the cade that generates the captcha image, also sets the name of the captcha session variable.
the image tag code on your page is:
<img src="webassist/captcha/wavt_captchasecurityimages.php?field=Sec_Captcha_2&transparent=true&font=fonts/MOM_T___.TTF" alt="Security Code" class="Captcha" />
the captcha session variable name is taken from the field querystring variable that is passed:
field=Sec_Captcha_2
the naming convention for the captcha session is "captcha_<Field URL Value>"