Jul-2020

Increase duty in tube side condensers

A retrofit is shown to improve several aspects of tube side condensation processes.

PETER DRÖGEMÜLLER
Calgavin Ltd

Article Summary

Tube side condensers are commonly used in refining. Allocation of the condensing fluid to the tube side can be driven by the requirement for low inventories, use of expensive corrosion resistant materials, or where air coolers are used. When condensing single component vapours, heat transfer coefficients are high. The situation changes when condensing multi-component mixtures or if condensation takes place with inert components. Under those conditions, the performance of condensers is often controlled by additional mass transport limitations between the liquid and vapour interface. To maintain the condensation process, the multi-component vapour must be cooled. This becomes a challenge since, particularly at low vapour velocities, the resulting sensible vapour cooling coefficient is low. In the case of horizontal in-tube condensation, as is found in air cooled condensers, at low vapour velocities towards the end of the condensation process stratified gravity-controlled flow is encountered. This can lead to thermodynamic non-equilibrium conditions which makes it difficult to correlate with standard heat exchanger design software.

Use of the hiTRAN thermal system improves several aspects of the tube side condensation process. The main benefit is the increased sensible vapour cooling coefficient. The wires also promote increased turbulence in the condensate film; in addition, film and vapour mixing contributes to a reduction in mass transport resistance. After explaining the underlying fundamentals of the use of hiTRAN thermal systems in multicomponent condensation, an industrial case study is presented where the systems were used to increase the performance of the condenser.

When condensing multi-component mixtures, or if condensation takes place with inert components, there are two major differences compared to pure component condensation.

Temperature profile along condensing path
When condensing a pure vapour, the condensation temperature is closely linked to the total pressure in the tube increment. It is therefore only influenced by frictional pressure losses and momentum losses or gains. In general, the condensation temperature remains almost constant along the tube length/condensation path (see Figure 1).

The situation is very different when condensing vapours containing non-condensable components in the applied temperature range. The condensing component depletes along the condensation path and, as a result, the partial pressure of this component and the associated saturation temperature reduces. To maintain the condensation process, the multi-component vapour must be cooled accordingly. With constant cooling temperature this also equates to a loss in driving temperature difference between condensing vapour and cooling medium (see Figure 1).

Due to the reduction in volume, vapour velocities in condensers are lower towards the exchanger exit. With low resulting Reynolds numbers, the sensible vapour cooling coefficients will be low in this region. Therefore, heat transfer enhancement provides the greatest benefit towards the exit of the exchanger.

As the subsequent case study shows, hiTran enhancement can be installed partially. It can therefore target locations which benefit most from tube side enhancement.

Temperature profile in a tube cross section
There is also a change in temperature profile over the tube cross section compared with pure component condensation.

The temperature profile in the plane along the condensate film and vapour flow in the presence of non-condensables is shown in Figure 2. Since condensable vapour components condense at the cold film, the mole fraction of inerts and more volatile vapour components increases at the interface between condensate film and vapour. As a result, the partial pressure of the condensing component reduces with reduced concentration nearer to the interface (light blue solid line). Since the saturation temperature of the condensing component is a direct reflection of local partial pressure, a characteristic temperature profile in the liquid vapour interface is formed (dark blue solid line). Without a concentration gradient, the condensing temperature would be constant over the cross section of the tube and shifted to higher values (dark blue dotted line).1 This indicates that, by ideal redistribution of the inert components to the bulk flow, the reduction in driving temperature difference (Δ tloss) could be prevented. In highly turbulent vapour liquid interfaces, for instance with high vapour velocities, the concentration gradient is reduced. Again, at lower vapour velocities, towards the exit of the exchanger, concentration gradients are more noticeable.
Case studies

Retrofit of underperforming horizontal condenser (tube side)
The end user reported unwanted hydrocarbon vapour carry-over. Condensation took place on the tube side with evaporating water at a pressure of 2 bar (128.3°C) on the shell side.

The main condensing component, at over 98% mass, was aromatic hydrocarbons. The remaining vapour contained >1% carbon disulphide (CS2) and some water content. Operating pressure and temperature were reported as 120 kPa and 151°C inlet temperature respectively. At this temperature and pressure level, CS2 acted as a non-condensable. Design mass flow was given with 25500 kg/hr. The exchanger was designed as a horizontal inclined BXM type, with 490 x 5m x 25mm x 2mm tubes.

The goal was to reduce hydrocarbon vapour carry-over by improving the cooling duty of the condenser.
Initial evaluations regarding the reported conditions were undertaken using HTRI Xchanger Suite² and Aspen Exchangers Design & Rating³ software. The calculated results were similar and did also reasonably reflect the reported plant data.

In Figure 3, the integral condensation curve as a plot of temperature against cumulative heat removal rate is shown as a red solid line. The dotted line with the yellow/green markers shows the corresponding vapour fraction on the secondary y-axis. In addition, the constant shell side cooling temperature of the evaporating water is shown as a blue solid line. Over the first 3m, almost 85% of the vapour condenses. The depletion of condensing components causes a reduction in partial pressure (condensing temperature) which becomes very pronounced towards the end of the condensation process. This is reflected in a considerable reduction in bulk vapour temperature after 3m. The large yellow/green marker indicates that the calculated outlet vapour content was about 4.6%. The outlet temperature corresponds to about 141°C. Further reduction in vapour will require further vapour cooling and associated loss in driving temperature difference to the cooling medium.

In addition, the capability to transfer heat is reduced by a change in flow regimes; this is also shown in the top section of the graph. Simulation results with both heat exchanger design packages indicate shear controlled annular and transitional flow regimes, induced by high vapour velocity, in the entrance section. After about 2.5m, this changes to a gravity controlled stratified wavy flow regime. For the last 500mm, Aspen reports even worse heat transfer conditions with stratified smooth flow.

In Figure 4 the corresponding liquid and vapour Reynolds numbers are shown. It is evident that towards the end of the condensation process, the vapour velocity and associated Reynolds number are reduced considerably; in turn, the condensate flow is increased, leading to an increased liquid Reynolds number. However, since the density of the liquid is about 250 times higher compared to the vapour density, the condensate velocity and therefore the Reynolds number remain low. The liquid flow can be described as laminar to transitional. These conditions lead to very low heat transfer coefficients towards the end of the condenser.