Apr-2010

Calculating column relief loads

Conventional, steady-state and dynamic simulation techniques are compared in a study of relief loads for failure modes applied to a distillation column

Haribabu Chittibabu, Amudha Valli and Vineet Khanna, Bechtel india PVE Ltd
Dipanjan Bhattacharya, Bechtel Corporation

Article Summary

Emergency relief in the process industries aims to protect equipment, the environment and operating personnel from abnormal conditions. Appropriate estimation of relief loads under extreme conditions is important for the correct sizing of relief valves and flare headers, and for the selection of disposal media. In addition, during debottlenecking or revamping of process units, adding a new relief valve and modifying the relief system can be very costly and, in terms of construction, difficult to implement.

Estimating accurate relief loads for distillation columns under various conditions is more complex because of compositional changes along the column height. The conventional method of estimating relief load (unbalanced heat method) is normally conservative and leads to bigger relief valves and flare headers, but it is the approach most widely practised. With increasing computing speed and software reliability, process simulation is increasingly used as an important tool for estimating relief load and properties. Steady-state simulation can also be used to estimate the relief load within limitations and can overcome some of the assumptions envisaged in the conventional method. Dynamic simulation provides an alternative method for determining relief load under abnormal conditions.

This article considers different methods for estimating relief load for a distillation column — a debutaniser in this case — and discusses the strengths and weaknesses of each method. There are many emergency cases that apply to a distillation column, and estimation of the maximum possible relief load requires an understanding of plant behaviour and identification of the worst case.

Case study: a debutaniser
The debutaniser column separates liquified petroleum gas (LPG) components from light naphtha. The overhead includes a cooling water total condenser, reflux drum and off-gas valve, which is normally closed. The debutaniser operates at 174 psia and relief is set at 214 psia. The debutaniser bottom is heated by a thermosyphon reboiler utilising medium-pressure steam. Figure 1 shows a flow diagram of the debutaniser under evaluation. Major relief conditions or plant situations identified for the debutaniser are loss of reflux, loss of feed and site-wide power failure.

Conventional method
The conventional approach is also known as the unbalanced heat method, where a mass and energy balance is developed under relief conditions, based on the scenario under consideration, to determine if there is any unbalanced or excess heat. The unbalanced heat is divided by the latent heat of vapourisation of the top tray liquid to give the relief load:

Relief load = Qunbalanced (excess) / λ

The conventional method for determining the relief load of a column is available in various literature1 and hence is not covered in detail here.
There are several assumptions in determining relief loads:
• Feed, products, reflux and top tray liquid compositions are unaltered during the relief condition
• Feed, product, reflux and stripping medium will continue at the normal rate unless the hydraulics at the relieving condition determine otherwise
• Enthalpy is balanced on the top tray and all unbalanced heat will reach and act upon the top tray liquid
• There is enough top tray liquid available to generate vapour during upset conditions.

To determine Qunbalanced, the first step is to develop a sketch around the affected system (see Figure 2) and perform a mass and energy balance in line with the upset condition:

R = Qunbalanced (excess) / λ
where
F   =  Debutaniser or column feed rate at relief
hF   =  Specific enthalpy of feed at relief
B   =  Debutaniser or column bottom rate at relief
hB   =  Specific enthalpy of bottom at relief
D   =  Debutaniser distillate rate at relief
hD   =  Specific enthalpy of distillate at relief
QR   =  Reboiler heat input at relief
QC   =  Condenser duty at relief (generally, the design duty can be considered)
hL   =  Specific enthalpy of top tray liquid
λ   =  Latent heat of vapourisation of top tray liquid
R   =  Relief load

Credit may be taken for reboiler pinch. At relieving pressure, the column temperature rises and the reboiler temperature difference may fall, leading to lower heat input to the column. This is reboiler pinch.2 Assume that the volume of the sump is sufficient to maintain a constant reboiler circulation rate and to re-rate the reboiler to obtain duty at relief condition. If there was a significant reduction in the reboiler duty at relief, the lighter components would begin travelling towards the bottom, causing the duty to rise again. Many designers re-rate the reboiler with feed composition instead of bottoms composition in these circumstances, to maintain a more conservative/realistic reboiler duty at relief.

Loss of reflux
• Reflux stops immediately
• The reflux drum and the condenser flood, restricting the overhead vapour path and pressurising the column
• The feed is pumped and sufficient head is available to maintain the feed flow rate at relief condition
• Bottom product continues at the same rate. Therefore: