噴射染色設(shè)備上影響染料吸收因素的分析--英文版.docx

此文檔收集于網(wǎng)絡(luò)，如有侵權(quán)，請聯(lián)系網(wǎng)站刪除Analysis of the factors influencing dye uptake on jet dyeing equipmentIntroduction An important objective of sample colorations in a textile dyehouse laboratory is to simulate production-scale conditions. While typical laboratory equipment, e.g. a rotating beaker-type machine, is useful for colour matching purposes, a more detailed process analysis in the laboratory might require machines that operate more closely like those in bulk production. One important question in this context is if pilot-scale machines can be used to accurately predict the dye exhaustion of bulk dyeing machines.The influence of the liquor flow rate on the exhaustion rate, initially established on package dyeing machines, has been known for a long time and has been confirmed several times since then, also for other types of dyeing equipment . There are at least three possible factors, which alone or in combination, can explain this sensitivity of the dye-uptake rate. All three factors contribute to a reduced dye supply at the fibre surface, thereby possibly leading to a process in which the ratedetermining step is no longer the diffusion of the dye inside the fibre (film diffusion-controlled process) but the diffusion of the dye from the dyebath to the fibre surface (liquid-diffusion limited). All three factors also gain in importance as the fluid velocity drops. In the first case, it is known that as the fluid velocity drops, a diffusional boundary layer of increasing thickness develops at the substrate surface which reduces masstransfer to the individual fibre . In the second case, the primary cause is a much diminished fluid velocity near the fibre surface compared with the macroscopic average dyebath flow rate. This is caused by flow resistance in the fibre assembly (microscopic depletion) . The resistance can occur, for example, in the intra-yarn pores or at the intersection of yarns in assemblies. This phenomenon is not to be confused with boundary layer effects which, when flow direction is perpendicular to the fibre axis, vary little with flow rate. Thirdly, the dyebath depletes as it traverses the package or fabric pile, thus lowering the dye supply, and consequently the uptake rate of subsequent substrate layers (macroscopic depletion). In the third case, it is the macroscopic dye supply within the entire package or pile, as opposed to the dye supply with the yarn in the second case, which limits the overall uptake rate. Thus, the second phenomenon can arise in a single fabric layer, or even a single yarn, whereas the third phenomenon requires the existence of several layers of fabric or yarn. In this study, the term liquid diffusion effects is used when reference is made to all three factors, without making specific reference to either factor.In a jet dyeing machine, fabric and dyebath are circulating. A fabric circulation in a jet dyeing machine may be divided into the following three phases: Passage through the jet nozzle and, possibly, an attached tube: In the nozzle, fabric, water and air-flows meet to create a situation of high fabricliquor interchange. The nozzles design and the fluid velocity determine how much kinetic energy is transferred to the fabric. Fabric propulsion on bulk machines, especially at higher fabric speeds, is mainly achieved by the impulse transfer in the jet with the reel providing only a minor contribution. In order to ensure adequate propulsion of the fabric, the liquor speed in the nozzle must be higher than the fabric speed. Retention, normally partially flooded, in the storage chamber: In the storage chamber, there is some fabric liquor interchange as the liquor travels over and through the packed fabric to the suction side of the main circulation pump, from where the dyebath is propelled back into the nozzle. On modern low liquor ratio machines, which allow the processing of cotton at liquor-to-goods ratio of around 5:1, the dyebath level is so low that only a fraction of the fabric in the storage chamber is submerged. Additional dye transfer occurs between the surfaces of the wet, compressed fabric layers. Acceleration from the storage chamber over the reel and towards the nozzle: On its way from the storage chamber, where the speed is zero or almost zero, to the reel, the fabric is accelerated quickly to its maximum speed. This acceleration and the gentle squeezing effect of the reel reduce the amount of water in the fabric rope, which in the case of cotton, for example, is around three times the fabric weight. Both actions contribute to a better overall fabricliquor interchange as mechanical agitation assists dye penetration and because more water is subsequently replenished in the nozzle.Based upon this general description of the fabric path in the machine, it would be expected that the following machine parameters could influence the dye-exhaustion rate.Jet nozzle size and designThe water exiting the nozzle hits the fabric rope at an angle, but interrupting the dyeing process after one or two fabric circulations showed that, on the pilot-scale equipment described in the Experimental section, there was nevertheless only a little dye penetration of the rope. Therefore, conditions in the jet nozzle could be described in a first approximation as a cylinder or, even simpler, a sheet (the fabric rope) immersed in a flow parallel to its axis. A suitable and well-established model describes the mass-transfer from the solution to the fibre with the help of a diffusional boundary layer, which increases in thickness with the length of the cylinder/sheet and which decreases in thickness with increasing fluid velocities. As the thickness of the boundary layer increases, dye transfer from the solution to the substrate reduces and, possibly, so does the dye-exhaustion rate.For a sheet immersed in a flow parallel to its axis, fluid velocity and average boundary layer thickness can be correlated using Eqn 1. The term average boundary layer thickness is used to indicate that dD is a function of the plate length, k, and that dD is averaged over k (definitions of the symbols used in this study, along with their corresponding units, are shown in Appendix.It is therefore conceivable that a higher liquor flow rate in the nozzle and/or a more intensive fabricliquor interchange, e.g. by attaching a tube to the nozzle which increases the time of proximity between fabric and dyebath, could reduce the boundary layer thickness and increase the exhaustion rate.Fabricliquor interchange in the storage chamberConditions in the storage chamber resemble a plug flow situation as the liquor from the jet nozzle flows over and through the fabric pile, i.e. by flowing perpendicularly to the fibre axis. However, there are important deviations from the plug flow model. For example, a significant but unknown portion of the liquor from the jet nozzle bypasses the pile and flows directly to the suction side of the main circulation pump. Furthermore, most of the fabric in the storage chamber is not submerged, neither on the pilot-scale machine and nor on the production equipment, as the liquor level is so low. Nevertheless, the plug flow model can serve to illustrate that both microscopic depletion and macroscopic depletion could influence exhaustion kinetics. First, there is an experimental evidence that masstransfer to the individual fibre in the fabric may be significantly reduced compared with the individual fibre caused by microscopic depletion. As a result, dye supply may be higher than dye uptake by the fibre only in the easily accessible outer portions of the yarn. Secondly, as a result of macroscopic depletion in the fabric pile consisting of several fabric layers, dye concentration continues to decrease as the dyebath flows through the layers and as dye is taken up by each layer so that dye supply is reduced for inner, more remote layers of the fabric pile . The exhaustion rate in the storage chamber could thus drop with lower dyebath flow rates as the liquid travels more slowly over and through the piled-up fabric, leading to a liquid diffusion-controlled process because of a reduced dye supply. Also, the packing density of the fabric presumably plays a role. At higher densities, the fibre surface becomes less accessible , but on the other hand, higher pressure increases the dye transfer between the wet, compressed fabric layers and the degree of de-watering is also higher. The latter contributes to a better fabricliquor interchange by increasing the pick-up of fresh dyebath in the nozzle. Fabric circulation frequency The possible influence of the fabric circulation frequency is related to the fact that the fabric loses some of the adhering water on its way from the storage chamber to the nozzle. Experimental work on winch dyeing machines has shown that the amount of dye on the fibre surface decreases on the way from the storage chamber to the reel. The main reason for this decrease is gravitational drainage of the dyebath from the fabric once the fabric has left the dyebath at the bottom of the machine. As it is the amount of water adhering to fibre surface that is reduced, it is accurate to refer to a reduction in the dye amount (g dye kg-1 fibre), which may or may not go hand in hand with a reduction in the dye concentration (g dye dm-3 water). When fresh dye solution is supplied in the nozzle, the dye amount rises again. A similar effect has been found for continuous contacting treatment with squeezing rollers. These experiments showed that the fabricfluid interaction is limited to the moments immediately before and after the squeezing when the dyebath is forced out of the fabric or replenished. This effect is qualitatively depicted in Figure 1. It shows a decreasing bulk dye solution concentration, arbitrarily chosen to diminish linearly with time. The dye solution concentration at the fibre surface will be identical to the bulk concentration until the fabric is lifted out of the dyebath if it is assumed that no liquid diffusion effects exist and if dye amount gradients in the substrate rope are ignored. The amount of dye adhering to the fibre surface then drops, as indicated by the grey line, until the solution is replenished in the nozzle. At half the fabric speed, the drop in the amount of dye occurs only half as often, but its duration is twice as long and the drop is also deeper (dotted line). The lower the fabric circulation frequency, it could therefore be argued, the lower the average amount of dye at the fibre surface and the lower the dye-uptake rate in the case of a liquid diffusion-controlled process.Dyebath circulation frequencyHigher dyebath flow rates lead to higher fluid velocities in the nozzle and in the storage chamber and, therefore, to a better dye supply in the inner layers of the fabric rope, possibly leading to a higher exhaustion rate.Liquor ratioThe liquor ratio is especially significant in cellulosic dyeing when it can strongly influence the rate of exhaustion which tends to increase as the liquor ratio decreases.To the authors knowledge, the effect that dyebath and fabric velocities of jet dyeing machinery have on exhaustion kinetics has never been examined systematically. It is the intention of this study to provide some answers in this context including semiquantitative explanations for the observed influences.ExperimentalDyeings were carried out on a pilot-scale jet dyeing machine type Mathis JFO with a jet nozzle diameter of 55 mm and an average fabric load of 0.85 kg. The fabric was stored in a freely rotating, perforated stainless steel drum that prevented direct contact between the fabric and the heated vessel wall. The fabric piled up in the drum was only partially submerged by the dyebath level at the employed liquor-to-goods ratio of 8:1. The fabric speed could be continuously adjusted between 0 and 30 m min-1. Fabric circulation times were recorded with the help of a magnetic detector installed next to the circulation chamber. The dyebath flow rate was adjustable between 0 and 200 dm3 min-1 and was monitored by a magnetic flow meter in the circulation line. Dyebath samples were taken during the process via a sampling port in the circulation line.For the experiments, a reactant fixable dye, Optisal Yellow 2RL from Clariant (CI Direct Yellow 162; 1) was used in its commercially available form. It is a metal-free azo dye with low migration tendency Society of Dyers and Colourists (SDC) class B/C. The dye has a molecular weight of 1237 g mol-1 and possesses four sulphonic acid groups and two azo bonds. The impurities, mostly sodium sulphate, constituted 56% of the commercial dye. Sodium chloride was used as electrolyte.All the experiments took place at 65 C at 0.45% owf dye concentration and with 10 g dm-3 salt. These conditions create a high substantivity environment, which is most favourable for the detection of liquid diffusion influences on the dye-uptake rate. The fabric was a pure cotton jersey with a dry weight of 136 g m-1 which had previously undergone an alkaline scouring treatment on a bulk machine in order to minimise lot-tolot differences. All the salt was added first and subsequently dye addition occurred by using either a pressurised 0.5 dm3 tank or a 1 dm3 open tank which connects to the main circulation line via a gear pump. Times were recorded from the moment when the dye was added. Samples were normally taken after 3, 5, 15, 30, 45, 60, 90 and 120 min. In some instances, the intervals were changed to 3, 5, 7, 10, 15, 30, 60 and 120 min.Dye exhaustion was determined indirectly via analysis of the dyebath. The dyebath absorbance was measured at 410 nm on a K-Tron Uvicon 860 dual beam spectrophotometer using cells of 10 mm pathlength and then converted into a concentration based upon a previously determined calibration curve. Three repeat dyeings yielded an average variation in the exhaustion value of 1%.Initial tests had shown that, after the dye addition, a small percentage of the dye remained in the tanks and their adjacent pipes. In the calculations, these dye losses were taken into account.Results and DiscussionSimulation of production-scale jet dyeing conditions on pilot scaleBefore the pilot-scale dyeings were carried out, attempts were made to establish the pilot-scale machine parameters that would most appropriately scale down those found in bulk production.Liquor ratioAchieving the same low liquor ratio as on production machines is a major challenge for pilot-scale jet machine designers. The main reason for the difficulty lies in the higher proportion of dye liquor contained in the pipework of the pilot-scale machine. The pipe size cannot be scaled down proportionally because the pilot plants are usually specified to run normal width fabric. Therefore, the nozzle and also the attached pipe have to be of a similar size as that of the production unit. The pilot-scale unit was, for the fabric type and load used in the tests and at the specified flow rates, capable of dyeing at a minimum liquor ratio of 8:1, which is equal to or slightly higher than the liquor ratio employed by a typical modern cotton jet dyehouse.Fabric circulation rateBulk-scale jet dyeing machines typically run at speeds between 150 and 600 m min-1 , resulting in a fabric rope circulation time of 60180 s, depending on the length of fabric rope. The 0.85 kg of fabric used on the pilot-scale machine had a length of 6.25 m. Therefore, if the same fabric circulation times were to be achieved on the pilotscale jet, a fabric speed of between 2 and 6 m min-1 would seem appropriate.Dyebath circulation rate Dyebath circulation times for a modern production jet dyeing machine vary between 60 and 120 s, although values of up to 360 s are possible. With a load of 0.85 kg on the pilot-scale jet and a liquor ratio of 8:1, the average dyebath volume was calculated to be around 7 dm3 . If the same dyebath circulation times were to be achieved on pilot scale, the dyebath flow rate would have to be between 1 and 7 dm3 min-1 .Fabricliquor interchange in storage chamber It is impossible to replicate the situation of a bulk machine because its package density is much higher. The fabric in the pilot-scale machine should therefore be more accessible for the dyebath, but this effect might be offset by a reduced dye transfer between fabric layers. The lower compression and the much-reduced acceleration in the case of the pilot-scale unit lead to less de-watering of the fabric before it enters the nozzle. The overall influence of these different parameters is unknown and it would appear that the interchange in the storage chamber of the pilot-scale machine could be smaller, equal or greater than that of a production machine.Nozzle fluid velocity and fabricliquor interchange The pilot-scale machine came with a 55 mm diameter circular nozzle, having a length of 75 mm and a slit width for the liquor of around 3 mm. As the slit area of the nozzle is known, the average liquor speed in the nozzle can be calculated asa function of the dyebath flow rate (Table 1). Production jets are reported to have a liquor speed in the nozzle between 200 m min-1 on soft flow machines and up to 1400 m min-1 on pure jets, i.e. between 1.5 and 3 times higher than the fabric speed. The liquor in the pilot jet nozzle would therefore have to reach a speed of between 3 and 18 m min-1 for fabric speeds of 2 and 6 m min-1 , respectively.Altogether, it would therefore appear that the pilot machine settings listed in Table 2 appropriately scale down production equipment conditions. There are, however, two important differences between the pilot jet and the production jet.First, the jet of the pilot machine provides much le