Mini-channel heat exchangers for industrial distillation processes

Mini-channel heat exchangers for industrial distillation processes

Van de Bor, D.M.

Vlugt, T.J.H. (promotor)

Mechanical, Maritime and Materials Engineering

Process and Energy

2014-03-03

In this thesis the technical and economic performance of compression-resorption heat pumps has been investigated. The main objective of this thesis was to improve the performance and reduce the investment costs of compression-resorption heat pumps applied in process industry. A model that is able to capture the performance of most heat pumps based on the Carnot and Lorentz COP by only knowing the temperature driving forces and estimating the compressor isentropic efficiency has been developed. By including an economic model for the investment costs of compressors and heat exchangers and including costs for electricity and heating, the model was capable of predicting the payback period for conventional systems. For larger systems of 10 MWth applied to a standard distillation column where compression-resorption heat pumps could use 50% of the lift as temperature glide, predictions show that such a heat pump will have a minimum payback period of approximately 3 years, while systems with a capacity of 2.8 MWth require about 5 years, clearly demonstrating the effect of system size. A more detailed model was implemented to investigate the performance of a compression-resorption heat pump applied to distillation processes in the Dutch industry. In this case, the heat pump could only make use of the temperature glide available in reboilers and condensers, which is much smaller than using the temperature glide in the columns. This is limiting the possible performance of the heat pump. The results showed that for most cases the performance was such that both energetic and cost advantages can be obtained with the implementation of such heat pumps. Further the model showed that in case of wet compression the inlet of the absorber should be a saturated vapor to reach maximum efficiency. To increase the performance of the compression-resorption heat pump and decrease the investment cost, the performance of mini-channel heat exchangers operating with a two-phase ammonia-water mixture was experimentally investigated. Initial research focused on the absorber performance in a mini channel annulus with a hydraulic diameter of 0.4 mm and a length of 0.8 m. Absorption side heat transfer coefficients in the range of 1000 to 10000 W m-2 K-1 were obtained for mass fluxes between 75 and 350 kg m-2 s-1 while the average vapor quality ranged from 0.2 to 0.6. The pressure drop varied between 0.2 and 1.6 bar under the given conditions and correlated with literature models within +25% / -25%. The heat transferred from shell to tube side ranged between 50 and 300 W. At low vapor qualities the heat transfer coefficient increases sharply between mass fluxes of 100 and 175 kg m-2 s-1. This behavior was less profound during experiments at higher vapor qualities. The tube side of the same heat exchanger was also investigated using the ammonia-water mixture during a desorption process. The tube side had a diameter of 1.1 mm and a length of 0.8 m. The obtained desorption side heat transfer coefficients lie in the range between 5500 and 10500 W m-2 K-1. The mass fluxes ranged from 150 to 300 kg m-2 s-1 and the average vapor quality ranged from 0.2 to 0.5. The heat transfer performance was well predicted by a model from literature after one of the empirical constants was adjusted. Due to ongoing deposition of debris in and in front of the channel the pressure drop increased over time such that a clear trend in pressure drop as function of mass flux and vapor quality could not be derived. The heat transferred in the heat exchanger under the given conditions ranged from 50 to 250 W. 116 One of the problems with mini-channel heat exchangers is upscaling. One tube can deliver up to 250 W, so for a system of 10 MWth 40 000 tubes are required. Further problems arise with flow distribution: one wants to distribute the flow such that each tube gets the same amount of liquid and vapor. As an intermediate step a heat exchanger was designed comprising of 116 tubes with a diameter of 0.5 mm. The shell side has a hydraulic diameter of 1.8 mm and an inner diameter of 21 mm. The flow distribution for single phase flows was first analyzed using a model which could capture the effects of contraction and expansions in the distributor and a similar design of the collector. Modeling the pressure drop in the shell side of a heat exchanger with the given geometry is complex. To simplify the approach, the Chilton-Colburn method has been chosen to be able to predict the friction factor in the shell side. The results from the model using water as working fluid showed that the flow is distributed evenly over all the tubes, deviations from the average were smaller than 0.1%. The heat transfer coefficients obtained with water as the working fluid on both sides lie between 750 and 850 W m-2 K-1 for mass fluxes between 5 and 30 kg m-2 s-1. The heat transfer model predicts 800 W m-2 K-1 for all heat transfer experiments, and the variation between the experiments can be merely seen as the deviations possible in the measurements and data reduction. Heat transfer experiments using the ammonia-water mixture have been conducted on this heat exchanger. When using this mixture in the shell side of the heat exchanger, it becomes clear that the heat transfer performance is lower compared to the same unit working with water in both shell and tube side. The shell side heat transfer coefficient is the limiting factor during the experiments. The overall heat transfer coefficient during the experiments ranged from 150 to 600 W m-2 K-1 for shell side mass fluxes from 2.7 to 8.1 kg m-2 s-1. The overall heat transfer coefficient now shows an increasing trend with mass flux, while during the water experiments the trend of increasing heat transfer coefficient with increasing mass flux remained within the error of the measurements. During all measurements the flow condition in the tube side was in all cases laminar flow. The ammonia-water mixture has also been put as the working fluid in the tube side of the heat exchanger in such a way that both shell and tube sides operate within the two-phase region. The overall heat transfer coefficients ranged between 300 and 1800 W m-2 K-1. The maximum attainable heat transfer coefficient increased because the mass flux could be increased from 8.1 kg m-2 s-1 to 16 kg m-2 s-1. Again the trend of increasing heat transfer coefficient with increasing mass flux was obtained. By increasing the inlet temperature of the absorber, the average vapor quality increases and the heat transfer coefficients also. Pressure drop ranged from 0.01 bar to 0.3 bar for tube side mass fluxes between 25 and 200 kg m-2 s-1, while the pressure drop on the shell side varied between 0.04 to 0.5 bar at mass fluxes between 2 and 16 kg m-2 s-1. During all two-phase measurements oscillations in flow rate and pressure drop have been identified, while they were stable during single phase flow. These oscillations are most likely caused by Taylor- and hydro-dynamic instabilities. During the desorption processes in the tube side of the heat exchangers the oscillations were larger, however, experiments

compression-resorption heat pump
mini-channel heat exchanger
ammonia-water
heat transfer coefficient
pressure drop

https://doi.org/10.4233/uuid:737def3e-897a-4153-8f57-791a8afa14a8

2014-05-28

9789462590809

Institutional Repository

doctoral thesis

(c) 2014 Van de Bor, D.M.