Process and system for automated online fouling prevention of vertical shell and tube gas-to-gas heat exchangers

1 . A process for on-line prevention of fouling of a gas-to-gas heat exchanger wherein the gas-to-gas heat exchanger is a shell and tube gas-to-gas heat exchanger having a tube side and shell side, contaminated gas flows through the tube side and clean gas through the shell side, the process comprising:

a) measuring gas temperatures, composition and flowrates;

b) calculating a heat exchange coefficient from measured temperatures, composition and flowrates of gases in the gas-to-gas heat exchanger to obtain a measured heat exchange coefficient;

c) comparing the measured heat exchange coefficient to a predetermined heat exchange coefficient; and

d) if measured heat exchange coefficient is less than the predetermined heat exchange coefficient, add grit to the contaminated gas;

wherein the contaminated gas has a flow velocity in the tubes sufficient for abrasive cleaning.

2 . The process of claim 1 , wherein the grit is pressurized grit or wherein the grit is provided as in a bag of friable material, as pellets or a compressed solid mass.

3 . The process of claim 1 , wherein step a) comprises:

measuring temperature of the contaminated gas entering the tube side of the gas-to-gas heat exchanger to obtain T hot in ;

measuring temperature of the contaminated gas exiting the tube side to obtain T hot out ;

measuring temperature of the clean gas entering the shell side to obtain T cold in

measuring temperature of the clean gas exiting the shell side to obtain T cold out

measuring the composition of the contaminated gas,

measuring the flowrate of the contaminated gas; and

measuring the flowrate of the clean gas.

4 . The process of claim 3 , wherein the heat exchange coefficient is calculated based on the equation:

μ=q/A LMTD, where μ is the overall heat transfer coefficient, q is the heat transferred, and LMTD is the Log Mean Temperature Differential and wherein the LMTD=(ΔT1−ΔT2)/ln((ΔT1/ΔT2), where ΔT1=T hot in −T cold out and ΔT2=T hot out −T cold in .

5 . The process of claim 1 , wherein the heat exchange coefficient is calculated in real time.

6 . The process of claim 5 , wherein the predetermined heat exchange coefficient is a predetermined percentage of the heat exchange coefficient of the heat exchange in new condition; wherein the percentage is 70-95%.

7 . The process of claim 1 , further comprising repeating steps a) to d).

8 . The process of claim 1 , wherein the grit is hard with sharp edges, wherein the grit is sand, cyclone ash or slag particulates.

9 . The process of claim 1 , further comprising measuring gas velocity.

10 . A system for on-line prevention of fouling of a gas-to-gas heat exchanger wherein the gas-to-gas heat exchanger is a shell and tube gas-to-gas heat exchanger having a tube side and shell side, contaminated gas flows through the tube side and clean gas through the shell side, the system comprising:

a grit injection apparatus, and;

a control system configured to:

receive temperature measurements of the contaminated gas and the clean gas,

receive composition measurements of the contaminated gas and the clean gas,

receive flowrate measurements of the contaminated gas and the clean gas,

calculate a heat exchange coefficient from measured temperatures, composition and flowrates of gases in the gas-to-gas heat exchanger to obtain a measured heat exchange coefficient;

comparing the measured heat exchange coefficient to a predetermined heat exchange coefficient; and

if measured heat exchange coefficient is less than the predetermined heat exchange coefficient, signal the grit injection apparatus to add grit to the contaminated gas;

wherein the temperatures are measured by a plurality of thermocouples.

11 . The system for on-line prevention of fouling of the gas-to-gas heat exchanger of claim 10 ; wherein

the grit injection apparatus comprises a distribution and pressure charging hopper operatively connected to a main hopper and a mechanical pressurization means or pressurizing gas line;

at least one vertical pipe extending from the distribution and pressure charging hopper;

wherein the distribution and pressure charging hopper and the at least one vertical pipe are configured to be pressurized; and

a grit outlet pipe operatively connected to the at least one vertical pipe and extending into an upper plenum of the gas-to-gas heat exchanger;

wherein the grit injection apparatus is configured to provide pressurized grit to the upper plenum.

12 . A heat exchange system comprising:

a shell and tube gas-to-gas heat exchanger having a tube side and shell side, wherein hot contaminated gas flows through the tube side and clean gas through the shell side;

at least one grit injection apparatus, each grit injection apparatus comprising a distribution and pressure charging hopper operatively connected to a main hopper and a mechanical pressurization means or pressurizing gas line;

at least one vertical pipe extending from the distribution and pressure charging hopper;

wherein the distribution and pressure charging hopper and the at least one vertical pipe are configured to be pressurized;

a grit outlet pipe operatively connected to the at least one vertical pipe and extending into an upper plenum of the shell and tube gas-to-gas heat exchanger;

wherein the grit injection apparatus is configured to provide pressurized grit to the upper plenum; and

a control system configured to:

receive temperature measurements of the contaminated gas and the clean gas,

receive composition measurements of the contaminated gas and the clean gas,

receive flowrate measurements of the contaminated gas and the clean gas,

calculate a heat exchange coefficient from measured temperatures, composition and flowrates of gases in the shell and tube gas-to-gas heat exchanger to obtain a measured heat exchange coefficient;

comparing the measured heat exchange coefficient to a predetermined heat exchange coefficient; and

if measured heat exchange coefficient is less than the predetermined heat exchange coefficient, signal the grit injection apparatus to add grit to the contaminated gas.

13 . The heat exchange system of claim 12 , wherein the shell and tube gas-to-gas heat exchanger is a vertical heat exchanger.

14 . The heat exchange system of claim 13 , wherein the vertical heat exchanger has an axially aligned vertical inlet hot gas pipe and grit injection is into the axially aligned vertical inlet hot gas pipe.

15 . The heat exchange system of claim 13 , wherein the vertical heat exchanger has a side inlet hot gas pipe; wherein the grit injection apparatus is axially aligned with the vertical heat exchanger or wherein grit injection is into the side inlet hot gas pipe.

16 . The heat exchanger system of claim 12 , comprising at least two grit injection apparatus.

17 . The heat exchanger system of claim 16 , wherein the at least two grit injection apparatus are configured to distribute grit relatively uniformly across the whole cross section of the shell and tube gas-to-gas heat exchanger.

18 . The heat exchanger system of claim 16 , wherein the at least two grit injection apparatus are configured to distribute grit preferentially to heat exchange tubes that are prone to fouling.