calculating column relief loads

Calculating column relief loads

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 selec-tion of disposal media. In addition, during debottlenecking or revamp-ing 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 vari-ous conditions is more complex

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 Ltddipanjan bHattaCHarya Bechtel Corporation

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 limi-tations 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 debu-taniser in this case and discusses the strengths and weaknesses of each method. There are many emer-gency 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 debutaniserThe debutaniser column separates liquified petroleum gas (LPG) components from light naphtha.

www.digitalrefining.com/article/1000487 PTQ Q2 2010 55

Refluxpump

Productpump

Feedpump

Reboiler

SteamCondensate

CWS CWR

CWS CWR

To flare, R 135F174 psia

391F178 psia

391F

412F

Pset = 214 psiaOff gas

Sour water

Bottom, naphtha product, B

615600 lb/hr, 391F

Distillate, sour LPG, D

58120 lb/hr, 104F

Feed, F

673700 lb/hr, 301F

196000 lb/hrDebutaniser

Refluxdrum

LC

PC

FC

FCTC

LCLC

FC

FC

PDC

Figure 1 Distillation column (debutaniser)

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 debutan-iser bottom is heated by a thermosyphon reboiler utilising medium-pressure steam. Figure 1 shows a flow diagram of the debu-taniser 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 consid-eration, 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)

/

The conventional method for determining the relief load of a

56 PTQ Q2 2010 www.digitalrefining.com/article/1000487

column is available in various liter-ature1 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 unal-tered during the relief condition Feed, product, reflux and strip-ping 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)

/ where F = Debutaniser or column feed rate at reliefhF = Specific enthalpy of feed at reliefB = Debutaniser or column bottom rate at reliefhB = Specific enthalpy of bottom at reliefD = Debutaniser distillate rate at reliefhD = Specific enthalpy of distillate at reliefQR = Reboiler heat input at reliefQC = Condenser duty at relief (generally, the design duty can be considered)hL = Specific enthalpy of top tray liquid = Latent heat of vapourisation of top tray liquidR = 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

Top tray

Excess heat

R

F, hFD, hD

B, hB

QR

QC

Debutaniser

Refluxdrum

Figure 2 Distillation column: unbalanced heat envelope

Relief valveopensReflux stops

4IMEMIN

RHBLETA RWOL&

&EED"OTTOMS

2ELIEF

2EFLUX

/VERHEADFROMCOLUMN

$ISTILLATE

Reflux drum fills

Figure 3 Loss of reflux: flow vs time

Qunbalanced

= F hF - B h

B - D h

D + Q

R - Q

C - (F - B - D) h

L

column sump level, and finally reaches zero The column overhead vapour rate decreases, the reflux drum level drops, and the distillate rate decreases to maintain the condenser level and finally becomes zero. Therefore:

Site-wide power failure (SWpF) All electrical equipment fails, therefore the feed pump, the debu-taniser bottom pumps and the reflux pumps stop Assuming all cooling water pumps are electrically driven, the condensing duty is also immedi-ately lost Steam is assumed to flow contin-uously to the reboiler. Therefore:

dynamic simulation of reliefconditionsChemical plants and refineries are never truly at a steady state and this is the case during relief. The transient behaviour of a column is best studied by means of dynamic simulation, which has gained in importance since the 1990s and has been used increasingly successfully as the reliability of simulation soft-ware has increased. The equations for material, energy and composi-tion balances include an additional accumulation term, which is differentiated with respect to time. The inclusion of an accumulation term enables the dynamic model to rigorously calculate compositional changes at each stage and to modify vapour/liquid equilibrium over time.

Unlike steady-state simulation, dynamic simulation works within a Pressure-Flow (P-F) network with two basic equations: resistance and volume balance. The resistance equation defines flow between pres-sure hold-ups, and the volume balance equation defines material balance at pressure hold-ups.

For the case under consideration, the accuracy of dynamic simulation


4IMEMIN

AISPERUSSERP F EI LE2

2ELIEFPRESSURE2ELIEFVALVESETPRESSURE2ELIEFVALVEACCUMULATEDPRESSURE

Peak pressure

Figure 4 Loss of reflux: relief pressure vs time

4IMEMIN

LEVELPU DLO(

2EBOILERSUMP2EFLUXDRUM#OLUMNSUMP

Reflux drum fills

Figure 5 Loss of reflux: holdup level vs time

upset condition relief load, lb/hr temperature, F molecular weightLoss of reflux 124 980 164 49.28Loss of feed 43 650 164 49.28Site-wide power failure 342 796 164 49.28

relief load calculated by conventional method

loss of feed Feed stops immediately After some time, when the column level drops, the bottom product decreases to maintain the

table 1

O O

Qunbalanced

= F hF - B h

B - D h

D + Q

R - Q

C - (F - B - D) h

L

O

Qunbalanced

= F hF - B h

B - D h

D + Q

R - Q

C - (F - B - D) h

L

O O O O O O

O O O O O

Qunbalanced

= F hF - B h

B - D h

D + Q

R - Q

C - (F - B - D) h

L

O

condenser flood, restricting the overhead vapour path and pressu-rising the column The feed is pumped and suffi-cient head is available to maintain the feed flow rate at relief condition Bottom product continues at the same rate. Therefore:

provides extra inputs compared with steady-state simulation: Dimensions, especially volumes, for all static equipment; column bottom and reflux drum levels are set to normal to simulate hold-ups A vendor curve for pressure flow relationships for rotating equipment Specific conductance for control valves (Cv value) for pressure flow relationships, and an actuator mode and rate for valve actuator dynamics Detailed exchanger thermal design for calculation of pressure drop and heat transfer coefficient. If detailed design is not available, a resistance term for the pressure flow relationship and overall UA can be specified Actual tray information such as diameter, flow path, distributor details, weir length and height are required for column hydraulic performance Controller for determining control actions during transitions. Credit is not taken for the control action, which reduces the relief load; for example, the column bottom temperature controller reduces the steam flow rate when the column bottom temperature rises at the relief condition.

loss of reflux condition The reflux pump is stopped in five minutes (see Figure 3). The level in the reflux drum starts to increase (see Figure 5). The overhead vapour from the column continues to flow through the condenser and fill the reflux drum. After 17 minutes, the reflux drum floods and the flow to the condenser is blocked; the column pressure starts to increase (see Figure 4). When the column reaches the set pressure, after about 21 minutes, the relief valve starts to open. Note that the pressure did not reach the maxi-mum accumulated pressure for the given orifice area of the relief valve.

Initially, the level in the column bottom sump decreases as the reflux is stopped, and the bottoms product level control valve closes to maintain the column sump level. The feed continues at a constant rate, since its pressure upstream of


&EED"OTTOMS

2ELIEF

2EFLUX

/VERHEADFROMCOLUMN

$ISTILLATE

Relief valveopen

Relief valveclose

Relief flow

Feed stops

4IMEMIN

RHBLETA RWOL&

Bottom & distillateflow zero

Figure 7 Loss of feed: flow vs time

4IMEMIN

AISPERUSSERPFEIL E2

2ELIEFVALVESETPRESSURE2ELIEFPRESSURE

Peak pressure

2ELIFVALVEACCUMULATEDPRESSURE

Figure 8 Loss of feed: relief pressure vs time

4IMEMIN

THGIEWRALUCELO -

2EBOILERDUTY#OLUMNSUMPMOLECULARWEIGHT

Pinched reboiler duty

$UTY"45HR

Figure 6 Loss of reflux: reboiler duty and molecular weight vs time

the control valve is higher than the relief pressure.

Figure 6 shows the reboiler duty and column sump molecular weight during this relief condition. As soon as the reflux is stopped, the molecu-lar weight in the column sump increases, leading to an increase in the boiling temperature of the column bottoms, finally resulting in reduced reboiler duty.

After 17 minutes, when the path for the overhead vapour was blocked (condenser flooded), lighter components started to fill the column sump and reboiler duty again started to increase. After 21 minutes, when the relief valve started to open, reboiler duty settled, based on the column sump composition at relief condition.

loss of feed condition The feed pump stops after five minutes (see Figure 7). After 10 minutes, the column sump level drops (see Figure 9) and the bottom flow is reduced to maintain the column sump level. As the column overhead vapour starts to decrease (see Figure 7), the reflux drum level decreases and the distillate flow reduces to maintain the reflux drum level. After 20 minutes, when distil-late and bottoms stop completely, only the vapour generated by the reboiler is condensed by the condenser. Figure 10 shows the pinched reboiler duty, condenser duty and column sump molecular weight.

During loss of feed, the column sump molecular weight increases, resulting in reduced reboiler duty. Since the top reflux is maintained at normal flow, the lighter compo-nents start migrating towards the bottom. The column profile starts becoming lighter and the tempera-ture profile starts lowering. This also results in the lower molecular weight of the column overhead vapour. After about 11 minutes, the condenser is not able to fully condense the overhead vapour due to its lower molecular weight, resulting in a rise in column pres-sure (see Figure 8). When the column reaches the set pressure, after about 23 minutes, the relief valve starts to open. Note that the


4IMEMIN

LEVELPUDLO (

2EFLUXDRUM#OLUMNSUMP

Reboiler sump level drops

Column sump level drops2EBOILERSUMP

Figure 9 Loss of feed: holdup level vs time

Pinched reboiler duty

Condenserduty

4IMEMIN

RH"45

YTU$

#ONDENSERDUTY#OLUMNSUMPMOLECULARWEIGHT

2EBOILERDUTY

Figure 10 Loss of feed: reboiler duty and molecular weight vs time

&EED"OTTOMS

2ELIEF

2EFLUX

/VERHEADFROMCOLUMN

$ISTILLATE

Relief valve open

Site-wide power failure

4IMEMIN

RHBLETAR WOL&

Figure 11 Site-wide power failure: flow vs time

pinched reboiler duty at this time is higher because of the lower molec-ular weight in the column sump. After about 35 minutes, all non-condensable or lighter components exit the column, reboiler duty reduces again to about 42% of normal, and the column stabilises at total reflux mode.

Site-wide power failure conditionAssume that site-wide power fail-ure occurs after five minutes (see Figure 11). During the power fail-ure, the feed pump, column bottom pump, reflux pump and cooling water pump stop, and their respec-tive flows become zero immediately. The column sump level increases immediately as the tray inventories are dumped to the bottom (see Figure 13).

As the flows of feed, distillate, bottoms and cooling water are cut, the vapours generated by the reboiler cause the column pressure to increase (see Figure 12). After 11 minutes, the relief valve opens. Initially, there is mass transfer between the vapours from the reboiler and the residual liquid on the trays; progressively, as the trays dry up, the temperature and molec-ular weight of the overhead (relieving) vapour increase. The bottoms progressively become heavier, resulting in a continuous decrease in the reboiler duty (see Figure 14). As the pinched reboiler duty carries on decreasing, the relief valve will eventually close.

During power failure, the relief load is relatively low compared with the loss of feed condition because the pinched reboiler duty is much less due to the high molec-ular weight in the column. During loss of feed, continuing reflux makes the column relatively lighter. The time taken to pressure up the column is much higher in the loss of feed scenario because the condenser is available, compared to the loss of power condition, where condensing duty was lost immediately.

Summaryloss of reflux conditionFigure 15 shows a comparison of relief load values obtained for loss


4IMEMIN

LEVELPUDLO(

2EFLUXDRUM#OLUMNSUMP

Column sumplevel increases

2EBOILERSUMP

Figure 13 Site-wide power failure: hold-up level vs time

4IMEMIN

RH"45

YTU $

THGIE WR AL UC ELO-

2EBOILERDUTY#ONDENSERDUTY

Increasing columnsump molecular weight

Reboiler dutydecreases

-OLECULARWEIGHT

Figure 14 Site-wide power failure: reboiler duty and molecular weight vs time

4IMEMIN

AISPERUSSERPFEILE2

2ELIEFVALVESETPRESSURE2ELIEFPRESSURE

Peak pressure

2ELIFVALVEACCUMULATEDPRESSURE

Figure 12 Site-wide power failure: relief pressure vs time

of reflux. According to the conven-tional method, the predicted relief load is higher than the value obtained by dynamic simulation. In the conventional method, the

assumption is that all of the unbal-anced heat will vapourise the top tray liquid, which has a lower specific enthalpy. The molecular weight and temperature are lower

for the top tray at bubble point and relief pressure when compared to dynamic simulation, which simu-lates reflux failure, resulting in a higher temperature and molecular weight.

In a dynamic simulation of loss of reflux, the column almost reaches a new steady-state condition after 25 minutes. The rectifying section of the column goes dry and only the stripping section is involved in mass transfer. This new steady state can also be reasonably simulated using a steady-state simulator (see Steady-state simulation to obtain relief load and properties).

There is a marginal difference in the relief load obtained by steady-state simulation and dynamic simulation because, in steady-state simulation, the column pressure has been raised to an accumulation pressure (set pressure +10% or +16% based on the scenario), whereas in dynamic simulation the pressure safety valve starts opening at its set pressure and the pressure does not reach the maximum accu-mulated pressure for the selected orifice area. Note that the conven-tional method and steady-state simulation are not time dependent, so the relief load appears constant in comparison with the dynamic simulation relief load.

loss of feed Figure 16 shows a comparison of relief load obtained for loss of feed. The relief load calculated by the conventional method is lower than by dynamic simulation. In the conventional method, the condenser duty equals the design duty and the cooling effect is predominant. In dynamic simulation, the condenser duty is not fixed and the hold-up of the individual compo-nents in the column determines the behaviour of the condenser. Initially, during loss of feed, the reboiler duty decreases due to pinch and the lighter components subsequently travel to the bottoms and the whole column profile becomes lighter. Eventually, the reboiler duty again starts to raise due to the decrease in molecular weight. This phenomenon cannot be evaluated with the conventional


4IMEMIN

RHBLDAOLFE ILE2

Steady-state simulation

Dynamic simulation

Conventional method

Figure 15 Loss of reflux: relief load vs time

4IMEMIN

RHBLDAOLFEIL E2

Dynamicsimulation

Conventional method

Figure 16 Loss of feed: relief load vs time

upset condition relief load, lb/hr temperature, F molecular weightLoss of reflux 90 800 310 62.5Loss of feed 93 500 117 44.2Site-wide power failure 29 250 290 76

relief load calculated by dynamic simulation

table 2

method, but validates the hypothe-sis that, if the pinched duty is too low, the designer should re-evalu-ate the reboiler duty, assuming lighter composition in the column bottoms.

Site-wide power failure Figure 17 shows a comparison of relief load obtained for site-wide power failure. In dynamic simula-tion, the relief load obtained is

much lower than by the conven-tional method. In reality, during this condition, after the trays dry up the column simply acts as a boil-ing pot without mass transfer. The reboiler duty continuously decreases as the contents become heavier with time. According to the conventional approach, reboiler duty and relief rate are calculated at one instant, which is at the start of the emergency (not at the start of

relief). This results in a conservative estimate. The effect of hold-up volumes and time taken to pressu-rise is normally ignored.

The conventional method is the most conservative and requires less effort during design. Steady-state simulation to determine the relief load has limited applicability. For grassroots designs, the conventional method may be the most appropri-ate, as detailed design and/or complete vendor information may not be available at the time of the relief systems design. It also helps to build in inherent design margins for any possible future expansion/debottlenecking operation, and to minimise changes during the late stages of the project due to any unforeseen design development.

Dynamic simulation models the system rigorously and tends to provide more accurate results, taking into account actual system dynamics and configuration. It tries to emulate plant behaviour, which usually results in lower relief loads. Dynamic simulation also provides relief loads based on time, which can be further analysed for optimis-ing the relief systems design. Dynamic simulation can be particu-larly useful in unit revamps, to limit the capital cost involved in relief system modifications.

Steady-state simulation to obtainrelief load and properties Simulate the distillation column into three sections: column, column overhead system and reboiler system The column can also be simulated as a reboiled column (column with a reboiler) with theoretical stages and normal operating pressure Define a reflux stream and feed it to the top tray Define the feed stream and assign an appropriate feed location. Give a normal pressure drop across the column Fix the normal reboiler duty to the energy stream and normal boil-up ratio (as a specification) Converge the column The column overhead system includes a pressure safety valve (PSV), cooling water condenser and reflux drum


4IMEMIN

RHBLD AOL FEILE2

Dynamic simulation

Conventional method

Figure 17 Site-wide power failure: relief load vs time

Refluxpump

CWS

RefluxRecycle

Internal energy duty = external reboiler duty

Set

To condenser

To relief

CWR

Steam

To externalreboiler

Total liquid fromcolumn bottom stage

(internal stream)

Condensate

Off gas

Sour water

Distillate

Bottom

Twinnedcolumn bottom

Feed

Internalenergystream

Debutaniser

Refluxdrum

Externalreboiler

Figure 18 Distillation column steady-state simulation relief condition

Split the overhead vapour from the column to relief and to condenser, and set the relief flow rate to zero Simulate the condenser as a shell and tube exchanger with cool-ing water on the tube side and overhead vapour totally condensed. Simulate the reflux drum, reflux pump, distillate product and reflux The reflux from the reflux pump should be same as the defined reflux stream to the top tray, so connect them through a recycle block The reboiler system should be simulated as a separate shell and tube heat exchanger (external reboiler) in order to study reboiler pinch at relieving conditions Create an internal stream of the total liquid from the bottom stage in the column. The internal stream minus the column bottoms is the feed to the external reboiler, so split the internal stream to the external reboiler and twinned column bottoms. Set the column bottoms flow rate to the twinned column bottoms stream Specify the normal UA to the external reboiler Specify the hot side of the external reboiler. For the case under consideration, the hot-side inlet is steam at its saturation condition and the hot-side outlet is total condensate Increase the column pressure to relief pressure (PSV set pressure + allowable accumulation). Since the bottom pressure is higher (relief

pressure + normal P), the bubble point of the column bottom increases. The temperature differ-ence across the external reboiler reduces, leading to lower external reboiler duty (pinch). The calcu-lated duty of the external reboiler should be equal to the energy stream attached to the column (internal energy stream). Iterate the column internal energy stream so that it matches the external reboiler duty. Even though the LMTD tends to increase in the condenser, many designers tend to restrict the maxi-mum condenser duty to design duty due to uncertainties in the calculation. For this exercise, the condenser duty is limited to the design duty only.

Now the column is at relieving pressure, giving an idea of the reduced reboiler duty and the amount of overhead vapour. The next step is to simulate the cause of overpressure to the maximum convergence of the column. For loss of reflux, increase the flow to relief, so that flow to the condenser is reduced and, ultimately, the flow to the reflux is reduced. Simultaneously reduce the distillate flow step-wise as the reflux pump is stopped. At the same time, keep iterating the column internal energy stream so that it matches the exter-nal reboiler duty. Ultimately, when the reflux and distillate are zero, all the overhead vapour from the column is the relieving flow.

The above methodology can also be extended to other emergencies,


where it is expected that the reliev-ing scenario could approach the steady-state condition.

references1 Sengupta M, Staats F Y, A new approach to relief valve load calculations, May 1978.2 Rahimi Mofrad S, Tower pressure relief calculation, Hydrocarbon Processing, Sep 2008.

Haribabu Chittibabu is an Engineering Specialist in the Advanced Simulation and Analysis group at Bechtel India. He has a bachelors degree in chemical engineering from University of Madras and a masters in petroleum refining and petrochemicals from Anna University, India. Email: [email protected] Valli is an Engineering Specialist in the Advanced Simulation and Analysis group at Bechtel, India. She has a bachelors degree in chemical engineering from Coimbatore Institute of Technology, India, and a masters in chemical engineering from Anna University, India. Email: [email protected] Khanna is Project Engineering Manager with Bechtel India. He has a bachelors degree in chemical engineering from the Indian Institute of Technology, Delhi, India. Email: [email protected] bhattacharya is an Engineering Specialist in the Advanced Simulation and Analysis group at Bechtel, Houston. He has a bachelors degree in chemical engineering from Jadavpur University, India, and masters in chemical engineering from University of Oklahoma. Email: [email protected]

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