion polymerisation which has recently been developed for the manufacture of commercially important latexes for emulsion paints. In this process instead of producing the polymer in batches in a tank polymer is produced continuously in a reactor that consists of a continuous tube coiled to a convenient shape. 

The major advantage of the loop process is that the equipment required to run it is small, lightweight, and hence of low cost. It needs minimal supervision yet can produce significant volumes of latex over the course of a year, and so is becoming increasingly important in industry. 

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A continuous tubular-loop process has been patented (10) and used for relatively small-scale production. The loop process consists of a tube-pump system in which the rate of latex circulation around the tube loop is considerably greater than the throughput rate. Thus, the distribution of residence times should be nearly the same as that of a single CSTR. 

When we use continuous analog controllers, all signals in a loop are continuous in time. Then the dynamic behavior of each component in the loop (process, measuring device, controller, final control element), as well as the response of the overall control system, can be effectively analyzed by continuous models (differential equations in the time domain or transfer functions in the Laplace domain). 

One alternative is the Loop process [1-3]. This employs a rather simple principle. A small volume of reaction mixture is recirculated, while streams of monomer and water phase [a stabilizer solution such as aqueous poly(vinyl alcohol)] are pumped into the reactor in the correct proportions. The reactor is fully filled and a balancing volume of product is released through a pressure-sustaining valve. Any unreacted monomer remaining in the outlet stream polymerizes on the way to the cooling tank or over a few hours, prior to packing. The volume of this type of reactor is only 40 to 80 L compared to 3000 to 100,000 L for a batch reactor. 

One of the greatest contrasts between the processes is the residence time within the reactor. It has already been noted that the poly(vinyl alcohol) in the batch process is usually present from the start. The residence time is therefore several hours. In contrast, the mean residence time of materials in the Loop process is around 2 to 10 min. This has obvious advantages in terms of minimizing degradation of colloids but will also restrict grafting between colloid and monomer. 

Aside from process comparisons, the main contrast between the systems is that of size, weight, and cost, especially for pressurized systems. Construction of batch reactors for use with ethylene at pressures of 1000 psi (70 atm) and upward has to be massive. The simple construction of the Loop process just pumps and pipework blends itself to use at high pressures. Apart from cost and weight, the small volume of the Loop reactor has obvious safety advantages. Despite these attractions, the Loop reactor system has so far been used successfully only for low-pressure systems such as poly(vinyl acetate) homopolymer for adhesives and copolymers for paint. Large-scale production of ethylene-vinyl acetate copolymers has yet to be demonstrated. 

Vinyl acetate homopolymers are simply-made adhesive bases manufactured by addition polymerization in the presence of water and stabilizers. They are made commercially by the batch reactor process or by the Loop reactor continuous process. External plasticizers such as dibutyl phthalate are often added to confer flexibility and to lower the temperature at which they form a film on drying. Higher-quality products may be made by the copolymerization of ethylene with vinyl acetate to form an EVA. This involves the safe handling of ethylene gas under high pressure, and the plant required is more complex and considerably more costly. The Loop process has considerable attraction in the field of pressure polymerization. 

The driving force for developing ethylene polymerization in a loop reactor was problems encountered in the autoclave process with fouling and related problems with heat removal. In the loop process two important features support effective heat removal the high surface-to-volume ratio offered by the pipe and the turbulent flow... 

There are several transition methods used for the loop process. 

V (the potential) is identified with the enthalpy, i.e. the number n of base pairings (contacts), and T corresponds to the entropy. At each stage in the folding process, as many as possible new favourable intramolecular interactions are fonned, while minimizing the loss of confonnational freedom (the principle of sequential minimization of entropy loss, SMEL). The entropy loss associated with loop closure is (and the rate of loop closure exp... 

The simulations also revealed that flapping motions of one of the loops of the avidin monomer play a crucial role in the mechanism of the unbinding of biotin. The fluctuation time for this loop as well as the relaxation time for many of the processes in proteins can be on the order of microseconds and longer (Eaton et al., 1997). The loop has enough time to fluctuate into an open state on experimental time scales (1 ms), but the fluctuation time is too long for this event to take place on the nanosecond time scale of simulations. To facilitate the exit of biotin from its binding pocket, the conformation of this loop was altered (Izrailev et al., 1997) using the interactive molecular dynamics features of MDScope (Nelson et al., 1995 Nelson et al., 1996 Humphrey et al., 1996). 

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

The limits of pore size corresponding to each process will, of course, depend both on the pore geometry and the size of the adsorbate molecule. For slit-shaped pores the primary process will be expected to be limited to widths below la, and the secondary to widths between 2a and 5ff. For more complicated shapes such as interstices between small spheres, the equivalent diameter will be somewhat higher, because of the more effective overlap of adsorption fields from neighbouring parts of the pore walls. The tertiary process—the reversible capillary condensation—will not be able to occur at all in slits if the walls are exactly parallel in other pores, this condensation will take place in the region between 5hysteresis loop and in a pore system containing a variety of pore shapes, reversible capillary condensation occurs in such pores as have a suitable shape alongside the irreversible condensation in the main body of pores. 

The first stage in the interpretation of a physisorption isotherm is to identify the isotherm type and hence the nature of the adsorption process(es) monolayer-multilayer adsorption, capillary condensation or micropore filling. If the isotherm exhibits low-pressure hysteresis (i.e. at p/p° < 0 4, with nitrogen at 77 K) the technique should be checked to establish the degree of accuracy and reproducibility of the measurements. In certain cases it is possible to relate the hysteresis loop to the morphology of the adsorbent (e.g. a Type B loop can be associated with slit-shaped pores or platey particles). 

Knitted fabrics are produced from one set of yams by looping and interlocking processes to form a planar stmcture. The pores in knitted fabrics are usually not uniform in size and shape, and again depend largely on yam dimensions and on the numerous variables of the knitting process. Knitted fabrics are normally quite deformable, and again physical properties are strongly dependent on the test direction. 

The air jet textured yam process is based on overfeeding a yam into a turbulent air jet so that the excess length forms into loops that are trapped in the yam stmcture. The air flow is unheated, turbulent, and asymmetrically impinges the yam. The process includes a heat stabilization zone. Key process variables include texturing speed, air pressure, percentage overfeed, filament linear density, air flow, spin finish, and fiber modulus (100). The loops create visual and tactile aesthetics similar to false twist textured and staple spun yams. 

A modification of the direct process has recentiy been reported usiag a ckculating reactor of the Buss Loop design (11). In addition to employing lower temperatures, this process is claimed to have lower steam and electricity utihty requirements than a more traditional reactor (12) for the direct carbonylation, although cooling water requirements are higher. The reaction can also be performed ia the presence of an amidine catalyst (13). Related processes have been reported that utilize a mixture of methylamines as the feed, but require transition-metal catalysts (14). 

The term channel induction furnace is appHed to those in which the energy for the process is produced in a channel of molten metal that forms the secondary circuit of an iron core transformer. The primary circuit consists of a copper cod which also encircles the core. This arrangement is quite similar to that used in a utdity transformer. Metal is heated within the loop by the passage of electric current and circulates to the hearth above to overcome the thermal losses of the furnace and provide power to melt additional metal as it is added. Figure 9 illustrates the simplest configuration of a single-channel induction melting furnace. Multiple inductors are also used for appHcations where additional power is required or increased rehabdity is necessary for continuous operation (11). 

An alternative starting network is one without stream spHts. The networks from the TI method maximize energy recovery and may introduce heat-load loops. Stream spHts ate not made in the initial steps of network invention. The ED method is proposed to be one in which heuristic rules and strategies would be used to improve the networks developed by the TI method. The importance of a thermodynamic base for evolutionary rules is stressed in this proposal, but there is no expHcit guidance for the evolutionary process. 

Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

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